2D single-faceted IrO2(101) monolayer enabling high-performing proton exchange membrane water electrolysis beyond 8,000 h stability at 1.5 A cm-2

2D single-faceted IrO2(101) monolayer enabling high-performing proton exchange membrane water electrolysis beyond 8,000 h stability at 1.5 A cm-2 | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article 2D single-faceted IrO2(101) monolayer enabling high-performing proton exchange membrane water electrolysis beyond 8,000 h stability at 1.5 A cm-2 Jianguo Liu, Deren Yang, Shouwei Zuo, Aidong Tan, Chunyang Zhang, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5187955/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 06 Aug, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Both commercially available and laboratory-synthesized IrO 2 catalysts typically possess rutile-type structures and diverse facet orientations. According to the theoretical results from density functional theory calculations, distinct IrO 2 facets will result in divergent electrocatalytic properties, among which the (101) crystal facet is theoretically predicted as the most energetically favorable for oxygen evolution reaction (OER) owing to its lowest energy barrier. Maintaining a single-unit-cell thickness while exposing a desired facet of 2D IrO 2 presents a significant opportunity and challenge for the development of high-performance OER anode catalysts. Herein, we develop an ammonia-induced facet engineering for oriented modulation of crystal facets in the ultimate limit of monolayer thickness, and successfully synthesize 2D monolayer IrO 2 exposing unique (101) facet. At the current density of 10 mA cm - 2 geo , an ultralow overpotential of 230 mV has been achieved on the highly activated (101) facet in a three-electrode system. More importantly, in a proton exchange membrane (PEM) electrolyzer, the IrO 2 anode reaches a low voltage of 1.74 V at an industrial-level current density of 2 A cm - 2 geo , much lower than that of all commercial IrO 2 electrocatalysts. Though facet engineering primarily contributes to modulating the intrinsic activity rather than stability, the as-prepared IrO 2 (101) monolayer performs over 8,000 hours of PEM water electrolysis (PEMWE) stability at constant 1.5 A cm - 2 geo , with a negligible decay rate of 4.0 mV kh - 1 . Furthermore, even a long-term PEMWE test of 1000 h using the membrane electrode assembly (MEA) with ultra-low Ir loading of 0.2 mg Ir cm - 2 geo under fluctuating operating conditions is performed, E Cell remains highly electrochemically stable over time at 1.5 A cm - 2 geo , without any signs of catalyst degradation. This work proposes that ammonia-induced facet engineering of 2D monolayer IrO 2 could represent a novel approach to selectively expose the desired (101) facet, thereby enabling unique facet-dependent OER performance and ultrahigh stability in industrial-scale PEM electrolysis, even under voltage fluctuations generated by solar and wind power. Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis Physical sciences/Materials science/Nanoscale materials/Two-dimensional materials Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1 Introduction Hydrogen (H 2 ), benefiting from its low-carbon intensive, high energy capacity and easy recyclability, is widely applied as an ideal energy carrier to ease the burden of energy and climate issues. Large-scale production of green hydrogen is highly dependent on the conversion of surplus electricity from renewable but fluctuating sources like solar, wind, and hydro power 1 . Coupling proton exchange membrane water electrolysis (PEMWE) technology with fluctuating renewables presents a promising avenue for producing green hydrogen, which is highly desired to achieve the international goal of ‘‘carbon neutrality’’. The sluggish kinetics of the oxygen evolution reaction (OER) at the anode are significantly hindered by electrocatalysts, posing a primary challenge to PEMWE commercialization 2 , 3 . An even more critical impediment lies in the limited stability of electrocatalysts under both steady-state and fluctuating operational conditions, especially in a strongly acidic environment. To fulfill the industrial demands of PEMWE systems, there is an urgent need to develop ultrahigh stable and active electrocatalysts, particularly anodic OER catalysts that can operate robustly at industrial-level constant current density or even highly volatile current densities, as generated by fluctuating renewables such as solar, wind, and hydro power 4 . To date, operating current density of 3 A cm − 2 geo under a low electrolytic voltage of 1.80 V is a basic but harsh guideline on OER electrocatalysts for the US Department of Energy (DOE) 2026 technical target. However, extensive current researches reveal an inevitable trend that most IrO 2 -based nanostructured catalysts hardly maintain OER efficiency beyond 120 h even at a low current density of 10 mA cm − 2 geo in a three-electrode system 5 . According to the DOE 2022 durability targets for both PEM electrolyzer stacks and systems, the commercial use of electrocatalyst must allow for long-term operation exceeding 40,000 hours with a decay rate of less than 4.8 mV kh − 1 . The huge research gap between DOE durability targets and the current status of anode electrocatalysts evaluated in the laboratory becomes the most imposing barrier preventing large-scale PEMWE commercialization 6 . Thus, developing novel IrO 2 system to balance OER activity and stability remains a significant challenge towards electrocatalyst design and application, thus leaving a large room for further study. Until now, IrO 2 has been widely deemed as the best anodic catalyst considering the balance between OER activity and stability 7 . Doping engineering, support engineering, crystal phase engineering and morphology engineering have been adopted as effective strategies to maximize intrinsic Ir activity and minimize Ir loading amount 8 . For this reason, a series of novel IrO 2 -based materials, including nanoparticles 9 , nanoneedles 10 , nanotubes 11 , nanosheets 12 , 13 , nanoribbons 14 , and even single atom 15 , 16 , 17 , have been designed, synthesized, and applied for acidic OER. Although various morphology engineering strategies have been implemented to develop highly active IrO 2 electrocatalysts, both laboratory-synthesized and commercial IrO 2 mainly exist in a conventional rutile phase with (110), (101), (200) and (211) crystal facets. Selecting an optimal crystal facet as the growth orientation to construct IrO 2 may offer an alternative pathway for the direct synthesis of high-performance anode electrocatalyst. Through 2D edge epitaxial growth, it is possible for us to achieve the unique crystal orientation in the monolayer limit without substrate persist. Accordingly, 2D monolayer IrO 2 with a unique (101) facet (denoted as IrO 2 (101) monolayer) hold great promise in OER catalysis due to several structural advantages. First, the 2D monolayer nanostructure makes each Ir atom easily accessible to H 2 O molecules during OER. Second, the completely uniform and periodic structure makes all surface atomic sites are equally active and equally accessible, which is close to homogeneous catalyst. Third, the desired (101) facet can be well exposed via altering the ammonia-induced epitaxial growth at a monolayer thickness. As anticipated, the single-faceted rutile structure may simultaneously maximize the intrinsic activity and stability of IrO 2 in both three-electrode cells and PEM device. However, most IrO 2 electrocatalysts are synthesized from the high-temperature oxidation of Ir salts using a traditional Adams combustion method, making facet engineering at a monolayer thickness more challenging 18 . Here, by developing an ammonia-induced facet engineering, we synthesize 2D single-faceted IrO 2 (101) monolayer and provide an insight into the long-term PEMWE applications under various operating conditions. Taking advantages of 2D monolayer structure and highly activated (101) facet, IrO 2 electrocatalyst exhibits a lowest overpotential of 237 mV at 10 mA cm − 2 geo for OER than that of commercial IrO 2 (C-IrO 2 ). More importantly, the PEM electrolyzer employing IrO 2 (101) monolayer as anode catalyst enables over 8,000 h stability test at 1.5 A cm − 2 geo . Even at a low Ir loading of 0.2 mg Ir cm − 2 geo , IrO 2 (101) monolayer anode achieves a low overpotential of only 552 mV and maintains stability for over 2,000 hours stability at industrially-relevant 2 A cm − 2 geo . The ultrahigh OER stability derived from the single (101) facet was also confirmed in the next 1000 h fluctuating test for PEMWE. 2 DFT-guided facet selection Conventional IrO 2 electrocatalysts obtained through the high-temperature Adams fusion method commonly exist as irregular rutile-type nanoparticles with multiple exposed facets 19 . The rutile phase of IrO 2 typically exhibits three primary crystallographic facets: (110), (200) and (101) (Fig. 1 a). Different catalytic facets will lead to different catalytic pathways, ultimately exhibiting different catalytic performances 20 . As a result, the observed OER catalytic activity of IrO 2 often represents an average of these various facet-dependent activities. Achieving optimal OER activity through facet engineering of IrO 2 along a preferential orientation remains a significant challenge. To gain deeper insights into facet-dependent catalytic performance and subsequently guide the synthesis towards desired facet orientation, we conducted comprehensive density functional theory (DFT) calculations using the Vienna Ab initio Simulation Package (VASP). We constructed adsorption models for the (101), (110), and (200) facets of rutile IrO 2 , and calculated the Gibbs free energies (ΔG) of oxygen intermediates (*OH, *O, and *OOH) to quantitatively assess their adsorption strengths based on both the adsorbate evolution mechanism (AEM) and the lattice oxygen mechanism (LOM) 21 , 22 . Our calculations reveal that, for the AEM, the (101) facet exhibits the lowest energy barrier of 0.53 eV (at U = 1.23 V), surpassing the (200) and (110) facets with barriers of 0.64 eV and 0.72 eV, respectively (Fig. 1 b). A similar trend is observed in Fig. 1 c for the LOM, with the (101) facet again demonstrating the lowest energy barrier. Notably, for the (101) facet, the energy barriers for both AEM and LOM are identical, suggesting that during the OER process on this facet, both reaction mechanisms could potentially occur simultaneously or interchangeably. To elucidate the rate-determining step (RDS) across different facets, we present the reaction pathways in Fig. 1 d-f. For the (110) and (200) facets, the RDS in both AEM and LOM is the desorption of O 2 species (step 4 in AEM and step 3 in LOM), indicating strong interactions with O intermediates. In contrast, for the (101) facet, the RDS is identified as the oxidation of *OH to *O (step 2 in both AEM and LOM), while the desorption steps exhibit significantly lower energy barriers compared to the other facets. Analysis of the d-orbital projected density of states (PDOS) of the exposed Ir atoms in the three models reveals that the Ir atoms in the (101) facet exhibit the lowest d-band center at -2.46 eV, followed by the (200) facet at -2.3 eV, while the (110) facet displays the highest d-band center at -2.13 eV (Supplementary Fig. 1). Previous studies have demonstrated that rutile IrO 2 typically exhibits strong binding to O-based intermediates, which impedes the dissociation and desorption of these species, limiting OER activity 14 , 23 , 24 . The downshift of the d-band center can effectively weaken the interaction between the catalytic site and intermediates by increasing the probability of anti-bonding orbital occupation, which is primarily responsible for the optimized OER activity observed on the (101) facet. Based on these findings, constructing IrO 2 catalysts with a preferential (101) facet orientation holds the potential to significantly enhance performance and provide novel mechanistic insights. Nevertheless, facet-controlled synthesis of 2D IrO 2 materials with the desired orientation remains a challenge, particularly at high calcination temperatures during the traditional molten salt method. Guided by DTF prediction, we propose an ammonia-induced crystal phase engineering approach for the synthesis of the rutile IrO 2 monolayer featuring a single (101) facet. 3 Characterization of 2D IrO(101) monolayer In this work, we have successfully synthesized 2D rutile IrO 2 in the monolayer limit, uniquely oriented along the (101) facet. As shown in Fig. 2 a, as-prepared IrO 2 is a freestanding ultrathin 2D material that can be successfully obtained via ammoniating process with K 2 IrCl 6 , KNO 3 and NH 3 ·H 2 O. TEM images reveal the uniform planar structure and graphene-like morphology with an average size of 200 nm (Supplementary Fig. 2). The ultrathin character greatly improves their mechanical flexibility, and further causes surface corrugations and crumples to form on 2D IrO 2 (Fig. 2 b). The high-resolution transmission electron microscopy (HRTEM) image shown in Fig. 2 c further displays the 2D ultrathin structure of rutile IrO 2 with dominant (101) facet. The X-ray diffraction (XRD) technique was then carried out to analyze its crystalline phase and lattice fringe spacing. The selective-area electron diffraction (SAED) pattern marked by red solid box shows a single bright polycrystalline ring (inset in Fig. 2 c). D-values were measured in the calibration patterns with a graphical tool by clicking on the two shortest non-collinear spots. Five obvious diffraction spots selected from this diffraction ring correspond to d-values of 0.2518, 0.2513, 0.2530, 0.2526, and 0.2547 nm, respectively, which are in agreement with the (101) lattice spacing 25 . The corresponding intensity profile along the blue rectangle in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image further exhibits an average space distance of ∼ 2.59 Å, which is almost same as the lattice parameter of 2.58 Å on the rutile-IrO 2 (101) facet (Fig. 2 d) 9 , 26 . All the crystalline characteristics of 2D IrO 2 monolayer confirm a preferential crystal orientation growth along the (101) direction. EDS mapping and line-scanning spectra further display Ir, O and N atoms are well-dispersed over the monolayer structure (Fig. 2 e and Supplementary Fig. 3). The percentages of Ir, O, and N atoms were measured to be 28.52%, 59.33%, and 12.15%, respectively, with an atomic ratio of Ir and O close to the theoretical value of 1:2 (Supplementary Fig. 4 and Supplementary Table 1). According to atomic force microscopy, 2D IrO 2 (101) crystals are approximately 2 ~ 3 nm in height, which is close to the theorical thickness of an IrO 2 (101) -(1×1) unit cell, and exhibit a uniform and smooth surface as measured by 3D structure simulation (Fig. 2 f and Supplementary Fig. 5). In general, commercially available IrO 2 exists in the rutile phase with (110), (101), (200) and (211) characteristic peaks in XRD pattern (Fig. 3 a) 27 . Different crystal facets will bring about different OER pathways, and finally display different OER performances 28 . Obtaining a unique crystal orientation of IrO 2 via the Adams fusion method remains challenging, owing to the isotropic growth that occurs under high-temperature treatment. Interestingly, with the introduction of amine, only a single (101) diffraction peak are observed on the 2D IrO 2 (101) monolayer, indicating the successful construction of a single-faceted structure through ammonia-induced preferred orientation growth. From a macroscopic perspective, the catalyst powder of IrO 2 (101) monolayer exhibits a distinctly different indigo, green color, which contrasts with the grayish powder of C-IrO 2 purchased from Alfa Aesar and Umicore (Supplementary Fig. 6). As shown in Fig. 3 b and Supplementary Fig. 7, the X-ray photoelectron spectroscopy (XPS) measurements were conducted to compare the electronic structures of as-prepared IrO 2 and C-IrO 2 . Except Ir4f and O1s signals, a negligible N1s peak is also detected in XPS spectrum of IrO 2 (101) monolayer, revealing trace N dopant during the ammonia-induced growth process. The Ir4f spectra can be deconvoluted into four peaks, including Ir4f 7/2 , Ir4f 5/2 and two accompanying satellite peaks (Fig. 3 c and Supplementary Fig. 7a) 29 , 30 . The Ir4f binding energies of IrO 2 (101) monolayer and IrO 2 NP are almost the same as that of C-IrO 2 , indicating that the major oxidation state of Ir is + 4. Based on the O1s XPS spectra in Fig. 3 d, the O1s peaks from corresponding lattice oxygen (Ir-O), unsaturated oxygen (Ovancy) and adsorbed oxygen (Oadsorbed) are clearly observed in both as-prepared IrO 2 and C-IrO 2 14 . Interestingly, both the Ir4f 7/2 and the O1s binding energies of IrO 2 (101) monolayer exhibit obvious positive shifts towards higher binding energy than those of IrO 2 NP and C-IrO 2 (Supplementary Fig. 7b and Supplementary Table 2), suggesting a stronger electron-donating effect of 2D monolayer structure than that of nanoparticle structures. Raman scattering is a powerful technique to characterize 2D ultrathin materials because of its strong sensitivity to the electronic structure. The peak positions of the Raman features are summarized in Supplementary Table 3. Interestingly, two major Raman peaks, namely the Eg and A1g modes for rutile IrO 2 NP and C-IrO 2 are identified, respectively, while only an Eg peak is detected at 540.09 cm-1 in the Raman spectrum of IrO 2 (101) monolayer (Fig. 3 e) 31 , 32 . The disappearance of A1g peak clearly reveals that the lattice vibrations of rutile IrO 2 are completely