pH-dependent Interaction of Interfacial Water with Pt in Electrocatalytic Hydrogen Oxidation Reaction

pH-dependent Interaction of Interfacial Water with Pt in Electrocatalytic Hydrogen Oxidation Reaction | 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 pH-dependent Interaction of Interfacial Water with Pt in Electrocatalytic Hydrogen Oxidation Reaction Yao Zhou, Wen-Xuan Liang, De-quan Cao, Ze-Tong Jia, Zhi-You Zhou, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7331057/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Electrocatalytic hydrogen oxidation reaction (HOR) on Pt depends strongly on pH of electrolyte; its kinetics is orders of magnitude slower in alkaline solutions referring to that in acid solutions. Recent studies reveal that interfacial water plays an important role in HOR on Pt, whether in acid or alkaline media. In this study, by measuring the ratio of mass change to charge change involved in oxidation of Hupd/H2/D2 on Pt (and also on PtRu) in acid and alkaline media, we discovered that the oxidative desorption of adsorbed hydrogen (Had) off Pt is coupled with adsorption of water molecules on Pt in acid solutions, whereas it occurs directly without obvious ad/desorption of H2O or OH- up to ca. 0.15 V (vs. RHE) in alkaline solutions. AIMD calculations further reveal that the dynamic processes of interfacial water reorientation and adsorption critically modulate the energy barrier of oxidative desorption of Had. In acidic environments the interfacial water demonstrates rotational flexibility, which actively interact with Had on the Pt surface, providing a favorable energy motif for Had stripping off Pt. In comparison, in alkaline solutions, owing to dielectric saturation, re-orientation and interaction of the interfacial water with Had suffers from higher energy barrier, constituting a key mechanistic origin of the sluggish alkaline HOR kinetics on Pt electrode. Physical sciences/Chemistry/Electrochemistry/Electrocatalysis Physical sciences/Energy science and technology/Fuel cells Physical sciences/Chemistry/Physical chemistry/Reaction kinetics and dynamics Physical sciences/Chemistry/Catalysis/Catalytic mechanisms Physical sciences/Chemistry/Catalysis/Electrocatalysis Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Hydrogen-to-electricity interconversion is part of the core technology for developing green hydrogen economy 1 . Over the course of pursuing high performance electrocatalysts for electrocatalytic hydrogen oxidation reaction (HOR), the exchange current density in alkaline media for HOR is about two orders of magnitude smaller than that in acid ones on Pt 2,3 . Considering that affluent OH - in alkaline media can neutralize the as-formed protons which shall thus favor hydrogen oxidation, such pH kinetic effect is somehow counter-intuitive. To understand the origin of the above phenomenon, the effect of pH value on hydrogen-binding energy (HBE) 4 or hydroxide binding energy (OHBE) 5,6 have been explored, for instance by voltametric or in-situ spectroscopic techniques or through DFT calculations 7 . The correlation between HBE and/or OHBE and the pH kinetic effect however is controversial 8-11 . It should also be mentioned that spectroscopic observation of the interfacial behaviors under alkaline conditions within the HOR range (typically 0.0-0.2 V vs RHE, which overlaps with the H upd potential range) is challenging 12 , though successful in-situ spectroscopic study of H ad and interfacial water behaviors in acid media on Pt within the hydrogen evolution range can trace back to last century 13-15 . In addition to the HBE and OHBE theory, accompanied by the development of modern computational chemistry 16,17 , the pH effect on electrical double layer (EDL) has regained enormous attention. On the one hand, the interfacial pH 18 and the potential of zero charge (PZC) 19-20 that play an essential role in regulating the EDL structure have been concerned 21,22 ; on the other hand, the influence of interfacial water, such as their reorganization energy 21,23-25 or the hydrogen bond network connectivity 16 , both of which highly depend on the EDL structure, have been studied by computational simulations. In general, the Volmer step namely the oxidation of H ad and the desorption of the resulting H + from the surface to the EDL (and then transported to the bulk solution), is one of the major concerns in discussing the pH kinetic effect on HOR 10 . The Volmer step, by default, in acid media (*H à * + H + + e - ) was long believed to involve straightforward oxidation of H ad into H + ; in alkaline media (*H + OH - à * + H 2 O + e - ), it was thought to be associated with (adsorbed) OH - 26,27 . In such conventional perception, however, the accompanying non-Faradaic interfacial behaviors, specifically the ad/desorption of interfacial H 2 O on/off the Pt surface are regrettably missing. Electrochemical quartz crystal microbalance (EQCM) or nanobalance (EQCN) can measure the interfacial mass changes in an in-situ manner in the ng cm -2 range 28-32 . As the molar weight of H 2 O far outweighs that of H + or H ad , participation of interfacial water (ad/desorption) in the Volmer step can lead to mass change features suitable for in-situ nanogravimetry measurement. Herein, by combining the voltametric and nanogravimetric techniques, both charge change and mass variation within the HOR range on Pt/PtRu in Ar/H 2 /D 2 were measured. The results reveal that interaction behaviors of interfacial water with polycrystalline Pt during oxidation of H upd or HOR is regulated by the pH of the electrolyte. By calculating the mass-to-charge ratio, in acid media, the oxidative desorption of H ad is found being accompanied by unignorable adsorption of water molecules onto the Pt electrode; in sharp comparison, when occurring in alkaline media, ad/ desorption of H 2 O or OH - on Pt is almost negligible up to 0.15 V (vs RHE, similarly thereafter unless otherwise specified). The observed pH-dependent interfacial water ad/desorption behaviors, corroborated by Ab Initio Molecular Dynamics (AIMD) simulations, indicate that the EDL on Pt in alkaline media within the HOR potential range is predominantly composed of rigid, discrete hydrated cation domains . Concurrently, slow-growth simulation results reveal that the Volmer step in both acidic and alkaline environments involves coupled water reorientation , which facilitates its role as a proton acceptor; consequently, the dynamic processes of interfacial water reorientation and adsorption critically modulate the energy barrier of the Volmer step. The EDL structure in alkaline media is anticipated to impede the transport of continuously generated H⁺ from the Pt surface to the bulk solution, constituting a key mechanistic origin of the sluggish alkaline HOR kinetics on Pt. Critically, the interfacial water structure, particularly its enhanced rotational flexibility in acidic environments, promotes more facile reaction kinetics. Results A homemade electrical cell (Supplementary Fig. 1) which is air-tight and equipped with pressure control system (Supplementary Fig. 2 and 3) was assembled for EQCM measurement, with a 7.995 MHz titanium/platinum coated quartz crystal as the working electrode (Supplementary Fig. 4 and Supplementary Note 1). In brief, when a potential is applied to the working electrode, EQCM can simultaneously record the charge change ( Q ) and frequency change ( ∆f ) occurring upon the electrode. And according to the Sauerbrey equation, the measured frequency change ∆ f is oppositely linear to the mass variation ∆ m 33,34 ; the conversion of ∆ f to ∆ m was conducted in each test (Supplementary Fig. 5 and Supplementary Note 2) 35 . The measurement was first conducted in Ar-purged acid or base solutions (Supplementary method). Fig. 1a and 1d show the typical Cyclic voltammetry (CV) profiles in acid (0.1 M H 2 SO 4 ) and alkaline (0.1 M KOH) media, where the oxidative desorption of H upd occurs within around 0 - 0.4 V. This process corresponds to the Volmer step ( * H à * + H + + e - ). Meanwhile, the relevant ∆ f was also obtained, from which the mass variation profiles (MV) were obtained, both of which are shown in Fig. 1b and 1e . The MV within the H upd potential range was interesting. As depicted in Fig. 1b and the inset, in acid media, the ∆m was found decreasing almost linearly with increasing potential within from 0 to 0.2 V. In sharp contrast, the Pt electrode in alkaline media experiences slight mass decrease with potential increasing up to ~0.15 V ( Fig. 1e ). Such differentiated trend also occurs when the same experiment was conducted in HClO 4 (Supplementary Fig. 6) and NaOH solutions (Supplementary Fig. 7) or with different scan rates (Supplementary Fig. 8 and 9). The relevant accumulated charge Q was then calculated, which is found linear to the mass variation ∆m ( Fig. 1c and 1f ); from the resulting