Suppressing lattice expansion via engineering oxygen vacancy into sub-5nm NiO for durable methanol electrooxidation

Suppressing lattice expansion via engineering oxygen vacancy into sub-5nm NiO for durable methanol electrooxidation | 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 Suppressing lattice expansion via engineering oxygen vacancy into sub-5nm NiO for durable methanol electrooxidation Tao Chen, Zhiying Lyu, Jiaxing Song This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8841186/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Replacing the sluggish oxygen evolution reaction (OER) with the electrocatalytic methanol oxidation reaction (MOR) is a promising energy-saving hydrogen production strategy with high-value chemical co-production. However, the electrocatalytic reconstruction at high reaction potentials is prone to catalyst structure collapse due to over-oxidation and lattice expansion, resulting in the loss of active sites and decreased stability. Herein, we presented an oxygen vacancy-rich sub-5 nm electrocatalyst (Nano-Vo-NiO) through a sol-gel-assisted hydrothermal-calcination process integrated with low-temperature plasma modification. The champion catalyst Nano-Vo-NiO operated more than 1000 hours at an industrial-level current density of ~ 500 mA cm − 2 , with a Faradaic efficiency of formate above 94.1% in a membrane electrode assembly (MEA). Operando X-ray absorption spectroscopy (XAS) demonstrated that the vacancy structure remains stable throughout the electrocatalytic process, inhibiting the elongation of Ni-Ni bonds and restricting lattice expansion during the electrocatalytic process. Density functional theory (DFT) calculations revealed that the vacancy structure reduces the proton deintercalation energy barrier, thus achieving efficient MOR activity and stability. This study highlights oxygen vacancies as a critical factor in nanoelectrocatalysis and delivers novel design principles and pathway regulation strategies for efficient electrocatalytic organic oxidation. Physical sciences/Chemistry/Catalysis/Electrocatalysis Physical sciences/Materials science/Nanoscale materials/Nanoparticles Oxygen vacancy sub-nanoscale size Methanol electro-oxidation Lattice expansion Durability Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Tackling the global energy crisis demands the development of clean alternative energy sources 1,2 . Hydrogen, an ideal energy carrier, boasts the merits of abundant resources, high combustion enthalpy, and carbon-neutral emission potential 3,4 . Nevertheless, conventional hydrogen production is predominantly dependent on fossil fuels, causing substantial greenhouse gas emissions 1 . Water electrolysis, an eco-friendly hydrogen production route, is limited by sluggish anodic oxygen evolution reaction (OER) kinetics and high energy consumption 5–7 . To overcome these drawbacks, integrating the electrocatalytic methanol oxidation reaction (MOR) with hydrogen evolution has emerged as a viable alternative 4,8–10 . Replacing OER with the thermodynamically more favorable MOR reduces overall cell voltage and energy consumption, enabling efficient co-production of hydrogen 8–10 . Notably, MOR produces high-value formate, a direct fuel for formic acid fuel cells and an efficient liquid-phase hydrogen carrier 11,12 . This strategy integrates energy-efficient hydrogen production with value-added chemical synthesis, holding great potential for sustainable energy applications. Nickel-based catalysts stand out as leading candidates for the electrocatalytic MOR, benefiting from their earth’s abundance and prominent electrocatalytic activity 13 . The currently accepted electrochemical–chemical (E-C) mechanism for MOR revolves around two pivotal steps: the reconstructive deprotonation of the catalyst and hydrogen transfer in the subsequent spontaneous chemical process 14–16 . Notably, the electrocatalytic reconstruction process represents a critical bottleneck for nickel-based catalysts, primarily arising from the high reaction energy barrier associated with reconstructive deprotonation 17 . The electrochemical reconfiguration process drives excessive lattice expansion, which in turn triggers structural collapse and Ni ion leaching and ultimately leads to compromised catalytic activity and stability 14,18,19 . Thus, advancing nickel-based MOR electrocatalysis necessitates overcoming the energy barrier constraint during reconstruction, mitigating lattice expansion and structural degradation, and addressing the inherent activity-stability trade-off. Among the various strategies for enhancing electrocatalytic oxidation activity and stability, nanonization can increase the surface atomic ratio via the size effect, thereby strengthening the interaction with substrates 20,21 . Meanwhile, nanoelectrocatalysts enable shortened stress transfer paths during the electrocatalytic reconstruction process 6,21 . Furthermore, oxygen defect engineering has been extensively investigated due to its positive effects on regulating reaction activity and electronic structures 22 . Specifically, the construction of oxygen vacancies to modulate unsaturated coordination sites and local electronic structures can effectively lower the oxidation potential of Ni and suppress overoxidation 23–25 . Notably, the presence of oxygen vacancies is expected to significantly relieve the stress induced by lattice expansion during reconstruction, thus maintaining the stability of the reconstructed structure 26,27 . Inspired by the above design principles, designing nano-electrocatalysts with oxygen vacancy structures by virtue of oxygen vacancy and nanosize effects to reduce the potential during reconstruction and inhibit lattice expansion is a promising solution for achieving high activity and high stability. Herein, we report the fabrication of nickel oxide via a sol-gel hydrothermal-calcination route with low-temperature plasma treatment. The low-temperature plasma technique effectively facilitated the creation of oxygen vacancies and suppressed the aggregation of nanoparticles, thereby successfully synthesizing a nanoelectrocatalyst Nano-Vo-NiO for electrocatalytic MOR. Experimental investigations revealed that Nano-Vo-NiO achieved a formate Faradaic efficiency exceeding 92% within a broad voltage range of 1.42–1.67 V vs. RHE in an H-type cell. Moreover, Nano-Vo-NiO demonstrated stable operation for over 1000 hours at an industrial-grade current density of ~ 500 mA cm − 2 , with the formate Faradaic efficiency consistently maintained above 94.1% into a membrane electrode assembly (MEA). In-situ Raman and operando X-ray absorption spectroscopy (XAS) demonstrated that the presence of oxygen vacancies effectively lowered the oxidation potential of Ni, accelerated the formation of the active NiOOH phase, and suppressed the elongation of Ni-Ni bonds while restricting lattice expansion during the electrochemical process. Density functional theory (DFT) calculations elucidated that oxygen vacancy induces an upward shift of the Ni d-band center, facilitating the deprotonation during catalyst reconstruction and effectively lowering the activation energy barrier of the *CH 3 OH→*CH 3 O dehydrogenation step. This work underscores the pivotal role of oxygen vacancies in nanoelectrocatalysts and offers novel design concepts and pathway modulation strategies for stable electrocatalytic organic oxidation. Results and Discussion Synthesis and Characterization The Nano-Vo-NiO catalyst was fabricated via a two-step synthetic strategy integrating the sol-gel hydrothermal-calcination process and low-temperature plasma treatment, with the schematic illustration of the synthesis protocol depicted in Fig. 1a. The catalyst is coated on the hydrophilic porous carbon paper 28 , and X-ray diffraction (XRD) patterns revealed that both Nano-Vo-NiO and the reference sample (Nano-NiO) exhibited distinct diffraction peaks at 37.2°, 43.3°, 62.9°, 75.4° and 79.4°, which are characteristic of the NiO crystalline phase (JCPDS No. 47-1049), corresponding to the (111), (200), (220), (311) and (222) crystal planes, respectively (Supplementary Figs. 1–3) 29,30 . Notably, the XRD diffraction peaks of Nano-Vo-NiO were broader and shifted toward lower angles, ascribed to the introduction of oxygen vacancies (Supplementary Fig. 3b) 31,32 . Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) images demonstrated that the particle size of Nano-Vo-NiO was comparable to that of Nano-NiO, approximately 4 nm, indicating that the plasma treatment did not induce obvious particle agglomeration (Fig. 1b and Supplementary Figs. 4–6). The detailed geometric and phase structures of the catalysts were probed by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). For Nano-Vo-NiO, the observed lattice fringe spacings of ~ 0.241 nm and ~ 0.148 nm correspond to the (111) and (220) crystal planes of NiO, respectively (Fig. 1c and Supplementary Fig. 7) 33,34 . Distinct amorphous regions (marked in yellow) are also discernible, indicative of the presence of oxygen vacancies (Fig. 1c) 31 . In contrast, a well-resolved lattice fringe spacing of ~ 0.21 nm is observed in Nano-NiO, which can be assigned to the (200) crystal plane of NiO, confirming the higher crystallinity of Nano-NiO (Supplementary Figs. 8–9) 35 . These observations are further corroborated by the selected area electron diffraction (SAED) patterns, which are consistent with those obtained from X-ray diffraction (XRD) results (Fig. 1c and Supplementary Fig. 10). Additionally, energy-dispersive X-ray spectroscopy (EDS) elemental mapping revealed that Ni and O elements are uniformly distributed across the entire architecture (Fig. 1d and Supplementary Fig. 11). X-ray photoelectron spectroscopy (XPS) was employed to analyze the valence states and electronic structures of catalysts. The survey scan XPS spectrum verified that Nano-Vo-NiO consists solely of Ni and O elements without any detectable impurities (Supplementary Fig. 12). The high-resolution Ni 2p spectrum, three sets of characteristic peaks were observed, which are associated with the spin-orbit splitting characteristics of Ni 3+ (Ni 2p 3/2 and Ni 2p 1/2 ) and Ni 2+ (Ni 2p 3/2 and Ni 2p 1/2 ), as well as a pair of broad satellite peaks (Supplementary Fig. 13a) 34,35 . Notably, the binding energy of Ni in Nano-Vo-NiO is lower than that in Nano-NiO, indicating a reduced valence state of Ni in the former. Furthermore, the high-resolution O 1s XPS spectrum, a distinct characteristic peak attributed to oxygen vacancy emerged at ~ 532 eV, which further corroborates the existence of oxygen vacancies in Nano-Vo-NiO (Supplementary Fig. 13b) 24 . X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements were subsequently performed to elucidate the electronic structures and coordination configurations. The Ni K-edge XANES spectra revealed that the absorption edge positions of both Nano-NiO and Nano-Vo-NiO lie between those of Ni foil and standard NiO, implying that the valence state of Ni in these two samples falls within the range of 0–2 (Fig. 1e) 9,18 . Specifically, the absorption edge of Nano-Vo-NiO is closer to that of Ni foil, whereas Nano-NiO exhibits an absorption edge position more proximate to standard NiO. This observation demonstrated that the oxygen vacancies lead to a lower valence state of Ni, which is corroborated by the XPS spectra 8,10 . In addition, the EXAFS spectra exhibited two distinct characteristic peaks at ~ 1.98 Å and ~ 2.92 Å, assigned to the Ni–O and Ni–Ni bonds, respectively (Fig. 1f, Supplementary Figs. 14–17 and Supplementary Table 1) 9,30 . Evidently, the intensity of the Ni–O peak in Nano-Vo-NiO is weaker than that in Nano-NiO, suggesting a lower Ni coordination number in Nano-Vo-NiO. Meanwhile, the Ni–O bond length in Nano-Vo-NiO is shorter than that in Nano-NiO (2.08 Å vs. 2.07 Å), providing additional evidence for the presence of oxygen vacancies (Supplementary Figs. 14–17 and Supplementary Table 1) 36 . The high-resolution wavelet transform (WT) was performed on the phase-uncorrected EXAFS data in both R and k spaces. For Nano-Vo-NiO, the most prominent WT signals appear at approximately 5 Å −1 and 7.2 Å −1 , which correspond to the initial existence of Ni − O and Ni − Ni bonds, respectively (Fig. 1g and Supplementary Figs. 18–21). 