suppressed in the 2D single-faceted monolayer structure. Furthermore, the 2D monolayer nanostructure exhibits highest specific surface area of 353.8 m 2 g − 1 , almost 3 and 50 times higher than that of nanoparticle counterpart (114.2 m 2 g − 1 ) and C-IrO 2 (7.7 m 2 g − 1 ), respectively. As shown in Fig. 3 f, the N 2 adsorption-desorption isotherm of IrO 2 (101) monolayer can be categorized as type IV with a hysteresis loop. The pore-size distribution further exhibits the presence of mesopores with an average diameter of 3.29 nm on the 2D monolayer, which is attributed to the ammonia-induced pore-forming process (Supplementary Fig. 8 and Supplementary Table 4) 9 . The coordination information of the 2D single-faceted monolayer structure was also corroborated by synchrotron X-ray absorption spectroscopy using the Ir L 3 -edge. As shown in Fig. 4 a, the X-ray absorption near-edge spectroscopy (XANES) curve of IrO 2 (101) monolayer is in good accordance with that of C-IrO 2 , indicating a similarity in the rutile phase. Meanwhile, IrO 2 (101) monolayer exhibits a higher absorption energy (11219.7 eV) compared to C-IrO 2 (11219.6 eV), indicating Ir species mainly exist as a more oxidation state. This result is also consistent with those measured from XPS analysis. Figure 4 b compares the Fourier transform (FT) k 2 -weighted EXAFS spectra of IrO 2 (101) monolayer, C-IrO 2 and Ir foil. As a reference sample, Ir foil exhibits a strong peak at 2.7 Å, which corresponds to the Ir-Ir coordination 33 . In contrast to Ir foil, no obvious peaks of Ir-Ir coordination were detected in IrO 2 (101) monolayer. The scattering paths of the Ir centers in IrO 2 (101) monolayer and C-IrO 2 exhibit a prominent peak of Ir-O coordination at 2.00 and 1.97 Å, respectively. Accordingly, the Ir L 3 -edge EXAFS reveals that the length of the Ir-O bond in IrO 2 (101) monolayer is slightly longer than that of C-IrO 2 . The increase in Ir-O bond length may be attributed to lattice expansion induced by the ultrathin monolayer structure and unsaturated coordination environment 34 , 35 . Furthermore, the WT-EXAFS spectrum of IrO 2 (101) monolayer shows that the shell area for Ir-O scattering at R = 1.50 Å and k = 4.50 Å-1, and no shell area for Ir-Ir scattering is observed, implying neither Ir nanoclusters nor nanoparticles exist (Fig. 4 c). The extended X-ray absorption fine structure (EXAFS) results were further Fourier transformed to simulate the real crystal model of IrO 2 (101) monolayer. Figure 4 d exhibits the fitting curve of single (101) facet, which is perfectly reproduced by the experimental FT-EXAFS data. According to Ir L 3 -edge EXAFS fitting parameters in Supplementary Table 5, the first-shell area for Ir-O scatter at R = 2.00 Å, the coordination number (Ir-O) is 5.928 and the bond length is 2.00140 Å. Combining the above XAFS results and previous studies, a schematic model of IrO 2 (101) monolayer is presented in Fig. 4 e. Each Ir atom coordinates with six O atoms to form a single layer of 2D rutile IrO 2 crystal along (101) orientation. 4 The synthesis of IrO(101) monolayer Although many facet engineering strategies have been proposed for tailoring the crystal facets under mild conditions, such as the additive-assisted solvothermal method and template approach, there are few studies on the facet engineering of rutile IrO 2 36 . As a result of the traditional Adams fusion method, rutile IrO 2 tends to grow along various orientations at high oxidation temperatures (400–800°C) and eventually expose different crystal facets. When high oxidation temperature is essential, selecting the optimal crystal facet while maintaining the monolayer thickness of IrO 2 becomes an enormous but meaningful challenge. Considering the above inevitable synthetic difficulties, we developed a novel ammonia-induced facet engineering for oriented regulation of the IrO 2 facets at the ultimate monolayer thickness limit. The key point of this facet-controlled synthesis has been investigated via modulating the synthesis parameters, elucidating that the nucleation and growth of IrO 2 (101) monolayer are dominantly guided by NH 3 ·H 2 O and temperature. During the annealing process, NH 3 ·H 2 O is introduced as a potent crystal facet growth inducer for exposing desired (101) facet. As depicted in Supplementary Fig. 9, IrO 2 exhibits a classic rutile structure with strong (110), (101), (200) and (211) diffraction peaks in the absence of NH 3 ·H 2 O. Upon the addition of 3.2 g NH 3 ·H 2 O, the (110), (200) and (211) signals are significantly suppressed, while the (101) facet emerges as the dominant crystal surface, revealing the role of NH 3 ·H 2 O in facilitating the unidirectionally oriented growth of IrO 2 grains. Particularly, as the NH 3 ·H 2 O amount is further increased to 12.8 g, the characteristic peaks corresponding to the (110), (200) and (211) facets almost disappear. Therefore, IrO 2 (101) monolayer has been successfully constructed through ammonia-induced growth, in which IrO 2 nucleation preferentially occurs along the (101) direction, further demonstrating the effect of NH 3 ·H 2 O on promoting the preferred growth of the (101) facet. Analogously, the morphological evolution of IrO 2 from irregular nanoparticles to 2D monolayers is accompanied by the concurrent crystal evolution, as evidenced by the increased ammonia amount. The successful synthesis of single-(101)-faceted monolayer validates the intimate correlation between the morphological and crystallographic changes, where the preferential growth along the (101) facet guides the formation of the 2D monolayer structure (Supplementary Fig. 10). In addition to NH 3 ·H 2 O, the unidirectionally oriented growth is also highly dependent on the annealing temperature, making it challenging to expose the unique (101) facet at temperatures exceeding 500°C. To understand the role of temperature in crystal evolution, XRD characterization was then conducted at different temperatures to simulate the IrO 2 growth (Supplementary Fig. 11). The heat temperatures of 350°C, 450°C and 500°C are crucial to its crystal growth, during which the crystal facet and structure undergo distinct transformations (labeled as IrO 2 (101)-350, IrO 2 (101) monolayer and IrO 2 (101)-500, respectively). According to the kinetic growth of IrO 2 , the relative nucleation barriers (ΔE) of other facets are much higher than that of IrO 2 (101) under 350°C and 450°C, resulting in the (101) facet becoming the dominating one. As temperature increases to 500°C, the growth velocities of (110), (200) and (211) facets accelerate significantly, which contribute to the isotropic growth behavior. Consequently, a conventional rutile IrO 2 structure with various crystalline facets will evolve preferentially. As evidenced by TEM images in Supplementary Fig. 12, 2D IrO 2 obtained at 350°C, 450°C, and 500°C exhibit average sizes of 100 nm, 500 nm and 2 µm, respectively, which correspond to the bottom-up growth. At 350°C, IrO 2 species tend to first nucleate at the sharp edges of the monolayer. Under a relatively high temperature, the growth rate of the monolayer edge become faster, and then the high-temperature crystal growth process facilitates the precise formation of micrometer-size monolayer structure along the (101) direction. As a result, the single-(101)-faceted structure can be well controlled via 2D edge epitaxial growth in the monolayer limit even without substrate persist. At a temperature of 500°C, the thermodynamically favored isotropic growth poses a major challenge to maintaining the 2D monolayer structure. Consequently, irregular IrO 2 nanoparticles inevitably form on the monolayer IrO 2 (101) surface. 5 OER performance in three-electrode cell In order to study the structural and performance advantages, the OER measurements of IrO 2 (101) monolayer, IrO 2 NP, C-IrO 2 (Alfa Aesar) and C-Ir black were initially carried out in a three-electrode configuration using 0.1 M HClO 4 as the aqueous electrolyte. All reference electrodes were calibrated before OER tests (Supplementary Fig. 13). As plotted in Fig. 5 a, IrO 2 (101) monolayer displays the lowest overpotential of 227 mV at a current density of 10 mA cmgeo − 2 , compared to 307, 311, and 301 mV of IrO 2 NP, C-IrO 2 and C-Ir black (Supplementary Fig. 14a). Even at 50 mA cm geo −2 , IrO 2 (101) monolayer maintains an ultralow overpotential of 272 mV, nearly 200 mV lower than its nanoparticle counterparts (Supplementary Fig. 14b). The most active linear sweep voltammetry (LSV) curve confirms that IrO 2 (101) facet plays the most vital role in optimizing the OER performance over other facets. Tafel slope is another important parameter to probe the response of IrO 2 catalyst to the interfacial charge transfer towards the working potential. As depicted in Fig. 5 b, IrO 2 (101) monolayer has the lowest Tafel slope of 44.73 mV dec − 1 within the potential range of 1.45–1.47 V. For comparison, higher Tafel slopes are obtained by IrO 2 NP, C-IrO 2 and C-Ir black, with delivering 83.06, 70.28 and 80.97 mV dec − 1 , respectively. The OER kinetic on the (101) facet is much faster than those of nanoparticle counterparts, confirming that the 2D single-facet monolayer structure is more favorable for OER pathway 37 . The kinetics of OER processes on IrO 2 (101) monolayer was further evaluated by EIS measurement (Supplementary Fig. 15 and Supplementary Table 6). Fitting EIS curves reveals that IrO 2 (101) monolayer has the smallest charge transfer resistance (RCT) of 10.32 ohm, compared to those measured for IrO 2 NP (109.60 ohm), C-IrO 2 (88.49 ohm), and C-Ir/C (18.06 ohm). We attribute the lowest high frequency resistance to the maximized charge transfer process resulted from the monolayer thickness and highly oriented facet, which is the main origin of the superior OER performance. To identify the origin of monolayer structural advantages in mass transfer, we quantified the surface superaerophobicity of IrO 2 catalysts by assessing the adhesive force between bubbles and the surface (Supplementary Fig. 16). Interestingly, the adhesive force measured on the monolayer IrO 2 (101) surface was found to be merely 17.7 µN, featuring excellent superaerophobicity in electrolyte. Employing IrO 2 NP and C-IrO 2 as contrastive samples, the values of adhesive force were measured to be 52.4 µN and 32.0 µN, respectively. Therefore, the bubble releases from the MEA surface of monolayer IrO 2 (101) seems to be easier in the solution, which can be attributed to the exceptionally low contact region between the bubbles and the uniform monolayer surface and thus low adhesive force 38 , 39 . Above results demonstrate that the fine construction of 2D monolayer interface featuring a specific facet simultaneously optimizes both its kinetic behavior and wetting behavior, thereby facilitating efficient charge and mass transfer from the IrO 2 surface for OER. We further calculated the specific current densities at 1.50 V vs. RHE to study their OER performances. To present a comprehensive performance evaluation of different catalysts, the OER performances were evaluated by normalizing surface areas obtained using different approaches, such as GCE (glassy carbon electrode) geometric areas and electrochemically active surface areas (ECSA) 40 . IrO 2 (101) monolayer exhibits both the highest geometrical activity of 47.75 mA cm geo −2 and ECSA-based activity of 0.027 mA cm ECSA −2 at 1.50 V vs. RHE, nearly 10 times higher than those of IrO 2 NP, C-IrO 2 , and C-Ir/C (Supplementary Fig. 17a, b). The methods to determine the ECSA-based surface areas of different catalysts and clean GCE are summarized in Methods section (Supplementary Figs. 18 and 19). In addition, the mass activities at 1.50 V vs. RHE were calculated to compare their intrinsic activities. By normalizing the Ir loading, rationally designed IrO 2 (101) monolayer performs 238.75 mA mg Ir −1 at 1.5 V vs. RHE, almost 10 times higher than that of other rutile IrO 2 (Fig. 5 c). To gain a comprehensive view of the intrinsic performance, the OER performance of IrO 2 (101) monolayer was also evaluated using 0.5 M H 2 SO 4 as the electrolyte. IrO 2 (101) monolayer exhibits the lowest overpotential of 245 mV at the current density of 10 mA cm geo −2 , compared to 304, 344, and 305 mV for IrO 2 NP, C-IrO 2 , and C-Ir/C, respectively (Supplementary Fig. 20a). The Tafel slope of IrO 2 (101) monolayer in 0.5 M H 2 SO 4 was measured to be only 47.47 mV dec − 1 (Supplementary Fig. 20b). In addition, both mass activities and EIS results illustrate that IrO 2 (101) monolayer shows the highest intrinsic activity and lowest electron transfer resistance (Supplementary Fig. 20c, d). Above OER measurements conducted in 0.5 M H 2 SO 4 electrolyte further demonstrates that the IrO 2 (101) facet provide an ideal catalytic platform for OER process by favoring mass transport, mechanical behavior, and even electron transfer. In the nucleation and growth process, both NH 3 ·H 2 O amount and temperature play vital roles in guiding the (101)-oriented growth of IrO 2 nuclei at a monolayer thickness. A perfect IrO 2 (101) monolayer is predicted to possess highest electrocatalytic performance for OER compared to other traditional nanostructures, due to the ~ 100% exposure of the (101) facet on the monolayer. LSV curves reveal that IrO 2 (101) monolayer exhibits the lowest overpotential of 227 mV to attain a current density of 10 mA cm − 2 geo , which is 80 mV, 36 mV and 22 mV lower, respectively, than its counterparts synthesized with NH 3 ·H 2 O amounts of 0 g, 3.2 g and 12.8 g (Supplementary Fig. 21). Insufficient ammonization is unfavorable for the preferred orientation growth of the highly active (101) facet, whereas excessive ammonization may disrupt the orderliness and integrity of the (101) crystal facet. In terms of annealing temperature, the overpotential of IrO 2 (101)-350 and IrO 2 (101)-500 were measured to be 257 mV and 357 mV at 10 mA cm − 2 geo , respectively, significantly higher than that of IrO 2 (101) monolayer (227 mV) (Supplementary Fig. 22). Thus, 450 o C is identified as the optimal temperature to favor the (101)-oriented growth of 2D monolayer structure. In addition to activity, balancing the operating stability in acidic media poses another significant challenge when considering the structural stability at industrial-level current densities under practical OER condition 41 , 42 . In a three-electrode system, both IrO 2 NPs and C-IrO 2 suffer from dramatically activity loss during the 100 h chronopotentiometry (CP) tests while the working potential of IrO 2 (101) monolayer at 50 mA cm − 2 geo maintains a consistent value after 910 h (Fig. 5 d). The overpotential decay rate was calculated to be only 15 µV h − 1 , demonstrating the superior stability of IrO 2 (101) monolayer. The inset in Fig. 5 d further exhibits that the linear polarization curves of IrO 2 (101) monolayer before and after the CP test perfectly overlap. It also operates stably at the high current of 200 mA cm − 2 geo during a 167 h stability test, confirming its great potential for commercial application (Supplementary Fig. 23). We also found that the 2D monolayer structure seems to be more insensitive to the working potential than previously reported nanostructured IrO 2 . For instance, a strong redox peak of Ir Ⅲ/Ⅳ appears at 0.86 V vs. RHE once an anodic potential is applied, indicating that Ir 3+ has been oxidized to Ir 4+ or an even higher state (Supplementary Fig. 24). Even after 10000 CV cycles, no obvious shift in the redox peak has been detected 43 . The polarization curves and overpotentials of IrO 2 (101) monolayer before and after the 2,000th /10,000th CV cycles tests also prove that its activity loss is negligible despite being a 2D monolayer structure (Supplementary Fig. 25). To provide a comprehensive evaluation of the stability property of IrO 2 (101) monolayer, the inductively coupled plasma-optical emission spectrometer (ICP-OES) was further operated to calculate the dissolution amount of the Ir element after 910 h durability test at 50 mA cm − 2 geo . Form the ICP-OES data in Supplementary Table 7, the S-number for each IrO 2 catalyst was calculated as the ratio of the molar amount of evolved O 2 to the molar amount of dissolved Ir 44 . The S-number of IrO 2 (101) monolayer after the 910 h OER was calculated to be 4.64×10 6 , over 11 and 40 times higher than the S-numbers of IrO 2 NP (4.18×10 5 ) and C-IrO 2 (1.14×10 5 ), respectively (Supplementary Table 8). In addition, IrO 2 (101) monolayer exhibits superior stability compared to other recently reported Ir-based catalysts, including pristine Ir or Ir oxides (1 ~ 9.2×10 5 ) 45 , Ir-based alloys (5 ~ 2.49×10 5 ) 41 , and IrO 2 @TiO 2 nanoparticles (1.0×10 4 ) 46 . Given the high current density and the superlong test time, the performance stability of 2D IrO 2 (101) monolayer in acidic media is further highlighted, pointing to its significant potential for commercialization even at monolayer thickness. As shown in Supplementary Figs. 26 and 27, the monolayer structure and uniform (101) facet orientation of the IrO 2 (101) monolayer remain intact after ~ 1000 h stability test at 50 mA cm − 2 geo . TEM analysis reveals an initial sheet-like morphology with no reconstructed nanoparticles or nanoclusters on the surface. XRD characterization further confirms the presence of only a prominent (101) diffraction peak at 34.93°, without any signals from other rutile facets, demonstrating high stability in the crystallographic orientation during long-term operation. To gain insights into the electronic structure changes after the stability test, XPS analysis was performed. As shown in Supplementary Fig. 28, XPS spectrum of the IrO 2 (101) monolayer exhibits a slight blue shift of 0.15 eV compared to the initial state, suggesting the partial oxidation of surface Ir atoms under acidic OER conditions. Above characterizations after OER stability test confirm that the single-(101)-facet structure realizes a win-win strategy in terms of activity and stability, positioning IrO 2 (101) monolayer as a promising candidate for PEM applications. 