slope, the mpe value, namely the mass change for per mole electrons transferred, was obtained (Supplementary Fig. 10 and Supplementary Note 3). In acid media (within ca. 0-0.28 V), as depicted in Scheme 1a , the desorption of H ad off Pt was often assumed to occur without de/adsorption of water molecules or OH - anions. If this is the real situation, it shall have given a mpe value of -1 g mol -1 . However, a mpe of around +7.85 g mol -1 is obtained ( Fig. 1c ). That is, for desorption of per mole H upd in acid solutions, the electrode experiences a mass increase ca. 8 g. This thus leads to a different Volmer step which is depicted in Scheme 1b and 1c . Specifically, the desorption of every two H upd in acid solution is coupled with adsorption of around one H 2 O molecule on the Pt surface, which hence yields a measured mpe of +7.85 g mol -1 . Meanwhile, when the same process was conducted in HClO 4 solution (Supplementary Fig. 6c), though a slightly different mpe value of +5.62 g mol -1 was obtained, it also suggests obvious adsorption of water molecules onto Pt during dissociation of H upd . Meanwhile, in alkaline solutions, it is often assumed that desorption of H ad involves (adsorbed) OH - , which is also known as the bifunctional mechanism ( Scheme 1d ). Had this been the situation, the process would have yielded a mpe value of -18 g mol -1 . However, shown in Fig. 1f , a negative mpe value of -0.68 g mol -1 is obtained for this process within ca. 0-0.15 V. Such a mpe, close to -1 g mol -1 , reveals that, the desorption of H upd off Pt in alkaline solutions does not involve observable ad/desorption of H 2 O or OH - molecules, as depicted in Scheme 1e and 1f . Otherwise, the resulting mpe would be significantly different, since the molecular weight of OH - or H 2 O is much larger than that of H + . To study the real HOR process on Pt, a H/H 2 stripping experiment was designed by referring to the classical CO-stripping process. As shown in Fig. 2a , hydrogen was preloaded onto the Pt under open circuit potential; followed by Ar-purging for a period of time, the H/H 2 -preloaded Pt then was subjected to linear sweep voltametric test (LSV, Supplementary Methods). Shown in Fig. 2b and 2d , in both acid and alkaline media, the current was much larger than the previous results obtained in the solutions purged with only Ar (as compared in Supplementary Fig. 11 and 12), confirming the oxidation of pre-adsorbed H 2 in addition to the H upd . Importantly, for both acid and base solutions, the MV within the HOR potential range demonstrates the same trend identical to those purged with only Ar ( Fig. 2c and 2e ). In acid solution, mass increase was observed, indicating obvious adsorption of water molecules on Pt during oxidation of the preloaded H 2 /H ( Fig. 2c ). In base solution, mass loss is found ( Fig. 2e ); moreover, this mass loss is obviously larger than that measured in the base solutions purged with only Ar, consistent with its much larger current. A similar experiment was further conducted in D 2 in alkaline media ( Fig. 2f ). When generating the same current, the mass loss from D/D 2 stripping is around two times of that from H/H 2 stripping ( Fig. 2g ). Such isotopic experiment confirms that the mass loss in the H/H 2 stripping in alkaline media indeed originates from surface adsorbed H ad /H 2 (or D ad /D 2 ). The interfacial process for HOR in alkaline solution was further compared in different experimental settings. Shown in Fig. 3a , H 2 was first purged into the bulk solution under open circuit potential for 30 mins; then H 2 was supplied continuously in the upper chamber of the cell during repetitive CV scans within 0-0.2 V (Supplementary Methods). As shown in Fig. 3b , the anodic current of the 1 st cycle was the largest, which then declined as the level of H 2 saturation in the base solution decreased continuously with increasing CV cycles in the scenario. Accordingly, we found that the mass loss for the 1 st anodic scan is obviously larger than that of the 10 th cycle, confirming that the measured mass loss was mainly from oxidation of pre-adsorbed H/H 2 on the electrode before each anodic scan ( Fig. 3c ). The above experiment was further verified in alkaline solution using D 2 . When oxidation of D 2 yields similar current density as that of H 2 ( Fig. 3d ), the mass loss for DOR approaches two times of that for HOR in the same alkaline solution ( Fig. 3e ). The experiment was also conducted in acid solution, mass increase was observed, a similar trend as that displayed in Ar-purged acid solution (Supplementary Fig. 13). These results render further support to the new Volmer step proposed in Scheme 1 . The HOR rate in alkaline media can be generally improved when Pt is alloyed with Ru 9,36,37 . Herein a layer of PtRu alloy was deposited on a Au working electrode of EQCM (Supplementary Fig. 14 and Supplementary method). In both Ar-purged acid and base solutions, Δ m for the PtRu electrode increases linearly with potential increasing up to ca. 0.2 V (Supplementary Fig. 15 and 16), showing different interfacial behavior from Pt. The HOR experiment depicted in Fig. 3a was also investigated on PtRu in alkaline media. Shown in Fig. 3f and the inset, the alkaline HOR rate on PtRu is much faster than on Pt; what’s more, contrary to the above-discussed situation of Pt, the PtRu demonstrates a positive ∆ m during alkaline HOR ( Fig. 3g ) . This indicates that alkaline HOR on PtRu is accompanied by adsorption of OH - or H 2 O on its surface, which can be one of the reasons contributing to its faster HOR rate. Discussion Such pH-dependent interfacial interaction between the EDL and the Pt surface demonstrated in Scheme 1c and 1f shall be associated with their different EDL structures. With increasing pH, at RHE scale, the HOR occurs far below the potential of zero charge (PZC) of Pt at pH 13 compared to that at the pH 1 (Supplementary Note 4, Supplementary Fig. 17-20) 38 . Hence, during alkaline HOR, solvated cations (e.g., [K(H 2 O) n ] + ), rather than free OH - anions or hydrogen-bonded water network, are expected to dominate the EDL. To investigate the impact of EDL configuration differences and provide mechanistic understanding, we performed AIMD simulations. The Pt (111) surface was chosen as a model electrode 21 , explicitly solvated with water molecules to construct the EDL. The simulated PZC (PZC sim ) of the Pt (111)-water interface is 0.16 V vs SHE, which agrees well with the literature value of 0.21 V vs SHE (Supplementary Fig. S21) 39-41 . Additionally, H ad coverage on Pt(111) (Pt-H x where x denotes the coverage) was systematically varied, and the associated change in the work function was extracted (Supplementary Fig. S22 and S23). As demonstrated by Li et al. 16 , varying the H ad coverage leads to a volcano-shaped dependence of work function, which in turn modulates the interfacial potential. To mimic acidic and alkaline environments, we modulated the electrode potential by adding proton–electron (H + + e - ) or sodium–electron (Na + + e - ) pairs. For the acidic system, one H + + e - pair was introduced and surface H ad coverage was set to 0.88, resulting in potential of 0.09 V vs SHE. For the alkaline system, three Na + + e - pairs were added; when the H ad coverage was set to 0.25, the potential was -0.65 V vs SHE; when the H ad coverage was 0, the potential increased to -0.41 V vs SHE (Supplementary Table 3). This model yielded a Na + concentration of 2.78 mol L -1 at the interface (Supplementary Fig. S24, S25), in good agreement with the interfacial cation concentrations reported by Koper and co-workers, who observed substantial cation accumulation within the EDL region due to electrostatic interactions and specific adsorption tendencies 21 . Fig. 5a and 5b illustrate the interfacial water structures on the bare or H-covered Pt under acidic and alkaline conditions, respectively, as revealed by our AIMD simulations and supported by EQCM experimental results. Under acidic conditions, the interface potential of the bare Pt was 0.16 V vs RHE, which closely matches the calculated and experimental PZC of Pt(111). For the alkaline system (pH = 14), the potential of the Pt-H 0.25 was 0.18 V vs RHE. We used the cosine of the angle between the water dipole and the surface normal (Z-axis) to represent water orientation. A cosine value close to +1 indicates an O-down orientation, whereas -1 corresponds to an H-down orientation ( Fig. 5c ). Fig. 5d quantifies the average distribution of water orientations along the Z-axis. Under acidic conditions, as the H ad coverage decreases, the surface water layer closest to Pt (111) transitions from H-down to O-down orientations. This transition indicates increased chemical adsorption of O-down water on Pt (111), especially at higher potentials. In contrast, in alkaline environments, the water orientation remains largely H-down