Polyoxometalate-modified platinum enables enhanced hydrogen evolution in buffered electrolytes

Abstract

Hydrogen evolution reaction (HER) under non-extreme pH conditions is attractive but remains kinetically challenging, even for benchmark platinum (Pt) catalysts. Herein, we report that modification with a SiW9 polyoxometalate significantly enhances the HER activity of Pt in near neutral media, as demonstrated in a phosphate-borate buffer (pH 9.2). This enhancement originates from a synergistic co-tuning of the electronic structure and particle size of Pt.

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Water electrolysis is a promising technology for green energy storage and represents a viable route for H₂ production. Current mainstream systems are polymer electrolyte membrane water electrolyzers (PEMWEs)^1–4 and alkaline water electrolyzers (AWEs),^5,6 where the electrolysis is performed under acidic and alkaline conditions, respectively, enabled by advances in catalyst design and the high proton/hydroxide conductivities of the electrolytes. Yet, material corrosion under such extreme pH conditions remains a critical issue, as earth-abundant metal materials are prone to degradation in acidic and/or alkaline environments. This limitation restricts the choice of applicable elements, thereby increasing the cost of the electrolyzers.⁷

An electrolyzer that promotes water splitting under non-extreme pH conditions, where earth-abundant elements such as Fe can be utilized, is highly desirable for reducing the capital costs. Our electrolyte engineering approach focuses on designing the local reaction environment to sustain efficient proton-coupled electron transfer beyond conventional extreme pH conditions.⁸ By tailoring buffer identity, concentration, and transport properties, we maximize the diffusional supply of proton donors and acceptors to the electrode interface. This enhances local proton activity, mitigates transport limitations at high current densities, and establishes the electrolyte as an active component that governs reaction kinetics through controlled proton and buffer ion flux.

Compared to the extensive development of catalysts operating under acidic or alkaline conditions, the optimization of the catalyst materials for water electrolysis under near neutral-pH conditions remains underexplored. Owing to the lack of established design principles for catalysts in non-extreme pH environments, largely due to the limited number of studies to date, we considered the modification and tuning of well-established catalysts developed for acidic or alkaline conditions as a primary strategy. Among these, platinum (Pt) nanoparticles are benchmark catalysts for water electrolysis, owing to nearly ideal hydrogen binding energy.⁹

Polyoxometalate (POM), a class of anionic metal oxide clusters with well-defined structures and tunable properties, have been widely used over the past decades to modify the properties of various metal nanoparticle catalysts.^10–14 We demonstrated that robust electronic interactions between surface-anchored multidentate POM [SiW₉O₃₄]¹⁰⁻ (SiW9) and metal nanoparticles, such as gold,^15,16 palladium,¹⁷ and gold–silver alloy,¹⁸ enhance their catalytic activity and stability. This enhancement arises from the high negative charge and multiple coordination sites of SiW9 (Fig. 1a). Following this sol–gel strategy, Wei et al. validated the HER performance of SiW9-protected-Pt nanoparticle catalyst in PEMWEs.¹⁹ However, this sol–gel synthesis requires careful control of reaction parameters such as concentration, temperature, and loading amount (Fig. 1a), which limits its extensions to practical applications.

Fig. 1 (a) Conventional sol–gel synthesis of POM-protected metal nanoparticle catalysts. (b) This work: facile post-modification of metal nanoparticle catalysts with multidentate POMs.

Here, we report the enhanced HER activity of Pt modified with SiW9 in a near neutral buffer (pH 9.2). Commercial Pt nanoparticles (Pt/Vulcan) were simply immersed in an aqueous solution of SiW9 to afford the modified catalyst, denoted as SiW9-Pt/Vulcan (Fig. 1b). Comparison of the adsorption behavior of SiW9 with that of plenary POMs, based on STEM-EDS mapping and elemental analysis, suggests that the coordinative structure of SiW9 provides an essential anchoring function onto Pt nanoparticles. The pronounced HER performance of SiW9-Pt/Vulcan was confirmed by diffusion-controlled electrolysis using a rotating disk electrode (RDE) and device-oriented electrolysis using carbon paper. This method was applied to another benchmark catalyst, Ru/Vulcan, for enhanced HER performance, demonstrating the applicability of this approach. Electrochemical analysis, IR, TEM, STEM-EDS, and XPS reveal that the significant improvement in HER activity of Pt—already near the apex of the HER volcano plot²⁰—is likely attributed to a co-tuning effect of electronic states and particle size induced by SiW9 adsorption. This facile strategy not only elucidates the intrinsic effect of multidentate POMs but also provides an effective approach for optimizing a wide range of HER catalysts via POM adsorption, even when they are already close to the apex of the catalytic volcano plots.