6 OER performance in PEM cell The catalyst research evolving from the simple modulation of IrO 2 composition/size on the macroscopic scale to the precise design of IrO 2 structure/morphology at the nanoscale suffer from the same corrosion problem stemming from the dissolution of IrO 4 2− anions into the electrolyte 42 . In general, most previously reported high-performance OER electrocatalysts can only survive in a three-electrode system, while their activity loss usually become magnified in PEM devices after 100 h, which starkly contrasts with the stability results observed on GCE 47 . For the industrial application of PEM electrolyzers, searching an advanced IrO₂ material structure that can meet the technical targets for PEM electrolyzer—high current densities of 2 A cm − 2 geo at low overpotentials (≤ 1.9 V) and long-term stability exceeding 40,000 h at industrial-scale current density with a decay rate below 4.8 µV h − 1 —presents a great opportunity and challenge for a sustainable PEM future 6 , 48 , 49 . As a proof-of-concept of its commercial value, the OER performance of IrO 2 (101) monolayer was evaluated in PEM devices under various operating conditions (Supplementary Fig. 29). As depicted in Fig. 6 a, the catalyst-coated membranes (CCMs) for PEMWE were prepared using IrO 2 (101) monolayer as the anode catalyst (0.2 mg Ir cm − 2 geo or 1.5 mg Ir cm − 2 geo ) and commercial Pt/C (60 wt % Pt, Tanaka Kikinzoku Group) as the cathode catalyst via spray coating on both sides of the Nafion 115 membrane. After assembly, MEAs with IrO 2 (101) monolayer were tested in a 4 cm 2 active area PEMWE single cell at 80°C and ambient pressure, with a preheated Milli-Q ultrapure water flow of 10 mL min − 1 onto the anode. IrO 2 (101) monolayer performs remarkable PEM activity, achieving the standard current density of 2 A cm − 2 geo at a low potential of 1.74 V, significantly lower than E cell of IrO 2 NP (1.82 V@2 A cm − 2 geo ) and C-IrO 2 (1.84 V@2 A cm − 2 geo ), respectively (Fig. 6 b). Additionally, lowing the Ir loading in CCMs below 0.5 mg Ir cm − 2 geo is another crucial issue to meet the deployment and upscaling of PEM electrolyzers, which can be ascribed to the ultralow-earth abundance and extremely high cost ( $ 4700 per oz in 2024) 18 , 37 . Thus, PEMWEs with ultralow-Ir-loading MEA (0.2 mg Ir cm − 2 geo ) were recorded to evaluate catalyst performance. As shown in the inset of Fig. 6 b, E cell reaches 1.78 V at 2 A cm − 2 geo using ultralow-Ir-loading MEA, which is comparable to the best PEMWEs employing same MEAs with substantially higher Ir loadings (1 ~ 2 mg Ir cm − 2 geo ) 50, 51 , 52 . Demonstrating the catalyst stability in PEM device is a more challenging task than achieving high PEM activity. To date, most OER catalyst stabilities are still tested in a simple three-electrode system utilizing a low working current density below 200 mA cm − 2 geo , which perform far from their practical working conditions. Although some recently reported catalysts have demonstrated operation in PEM system for over 100 h at a constant current density of 1 ~ 2 A cm − 2 geo , evaluating their comprehensive working stabilities in all aspects including the stability response to fluctuations in wind/solar energy inputs or ultralow Ir loading, is easy to ignore 6 . Therefore, many high-performance OER electrocatalysts developed from academia often rest in the laboratory, while commercially available IrO 2 is exploring to address technical problems encountered from real applications 53 . Compared to the steady-state condition, voltage fluctuation caused by wind or solar power can exacerbate Ir loss and even catalytic deterioration, ultimately compromising the overall PEM efficiency 54 , 55 . To bridge the research gap of stability tests, we use a square wave voltage toggling between 1.6 and 2.3 V to simulate the fluctuating power patterns of wind or solar photovoltaic sources. As depicted in Fig. 6 c, E Cell over time was measured at 1.6 and 2.3 V for 1 minute each. Notably, voltage switching exhibits little negative effect on PEM durability, suggesting IrO 2 (101) monolayer remains both activity and structural advantages even after 1,000 h fluctuating testing. As for steady-state condition, stability assessments of OER catalysts are typically conducted in a simple three-electrode or PEM system, where constant voltages are applied from external electrochemical workstation. This is a key criteria before we can commercialize it. Figure 6 d firstly compares the stability data of low-Ir-loading PEM electrolyzers. Even with Ir and Pt loading as low as 0.2 mg cm − 2 geo , E cell achieves over 2,000 h stability without obvious activity loss at 1.5 A cm − 2 geo . As shown in Fig. 6 e, IrO 2 (101) monolayer not only meets the DOE technical standards but also significantly outperforms previously reported catalysts in terms of both voltage efficiency and stability 10 , 12 , 33 , 46 , 47 , 56 , 57 , 58 , 59 , 60 , 61 . As evidenced in Fig. 6 f, the IrO 2 (101) monolayer anode cell (1.5 mg Ir cm − 2 geo ) maintains an ultrahigh OER stability of 8,000 h at 1.5 A cm − 2 geo with a negligible degradation rate of 4 µV h − 1 . Up to now, the PEM system is still operating stably. In contrast, for C-IrO 2 and IrO 2 NPs, their efficiency loss is drastically raised during the 500 h OER test, which may be attributed to the accelerated catalyst degradation. All these comprehensive stability tests confirm that IrO 2 (101) monolayer suffers less catalytic deterioration under both dynamic and constant power operation, despite its monolayer rutile (101) structure. Conclusion Guided by theoretical calculations, we have successfully synthesized 2D single-faceted IrO 2 (101) monolayer based on an ammonia-induced preferred orientation growth. Both XRD characterization and TEM observation confirm that IrO 2 (101) monolayer possesses a monolayer rutile structure featuring a single (101) facet. The highly activated (101) facet presents an ultralow overpotential of 237 mV at 10 mA cm − 2 geo in a three-electrode cell, and performs excellent PEM activity with a low E cell of 1.74 V at 2 A cm − 2 geo . Importantly, the practical operating stability of the IrO 2 (101) monolayer anode cell has been tested under various working conditions. Although IrO 2 crystal is constructed in the monolayer limit along a unique growth orientation, IrO 2 (101) monolayer still meets the DOE durability targets for PEMWE. Under steady-state condition, the long-term operation exceeds 8,000 hours with a negligible decay rate of less than 4.0 mV kh − 1 . Even at an ultralow Ir loading of 0.2 mg cm − 2 geo , the PEMWE test for IrO 2 (101) monolayer anode maintains stability for over 2,000 h at a constant 1.5 A cm − 2 geo . When coupled with simulated fluctuating electricity from solar/wind power sources, IrO 2 (101) monolayer anode cell also achieves long-term stability exceeding 1,000 h. This work proposes a novel phase engineering strategy for the design and synthesis of 2D single-(101)-faceted IrO 2 at monolayer thickness, demonstrating outstanding OER activity and long-term PEMWE stability under both steady-state condition and fluctuating condition. Methods Chemicals. Dipotassium hexachloroiridate (K 2 IrCl 6 , 99.9%) was purchased from Alfa Aesar Co. Iridium(IV) oxide (IrO 2 , 99%) was purchased from Alfa Aesar and Umicore. Ir Black (Ir, 99.9%) was purchased from Apinno Co. Ammonium hydroxide (NH 3 ·H 2 O, 25.0–30.0%) and isopropanol were purchased from Beijing InnoChem Science & Technology Co., Ltd. Sulphuric acid (H 2 SO 4 , 95.0–98.0%), perchloricacid (HClO 4 , 70.0–72.0%), and Potassium nitrate (KNO 3 , ≥ 99.0) were obtained form Sinopharm Chemical Reagent Co. Nafion solution (5 wt %) and Nafion N115 membrane was purchased from DuPont Co. Other reagents used were of analytical grade without further purification. Deionized water, supplied by Thermo Scientific Smart2Pure 6 UV Water Purification System, was used throughout the experiments. The PEM device was purchased from China Titanium Guochuang (Qingdao) Technology Co. Synthesis of 2D IrO 2 (101) monolayer. The synthesis of IrO 2 was carried out in a muffle furnace. 240 mg of K 2 IrCl 6 and 6.4 g of NH 3 ·H 2 O were completely dissolved in 40 mL deionized water. After the mixture had been reacting for 3 h ultrasonically, 4.848g of KNO 3 was added and stirred for 1 h. After that, the mixture was heated at 80°C and magnetically stirred until nearly complete evaporation of water was achieved. The resulting powder was then placed in a quartz boat and heated to 450°C at a rate of 5°C min − 1 and held at this temperature for 0.5 h in air. It was then allowed to cool naturally to room temperature. Finally, IrO 2 (101) monolayer was obtained by filtering and washing with deionized water, respectively, followed by drying at 60°C in a vacuum oven overnight. The comparison materials were prepared by altering the calcination temperatures to 350°C and 500°C, and by modulating the volume of ammonia solution to 0 mL, 3.2 mL, and 12.8 mL. Notably, IrO₂ NP was synthesized specifically using 0 mL of ammonia solution. Characterizations. The XRD patterns of all samples were recorded by X-ray powder diffraction (XRD, D8 advance) at an operation voltage of 40 kV and a current of 40 mA, using a Cu Kα radiation source. The TEM images and EDX data of all samples were characterized by a JEOL JEM-F200 transmission electron microscope with an accelerating voltage of 200 kV. AFM images were captured by the atomic force microscopy (AFM, Dimenson ICON). The XPS results were tested by X-ray photoelectron spectrometer (Thermo Fischer, ESCALAB250Xi) using an operation voltage of 12.5 kV and a filament current of 16 mA, with the excitation source using Alka rays ( hv = 1486.8 eV). And the charge correction is based on C1s at 284.80 eV as the energy reference. The BET specific surface areas were characterized using American Micromeritics 3Flex Version 5.00. The Ir dissolution concentration was determined by Agilent 7800 ICP-MS. The Raman spectroscopies were measured using a Laser Microscopic Confocal Raman Spectrometer (inVia™, Renishaw) under excitation by laser light at λ = 532 nm. The adhesion was recorded using an adhesion tester (DCAT 25, DataPhysics) at a line voltage of 24 V DC and an input power of 70 W. XAFS spectra were collected in fluorescence mode at the 1W1B X-ray absorption beamline of the Beijing Synchrotron Radiation Facility (BSRF) using a double-crystal Si (111) monochromator. Iridium foil was measured in transmission mode and used as a reference for energy calibration. Data were analyzed with the ATHENA and ARTEMIS (version 0.9.26) modules of the IFEFFIT software package. The EXAFS functions were Fourier transformed to R-space with a k-weight of 2, within the range of 3.16–12.40 Å⁻¹, using a Hanning window (dk = 1.0 Å⁻¹). Electrochemical measurements. All electrochemical experiments in the three electrode system were carried out using the CHI760E electrochemical workstation (Shanghai Chenhua, China). The glass carbon electrode (GCE, area: 0.196 cm 2 ) was used as the working electrode, calomel electrode was used as the reference electrode, and platinum gauze was used as the counter electrode. OER measurements were conducted in O 2 -saturated 0.1 M HClO 4 or 0.5 M H 2 SO 4 electrolyte. For measurement with iR-correction, R referred to the ohmic resistance arising from the electrolyte/contact resistance of the setup. The scan rate was set to 2 mV s⁻¹ for both LSV measurements ranging from 1.1 to 1.6 V vs. SCE, and Tafel plot measured from 1.1 to 1.4 V vs. SCE. The Electrochemical Impedance Spectroscopy (EIS) tests were conducted with a frequency range from 10 Hz (low frequency) to 100 kHz (high frequency). Chronopotentiometry measurements were performed using the Chronopotentiometry mode on the CHI760 electrochemical workstation, with the current set to constant values of 50 or 200 mA/cm². Cyclic voltammetry was performed using the CV mode on the CHI760 electrochemical workstation, with the voltage range set from 0.05 to 1.25 V vs SCE. The mass activities and ECSA-based specific activities of the catalysts were calculated based on the catalyst mass and electrochemical active surface areas (ECSAs) on the glassy carbon electrodes, respectively. The catalysts solutions were obtained by mixing 4.0 mg IrO 2 catalyst in a solution of 800 µL isopropanol, 200 µL deionized water and 40 µL Nafion solution (5 wt%), with sonication to form the homogenous catalyst ink. Then, 10 µL of the ink was dispersed on the GCE with drying naturally for testing. In the stability test, Ti felt (area: 1×1 cm 2 ) was used as the working electrode, and 0.1M HClO 4 saturated with O 2 was used as the electrolyte. The homogeneous catalyst ink was formed by ultrasonic mixing of 5.0 mg catalyst in 500 µL isopropanol, 500 µL deionized water and 10 µL Nafion solution (5 wt%). 200 µL of ink was dispersed on the Ti felt. For the PEM experiments, the anode catalyst ink was prepared by mixing 10 mg of IrO₂ catalyst with 108 µL of deionized water, 1327 µL of isopropanol, and 44.4 µL of 5 wt% Nafion solution. The cathode catalyst ink was prepared by mixing 10 mg of Pt/C catalyst with 128 µL of deionized water, 1528 µL of isopropanol, and 95.3 µL of 5 wt% Nafion solution. Both inks were then thoroughly mixed using an ultrasonic bath and a cell disruptor. After stirring for over 24 hours, the catalyst ink was uniformly sprayed on the central 2.5×2.5 cm 2 area of the N115 proton exchange membrane at 90 ℃ using ultrasonic spraying machine. Carbon paper and Ti felt were pressed on the outside of the catalytic layer using a hot press at a pressure of 450 kg and a temperature of 140 ℃ for ten minutes. The anode and cathode loadings of MEAs were calibrated using X-ray fluorescence (XRF) spectroscopy. After the MEA was prepared, PTFE sealing materials and Ti bipolar plates were assembled around the MEA to form an electrolyzer. For all PEM tests, deionized water was used as the electrolyte at a flow rate of 18.2 ml min − 1 , with a working temperature of 80°C, and voltage was supplied by an NGI programmable DC power source. The I-V curve was measured from 0 to 3 A cm − ², with 25 current points, each held for 30 seconds. Steady-state tests were conducted using a cross-flow configuration at 1.5 A cm − ². Non-steady-state tests were conducted using a square wave pattern with voltages of 1.6 V and 2.3 V, each held for 1 minute. The XRD, XPS, and TEM results of IrO 2 (101) monolayer after the stability test were obtained by analyzing the used MEAs. The ECSAs of the IrO 2 -NS were estimated from double layer capacitance (C DL ) and specific capacitance (C S ) using the following equation: $$\:\text{ECSA=}{\text{C}}_{\text{DL}}/{\text{C}}_{\text{S}}$$ The electrochemical double-layer capacitance (C DL ) at non-Faradic potential range was obtained by measuring the capacitance of double layer at solid-liquid interface employing cyclic voltammetry (CV) with different scan rates (5, 10, 20, 30, 40, 50, 60 and 70 mV s − 1 ) in a range from 1.25 to 1.35 V vs. RHE. The C S value was found to be 0.036 mF cm − 2 in 0.1 M HClO 4 electrolyte using a polished glassy carbon-rotating disk electrode from the CV curves in Supplementary Fig. 18. The C DL values of IrO 2 (101) monolayer, IrO 2 NP, C-IrO 2 and C-Ir black were estimated from the CV curves in Supplementary Figs. 18 and 19. DFT calculations Density functional theory (DFT) calculations were conducted using the Vienna Ab initio Simulation Package (VASP) 62 , 63 , 64 , 65 . The interactions between ion cores and valence electrons were described using the projector augmented wave (PAW) method 66 , 67 . For the exchange-correlation functional, we employed the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) formulation 68 . The wave functions at each k-point were expanded using a plane wave basis set, with the kinetic energy cutoff set to 500 eV. Geometric optimizations were performed using a force-based conjugate gradient method until the energy converged to 1.0×10 − 5 eV per atom, with the maximum force on each atom less than 0.05 eV Å −1 . To account for long-range van der Waals interactions, Grimme’s D3 dispersion correction was implemented 69 , 70 . The crystal structures of rutile-IrO 2 were used to construct slab models for the electrocatalytic study. Periodic slab models of the IrO 2 (110), (200), and (101) facets were constructed using 2×1, 2×2, and 1×2 surface supercells, respectively, each comprising four metal-oxide layers. The bottom layer was constrained to maintain bulk-like properties. A vacuum space of 15 Å was introduced between the slab and its periodic image to minimize inter-slab interactions. To evaluate the activity of the electrocatalysts, we estimated the free energy diagram using the following Eq. 7 1 : $$\:\text{ΔG=ΔE+ΔZPE-TΔS}$$ where ΔG is the change in Gibbs free energy, ΔE represents the total energy change derived from DFT calculations, ΔZPE is the change in zero-point energy, ΔS denotes the entropy change, and T is the temperature (298.15 K). Declarations Competing interests The authors declare no competing financial interests. Additional information Supplementary Information is available for this paper. Reprints and permissions information is available. Correspondence and requests for materials should be addressed to D.Y. or J.L. Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Author contributions J.L. led the whole project. D.Y. and C.Z. contributed equally to this work. D.Y. guided synthetic process, performed data analysis and wrote the manuscript. C.Z. synthesized the catalysts, performed TEM, XRD, XPS, Raman, ICP and electrochemical measures. A.T. performed DFT calculation and analysis. S.Z. performed XAFS measurements and analysis. Acknowledgements This work was supported by the National Key R&D Plan of China (2021YFB4000101), Interdisciplinary Innovation Program of North China Electric Power University, the National Natural Science Foundation of China (22205061), Hebei Province Outstanding Youth Fund (B2024502007), the Double First-class University Construction Project of North China Electric Power University (XM2412302) and the Fundamental Research Funds for the Central Universities (JB2024087). Data availability. The data that support the plots within this paper and other finding of this study are available from the corresponding author upon reasonable request. Received: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff)) References Liu R-T et al (2023) Recent advances in proton exchange membrane water electrolysis. Chem Soc Rev 52:5652–5683 Song J et al (2020) A review on fundamentals for designing oxygen evolution electrocatalysts. 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Phys Rev B Condens Matter 47:558–561 Kresse G, Hafner J (1994) Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium. Phys Rev B Condens Matter 49:14251–14269 Kresse G, FurthmГјller J (1996) Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys Rev B 54:11169 BlГ¶chl PE (1994) Projector augmented-wave method. Phys Rev B 50:17953 Kresse G, Joubert D (1999) From ultrasoft pseudopotentials to the projector augmented-wave method. Phys Rev B 59:1758 Perdew JP, Burke K, Ernzerhof M (1996) Generalized gradient approximation made simple. Phys Rev Lett 77:3865 Grimme S, Antony J, Ehrlich S, Krieg H (2010) A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J Chem Phys 132 Grimme S, Ehrlich S, Goerigk L (2011) Effect of the damping function in dispersion corrected density functional theory. J Comput Chem 32:1456–1465 Man IC et al (2011) Universality in oxygen evolution electrocatalysis on oxide surfaces. ChemCatChem 3:1159–1165 Additional Declarations There is NO Competing Interest. Supplementary Files SupportingInformation.docx Cite Share Download PDF Status: Published Journal Publication published 06 Aug, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5187955","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":362371369,"identity":"198f2aaf-1526-45c9-b072-f99c2031314d","order_by":0,"name":"Jianguo 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University","correspondingAuthor":false,"prefix":"","firstName":"Yang","middleName":"","lastName":"Yue","suffix":""},{"id":362371380,"identity":"bbcb13ef-d63b-426b-9f62-8c1e528945aa","order_by":11,"name":"Louyu Jin","email":"","orcid":"","institution":"North China Electric Power University","correspondingAuthor":false,"prefix":"","firstName":"Louyu","middleName":"","lastName":"Jin","suffix":""},{"id":362371381,"identity":"0a64298f-7d3f-4732-8217-ea529333e920","order_by":12,"name":"Yipeng Zhang","email":"","orcid":"","institution":"North China Electric Power University","correspondingAuthor":false,"prefix":"","firstName":"Yipeng","middleName":"","lastName":"Zhang","suffix":""}],"badges":[],"createdAt":"2024-10-01 14:21:35","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5187955/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5187955/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-025-62665-2","type":"published","date":"2025-08-06T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":67353317,"identity":"48d52d9c-35dc-4f58-b481-44e72163278c","added_by":"auto","created_at":"2024-10-24 04:31:33","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1283307,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTheoretical analysis of the OER for 2D IrO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e(101) monolayer.\u003c/strong\u003e \u003cstrong\u003ea \u003c/strong\u003eCrystal structure illustration of the (110), (200), and (101) facets of the rutile-phase IrO\u003csub\u003e2\u003c/sub\u003e. The Gibbs free energy profiles of OER process on different facets at the applied potentials of 1.23 V via the\u0026nbsp;(\u003cstrong\u003eb\u003c/strong\u003e) AEM\u0026nbsp;and\u0026nbsp;(\u003cstrong\u003ec\u003c/strong\u003e) LOM\u0026nbsp;pathways, respectively. The AEM reaction pathways over the (\u003cstrong\u003ed\u003c/strong\u003e) (110), (\u003cstrong\u003ee\u003c/strong\u003e) (200) and (\u003cstrong\u003ef\u003c/strong\u003e) (101) facets, respectively. The Ir, O, and H atoms are represented with the blue, red, and pink circles, respectively.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-5187955/v1/60b89cfa8e6eae1afb528c4e.png"},{"id":67353304,"identity":"3d9f21ef-84ba-4fc3-933b-28839654afe7","added_by":"auto","created_at":"2024-10-24 04:31:32","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":6384446,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eNanostructure characterization of 2D IrO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2(\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e101) monolayer.\u003c/strong\u003e a TEM image of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer. b HRTEM image of surface corrugation on IrO\u003csub\u003e2\u003c/sub\u003e. c HRTEM image of the (101) facet. The inset shows the SAED pattern of the marked regions. d HRTEM image and the corresponding intensity profile along the blue rectangle in d. e STEM elemental mapping of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer. f AFM image of 2D monolayer IrO\u003csub\u003e2\u003c/sub\u003e with a thickness of ~3 nm. The scale bars in a, b, c, e and f are 100 nm, 10 nm, 5 nm, 50 nm and 100 nm, respectively.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-5187955/v1/4c8c28650680dbed2bbf68cc.png"},{"id":67353313,"identity":"3f904405-71df-4d06-b134-dcfab7d22fb5","added_by":"auto","created_at":"2024-10-24 04:31:33","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1024703,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSpectroscopic characterization of 2D IrO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e(101) monolayer\u003c/strong\u003e.\u003cstrong\u003e a \u003c/strong\u003eXRD curves and \u003cstrong\u003eb\u003c/strong\u003e XPS spectra of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, IrO\u003csub\u003e2\u003c/sub\u003e NP and C-IrO\u003csub\u003e2\u003c/sub\u003e, respectively.\u003cstrong\u003e c \u003c/strong\u003eIr 4f XPS spectra and \u003cstrong\u003ed \u003c/strong\u003eO 1s XPS spectra of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer and C-IrO\u003csub\u003e2\u003c/sub\u003e. \u003cstrong\u003ee \u003c/strong\u003eRaman spectra of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, IrO\u003csub\u003e2\u003c/sub\u003e NP and C-IrO\u003csub\u003e2\u003c/sub\u003e.\u003cstrong\u003e f \u003c/strong\u003eNitrogen adsorption-desorption isotherms of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, IrO\u003csub\u003e2\u003c/sub\u003e NP and C-IrO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-5187955/v1/0d4023be8b7c032c54c80831.png"},{"id":67353774,"identity":"d6671b03-03e5-4358-9dc5-a159021a37d6","added_by":"auto","created_at":"2024-10-24 04:39:33","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1656917,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructure representations of 2D IrO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e(101) monolayer.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e Ir L\u003csub\u003e3\u003c/sub\u003e-edge XANES spectra of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, IrO\u003csub\u003e2\u003c/sub\u003e NP and Ir foil. \u003cstrong\u003eb\u003c/strong\u003e FT-EXAFS spectra of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, IrO\u003csub\u003e2\u003c/sub\u003e NP and Ir foil at Ir L\u003csub\u003e3\u003c/sub\u003e-edge. \u003cstrong\u003ec\u003c/strong\u003e Corresponding wavelet transforms for the k\u003csup\u003e2\u003c/sup\u003e-weighted Ir L\u003csub\u003e3\u003c/sub\u003e-edge EXAFS signals. \u003cstrong\u003ed \u003c/strong\u003eThe FT-EXAFS r space fitting curve of crystal model in \u003cstrong\u003ee\u003c/strong\u003e. \u003cstrong\u003ee\u003c/strong\u003e Crystal structure of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer. The refined structure model is simulated based on the XRD, SAED and EXAFS analysis, Ir (yellow), O (red).\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-5187955/v1/44e78069f5fc5692da01c9ce.png"},{"id":67353306,"identity":"5da94c9c-9e1c-40d3-ab5b-2b3c88608342","added_by":"auto","created_at":"2024-10-24 04:31:32","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":450039,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrochemical OER performance in three-electrode cell.\u003c/strong\u003e \u003cstrong\u003ea \u003c/strong\u003eLSV curves of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, IrO\u003csub\u003e2\u003c/sub\u003e NP, C-IrO\u003csub\u003e2\u003c/sub\u003e and C-Ir black scanned at 5 mv s\u003csup\u003e-1\u003c/sup\u003e in 0.1 M HClO\u003csub\u003e4\u003c/sub\u003e. \u003cstrong\u003eb \u003c/strong\u003eTOF curves of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, IrO\u003csub\u003e2\u003c/sub\u003e NP, C-IrO\u003csub\u003e2\u003c/sub\u003e and C-Ir black. \u003cstrong\u003ec \u003c/strong\u003eCorresponding mass activities of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, IrO\u003csub\u003e2\u003c/sub\u003e NP, C-IrO\u003csub\u003e2\u003c/sub\u003e and C-Ir black. \u003cstrong\u003ed \u003c/strong\u003eChronopotentiometric measurements of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, IrO\u003csub\u003e2\u003c/sub\u003e NP, C-IrO\u003csub\u003e2\u003c/sub\u003e at a high constant current density of 50 mA cm\u003csub\u003egeo\u003c/sub\u003e\u003csup\u003e-2\u003c/sup\u003e, where IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer performs 910 h stability. Inset shows LSV curves of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer before and after stability test.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-5187955/v1/823d43451180b15841bfbcb8.png"},{"id":67353297,"identity":"edb63eb8-6507-4ae9-9808-67fb9f4bd596","added_by":"auto","created_at":"2024-10-24 04:31:30","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":735860,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrochemical OER performance in PEM device under various operating conditions.\u003c/strong\u003e \u003cstrong\u003ea \u003c/strong\u003eSchematic diagram of the PEM cell. \u003cstrong\u003eb \u003c/strong\u003eCell voltage (E\u003csub\u003ecell\u003c/sub\u003e) with respect to the current density with the anode loading of 1.5 mg\u003csub\u003eIr\u003c/sub\u003e cm\u003csup\u003e-2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e and the cathode loading of 0.3 mg\u003csub\u003ePt\u003c/sub\u003e cm\u003csup\u003e-2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e. Inset shows\u003cstrong\u003e \u003c/strong\u003eI-V curves of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer and C-IrO\u003csub\u003e2\u003c/sub\u003e with an ultralow anode/cathode loading of 0.2 mg\u003csub\u003eIr/Pt\u003c/sub\u003e cm\u003csup\u003e-2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e. \u003cstrong\u003ec \u003c/strong\u003eElectrolysis durability test of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer with low Ir loading under simulated fluctuating condition. \u003cstrong\u003ed \u003c/strong\u003eElectrolysis durability test of low-Ir-loading MEAs at a constant 1.5 A cm\u003csup\u003e−2\u003c/sup\u003e. \u003cstrong\u003ee\u003c/strong\u003e Comparison of PEM performance in this work with those in previous studies. \u003cstrong\u003ef\u003c/strong\u003e Electrolysis durability test of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer under steady-state condition, which achieves over 8,000 h OER stability at a constant 1.5 A cm\u003csup\u003e−2\u003c/sup\u003e. PEM devices were carried out at 80 °C and ambient pressure.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-5187955/v1/ce4e18a8ed5622ba475ebd0c.png"},{"id":88509414,"identity":"88c68a0e-d3dd-4d5e-a53d-a3eef52bed0e","added_by":"auto","created_at":"2025-08-07 07:48:45","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":11778814,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5187955/v1/abb5c16b-117f-4966-a4b2-d50654e0ed6b.pdf"},{"id":67353302,"identity":"5452296d-4de1-4cfd-a55e-5ed045893bfa","added_by":"auto","created_at":"2024-10-24 04:31:32","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":36115124,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-5187955/v1/54870847330d9782929b2c8b.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"2D single-faceted IrO2(101) monolayer enabling high-performing proton exchange membrane water electrolysis beyond 8,000 h stability at 1.5 A cm-2","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eHydrogen (H\u003csub\u003e2\u003c/sub\u003e), benefiting from its low-carbon intensive, high energy capacity and easy recyclability, is widely applied as an ideal energy carrier to ease the burden of energy and climate issues. Large-scale production of green hydrogen is highly dependent on the conversion of surplus electricity from renewable but fluctuating sources like solar, wind, and hydro power\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Coupling proton exchange membrane water electrolysis (PEMWE) technology with fluctuating renewables presents a promising avenue for producing green hydrogen, which is highly desired to achieve the international goal of \u0026lsquo;\u0026lsquo;carbon neutrality\u0026rsquo;\u0026rsquo;. The sluggish kinetics of the oxygen evolution reaction (OER) at the anode are significantly hindered by electrocatalysts, posing a primary challenge to PEMWE commercialization\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. An even more critical impediment lies in the limited stability of electrocatalysts under both steady-state and fluctuating operational conditions, especially in a strongly acidic environment. To fulfill the industrial demands of PEMWE systems, there is an urgent need to develop ultrahigh stable and active electrocatalysts, particularly anodic OER catalysts that can operate robustly at industrial-level constant current density or even highly volatile current densities, as generated by fluctuating renewables such as solar, wind, and hydro power\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTo date, operating current density of 3 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e under a low electrolytic voltage of 1.80 V is a basic but harsh guideline on OER electrocatalysts for the US Department of Energy (DOE) 2026 technical target. However, extensive current researches reveal an inevitable trend that most IrO\u003csub\u003e2\u003c/sub\u003e-based nanostructured catalysts hardly maintain OER efficiency beyond 120 h even at a low current density of 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e in a three-electrode system\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. According to the DOE 2022 durability targets for both PEM electrolyzer stacks and systems, the commercial use of electrocatalyst must allow for long-term operation exceeding 40,000 hours with a decay rate of less than 4.8 mV kh\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The huge research gap between DOE durability targets and the current status of anode electrocatalysts evaluated in the laboratory becomes the most imposing barrier preventing large-scale PEMWE commercialization\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. Thus, developing novel IrO\u003csub\u003e2\u003c/sub\u003e system to balance OER activity and stability remains a significant challenge towards electrocatalyst design and application, thus leaving a large room for further study.