9 The electron paramagnetic resonance (EPR) spectroscopy results revealed that Nano-Vo-NiO exhibits a distinct EPR signal (g = 2.0037), corresponding to the electrons trapped by oxygen vacancies, which is consistent with XPS results (Supplementary Fig. 22) 32 . Moreover, the oxygen vacancy-enriched Nano-Vo-NiO exhibits a lower contact angle, indicating that oxygen vacancies contribute to enhanced hydrophilicity and thereby facilitate improved substrate accessibility during the electrocatalytic reaction (Supplementary Fig. 23) 37 . To elucidate surface electronic properties, zeta potential measurements were performed. Plasma treatment reversed the catalyst’s potential from negative to positive, arising from oxygen vacancy-driven charge redistribution in NiO where positive defects counteract (Supplementary Fig. 24) 38 . Electrocatalytic methanol oxidation To further explore the performance of electrocatalysts for the electrocatalytic MOR, the synthesized materials were tested in an H-type electrochemical cell. A standard three-electrode system was adopted, including a catalyst-based working electrode, a Pt mesh counter electrode, and an Hg/HgO reference electrode, separated by an anion-exchange membrane. The electrolyte was 1 M KOH with or without 1 M methanol (MeOH). High-performance liquid chromatography (HPLC) was utilized to quantify formate yield and determine the MOR Faradaic efficiency (FE) (Supplementary Fig. 25). Figure 2a displays the linear sweep voltammetry (LSV) curves recorded in 1 M KOH + 1 M MeOH. Nano-Vo-NiO exhibits an onset potential of 1.39 V vs. RHE at a current density of 10 mA cm − 2 , which is slightly lower than that of pristine Nano-NiO (1.42 V vs. RHE). The LSV curves in the presence of methanol showed a marked increase in current density compared to those in 1 M KOH, accompanied by the disappearance of Ni 3 ⁺ species. This observation reveals that methanol oxidation proceeds preferentially via an electrochemical–chemical (E–C) pathway, which is thermodynamically more favorable than OER 15,16 . Notably, at 1.6 V vs. RHE, Nano-Vo-NiO delivers a current density of 132 mA cm − 2 , higher than that of Nano-NiO (56 mA cm − 2 ), as shown in Fig. 2b, confirming that oxygen vacancies promote the formation of highly active NiOOH species for MOR 39,40 . The influence of synthesis parameters, plasma treatment powers, on MOR activity was systematically examined (Supplementary Figs. 26–27). As illustrated in Fig. 3c, the Tafel slope of Nano-Vo-NiO is 48.1 mV dec − 1 , significantly lower than that of Nano-NiO (72.8 mV dec − 1 ), indicating accelerated reaction kinetics and reduced charge-transfer resistance 41,42 . This conclusion is further supported by electrochemical impedance spectroscopy (EIS) measurements (Supplementary Fig. 28), where Nano-Vo-NiO exhibits the smallest semicircle diameter in the Nyquist plot, consistent with enhanced interfacial charge transfer 30,43 . As presented in Supplementary Figs. 29–30, normalization of the current response by the double-layer capacitance (C dl ) reveals a substantially larger electrochemically active surface area (ECSA) for Nano-Vo-NiO (2.7 mF cm − 2 ) compared to Nano-NiO (1.8 mF cm − 2 ). Figure 2d presents the potential-dependent Faradaic efficiency for formate production 23,44 . As the applied potential increases, the FE for formate gradually decreases due to the competitive oxygen evolution reaction (OER) at higher potentials (Supplementary Fig. 31). Remarkably, Nano-Vo-NiO maintains a formate FE above 90% across a broad potential window of 1.42–1.67 V vs. RHE (Fig. 2d, Supplementary Fig. 31). Within this range, the formate production rate increases monotonically with potential, reaching 1.254 mmol cm − 2 h − 1 for Nano-Vo-NiO at 1.67 V vs. RHE (Fig. 2e). Beyond 1.72 V vs. RHE, the formate production rate declines as OER becomes dominant, and without other liquid products were detected (Supplementary Fig. 32) 40,45 . Meanwhile, the FE for hydrogen evolution at the cathode remains close to 100% throughout the potential range (1.42–1.72 V vs. RHE) (Supplementary Fig. 33). At 1.67 V vs. RHE, the hydrogen evolution rate with Nano-Vo-NiO is 2.42 times higher than that with Nano-NiO ((Fig. 2f). Chronoamperometric measurements were performed to assess the long-term stability, as depicted in Fig. 2g. Nano-Vo-NiO demonstrated exceptional operational stability, maintaining a high and consistent Faradaic efficiency (FE) for formate production (~ 94%) over 120 hours at current densities exceeding 100 mA cm − 2 . In contrast, Nano-NiO suffers from significant degradation under identical conditions, exhibiting a continuous decline in both current density and FE (Fig. 2g). 1 H NMR spectroscopy indicated that only formate was detected as the product (Supplementary Fig. 34). The results of XRD, XPS and SEM characterizations of the post-reaction catalyst demonstrated that the catalyst structure remained basically intact (Supplementary Figs. 35–38). The attenuation corresponding to the LSV curves was negligible (Supplementary Fig. 39). Inductively coupled plasma (ICP) analysis of post-reaction electrolytes showed that leached Ni concentration from Nano-NiO was ~ 5-fold higher than that from Nano-Vo-NiO, indicating oxygen vacancies effectively mitigate structural degradation and suppress active Ni dissolution during prolonged electrolysis (Supplementary Table 2). Mechanistic Studies of electrochemical reconstruction. To further investigate the electrochemical reconstruction of the catalyst, multi-potential step measurements were performed (Supplementary Figs. 40 and 41). Specifically, a potential of 1.6 V (vs. RHE) was applied from 0 to 40 s to generate Ni 3+ species; the circuit was then held open from 40 to 90 s, followed by applying a potential of 1.1 V (vs. RHE) from 90 to 150 s. Upon injection of 1 M methanol, the reduction current associated with Ni 3+ dropped sharply, which can be attributed to the direct consumption of Ni 3+ species by methanol, demonstrating that the MOR via an electrochemical–chemical (E–C) pathway (Supplementary Fig. 40) 4 . Compared to Nano-NiO, Nano-Vo-NiO exhibited a stronger reduction peak corresponding to Ni 3+ after open-circuit potential testing in 1 M KOH. Furthermore, in 1 M KOH containing 1 M methanol, it displayed a higher current during the 0–40 s interval but a weaker Ni 3+ reduction peak in the 90–150 s period (Supplementary Fig. 41). This further confirms that vacancy engineering accelerates the generation of high-valence nickel active species and simultaneously enhances methanol adsorption, thereby contributing to improved catalytic activity 9,45 . In-situ Raman spectra were employed to investigate the reconstruction mechanisms of Nano-NiO and Nano-Vo-NiO. Distinct characteristic peaks at approximately 472 cm − 1 and 558 cm − 1 emerged clearly with increasing applied potential, corresponding to the ν(Ni 3+ –O) and δ(Ni 3+ –O) vibrational modes, respectively, confirming the gradual formation of NiOOH species (Figs. 3a and 3b) 19,29 . Nano-Vo-NiO exhibited stronger signals, indicating the generation of a greater amount of active nickel species. Furthermore, as the potential increased, the δ(Ni³⁺–O) peak from NiOOH showed a red shift (Fig. 3c), signifying lattice expansion due to bond elongation. In contrast, Nano-Vo-NiO displayed no significant red shift, demonstrating suppressed lattice expansion during its reconstruction process 46 . This mitigates the internal stress caused by repeated lattice expansion and contraction during electrochemical reconstruction, reflecting restricted distortion of the NiO 6 octahedral units and thereby inhibiting the loss of active sites 37 . To gain deeper insight into the dynamic reconstruction process and the differences in fine structural evolution, Operando electrochemical X-ray absorption spectra (XAS) were carried out. X-ray absorption near-edge structure (XANES) spectra revealed that as the applied potential increased from OCP to 1.6 V (vs. RHE), the Ni K-edge absorption energy for both Nano-NiO and Nano-Vo-NiO systematically shifted to higher energy, demonstrating the gradual accumulation of high-valence nickel species (Figs. 3d and 3e) 23,47 . Notably, the Ni K-edge absorption edge of Nano-Vo-NiO shifted more significantly, revealing that electrochemical reconstruction generates more active nickel species (Fig. 3f) 48 . The Ni K-edge absorption energy of Nano-Vo-NiO contrasts with that of Nano-NiO, which exhibits a distinct energy upshift (Fig. 3f). This observation points to a higher oxidation state in Nano-NiO, originating from the prevalence of unsaturated coordinated Ni sites, and aligns with previous literature 23,31 . Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) spectra recorded in R-space at OCP, 1.2 V, 1.4 V, and 1.6 V (vs. RHE) exhibited two distinct peaks for both Nano-NiO and Nano-Vo-NiO at ~ 2.85 Å and ~ 2 Å, corresponding to Ni–Ni and Ni–O bonds, respectively (Figs. 3g and 3h) 18,48 . With increasing applied potential, the Ni–O bond lengths for both catalysts significantly shortened, decreasing from 2.1 Å to 2.07 Å for Nano-NiO and from 2.06 Å to 2.0 Å for Nano-Vo-NiO, respectively (Figs. 3g and 3h, Supplementary Table 3 and Table 4). This is attributed to the interaction between high-valence nickel and surrounding oxygen atoms, further verifying the formation of NiOOH 30,49 . It can be clearly observed that the coordination numbers (CN) of the Ni–O bonds remained relatively stable throughout the reconstruction process (Supplementary Table 3 and Table 4). The CN of Ni–O bonds in Nano-NiO and Nano-Vo-NiO stabilized at approximately 6 and 4, respectively, demonstrating that oxygen vacancies remain stable during the entire catalytic process. These stably existing defect structures are believed to enhance chemisorption and promote electrocatalytic oxidation processes 39 . Notably, for Nano-Vo-NiO, the Ni–Ni bond length increased from 2.78 Å to 2.88 Å with the potential from OCP to 1.6 V (vs. RHE). In contrast, for the Nano-NiO, the Ni–Ni bond length increased from 2.77 Å to 2.93 Å (Fig. 3i and Supplementary Figs. 42–43). A longer Ni–Ni bond illustrates a larger crystalline phase size, demonstrating lattice distortion during reconstruction and the twisting of NiO 6 octahedral units 48–50 . As presented in Fig. 3i, the change in the Ni–Ni bond length for Nano-Vo-NiO was significantly smaller than that for Nano-NiO, further proving that the construction of vacancies can suppress lattice expansion during electrocatalytic process (Fig. 3j). DFT calculation and mechanism clarification for enhanced activity Based on density functional theory (DFT) calculations, this study further elucidates the changes in electronic structure and energy during the enhancement of electrocatalytic MOR. With reference to the reported literature, NiOOH was selected as the active site for MOR 8,15 . Theoretical models featuring four-coordinated and saturated coordinated Ni sites as active centers were constructed, corresponding to Nano-NiO and oxygen vacancy (Vo)-rich Nano-Vo-NiO, respectively. Their structural configurations and side views are presented in Fig. 4a and Supplementary Fig. 44. Model optimization results revealed that the Ni–Ni and Ni-O bond lengths in Nano-NiO and Nano-Vo-NiO are consistent with experimental Operando XAS in 1.6 V vs. RHE. Compared with Nano-NiO, unsaturated coordination in Nano-V₀-NiO shifts the Ni d-band center closer to the Fermi level (Fig. 4b), markedly enhancing the adsorption capacity for hydroxylated oxide active species 40,51 . The adsorption energy of Nano-Vo-NiO for CH 3 OH* and OH* are − 0.50 eV and − 2.68 eV, respectively, which are stronger than those of Nano-NiO (-0.26 eV and − 2.09 eV, respectively). This result confirmed that oxygen vacancies can effectively enhance the catalyst’s adsorption capacity toward reactants and reduce mass transfer energy barriers (Fig. 4c and Supplementary Fig. 45) 34,41 . Subsequent calculations on the deprotonation energy barrier for NiOOH formation indicated that Nano-Vo-NiO exhibits a lower deprotonation energy barrier, implying that oxygen vacancies can accelerate the self-reconstruction process for active site formation (Fig. 4d and Supplementary Fig. 46) 17 . The Gibbs free energy profiles and the corresponding configurations of Nano-NiO and Nano-Vo-NiO are presented in Fig. 4e and Supplementary Figs. 47–48, respectively. For the Nano-Vo-NiO catalyst with abundant unsaturated coordinated Ni sites, the energy barrier of the rate-determining step (0.65 eV) is remarkably lower than that of Nano-NiO (1.14 eV), which can effectively facilitate the dehydrogenation process of *CH 3 OH→*CH 3 O and thus endow the catalyst with excellent electrocatalytic methanol oxidation performance 45,52 . Coupling Electrochemical MOR and HER Owing to the excellent electrocatalytic activity and stability of Nano-Vo-NiO, it was employed together with Pt/C as electrocatalysts for the MOR coupled with HER, respectively, in a zero-potential membrane electrode assembly (MEA) system. The anodic electrolyte was a 1 M KOH containing 1 M methanol, while the cathodic electrolyte was a 1 M KOH; a schematic diagram of the relevant system is shown in Fig. 5a. As observed from the linear sweep voltammetry (LSV) curves (Fig. 5b), compared with the OER || HER system, the reaction overpotential decreased significantly in the presence of methanol, demonstrating that the system tends to undergo the MOR preferentially 53 . At a current density of 1 A cm − 2 , the cell voltage of the entire reaction decreased by 210 mV (Fig. 5c). Under galvanostatic electrochemical conditions, Nano-Vo-NiO delivers a FE for formate exceeding 80% over a broad current density window of 0.2–1.2 A cm − 2 , while Nano-NiO affords a markedly lower FE (Supplementary Fig. 49). Notably, the champion catalyst, Nano-Vo-NiO, delivered a formate production rate of 9.436 mol cm − 2 h − 1 with a Faradaic efficiency of 84.3% (Fig. 