Metal-Dependent Anchoring and Catalytic Behavior of Graphene-Supported Iridium and Nickel Single-Atom Anode Catalysts for Anion Exchange Membrane Water Electrolysis

Metal-Dependent Anchoring and Catalytic Behavior of Graphene-Supported Iridium and Nickel Single-Atom Anode Catalysts for Anion Exchange Membrane Water Electrolysis | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 15 January 2026 V1 Latest version Share on Metal-Dependent Anchoring and Catalytic Behavior of Graphene-Supported Iridium and Nickel Single-Atom Anode Catalysts for Anion Exchange Membrane Water Electrolysis Authors : Ji Hoon Lee 0009-0002-8494-5994 , Eunju Yang , Jeongeun Song , Bong-Gyeong Shin , Yukwon Jeon 0000-0001-7050-2274 , Qilin Li , and Yeojoon Yoon [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.176850556.68081151/v1 186 views 101 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Anion exchange membrane water electrolysis (AEMWE) is a promising platform for sustainable hydrogen production because it enables the use of non-noble metal catalysts in alkaline environments. However, conventional carbon-based anodes are prone to corrosion and structural degradation under oxidizing conditions, hindering the development of durable and active anode catalysts. To address this limitation, a graphene framework was employed as the anode, with iridium- and nickel-based single-atom catalysts (SACs) stabilizing the active sites and tailoring the interfacial environments. The impact of atomic anchoring sites, i.e., graphene edges versus basal planes, on catalytic performance and durability of the anode was examined. To deliberately control the anchoring preference of metal atoms, a post-calcination air-injection procedure is introduced, inducing moderate surface hydroxylation and stabilizing single-atom configurations on graphene. Advanced characterizations (high-angle annular dark-field scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure) reveal that iridium atoms preferentially anchor at the hydroxyl-group-rich graphene edges. In contrast, nickel atoms incorporate into the basal-plane carbon or nitrogen coordination sites. Edge-anchored iridium SACs demonstrate higher oxygen evolution activity but lower stability, whereas basal-anchored nickel SACs exhibit improved durability at the expense of catalytic activity. These findings establish a structure–property relationship between atomic coordination and electrochemical behavior, providing a design principle for graphene-based anode catalysts that can simultaneously enhance durability and reduce the cost of hydrogen production. Article category: Research Article Subcategory: Electrocatalysis Metal-Dependent Anchoring and Catalytic Behavior of Graphene-Supported Iridium and Nickel Single-Atom Anode Catalysts for Anion Exchange Membrane Water Electrolysis Ji Hoon Lee, Eunju Yang, Jeongeun Song, Bong-Gyeong Shin, Prof. Yukwon Jeon, Prof. Qilin Li, Prof. Yeojoon Yoon* J. H. Lee, E. Yang, J. Song, B.-G. Shin, Prof. Y. Jeon, Prof. Y. Yoon Department of Environmental and Energy Engineering, Yonsei University, Wonju, Republic of Korea E-mail: [email protected] J. H. Lee, Prof. Q. Li Department of Civil and Environmental Engineering, Rice University, Houston, TX, United States Keywords: graphene edge, basal plane, single-atom catalyst, surface hydroxylation, hydrogen production Anion exchange membrane water electrolysis (AEMWE) is a promising platform for sustainable hydrogen production because it enables the use of non-noble metal catalysts in alkaline environments. However, conventional carbon-based anodes are prone to corrosion and structural degradation under oxidizing conditions, hindering the development of durable and active anode catalysts. To address this limitation, a graphene framework was employed as the anode, with iridium- and nickel-based single-atom catalysts (SACs) stabilizing the active sites and tailoring the interfacial environments. The impact of atomic anchoring sites, i.e., graphene edges versus basal planes, on catalytic performance and durability of the anode was examined. To deliberately control the anchoring preference of metal atoms, a post-calcination air-injection procedure is introduced, inducing moderate surface hydroxylation and stabilizing single-atom configurations on graphene. Advanced characterizations (high-angle annular dark-field scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure) reveal that iridium atoms preferentially anchor at the hydroxyl-group-rich graphene edges. In contrast, nickel atoms incorporate into the basal-plane carbon or nitrogen coordination sites. Edge-anchored iridium SACs demonstrate higher oxygen evolution activity but lower stability, whereas basal-anchored nickel SACs exhibit improved durability at the expense of catalytic activity. These findings establish a structure–property relationship between atomic coordination and electrochemical behavior, providing a design principle for graphene-based anode catalysts that can simultaneously enhance durability and reduce the cost of hydrogen production. 