regardless of the H ad coverage. These orientation trends are consistent with EQCM observations, suggesting that acidic interfaces enable easier water reorientation due to proximity to the PZC, whereas alkaline interfaces restrict reorientation due to dielectric saturation. While the acidic/alkaline interface covered with H ad exhibits dominant H-down configurations overall, the interfacial property differs significantly. In acidic media, the system is closer to the PZC, resulting in a higher interfacial dielectric constant and increased flexibility of water orientation. Conversely, alkaline systems lie far from the PZC, leading to dielectric saturation, a lower interfacial dielectric constant, and hindered rotational freedom of water molecules 21,42-46 . To examine how these EDL differences influence the Volmer step, we conducted slow-growth MD. The reaction coordinate was defined as the proton transfer distance between the transferring H ad and adjacent O atoms (Supplementary Fig. S26). The calculated free energy, which is from the integration of the forces acting on the reacting atoms (Supplementary Fig. S27), reveal a barrier of 0.92 eV for the acidic interface ( Fig. 6a ) and 1.19 eV for the alkaline interface ( Fig. 6c ). In both environments, water reorientation occurs during proton transfer, as seen in the intermediate structures (Supplementary Fig. S28). Meanwhile, the H ad –O distance in the transition state is 1.22 Å for the acidic EDL and 1.18 Å for the alkaline interface ( Fig. 6b and 6d ), respectively, both exceeding typical O–H bonding ranges (0.9–1.1 Å). Considering the similar H ad -O distance of the transition state in acid and base solutions, their calculated energy difference up to 0.27 eV shall originate from the different EDL structures. This suggests the hydrogen-bonding interaction between the interfacial water and the H ad is blocked in the alkaline media, likely due to hindered water reorientation. This elevated barrier is attributed to the strong surface electric field induced by cation aggregation in the alkaline EDL, which causes the water molecules in the EDL to reach dielectric saturation, thereby affecting their dielectric response 47,48 . Although dielectric saturation would reduce the solvent rearrangement energy within the framework of Marcus theory, due to the fact that this step is coupled with the coordinate changes in the reaction process between the interfacial water and surface adsorbed H ad , the reaction energy barrier increases. Hence, these AIMD results, together with the EQCM observation, show that, in acid HOR process, the adsorption of interfacial water participates actively in the oxidative stripping of adsorbed H ad off Pt, whereas such a process is much less obvious in alkaline HOR. Such pH-differentiated interaction behaviors of interfacial water with the Pt surface is expected to be an important cause for the pH kinetic effect on HOR. Conclusion In summary, the pH-differentiated interaction of interfacial water with Pt surface within the H upd or HOR potential range is investigated. Combining voltametric and nanogravimetric techniques, we discovered that, the oxidation and desorption of H ad in acid solution is accompanied actively with adsorption of water molecules on Pt surface; however, in alkaline media, ad/desorption of water molecules or OH - anions on Pt is negligible during desorption of H ad . By simulating Pt-water structure variations under different H ad coverages, the AIMD calculations confirmed the quantitative results of EQCM measurement: the water molecules exhibit greater rotational freedom in acid solutions, leading to the formation of O-down chemisorbed water during the Volmer step, whereas it suffers from large energy barrier in alkaline media. Slow-growth MD further demonstrated that this increased rotational freedom and the resulting interfacial water adsorption facilitate H ad stripping off Pt surface, resulting in a reduced energy barrier for the Volmer step in HOR. Methods Cyclic voltammetry (CV) or liner sweep voltammetry (LSV) and mass variation (MV) measurements were performed using a 440C Quartz Crystal Microbalance (CHI) with 7.995 MHz titanium/platinum coated quartz crystal electrodes (Renluxcrystal) as the working electrode, Pt wire as the counter electrode, and saturated calomel electrode (saturated KCl solution, in acid) or Hg/HgO (1 M KOH, in base) as the reference electrode. The whole setup was placed on an anti-vibration table, and the experiment was carried out under constant room temperature (298 ± 1 K) and humidity (45 ± 2%). Before every test, the working electrode shall be polished and activated following a rigorous procedure (Supplementary method); the electrode before and after the test was characterized to ensure its structural stability (Supplementary Note 1). In general, to measure the current and the frequency change ∆ f versus the applied potential in different gas atmosphere, after activation of the working electrode, Ar was purged directly into the fresh electrolyte for 30 mins with a flow rate of 15 mL min -1 to expel the dissolved impurity gas (e.g., O 2 ). For measurement in H 2 or D 2 , the electrolyte was then purged with H 2 /D 2 for at least 10 minutes with a 15 mL min -1 to saturate the electrolyte with the reactant gas. After solution purging and before the electrochemical test, the cell shall sit for at least one min to resume a steady frequency baseline. During the electrochemical test where gas purging was necessary, instead of purging into the electrolyte, the gas was supplied in the upper chamber of the cell continuously, with a flow rate decreased to 5 mL min -1 , to minimize disturbing the frequency change (Supplementary method). All AIMD simulations were conducted employing the Vienna Ab initio Simulation Package (VASP) 49,50 , utilizing a plane-wave basis set with pseudopotentials. The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation functional 51 treated exchange-correlation effects. Wavefunctions were expanded with a kinetic energy cutoff of 400 eV, and partial occupancies were handled using the first-order Methfessel-Paxton method (smearing width = 0.05 eV). Semi-empirical van der Waals interactions were incorporated via Grimme's D3 dispersion correction scheme. All simulations were performed without spin polarization due to its negligible impact on total energies. More details can be found in Supplementary method. Free energy profiles for the Volmer reaction were computed using the slow-growth approach 52 within constrained molecular dynamics, the Chan-Nørskov correction scheme was applied to mitigate work function artifacts. Theoretical foundations for the slow-growth method and potential corrections are detailed in Supplementary Methods. Declarations Data availability All relevant data are included in the Article and its Supplementary Information or are available from the corresponding authors upon reasonable request. Acknowledgements The National Key Research and Development Program of China (No. 2021YFA1502000) and the National Natural Science Foundation of China (No. 22472142, 22288102, 22322202) were acknowledged. Author contributions W.L., D.C. and Z.J. contributed equally to this work. Y.Z., Z.Z, C.Z., J.L., T.W. and S.S. supervised the project and provided guidance on the project. Y.Z., W.L. and D.C. conceived and designed the study. W.L. and D.C. performed the experiments. Z.J. and T.W. performed the AIMD studies. W.L., D.C., Z.J., Y.Z., T.W. and S.S. wrote and revised the paper. All authors contributed to the analysis. Competing interests The authors declare no competing interests. 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Phys. 151 , 160902 (2019). https://doi.org/10.1063/1.5124878 Partanen, L. & Laasonen, K. Ab initio molecular dynamics investigation of the Pt(111)–water interface structure in an alkaline environment with high surface OH-coverages. Phys. Chem. Chem. Phys. 26 , 18233-18243 (2024). https://doi.org/10.1039/D4CP01100G Huang, J. Hybrid density-potential functional theory of electric double layers. Electrochim. Acta 389 , 138720 (2021). https://doi.org/https://doi.org/10.1016/j.electacta.2021.138720 Tang, W., Zhao, S. & Huang, J. Origin of Solvent Dependency of the Potential of Zero Charge. JACS Au 3 , 3381-3390 (2023). https://doi.org/10.1021/jacsau.3c00552 Kresse, G. & Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B 47 , 558-561 (1993). https://doi.org/10.1103/PhysRevB.47.558 Kresse, G. & Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6 , 15-50 (1996). https://doi.org/https://doi.org/10.1016/0927-0256(96)00008-0 Perdew, J. P. et al. Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation. Phys. Rev. B 46 , 6671-6687 (1992). https://doi.org/10.1103/PhysRevB.46.6671 Oberhofer, H., Dellago, C. & Geissler, P. L. Biased Sampling of Nonequilibrium Trajectories: Can Fast Switching Simulations Outperform Conventional Free Energy Calculation Methods? J. Phys. Chem. B 109 , 6902-6915 (2005). https://doi.org/10.1021/jp044556a Scheme 1 Scheme 1 is available in the Supplementary Files section. Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryInformation20250808.docx Supplementary Information scheme1.jpg Scheme 1| pH-differentiated interaction of interfacial water with Pt in the H upd potential range. (a, d) The conventional assumption of the Volmer step; (b, e) the Volmer step and (c, f) the relevant scheme proposed in this study which considers the interaction of interfacial water with Pt based on the measured mpe in acid and alkaline media. Cite Share Download PDF Status: Under Review 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-7331057","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":515523768,"identity":"2b1da09a-a578-4f6a-81a5-7ce2d6257cfc","order_by":0,"name":"Yao 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University","correspondingAuthor":false,"prefix":"","firstName":"Shi-Gang","middleName":"","lastName":"Sun","suffix":""}],"badges":[],"createdAt":"2025-08-09 03:50:08","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7331057/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7331057/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":91495445,"identity":"0512debe-b7c7-4969-a180-541a344088a0","added_by":"auto","created_at":"2025-09-17 06:15:25","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":187241,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCV and MV for desorption of H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eupd\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e on Pt in Ar-purged acid and alkaline media.\u003c/strong\u003e (\u003cstrong\u003ea, d\u003c/strong\u003e) The CV profiles, (\u003cstrong\u003eb, e\u003c/strong\u003e) the MV profiles where both the \u003cem\u003e∆f\u003c/em\u003e and the relevant \u003cem\u003e∆m\u003c/em\u003e are shown, (\u003cstrong\u003ec, f\u003c/strong\u003e) the relevant \u003cem\u003e∆m\u003c/em\u003e vs accumulative charge (\u003cem\u003eQ\u003c/em\u003e) on Pt in Ar-purged (\u003cstrong\u003ea, b, c\u003c/strong\u003e) acid solution and (\u003cstrong\u003ed, e, f\u003c/strong\u003e) base solutions.\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7331057/v1/17d182a74d55da16494096e0.jpg"},{"id":91493523,"identity":"f0ffbaf1-c7e0-4807-891c-ba2f025b4cc1","added_by":"auto","created_at":"2025-09-17 06:07:25","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":122160,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStripping of preloaded H/H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2 \u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eor D/D\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e on Pt in acid and alkaline media.\u003c/strong\u003e (\u003cstrong\u003ea\u003c/strong\u003e) Procedures of the H/H\u003csub\u003e2 \u003c/sub\u003eor D/D\u003csub\u003e2 \u003c/sub\u003estripping\u003csub\u003e \u003c/sub\u003eexperiment where H\u003csub\u003e2\u003c/sub\u003e/D\u003csub\u003e2 \u003c/sub\u003ewas preloaded on Pt, followed by Ar purging before the electrochemical test; (\u003cstrong\u003eb, d\u003c/strong\u003e) the resulting LSV and (\u003cstrong\u003ec, e\u003c/strong\u003e) MV profiles for H/H\u003csub\u003e2\u003c/sub\u003e stripping in (\u003cstrong\u003eb, c\u003c/strong\u003e) acid or (\u003cstrong\u003ed,e\u003c/strong\u003e) base solutions; (\u003cstrong\u003ef\u003c/strong\u003e) LSV and (g) MV profiles for stripping of D/D\u003csub\u003e2\u003c/sub\u003e in base solutions.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7331057/v1/6529bb7be9f9f86421df9dfa.jpg"},{"id":91493524,"identity":"840f7de7-e549-4210-a6f2-dddd4054c94c","added_by":"auto","created_at":"2025-09-17 06:07:25","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":155473,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOxidation of H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e/D\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e in base solutions:\u003c/strong\u003e (\u003cstrong\u003ea\u003c/strong\u003e) Procedures of HOR in alkaline solutions\u003csub\u003e \u003c/sub\u003ewhere H\u003csub\u003e2 \u003c/sub\u003ewas first purged into base solution and then was supplied in the upper chamber throughout the whole process; the resulting (\u003cstrong\u003eb\u003c/strong\u003e)\u003cem\u003e \u003c/em\u003eLSV and (\u003cstrong\u003ec\u003c/strong\u003e) MV profiles for the 1\u003csup\u003est\u003c/sup\u003e and 10\u003csup\u003eth\u003c/sup\u003e cycle; comparison of (\u003cstrong\u003ed\u003c/strong\u003e) LSV\u003cem\u003e \u003c/em\u003eand (\u003cstrong\u003ee\u003c/strong\u003e) MV profiles in\u003cstrong\u003e \u003c/strong\u003eH\u003csub\u003e2\u003c/sub\u003e or D\u003csub\u003e2\u003c/sub\u003e-purged base solution on Pt; comparison of (\u003cstrong\u003ef\u003c/strong\u003e) LSV\u003cem\u003e \u003c/em\u003eand (\u003cstrong\u003eg\u003c/strong\u003e) the MV in base solutions on Pt or PtRu.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7331057/v1/37cf0d8163fc2b92cca5bf08.jpg"},{"id":91493525,"identity":"58b33e11-d345-4f01-ad32-2d60f5465e98","added_by":"auto","created_at":"2025-09-17 06:07:25","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":155772,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFig. 5| Different behaviors of water molecules between Acid and Alkaline Pt(111)-water EDLs. \u003c/strong\u003e(\u003cstrong\u003ea\u003c/strong\u003e) Pt (111)-water acidic interface (Pt, gray; H, white; O, red; Na, purple). (b) Pt (111)-water alkaline interface. (c) Diagram showing the cosine value of the angle between the dipole moment of water molecules and the normal vector of the Pt (111) surface. (d) Average orientation distribution of acid surface water molecules along the Z-axis (represented by the cosine of the angle between the dipole of water molecules and the Z-axis); (e) Average orientation distribution of alkaline surface water molecules along the Z-axis.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7331057/v1/a9ab9f117c19e9baa984b478.jpg"},{"id":91495444,"identity":"a3a5174a-a343-4c57-a868-8926e2a2be7a","added_by":"auto","created_at":"2025-09-17 06:15:25","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":120687,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFig. 6| Free energy of Volmer step in Pt (111)-water EDLs in acid and alkaline media.\u003c/strong\u003e (a) and (c) The curve of free energy change in the Volmer step of the acid and alkaline HOR reaction. (b) and (d). The typical initial, transition, and final local structures of water-at-interface (Pt, gray; H, white; O, red and yellow; Na, purple; Other water molecules are represented by line models, while Pt, Na, the water molecules and the H atoms involved in the reaction are depicted using ball-and-stick models).\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7331057/v1/2a0f5acf9d271d85e46518cc.jpg"},{"id":91496000,"identity":"0b80eb19-e31f-4490-a469-015177827f5b","added_by":"auto","created_at":"2025-09-17 06:23:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1709538,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7331057/v1/19e6cdf3-83fb-4985-9a19-c480a9ef1d68.pdf"},{"id":91493530,"identity":"6e6af4d1-9ed8-42ac-8774-090536603996","added_by":"auto","created_at":"2025-09-17 06:07:29","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":61879339,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SupplementaryInformation20250808.docx","url":"https://assets-eu.researchsquare.com/files/rs-7331057/v1/f2129933e627b37f5826437e.docx"},{"id":91493528,"identity":"dc0f0515-1725-41a3-a4cb-c3195d866b46","added_by":"auto","created_at":"2025-09-17 06:07:25","extension":"jpg","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":155779,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eScheme 1| pH-differentiated interaction of interfacial water with Pt in the H\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003eupd\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e potential range.\u003c/strong\u003e (\u003cstrong\u003ea, d\u003c/strong\u003e) The conventional assumption of the Volmer step; (\u003cstrong\u003eb, e\u003c/strong\u003e) the Volmer step and (\u003cstrong\u003ec, f\u003c/strong\u003e) the relevant scheme proposed in this study which considers the interaction of interfacial water with Pt based on the measured \u003cem\u003empe\u003c/em\u003e in acid and alkaline media.