5d). Simultaneously, upon probing the FE of the HER under variant current density, it was observed that the FE of hydrogen approached 100% (Supplementary Fig. 50). The FE of formate remained stable above 94% after a stability test of over 1000 h at current density of ~ 500 mA cm − 2 , exhibiting excellent electrocatalytic stability (Fig. 5e). This superior performance places the catalyst at the top level among all state-of-the-art electrocatalysts ever reported for electrocatalytic MOR (Fig. 5f and Supplementary Table 5) 9,42–45,50,51,54 . Only formate was detected as the product in the 1 H NMR spectrum of the electrolyte in the two-electrode MEA after long-term test (Supplementary Fig. 51). Techno-economic analysis (TEA) indicated that the MOR || HER system employing Nano-Vo-NiO as the catalyst is economically viable (Supplementary Fig. 52 and Tables 6–8), generating a revenue exceeding USD 690 per ton of methanol processed at an operating current density of 300 mA cm − 2 , thereby demonstrating significant practical application potential (Fig. 5g and Supplementary Note 1) 55 . Conclusion In summary, we successfully fabricated an oxygen vacancy-rich sub-5nm electrocatalyst (Nano-Vo-NiO) via the integration of sol-gel-assisted hydrothermal-calcination with low-temperature plasma treatment, and employed it for electrocatalytic MOR. The catalyst exhibited a formate Faradaic efficiency exceeding 92% over a wide potential range of 1.42–1.67 V vs. RHE. Further integration of this catalyst into a membrane electrode assembly achieved stable operation for over 1000 h at a current density of ~ 500 mA cm − 2 , with the formate Faradaic efficiency maintained above 94%. In-situ characterizations and DFT calculations revealed that oxygen vacancies not only upshift the d-band center of Ni sites but also reduce the energy barrier for electrocatalytic reconstruction. Notably, the retained oxygen vacancies suppress Ni-Ni bond elongation and restrict lattice expansion during electrochemical reconstruction, thereby enhancing the catalyst's long-term stability. This work underscores the pivotal role of oxygen vacancies in nanoelectrocatalysts and establishes innovative design principles and pathway regulation strategies for durable electrocatalytic organic oxidation. Methods Chemicals Methanol (CH 3 OH, 99.5%) and Nickel nitrate hexahydrate (Ni(NO 3 ) 2 ·6H 2 O, 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Pt/C (20% Pt), Sodium hydroxide (NaOH, 97%) and potassium hydroxide (KOH, 90%) were obtained from Aladdin Chemical Reagent Co., Ltd. Formic acid (HCOOH, 98%), sulfuric acid (H 2 SO 4 , > 99%), and hydrochloric acid (HCl, ~ 37%) were supplied by Shanghai Lingfeng Chemical Reagent Co., Ltd. Deionized water was acquired from Wahaha Group Co., Ltd. All reagents were used as received without further purification. Synthesis of catalysts 3 g of nickel nitrate hexahydrate (Ni(NO 3 ) 2 ·6H 2 O) was weighed and dissolved in 50 mL of deionized water, followed by stirring for 30 minutes to ensure complete dissolution. Subsequently, a 1 M NaOH solution was slowly added dropwise into the above solution, with the stirring speed maintained at 600 rpm. The pH value of the solution was adjusted to approximately 10. Then, stirring was continued for 1 hour, during which green precipitates were observed. The precipitates were centrifugally washed three times with ethanol and deionized water, respectively, at a centrifugal speed of 9000 rpm and a washing duration of 3 minutes each time. The obtained green precipitates were freeze-dried under vacuum for 24 hours. Afterwards, the dried sample was placed in a muffle furnace and annealed in air at 270°C for 2 hours, yielding the sample denoted as Nano-NiO. Low-temperature plasma treatment was employed to construct oxygen vacancies. Approximately 40 mg of Nano-NiO catalyst was placed on a cooling stage for plasma treatment. The argon gas flow rate was maintained at 50 mL/min, with the vacuum stabilized around 30 Pa. The plasma treatment power (300, 400, 500, 600 W) and duration (0.5, 1, 1.5, 2 h) were adjusted accordingly. The catalyst exhibiting the optimal electrocatalytic performance was denoted as Nano‑Vo‑NiO. The catalyst was deposited onto a hydrophilic carbon paper, where its porous architecture facilitates the adsorption and diffusion of reaction substrates. Before conducting electrocatalytic tests on the prepared catalyst, the catalyst was fully activated. The typical electrode preparation process was as follows: 5 mg of the catalyst was weighed and dispersed in 1 mL of a mixed solution of ethanol and water (volume ratio V1:V2 = 50:50). Then, 40 µL of 5 wt.% Nafion solution was added to the dispersion, which was subsequently sonicated for 30 minutes. Using carbon paper as the gas diffusion electrode, 88 µL of catalyst ink was dropped onto the carbon cloth (1 cm²) and dried in ambient air. In a three-electrode system, a Hg/HgO electrode and a Pt electrode served as the reference electrode and counter electrode, respectively, and the electrolyte was a 1 M KOH solution. Within the potential range of 0.076 ~ 0.776 V (vs. RHE), 30 cycles of cyclic voltammetry (CV) activation tests were performed on the catalyst at a scan rate of 5 mV/s, followed by electrochemical measurements. Characterizations The phase and crystal structure of the catalyst were characterized using a Rigaku 9 kW X-ray diffractometer (Cu rotating anode source) with XRD patterns collected in the range of 5°–80° at a scanning rate of 10°/min. Crystallinity, lattice fringes, and grain boundary distribution were further analyzed via a Thermo Fisher Talos F200X G2 high-resolution transmission electron microscope (HR-TEM). Morphological features and elemental characteristics were investigated with a ZEISS Sigma 300 scanning electron microscope to observe surface morphology, particle dispersibility, and elemental distribution. X-ray absorption fine structure (XAFS) spectra were collected at the beamline BL20U1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF). Surface chemical properties were analyzed by a Kratos AXIS Supra X-ray photoelectron spectrometer (Al Kα radiation, 1486.6 eV) calibrated with the C 1s peak at 284.6 eV for determining composition and valence states. The hydrophilicity/hydrophobicity of the catalyst was measured using a Dataphysics OCA 20 contact angle meter, while Zeta potential and colloidal stability were evaluated via a Malvern Zetasizer Nano ZS90. Oxygen vacancies in the catalyst were monitored by a Bruker EMX10/12 electron paramagnetic resonance spectrometer. A Thermo Fisher DXR3xi Raman spectrometer (532 nm laser, 5 mW, 200 ~ 3000 cm − 1 ) was used to analyze the specific structure of the catalyst. A Bruker AV500 nuclear magnetic resonance spectrometer ( 1 H NMR) was employed to study the product composition and catalytic selectivity after the electrocatalytic reaction. Electrochemical measurement All electrochemical tests were conducted at room temperature. Electrochemical double-layer capacitance (C dl ) was measured via cyclic voltammetry (CV) in a 1 M KOH electrolyte, with data in the non-Faradaic region collected at different scan rates ranging from 20 mV s − 1 to 100 mV s − 1 . For the electrochemical impedance spectroscopy (EIS) test, the frequency range was 0.01 Hz to 10,000 Hz, using a Pt sheet as the counter electrode and Hg/HgO as the reference electrode. Before the test, the catalyst was activated via cyclic voltammetry, and the electrolyte was saturated with Ar for 30 minutes. The Tafel slope was obtained from the linear sweep voltammetry (LSV) curves within a potential window of 10 mV, and plotted against the average current or potential. The calculation formula for the Tafel slope is as follows: η = a + b × log( J ) Where η (mV) denotes overpotential, a is the exchange current density; J (mA cm − 2 ) represents current density; and b is the Tafel slope (mV dec − 1 ). The membrane electrode assembly (MEA) was carried out employing spray coating and hot-pressing transfer methods. Specifically, the catalyst inks for the anode (Nano-Vo-NiO for MOR) and the cathode (20% Pt/C for HER) were uniformly coated onto PET substrates using a spray coater, followed by vacuum drying at 60°C. The dried anode and cathode catalyst layers were then aligned facing opposite sides of a cation exchange membrane (SF-C120) and subjected to hot-pressing under the following conditions: temperature of 120°C, pressure of 2 MPa, and duration of 60 seconds. Under these parameters, the catalyst layers were successfully transferred and firmly bonded to both sides of the ion exchange membrane. Titanium fiber felts were placed on both sides of the electrode as gas diffusion and protect layers for the cathode and anode, with an active area of 1 cm 2 . Both end plates were made of titanium and featured serpentine flow fields. An SF-C120 ion exchange membrane was used as the separator. Before testing, the cell components were tightly fastened using a torque wrench at 5 N·m to ensure stable contact resistance at the electrode–membrane interfaces, thereby reducing the overall internal resistance of the cell to below 0.4 Ω and ensuring long-term stability during testing. All electrochemical tests were conducted at room temperature. The anolyte consisted of a 1 M KOH solution with 1 M methanol, while the catholyte was a 1 M KOH solution. Both electrolytes were circulated using gas–liquid mixed flow pumps. All electrochemical data were recorded without iR compensation. Product quantification The electrocatalytic performance for the methanol oxidation reaction (MOR) was evaluated using a three-electrode configuration within an H-type cell, connected to a Wuhan Corrtest electrochemical workstation (Corrtest C350M). A Hg/HgO electrode and a Pt electrode served as the reference and counter electrode, respectively. The anodic compartment contained 10 mL of 1 M methanol in 1 M KOH aqueous solution, while the cathodic compartment was filled with 1 M KOH solution. The two chambers were separated by an FAA-3-PK-75 ion exchange membrane. All applied potentials were converted to the reversible hydrogen electrode (RHE) scale using the following equation: $$\:{E}_{RHE}={E}_{\text{H}\text{g}/\text{H}\text{g}\text{O}}+0.0592\text{*}\text{p}\text{H}+0.098$$ Product analysis was conducted via high-performance liquid chromatography (HPLC). After electrolysis, 1 mL of the electrolyte was collected, acidified with 2 M HCl, and diluted before injection. The HPLC separation was performed on a Shodex SUGAR SC1011 column maintained at 80°C, with a mobile phase of 5 mM H₂SO₄ flowing at 0.7 mL/min. Detection was carried out at a wavelength of 210 nm. For stability assessments, chronopotentiometric measurements were performed at a constant current density of 100 mA cm -2 . The resulting electrolyte was similarly acidified, diluted, and analyzed by HPLC to quantify products. The Faradaic efficiency (FE) for formate was calculated using the formula: FE (%) = [ \(\:\frac{moles\:of\:product\:\times\:\:N\:\times\:\:F}{\:\text{t}\text{o}\text{t}\text{a}\text{l}\:\text{c}\text{h}\text{a}\text{r}\text{g}\text{e}\:\text{p}\text{a}\text{s}\text{s}\text{e}\text{d}}\) ] × 100% where F represents Faraday’s constant (96485 C mol -1 ), and N denotes the number of electron transfers per methanol molecule oxidized to formate, which is 4. Operando measurements For standard electrochemical in-situ Raman spectroscopy measurements, Nano-NiO and Nano-Vo-NiO were fully activated separately, with the preparation and activation processes of the catalysts provided in Section 1.2. Measurements were conducted using a home-made in-situ Raman cell, employing a 514 nm He laser as the excitation source. Herein, a carbon rod served as the counter electrode, and Hg/HgO as the reference electrode. The electrolyte was pre-purged with saturated Ar, and then kept flowing via a peristaltic pump. Each target potential was held for 3 minutes, followed by immediate collection of the Raman spectra. Before the measurement, the Raman spectral shifts were calibrated against the 520.7 cm − 1 peak of a silicon wafer. Operando Ni K-edge X-ray absorption fine structure (XAFS) spectra were recorded using the BL20U1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF) and a custom-designed in-situ electrochemical cell. Data collection in fluorescence mode was employed to track the dynamic structural evolution of the catalyst during electrocatalytic reconstruction, with a data acquisition duration of 40 minutes for each reaction potential. The X-ray absorption near-edge structure (XANES) spectra were energy-calibrated using a Ni foil as the reference. The catalyst was deposited onto the surface of a gas diffusion electrode (GDE), with the working electrode fabricated following the protocol detailed in Section 1.2. The XANES and extended X-ray absorption fine structure (EXAFS) datasets were processed in accordance with standard protocols via the Athena software embedded within the Demeter package. EXAFS fitting analyses were conducted within the Artemis module utilizing the FEFF6 code for theoretical scattering path calculations. Calculation