1. Introduction Hydrogen production via water electrolysis powered by intermittent renewable energy sources is the cornerstone of the emerging hydrogen economy. [1] Among the leading technologies, anion exchange membrane water electrolysis (AEMWE) has emerged as a promising solution that bridges the cost-effectiveness of conventional alkaline water electrolysis with the high power density of proton exchange membrane water electrolysis. [2–4] Under alkaline conditions, AEMWE enables the use of earth-abundant, non-precious metal catalysts, providing a promising pathway toward scalable and economically viable hydrogen production. However, the practical implementation of AEMWE under alkaline conditions is hindered by slow reaction kinetics, particularly the oxygen evolution reaction (OER). [5] The kinetic limitations result in substantial overpotentials and efficiency losses during high current density operations. Conventional nanoparticle-based catalysts suffer from low atom-utilization efficiency and insufficient structural stability, including particle dissolution and aggregation, in harsh electrochemical environments. [6] Thus, catalyst architectures that maximize active site exposure while maintaining robust chemical stability during long-term operation are urgently required. Single-atom catalysts (SACs) have emerged as transformative platforms that enable near-complete atom utilization and provide well-defined active sites. However, the high surface energy of isolated metal atoms makes them thermodynamically unstable, necessitating suitable supports to prevent aggregation and maintain atomic dispersion under electrochemical operating conditions. Accordingly, carbon-based materials have been extensively explored as supports for SACs, with graphene attracting particular attention owing to its high electrical conductivity, chemical stability, and tunable surface chemistry. [7] Graphene also provides distinct coordination environments, such as basal planes and edge sites, that can significantly influence the electronic structure and catalytic behavior of the single metal atoms anchored to it. Despite extensive progress in SAC research, [7–11] a critical knowledge gap remains regarding the site-specific anchoring behaviors of different metal species on graphene. [12] Systematic comparisons between noble metals, such as iridium (Ir), and non-noble transition metals, such as nickel (Ni), supported on identical graphene platforms, are scarce. Therefore, understanding how these metals coordinate with basal versus edge sites and how such coordination differences affect the intrinsic activity and durability of the catalysts under AEMWE conditions is essential for the rational design of advanced electrocatalysts. In this study, we conduct a comparative investigation of graphene-supported iridium and nickel SACs as anode catalysts for AEMWE. Stable atomic dispersion of Ir and Ni on graphene was achieved using a controlled impregnation-based synthesis strategy. The Ir and Ni SACs were stabilized via distinct metal–support coordination environments. Through combined spectroscopic and electrochemical analyses, we elucidated the relationship between the metal–support coordination environment and OER performance. We further demonstrated how metal identity and local chemical structure influence charge-transfer resistance and long-term stability at high current densities. These findings establish a mechanistic framework for the rational design of carbon-supported SACs, offering practical guidelines for the development of high-performance and cost-effective AEMWE systems. 