Rational design of a PMo₁₂–SiW₁₂ coupled catalytic system toward energy-efficient methanol-to-hydrogen conversion

Abstract

Methanol is a promising hydrogen carrier for sustainable hydrogen production, but conventional electrocatalytic methanol oxidation for the hydrogen evolution reaction (HER) is limited by sluggish kinetics and susceptibility to catalyst poisoning. We developed a novel bifunctional Pt/Super-Electrically Activated Carbon (SEAC) catalyst coupled with a synergistic H₃PMo₁₂O₄₀ (PMo₁₂) and H₄SiW₁₂O₄₀ (SiW₁₂) system for hydrogen production from methanol. The integrated PMo₁₂–SiW₁₂ system can operate at a low voltage of 0.89 V to achieve 1 A cm⁻², reducing energy consumption to 1.73 kW h Nm⁻³ H₂, which is only 38.02% of that for conventional alkaline water electrolysis. This PMo₁₂–SiW₁₂ mediated approach can also be extended to other organic hydrogen carriers like ethanol, offering a low-cost, energy-efficient pathway for sustainable hydrogen production.

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Green foundation

1. We report a redox-mediated strategy for sustainable methanol-to-hydrogen production through a bifunctional Pt/SEAC catalyst integrated with a synergistic PMo₁₂–SiW₁₂ coupled system.

2. The catalytic system achieves a current density of 1 A cm⁻² at a low voltage of 0.89 V, reducing energy consumption to 1.73 kW h Nm⁻³ H₂, ensuring high energy efficiency and cost-effectiveness.

3. By mitigating catalyst detachment and poisoning, the redox-mediated system ensures long-term stability and enables the use of various organic hydrogen carriers, such as ethanol and isopropanol, for sustainable hydrogen production.

1. Introduction

With the acceleration of the energy transition, hydrogen energy, as a zero-carbon energy carrier, offers significant decarbonization potential in high-carbon-emission sectors such as steel production, shipping, and other heavy industries.^1–6 However, ∼95% of the current global hydrogen supply (∼15 trillion mol per year) is derived from fossil fuel reforming, which is unsustainable and contributes significantly to atmospheric CO₂ emissions.⁷ Water electrolysis, powered by renewable energy, offers a sustainable technology for green hydrogen production.^8–10 However, conventional water electrolysis for hydrogen production faces the challenges of high renewable energy input (4.5–5 kW h Nm⁻³ H₂) and transport issues due to the extremely low volumetric energy density of gaseous hydrogen.¹¹ These challenges highlight the need for energy-efficient hydrogen production and hydrogen storage technologies.

Methanol (CH₃OH) is an attractive liquid hydrogen carrier due to its high volumetric energy density (15.75 MJ L⁻¹, 1.8 times that of liquid H₂), stability under ambient conditions, and low-cost production from diverse feedstocks, such as CO₂ or biomass.^12–15 Thermocatalytic methanol steam reforming (MSR) is widely used owing to its high hydrogen yield and technological maturity.^16,17 However, MSR produces H₂ mixed with CO₂, CO, and other byproducts, requiring complex purification to meet fuel cell standards (<10 ppm CO).^16,18,19 MSR also faces challenges such as high carbon emissions, catalyst deactivation, and safety risks.²⁰ Electrocatalytic methanol-to-hydrogen conversion offers a milder, more controllable alternative, combining energy efficiency with environmental benefits.²¹ In this system, the methanol oxidation on the anode replaces the energy-intensive oxygen evolution reaction, while the cathode drives the HER. However, sluggish methanol oxidation reaction (MOR) kinetics limit its scalability, and high HER overpotentials reduce system efficiency, necessitating advanced catalyst designs.²²