SiW9 was selectively adsorbed onto Pt nanoparticles by a simple immersion method. Specifically, Pt/Vulcan was immersed in an aqueous solution of the sodium salt of SiW9 (Na₁₀SiW₉O₃₄) at room temperature, followed by filtration and washing with excessive water to obtain SiW9-Pt/Vulcan (Fig. 1b). The SiW9 retrieved from the filtrate was subjected to IR analysis, and characteristic bands derived from SiW9 exhibited no significant peak shift, demonstrating that its structural integrity was maintained during the catalyst modification process (Fig. S1). TEM images revealed that SiW9-Pt/Vulcan retained a particle size of 2.3 ± 0.4 nm (Fig. 2a, b and Fig. S2) without noticeable agglomeration, thereby minimizing the influence of particle size on subsequent HER performance evaluations. Then, STEM-EDS mapping images showed a homogeneous distribution of Pt and W in SiW9-Pt/Vulcan (Fig. 2c), confirming the successful adsorption of SiW9 onto Pt nanoparticles.

Fig. 2 TEM images of (a) Pt/Vulcan and (b) SiW9-Pt/Vulcan. (c) STEM-EDS mapping images of SiW9-Pt/Vulcan.

According to ICP-AES analysis, the loading amount of the Pt catalysts were confirmed: Pt/Vulcan contained 18.6 wt% Pt, while SiW9-Pt/Vulcan contained 17.4 wt% Pt and 4.5 wt% W, indicating that no loss of Pt occurred during the POM modification (Table S1). Notably, the molar ratio of SiW9 to Pt was determined to be 3.1 : 100, corresponding to an estimated surface coverage of approximately 50% (Fig. S3; see SI for details), which is consistent with our previous reports on SiW9-modified Au nanoparticles.^15,16 On the contrary, when plenary POMs such as [SiW₁₂O₄₀]⁴⁻ (SiW12) or [Nb₆O₁₉]⁸⁻ (Nb6) were utilized in place of SiW9 in this simple immersion method, almost no adsorption of these POMs was observed, as evidenced by ICP-AES analysis and STEM-EDS mapping (Fig. S4 and Table S2). Considering the structural similarity of SiW12 and the comparable charge density of Nb6 to those of SiW9, this difference is attributed to the specific interaction between metal nanoparticles and multidentate POMs, as established in previous studies by our group and others.^15–19 Furthermore, SiW9-Pt/Vulcan was immersed in the electrolyte solution (pH 9.2, 0.5 mol kg⁻¹ K-phosphate + 1.5 mol kg⁻¹ K-borate),²¹ and ICP-AES analysis confirmed that negligible amounts of Pt and W leached into the solution, indicating the high stability of the modified catalyst under operating conditions (Table S3).

The HER activity of Pt/Vulcan with and without SiW9 modification was examined using chronopotentiometry (CP).²¹ To rule out the possibility of diffusion limitations and to isolate the effects of changes in electronic nature, a rotating disk electrode (RDE) system was employed, allowing precise control of mass transport. The rotation rate was set to 3600 rpm, at which the rotation speed had no significant effect on the required potential (Fig. 3a), thereby excluding contributions from ion diffusion-related phenomena. The CP results, in the presence and absence of SiW9, demonstrate a substantial enhancement in the catalytic activity. The overpotential at 10 mA cm⁻² decreased from 130 to 81 mV (Fig. 3b), indicating that this post-modification with SiW9 significantly improves the HER performance of Pt nanoparticle catalysts.

Fig. 3 (a) Independence of the HER rate on rotation speed for Pt/Vulcan at 100 mA cm⁻² in an aqueous buffer solution at pH 9.2 of 0.5 mol kg⁻¹ K-phosphate + 1.5 mol kg⁻¹ K-borate at 80 °C with H₂ bubbling. Note: in order to obtain a stable state of the potential applied and exclude the effect of possible catalyst degradation, CP was performed at 100 mA cm⁻² for 30 min before changing the rotation rate of the RDE. As no obvious effect of rotation speed was observed in 100 mA cm⁻², a much higher current than those in the Tafel plot region, it is clear that no effect of rotation speed for Pt/Vulcan exists at 10 mA cm⁻². Since Pt/Vulcan possessed lower activity than SiW9-Pt/Vulcan, it is evident that there is also no effect of rotation speed at 10 mA cm⁻² for SiW9-Pt/Vulcan. (b) Tafel plots of the Pt/Vulcan/GC RDE electrode (red) and the SiW9-Pt/Vulcan/GC electrode (blue) in an aqueous buffer solution at pH 9.2 of 0.5 mol kg⁻¹ K-phosphate + 1.5 mol kg⁻¹ K-borate at 70 °C with H₂ bubbling, based on chronopotentiometry measurements. The loading amount of Pt on RDE is 2.5 µg cm⁻².