details First-principles calculations with spin polarization were carried out using the Vienna Ab initio Simulation Package (VASP). The projector-augmented wave (PAW) pseudopotentials were employed to describe the electron-ion interactions 56 . The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional was adopted for the calculation of electronic structures 57,58 . A plane-wave kinetic energy cutoff of 520 eV was set for all calculations. For Brillouin zone integration, a Gaussian smearing width of 0.05 eV was applied to guarantee numerical stability during self-consistent field (SCF) iterations 59 . The convergence criteria were set as follows: the electronic convergence was achieved when the total energy difference between consecutive SCF cycles was less than 10 eV. Structural relaxation was terminated until the residual force on each atom was reduced to below 0.02 eV·Å. To eliminate interlayer periodic interactions, a vacuum layer of 18 Å was introduced perpendicular to the surface plane. The semi-empirical DFT-D3 dispersion correction proposed by Grimme was incorporated to account for non-covalent interactions. During the electrocatalytic reconstruction process, hydroxylated nickel oxide species were identified by in-situ Raman spectroscopy, which motivated the establishment of a NiOOH model as the reference system with saturated coordination number (Nano-NiO). In-situ XAS spectra confirmed the existence of oxygen vacancies in the Nano-Vo-NiO sample with a four-coordinated coordination number. The adsorption energy (E ad ) was computed via the equation below: E ad = E molecule + surface – E surface – E molecule Where E surface is the energy of the catalyst model, E molecule represents the energy of the adsorbate, and E molecule+surface represents the total energy of the adducts of adsorbent and adsorbate. The proton desorption energy ( E H ) was calculated using the following expression: E H = E V−H + 1/2 E H2 − E slab Where E V−H , E slab , and E H2 are the energy for the relaxed slab after H atom desorption, the relaxed slab, and the gas phase H 2 , respectively. The Gibbs free energy change (ΔG) for each elementary step was estimated by the formula: 60 Δ G = Δ E DFT + Δ E ZPE – T Δ S Where Δ E DFT is the electronic energy difference obtained from DFT calculations, Δ E ZPE is the zero-point energy change, T is the temperature (set to 298.15 K), and Δ S is the entropy change. The Gibbs free energy change was calculated for each reaction step. For the standard hydrogen electrode (SHE), the free energy of (H + e) was equivalent to that of 1/2 H(g). Declarations Acknowledgements We thank L. Zuo (State Key Laboratory of Silicon and Advanced Semiconductor Materials, Department of Polymer Science and Engineering, Zhejiang University) for his invaluable advice during the revision process. This work was supported by the National Natural Science Foundation of China (No. 62404084) and the China Postdoctoral Science Foundation (2025M781129). The authors would like to thank Scientific Compass (www.shiyanjia.com) for the support of SEM tests and Scixas Lab (www.scixas.com) for the TEM tests. Author contributions T. C. performed the experiments, density functional theory (DFT) calculations, in-situ spectroscopy tests, and wrote the manuscript. Z. Lyu. contributed to the discussion of the technoeconomic analysis (TEA). T. C. and J. S. supervised the project, conceived the research idea, analyzed the data, and acquired funding. All authors participated in data analysis and engaged in constructive discussions throughout the work. Competing Interests Statement The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Additional information Supplementary Information References Gunathilake, C. et al. A comprehensive review on hydrogen production, storage, and applications. Chem. Soc. Rev. 53 , 10900-10969, (2024). 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Supplementary Files SupportingInformation.pdf Supporting Information Graphicalabstract.docx Cite Share Download PDF Status: Posted 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-8841186","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":593904176,"identity":"dd945391-9efe-4583-adc9-c4a703f3fad6","order_by":0,"name":"Tao Chen","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA40lEQVRIie3RMQrCMBSA4VeEukS6xqVe4ZWO9ggeoiK0U0EQimNEqIvgGnDwDOIFIoFOpXNBB4+guHRRtDo4Jm6C+bfA+8gjATCZfjC7PRd+PQ1cANEcNYhD8nACReTrky4f4NXK5JC9L9UgWIG3G9tRvJkVCOdUgrNmCnJgI5+TIJmxAi1eSqBHoSBHkSOhUTKHAludTALSULXYcFETlLHdkJsO6fIR+CSUIWmIpUOejwweF5HHIR/vl2VMaKUgdnt1xss96PW43J7qtO86XEE+UfH6TKI736zIvhg2mUymv+oBACZIN0GKtVgAAAAASUVORK5CYII=","orcid":"https://orcid.org/0009-0000-4889-5868","institution":"State Key Laboratory of Silicon and Advanced Semiconductor Materials, Department of Polymer Science and Engineering, Zhejiang University","correspondingAuthor":true,"prefix":"","firstName":"Tao","middleName":"","lastName":"Chen","suffix":""},{"id":593904177,"identity":"35e4debf-c2a9-462a-b974-c8fd7a1ecfec","order_by":1,"name":"Zhiying Lyu","email":"","orcid":"","institution":"RWTH International Academy, RWTH Aachen University","correspondingAuthor":false,"prefix":"","firstName":"Zhiying","middleName":"","lastName":"Lyu","suffix":""},{"id":593904178,"identity":"005c9dd0-d93a-4aaa-8685-4be76133b0a6","order_by":2,"name":"Jiaxing Song","email":"","orcid":"","institution":"Jiaxing University","correspondingAuthor":false,"prefix":"","firstName":"Jiaxing","middleName":"","lastName":"Song","suffix":""}],"badges":[],"createdAt":"2026-02-10 12:45:20","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8841186/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8841186/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103062810,"identity":"f68ec150-49ca-44a6-a52a-1b18da81c1df","added_by":"auto","created_at":"2026-02-20 10:33:21","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":609151,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSynthesis scheme and structural characterizations of Nano-Vo-NiO.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e Schematic illustration of the catalyst’s preparation process. \u003cstrong\u003eb\u003c/strong\u003e HAADF-STEM image (inset: mean size) and \u003cstrong\u003ec\u003c/strong\u003e HR-TEM images with clear lattice fringes of Nano-Vo-NiO (inset: SAED image). \u003cstrong\u003ed\u003c/strong\u003e HAADF-STEM image and the corresponding EDS elemental mapping images on Nano-Vo-NiO (scan bar: 20 nm). \u003cstrong\u003ee\u003c/strong\u003e Normalized Ni \u003cem\u003eK\u003c/em\u003e-edge XANES spectra and \u003cstrong\u003ef\u003c/strong\u003e k\u003csup\u003e3\u003c/sup\u003e-weighted EXAFS spectra of Ni \u003cem\u003eK\u003c/em\u003e-edge from Nano-NiO and Nano-Vo-NiO, together with Ni foil, NiO as references. \u003cstrong\u003eg \u003c/strong\u003eWavelet transform of Ni \u003cem\u003eK\u003c/em\u003e-edge on Nano-Vo-NiO.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8841186/v1/72ce8fa8bbebbc830d1103c2.png"},{"id":103062811,"identity":"2ab46b8c-f48a-410f-80ae-48bdc4c57a9f","added_by":"auto","created_at":"2026-02-20 10:33:21","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":78117,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMOR performances of Nano-Vo-NiO electrocatalyst.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e LSV curves in 1 M KOH with or without 1 M CH\u003csub\u003e3\u003c/sub\u003eOH of Nano-NiO and Nano-Vo-NiO. \u003cstrong\u003eb\u003c/strong\u003e Comparison of electrocatalytic methanol oxidation current density at various potentials of Nano-NiO and Nano-Vo-NiO. \u003cstrong\u003ec\u003c/strong\u003e Tafel curves of Nano-NiO and Nano-Vo-NiO for the electrocatalytic methanol oxidation reaction. \u003cstrong\u003ed\u003c/strong\u003e FE of formate and \u003cstrong\u003ee\u003c/strong\u003e Formate production rate at varied voltages of Nano-NiO and Nano-Vo-NiO. \u003cstrong\u003ef\u003c/strong\u003e Comparison of hydrogen productivity of Nano-NiO and Nano-Vo-NiO at the potential of 1.62 V vs. RHE. \u003cstrong\u003eg\u003c/strong\u003e Chronopotentiometry curves, corresponding FE of formate in a long-term stability test at the potential of 1.6 V vs. RHE.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8841186/v1/225b4cdca601cf337502352a.png"},{"id":103062816,"identity":"fd9b2ccd-f866-4e75-aa2a-f95801b5e069","added_by":"auto","created_at":"2026-02-20 10:33:21","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":223665,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMOR mechanism analyses.\u003c/strong\u003e \u003cem\u003eIn-situ\u003c/em\u003e Raman spectra of \u003cstrong\u003ea\u003c/strong\u003e Nano-NiO and \u003cstrong\u003eb\u003c/strong\u003e Nano-Vo-NiO in 1 M KOH from OCP V to 1.7 V (vs. RHE). \u003cstrong\u003ec\u003c/strong\u003e The plot of δ(Ni\u003csup\u003eIII\u003c/sup\u003e-O) Raman shift as a function of voltage in \u003cem\u003ein\u003c/em\u003e-\u003cem\u003esitu\u003c/em\u003e Raman spectra. \u003cem\u003eOperando\u003c/em\u003e Ni K-edge XANES spectra of \u003cstrong\u003ed\u003c/strong\u003e Nano-NiO and \u003cstrong\u003ee \u003c/strong\u003eNano-Vo-NiO in 1 M KOH at different potentials. The insets show the magnified spectra of the highlighted squares. \u003cstrong\u003ef \u003c/strong\u003eNormalized absorption edge energy [χμ(E) = 0.5] of Nano-NiO and Nano-Vo-NiO as a function of applied potential. \u003cem\u003eOperando \u003c/em\u003eNi K-edge FT-EXAFS spectra of \u003cstrong\u003eg\u003c/strong\u003e Nano-NiO and \u003cstrong\u003eh\u003c/strong\u003e Nano-Vo-NiO in 1 M KOH at different potentials and \u003cstrong\u003ei\u003c/strong\u003e Ni–Ni bond lengths of Nano-NiO and Nano-Vo-NiO as a function of applied potential. \u003cstrong\u003ej\u003c/strong\u003e Schematic diagram of the reconfiguration process of Nano-NiO and Nano-Vo-NiO during the electrochemical MOR.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8841186/v1/f083859f2ad29769db9462c5.png"},{"id":103062812,"identity":"7ab78a36-ea7e-44f0-9731-cbe59e054350","added_by":"auto","created_at":"2026-02-20 10:33:21","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":314466,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDFT calculations. \u003c/strong\u003eTop- and side-view models of \u003cstrong\u003ea\u003c/strong\u003e Nano-Vo-NiO. \u003cstrong\u003eb\u003c/strong\u003e d-band centers of Nano-NiO and Nano-Vo-NiO. The Fermi level is set to zero. \u003cstrong\u003ec\u003c/strong\u003e Adsorption energy of OH and methanol on the surface of Nano-NiO and Nano-Vo-NiO (inset: Adsorption configurations of OH* and CH\u003csub\u003e3\u003c/sub\u003eOH* on Nano-Vo-NiO. The blue, light pink, brown and red balls represent the Ni, H, C and O atoms, respectively). \u003cstrong\u003ed \u003c/strong\u003eFree energy profiles for deintercalation of the proton of Nano-NiO and Nano-Vo-NiO (inset: The configuration profiles for deintercalation of the proton over Nano-Vo-NiO). \u003cstrong\u003ee\u003c/strong\u003e Th\u003cstrong\u003ee\u003c/strong\u003e Gibbs free energy diagram for electrocatalytic methanol oxidation on Nano-NiO and Nano-Vo-NiO.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8841186/v1/2fdd6547f1118dce263c5f19.png"},{"id":103503862,"identity":"e909dd9f-6d86-4579-9c5b-0214c221281c","added_by":"auto","created_at":"2026-02-26 13:03:40","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":132502,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePerformances of methanol-assisted water splitting and TEA analysis. a\u003c/strong\u003e Schematic diagram of electrocatalytic MOR coupling HER in MEA electrolyzer using Nano-Vo-NiO and Pt/C as the electrocatalysts. \u003cstrong\u003eb\u003c/strong\u003e LSV curves in the MOR || HER system and OER || HER system of Nano-NiO and Nano-Vo-NiO. \u003cstrong\u003ec\u003c/strong\u003e Comparison of the cell voltage of Nano-NiO and Nano-Vo-NiO in the MOR || HER system at different current densities. \u003cstrong\u003ed\u003c/strong\u003e Formate production rate and FE\u003csub\u003eformate\u003c/sub\u003e at varied current density of Nano-Vo-NiO. \u003cstrong\u003ee\u003c/strong\u003e Long-term stability of MOR || HER electrolysis system with Nano-Vo-NiO and Pt/C as the electrocatalyst. \u003cstrong\u003ef\u003c/strong\u003e Comparison of the current density and stability time with recently reported methanol-assisted water electrolysis. \u003cstrong\u003eg\u003c/strong\u003e Estimating production costs and revenue by techno-economic analysis.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8841186/v1/fbcf69a0a9e535de086c9d5b.png"},{"id":107488665,"identity":"3cf9c3a2-a886-4cb1-8600-25e46719143d","added_by":"auto","created_at":"2026-04-22 02:45:30","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1826918,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8841186/v1/e57f9805-1c88-4e86-8d2f-46ef00299d29.pdf"},{"id":103062814,"identity":"83ac2720-f37f-43b8-927b-140032894e7c","added_by":"auto","created_at":"2026-02-20 10:33:21","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":6281133,"visible":true,"origin":"","legend":"Supporting Information","description":"","filename":"SupportingInformation.