2. Results and Discussion 2.1 Configuration of Single Atoms The morphologies and elemental compositions of Ir/reduced graphene oxide (rGO) and Ni/rGO were investigated using field-emission scanning electron microscopy (FE-SEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), respectively. Figure S1 presents the FE-SEM image of an Ir/rGO sheet with a few layers, revealing that its morphology is comparable to that of pristine rGO. The TEM images (Figures 1A,C,E,F) revealed sub-nanometer metal species that were homogeneously dispersed on the rGO sheets for both the Ir/rGO and Ni/rGO samples, whereas pristine rGO exhibited no such features. HAADF-STEM images shown in Figures 1B and 1D revealed that the metal catalysts were dispersed as single atoms. Elemental mapping of C, N, Ir and Ni by energy-dispersive X-ray spectroscopy (EDS) confirmed the uniform presence of iridium and nickel on the graphene surfaces. The quantitative elemental analysis revealed that the 1.0 Ir/rGO catalyst contained 5.97 wt% iridium (0.40 at%), whereas the 1.0 Ni/rGO sample contained 2.94 wt% nickel (0.63 at%), confirming efficient metal utilization with minimal loading. For the Ni/rGO catalysts, thermogravimetric analysis (Figure S4) showed minimal residual mass after complete carbon oxidation, confirming the incorporation of nickel species at low concentrations. The absence of the multi-step decomposition behavior typically observed in multi-atom metal aggregates further suggests that nickel was atomically dispersed on the graphene scaffold. Figure 1. TEM images of Ir/rGO and Ni/rGO. (A) Ir/rGO STEM image; (B) Ir/rGO HAADF-STEM image; (C) Ni/rGO STEM image; (D) Ni/rGO HAADF-STEM image; (E) TEM-EDS elemental mapping image of 0.40 at% iridium loading on the graphene support; (F) TEM-EDS elemental mapping image of 0.63 at% nickel loading on the graphene support. Further characterization using X-ray absorption spectroscopy (XAS) techniques, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), was conducted to elucidate the atomic structure and chemical environment, respectively, of the Ir/rGO and Ni/rGO catalysts. XANES analyses provided information on the oxidation state and local symmetry of absorbing atoms, while EXAFS analyses enabled the quantitative determination of the coordination numbers, interatomic distances, and types of neighboring atoms. For Ir/rGO, the XANES spectrum (Figure S5) exhibited an absorption edge similar to that of the iridium powder, indicating a comparable oxidation state. However, its EXAFS spectrum revealed a more complex oscillation pattern in the k-space (Figure 2a). In particular, adding the rGO support resulted in broader and damped oscillations compared to the Ir powder. Moreover, the EXAFS spectrum exhibited a pronounced Ir–OH peak in the r-space (Figure 2b), suggesting strong interactions between the Ir atoms and hydroxyl groups on the graphene surface. [13–15] In contrast, the Ni/rGO sample exhibited XANES edge features similar to those of metallic Ni foil (Figure S6), indicating a predominantly metallic character. The corresponding EXAFS spectrum of Ni/rGO (Figure 2c–d) exhibited irregular oscillations in the k-space and dominant Ni–C coordination in the r-space, suggesting that the Ni atoms were anchored through covalent interactions with carbon atoms in the graphene lattice. [16–18] These results suggest that Ir single atoms preferentially formed covalent bonds with hydroxyl groups located at the defective edge sites of graphene, while nickel single atoms tended to coordinate with carbon atoms on the basal plane. [19,20] This site-selective incorporation is attributed to the distinct electronic and coordination characteristics of each metal: with a stronger oxidative tendency and higher affinity for oxygen-containing functional groups, Ir was stabilized more effectively at the oxygen-rich edge regions through Ir–OH bonding. In contrast, nickel favors planar coordination with sp²-hybridized carbon or nitrogen atoms on the graphene basal plane because of its lower electronegativity [21–23] and preference for metallic bonding environments. Consequently, the synthesis conditions naturally guided Ir atoms toward edge anchoring and Ni atoms toward basal-plane incorporation, establishing a clear structure–site relationship between the metal identity and anchoring configuration. Figure 2. EXAFS k- and r-spaces of Ir/rGO and Ni/rGO. (A) k-Spaces of Ir powder and Ir/rGO; (B) r-Spaces of Ir powder and Ir/rGO showing peak shifts to a lower r-space; (C) k-Spaces of Ni foil and Ni/rGO; (D) r-Spaces of Ni foil and Ni/rGO showing peak shifts to a lower r-space. 