The change in Tafel slopes indicates a kinetic modification induced by the presence of SiW9 on Pt nanoparticles. Although buffered electrolytes do not completely eliminate diffusion-induced overpotentials,^22,23 the apparent Tafel slopes were compared to elucidate the effects of SiW9. The apparent Tafel slopes in the linear Tafel region (log|j| = 0.7–1.1) were 107.8 mV dec⁻¹ and 160.1 mV dec⁻¹ with and without SiW9 adsorption, respectively (Fig. 3b). The significant decrease in the Tafel slope can be rationalized by either an increase in the coverage of reaction intermediates with increasing overpotential, an intrinsic increase in the charge transfer coefficient of the rate-determining step (RDS), or a switch of the RDS to another elementary step. The surface coverage is governed by the relative rates of formation and consumption of the corresponding intermediates, while the charge transfer coefficient reflects the potential sensitivity of the reaction barrier. Likewise, a switch in the RDS arises from the changes in the relative rates of competing elementary steps. Therefore, although the precise origin of the Tafel slope change cannot be uniquely identified in the present study, all of these possibilities are closely related to kinetic phenomena governed by the electronic properties of the catalyst.

Having demonstrated that SiW9 enhances the HER activity of Pt nanoparticles, we evaluated the availability of this method under industrially relevant conditions (Fig. S5). Despite the involvement of the diffusion effect—particularly given the high activity of SiW9-Pt/Vulcan—the adsorption of SiW9 onto the Pt nanoparticle catalyst still led to a significant improvement in catalytic performance. Specifically, the overpotential decreased from −0.64 to −0.55 V vs RHE at a current density of 1 A cm⁻² (Fig. S5), which is qualitatively consistent with the behaviors observed in the RDE system. The relatively large Tafel slope values in the high-current-density region (100–1000 mA cm⁻²; >400 mV dec⁻¹ with or without SiW9) suggest that mass-transport-related phenomena are involved, likely associated with local pH shifts at the electrode surface and corresponding shifts in the equilibrium potential of the HER.²⁴ The effects of POM may include mitigating local pH changes even at high current densities, effectively mimicking the function of an ionomer by acting as a surface-confined buffer anion species.

The durability of SiW9-Pt/Vulcan under water electrolysis conditions relevant to practical applications was further evaluated by chronopotentiometry (Fig. 4). The results confirm a clear and sustained improvement in the catalytic performance of Pt for at least 5 h. Particularly, the potential loss at 400 mA cm⁻² was nearly negligible for SiW9-Pt/Vulcan, whereas a loss of approximately 15% was observed for Pt/Vulcan (Fig. S6). In addition, SiW9-Pt/Vulcan showed improved resistance toward repeated ON/OFF operation compared with Pt/Vulcan (Fig. S7). These results indicate that the adsorption of POMs onto Pt nanoparticles is sufficiently robust over this timescale, even under the industrial level-current density conditions.

Fig. 4 The durability test of the time-dependent potential curve of Pt/Vulcan/carbon paper (red) and the SiW9-Pt/Vulcan/carbon paper (blue) in an aqueous buffer solution at pH 9.2 containing 0.5 mol kg⁻¹ K-phosphate + 1.5 mol kg⁻¹ K-borate at 80 °C with H₂ bubbling, based on chronopotentiometry measurements. The loading amount of Pt on the carbon paper is 0.3 mg cm⁻². The Pt ratio in the loaded Pt/Vulcan is 40% (w/w).