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8841186/v1/0d862ac08138601909b80e75.pdf"},{"id":103504128,"identity":"e12bd2ae-aaaa-4c59-a5ba-0d33290b99f7","added_by":"auto","created_at":"2026-02-26 13:17:35","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":377610,"visible":true,"origin":"","legend":"","description":"","filename":"Graphicalabstract.docx","url":"https://assets-eu.researchsquare.com/files/rs-8841186/v1/c6e33839e6e515a341223f3a.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Suppressing lattice expansion via engineering oxygen vacancy into sub-5nm NiO for durable methanol electrooxidation","fulltext":[{"header":"Introduction","content":"\u003cp\u003eTackling the global energy crisis demands the development of clean alternative energy sources\u003csup\u003e1,2\u003c/sup\u003e. Hydrogen, an ideal energy carrier, boasts the merits of abundant resources, high combustion enthalpy, and carbon-neutral emission potential\u003csup\u003e3,4\u003c/sup\u003e. Nevertheless, conventional hydrogen production is predominantly dependent on fossil fuels, causing substantial greenhouse gas emissions\u003csup\u003e1\u003c/sup\u003e. Water electrolysis, an eco-friendly hydrogen production route, is limited by sluggish anodic oxygen evolution reaction (OER) kinetics and high energy consumption\u003csup\u003e5\u0026ndash;7\u003c/sup\u003e. To overcome these drawbacks, integrating the electrocatalytic methanol oxidation reaction (MOR) with hydrogen evolution has emerged as a viable alternative\u003csup\u003e4,8\u0026ndash;10\u003c/sup\u003e. Replacing OER with the thermodynamically more favorable MOR reduces overall cell voltage and energy consumption, enabling efficient co-production of hydrogen\u003csup\u003e8\u0026ndash;10\u003c/sup\u003e. Notably, MOR produces high-value formate, a direct fuel for formic acid fuel cells and an efficient liquid-phase hydrogen carrier\u003csup\u003e11,12\u003c/sup\u003e. This strategy integrates energy-efficient hydrogen production with value-added chemical synthesis, holding great potential for sustainable energy applications.\u003c/p\u003e \u003cp\u003eNickel-based catalysts stand out as leading candidates for the electrocatalytic MOR, benefiting from their earth\u0026rsquo;s abundance and prominent electrocatalytic activity\u003csup\u003e13\u003c/sup\u003e. The currently accepted electrochemical\u0026ndash;chemical (E-C) mechanism for MOR revolves around two pivotal steps: the reconstructive deprotonation of the catalyst and hydrogen transfer in the subsequent spontaneous chemical process\u003csup\u003e14\u0026ndash;16\u003c/sup\u003e. Notably, the electrocatalytic reconstruction process represents a critical bottleneck for nickel-based catalysts, primarily arising from the high reaction energy barrier associated with reconstructive deprotonation\u003csup\u003e17\u003c/sup\u003e. The electrochemical reconfiguration process drives excessive lattice expansion, which in turn triggers structural collapse and Ni ion leaching and ultimately leads to compromised catalytic activity and stability\u003csup\u003e14,18,19\u003c/sup\u003e. Thus, advancing nickel-based MOR electrocatalysis necessitates overcoming the energy barrier constraint during reconstruction, mitigating lattice expansion and structural degradation, and addressing the inherent activity-stability trade-off.\u003c/p\u003e \u003cp\u003eAmong the various strategies for enhancing electrocatalytic oxidation activity and stability, nanonization can increase the surface atomic ratio \u003cem\u003evia\u003c/em\u003e the size effect, thereby strengthening the interaction with substrates\u003csup\u003e20,21\u003c/sup\u003e. Meanwhile, nanoelectrocatalysts enable shortened stress transfer paths during the electrocatalytic reconstruction process\u003csup\u003e6,21\u003c/sup\u003e. Furthermore, oxygen defect engineering has been extensively investigated due to its positive effects on regulating reaction activity and electronic structures\u003csup\u003e22\u003c/sup\u003e. Specifically, the construction of oxygen vacancies to modulate unsaturated coordination sites and local electronic structures can effectively lower the oxidation potential of Ni and suppress overoxidation\u003csup\u003e23\u0026ndash;25\u003c/sup\u003e. Notably, the presence of oxygen vacancies is expected to significantly relieve the stress induced by lattice expansion during reconstruction, thus maintaining the stability of the reconstructed structure\u003csup\u003e26,27\u003c/sup\u003e. Inspired by the above design principles, designing nano-electrocatalysts with oxygen vacancy structures by virtue of oxygen vacancy and nanosize effects to reduce the potential during reconstruction and inhibit lattice expansion is a promising solution for achieving high activity and high stability.\u003c/p\u003e \u003cp\u003eHerein, we report the fabrication of nickel oxide \u003cem\u003evia\u003c/em\u003e a sol-gel hydrothermal-calcination route with low-temperature plasma treatment. The low-temperature plasma technique effectively facilitated the creation of oxygen vacancies and suppressed the aggregation of nanoparticles, thereby successfully synthesizing a nanoelectrocatalyst Nano-Vo-NiO for electrocatalytic MOR. Experimental investigations revealed that Nano-Vo-NiO achieved a formate Faradaic efficiency exceeding 92% within a broad voltage range of 1.42\u0026ndash;1.67 V vs. RHE in an H-type cell. Moreover, Nano-Vo-NiO demonstrated stable operation for over 1000 hours at an industrial-grade current density of ~\u0026thinsp;500 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, with the formate Faradaic efficiency consistently maintained above 94.1% into a membrane electrode assembly (MEA). \u003cem\u003eIn-situ\u003c/em\u003e Raman and \u003cem\u003eoperando\u003c/em\u003e X-ray absorption spectroscopy (XAS) demonstrated that the presence of oxygen vacancies effectively lowered the oxidation potential of Ni, accelerated the formation of the active NiOOH phase, and suppressed the elongation of Ni-Ni bonds while restricting lattice expansion during the electrochemical process. Density functional theory (DFT) calculations elucidated that oxygen vacancy induces an upward shift of the Ni d-band center, facilitating the deprotonation during catalyst reconstruction and effectively lowering the activation energy barrier of the *CH\u003csub\u003e3\u003c/sub\u003eOH\u0026rarr;*CH\u003csub\u003e3\u003c/sub\u003eO dehydrogenation step. This work underscores the pivotal role of oxygen vacancies in nanoelectrocatalysts and offers novel design concepts and pathway modulation strategies for stable electrocatalytic organic oxidation.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\"\u003e\n \u003ch2\u003eSynthesis and Characterization\u003c/h2\u003e\n \u003cp\u003eThe Nano-Vo-NiO catalyst was fabricated \u003cem\u003evia\u003c/em\u003e a two-step synthetic strategy integrating the sol-gel hydrothermal-calcination process and low-temperature plasma treatment, with the schematic illustration of the synthesis protocol depicted in Fig. 1a. The catalyst is coated on the hydrophilic porous carbon paper\u003csup\u003e28\u003c/sup\u003e, and X-ray diffraction (XRD) patterns revealed that both Nano-Vo-NiO and the reference sample (Nano-NiO) exhibited distinct diffraction peaks at 37.2°, 43.3°, 62.9°, 75.4° and 79.4°, which are characteristic of the NiO crystalline phase (JCPDS No. 47-1049), corresponding to the (111), (200), (220), (311) and (222) crystal planes, respectively (Supplementary Figs. 1–3)\u003csup\u003e29,30\u003c/sup\u003e. Notably, the XRD diffraction peaks of Nano-Vo-NiO were broader and shifted toward lower angles, ascribed to the introduction of oxygen vacancies (Supplementary Fig.\u0026nbsp;3b)\u003csup\u003e31,32\u003c/sup\u003e. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) images demonstrated that the particle size of Nano-Vo-NiO was comparable to that of Nano-NiO, approximately 4 nm, indicating that the plasma treatment did not induce obvious particle agglomeration (Fig. 1b and Supplementary Figs. 4–6). The detailed geometric and phase structures of the catalysts were probed by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). For Nano-Vo-NiO, the observed lattice fringe spacings of ~ 0.241 nm and ~ 0.148 nm correspond to the (111) and (220) crystal planes of NiO, respectively (Fig. 1c and Supplementary Fig.\u0026nbsp;7)\u003csup\u003e33,34\u003c/sup\u003e. Distinct amorphous regions (marked in yellow) are also discernible, indicative of the presence of oxygen vacancies (Fig. 1c)\u003csup\u003e31\u003c/sup\u003e. In contrast, a well-resolved lattice fringe spacing of ~ 0.21 nm is observed in Nano-NiO, which can be assigned to the (200) crystal plane of NiO, confirming the higher crystallinity of Nano-NiO (Supplementary Figs.\u0026nbsp;8–9)\u003csup\u003e35\u003c/sup\u003e. These observations are further corroborated by the selected area electron diffraction (SAED) patterns, which are consistent with those obtained from X-ray diffraction (XRD) results (Fig. 1c and Supplementary Fig. 10). Additionally, energy-dispersive X-ray spectroscopy (EDS) elemental mapping revealed that Ni and O elements are uniformly distributed across the entire architecture (Fig. 1d and Supplementary Fig.\u0026nbsp;11).\u003c/p\u003e\n \u003cp\u003eX-ray photoelectron spectroscopy (XPS) was employed to analyze the valence states and electronic structures of catalysts. The survey scan XPS spectrum verified that Nano-Vo-NiO consists solely of Ni and O elements without any detectable impurities (Supplementary Fig.\u0026nbsp;12). The high-resolution Ni 2p spectrum, three sets of characteristic peaks were observed, which are associated with the spin-orbit splitting characteristics of Ni\u003csup\u003e3+\u003c/sup\u003e (Ni 2p\u003csub\u003e3/2\u003c/sub\u003e and Ni 2p\u003csub\u003e1/2\u003c/sub\u003e) and Ni\u003csup\u003e2+\u003c/sup\u003e (Ni 2p\u003csub\u003e3/2\u003c/sub\u003e and Ni 2p\u003csub\u003e1/2\u003c/sub\u003e), as well as a pair of broad satellite peaks (Supplementary Fig. 13a)\u003csup\u003e34,35\u003c/sup\u003e. Notably, the binding energy of Ni in Nano-Vo-NiO is lower than that in Nano-NiO, indicating a reduced valence state of Ni in the former. Furthermore, the high-resolution O 1s XPS spectrum, a distinct characteristic peak attributed to oxygen vacancy emerged at ~ 532 eV, which further corroborates the existence of oxygen vacancies in Nano-Vo-NiO (Supplementary Fig.\u0026nbsp;13b)\u003csup\u003e24\u003c/sup\u003e.\u003c/p\u003e\n \u003cp\u003eX-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements were subsequently performed to elucidate the electronic structures and coordination configurations. The Ni K-edge XANES spectra revealed that the absorption edge positions of both Nano-NiO and Nano-Vo-NiO lie between those of Ni foil and standard NiO, implying that the valence state of Ni in these two samples falls within the range of 0–2 (Fig. 1e)\u003csup\u003e9,18\u003c/sup\u003e. Specifically, the absorption edge of Nano-Vo-NiO is closer to that of Ni foil, whereas Nano-NiO exhibits an absorption edge position more proximate to standard NiO. This observation demonstrated that the oxygen vacancies lead to a lower valence state of Ni, which is corroborated by the XPS spectra\u003csup\u003e8,10\u003c/sup\u003e. In addition, the EXAFS spectra exhibited two distinct characteristic peaks at ~ 1.98 Å and ~ 2.92 Å, assigned to the Ni–O and Ni–Ni bonds, respectively (Fig. 1f, Supplementary Figs.\u0026nbsp;14–17 and Supplementary Table\u0026nbsp;1)\u003csup\u003e9,30\u003c/sup\u003e. Evidently, the intensity of the Ni–O peak in Nano-Vo-NiO is weaker than that in Nano-NiO, suggesting a lower Ni coordination number in Nano-Vo-NiO. Meanwhile, the Ni–O bond length in Nano-Vo-NiO is shorter than that in Nano-NiO (2.08 Å vs. 2.07 Å), providing additional evidence for the presence of oxygen vacancies (Supplementary Figs.\u0026nbsp;14–17 and Supplementary Table\u0026nbsp;1)\u003csup\u003e36\u003c/sup\u003e. The high-resolution wavelet transform (WT) was performed on the phase-uncorrected EXAFS data in both R and \u003cem\u003ek\u003c/em\u003e spaces. For Nano-Vo-NiO, the most prominent WT signals appear at approximately 5 Å\u003csup\u003e−1\u003c/sup\u003e and 7.2 Å\u003csup\u003e−1\u003c/sup\u003e, which correspond to the initial existence of Ni − O and Ni − Ni bonds, respectively (Fig. 1g and Supplementary Figs.\u0026nbsp;18–21).\u003csup\u003e9\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003eThe electron paramagnetic resonance (EPR) spectroscopy results revealed that Nano-Vo-NiO exhibits a distinct EPR signal (g = 2.0037), corresponding to the electrons trapped by oxygen vacancies, which is consistent with XPS results (Supplementary Fig.\u0026nbsp;22)\u003csup\u003e32\u003c/sup\u003e. Moreover, the oxygen vacancy-enriched Nano-Vo-NiO exhibits a lower contact angle, indicating that oxygen vacancies contribute to enhanced hydrophilicity and thereby facilitate improved substrate accessibility during the electrocatalytic reaction (Supplementary Fig.