2.2 XPS Analysis To further clarify the site-dependent coordination of iridium and nickel on graphene, X-ray photoelectron spectroscopy (XPS) was performed to analyze the electronic states and local bonding environments of the synthesized catalysts. The survey spectrum of pristine rGO exhibited only C 1s and O 1s peaks, confirming that all metal-related signals originated from the introduced single-atom species rather than from the substrate. For Ir/rGO, the high-resolution O 1s spectrum (Figure 3b) revealed a pronounced Ir–OH component, indicating a strong interaction between the Ir atoms and oxygen-containing functional groups located at the graphene edges. This feature was most prominent in Ir/rGO(4), suggesting that a moderate level of air exposure after calcination promoted surface hydroxylation without excessive oxidation. In contrast, Ir/rGO(0) (no air exposure) and Ir/rGO(8) (excess exposure) displayed weaker Ir–OH intensities, implying insufficient and over-oxidative conditions, respectively (Figure 3c). Thus, post-calcination air injection controlled the hydroxyl coverage and selectively stabilized Ir atoms at the oxygen-rich edge sites—a coordination environment that is expected to strongly influence the OER behavior. For Ni/rGO, the Ni 2p region (Figure 3f) exhibited broad and low-intensity features that remained consistent across the loadings. Such spectral broadening is characteristic of isolated nickel centers and results from multiplet splitting and shake-up satellite effects. The absence of sharp Ni–O peaks indicated that nickel did not form oxide clusters or NiO nanoparticles. Instead, an additional C 1s component at approximately 285–286 eV, consistent with C–Ni coordination, [24] was observed (Figure 3e). These results suggest that Ni was preferentially incorporated into graphene basal-plane defects rather than edge-oxygen sites. Unlike Ir, the contributions of Ni–OH were minimal, reflecting the thermodynamic preference of Ni for basal-plane coordination. Post-treatment attempts aimed at enhancing Ni–OH formation did not alter the XPS spectra or improve the OER activity, indicating that the intrinsic bonding preference of Ni single atoms remains unchanged. Combined with the XANES and EXAFS results, the XPS analyses provide compelling evidence for the single-atom nature of the catalysts. Notably, the EXAFS spectra show no detectable Ir-Ir or Ni-Ni coordination signals, confirming the absence of metal-metal bonding and ruling out the formation of metal clusters or nanoparticles. Instead, the spectroscopic results demonstrated that Ir atoms were predominantly anchored to oxygen-rich edge sites via Ir–OH coordination, whereas nickel atoms occupied the basal-plane defect sites through Ni-C interactions. These distinct anchoring environments provided a structural basis for the fundamentally different electrochemical behaviors of Ir/rGO and Ni/rGO observed in the subsequent rotating disk electrode (RDE) and AEMWE evaluations. Figure 3. XPS analysis of Ir/rGO and Ni/rGO: (a) Survey scan demonstrating the presence of iridium on graphene support; (b) O 1s spectra showing that the Ir–OH bonding energy is larger than that for Ir–O bonding; (c) O 1s spectra showing enhanced Ir–OH bonding in the catalyst prepared with 4 cm³/min air flow; (d) Survey spectrum showing characteristic peaks of rGO with prominent carbon and oxygen signals; (e) C 1s spectrum revealing various carbon bonding states, including C–C, C–H, and C-Ni bonds; (f) Ni 2p spectrum demonstrating Ni–O coordination and metallic nickel states. 2.3 CV and LSV Analysis The electrochemical behaviors of the catalysts were evaluated using RDE measurements, including cyclic voltammetry (CV) and linear sweep voltammetry (LSV). Figure 4 presents a comparative analysis of the electrochemical behaviors of various graphene-based SACs, as measured by CV and LSV over 40–80 cycles. The goal of this investigation was to elucidate how different metal anchoring sites (Ir vs. Ni) and air-injection conditions influenced the overall electrochemical performance of catalysts prepared at a 1:1 metal-to-graphene ratio. The distinct CV profiles of the samples (Figure 4a) confirmed the successful synthesis of unique SAC configurations on the graphene framework. [25,26] A notable difference was observed in the LSV performance during repeated cycling. For the Ir/rGO sample, the overpotential significantly increased from 0.43 V at 40 cycles to 0.55 V at 80 cycles, [27] indicating a gradual structural and electronic degradation of the active sites under prolonged OER