The sizes of Pt nanoparticles after 5 h of electrolysis at 400 mA cm⁻² were evaluated by TEM. No significant change in particle size was observed for SiW9-Pt/Vulcan (2.4 ± 0.4 nm), whereas pronounced agglomeration of Pt nanoparticles was observed for Pt/Vulcan (4.4 ± 1.1 nm, Fig. 2a, b and Fig. S8). Furthermore, STEM-EDS mapping images revealed that SiW9-Pt/Vulcan remained its core–shell-like structure and that no obvious aggregation of SiW9 into WO_x nanoparticles occurred after electrolysis (Fig. S9). Meanwhile, the recovered species in SiW9-Pt/Vulcan after the reaction using a salting-out method,¹⁹ maintained the characteristic bands derived from SiW9 (Fig. S10), demonstrating that the structural integrity of SiW9 was maintained during the reaction. These results indicate that the adsorption of SiW9 not only enhances the catalytic activity of Pt but also improves its stability during electrolysis. Although Pt/Vulcan has been widely employed as a cathode catalyst, the observed agglomeration of Pt nanoparticles in the near neutral buffer media (0.5 mol kg⁻¹ K-phosphate + 1.5 mol kg⁻¹ K-borate) highlights the necessity of POM-based modification under near neutral pH conditions.

In addition, this facile adsorption method was extended to another commercial catalyst, Ru/Vulcan. TEM images revealed that the Ru nanoparticles retained their particle size at approximately 2.7 nm (Fig. S11), while enhanced HER activity was observed using an RDE system (Fig. S12). An increase in activity was also observed when a Ru/Vulcan-deposited glassy carbon electrode was immersed in a solution of SiW9 (Fig. S13), supporting the effectiveness of this POM modification strategy.

XPS is widely used to investigate the electronic structures of metal nanoparticles, and shifts to higher binding energy (BE) are commonly interpreted as indicative of decreased electron density.^16,19,25,26 The peaks observed at around 72 eV and 75 eV in the Pt 4f region, corresponding to 4f_5/2 and 4f_7/2, respectively, can be attributed to a mixed oxidation state of Pt⁰ and Pt^δ+ (Fig. 5),²⁶ suggesting partial oxidation of Pt upon exposure to air. Compared to Pt/Vulcan (BE = 71.4 eV for Pt⁰ and 72.6 eV for Pt^δ+), SiW9-Pt/Vulcan exhibited higher BE values (71.6 eV for Pt⁰ and 72.7 eV for Pt^δ+), indicating a decrease in the electronic density of Pt after SiW9 modification. Consistently, the Pt^δ+/Pt⁰ ratio (0.89) was significantly higher than that for Pt/Vulcan (0.58), supporting this conclusion. Since no significant change in particle size was observed upon SiW9 adsorption (Fig. 2), the BE shift is attributed to electron transfer from Pt nanoparticles to the SiW9 ligands, consistent with observations in sol–gel-derived systems.¹⁹ A similar modulation of electronic states was observed for Ru/Vulcan after SiW9 modification, as evidenced by XPS, further supporting this interpretation (Fig. S11).

Fig. 5 XPS spectra of Pt 4f orbital for (a) Pt/Vulcan and (b) SiW9-Pt/Vulcan.

Finally, we discuss the enhancement effect of SiW9 on the HER performance of Pt nanoparticles in terms of three key factors. The first is the suppression of particle agglomeration. TEM images after electrolysis reveal that SiW9 serves as a dual role in stabilizing Pt nanoparticles while maintaining their high catalytic activity. The second factor is the electron-withdrawing nature of SiW9, as reflected in changes in kinetic parameters observed in electrochemical measurements. XPS analysis indicates a decrease in the electron density of Pt nanoparticles after SiW9 adsorption, which may optimize the Pt–H binding strength and thereby intrinsically enhance HER activity. The third factor is the unique multielectron transfer capability of POMs, which may further promote H₂ production by facilitating mass transport.^27–29 Overall, given that Pt is widely recognized to lie near the apex of the HER volcano plot—where hydrogen adsorption on Pt is slightly exothermic—the present POM-modification strategy, which fine-tunes the Pt–H binding interaction, represents a crucial yet challenging step toward further approaching the ideal apex of HER catalysts.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6cc02380k.

Acknowledgements

We gratefully acknowledge the financial support from GteX Program Japan (Grant Number JPMJGX23H2), JSPS KAKENHI (24KJ0563 and 22H04971), and Mizuho Foundation for the Promotion of Sciences. A part of this work was supported by Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), JPMXP1224UT0038 and JPMXP1225UT0084. We thank Ms Mari Morita and Mr Hiroyuki Oshikawa (The University of Tokyo) for their assistance with the STEM-EDS analysis. We also thank Ms Vo Tran Thanh Ngoc (Vietnam National University), Ms Atmadja Kimberly (Nanyang Technological University), and Mr Kumar Srikant (Indian Institute of Technology Roorkee) for their experimental support on electrochemical measurements.

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Footnotes

† Co-first author.

‡ Current address: National Institute for Materials Science, Research Center for Energy and Environmental Materials, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan.

§ Current address: Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan.

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