\u0026nbsp;23)\u003csup\u003e37\u003c/sup\u003e. To elucidate surface electronic properties, zeta potential measurements were performed. Plasma treatment reversed the catalyst’s potential from negative to positive, arising from oxygen vacancy-driven charge redistribution in NiO where positive defects counteract (Supplementary Fig.\u0026nbsp;24)\u003csup\u003e38\u003c/sup\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eElectrocatalytic methanol oxidation\u003c/h3\u003e\n\u003cp\u003eTo further explore the performance of electrocatalysts for the electrocatalytic MOR, the synthesized materials were tested in an H-type electrochemical cell. A standard three-electrode system was adopted, including a catalyst-based working electrode, a Pt mesh counter electrode, and an Hg/HgO reference electrode, separated by an anion-exchange membrane. The electrolyte was 1 M KOH with or without 1 M methanol (MeOH). High-performance liquid chromatography (HPLC) was utilized to quantify formate yield and determine the MOR Faradaic efficiency (FE) (Supplementary Fig. 25). Figure 2a displays the linear sweep voltammetry (LSV) curves recorded in 1 M KOH + 1 M MeOH. Nano-Vo-NiO exhibits an onset potential of 1.39 V vs. RHE at a current density of 10 mA cm\u003csup\u003e− 2\u003c/sup\u003e, which is slightly lower than that of pristine Nano-NiO (1.42 V vs. RHE). The LSV curves in the presence of methanol showed a marked increase in current density compared to those in 1 M KOH, accompanied by the disappearance of Ni\u003csup\u003e3\u003c/sup\u003e⁺ species. This observation reveals that methanol oxidation proceeds preferentially \u003cem\u003evia\u003c/em\u003e an electrochemical–chemical (E–C) pathway, which is thermodynamically more favorable than OER\u003csup\u003e15,16\u003c/sup\u003e. Notably, at 1.6 V vs. RHE, Nano-Vo-NiO delivers a current density of 132 mA cm\u003csup\u003e− 2\u003c/sup\u003e, higher than that of Nano-NiO (56 mA cm\u003csup\u003e− 2\u003c/sup\u003e), as shown in Fig. 2b, confirming that oxygen vacancies promote the formation of highly active NiOOH species for MOR\u003csup\u003e39,40\u003c/sup\u003e. The influence of synthesis parameters, plasma treatment powers, on MOR activity was systematically examined (Supplementary Figs. 26–27). As illustrated in Fig. 3c, the Tafel slope of Nano-Vo-NiO is 48.1 mV dec\u003csup\u003e− 1\u003c/sup\u003e, significantly lower than that of Nano-NiO (72.8 mV dec\u003csup\u003e− 1\u003c/sup\u003e), indicating accelerated reaction kinetics and reduced charge-transfer resistance\u003csup\u003e41,42\u003c/sup\u003e. This conclusion is further supported by electrochemical impedance spectroscopy (EIS) measurements (Supplementary Fig.\u0026nbsp;28), where Nano-Vo-NiO exhibits the smallest semicircle diameter in the Nyquist plot, consistent with enhanced interfacial charge transfer\u003csup\u003e30,43\u003c/sup\u003e. As presented in Supplementary Figs.\u0026nbsp;29–30, normalization of the current response by the double-layer capacitance (C\u003csub\u003edl\u003c/sub\u003e) reveals a substantially larger electrochemically active surface area (ECSA) for Nano-Vo-NiO (2.7 mF cm\u003csup\u003e− 2\u003c/sup\u003e) compared to Nano-NiO (1.8 mF cm\u003csup\u003e− 2\u003c/sup\u003e). Figure 2d presents the potential-dependent Faradaic efficiency for formate production\u003csup\u003e23,44\u003c/sup\u003e. As the applied potential increases, the FE for formate gradually decreases due to the competitive oxygen evolution reaction (OER) at higher potentials (Supplementary Fig. 31). Remarkably, Nano-Vo-NiO maintains a formate FE above 90% across a broad potential window of 1.42–1.67 V vs. RHE (Fig. 2d, Supplementary Fig.\u0026nbsp;31). Within this range, the formate production rate increases monotonically with potential, reaching 1.254 mmol cm\u003csup\u003e− 2\u003c/sup\u003e h\u003csup\u003e− 1\u003c/sup\u003e for Nano-Vo-NiO at 1.67 V vs. RHE (Fig. 2e). Beyond 1.72 V vs. RHE, the formate production rate declines as OER becomes dominant, and without other liquid products were detected (Supplementary Fig.\u0026nbsp;32)\u003csup\u003e40,45\u003c/sup\u003e. Meanwhile, the FE for hydrogen evolution at the cathode remains close to 100% throughout the potential range (1.42–1.72 V vs. RHE) (Supplementary Fig. 33). At 1.67 V vs. RHE, the hydrogen evolution rate with Nano-Vo-NiO is 2.42 times higher than that with Nano-NiO ((Fig. 2f). Chronoamperometric measurements were performed to assess the long-term stability, as depicted in Fig. 2g. Nano-Vo-NiO demonstrated exceptional operational stability, maintaining a high and consistent Faradaic efficiency (FE) for formate production (~ 94%) over 120 hours at current densities exceeding 100 mA cm\u003csup\u003e− 2\u003c/sup\u003e. In contrast, Nano-NiO suffers from significant degradation under identical conditions, exhibiting a continuous decline in both current density and FE (Fig. 2g). \u003csup\u003e1\u003c/sup\u003eH NMR spectroscopy indicated that only formate was detected as the product (Supplementary Fig. 34). The results of XRD, XPS and SEM characterizations of the post-reaction catalyst demonstrated that the catalyst structure remained basically intact (Supplementary Figs. 35–38). The attenuation corresponding to the LSV curves was negligible (Supplementary Fig. 39). Inductively coupled plasma (ICP) analysis of post-reaction electrolytes showed that leached Ni concentration from Nano-NiO was ~ 5-fold higher than that from Nano-Vo-NiO, indicating oxygen vacancies effectively mitigate structural degradation and suppress active Ni dissolution during prolonged electrolysis (Supplementary Table 2).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMechanistic Studies of electrochemical reconstruction.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo further investigate the electrochemical reconstruction of the catalyst, multi-potential step measurements were performed (Supplementary Figs. 40 and 41). Specifically, a potential of 1.6 V (vs. RHE) was applied from 0 to 40 s to generate Ni\u003csup\u003e3+\u003c/sup\u003e species; the circuit was then held open from 40 to 90 s, followed by applying a potential of 1.1 V (vs. RHE) from 90 to 150 s. Upon injection of 1 M methanol, the reduction current associated with Ni\u003csup\u003e3+\u003c/sup\u003e dropped sharply, which can be attributed to the direct consumption of Ni\u003csup\u003e3+\u003c/sup\u003e species by methanol, demonstrating that the MOR \u003cem\u003evia\u003c/em\u003e an electrochemical–chemical (E–C) pathway (Supplementary Fig. 40)\u003csup\u003e4\u003c/sup\u003e. Compared to Nano-NiO, Nano-Vo-NiO exhibited a stronger reduction peak corresponding to Ni\u003csup\u003e3+\u003c/sup\u003e after open-circuit potential testing in 1 M KOH. Furthermore, in 1 M KOH containing 1 M methanol, it displayed a higher current during the 0–40 s interval but a weaker Ni\u003csup\u003e3+\u003c/sup\u003e reduction peak in the 90–150 s period (Supplementary Fig. 41). This further confirms that vacancy engineering accelerates the generation of high-valence nickel active species and simultaneously enhances methanol adsorption, thereby contributing to improved catalytic activity\u003csup\u003e9,45\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eIn-situ\u003c/em\u003e Raman spectra were employed to investigate the reconstruction mechanisms of Nano-NiO and Nano-Vo-NiO. Distinct characteristic peaks at approximately 472 cm\u003csup\u003e− 1\u003c/sup\u003e and 558 cm\u003csup\u003e− 1\u003c/sup\u003e emerged clearly with increasing applied potential, corresponding to the ν(Ni\u003csup\u003e3+\u003c/sup\u003e–O) and δ(Ni\u003csup\u003e3+\u003c/sup\u003e–O) vibrational modes, respectively, confirming the gradual formation of NiOOH species (Figs. 3a and 3b)\u003csup\u003e19,29\u003c/sup\u003e. Nano-Vo-NiO exhibited stronger signals, indicating the generation of a greater amount of active nickel species. Furthermore, as the potential increased, the δ(Ni³⁺–O) peak from NiOOH showed a red shift (Fig. 3c), signifying lattice expansion due to bond elongation. In contrast, Nano-Vo-NiO displayed no significant red shift, demonstrating suppressed lattice expansion during its reconstruction process\u003csup\u003e46\u003c/sup\u003e. This mitigates the internal stress caused by repeated lattice expansion and contraction during electrochemical reconstruction, reflecting restricted distortion of the NiO\u003csub\u003e6\u003c/sub\u003e octahedral units and thereby inhibiting the loss of active sites\u003csup\u003e37\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eTo gain deeper insight into the dynamic reconstruction process and the differences in fine structural evolution, \u003cem\u003eOperando\u003c/em\u003e electrochemical X-ray absorption spectra (XAS) were carried out. X-ray absorption near-edge structure (XANES) spectra revealed that as the applied potential increased from OCP to 1.6 V (vs. RHE), the Ni K-edge absorption energy for both Nano-NiO and Nano-Vo-NiO systematically shifted to higher energy, demonstrating the gradual accumulation of high-valence nickel species (Figs.\u0026nbsp;3d and 3e)\u003csup\u003e23,47\u003c/sup\u003e. Notably, the Ni K-edge absorption edge of Nano-Vo-NiO shifted more significantly, revealing that electrochemical reconstruction generates more active nickel species (Fig.\u0026nbsp;3f)\u003csup\u003e48\u003c/sup\u003e. The Ni K-edge absorption energy of Nano-Vo-NiO contrasts with that of Nano-NiO, which exhibits a distinct energy upshift (Fig.\u0026nbsp;3f). This observation points to a higher oxidation state in Nano-NiO, originating from the prevalence of unsaturated coordinated Ni sites, and aligns with previous literature\u003csup\u003e23,31\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eFourier-transformed extended X-ray absorption fine structure (FT-EXAFS) spectra recorded in R-space at OCP, 1.2 V, 1.4 V, and 1.6 V (vs. RHE) exhibited two distinct peaks for both Nano-NiO and Nano-Vo-NiO at ~ 2.85 Å and ~ 2 Å, corresponding to Ni–Ni and Ni–O bonds, respectively (Figs.\u0026nbsp;3g and 3h)\u003csup\u003e18,48\u003c/sup\u003e. With increasing applied potential, the Ni–O bond lengths for both catalysts significantly shortened, decreasing from 2.1 Å to 2.07 Å for Nano-NiO and from 2.06 Å to 2.0 Å for Nano-Vo-NiO, respectively (Figs.\u0026nbsp;3g and 3h, Supplementary Table\u0026nbsp;3 and Table\u0026nbsp;4). This is attributed to the interaction between high-valence nickel and surrounding oxygen atoms, further verifying the formation of NiOOH\u003csup\u003e30,49\u003c/sup\u003e. It can be clearly observed that the coordination numbers (CN) of the Ni–O bonds remained relatively stable throughout the reconstruction process (Supplementary Table\u0026nbsp;3 and Table\u0026nbsp;4). The CN of Ni–O bonds in Nano-NiO and Nano-Vo-NiO stabilized at approximately 6 and 4, respectively, demonstrating that oxygen vacancies remain stable during the entire catalytic process. These stably existing defect structures are believed to enhance chemisorption and promote electrocatalytic oxidation processes\u003csup\u003e39\u003c/sup\u003e. Notably, for Nano-Vo-NiO, the Ni–Ni bond length increased from 2.78 Å to 2.88 Å with the potential from OCP to 1.6 V (vs. RHE). In contrast, for the Nano-NiO, the Ni–Ni bond length increased from 2.77 Å to 2.93 Å (Fig.\u0026nbsp;3i and Supplementary Figs.\u0026nbsp;42–43). A longer Ni–Ni bond illustrates a larger crystalline phase size, demonstrating lattice distortion during reconstruction and the twisting of NiO\u003csub\u003e6\u003c/sub\u003e octahedral units\u003csup\u003e48–50\u003c/sup\u003e. As presented in Fig.\u0026nbsp;3i, the change in the Ni–Ni bond length for Nano-Vo-NiO was significantly smaller than that for Nano-NiO, further proving that the construction of vacancies can suppress lattice expansion during electrocatalytic process (Fig.\u0026nbsp;3j).\u003c/p\u003e\n\u003ch3\u003eDFT calculation and mechanism clarification for enhanced activity\u003c/h3\u003e\n\u003cp\u003eBased on density functional theory (DFT) calculations, this study further elucidates the changes in electronic structure and energy during the enhancement of electrocatalytic MOR. With reference to the reported literature, NiOOH was selected as the active site for MOR\u003csup\u003e8,15\u003c/sup\u003e. Theoretical models featuring four-coordinated and saturated coordinated Ni sites as active centers were constructed, corresponding to Nano-NiO and oxygen vacancy (Vo)-rich Nano-Vo-NiO, respectively. Their structural configurations and side views are presented in Fig. 4a and Supplementary Fig. 44. Model optimization results revealed that the Ni–Ni and Ni-O bond lengths in Nano-NiO and Nano-Vo-NiO are consistent with experimental \u003cem\u003eOperando\u003c/em\u003e XAS in 1.6 V vs. RHE. Compared with Nano-NiO, unsaturated coordination in Nano-V₀-NiO shifts the Ni d-band center closer to the Fermi level (Fig. 4b), markedly enhancing the adsorption capacity for hydroxylated oxide active species\u003csup\u003e40,51\u003c/sup\u003e. The adsorption energy of Nano-Vo-NiO for CH\u003csub\u003e3\u003c/sub\u003eOH* and OH* are − 0.50 eV and − 2.68 eV, respectively, which are stronger than those of Nano-NiO (-0.26 eV and − 2.09 eV, respectively). This result confirmed that oxygen vacancies can effectively enhance the catalyst’s adsorption capacity toward reactants and reduce mass transfer energy barriers (Fig. 4c and Supplementary Fig.