operation. This phenomenon is associated with localized thermal accumulation (“thermal runaway”), [28–30] where oxygen evolution at edge sites generates heat and ROS that compromise the structural integrity of the graphene support. [31,32] Although Ni/rGO exhibited a higher overpotential than Ir/rGO during the initial stages, its cycling stability surpassed that of the Ir/rGO catalyst. This contrast was rationalized by the distinct atomic anchoring environments revealed by the XPS and EXAFS analyses. Edge-anchored Ir atoms stabilized through Ir–OH coordination provided highly active but dynamically evolving catalytic centers that gradually underwent structural relaxation and oxidation during repeated OER cycles. In contrast, Ni atoms incorporated into the graphene basal plane formed a stronger Ni–C coordination, resulting in a more rigid and thermodynamically stable configuration that suppressed degradation and led to superior long-term durability, despite their intrinsically higher activation energy. Figure 4. Electrochemical characterizations of the graphene-based SACs (Ir/rGO and Ni/rGO) compared with commercial Ir/C and rGO. (a) CV curves; (b) LSV curves. To further optimize the anode performance, catalysts synthesized under varying air-injection volumes were compared (Figure 5). The CV profiles of Ir/rGO(0), Ir/rGO(4), and Ir/rGO(8) (Figure 5a) exhibited similar overall shapes; however, Ir/rGO(4) demonstrated a distinctly higher anodic peak, indicating enhanced oxygen evolution kinetics. This improvement was attributed to the controlled air exposure (4 cm³/min) during synthesis, which promoted moderate surface hydroxylation and uniform metal dispersion on rGO without excessive oxidation. The corresponding LSV results (Figure 5b) confirmed that Ir/rGO(4) exhibited the lowest overpotential for the OER, [33] validating that a 4 cm³/min air injection provided optimal catalyst structure and performance. Thus, controlled air injection during synthesis played a critical role in tailoring the surface chemistry and electronic configuration of iridium-based SACs, directly affecting their electrochemical durability and activity under AEMWE operating conditions. Figure 5 . Electrochemical characterization of Ir/rGO with varying air flow rates and iridium precursor loadings. (a) CV curves; (b) LSV curves. For iridium-based SACs, the formation and cleavage of iridium–hydroxyl (Ir–OH) bonds played a crucial role in determining the OER pathway under alkaline conditions. The general reaction mechanism of iridium during electrochemical water-splitting is described in Equations (1)–(4): [34–37] Ir + H₂O → Ir–OH + H⁺ + e⁻ (1) Ir–OH → Ir–O + H⁺ + e⁻ (2) Ir–O + H₂O → Ir–OOH + H⁺ + e⁻ (3) Ir–OOH → Ir + O₂ + H⁺ + e⁻ (4) The free-energy changes at each step were evaluated to determine the thermodynamic overpotential of the catalyst. For the 1.0 Ir/rGO sample, the computed free-energy changes were 1.45, 1.57, and 1.81 V for Equations (1), (2), and (3), respectively. Based on the standard potential of U°(RHE) = 1.23 V, the estimated corresponding overpotential was η = (ΔGₘₐₓ/e) − 1.23 = 0.41 V. The final oxygen-release step, as shown in Equation (4), typically exhibits a lower energy barrier (≈ 0.30 V), ensuring the closure of the catalytic cycles. Thus, the Ir–OH intermediate required less energy for its conversion to Ir–O and O₂ compared to other possible pathways. Consequently, catalysts containing a higher concentration of Ir–OH bonds exhibit a lower overpotential and enhanced OER activity. [38] Optimizing the hydroxyl coordination environment of iridium represents an effective strategy for improving the catalytic efficiency of graphene-based single-atom systems. However, due to the localization of Ir atoms at oxygen-rich edge sites, these active centers are prone to oxidation and structural rearrangements under prolonged cycling, which can compromise their long-term stability. The OER mechanisms of Ni SACs significantly differ under alkaline (KOH) electrolyte conditions. When the nickel surface interacts with hydroxide ions, the active phase is generally understood to cycle between Ni(OH)₂ and NiOOH, with highly reactive oxygen-containing intermediates (e.g., NiOO⁻ and NiOOH⁻) [39,40] have been proposed in electrochemical and theoretical studies, as shown in Equations (5)–(8): [41–45] Ni(OH)₂ + OH⁻ → NiOOH + H₂O + e⁻ (5) NiOOH + OH⁻ → NiOO⁻ + H₂O (6) NiOO⁻ + H₂O → NiOOH + OH⁻ (7) 4 NiOOH → 4 Ni(OH)₂ + O₂ (8) In this system, the