\u003c/p\u003e","description":"","filename":"scheme1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7331057/v1/0bbf5475e4c3a85c6e57f919.jpg"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"pH-dependent Interaction of Interfacial Water with Pt in Electrocatalytic Hydrogen Oxidation Reaction","fulltext":[{"header":"Introduction ","content":"\u003cp\u003eHydrogen-to-electricity interconversion is part of the core technology for developing green hydrogen economy\u003csup\u003e1\u003c/sup\u003e. Over the course of pursuing high performance electrocatalysts for electrocatalytic hydrogen oxidation reaction (HOR), the exchange current density in alkaline media for HOR is about two orders of magnitude smaller than that in acid ones on Pt\u003csup\u003e2,3\u003c/sup\u003e. Considering that affluent OH\u003csup\u003e-\u0026nbsp;\u003c/sup\u003ein alkaline media can neutralize the as-formed protons which shall thus favor hydrogen oxidation, such pH kinetic effect is somehow counter-intuitive.\u003c/p\u003e\n\u003cp\u003eTo understand the origin of the above phenomenon, the effect of pH value on hydrogen-binding energy (HBE)\u003csup\u003e4\u003c/sup\u003e or hydroxide binding energy (OHBE)\u003csup\u003e5,6\u003c/sup\u003e have been explored, for instance by voltametric or in-situ spectroscopic techniques or through DFT calculations\u003csup\u003e7\u003c/sup\u003e. The correlation between HBE and/or OHBE and the pH kinetic effect however is controversial\u003csup\u003e8-11\u003c/sup\u003e. It should also be mentioned that spectroscopic observation of the interfacial behaviors under alkaline conditions within the HOR range (typically 0.0-0.2 V vs RHE, which overlaps with the H\u003csub\u003eupd\u003c/sub\u003e potential range) is challenging\u003csup\u003e12\u003c/sup\u003e, though successful in-situ spectroscopic study of H\u003csub\u003ead\u003c/sub\u003e and interfacial water behaviors in acid media on Pt within the hydrogen evolution range can trace back to last century\u003csup\u003e13-15\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eIn addition to the HBE and OHBE theory, accompanied by the development of modern computational chemistry\u003csup\u003e16,17\u003c/sup\u003e, the pH effect on electrical double layer (EDL) has regained enormous attention. On the one hand, the interfacial pH\u003csup\u003e18\u003c/sup\u003e and the potential of zero charge (PZC)\u003csup\u003e19-20\u003c/sup\u003e that play an essential role in regulating the EDL structure have been concerned\u003csup\u003e21,22\u003c/sup\u003e; on the other hand, the influence of interfacial water, such as their reorganization energy\u003csup\u003e21,23-25\u003c/sup\u003e or the hydrogen bond network connectivity\u003csup\u003e16\u003c/sup\u003e, both of which highly depend on the EDL structure, have been studied by computational simulations. In general, the Volmer step namely the oxidation of H\u003csub\u003ead\u003c/sub\u003e and the desorption of the resulting H\u003csup\u003e+\u003c/sup\u003e from the surface to the EDL (and then transported to the bulk solution), is one of the major concerns in discussing the pH kinetic effect on HOR\u003csup\u003e10\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe Volmer step, by default, in acid media (*H\u0026nbsp;\u0026agrave;\u0026nbsp;* + H\u003csup\u003e+\u003c/sup\u003e + e\u003csup\u003e-\u003c/sup\u003e) was long believed to involve straightforward oxidation of H\u003csub\u003ead\u003c/sub\u003e into H\u003csup\u003e+\u003c/sup\u003e; in alkaline media (*H + OH\u003csup\u003e-\u0026nbsp;\u003c/sup\u003e\u0026agrave;\u0026nbsp;* + H\u003csub\u003e2\u003c/sub\u003eO + e\u003csup\u003e-\u003c/sup\u003e), it was thought to be associated with (adsorbed) OH\u003csup\u003e-\u003c/sup\u003e \u003csup\u003e26,27\u003c/sup\u003e. In such conventional perception, however, the accompanying non-Faradaic interfacial behaviors, specifically the ad/desorption of interfacial H\u003csub\u003e2\u003c/sub\u003eO on/off the Pt surface are regrettably missing. Electrochemical quartz crystal microbalance (EQCM) or nanobalance (EQCN) can measure the interfacial mass changes in an in-situ manner in the ng cm\u003csup\u003e-2\u003c/sup\u003e range\u003csup\u003e28-32\u003c/sup\u003e. As the molar weight of H\u003csub\u003e2\u003c/sub\u003eO far outweighs that of H\u003csup\u003e+\u003c/sup\u003e or H\u003csub\u003ead\u003c/sub\u003e, participation of interfacial water (ad/desorption) in the Volmer step can lead to mass change features suitable for in-situ nanogravimetry measurement.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHerein, by combining the voltametric and nanogravimetric techniques, both charge change and mass variation within the HOR range on Pt/PtRu in Ar/H\u003csub\u003e2\u003c/sub\u003e/D\u003csub\u003e2\u003c/sub\u003e were measured. The results reveal that interaction behaviors of interfacial water with polycrystalline Pt during oxidation of H\u003csub\u003eupd\u003c/sub\u003e or HOR is regulated by the pH of the electrolyte. By calculating the mass-to-charge ratio, in acid media, the oxidative desorption of H\u003csub\u003ead\u003c/sub\u003e is found being accompanied by unignorable adsorption of water molecules onto the Pt electrode; in sharp comparison, when occurring in alkaline media, ad/ desorption of H\u003csub\u003e2\u003c/sub\u003eO or OH\u003csup\u003e-\u003c/sup\u003e on Pt is almost negligible up to 0.15 V (vs RHE, similarly thereafter unless otherwise specified). The observed pH-dependent interfacial water ad/desorption behaviors, corroborated by Ab Initio Molecular Dynamics (AIMD) simulations, indicate that the EDL on Pt in alkaline media within the HOR potential range is \u003cstrong\u003epredominantly composed of rigid, discrete hydrated cation domains\u003c/strong\u003e. Concurrently, slow-growth simulation results reveal that the Volmer step in both acidic and alkaline environments involves \u003cstrong\u003ecoupled water reorientation\u003c/strong\u003e, which facilitates its role as a proton acceptor; consequently, the dynamic processes of interfacial water reorientation and adsorption critically modulate the energy barrier of the Volmer step. The EDL structure in alkaline media is anticipated to impede the transport of continuously generated H⁺ from the Pt surface to the bulk solution, constituting a key mechanistic origin of the sluggish alkaline HOR kinetics on Pt. Critically, the interfacial water structure, particularly its enhanced rotational flexibility in acidic environments, promotes more facile reaction kinetics.\u003c/p\u003e"},{"header":"Results ","content":"\u003cp\u003eA homemade electrical cell (Supplementary Fig. 1) which is air-tight and equipped with pressure control system (Supplementary Fig. 2 and 3) was assembled for EQCM measurement, with a 7.995 MHz titanium/platinum coated quartz crystal as the working electrode (Supplementary Fig. 4 and Supplementary Note 1).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn brief, when a potential is applied to the working electrode, EQCM can simultaneously record the charge change (\u003cem\u003eQ\u003c/em\u003e) and frequency change (\u003cem\u003e∆f\u003c/em\u003e) occurring upon the electrode. And according to the Sauerbrey equation, the measured frequency change ∆\u003cem\u003ef\u003c/em\u003e is oppositely linear to the mass variation ∆\u003cem\u003em\u0026nbsp;\u003c/em\u003e\u003csup\u003e33,34\u003c/sup\u003e; the conversion of ∆\u003cem\u003ef\u003c/em\u003e to ∆\u003cem\u003em\u003c/em\u003e was conducted in each test (Supplementary Fig. 5 and Supplementary Note 2)\u003csup\u003e35\u003c/sup\u003e. \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe measurement was first conducted in Ar-purged acid or base solutions (Supplementary method). \u003cstrong\u003eFig. 1a and 1d\u003c/strong\u003e show the typical Cyclic voltammetry (CV) profiles in acid (0.1 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) and alkaline (0.1 M KOH) media, where the oxidative desorption of H\u003csub\u003eupd\u003c/sub\u003e occurs within around 0 - 0.4 V. This process corresponds to the Volmer step (\u003csup\u003e*\u003c/sup\u003eH\u0026nbsp;\u0026agrave;\u0026nbsp;* + H\u003csup\u003e+\u003c/sup\u003e + e\u003csup\u003e-\u003c/sup\u003e). Meanwhile, the relevant ∆\u003cem\u003ef\u003c/em\u003e was also obtained, from which the mass variation profiles (MV) were obtained, both of which are shown in \u003cstrong\u003eFig. 1b and 1e\u003c/strong\u003e.\u003c/p\u003e\n\u003cp\u003eThe MV within the H\u003csub\u003eupd\u003c/sub\u003e potential range was interesting. As depicted in \u003cstrong\u003eFig. 1b\u003c/strong\u003e and the inset, in acid media, the \u003cem\u003e∆m\u003c/em\u003e was found decreasing almost linearly with increasing potential within from 0 to 0.2 V. In sharp contrast, the Pt electrode in alkaline media experiences slight mass decrease with potential increasing up to ~0.15 V (\u003cstrong\u003eFig. 1e\u003c/strong\u003e).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSuch differentiated trend also occurs when the same experiment was conducted in HClO\u003csub\u003e4\u003c/sub\u003e (Supplementary Fig. 6) and NaOH solutions (Supplementary Fig. 7) or with different scan rates (Supplementary Fig. 8 and 9).