\u003c/p\u003e \u003cp\u003eUntil now, IrO\u003csub\u003e2\u003c/sub\u003e has been widely deemed as the best anodic catalyst considering the balance between OER activity and stability\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Doping engineering, support engineering, crystal phase engineering and morphology engineering have been adopted as effective strategies to maximize intrinsic Ir activity and minimize Ir loading amount\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. For this reason, a series of novel IrO\u003csub\u003e2\u003c/sub\u003e-based materials, including nanoparticles\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e, nanoneedles\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, nanotubes\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e, nanosheets\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e, nanoribbons\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e, and even single atom\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e, have been designed, synthesized, and applied for acidic OER. Although various morphology engineering strategies have been implemented to develop highly active IrO\u003csub\u003e2\u003c/sub\u003e electrocatalysts, both laboratory-synthesized and commercial IrO\u003csub\u003e2\u003c/sub\u003e mainly exist in a conventional rutile phase with (110), (101), (200) and (211) crystal facets. Selecting an optimal crystal facet as the growth orientation to construct IrO\u003csub\u003e2\u003c/sub\u003e may offer an alternative pathway for the direct synthesis of high-performance anode electrocatalyst. Through 2D edge epitaxial growth, it is possible for us to achieve the unique crystal orientation in the monolayer limit without substrate persist. Accordingly, 2D monolayer IrO\u003csub\u003e2\u003c/sub\u003e with a unique (101) facet (denoted as IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer) hold great promise in OER catalysis due to several structural advantages. First, the 2D monolayer nanostructure makes each Ir atom easily accessible to H\u003csub\u003e2\u003c/sub\u003eO molecules during OER. Second, the completely uniform and periodic structure makes all surface atomic sites are equally active and equally accessible, which is close to homogeneous catalyst. Third, the desired (101) facet can be well exposed \u003cem\u003evia\u003c/em\u003e altering the ammonia-induced epitaxial growth at a monolayer thickness. As anticipated, the single-faceted rutile structure may simultaneously maximize the intrinsic activity and stability of IrO\u003csub\u003e2\u003c/sub\u003e in both three-electrode cells and PEM device. However, most IrO\u003csub\u003e2\u003c/sub\u003e electrocatalysts are synthesized from the high-temperature oxidation of Ir salts using a traditional Adams combustion method, making facet engineering at a monolayer thickness more challenging\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eHere, by developing an ammonia-induced facet engineering, we synthesize 2D single-faceted IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer and provide an insight into the long-term PEMWE applications under various operating conditions. Taking advantages of 2D monolayer structure and highly activated (101) facet, IrO\u003csub\u003e2\u003c/sub\u003e electrocatalyst exhibits a lowest overpotential of 237 mV at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e for OER than that of commercial IrO\u003csub\u003e2\u003c/sub\u003e (C-IrO\u003csub\u003e2\u003c/sub\u003e). More importantly, the PEM electrolyzer employing IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer as anode catalyst enables over 8,000 h stability test at 1.5 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e. Even at a low Ir loading of 0.2 mg\u003csub\u003eIr\u003c/sub\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e, IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer anode achieves a low overpotential of only 552 mV and maintains stability for over 2,000 hours stability at industrially-relevant 2 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e. The ultrahigh OER stability derived from the single (101) facet was also confirmed in the next 1000 h fluctuating test for PEMWE.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"2 DFT-guided facet selection","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eConventional IrO\u003csub\u003e2\u003c/sub\u003e electrocatalysts obtained through the high-temperature Adams fusion method commonly exist as irregular rutile-type nanoparticles with multiple exposed facets\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. The rutile phase of IrO\u003csub\u003e2\u003c/sub\u003e typically exhibits three primary crystallographic facets: (110), (200) and (101) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Different catalytic facets will lead to different catalytic pathways, ultimately exhibiting different catalytic performances\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. As a result, the observed OER catalytic activity of IrO\u003csub\u003e2\u003c/sub\u003e often represents an average of these various facet-dependent activities. Achieving optimal OER activity through facet engineering of IrO\u003csub\u003e2\u003c/sub\u003e along a preferential orientation remains a significant challenge.\u003c/p\u003e \u003cp\u003eTo gain deeper insights into facet-dependent catalytic performance and subsequently guide the synthesis towards desired facet orientation, we conducted comprehensive density functional theory (DFT) calculations using the Vienna Ab initio Simulation Package (VASP). We constructed adsorption models for the (101), (110), and (200) facets of rutile IrO\u003csub\u003e2\u003c/sub\u003e, and calculated the Gibbs free energies (ΔG) of oxygen intermediates (*OH, *O, and *OOH) to quantitatively assess their adsorption strengths based on both the adsorbate evolution mechanism (AEM) and the lattice oxygen mechanism (LOM)\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Our calculations reveal that, for the AEM, the (101) facet exhibits the lowest energy barrier of 0.53 eV (at U\u0026thinsp;=\u0026thinsp;1.23 V), surpassing the (200) and (110) facets with barriers of 0.64 eV and 0.72 eV, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). A similar trend is observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec for the LOM, with the (101) facet again demonstrating the lowest energy barrier. Notably, for the (101) facet, the energy barriers for both AEM and LOM are identical, suggesting that during the OER process on this facet, both reaction mechanisms could potentially occur simultaneously or interchangeably. To elucidate the rate-determining step (RDS) across different facets, we present the reaction pathways in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed-f. For the (110) and (200) facets, the RDS in both AEM and LOM is the desorption of O\u003csub\u003e2\u003c/sub\u003e species (step 4 in AEM and step 3 in LOM), indicating strong interactions with O intermediates. In contrast, for the (101) facet, the RDS is identified as the oxidation of *OH to *O (step 2 in both AEM and LOM), while the desorption steps exhibit significantly lower energy barriers compared to the other facets.\u003c/p\u003e \u003cp\u003eAnalysis of the d-orbital projected density of states (PDOS) of the exposed Ir atoms in the three models reveals that the Ir atoms in the (101) facet exhibit the lowest d-band center at -2.46 eV, followed by the (200) facet at -2.3 eV, while the (110) facet displays the highest d-band center at -2.13 eV (Supplementary Fig.\u0026nbsp;1). Previous studies have demonstrated that rutile IrO\u003csub\u003e2\u003c/sub\u003e typically exhibits strong binding to O-based intermediates, which impedes the dissociation and desorption of these species, limiting OER activity\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. The downshift of the d-band center can effectively weaken the interaction between the catalytic site and intermediates by increasing the probability of anti-bonding orbital occupation, which is primarily responsible for the optimized OER activity observed on the (101) facet.\u003c/p\u003e \u003cp\u003eBased on these findings, constructing IrO\u003csub\u003e2\u003c/sub\u003e catalysts with a preferential (101) facet orientation holds the potential to significantly enhance performance and provide novel mechanistic insights. Nevertheless, facet-controlled synthesis of 2D IrO\u003csub\u003e2\u003c/sub\u003e materials with the desired orientation remains a challenge, particularly at high calcination temperatures during the traditional molten salt method. Guided by DTF prediction, we propose an ammonia-induced crystal phase engineering approach for the synthesis of the rutile IrO\u003csub\u003e2\u003c/sub\u003e monolayer featuring a single (101) facet.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"3 Characterization of 2D IrO(101) monolayer","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eIn this work, we have successfully synthesized 2D rutile IrO\u003csub\u003e2\u003c/sub\u003e in the monolayer limit, uniquely oriented along the (101) facet. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, as-prepared IrO\u003csub\u003e2\u003c/sub\u003e is a freestanding ultrathin 2D material that can be successfully obtained via ammoniating process with K\u003csub\u003e2\u003c/sub\u003eIrCl\u003csub\u003e6\u003c/sub\u003e, KNO\u003csub\u003e3\u003c/sub\u003e and NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO. TEM images reveal the uniform planar structure and graphene-like morphology with an average size of 200 nm (Supplementary Fig.\u0026nbsp;2). The ultrathin character greatly improves their mechanical flexibility, and further causes surface corrugations and crumples to form on 2D IrO\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). The high-resolution transmission electron microscopy (HRTEM) image shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec further displays the 2D ultrathin structure of rutile IrO\u003csub\u003e2\u003c/sub\u003e with dominant (101) facet. The X-ray diffraction (XRD) technique was then carried out to analyze its crystalline phase and lattice fringe spacing. The selective-area electron diffraction (SAED) pattern marked by red solid box shows a single bright polycrystalline ring (inset in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). D-values were measured in the calibration patterns with a graphical tool by clicking on the two shortest non-collinear spots. Five obvious diffraction spots selected from this diffraction ring correspond to d-values of 0.2518, 0.2513, 0.2530, 0.2526, and 0.2547 nm, respectively, which are in agreement with the (101) lattice spacing\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. The corresponding intensity profile along the blue rectangle in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image further exhibits an average space distance of \u0026sim; 2.59 \u0026Aring;, which is almost same as the lattice parameter of 2.58 \u0026Aring; on the rutile-IrO\u003csub\u003e2\u003c/sub\u003e (101) facet (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed)\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. All the crystalline characteristics of 2D IrO\u003csub\u003e2\u003c/sub\u003e monolayer confirm a preferential crystal orientation growth along the (101) direction. EDS mapping and line-scanning spectra further display Ir, O and N atoms are well-dispersed over the monolayer structure (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee and Supplementary Fig.\u0026nbsp;3). The percentages of Ir, O, and N atoms were measured to be 28.52%, 59.33%, and 12.15%, respectively, with an atomic ratio of Ir and O close to the theoretical value of 1:2 (Supplementary Fig.\u0026nbsp;4 and Supplementary Table\u0026nbsp;1). According to atomic force microscopy, 2D IrO\u003csub\u003e2\u003c/sub\u003e(101) crystals are approximately 2\u0026thinsp;~\u0026thinsp;3 nm in height, which is close to the theorical thickness of an IrO\u003csub\u003e2\u003c/sub\u003e(101) -(1\u0026times;1) unit cell, and exhibit a uniform and smooth surface as measured by 3D structure simulation (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef and Supplementary Fig.\u0026nbsp;5).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eIn general, commercially available IrO\u003csub\u003e2\u003c/sub\u003e exists in the rutile phase with (110), (101), (200) and (211) characteristic peaks in XRD pattern (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea)\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Different crystal facets will bring about different OER pathways, and finally display different OER performances\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. Obtaining a unique crystal orientation of IrO\u003csub\u003e2\u003c/sub\u003e via the Adams fusion method remains challenging, owing to the isotropic growth that occurs under high-temperature treatment. Interestingly, with the introduction of amine, only a single (101) diffraction peak are observed on the 2D IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, indicating the successful construction of a single-faceted structure through ammonia-induced preferred orientation growth. From a macroscopic perspective, the catalyst powder of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer exhibits a distinctly different indigo, green color, which contrasts with the grayish powder of C-IrO\u003csub\u003e2\u003c/sub\u003e purchased from Alfa Aesar and Umicore (Supplementary Fig.\u0026nbsp;6).\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb and Supplementary Fig.\u0026nbsp;7, the X-ray photoelectron spectroscopy (XPS) measurements were conducted to compare the electronic structures of as-prepared IrO\u003csub\u003e2\u003c/sub\u003e and C-IrO\u003csub\u003e2\u003c/sub\u003e. Except Ir4f and O1s signals, a negligible N1s peak is also detected in XPS spectrum of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, revealing trace N dopant during the ammonia-induced growth process. The Ir4f spectra can be deconvoluted into four peaks, including Ir4f\u003csub\u003e7/2\u003c/sub\u003e, Ir4f\u003csub\u003e5/2\u003c/sub\u003e and two accompanying satellite peaks (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec and Supplementary Fig.\u0026nbsp;7a)\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. The Ir4f binding energies of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer and IrO\u003csub\u003e2\u003c/sub\u003e NP are almost the same as that of C-IrO\u003csub\u003e2\u003c/sub\u003e, indicating that the major oxidation state of Ir is +\u0026thinsp;4. Based on the O1s XPS spectra in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed, the O1s peaks from corresponding lattice oxygen (Ir-O), unsaturated oxygen (Ovancy) and adsorbed oxygen (Oadsorbed) are clearly observed in both as-prepared IrO\u003csub\u003e2\u003c/sub\u003e and C-IrO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e14\u003c/sup\u003e. Interestingly, both the Ir4f\u003csub\u003e7/2\u003c/sub\u003e and the O1s binding energies of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer exhibit obvious positive shifts towards higher binding energy than those of IrO\u003csub\u003e2\u003c/sub\u003e NP and C-IrO\u003csub\u003e2\u003c/sub\u003e (Supplementary Fig.\u0026nbsp;7b and Supplementary Table\u0026nbsp;2), suggesting a stronger electron-donating effect of 2D monolayer structure than that of nanoparticle structures.\u003c/p\u003e \u003cp\u003eRaman scattering is a powerful technique to characterize 2D ultrathin materials because of its strong sensitivity to the electronic structure. The peak positions of the Raman features are summarized in Supplementary Table\u0026nbsp;3. Interestingly, two major Raman peaks, namely the Eg and A1g modes for rutile IrO\u003csub\u003e2\u003c/sub\u003e NP and C-IrO\u003csub\u003e2\u003c/sub\u003e are identified, respectively, while only an Eg peak is detected at 540.09 cm-1 in the Raman spectrum of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee)\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. The disappearance of A1g peak clearly reveals that the lattice vibrations of rutile IrO\u003csub\u003e2\u003c/sub\u003e are completely suppressed in the 2D single-faceted monolayer structure. Furthermore, the 2D monolayer nanostructure exhibits highest specific surface area of 353.8 m\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, almost 3 and 50 times higher than that of nanoparticle counterpart (114.2 m\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and C-IrO\u003csub\u003e2\u003c/sub\u003e (7.7 m\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), respectively. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef, the N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherm of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer can be categorized as type IV with a hysteresis loop. The pore-size distribution further exhibits the presence of mesopores with an average diameter of 3.29 nm on the 2D monolayer, which is attributed to the ammonia-induced pore-forming process (Supplementary Fig.\u0026nbsp;8 and Supplementary Table\u0026nbsp;4)\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe coordination information of the 2D single-faceted monolayer structure was also corroborated by synchrotron X-ray absorption spectroscopy using the Ir L\u003csub\u003e3\u003c/sub\u003e-edge. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, the X-ray absorption near-edge spectroscopy (XANES) curve of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer is in good accordance with that of C-IrO\u003csub\u003e2\u003c/sub\u003e, indicating a similarity in the rutile phase. Meanwhile, IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer exhibits a higher absorption energy (11219.7 eV) compared to C-IrO\u003csub\u003e2\u003c/sub\u003e (11219.6 eV), indicating Ir species mainly exist as a more oxidation state. This result is also consistent with those measured from XPS analysis. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb compares the Fourier transform (FT) k\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e-weighted EXAFS spectra of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, C-IrO\u003csub\u003e2\u003c/sub\u003e and Ir foil. As a reference sample, Ir foil exhibits a strong peak at 2.7 \u0026Aring;, which corresponds to the Ir-Ir coordination\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. In contrast to Ir foil, no obvious peaks of Ir-Ir coordination were detected in IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer. The scattering paths of the Ir centers in IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer and C-IrO\u003csub\u003e2\u003c/sub\u003e exhibit a prominent peak of Ir-O coordination at 2.00 and 1.97 \u0026Aring;, respectively. Accordingly, the Ir L\u003csub\u003e3\u003c/sub\u003e-edge EXAFS reveals that the length of the Ir-O bond in IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer is slightly longer than that of C-IrO\u003csub\u003e2\u003c/sub\u003e. The increase in Ir-O bond length may be attributed to lattice expansion induced by the ultrathin monolayer structure and unsaturated coordination environment\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. Furthermore, the WT-EXAFS spectrum of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer shows that the shell area for Ir-O scattering at R\u0026thinsp;=\u0026thinsp;1.50 \u0026Aring; and k\u0026thinsp;=\u0026thinsp;4.50 \u0026Aring;-1, and no shell area for Ir-Ir scattering is observed, implying neither Ir nanoclusters nor nanoparticles exist (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). The extended X-ray absorption fine structure (EXAFS) results were further Fourier transformed to simulate the real crystal model of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed exhibits the fitting curve of single (101) facet, which is perfectly reproduced by the experimental FT-EXAFS data. According to Ir L\u003csub\u003e3\u003c/sub\u003e-edge EXAFS fitting parameters in Supplementary Table\u0026nbsp;5, the first-shell area for Ir-O scatter at R\u0026thinsp;=\u0026thinsp;2.00 \u0026Aring;, the coordination number (Ir-O) is 5.928 and the bond length is 2.00140 \u0026Aring;. Combining the above XAFS results and previous studies, a schematic model of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee. Each Ir atom coordinates with six O atoms to form a single layer of 2D rutile IrO\u003csub\u003e2\u003c/sub\u003e crystal along (101) orientation.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"4 The synthesis of IrO(101) monolayer","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eAlthough many facet engineering strategies have been proposed for tailoring the crystal facets under mild conditions, such as the additive-assisted solvothermal method and template approach, there are few studies on the facet engineering of rutile IrO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e36\u003c/sup\u003e. As a result of the traditional Adams fusion method, rutile IrO\u003csub\u003e2\u003c/sub\u003e tends to grow along various orientations at high oxidation temperatures (400\u0026ndash;800\u0026deg;C) and eventually expose different crystal facets. When high oxidation temperature is essential, selecting the optimal crystal facet while maintaining the monolayer thickness of IrO\u003csub\u003e2\u003c/sub\u003e becomes an enormous but meaningful challenge. Considering the above inevitable synthetic difficulties, we developed a novel ammonia-induced facet engineering for oriented regulation of the IrO\u003csub\u003e2\u003c/sub\u003e facets at the ultimate monolayer thickness limit.\u003c/p\u003e \u003cp\u003eThe key point of this facet-controlled synthesis has been investigated via modulating the synthesis parameters, elucidating that the nucleation and growth of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer are dominantly guided by NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO and temperature. During the annealing process, NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO is introduced as a potent crystal facet growth inducer for exposing desired (101) facet. As depicted in Supplementary Fig.\u0026nbsp;9, IrO\u003csub\u003e2\u003c/sub\u003e exhibits a classic rutile structure with strong (110), (101), (200) and (211) diffraction peaks in the absence of NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO. Upon the addition of 3.2 g NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO, the (110), (200) and (211) signals are significantly suppressed, while the (101) facet emerges as the dominant crystal surface, revealing the role of NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO in facilitating the unidirectionally oriented growth of IrO\u003csub\u003e2\u003c/sub\u003e grains. Particularly, as the NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO amount is further increased to 12.8 g, the characteristic peaks corresponding to the (110), (200) and (211) facets almost disappear. Therefore, IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer has been successfully constructed through ammonia-induced growth, in which IrO\u003csub\u003e2\u003c/sub\u003e nucleation preferentially occurs along the (101) direction, further demonstrating the effect of NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO on promoting the preferred growth of the (101) facet. Analogously, the morphological evolution of IrO\u003csub\u003e2\u003c/sub\u003e from irregular nanoparticles to 2D monolayers is accompanied by the concurrent crystal evolution, as evidenced by the increased ammonia amount. The successful synthesis of single-(101)-faceted monolayer validates the intimate correlation between the morphological and crystallographic changes, where the preferential growth along the (101) facet guides the formation of the 2D monolayer structure (Supplementary Fig.\u0026nbsp;10).\u003c/p\u003e \u003cp\u003eIn addition to NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO, the unidirectionally oriented growth is also highly dependent on the annealing temperature, making it challenging to expose the unique (101) facet at temperatures exceeding 500\u0026deg;C. To understand the role of temperature in crystal evolution, XRD characterization was then conducted at different temperatures to simulate the IrO\u003csub\u003e2\u003c/sub\u003e growth (Supplementary Fig.\u0026nbsp;11). The heat temperatures of 350\u0026deg;C, 450\u0026deg;C and 500\u0026deg;C are crucial to its crystal growth, during which the crystal facet and structure undergo distinct transformations (labeled as IrO\u003csub\u003e2\u003c/sub\u003e(101)-350, IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer and IrO\u003csub\u003e2\u003c/sub\u003e(101)-500, respectively). According to the kinetic growth of IrO\u003csub\u003e2\u003c/sub\u003e, the relative nucleation barriers (ΔE) of other facets are much higher than that of IrO\u003csub\u003e2\u003c/sub\u003e(101) under 350\u0026deg;C and 450\u0026deg;C, resulting in the (101) facet becoming the dominating one. As temperature increases to 500\u0026deg;C, the growth velocities of (110), (200) and (211) facets accelerate significantly, which contribute to the isotropic growth behavior. Consequently, a conventional rutile IrO\u003csub\u003e2\u003c/sub\u003e structure with various crystalline facets will evolve preferentially. As evidenced by TEM images in Supplementary Fig.\u0026nbsp;12, 2D IrO\u003csub\u003e2\u003c/sub\u003e obtained at 350\u0026deg;C, 450\u0026deg;C, and 500\u0026deg;C exhibit average sizes of 100 nm, 500 nm and 2 \u0026micro;m, respectively, which correspond to the bottom-up growth. At 350\u0026deg;C, IrO\u003csub\u003e2\u003c/sub\u003e species tend to first nucleate at the sharp edges of the monolayer. Under a relatively high temperature, the growth rate of the monolayer edge become faster, and then the high-temperature crystal growth process facilitates the precise formation of micrometer-size monolayer structure along the (101) direction. As a result, the single-(101)-faceted structure can be well controlled via 2D edge epitaxial growth in the monolayer limit even without substrate persist. At a temperature of 500\u0026deg;C, the thermodynamically favored isotropic growth poses a major challenge to maintaining the 2D monolayer structure. Consequently, irregular IrO\u003csub\u003e2\u003c/sub\u003e nanoparticles inevitably form on the monolayer IrO\u003csub\u003e2\u003c/sub\u003e(101) surface.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"5 OER performance in three-electrode cell","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eIn order to study the structural and performance advantages, the OER measurements of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, IrO\u003csub\u003e2\u003c/sub\u003e NP, C-IrO\u003csub\u003e2\u003c/sub\u003e (Alfa Aesar) and C-Ir black were initially carried out in a three-electrode configuration using 0.1 M HClO\u003csub\u003e4\u003c/sub\u003e as the aqueous electrolyte. All reference electrodes were calibrated before OER tests (Supplementary Fig.\u0026nbsp;13). As plotted in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer displays the lowest overpotential of 227 mV at a current density of 10 mA cmgeo\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, compared to 307, 311, and 301 mV of IrO\u003csub\u003e2\u003c/sub\u003e NP, C-IrO\u003csub\u003e2\u003c/sub\u003e and C-Ir black (Supplementary Fig.\u0026nbsp;14a). Even at 50 mA cm\u003csub\u003egeo\u003c/sub\u003e\u003csup\u003e\u0026minus;2\u003c/sup\u003e, IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer maintains an ultralow overpotential of 272 mV, nearly 200 mV lower than its nanoparticle counterparts (Supplementary Fig.\u0026nbsp;14b). The most active linear sweep voltammetry (LSV) curve confirms that IrO\u003csub\u003e2\u003c/sub\u003e(101) facet plays the most vital role in optimizing the OER performance over other facets. Tafel slope is another important parameter to probe the response of IrO\u003csub\u003e2\u003c/sub\u003e catalyst to the interfacial charge transfer towards the working potential. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb, IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer has the lowest Tafel slope of 44.73 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e within the potential range of 1.45\u0026ndash;1.47 V. For comparison, higher Tafel slopes are obtained by IrO\u003csub\u003e2\u003c/sub\u003e NP, C-IrO\u003csub\u003e2\u003c/sub\u003e and C-Ir black, with delivering 83.06, 70.28 and 80.97 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. The OER kinetic on the (101) facet is much faster than those of nanoparticle counterparts, confirming that the 2D single-facet monolayer structure is more favorable for OER pathway\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. The kinetics of OER processes on IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer was further evaluated by EIS measurement (Supplementary Fig.\u0026nbsp;15 and Supplementary Table\u0026nbsp;6). Fitting EIS curves reveals that IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer has the smallest charge transfer resistance (RCT) of 10.32 ohm, compared to those measured for IrO\u003csub\u003e2\u003c/sub\u003e NP (109.60 ohm), C-IrO\u003csub\u003e2\u003c/sub\u003e (88.49 ohm), and C-Ir/C (18.06 ohm). We attribute the lowest high frequency resistance to the maximized charge transfer process resulted from the monolayer thickness and highly oriented facet, which is the main origin of the superior OER performance. To identify the origin of monolayer structural advantages in mass transfer, we quantified the surface superaerophobicity of IrO\u003csub\u003e2\u003c/sub\u003e catalysts by assessing the adhesive force between bubbles and the surface (Supplementary Fig.\u0026nbsp;16). Interestingly, the adhesive force measured on the monolayer IrO\u003csub\u003e2\u003c/sub\u003e(101) surface was found to be merely 17.7 \u0026micro;N, featuring excellent superaerophobicity in electrolyte. Employing IrO\u003csub\u003e2\u003c/sub\u003e NP and C-IrO\u003csub\u003e2\u003c/sub\u003e as contrastive samples, the values of adhesive force were measured to be 52.4 \u0026micro;N and 32.0 \u0026micro;N, respectively. Therefore, the bubble releases from the MEA surface of monolayer IrO\u003csub\u003e2\u003c/sub\u003e(101) seems to be easier in the solution, which can be attributed to the exceptionally low contact region between the bubbles and the uniform monolayer surface and thus low adhesive force\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. Above results demonstrate that the fine construction of 2D monolayer interface featuring a specific facet simultaneously optimizes both its kinetic behavior and wetting behavior, thereby facilitating efficient charge and mass transfer from the IrO\u003csub\u003e2\u003c/sub\u003e surface for OER.\u003c/p\u003e \u003cp\u003eWe further calculated the specific current densities at 1.50 V vs. RHE to study their OER performances. To present a comprehensive performance evaluation of different catalysts, the OER performances were evaluated by normalizing surface areas obtained using different approaches, such as GCE (glassy carbon electrode) geometric areas and electrochemically active surface areas (ECSA)\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer exhibits both the highest geometrical activity of 47.75 mA cm\u003csub\u003egeo\u003c/sub\u003e\u003csup\u003e\u0026minus;2\u003c/sup\u003e and ECSA-based activity of 0.027 mA cm\u003csub\u003eECSA\u003c/sub\u003e\u003csup\u003e\u0026minus;2\u003c/sup\u003e at 1.50 V vs. RHE, nearly 10 times higher than those of IrO\u003csub\u003e2\u003c/sub\u003e NP, C-IrO\u003csub\u003e2\u003c/sub\u003e, and C-Ir/C (Supplementary Fig.\u0026nbsp;17a, b). The methods to determine the ECSA-based surface areas of different catalysts and clean GCE are summarized in Methods section (Supplementary Figs.\u0026nbsp;18 and 19). In addition, the mass activities at 1.50 V vs. RHE were calculated to compare their intrinsic activities. By normalizing the Ir loading, rationally designed IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer performs 238.75 mA mg\u003csub\u003eIr\u003c/sub\u003e\u003csup\u003e\u0026minus;1\u003c/sup\u003e at 1.5 V vs. RHE, almost 10 times higher than that of other rutile IrO\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). To gain a comprehensive view of the intrinsic performance, the OER performance of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer was also evaluated using 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e as the electrolyte. IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer exhibits the lowest overpotential of 245 mV at the current density of 10 mA cm\u003csub\u003egeo\u003c/sub\u003e\u003csup\u003e\u0026minus;2\u003c/sup\u003e, compared to 304, 344, and 305 mV for IrO\u003csub\u003e2\u003c/sub\u003e NP, C-IrO\u003csub\u003e2\u003c/sub\u003e, and C-Ir/C, respectively (Supplementary Fig.\u0026nbsp;20a). The Tafel slope of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e was measured to be only 47.47 mV dec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Supplementary Fig.\u0026nbsp;20b). In addition, both mass activities and EIS results illustrate that IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer shows the highest intrinsic activity and lowest electron transfer resistance (Supplementary Fig.\u0026nbsp;20c, d). Above OER measurements conducted in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte further demonstrates that the IrO\u003csub\u003e2\u003c/sub\u003e(101) facet provide an ideal catalytic platform for OER process by favoring mass transport, mechanical behavior, and even electron transfer.\u003c/p\u003e \u003cp\u003eIn the nucleation and growth process, both NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO amount and temperature play vital roles in guiding the (101)-oriented growth of IrO\u003csub\u003e2\u003c/sub\u003e nuclei at a monolayer thickness. A perfect IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer is predicted to possess highest electrocatalytic performance for OER compared to other traditional nanostructures, due to the ~\u0026thinsp;100% exposure of the (101) facet on the monolayer. LSV curves reveal that IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer exhibits the lowest overpotential of 227 mV to attain a current density of 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e, which is 80 mV, 36 mV and 22 mV lower, respectively, than its counterparts synthesized with NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO amounts of 0 g, 3.2 g and 12.8 g (Supplementary Fig.\u0026nbsp;21). Insufficient ammonization is unfavorable for the preferred orientation growth of the highly active (101) facet, whereas excessive ammonization may disrupt the orderliness and integrity of the (101) crystal facet. In terms of annealing temperature, the overpotential of IrO\u003csub\u003e2\u003c/sub\u003e(101)-350 and IrO\u003csub\u003e2\u003c/sub\u003e(101)-500 were measured to be 257 mV and 357 mV at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e, respectively, significantly higher than that of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer (227 mV) (Supplementary Fig.\u0026nbsp;22). Thus, 450 \u003csup\u003eo\u003c/sup\u003eC is identified as the optimal temperature to favor the (101)-oriented growth of 2D monolayer structure.\u003c/p\u003e \u003cp\u003eIn addition to activity, balancing the operating stability in acidic media poses another significant challenge when considering the structural stability at industrial-level current densities under practical OER condition\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e. In a three-electrode system, both IrO\u003csub\u003e2\u003c/sub\u003e NPs and C-IrO\u003csub\u003e2\u003c/sub\u003e suffer from dramatically activity loss during the 100 h chronopotentiometry (CP) tests while the working potential of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer at 50 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e maintains a consistent value after 910 h (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). The overpotential decay rate was calculated to be only 15 \u0026micro;V h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, demonstrating the superior stability of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer. The inset in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed further exhibits that the linear polarization curves of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer before and after the CP test perfectly overlap. It also operates stably at the high current of 200 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e during a 167 h stability test, confirming its great potential for commercial application (Supplementary Fig.