Ni–O bond serves as the primary active site, facilitating reversible redox conversion between Ni²⁺ and Ni³⁺ during the OER. Unlike iridium, which operates through edge-anchored hydroxyl intermediates, nickel atoms embedded in the basal plane of graphene exhibit higher structural integration and better long-term stability, although with slightly lower activity. Overall, these results highlight a clear trade-off between activity and stability in the two systems. Ir/rGO achieves a higher OER activity owing to its facile Ir–OH transition pathway but exhibits gradual edge-site degradation, while Ni/rGO offers superior structural stability through basal-plane anchoring and robust Ni–O coordination. Moreover, Ni/rGO exhibits a higher overpotential than Ir/rGO, reflecting its intrinsically lower OER activity of Ni-based active sites compared with edge-coordinated Ir sites. Despite this, such performance was achieved with only 2.94 wt% nickel, reflecting a high catalyst utilization efficiency even at extremely low metal loadings. To confirm whether these site-dependent characteristics persisted under realistic operating conditions, both catalysts were further evaluated in AEMWE full-cell configurations. 2.4 AEMWE System Following the successful configuration of iridium and nickel single atoms on graphene, we evaluated the performance of 1.0 Ir/rGO and 1.0 Ni/rGO in an AEMWE system, where hydroxide ion transport was governed by the anion exchange membrane in the presence of a circulating KOH electrolyte. The electrodes for the membrane electrode assemblies (MEAs) were fabricated using 10 wt% catalyst, 10 wt% carbon black, and 80 wt% activated carbon. [46–49] Ionomer contents of 10 and 30 wt% [50] were used in initial tests to compare performance. The 30 wt% ionomer MEA delivered slightly higher performance, achieving a maximum current density of 1.594 A cm⁻² and a current density of 0.870 A cm⁻² at a practical operating voltage of 1.9 V; therefore, it was selected for subsequent experiments. The effect of catalyst loading was subsequently investigated by comparing 10 and 100 wt% for both Ir/rGO and Ni/rGO. Ir/rGO demonstrated improved efficiency with a higher loading (100 wt%), attaining 1.631 A cm⁻² (max) and 1.055 A cm⁻² at 1.9 V. In contrast, Ni/rGO catalysts showed minimal performance variation at different Ni loadings, with 1.161 A cm⁻² (max) and 0.656 A cm⁻² at 1.9 V at both 10 and 100 wt% Ni/rGO loading. These distinct performance trends were consistent with our earlier findings, which revealed that iridium single atoms located at the graphene edges behaved as highly active and independent catalytic centers. Nickel single atoms embedded within the basal plane mainly enhanced structural stability, and increasing the Ni loading did not significantly increase the number of electrochemically active sites, resulting in negligible performance improvement. For practical AEMWE operation, we adopted a 10 wt% catalyst loading as a cost-effective condition, thus substantially reducing electrode cost while maintaining reliable performance. Both Ni/rGO and Ir/rGO exhibited lower overpotentials in the AEMWE full-cell configuration compared with their corresponding half-cell RDE measurements. Specifically, Ni/rGO showed an overpotential of 0.391 V in the AEMWE full cell, which was lower than the value obtained from the RDE measurement (0.515 V). Similarly, Ir/rGO exhibited an overpotential of 0.351 V in the AEMWE full cell, compared with 0.406 V measured in the RDE configuration. Although basal-plane integration (Ni/rGO) enhanced stability, edge-site bonding (Ir/rGO) yielded superior catalytic efficiency in AEMWE. This difference in atomic configuration and corresponding catalytic behavior offers valuable insights for future catalyst design. The performance of our graphene-based SACs, particularly the Ir/rGO system, highlights their strong potential for practical, efficient, and cost-effective applications in AEMWE. Figure 6 . I–V curve analysis of AEMWE with the platinum cathode at 70 ℃. (a) Varying ionomer content at fixed 10 wt% Ir/rGO loading; (b) Effect of Ir/rGO loading optimization using 10 wt% ionomer content; (c) I–V curves of Ni/rGO. 3. Conclusion This study elucidates the fundamental correlation between the atomic anchoring sites of single-atom catalysts (SACs) and their electrochemical performance in anion exchange membrane water electrolysis (AEMWE). We demonstrate that the divergent spatial distribution of