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe relevant accumulated charge \u003cem\u003eQ\u003c/em\u003e was then calculated, which is found linear to the mass variation \u003cem\u003e∆m\u003c/em\u003e (\u003cstrong\u003eFig. 1c and 1f\u003c/strong\u003e); from the resulting slope, the \u003cem\u003empe\u003c/em\u003e value, namely the mass change for per mole electrons transferred, was obtained (Supplementary Fig. 10 and Supplementary Note 3). \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn acid media (within \u003cem\u003eca.\u003c/em\u003e 0-0.28 V), as depicted in \u003cstrong\u003eScheme 1a\u003c/strong\u003e, the desorption of H\u003csub\u003ead\u003c/sub\u003e off Pt was often assumed to occur without de/adsorption of water molecules or OH\u003csup\u003e-\u003c/sup\u003e anions. If this is the real situation, it shall have given a \u003cem\u003empe\u0026nbsp;\u003c/em\u003evalue of -1 g mol\u003csup\u003e-1\u003c/sup\u003e. However, a \u003cem\u003empe\u003c/em\u003e of around +7.85 g mol\u003csup\u003e-1\u003c/sup\u003e is obtained (\u003cstrong\u003eFig. 1c\u003c/strong\u003e). That is, for desorption of per mole H\u003csub\u003eupd\u003c/sub\u003e in acid solutions, the electrode experiences a mass increase \u003cem\u003eca.\u003c/em\u003e 8 g. This thus leads to a different Volmer step which is depicted in \u003cstrong\u003eScheme 1b and 1c\u003c/strong\u003e. Specifically, the desorption of every two H\u003csub\u003eupd\u003c/sub\u003e in acid solution is coupled with adsorption of around one H\u003csub\u003e2\u003c/sub\u003eO molecule on the Pt surface, which hence yields a measured \u003cem\u003empe\u003c/em\u003e of +7.85 g mol\u003csup\u003e-1\u003c/sup\u003e. Meanwhile, when the same process was conducted in HClO\u003csub\u003e4\u003c/sub\u003e solution (Supplementary Fig. 6c), though a slightly different \u003cem\u003empe\u003c/em\u003e value of +5.62 g mol\u003csup\u003e-1\u003c/sup\u003e was obtained, it also suggests obvious adsorption of water molecules onto Pt during dissociation of H\u003csub\u003eupd\u003c/sub\u003e. \u0026nbsp; \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eMeanwhile, in alkaline solutions, it is often assumed that desorption of H\u003csub\u003ead\u003c/sub\u003e involves (adsorbed) OH\u003csup\u003e-\u003c/sup\u003e, which is also known as the bifunctional mechanism (\u003cstrong\u003eScheme 1d\u003c/strong\u003e). Had this been the situation, the process would have yielded a \u003cem\u003empe\u003c/em\u003e value of -18 g mol\u003csup\u003e-1\u003c/sup\u003e. However, shown in \u003cstrong\u003eFig. 1f\u003c/strong\u003e,\u003cem\u003e\u0026nbsp;\u003c/em\u003ea negative \u003cem\u003empe\u003c/em\u003e value of -0.68 g mol\u003csup\u003e-1\u003c/sup\u003e is obtained for this process within \u003cem\u003eca.\u003c/em\u003e 0-0.15 V. Such a \u003cem\u003empe,\u0026nbsp;\u003c/em\u003eclose to -1 g mol\u003csup\u003e-1\u003c/sup\u003e, reveals that, the desorption of H\u003csub\u003eupd\u0026nbsp;\u003c/sub\u003eoff Pt in alkaline solutions does not involve observable ad/desorption of H\u003csub\u003e2\u003c/sub\u003eO or OH\u003csup\u003e-\u003c/sup\u003e molecules, as depicted in \u003cstrong\u003eScheme 1e\u003c/strong\u003e \u003cstrong\u003eand 1f\u003c/strong\u003e. Otherwise, the resulting \u003cem\u003empe\u003c/em\u003e would be significantly different, since the molecular weight of OH\u003csup\u003e-\u003c/sup\u003e or H\u003csub\u003e2\u003c/sub\u003eO is much larger than that of H\u003csup\u003e+\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eTo study the real HOR process on Pt, a H/H\u003csub\u003e2\u003c/sub\u003e stripping experiment was designed by referring to the classical CO-stripping process. As shown in \u003cstrong\u003eFig. 2a\u003c/strong\u003e, hydrogen was preloaded onto the Pt under open circuit potential; followed by Ar-purging for a period of time, the H/H\u003csub\u003e2\u003c/sub\u003e-preloaded Pt then was subjected to linear sweep voltametric test (LSV, Supplementary Methods).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eShown in \u003cstrong\u003eFig. 2b and 2d\u003c/strong\u003e, in both acid and alkaline media, the current was much larger than the previous results obtained in the solutions purged with only Ar (as compared in Supplementary Fig. 11 and 12), confirming the oxidation of pre-adsorbed H\u003csub\u003e2\u003c/sub\u003e in addition to the H\u003csub\u003eupd\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eImportantly, for both acid and base solutions, the MV within the HOR potential range demonstrates the same trend identical to those purged with only Ar (\u003cstrong\u003eFig. 2c and 2e\u003c/strong\u003e). In acid solution, mass increase was observed, indicating obvious adsorption of water molecules on Pt during oxidation of the preloaded H\u003csub\u003e2\u003c/sub\u003e/H (\u003cstrong\u003eFig. 2c\u003c/strong\u003e). In base solution, mass loss is found (\u003cstrong\u003eFig. 2e\u003c/strong\u003e); moreover, this mass loss is obviously larger than that measured in the base solutions purged with only Ar, consistent with its much larger current.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eA similar experiment was further conducted in D\u003csub\u003e2\u003c/sub\u003e in alkaline media (\u003cstrong\u003eFig. 2f\u003c/strong\u003e). When generating the same current, the mass loss from D/D\u003csub\u003e2\u003c/sub\u003e stripping is around two times of that from H/H\u003csub\u003e2\u003c/sub\u003e stripping\u003csub\u003e\u0026nbsp;\u003c/sub\u003e(\u003cstrong\u003eFig. 2g\u003c/strong\u003e). Such isotopic experiment confirms that the mass loss in the H/H\u003csub\u003e2\u003c/sub\u003e stripping in alkaline media indeed originates from surface adsorbed H\u003csub\u003ead\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e (or D\u003csub\u003ead\u003c/sub\u003e/D\u003csub\u003e2\u003c/sub\u003e).\u003c/p\u003e\n\u003cp\u003eThe interfacial process for HOR in alkaline solution was further compared in different experimental settings. Shown in \u003cstrong\u003eFig. 3a\u003c/strong\u003e, H\u003csub\u003e2\u003c/sub\u003e was first purged into the bulk solution under open circuit potential for 30 mins; then H\u003csub\u003e2\u003c/sub\u003e was supplied continuously in the upper chamber of the cell during repetitive CV scans within 0-0.2 V (Supplementary Methods).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAs shown in \u003cstrong\u003eFig. 3b\u003c/strong\u003e, the anodic current of the 1\u003csup\u003est\u003c/sup\u003e cycle was the largest, which then declined as the level of H\u003csub\u003e2\u003c/sub\u003e saturation in the base solution decreased continuously with increasing CV cycles in the scenario. Accordingly, we found that the mass loss for the 1\u003csup\u003est\u003c/sup\u003e anodic scan is obviously larger than that of the 10\u003csup\u003eth\u003c/sup\u003e cycle, confirming that the measured mass loss was mainly from oxidation of pre-adsorbed H/H\u003csub\u003e2\u003c/sub\u003e on the electrode before each anodic scan (\u003cstrong\u003eFig. 3c\u003c/strong\u003e).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe above experiment was further verified in alkaline solution using D\u003csub\u003e2\u003c/sub\u003e. When oxidation of D\u003csub\u003e2\u003c/sub\u003e yields similar current density as that of H\u003csub\u003e2\u003c/sub\u003e (\u003cstrong\u003eFig. 3d\u003c/strong\u003e), the mass loss for DOR approaches two times of that for HOR in the same alkaline solution (\u003cstrong\u003eFig. 3e\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eThe experiment was also conducted in acid solution, mass increase was observed, a similar trend as that displayed in Ar-purged acid solution (Supplementary Fig. 13). These results render further support to the new Volmer step proposed in \u003cstrong\u003eScheme 1\u003c/strong\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe HOR rate in alkaline media can be generally improved when Pt is alloyed with Ru\u003csup\u003e9,36,37\u003c/sup\u003e. Herein a layer of PtRu alloy was deposited on a Au working electrode of EQCM (Supplementary Fig. 14 and Supplementary method). In both Ar-purged acid and base solutions, \u0026Delta;\u003cem\u003em\u003c/em\u003e for the PtRu electrode increases linearly with potential increasing up to\u003cem\u003e\u0026nbsp;ca.\u003c/em\u003e 0.2 V (Supplementary Fig. 15 and 16), showing different interfacial behavior from Pt.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe HOR experiment depicted in \u003cstrong\u003eFig. 3a\u003c/strong\u003e was also investigated on PtRu in alkaline media. Shown in \u003cstrong\u003eFig. 3f\u003c/strong\u003e and the inset, the alkaline HOR rate on PtRu is much faster than on Pt; what\u0026rsquo;s more, contrary to the above-discussed situation of Pt, the PtRu demonstrates a positive ∆\u003cem\u003em\u003c/em\u003e during alkaline HOR (\u003cstrong\u003eFig. 3g\u003c/strong\u003e)\u003cstrong\u003e.