\u0026nbsp;23). We also found that the 2D monolayer structure seems to be more insensitive to the working potential than previously reported nanostructured IrO\u003csub\u003e2\u003c/sub\u003e. For instance, a strong redox peak of Ir\u003csup\u003eⅢ/Ⅳ\u003c/sup\u003e appears at 0.86 V vs. RHE once an anodic potential is applied, indicating that Ir\u003csup\u003e3+\u003c/sup\u003e has been oxidized to Ir\u003csup\u003e4+\u003c/sup\u003e or an even higher state (Supplementary Fig.\u0026nbsp;24). Even after 10000 CV cycles, no obvious shift in the redox peak has been detected\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. The polarization curves and overpotentials of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer before and after the 2,000th /10,000th CV cycles tests also prove that its activity loss is negligible despite being a 2D monolayer structure (Supplementary Fig.\u0026nbsp;25). To provide a comprehensive evaluation of the stability property of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, the inductively coupled plasma-optical emission spectrometer (ICP-OES) was further operated to calculate the dissolution amount of the Ir element after 910 h durability test at 50 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e. Form the ICP-OES data in Supplementary Table\u0026nbsp;7, the S-number for each IrO\u003csub\u003e2\u003c/sub\u003e catalyst was calculated as the ratio of the molar amount of evolved O\u003csub\u003e2\u003c/sub\u003e to the molar amount of dissolved Ir\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. The S-number of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer after the 910 h OER was calculated to be 4.64\u0026times;10\u003csup\u003e6\u003c/sup\u003e, over 11 and 40 times higher than the S-numbers of IrO\u003csub\u003e2\u003c/sub\u003e NP (4.18\u0026times;10\u003csup\u003e5\u003c/sup\u003e) and C-IrO\u003csub\u003e2\u003c/sub\u003e (1.14\u0026times;10\u003csup\u003e5\u003c/sup\u003e), respectively (Supplementary Table\u0026nbsp;8). In addition, IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer exhibits superior stability compared to other recently reported Ir-based catalysts, including pristine Ir or Ir oxides (1\u0026thinsp;~\u0026thinsp;9.2\u0026times;10\u003csup\u003e5\u003c/sup\u003e)\u003csup\u003e45\u003c/sup\u003e, Ir-based alloys (5\u0026thinsp;~\u0026thinsp;2.49\u0026times;10\u003csup\u003e5\u003c/sup\u003e)\u003csup\u003e41\u003c/sup\u003e, and IrO\u003csub\u003e2\u003c/sub\u003e@TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles (1.0\u0026times;10\u003csup\u003e4\u003c/sup\u003e)\u003csup\u003e46\u003c/sup\u003e. Given the high current density and the superlong test time, the performance stability of 2D IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer in acidic media is further highlighted, pointing to its significant potential for commercialization even at monolayer thickness. As shown in Supplementary Figs.\u0026nbsp;26 and 27, the monolayer structure and uniform (101) facet orientation of the IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer remain intact after ~\u0026thinsp;1000 h stability test at 50 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e. TEM analysis reveals an initial sheet-like morphology with no reconstructed nanoparticles or nanoclusters on the surface. XRD characterization further confirms the presence of only a prominent (101) diffraction peak at 34.93\u0026deg;, without any signals from other rutile facets, demonstrating high stability in the crystallographic orientation during long-term operation. To gain insights into the electronic structure changes after the stability test, XPS analysis was performed. As shown in Supplementary Fig.\u0026nbsp;28, XPS spectrum of the IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer exhibits a slight blue shift of 0.15 eV compared to the initial state, suggesting the partial oxidation of surface Ir atoms under acidic OER conditions. Above characterizations after OER stability test confirm that the single-(101)-facet structure realizes a win-win strategy in terms of activity and stability, positioning IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer as a promising candidate for PEM applications.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"6 OER performance in PEM cell","content":"\u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe catalyst research evolving from the simple modulation of IrO\u003csub\u003e2\u003c/sub\u003e composition/size on the macroscopic scale to the precise design of IrO\u003csub\u003e2\u003c/sub\u003e structure/morphology at the nanoscale suffer from the same corrosion problem stemming from the dissolution of IrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e anions into the electrolyte\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e. In general, most previously reported high-performance OER electrocatalysts can only survive in a three-electrode system, while their activity loss usually become magnified in PEM devices after 100 h, which starkly contrasts with the stability results observed on GCE\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. For the industrial application of PEM electrolyzers, searching an advanced IrO₂ material structure that can meet the technical targets for PEM electrolyzer\u0026mdash;high current densities of 2 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e at low overpotentials (\u0026le;\u0026thinsp;1.9 V) and long-term stability exceeding 40,000 h at industrial-scale current density with a decay rate below 4.8 \u0026micro;V h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026mdash;presents a great opportunity and challenge for a sustainable PEM future\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. As a proof-of-concept of its commercial value, the OER performance of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer was evaluated in PEM devices under various operating conditions (Supplementary Fig.\u0026nbsp;29). As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, the catalyst-coated membranes (CCMs) for PEMWE were prepared using IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer as the anode catalyst (0.2 mg\u003csub\u003eIr\u003c/sub\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e or 1.5 mg\u003csub\u003eIr\u003c/sub\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e) and commercial Pt/C (60 wt % Pt, Tanaka Kikinzoku Group) as the cathode catalyst via spray coating on both sides of the Nafion 115 membrane. After assembly, MEAs with IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer were tested in a 4 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e active area PEMWE single cell at 80\u0026deg;C and ambient pressure, with a preheated Milli-Q ultrapure water flow of 10 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e onto the anode.\u003c/p\u003e \u003cp\u003eIrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer performs remarkable PEM activity, achieving the standard current density of 2 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e \u003csub\u003egeo\u003c/sub\u003e at a low potential of 1.74 V, significantly lower than E\u003csub\u003ecell\u003c/sub\u003e of IrO\u003csub\u003e2\u003c/sub\u003e NP (1.82 V@2 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e \u003csub\u003egeo\u003c/sub\u003e) and C-IrO\u003csub\u003e2\u003c/sub\u003e (1.84 V@2 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e \u003csub\u003egeo\u003c/sub\u003e), respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). Additionally, lowing the Ir loading in CCMs below 0.5 mg\u003csub\u003eIr\u003c/sub\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e \u003csub\u003egeo\u003c/sub\u003e is another crucial issue to meet the deployment and upscaling of PEM electrolyzers, which can be ascribed to the ultralow-earth abundance and extremely high cost (\u003cspan\u003e$\u003c/span\u003e4700 per oz in 2024)\u003csup\u003e \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e \u003c/sup\u003e. Thus, PEMWEs with ultralow-Ir-loading MEA (0.2 mg\u003csub\u003eIr\u003c/sub\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e \u003csub\u003egeo\u003c/sub\u003e) were recorded to evaluate catalyst performance. As shown in the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb, E\u003csub\u003ecell\u003c/sub\u003e reaches 1.78 V at 2 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e \u003csub\u003egeo\u003c/sub\u003e using ultralow-Ir-loading MEA, which is comparable to the best PEMWEs employing same MEAs with substantially higher Ir loadings (1\u0026thinsp;~\u0026thinsp;2 mg\u003csub\u003eIr\u003c/sub\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e \u003csub\u003egeo\u003c/sub\u003e)\u003csup\u003e50, \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e \u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eDemonstrating the catalyst stability in PEM device is a more challenging task than achieving high PEM activity. To date, most OER catalyst stabilities are still tested in a simple three-electrode system utilizing a low working current density below 200 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e, which perform far from their practical working conditions. Although some recently reported catalysts have demonstrated operation in PEM system for over 100 h at a constant current density of 1\u0026thinsp;~\u0026thinsp;2 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e, evaluating their comprehensive working stabilities in all aspects including the stability response to fluctuations in wind/solar energy inputs or ultralow Ir loading, is easy to ignore\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. Therefore, many high-performance OER electrocatalysts developed from academia often rest in the laboratory, while commercially available IrO\u003csub\u003e2\u003c/sub\u003e is exploring to address technical problems encountered from real applications\u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e. Compared to the steady-state condition, voltage fluctuation caused by wind or solar power can exacerbate Ir loss and even catalytic deterioration, ultimately compromising the overall PEM efficiency\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e. To bridge the research gap of stability tests, we use a square wave voltage toggling between 1.6 and 2.3 V to simulate the fluctuating power patterns of wind or solar photovoltaic sources. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec, E\u003csub\u003eCell\u003c/sub\u003e over time was measured at 1.6 and 2.3 V for 1 minute each. Notably, voltage switching exhibits little negative effect on PEM durability, suggesting IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer remains both activity and structural advantages even after 1,000 h fluctuating testing.\u003c/p\u003e \u003cp\u003eAs for steady-state condition, stability assessments of OER catalysts are typically conducted in a simple three-electrode or PEM system, where constant voltages are applied from external electrochemical workstation. This is a key criteria before we can commercialize it. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed firstly compares the stability data of low-Ir-loading PEM electrolyzers. Even with Ir and Pt loading as low as 0.2 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e, E\u003csub\u003ecell\u003c/sub\u003e achieves over 2,000 h stability without obvious activity loss at 1.5 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee, IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer not only meets the DOE technical standards but also significantly outperforms previously reported catalysts in terms of both voltage efficiency and stability\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e, \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e, \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e. As evidenced in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef, the IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer anode cell (1.5 mg\u003csub\u003eIr\u003c/sub\u003e cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e) maintains an ultrahigh OER stability of 8,000 h at 1.5 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e with a negligible degradation rate of 4 \u0026micro;V h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Up to now, the PEM system is still operating stably. In contrast, for C-IrO\u003csub\u003e2\u003c/sub\u003e and IrO\u003csub\u003e2\u003c/sub\u003e NPs, their efficiency loss is drastically raised during the 500 h OER test, which may be attributed to the accelerated catalyst degradation. All these comprehensive stability tests confirm that IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer suffers less catalytic deterioration under both dynamic and constant power operation, despite its monolayer rutile (101) structure.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e "},{"header":"Conclusion","content":"\u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eGuided by theoretical calculations, we have successfully synthesized 2D single-faceted IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer based on an ammonia-induced preferred orientation growth. Both XRD characterization and TEM observation confirm that IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer possesses a monolayer rutile structure featuring a single (101) facet. The highly activated (101) facet presents an ultralow overpotential of 237 mV at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e in a three-electrode cell, and performs excellent PEM activity with a low E\u003csub\u003ecell\u003c/sub\u003e of 1.74 V at 2 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e. Importantly, the practical operating stability of the IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer anode cell has been tested under various working conditions. Although IrO\u003csub\u003e2\u003c/sub\u003e crystal is constructed in the monolayer limit along a unique growth orientation, IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer still meets the DOE durability targets for PEMWE. Under steady-state condition, the long-term operation exceeds 8,000 hours with a negligible decay rate of less than 4.0 mV kh\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Even at an ultralow Ir loading of 0.2 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e, the PEMWE test for IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer anode maintains stability for over 2,000 h at a constant 1.5 A cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e. When coupled with simulated fluctuating electricity from solar/wind power sources, IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer anode cell also achieves long-term stability exceeding 1,000 h. This work proposes a novel phase engineering strategy for the design and synthesis of 2D single-(101)-faceted IrO\u003csub\u003e2\u003c/sub\u003e at monolayer thickness, demonstrating outstanding OER activity and long-term PEMWE stability under both steady-state condition and fluctuating condition.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e "},{"header":"Methods","content":"\u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003e \u003cb\u003eChemicals.\u003c/b\u003e Dipotassium hexachloroiridate (K\u003csub\u003e2\u003c/sub\u003eIrCl\u003csub\u003e6\u003c/sub\u003e, 99.9%) was purchased from Alfa Aesar Co. Iridium(IV) oxide (IrO\u003csub\u003e2\u003c/sub\u003e, 99%) was purchased from Alfa Aesar and Umicore. Ir Black (Ir, 99.9%) was purchased from Apinno Co. Ammonium hydroxide (NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO, 25.0\u0026ndash;30.0%) and isopropanol were purchased from Beijing InnoChem Science \u0026amp; Technology Co., Ltd. Sulphuric acid (H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 95.0\u0026ndash;98.0%), perchloricacid (HClO\u003csub\u003e4\u003c/sub\u003e, 70.0\u0026ndash;72.0%), and Potassium nitrate (KNO\u003csub\u003e3\u003c/sub\u003e, \u0026ge;\u0026thinsp;99.0) were obtained form Sinopharm Chemical Reagent Co. Nafion solution (5 wt %) and Nafion N115 membrane was purchased from DuPont Co. Other reagents used were of analytical grade without further purification. Deionized water, supplied by Thermo Scientific Smart2Pure 6 UV Water Purification System, was used throughout the experiments. The PEM device was purchased from China Titanium Guochuang (Qingdao) Technology Co.\u003c/p\u003e \u003cp\u003e \u003cb\u003eSynthesis of 2D IrO\u003c/b\u003e \u003csub\u003e \u003cb\u003e2\u003c/b\u003e \u003c/sub\u003e \u003cb\u003e(101) monolayer.\u003c/b\u003e The synthesis of IrO\u003csub\u003e2\u003c/sub\u003e was carried out in a muffle furnace. 240 mg of K\u003csub\u003e2\u003c/sub\u003eIrCl\u003csub\u003e6\u003c/sub\u003e and 6.4 g of NH\u003csub\u003e3\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO were completely dissolved in 40 mL deionized water. After the mixture had been reacting for 3 h ultrasonically, 4.848g of KNO\u003csub\u003e3\u003c/sub\u003e was added and stirred for 1 h. After that, the mixture was heated at 80\u0026deg;C and magnetically stirred until nearly complete evaporation of water was achieved. The resulting powder was then placed in a quartz boat and heated to 450\u0026deg;C at a rate of 5\u0026deg;C min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and held at this temperature for 0.5 h in air. It was then allowed to cool naturally to room temperature. Finally, IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer was obtained by filtering and washing with deionized water, respectively, followed by drying at 60\u0026deg;C in a vacuum oven overnight. The comparison materials were prepared by altering the calcination temperatures to 350\u0026deg;C and 500\u0026deg;C, and by modulating the volume of ammonia solution to 0 mL, 3.2 mL, and 12.8 mL. Notably, IrO₂ NP was synthesized specifically using 0 mL of ammonia solution.