metals—iridium at oxygen-rich edges and nickel within basal planes is driven by the distinct binding affinities and coordination energies of each metal relative to specific graphene defects. While the high-energy environment of oxygenated edge sites facilitates the potent OER activity of iridium, the stable coordination within the basal lattice provides nickel with superior long-term electrochemical stability. By identifying these site-specific anchoring mechanisms, this work advances beyond empirical observations to provide a mechanistic design principle: the strategic selection of metal–support configurations to balance the inherent trade-off between activity and stability. These insights establish a clear design principle for heterogeneous SACs, demonstrating that high activity and long-term stability can be achieved by matching the metal identity with an appropriate coordination environment, such as stabilizing O-coordinated noble metal atoms at graphene edge sites to maximize intrinsic activity or anchoring transition metal atoms within basal-plane defects to enhance structural durability. 4. Experimental Section Chemicals and Carbon Materials : Iridium(III) chloride hydrate (IrCl₃·xH₂O, 99.9 % trace metal basis) and Nafion 117 were purchased from Sigma-Aldrich (Germany). Nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O, 99 % trace metal basis) was obtained from Thermo Scientific (USA). Ethanol (99 %) was purchased from Duksan (Korea). All reagents were used as received without further purification. rGO was synthesized in three consecutive steps: (1) graphite powder (Ultra “F” purity, Alfa Aesar, USA) was oxidized into graphite oxide using a Couette–Taylor reactor combined with chemical exfoliation; [51] (2) the resulting graphite oxide (GO) was centrifuged and freeze-dried; [52] and (3) GO was chemically reduced with hydrazine to yield rGO. [53] This reduction process was critical because the removal of oxygen-containing functional groups, such as hydroxyl, carbonyl, and carboxyl groups, significantly enhances the electrical conductivity of graphene [54] Synthesis Method : The catalysts were synthesized using the impregnation method [55] , and the influence of metal identity (iridium vs. nickel) and post-calcination air treatment on the electrochemical performance of graphene-based SACs was thoroughly investigated. Iridium and nickel precursors were synthesized and added to deionized water, stirred vigorously for 12 hours to form dispersion A. The same weight of rGO was added to deionized water, sonicated, and then stirred vigorously to form dispersion B. Subsequently, dispersion A was added to dispersion B. The mixture was vigorously stirred at 60 ℃ for 8 h, filtered through a polyvinylidene difluoride 0.45 μm filter, and dehydrated under vacuum at 40 ℃ overnight. The moisture-free powder was then calcinated in a tube furnace at 450 ℃ for 2 h under a mixed N₂/H₂ atmosphere (4:1 ratio) to minimize the oxidation of both the metal precursors and graphene framework. Subsequently, a controlled airflow was introduced at 25 ~ 30 ℃ for 3 h to induce mild surface hydroxylation and to oxidize the metal sites, [56,57] which enabled controlled surface functionalization without causing excessive oxidation of the graphene framework. [58] Using the Ir/rGO catalyst, we systematically examined the effects of different air injection ratios on the structural and electrochemical characteristics. After calcination under an N₂/H₂ atmosphere (4:1 ratio), air was injected at controlled flow rates of 0, 4, and 8 cm³/min for 3 h at room temperature to tailor surface hydroxylation levels, which consequently governed the anchoring preference of iridium at the oxygen-rich graphene edge sites. Physicochemical Characterization : The physicochemical properties of graphene-based SACs were characterized using various advanced techniques. XANES and EXAFS measurements were performed at the XAS-KIST beamline of the Pohang Accelerator Laboratory (South Korea) to elucidate the local atomic structure and chemical environment, respectively, of the graphene-supported SACs. The surface morphologies of the catalysts were examined using FE-SEM (Hitachi S-4800, Japan). HAADF-STEM was performed using a JEM-ARM200F instrument (NEOARM, Japan) operated at 200 kV to directly visualize and confirm the presence of single atoms on the graphene support. For nickel-containing calculations, we performed thermogravimetric analysis using a Hitachi STA 