\u0026nbsp;\u003c/strong\u003eThis\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eindicates that alkaline HOR on PtRu is accompanied by adsorption of OH\u003csup\u003e-\u003c/sup\u003e or H\u003csub\u003e2\u003c/sub\u003eO on its surface, which can be one of the reasons contributing to its faster HOR rate.\u0026nbsp;\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eSuch pH-dependent interfacial interaction between the EDL and the Pt surface demonstrated in \u003cstrong\u003eScheme 1c and 1f\u003c/strong\u003e shall be associated with their different EDL structures. With increasing pH, at RHE scale, the HOR occurs far below the potential of zero charge (PZC) of Pt at pH 13 compared to that at the pH 1 (Supplementary Note 4, Supplementary Fig. 17-20)\u003csup\u003e38\u003c/sup\u003e. Hence, during alkaline HOR, solvated cations (e.g., [K(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003en\u003c/sub\u003e]\u003csup\u003e+\u003c/sup\u003e), rather than free OH\u003csup\u003e-\u003c/sup\u003e anions or hydrogen-bonded water network, are expected to dominate the EDL.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo investigate the impact of EDL configuration differences and provide mechanistic understanding, we performed AIMD simulations. The Pt (111) surface was chosen as a model electrode\u003csup\u003e21\u003c/sup\u003e, explicitly solvated with water molecules to construct the EDL. The simulated PZC (PZC\u003csub\u003esim\u003c/sub\u003e) of the Pt (111)-water interface is 0.16 V vs SHE, which agrees well with the literature value of 0.21 V vs SHE (Supplementary Fig. S21)\u003csup\u003e39-41\u003c/sup\u003e. Additionally, H\u003csub\u003ead\u003c/sub\u003e coverage on Pt(111) (Pt-H\u003csub\u003ex\u003c/sub\u003e where \u003cem\u003ex\u003c/em\u003e denotes the coverage) was systematically varied, and the associated change in the work function was extracted (Supplementary Fig. S22 and S23). As demonstrated by Li et al.\u003csup\u003e16\u003c/sup\u003e, varying the H\u003csub\u003ead\u003c/sub\u003e coverage leads to a volcano-shaped dependence of work function, which in turn modulates the interfacial potential.\u003c/p\u003e\n\u003cp\u003eTo mimic acidic and alkaline environments, we modulated the electrode potential by adding proton\u0026ndash;electron (H\u003csup\u003e+\u003c/sup\u003e + e\u003csup\u003e-\u003c/sup\u003e) or sodium\u0026ndash;electron (Na\u003csup\u003e+\u003c/sup\u003e + e\u003csup\u003e-\u003c/sup\u003e) pairs. For the acidic system, one H\u003csup\u003e+\u003c/sup\u003e + e\u003csup\u003e-\u003c/sup\u003e pair was introduced and surface H\u003csub\u003ead\u003c/sub\u003e coverage was set to 0.88, resulting in potential of 0.09 V vs SHE. For the alkaline system, three Na\u003csup\u003e+\u003c/sup\u003e + e\u003csup\u003e-\u003c/sup\u003e pairs were added; when the H\u003csub\u003ead\u003c/sub\u003e coverage was set to 0.25, the potential was -0.65 V vs SHE; when the H\u003csub\u003ead\u003c/sub\u003e coverage was 0, the potential increased to -0.41 V vs SHE (Supplementary Table 3).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThis model yielded a Na\u003csup\u003e+\u003c/sup\u003e concentration of 2.78 mol L\u003csup\u003e-1\u003c/sup\u003e at the interface (Supplementary Fig. S24, S25), in good agreement with the interfacial cation concentrations reported by Koper and co-workers, who observed substantial cation accumulation within the EDL region due to electrostatic interactions and specific adsorption tendencies\u003csup\u003e21\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFig. 5a and 5b\u003c/strong\u003e illustrate the interfacial water structures on the bare or H-covered Pt under acidic and alkaline conditions, respectively, as revealed by our AIMD simulations and supported by EQCM experimental results. Under acidic conditions, the interface potential of the bare Pt was 0.16 V vs RHE, which closely matches the calculated and experimental PZC of Pt(111). For the alkaline system (pH = 14), the potential of the Pt-H\u003csub\u003e0.25\u003c/sub\u003e was 0.18 V vs RHE.\u003c/p\u003e\n\u003cp\u003eWe used the cosine of the angle between the water dipole and the surface normal (Z-axis) to represent water orientation. A cosine value close to +1 indicates an O-down orientation, whereas -1 corresponds to an H-down orientation (\u003cstrong\u003eFig. 5c\u003c/strong\u003e). \u003cstrong\u003eFig. 5d\u003c/strong\u003e quantifies the average distribution of water orientations along the Z-axis.\u003c/p\u003e\n\u003cp\u003eUnder acidic conditions, as the H\u003csub\u003ead\u003c/sub\u003e coverage decreases, the surface water layer closest to Pt (111) transitions from H-down to O-down orientations. This transition indicates increased chemical adsorption of O-down water on Pt (111), especially at higher potentials. In contrast, in alkaline environments, the water orientation remains largely H-down regardless of the H\u003csub\u003ead\u003c/sub\u003e coverage. These orientation trends are consistent with EQCM observations, suggesting that acidic interfaces enable easier water reorientation due to proximity to the PZC, whereas alkaline interfaces restrict reorientation due to dielectric saturation.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWhile the acidic/alkaline interface covered with H\u003csub\u003ead\u003c/sub\u003e exhibits dominant H-down configurations overall, the interfacial property differs significantly. In acidic media, the system is closer to the PZC, resulting in a higher interfacial dielectric constant and increased flexibility of water orientation. Conversely, alkaline systems lie far from the PZC, leading to dielectric saturation, a lower interfacial dielectric constant, and hindered rotational freedom of water molecules\u003csup\u003e21,42-46\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eTo examine how these EDL differences influence the Volmer step, we conducted slow-growth MD. The reaction coordinate was defined as the proton transfer distance between the transferring H\u003csub\u003ead\u003c/sub\u003e and adjacent O atoms (Supplementary Fig. S26). The calculated free energy, which is from the integration of the forces acting on the reacting atoms (Supplementary Fig. S27), reveal a barrier of 0.92 eV for the acidic interface (\u003cstrong\u003eFig. 6a\u003c/strong\u003e) and 1.19 eV for the alkaline interface (\u003cstrong\u003eFig. 6c\u003c/strong\u003e). In both environments, water reorientation occurs during proton transfer, as seen in the intermediate structures (Supplementary Fig. S28). Meanwhile, the H\u003csub\u003ead\u003c/sub\u003e\u0026ndash;O distance in the transition state is 1.22 \u0026Aring; for the acidic EDL and 1.18 \u0026Aring; for the alkaline interface (\u003cstrong\u003eFig. 6b and 6d\u003c/strong\u003e), respectively, both exceeding typical O\u0026ndash;H bonding ranges (0.9\u0026ndash;1.1 \u0026Aring;). Considering the similar H\u003csub\u003ead\u003c/sub\u003e-O distance of the transition state in acid and base solutions, their calculated energy difference up to 0.27 eV shall originate from the different EDL structures. This suggests the hydrogen-bonding interaction between the interfacial water and the H\u003csub\u003ead\u003c/sub\u003e is blocked in the alkaline media, likely due to hindered water reorientation.\u003c/p\u003e\n\u003cp\u003eThis elevated barrier is attributed to the strong surface electric field induced by cation aggregation in the alkaline EDL, which causes the water molecules in the EDL to reach dielectric saturation, thereby affecting their dielectric response\u003csup\u003e47,48\u003c/sup\u003e. Although dielectric saturation would reduce the solvent rearrangement energy within the framework of Marcus theory, due to the fact that this step is coupled with the coordinate changes in the reaction process between the interfacial water and surface adsorbed H\u003csub\u003ead\u003c/sub\u003e, the reaction energy barrier increases.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHence, these AIMD results, together with the EQCM observation, show that, in acid HOR process, the adsorption of interfacial water participates actively in the oxidative stripping of adsorbed H\u003csub\u003ead\u003c/sub\u003e off Pt, whereas such a process is much less obvious in alkaline HOR. Such pH-differentiated interaction behaviors of interfacial water with the Pt surface is expected to be an important cause for the pH kinetic effect on HOR.