\u0026nbsp;45)\u003csup\u003e34,41\u003c/sup\u003e. Subsequent calculations on the deprotonation energy barrier for NiOOH formation indicated that Nano-Vo-NiO exhibits a lower deprotonation energy barrier, implying that oxygen vacancies can accelerate the self-reconstruction process for active site formation (Fig. 4d and Supplementary Fig.\u0026nbsp;46)\u003csup\u003e17\u003c/sup\u003e. The Gibbs free energy profiles and the corresponding configurations of Nano-NiO and Nano-Vo-NiO are presented in Fig. 4e and Supplementary Figs. 47–48, respectively. For the Nano-Vo-NiO catalyst with abundant unsaturated coordinated Ni sites, the energy barrier of the rate-determining step (0.65 eV) is remarkably lower than that of Nano-NiO (1.14 eV), which can effectively facilitate the dehydrogenation process of *CH\u003csub\u003e3\u003c/sub\u003eOH→*CH\u003csub\u003e3\u003c/sub\u003eO and thus endow the catalyst with excellent electrocatalytic methanol oxidation performance\u003csup\u003e45,52\u003c/sup\u003e.\u003c/p\u003e\n\u003ch3\u003eCoupling Electrochemical MOR and HER\u003c/h3\u003e\n\u003cp\u003eOwing to the excellent electrocatalytic activity and stability of Nano-Vo-NiO, it was employed together with Pt/C as electrocatalysts for the MOR coupled with HER, respectively, in a zero-potential membrane electrode assembly (MEA) system. The anodic electrolyte was a 1 M KOH containing 1 M methanol, while the cathodic electrolyte was a 1 M KOH; a schematic diagram of the relevant system is shown in Fig.\u0026nbsp;5a. As observed from the linear sweep voltammetry (LSV) curves (Fig.\u0026nbsp;5b), compared with the OER || HER system, the reaction overpotential decreased significantly in the presence of methanol, demonstrating that the system tends to undergo the MOR preferentially\u003csup\u003e53\u003c/sup\u003e. At a current density of 1 A cm\u003csup\u003e− 2\u003c/sup\u003e, the cell voltage of the entire reaction decreased by 210 mV (Fig.\u0026nbsp;5c). Under galvanostatic electrochemical conditions, Nano-Vo-NiO delivers a FE for formate exceeding 80% over a broad current density window of 0.2–1.2 A cm\u003csup\u003e− 2\u003c/sup\u003e, while Nano-NiO affords a markedly lower FE (Supplementary Fig.\u0026nbsp;49). Notably, the champion catalyst, Nano-Vo-NiO, delivered a formate production rate of 9.436 mol cm\u003csup\u003e− 2\u003c/sup\u003e h\u003csup\u003e− 1\u003c/sup\u003e with a Faradaic efficiency of 84.3% (Fig.\u0026nbsp;5d). Simultaneously, upon probing the FE of the HER under variant current density, it was observed that the FE of hydrogen approached 100% (Supplementary Fig.\u0026nbsp;50). The FE of formate remained stable above 94% after a stability test of over 1000 h at current density of ~ 500 mA cm\u003csup\u003e− 2\u003c/sup\u003e, exhibiting excellent electrocatalytic stability (Fig.\u0026nbsp;5e). This superior performance places the catalyst at the top level among all state-of-the-art electrocatalysts ever reported for electrocatalytic MOR (Fig.\u0026nbsp;5f and Supplementary Table\u0026nbsp;5)\u003csup\u003e9,42–45,50,51,54\u003c/sup\u003e. Only formate was detected as the product in the \u003csup\u003e1\u003c/sup\u003eH NMR spectrum of the electrolyte in the two-electrode MEA after long-term test (Supplementary Fig.\u0026nbsp;51). Techno-economic analysis (TEA) indicated that the MOR || HER system employing Nano-Vo-NiO as the catalyst is economically viable (Supplementary Fig.\u0026nbsp;52 and Tables\u0026nbsp;6–8), generating a revenue exceeding USD 690 per ton of methanol processed at an operating current density of 300 mA cm\u003csup\u003e− 2\u003c/sup\u003e, thereby demonstrating significant practical application potential (Fig.\u0026nbsp;5g and Supplementary Note 1)\u003csup\u003e55\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, we successfully fabricated an oxygen vacancy-rich sub-5nm electrocatalyst (Nano-Vo-NiO) \u003cem\u003evia\u003c/em\u003e the integration of sol-gel-assisted hydrothermal-calcination with low-temperature plasma treatment, and employed it for electrocatalytic MOR. The catalyst exhibited a formate Faradaic efficiency exceeding 92% over a wide potential range of 1.42–1.67 V vs. RHE. Further integration of this catalyst into a membrane electrode assembly achieved stable operation for over 1000 h at a current density of ~ 500 mA cm\u003csup\u003e− 2\u003c/sup\u003e, with the formate Faradaic efficiency maintained above 94%. \u003cem\u003eIn-situ\u003c/em\u003e characterizations and DFT calculations revealed that oxygen vacancies not only upshift the d-band center of Ni sites but also reduce the energy barrier for electrocatalytic reconstruction. Notably, the retained oxygen vacancies suppress Ni-Ni bond elongation and restrict lattice expansion during electrochemical reconstruction, thereby enhancing the catalyst's long-term stability. This work underscores the pivotal role of oxygen vacancies in nanoelectrocatalysts and establishes innovative design principles and pathway regulation strategies for durable electrocatalytic organic oxidation.\u003c/p\u003e"},{"header":"Methods","content":"\u003ch2\u003eChemicals\u003c/h2\u003e\u003cp\u003eMethanol (CH\u003csub\u003e3\u003c/sub\u003eOH, 99.5%) and Nickel nitrate hexahydrate (Ni(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e·6H\u003csub\u003e2\u003c/sub\u003eO, 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Pt/C (20% Pt), Sodium hydroxide (NaOH, 97%) and potassium hydroxide (KOH, 90%) were obtained from Aladdin Chemical Reagent Co., Ltd. Formic acid (HCOOH, 98%), sulfuric acid (H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, \u0026gt; 99%), and hydrochloric acid (HCl, ~ 37%) were supplied by Shanghai Lingfeng Chemical Reagent Co., Ltd. Deionized water was acquired from Wahaha Group Co., Ltd. All reagents were used as received without further purification.\u003c/p\u003e\u003ch3\u003eSynthesis of catalysts\u003c/h3\u003e\u003cp\u003e3 g of nickel nitrate hexahydrate (Ni(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e·6H\u003csub\u003e2\u003c/sub\u003eO) was weighed and dissolved in 50 mL of deionized water, followed by stirring for 30 minutes to ensure complete dissolution. Subsequently, a 1 M NaOH solution was slowly added dropwise into the above solution, with the stirring speed maintained at 600 rpm. The pH value of the solution was adjusted to approximately 10. Then, stirring was continued for 1 hour, during which green precipitates were observed. The precipitates were centrifugally washed three times with ethanol and deionized water, respectively, at a centrifugal speed of 9000 rpm and a washing duration of 3 minutes each time. The obtained green precipitates were freeze-dried under vacuum for 24 hours. Afterwards, the dried sample was placed in a muffle furnace and annealed in air at 270°C for 2 hours, yielding the sample denoted as Nano-NiO.\u003c/p\u003e\u003cp\u003eLow-temperature plasma treatment was employed to construct oxygen vacancies. Approximately 40 mg of Nano-NiO catalyst was placed on a cooling stage for plasma treatment. The argon gas flow rate was maintained at 50 mL/min, with the vacuum stabilized around 30 Pa. The plasma treatment power (300, 400, 500, 600 W) and duration (0.5, 1, 1.5, 2 h) were adjusted accordingly. The catalyst exhibiting the optimal electrocatalytic performance was denoted as Nano‑Vo‑NiO.\u003c/p\u003e\u003cp\u003eThe catalyst was deposited onto a hydrophilic carbon paper, where its porous architecture facilitates the adsorption and diffusion of reaction substrates. Before conducting electrocatalytic tests on the prepared catalyst, the catalyst was fully activated. The typical electrode preparation process was as follows: 5 mg of the catalyst was weighed and dispersed in 1 mL of a mixed solution of ethanol and water (volume ratio V1:V2 = 50:50). Then, 40 µL of 5 wt.% Nafion solution was added to the dispersion, which was subsequently sonicated for 30 minutes. Using carbon paper as the gas diffusion electrode, 88 µL of catalyst ink was dropped onto the carbon cloth (1 cm²) and dried in ambient air. In a three-electrode system, a Hg/HgO electrode and a Pt electrode served as the reference electrode and counter electrode, respectively, and the electrolyte was a 1 M KOH solution. Within the potential range of 0.076 ~ 0.776 V (vs. RHE), 30 cycles of cyclic voltammetry (CV) activation tests were performed on the catalyst at a scan rate of 5 mV/s, followed by electrochemical measurements.\u003c/p\u003e\u003ch2\u003eCharacterizations\u003c/h2\u003e\u003cp\u003eThe phase and crystal structure of the catalyst were characterized using a Rigaku 9 kW X-ray diffractometer (Cu rotating anode source) with XRD patterns collected in the range of 5°–80° at a scanning rate of 10°/min. Crystallinity, lattice fringes, and grain boundary distribution were further analyzed \u003cem\u003evia\u003c/em\u003e a Thermo Fisher Talos F200X G2 high-resolution transmission electron microscope (HR-TEM). Morphological features and elemental characteristics were investigated with a ZEISS Sigma 300 scanning electron microscope to observe surface morphology, particle dispersibility, and elemental distribution. X-ray absorption fine structure (XAFS) spectra were collected at the beamline BL20U1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF). Surface chemical properties were analyzed by a Kratos AXIS Supra X-ray photoelectron spectrometer (Al Kα radiation, 1486.6 eV) calibrated with the C 1s peak at 284.6 eV for determining composition and valence states. The hydrophilicity/hydrophobicity of the catalyst was measured using a Dataphysics OCA 20 contact angle meter, while Zeta potential and colloidal stability were evaluated via a Malvern Zetasizer Nano ZS90. Oxygen vacancies in the catalyst were monitored by a Bruker EMX10/12 electron paramagnetic resonance spectrometer. A Thermo Fisher DXR3xi Raman spectrometer (532 nm laser, 5 mW, 200 ~ 3000 cm\u003csup\u003e− 1\u003c/sup\u003e) was used to analyze the specific structure of the catalyst. A Bruker AV500 nuclear magnetic resonance spectrometer (\u003csup\u003e1\u003c/sup\u003eH NMR) was employed to study the product composition and catalytic selectivity after the electrocatalytic reaction.\u003c/p\u003e\u003ch2\u003eElectrochemical measurement\u003c/h2\u003e\u003cp\u003eAll electrochemical tests were conducted at room temperature. Electrochemical double-layer capacitance (C\u003csub\u003edl\u003c/sub\u003e) was measured \u003cem\u003evia\u003c/em\u003e cyclic voltammetry (CV) in a 1 M KOH electrolyte, with data in the non-Faradaic region collected at different scan rates ranging from 20 mV s\u003csup\u003e− 1\u003c/sup\u003e to 100 mV s\u003csup\u003e− 1\u003c/sup\u003e. For the electrochemical impedance spectroscopy (EIS) test, the frequency range was 0.01 Hz to 10,000 Hz, using a Pt sheet as the counter electrode and Hg/HgO as the reference electrode. Before the test, the catalyst was activated \u003cem\u003evia\u003c/em\u003e cyclic voltammetry, and the electrolyte was saturated with Ar for 30 minutes.\u003c/p\u003e\u003cp\u003eThe Tafel slope was obtained from the linear sweep voltammetry (LSV) curves within a potential window of 10 mV, and plotted against the average current or potential. The calculation formula for the Tafel slope is as follows:\u003c/p\u003e\u003cp\u003eη = a + b × log(\u003cem\u003eJ\u003c/em\u003e)\u003c/p\u003e\u003cp\u003eWhere η (mV) denotes overpotential, a is the exchange current density; \u003cem\u003eJ\u003c/em\u003e (mA cm\u003csup\u003e− 2\u003c/sup\u003e) represents current density; and b is the Tafel slope (mV dec\u003csup\u003e− 1\u003c/sup\u003e).\u003c/p\u003e\u003cp\u003eThe membrane electrode assembly (MEA) was carried out employing spray coating and hot-pressing transfer methods. Specifically, the catalyst inks for the anode (Nano-Vo-NiO for MOR) and the cathode (20% Pt/C for HER) were uniformly coated onto PET substrates using a spray coater, followed by vacuum drying at 60°C. The dried anode and cathode catalyst layers were then aligned facing opposite sides of a cation exchange membrane (SF-C120) and subjected to hot-pressing under the following conditions: temperature of 120°C, pressure of 2 MPa, and duration of 60 seconds. Under these parameters, the catalyst layers were successfully transferred and firmly bonded to both sides of the ion exchange membrane.\u003c/p\u003e\u003cp\u003eTitanium fiber felts were placed on both sides of the electrode as gas diffusion and protect layers for the cathode and anode, with an active area of 1 cm\u003csup\u003e2\u003c/sup\u003e. Both end plates were made of titanium and featured serpentine flow fields. An SF-C120 ion exchange membrane was used as the separator. Before testing, the cell components were tightly fastened using a torque wrench at 5 N·m to ensure stable contact resistance at the electrode–membrane interfaces, thereby reducing the overall internal resistance of the cell to below 0.4 Ω and ensuring long-term stability during testing. All electrochemical tests were conducted at room temperature. The anolyte consisted of a 1 M KOH solution with 1 M methanol, while the catholyte was a 1 M KOH solution. Both electrolytes were circulated using gas–liquid mixed flow pumps. All electrochemical data were recorded without iR compensation.