\u003c/p\u003e \u003cp\u003e \u003cb\u003eCharacterizations.\u003c/b\u003e The XRD patterns of all samples were recorded by X-ray powder diffraction (XRD, D8 advance) at an operation voltage of 40 kV and a current of 40 mA, using a Cu Kα radiation source. The TEM images and EDX data of all samples were characterized by a JEOL JEM-F200 transmission electron microscope with an accelerating voltage of 200 kV. AFM images were captured by the atomic force microscopy (AFM, Dimenson ICON). The XPS results were tested by X-ray photoelectron spectrometer (Thermo Fischer, ESCALAB250Xi) using an operation voltage of 12.5 kV and a filament current of 16 mA, with the excitation source using Alka rays (\u003cem\u003ehv\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1486.8 eV). And the charge correction is based on C1s at 284.80 eV as the energy reference. The BET specific surface areas were characterized using American Micromeritics 3Flex Version 5.00. The Ir dissolution concentration was determined by Agilent 7800 ICP-MS. The Raman spectroscopies were measured using a Laser Microscopic Confocal Raman Spectrometer (inVia\u0026trade;, Renishaw) under excitation by laser light at λ\u0026thinsp;=\u0026thinsp;532 nm. The adhesion was recorded using an adhesion tester (DCAT 25, DataPhysics) at a line voltage of 24 V DC and an input power of 70 W. XAFS spectra were collected in fluorescence mode at the 1W1B X-ray absorption beamline of the Beijing Synchrotron Radiation Facility (BSRF) using a double-crystal Si (111) monochromator. Iridium foil was measured in transmission mode and used as a reference for energy calibration. Data were analyzed with the ATHENA and ARTEMIS (version 0.9.26) modules of the IFEFFIT software package. The EXAFS functions were Fourier transformed to R-space with a k-weight of 2, within the range of 3.16\u0026ndash;12.40 \u0026Aring;⁻\u0026sup1;, using a Hanning window (dk\u0026thinsp;=\u0026thinsp;1.0 \u0026Aring;⁻\u0026sup1;).\u003c/p\u003e \u003cp\u003e \u003cb\u003eElectrochemical measurements.\u003c/b\u003e All electrochemical experiments in the three electrode system were carried out using the CHI760E electrochemical workstation (Shanghai Chenhua, China). The glass carbon electrode (GCE, area: 0.196 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e) was used as the working electrode, calomel electrode was used as the reference electrode, and platinum gauze was used as the counter electrode. OER measurements were conducted in O\u003csub\u003e2\u003c/sub\u003e-saturated 0.1 M HClO\u003csub\u003e4\u003c/sub\u003e or 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte. For measurement with iR-correction, R referred to the ohmic resistance arising from the electrolyte/contact resistance of the setup. The scan rate was set to 2 mV s⁻\u0026sup1; for both LSV measurements ranging from 1.1 to 1.6 V vs. SCE, and Tafel plot measured from 1.1 to 1.4 V vs. SCE. The Electrochemical Impedance Spectroscopy (EIS) tests were conducted with a frequency range from 10 Hz (low frequency) to 100 kHz (high frequency). Chronopotentiometry measurements were performed using the Chronopotentiometry mode on the CHI760 electrochemical workstation, with the current set to constant values of 50 or 200 mA/cm\u0026sup2;. Cyclic voltammetry was performed using the CV mode on the CHI760 electrochemical workstation, with the voltage range set from 0.05 to 1.25 V vs SCE. The mass activities and ECSA-based specific activities of the catalysts were calculated based on the catalyst mass and electrochemical active surface areas (ECSAs) on the glassy carbon electrodes, respectively. The catalysts solutions were obtained by mixing 4.0 mg IrO\u003csub\u003e2\u003c/sub\u003e catalyst in a solution of 800 \u0026micro;L isopropanol, 200 \u0026micro;L deionized water and 40 \u0026micro;L Nafion solution (5 wt%), with sonication to form the homogenous catalyst ink. Then, 10 \u0026micro;L of the ink was dispersed on the GCE with drying naturally for testing. In the stability test, Ti felt (area: 1\u0026times;1 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e) was used as the working electrode, and 0.1M HClO\u003csub\u003e4\u003c/sub\u003e saturated with O\u003csub\u003e2\u003c/sub\u003e was used as the electrolyte. The homogeneous catalyst ink was formed by ultrasonic mixing of 5.0 mg catalyst in 500 \u0026micro;L isopropanol, 500 \u0026micro;L deionized water and 10 \u0026micro;L Nafion solution (5 wt%). 200 \u0026micro;L of ink was dispersed on the Ti felt.\u003c/p\u003e \u003cp\u003eFor the PEM experiments, the anode catalyst ink was prepared by mixing 10 mg of IrO₂ catalyst with 108 \u0026micro;L of deionized water, 1327 \u0026micro;L of isopropanol, and 44.4 \u0026micro;L of 5 wt% Nafion solution. The cathode catalyst ink was prepared by mixing 10 mg of Pt/C catalyst with 128 \u0026micro;L of deionized water, 1528 \u0026micro;L of isopropanol, and 95.3 \u0026micro;L of 5 wt% Nafion solution. Both inks were then thoroughly mixed using an ultrasonic bath and a cell disruptor. After stirring for over 24 hours, the catalyst ink was uniformly sprayed on the central 2.5\u0026times;2.5 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e area of the N115 proton exchange membrane at 90 ℃ using ultrasonic spraying machine. Carbon paper and Ti felt were pressed on the outside of the catalytic layer using a hot press at a pressure of 450 kg and a temperature of 140 ℃ for ten minutes. The anode and cathode loadings of MEAs were calibrated using X-ray fluorescence (XRF) spectroscopy.\u003c/p\u003e \u003cp\u003eAfter the MEA was prepared, PTFE sealing materials and Ti bipolar plates were assembled around the MEA to form an electrolyzer. For all PEM tests, deionized water was used as the electrolyte at a flow rate of 18.2 ml min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, with a working temperature of 80\u0026deg;C, and voltage was supplied by an NGI programmable DC power source. The I-V curve was measured from 0 to 3 A cm\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup2;, with 25 current points, each held for 30 seconds. Steady-state tests were conducted using a cross-flow configuration at 1.5 A cm\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026sup2;. Non-steady-state tests were conducted using a square wave pattern with voltages of 1.6 V and 2.3 V, each held for 1 minute. The XRD, XPS, and TEM results of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer after the stability test were obtained by analyzing the used MEAs.\u003c/p\u003e \u003cp\u003eThe ECSAs of the IrO\u003csub\u003e2\u003c/sub\u003e-NS were estimated from double layer capacitance (C\u003csub\u003eDL\u003c/sub\u003e) and specific capacitance (C\u003csub\u003eS\u003c/sub\u003e) using the following equation:\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Equa\" class=\"Equation\"\u003e \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\text{ECSA=}{\\text{C}}_{\\text{DL}}/{\\text{C}}_{\\text{S}}$$\u003c/div\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe electrochemical double-layer capacitance (C\u003csub\u003eDL\u003c/sub\u003e) at non-Faradic potential range was obtained by measuring the capacitance of double layer at solid-liquid interface employing cyclic voltammetry (CV) with different scan rates (5, 10, 20, 30, 40, 50, 60 and 70 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) in a range from 1.25 to 1.35 V vs. RHE.\u003c/p\u003e \u003cp\u003eThe C\u003csub\u003eS\u003c/sub\u003e value was found to be 0.036 mF cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e in 0.1 M HClO\u003csub\u003e4\u003c/sub\u003e electrolyte using a polished glassy carbon-rotating disk electrode from the CV curves in Supplementary Fig.\u0026nbsp;18. The C\u003csub\u003eDL\u003c/sub\u003e values of IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer, IrO\u003csub\u003e2\u003c/sub\u003e NP, C-IrO\u003csub\u003e2\u003c/sub\u003e and C-Ir black were estimated from the CV curves in Supplementary Figs.\u0026nbsp;18 and 19.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eDFT calculations\u003c/b\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eDensity functional theory (DFT) calculations were conducted using the Vienna Ab initio Simulation Package (VASP)\u003csup\u003e\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e, \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e, \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e, \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e\u003c/sup\u003e. The interactions between ion cores and valence electrons were described using the projector augmented wave (PAW) method\u003csup\u003e\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e, \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e\u003c/sup\u003e. For the exchange-correlation functional, we employed the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) formulation\u003csup\u003e\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e\u003c/sup\u003e. The wave functions at each k-point were expanded using a plane wave basis set, with the kinetic energy cutoff set to 500 eV. Geometric optimizations were performed using a force-based conjugate gradient method until the energy converged to 1.0\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e eV per atom, with the maximum force on each atom less than 0.05 eV \u0026Aring;\u003csup\u003e\u0026minus;1\u003c/sup\u003e. To account for long-range van der Waals interactions, Grimme\u0026rsquo;s D3 dispersion correction was implemented\u003csup\u003e\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e, \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe crystal structures of rutile-IrO\u003csub\u003e2\u003c/sub\u003e were used to construct slab models for the electrocatalytic study. Periodic slab models of the IrO\u003csub\u003e2\u003c/sub\u003e(110), (200), and (101) facets were constructed using 2\u0026times;1, 2\u0026times;2, and 1\u0026times;2 surface supercells, respectively, each comprising four metal-oxide layers. The bottom layer was constrained to maintain bulk-like properties. A vacuum space of 15 \u0026Aring; was introduced between the slab and its periodic image to minimize inter-slab interactions. To evaluate the activity of the electrocatalysts, we estimated the free energy diagram using the following Eq.\u0026nbsp;7\u003csup\u003e1\u003c/sup\u003e:\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Equb\" class=\"Equation\"\u003e \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:\\text{\u0026Delta;G=\u0026Delta;E+\u0026Delta;ZPE-T\u0026Delta;S}$$\u003c/div\u003e \u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003ewhere ΔG is the change in Gibbs free energy, ΔE represents the total energy change derived from DFT calculations, ΔZPE is the change in zero-point energy, ΔS denotes the entropy change, and T is the temperature (298.15 K).\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eCompeting interests\u003c/h2\u003e\n\u003cp\u003eThe authors declare no competing financial interests.\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary Information\u003c/strong\u003e is available for this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReprints and permissions information \u003c/strong\u003eis available.\u003c/p\u003e\n\u003cp\u003eCorrespondence and requests for materials should be addressed to D.Y. or J.L.\u003c/p\u003e\n\u003cp\u003ePublisher\u0026rsquo;s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.\u003c/p\u003e\n\u003ch2\u003eAuthor contributions\u003c/h2\u003e\n\u003cp\u003eJ.L. led the whole project. D.Y. and C.Z. contributed equally to this work. D.Y. guided synthetic process, performed data analysis and wrote the manuscript. C.Z. synthesized the catalysts, performed TEM, XRD, XPS, Raman, ICP and electrochemical measures. A.T. performed DFT calculation and analysis. S.Z. performed XAFS measurements and analysis.\u003c/p\u003e\n\u003ch2\u003eAcknowledgements\u003c/h2\u003e\n\u003cp\u003eThis work was supported by the National Key R\u0026amp;D Plan of China (2021YFB4000101), Interdisciplinary Innovation Program of North China Electric Power University, the National Natural Science Foundation of China (22205061), Hebei Province Outstanding Youth Fund (B2024502007), the Double First-class University Construction Project of North China Electric Power University (XM2412302) and the Fundamental Research Funds for the Central Universities (JB2024087).\u003c/p\u003e\n\u003ch2\u003eData availability.\u003c/h2\u003e\n\u003cp\u003eThe data that support the plots within this paper and other finding of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003eReceived: ((will be filled in by the editorial staff))\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Published online: ((will be filled in by the editorial staff))\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLiu R-T et al (2023) Recent advances in proton exchange membrane water electrolysis. 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ChemCatChem 3:1159\u0026ndash;1165\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5187955/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5187955/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eBoth commercially available and laboratory-synthesized IrO\u003csub\u003e2\u003c/sub\u003e catalysts typically possess rutile-type structures and diverse facet orientations. According to the theoretical results from density functional theory calculations, distinct IrO\u003csub\u003e2\u003c/sub\u003e facets will result in divergent electrocatalytic properties, among which the (101) crystal facet is theoretically predicted as the most energetically favorable for oxygen evolution reaction (OER) owing to its lowest energy barrier. Maintaining a single-unit-cell thickness while exposing a desired facet of 2D IrO\u003csub\u003e2\u003c/sub\u003e presents a significant opportunity and challenge for the development of high-performance OER anode catalysts. Herein, we develop an ammonia-induced facet engineering for oriented modulation of crystal facets in the ultimate limit of monolayer thickness, and successfully synthesize 2D monolayer IrO\u003csub\u003e2\u003c/sub\u003e exposing unique (101) facet. At the current density of 10 mA cm\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e, an ultralow overpotential of 230 mV has been achieved on the highly activated (101) facet in a three-electrode system. More importantly, in a proton exchange membrane (PEM) electrolyzer, the IrO\u003csub\u003e2\u003c/sub\u003e anode reaches a low voltage of 1.74 V at an industrial-level current density of 2 A cm\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e, much lower than that of all commercial IrO\u003csub\u003e2\u003c/sub\u003e electrocatalysts. Though facet engineering primarily contributes to modulating the intrinsic activity rather than stability, the as-prepared IrO\u003csub\u003e2\u003c/sub\u003e(101) monolayer performs over 8,000 hours of PEM water electrolysis (PEMWE) stability at constant 1.5 A cm\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e, with a negligible decay rate of 4.0 mV kh\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Furthermore, even a long-term PEMWE test of 1000 h using the membrane electrode assembly (MEA) with ultra-low Ir loading of 0.2 mg\u003csub\u003eIr\u003c/sub\u003e cm\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e under fluctuating operating conditions is performed, \u003cem\u003eE\u003c/em\u003e\u003csub\u003eCell\u003c/sub\u003e remains highly electrochemically stable over time at 1.5 A cm\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e\u003csub\u003egeo\u003c/sub\u003e, without any signs of catalyst degradation. This work proposes that ammonia-induced facet engineering of 2D monolayer IrO\u003csub\u003e2\u003c/sub\u003e could represent a novel approach to selectively expose the desired (101) facet, thereby enabling unique facet-dependent OER performance and ultrahigh stability in industrial-scale PEM electrolysis, even under voltage fluctuations generated by solar and wind power.\u003c/p\u003e","manuscriptTitle":"2D single-faceted IrO2(101) monolayer enabling high-performing proton exchange membrane water electrolysis beyond 8,000 h stability at 1.5 A cm-2","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-10-24 04:31:21","doi":"10.21203/rs.3.rs-5187955/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"7986ed33-3384-4b49-a0ff-3088eb7a116a","owner":[],"postedDate":"October 24th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":38555572,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis"},{"id":38555573,"name":"Physical sciences/Materials science/Nanoscale materials/Two-dimensional materials"}],"tags":[],"updatedAt":"2025-08-07T07:33:25+00:00","versionOfRecord":{"articleIdentity":"rs-5187955","link":"https://doi.org/10.1038/s41467-025-62665-2","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-08-06 04:00:00","publishedOnDateReadable":"August 6th, 2025"},"versionCreatedAt":"2024-10-24 04:31:21","video":"","vorDoi":"10.1038/s41467-025-62665-2","vorDoiUrl":"https://doi.org/10.1038/s41467-025-62665-2","workflowStages":[]},"version":"v1","identity":"rs-5187955","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5187955","identity":"rs-5187955","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}