7300 instrument (Japan). XPS (Thermo VG K-Alpha+, USA) was employed to investigate the electronic states of nickel and iridium and to characterize the nature of the bonds between the single atoms and graphene. Electrochemical Measurement : The electrochemical performance was investigated using a Zive BP2F electrochemical workstation in a three-electrode configuration, which included a platinum wire counter electrode, a saturated Hg/HgO reference electrode in 1 M NaOH, and a glassy carbon working electrode. The catalysts (5 mg) and 45 μL of Nafion 117 solution were dispersed in 1,000 μL of ethanol [59,60] and ultrasonicated until a homogeneous catalyst ink was obtained. The catalysts were loaded onto the working electrode by drop-coating 10 μL of the ink onto a glassy carbon electrode, followed by air-drying at room temperature. CV, LSV, and OER tests were performed in a 0.1 M KOH solution with automatic iR compensation at a scan rate of 10 mV s⁻¹ and 1,600 rpm (315 xg) using a Pine WaveVortex 10. All the potentials were converted to the reversible hydrogen electrode (RHE) scale using Equations (9)–(10): [61] E(RHE) = E(Hg/HgO) + (2.303 × RT/F) × pH + E real (Hg/HgO)(1M NaOH/KOH) (9) E(RHE) = E(Hg/HgO) + 0.0592 × pH + E real (Hg/HgO)(1M NaOH/KOH) (10) where R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), T is the absolute temperature (298 K), and F is the Faraday constant (96,485 C mol⁻¹). The term (2.303 × RT/F) corresponds to 0.0592 V at 25 ℃. Application of AEMWE with Synthesized Catalysts : AEMWE was performed using a single-cell test kit (CNL Corp., Korea). A PiperION anion exchange membrane (20 µm, Versogen, USA) was used for all experiments. MEAs with an active area of 5 cm² were fabricated using the synthesized graphene-based SACs as anodes and a commercially available Pt/C cathode provided by CNL during MEA fabrication. To optimize MEA performance, various anode configurations were prepared by adjusting the catalyst loading and ionomer content in the catalyst ink. These evaluations were conducted to identify the optimal composition for efficient oxygen evolution and overall system stability. The electrolysis cell was operated at 70 °C using a temperature-controlled water circulation system, in which heated water was continuously supplied to maintain a constant cell temperature. An operating temperature of 70 °C was selected as it represents a practical compromise commonly employed in AEMWE systems, where elevated temperatures enhance reaction kinetics and hydroxide ion conductivity while avoiding accelerated degradation of anion exchange membranes. By systematically comparing the customized anode MEAs, we assessed the effectiveness of the synthesized catalysts in a realistic AEMWE configuration. This approach enabled us to elucidate the correlation between MEA composition, catalyst properties, and cell performance, providing valuable insights for designing high-performance and cost-effective anode catalysts for AEMWE systems [62] Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea Government(MSIT) (RS-2025-24534554) The authors acknowledge CNL for their technical assistance and support in the fabrication and testing of the AEMWE cells. Conflict of Interests The authors declare no conflict of interest. A patent related to this work has been filed. Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff)) References [1] P. M. Falcone, M. Hiete, A. 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Keywords basal plane graphene edge hydrogen production single-atom catalyst surface hydroxylation Authors Affiliations Ji Hoon Lee 0009-0002-8494-5994 Yonsei University View all articles by this author Eunju Yang Yonsei University View all articles by this author Jeongeun Song Yonsei University View all articles by this author Bong-Gyeong Shin Yonsei University View all articles by this author Yukwon Jeon 0000-0001-7050-2274 Yonsei University - Mirae Campus View all articles by this author Qilin Li Rice University View all articles by this author Yeojoon Yoon [email protected] Yonsei University View all articles by this author Metrics & Citations Metrics Article Usage 186 views 101 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Ji Hoon Lee, Eunju Yang, Jeongeun Song, et al. Metal-Dependent Anchoring and Catalytic Behavior of Graphene-Supported Iridium and Nickel Single-Atom Anode Catalysts for Anion Exchange Membrane Water Electrolysis. 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