\u003c/p\u003e\u003ch2\u003eProduct quantification\u003c/h2\u003e\u003cp\u003eThe electrocatalytic performance for the methanol oxidation reaction (MOR) was evaluated using a three-electrode configuration within an H-type cell, connected to a Wuhan Corrtest electrochemical workstation (Corrtest C350M). A Hg/HgO electrode and a Pt electrode served as the reference and counter electrode, respectively. The anodic compartment contained 10 mL of 1 M methanol in 1 M KOH aqueous solution, while the cathodic compartment was filled with 1 M KOH solution. The two chambers were separated by an FAA-3-PK-75 ion exchange membrane. All applied potentials were converted to the reversible hydrogen electrode (RHE) scale using the following equation:\u003c/p\u003e\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:{E}_{RHE}={E}_{\\text{H}\\text{g}/\\text{H}\\text{g}\\text{O}}+0.0592\\text{*}\\text{p}\\text{H}+0.098$$\u003c/div\u003e\u003c/div\u003e\u003cp\u003eProduct analysis was conducted via high-performance liquid chromatography (HPLC). After electrolysis, 1 mL of the electrolyte was collected, acidified with 2 M HCl, and diluted before injection. The HPLC separation was performed on a Shodex SUGAR SC1011 column maintained at 80°C, with a mobile phase of 5 mM H₂SO₄ flowing at 0.7 mL/min. Detection was carried out at a wavelength of 210 nm.\u003c/p\u003e\u003cp\u003eFor stability assessments, chronopotentiometric measurements were performed at a constant current density of 100 mA cm\u003csup\u003e-2\u003c/sup\u003e. The resulting electrolyte was similarly acidified, diluted, and analyzed by HPLC to quantify products.\u003c/p\u003e\u003cp\u003eThe Faradaic efficiency (FE) for formate was calculated using the formula:\u003c/p\u003e\u003cp\u003eFE (%) = [\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{moles\\:of\\:product\\:\\times\\:\\:N\\:\\times\\:\\:F}{\\:\\text{t}\\text{o}\\text{t}\\text{a}\\text{l}\\:\\text{c}\\text{h}\\text{a}\\text{r}\\text{g}\\text{e}\\:\\text{p}\\text{a}\\text{s}\\text{s}\\text{e}\\text{d}}\\)\u003c/span\u003e\u003c/span\u003e ] × 100%\u003c/p\u003e\u003cp\u003ewhere F represents Faraday’s constant (96485 C mol\u003csup\u003e-1\u003c/sup\u003e), and N denotes the number of electron transfers per methanol molecule oxidized to formate, which is 4.\u003c/p\u003e\u003cp\u003e \u003cb\u003eOperando\u003c/b\u003e \u003cb\u003emeasurements\u003c/b\u003e\u003c/p\u003e\u003cp\u003eFor standard electrochemical \u003cem\u003ein-situ\u003c/em\u003e Raman spectroscopy measurements, Nano-NiO and Nano-Vo-NiO were fully activated separately, with the preparation and activation processes of the catalysts provided in Section 1.2. Measurements were conducted using a home-made \u003cem\u003ein-situ\u003c/em\u003e Raman cell, employing a 514 nm He laser as the excitation source. Herein, a carbon rod served as the counter electrode, and Hg/HgO as the reference electrode. The electrolyte was pre-purged with saturated Ar, and then kept flowing \u003cem\u003evia\u003c/em\u003e a peristaltic pump. Each target potential was held for 3 minutes, followed by immediate collection of the Raman spectra. Before the measurement, the Raman spectral shifts were calibrated against the 520.7 cm\u003csup\u003e− 1\u003c/sup\u003e peak of a silicon wafer. \u003cem\u003eOperando\u003c/em\u003e Ni K-edge X-ray absorption fine structure (XAFS) spectra were recorded using the BL20U1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF) and a custom-designed \u003cem\u003ein-situ\u003c/em\u003e electrochemical cell. Data collection in fluorescence mode was employed to track the dynamic structural evolution of the catalyst during electrocatalytic reconstruction, with a data acquisition duration of 40 minutes for each reaction potential. The X-ray absorption near-edge structure (XANES) spectra were energy-calibrated using a Ni foil as the reference. The catalyst was deposited onto the surface of a gas diffusion electrode (GDE), with the working electrode fabricated following the protocol detailed in Section 1.2. The XANES and extended X-ray absorption fine structure (EXAFS) datasets were processed in accordance with standard protocols \u003cem\u003evia\u003c/em\u003e the Athena software embedded within the Demeter package. EXAFS fitting analyses were conducted within the Artemis module utilizing the FEFF6 code for theoretical scattering path calculations.\u003c/p\u003e\u003ch2\u003eCalculation details\u003c/h2\u003e\u003cp\u003eFirst-principles calculations with spin polarization were carried out using the Vienna Ab initio Simulation Package (VASP). The projector-augmented wave (PAW) pseudopotentials were employed to describe the electron-ion interactions\u003csup\u003e56\u003c/sup\u003e. The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional was adopted for the calculation of electronic structures\u003csup\u003e57,58\u003c/sup\u003e. A plane-wave kinetic energy cutoff of 520 eV was set for all calculations. For Brillouin zone integration, a Gaussian smearing width of 0.05 eV was applied to guarantee numerical stability during self-consistent field (SCF) iterations\u003csup\u003e59\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe convergence criteria were set as follows: the electronic convergence was achieved when the total energy difference between consecutive SCF cycles was less than 10 eV. Structural relaxation was terminated until the residual force on each atom was reduced to below 0.02 eV·Å. To eliminate interlayer periodic interactions, a vacuum layer of 18 Å was introduced perpendicular to the surface plane. The semi-empirical DFT-D3 dispersion correction proposed by Grimme was incorporated to account for non-covalent interactions.\u003c/p\u003e\u003cp\u003eDuring the electrocatalytic reconstruction process, hydroxylated nickel oxide species were identified by \u003cem\u003ein-situ\u003c/em\u003e Raman spectroscopy, which motivated the establishment of a NiOOH model as the reference system with saturated coordination number (Nano-NiO). \u003cem\u003eIn-situ\u003c/em\u003e XAS spectra confirmed the existence of oxygen vacancies in the Nano-Vo-NiO sample with a four-coordinated coordination number.\u003c/p\u003e\u003cp\u003eThe adsorption energy (E\u003csub\u003ead\u003c/sub\u003e) was computed \u003cem\u003evia\u003c/em\u003e the equation below:\u003c/p\u003e\u003cp\u003e \u003cem\u003eE\u003c/em\u003e \u003csub\u003ead\u003c/sub\u003e = \u003cem\u003eE\u003c/em\u003e\u003csub\u003emolecule\u003c/sub\u003e + \u003csub\u003esurface\u003c/sub\u003e – \u003cem\u003eE\u003c/em\u003e\u003csub\u003esurface\u003c/sub\u003e – \u003cem\u003eE\u003c/em\u003e\u003csub\u003emolecule\u003c/sub\u003e\u003c/p\u003e\u003cp\u003eWhere \u003cem\u003eE\u003c/em\u003e\u003csub\u003esurface\u003c/sub\u003e is the energy of the catalyst model, \u003cem\u003eE\u003c/em\u003e\u003csub\u003emolecule\u003c/sub\u003e represents the energy of the adsorbate, and \u003cem\u003eE\u003c/em\u003e\u003csub\u003emolecule+surface\u003c/sub\u003e represents the total energy of the adducts of adsorbent and adsorbate.\u003c/p\u003e\u003cp\u003eThe proton desorption energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eH\u003c/sub\u003e) was calculated using the following expression:\u003c/p\u003e\u003cp\u003e \u003cem\u003eE\u003c/em\u003e \u003csub\u003eH\u003c/sub\u003e = \u003cem\u003eE\u003c/em\u003e\u003csub\u003eV−H\u003c/sub\u003e + 1/2 \u003cem\u003eE\u003c/em\u003e\u003csub\u003eH2\u003c/sub\u003e − \u003cem\u003eE\u003c/em\u003e\u003csub\u003eslab\u003c/sub\u003e\u003c/p\u003e\u003cp\u003eWhere \u003cem\u003eE\u003c/em\u003e\u003csub\u003eV−H\u003c/sub\u003e, \u003cem\u003eE\u003c/em\u003e\u003csub\u003eslab\u003c/sub\u003e, and \u003cem\u003eE\u003c/em\u003e\u003csub\u003eH2\u003c/sub\u003e are the energy for the relaxed slab after H atom desorption, the relaxed slab, and the gas phase H\u003csub\u003e2\u003c/sub\u003e, respectively.\u003c/p\u003e\u003cp\u003eThe Gibbs free energy change (ΔG) for each elementary step was estimated by the formula:\u003csup\u003e60\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eΔ\u003cem\u003eG\u003c/em\u003e = Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eDFT\u003c/sub\u003e + Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eZPE\u003c/sub\u003e – \u003cem\u003eT\u003c/em\u003eΔ\u003cem\u003eS\u003c/em\u003e\u003c/p\u003e\u003cp\u003eWhere Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eDFT\u003c/sub\u003e is the electronic energy difference obtained from DFT calculations, Δ\u003cem\u003eE\u003c/em\u003e\u003csub\u003eZPE\u003c/sub\u003e is the zero-point energy change, \u003cem\u003eT\u003c/em\u003e is the temperature (set to 298.15 K), and Δ\u003cem\u003eS\u003c/em\u003e is the entropy change. The Gibbs free energy change was calculated for each reaction step. For the standard hydrogen electrode (SHE), the free energy of (H + e) was equivalent to that of 1/2 H(g).\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank L. Zuo (State Key Laboratory of Silicon and Advanced Semiconductor Materials, Department of Polymer Science and Engineering, Zhejiang University) for his invaluable advice during the revision process. This work was supported by the National Natural Science Foundation of China (No. 62404084) and the China Postdoctoral Science Foundation (2025M781129). The authors would like to thank Scientific Compass (www.shiyanjia.com) for the support of SEM tests and Scixas Lab (www.scixas.com) for the TEM tests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eT. C. performed the experiments, density functional theory (DFT) calculations, \u003cem\u003ein-situ\u003c/em\u003e spectroscopy tests, and wrote the manuscript. Z. Lyu. contributed to the discussion of the technoeconomic analysis (TEA). T. C. and J. S. supervised the project, conceived the research idea, analyzed the data, and acquired funding. All authors participated in data analysis and engaged in constructive discussions throughout the work.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests Statement\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupplementary Information\u003cstrong\u003e\u003cbr\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eGunathilake, C.\u003cem\u003e et al.\u003c/em\u003e A comprehensive review on hydrogen production, storage, and applications. \u003cem\u003eChem. Soc. 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However, the electrocatalytic reconstruction at high reaction potentials is prone to catalyst structure collapse due to over-oxidation and lattice expansion, resulting in the loss of active sites and decreased stability. Herein, we presented an oxygen vacancy-rich sub-5 nm electrocatalyst (Nano-Vo-NiO) through a sol-gel-assisted hydrothermal-calcination process integrated with low-temperature plasma modification. The champion catalyst Nano-Vo-NiO operated more than 1000 hours at an industrial-level current density of ~\u0026thinsp;500 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, with a Faradaic efficiency of formate above 94.1% in a membrane electrode assembly (MEA). \u003cem\u003eOperando\u003c/em\u003e X-ray absorption spectroscopy (XAS) demonstrated that the vacancy structure remains stable throughout the electrocatalytic process, inhibiting the elongation of Ni-Ni bonds and restricting lattice expansion during the electrocatalytic process. Density functional theory (DFT) calculations revealed that the vacancy structure reduces the proton deintercalation energy barrier, thus achieving efficient MOR activity and stability. This study highlights oxygen vacancies as a critical factor in nanoelectrocatalysis and delivers novel design principles and pathway regulation strategies for efficient electrocatalytic organic oxidation.\u003c/p\u003e","manuscriptTitle":"Suppressing lattice expansion via engineering oxygen vacancy into sub-5nm NiO for durable methanol electrooxidation","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-20 10:33:16","doi":"10.21203/rs.3.rs-8841186/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"dd6ac0bd-8ff3-4392-9e79-56247d9cab0e","owner":[],"postedDate":"February 20th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":63200343,"name":"Physical sciences/Chemistry/Catalysis/Electrocatalysis"},{"id":63200344,"name":"Physical sciences/Materials science/Nanoscale materials/Nanoparticles"}],"tags":[],"updatedAt":"2026-04-21T13:21:12+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-20 10:33:16","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8841186","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8841186","identity":"rs-8841186","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}