\u003c/p\u003e"},{"header":"Conclusion ","content":"\u003cp\u003eIn summary, the pH-differentiated interaction of interfacial water with Pt surface within the H\u003csub\u003eupd\u003c/sub\u003e or HOR potential range is investigated. Combining voltametric and nanogravimetric techniques, we discovered that, the oxidation and desorption of H\u003csub\u003ead\u003c/sub\u003e in acid solution is accompanied actively with adsorption of water molecules on Pt surface; however, in alkaline media, ad/desorption of water molecules or OH\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eanions on Pt is negligible during desorption of H\u003csub\u003ead\u003c/sub\u003e. By simulating Pt-water structure variations under different H\u003csub\u003ead\u003c/sub\u003e coverages, the AIMD calculations confirmed the quantitative results of EQCM measurement: the water molecules exhibit greater rotational freedom in acid solutions, leading to the formation of O-down chemisorbed water during the Volmer step, whereas it suffers from large energy barrier in alkaline media. Slow-growth MD further demonstrated that this increased rotational freedom and the resulting interfacial water adsorption facilitate H\u003csub\u003ead\u003c/sub\u003e stripping off Pt surface, resulting in a reduced energy barrier for the Volmer step in HOR.\u0026nbsp;\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eCyclic voltammetry (CV) or liner sweep voltammetry (LSV) and mass variation (MV) measurements were performed using a 440C Quartz Crystal Microbalance (CHI) with 7.995 MHz titanium/platinum coated quartz crystal electrodes (Renluxcrystal) as the working electrode, Pt wire as the counter electrode, and saturated calomel electrode (saturated KCl solution, in acid) or Hg/HgO (1 M KOH, in base) as the reference electrode. The whole setup was placed on an anti-vibration table, and the experiment was carried out under constant room temperature (298 \u0026plusmn; 1 K) and humidity (45 \u0026plusmn; 2%).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eBefore every test, the working electrode shall be polished and activated following a rigorous procedure (Supplementary method); the electrode before and after the test was characterized to ensure its structural stability (Supplementary Note 1).\u003c/p\u003e\n\u003cp\u003eIn general, to measure the current and the frequency change ∆\u003cem\u003ef\u003c/em\u003e versus the applied potential in different gas atmosphere, after activation of the working electrode, Ar was purged directly into the fresh electrolyte for 30 mins with a flow rate of 15 mL min\u003csup\u003e-1\u003c/sup\u003e to expel the dissolved impurity gas (e.g., O\u003csub\u003e2\u003c/sub\u003e). For measurement in H\u003csub\u003e2\u003c/sub\u003e or D\u003csub\u003e2\u003c/sub\u003e, the electrolyte was then purged with H\u003csub\u003e2\u003c/sub\u003e/D\u003csub\u003e2\u003c/sub\u003e for at least 10 minutes with a 15 mL min\u003csup\u003e-1\u003c/sup\u003e to saturate the electrolyte with the reactant gas. After solution purging and before the electrochemical test, the cell shall sit for at least one min to resume a steady frequency baseline. During the electrochemical test where gas purging was necessary, instead of purging into the electrolyte, the gas was supplied in the upper chamber of the cell continuously, with a flow rate decreased to 5 mL min\u003csup\u003e-1\u003c/sup\u003e, to minimize disturbing the frequency change (Supplementary method).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAll AIMD simulations were conducted employing the Vienna Ab initio Simulation Package (VASP)\u003csup\u003e49,50\u003c/sup\u003e, utilizing a plane-wave basis set with pseudopotentials. The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation functional\u003csup\u003e51\u003c/sup\u003e treated exchange-correlation effects. Wavefunctions were expanded with a kinetic energy cutoff of 400 eV, and partial occupancies were handled using the first-order Methfessel-Paxton method (smearing width = 0.05 eV). Semi-empirical van der Waals interactions were incorporated via Grimme\u0026apos;s D3 dispersion correction scheme. All simulations were performed without spin polarization due to its negligible impact on total energies. More details can be found in Supplementary method.\u003c/p\u003e\n\u003cp\u003eFree energy profiles for the Volmer reaction were computed using the slow-growth approach\u003csup\u003e52\u003c/sup\u003e within constrained molecular dynamics, the Chan-N\u0026oslash;rskov correction scheme was applied to mitigate work function artifacts. Theoretical foundations for the slow-growth method and potential corrections are detailed in Supplementary Methods.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll relevant data are included in the Article and its Supplementary Information or are available from the corresponding authors upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe National Key Research and Development Program of China (No. 2021YFA1502000) and the National Natural Science Foundation of China (No. 22472142, 22288102, 22322202) were acknowledged.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eW.L., D.C. and Z.J. contributed equally to this work. Y.Z., Z.Z, C.Z., J.L., T.W. and S.S. supervised the project and provided guidance on the project. Y.Z., W.L. and D.C. conceived and designed the study. W.L. and D.C. performed the experiments. Z.J. and T.W. performed the AIMD studies. W.L., D.C., Z.J., Y.Z., T.W. and S.S. wrote and revised the paper. All authors contributed to the analysis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eSeh, Z. W. et al. Combining theory and experiment in electrocatalysis: Insights into materials design. \u003cem\u003eScience\u003c/em\u003e \u003cstrong\u003e355\u003c/strong\u003e, eaad4998 (2017). https://doi.org/10.1126/science.aad4998\u003c/li\u003e\n\u003cli\u003eDurst, J. et al. New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism. \u003cem\u003eEnergy Environ. 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B\u003c/em\u003e \u003cstrong\u003e109\u003c/strong\u003e, 6902-6915 (2005). https://doi.org/10.1021/jp044556a\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Scheme 1","content":"\u003cp\u003eScheme 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7331057/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7331057/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Electrocatalytic hydrogen oxidation reaction (HOR) on Pt depends strongly on pH of electrolyte; its kinetics is orders of magnitude slower in alkaline solutions referring to that in acid solutions. Recent studies reveal that interfacial water plays an important role in HOR on Pt, whether in acid or alkaline media. In this study, by measuring the ratio of mass change to charge change involved in oxidation of Hupd/H2/D2 on Pt (and also on PtRu) in acid and alkaline media, we discovered that the oxidative desorption of adsorbed hydrogen (Had) off Pt is coupled with adsorption of water molecules on Pt in acid solutions, whereas it occurs directly without obvious ad/desorption of H2O or OH- up to ca. 0.15 V (vs. RHE) in alkaline solutions. AIMD calculations further reveal that the dynamic processes of interfacial water reorientation and adsorption critically modulate the energy barrier of oxidative desorption of Had. In acidic environments the interfacial water demonstrates rotational flexibility, which actively interact with Had on the Pt surface, providing a favorable energy motif for Had stripping off Pt. In comparison, in alkaline solutions, owing to dielectric saturation, re-orientation and interaction of the interfacial water with Had suffers from higher energy barrier, constituting a key mechanistic origin of the sluggish alkaline HOR kinetics on Pt electrode.","manuscriptTitle":"pH-dependent Interaction of Interfacial Water with Pt in Electrocatalytic Hydrogen Oxidation Reaction","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-17 06:07:20","doi":"10.21203/rs.3.rs-7331057/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-catalysis","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"natcatal","sideBox":"Learn more about [Nature Catalysis](http://www.nature.com/natcatal/)","snPcode":"","submissionUrl":"","title":"Nature Catalysis","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Research","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"48d1674b-b39f-476b-a529-af14e067169d","owner":[],"postedDate":"September 17th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":54762701,"name":"Physical sciences/Chemistry/Electrochemistry/Electrocatalysis"},{"id":54762702,"name":"Physical sciences/Energy science and technology/Fuel cells"},{"id":54762703,"name":"Physical sciences/Chemistry/Physical chemistry/Reaction kinetics and dynamics"},{"id":54762704,"name":"Physical sciences/Chemistry/Catalysis/Catalytic mechanisms"},{"id":54762705,"name":"Physical sciences/Chemistry/Catalysis/Electrocatalysis"}],"tags":[],"updatedAt":"2026-02-21T14:15:46+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-17 06:07:20","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7331057","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7331057","identity":"rs-7331057","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}