Conventional Pt-based catalysts exhibit excellent activity but suffer from CO poisoning and high cost due to high precious metal loading.²³ Recent advances have focused on improving Pt-based catalysts through structural engineering and doping. For example, dendritic Au/PtCu nanowires show high MOR activities for methanol oxidation via Cu doping, also demonstrating CO tolerance.²⁴ Multiple noble metals were incorporated into the synthesis of catalysts, such as Pt–Ir/C nanowires, in which the nanowire structure, synergizing with Ir doping, significantly enhanced their stability in acidic media compared to conventional Pt/C catalysts.²⁵ Additionally, high-entropy alloy PtRhGaNiW nanowires were proposed, where Ga/Ni and W modulated the d-band center shift, promoted surface electron redistribution, and accelerated oxidative removal of CO intermediates during methanol oxidation.²⁶ On the other hand, Pt-loaded catalysts, depending on the metal size, morphology, and carrier properties, show a wide range of applications in the MOR.²⁷ Different metal oxide supports, including CeO₂,²⁸ ZrO₂,²⁹ MgO,³⁰ TiO₂,³¹ and RuO₂,³² enhance the MOR activity through synergistic interactions with Pt. Carbon-based supports, due to their high surface area and excellent conductivity, such as activated carbon,^33,34 carbon nanotubes,^28,35 and graphene oxide,^36,37 have also been widely applied in MOR systems to increase the Pt dispersion and expose active sites. However, to improve the economic feasibility of the methanol-to-hydrogen technology, it is of great significance to establish a low Pt-loading and high-performance catalytic system.

Polyoxometalates (POMs) have demonstrated remarkable catalytic potential in Pt synergistic systems.^38–41 POMs, such as typical Keggin structural phosphomolybdic acid (H₃PMo₁₂O₄₀, abbreviated as PMo₁₂) and silicotungstic acid (H₄SiW₁₂O₄₀, abbreviated as SiW₁₂), are a class of nanoscale metal–oxygen clusters with excellent redox properties, enabling reversible multi-electron transfer.^42–44 For example, a Pt₁-PMo₁₂@NiCo-LDH catalyst was developed, where Pt atoms act as active sites for H₂O dissociation to generate reactive hydrogen (H*), and PMo₁₂ functions as an electron–proton shuttle to accelerate proton-coupled electron transfer (PCET) processes.⁴⁵ A single Pt catalyst (Pt₁-PMA/AC) using phosphomolybdic acid (PMA)-modified activated carbon as the support was also developed in the organic hydrogenation reactions with high activity and selectivity.⁴⁶ Rausch et al. proposed in Science a novel water-splitting strategy utilizing SiW₁₂ as a redox mediator.⁷ This system decoupled water oxidation from hydrogen evolution reactions outside the electrolyzer, enabling low-pressure hydrogen production.

Herein, we have developed a bifunctional Pt/SEAC catalyst integrated with a rationally designed co-catalytic system comprising PMo₁₂–SiW₁₂ for the efficient conversion of methanol to hydrogen. At the anode, the Pt/SEAC catalyst is coupled with a PMo₁₂ solution to accelerate methanol oxidation. PMo₁₂ functions as a soluble electron carrier, thereby preventing issues related to catalyst detachment and poisoning. At the cathode, the Pt/SEAC catalyst is paired with SiW₁₂, where SiW₁₂ acts as an electron shuttle to transfer electrons from the electrode surface to the Pt/SEAC catalyst for hydrogen evolution. Consequently, the bifunctional Pt/SEAC catalyst can concurrently promote methanol oxidation and hydrogen evolution reactions at both the anode and the cathode. This redox-mediated strategy, employing dual PMo₁₂–SiW₁₂, enables the separation of the Pt catalyst from the electrode, thus providing a scalable pathway for sustainable hydrogen production.

2. Experimental section

2.1. Synthesis of catalysts

Pt/SEAC was synthesized by the H₂ thermal reduction method using H₂PtCl₆·6H₂O as a precursor. 0.53 mg of H₂PtCl₆·6H₂O and 19 mg of PMo₁₂ were dissolved in 12 mL of acetone and then mixed with 0.2 g of SEAC under ultrasonication for 1 h. The suspension was dried in an oven at 80 °C for 1 h. The obtained powder was calcined under a H₂ atmosphere at 560 °C for 1 h to yield a 560 °C-Pt/SEAC catalyst. Catalysts with varying calcination temperatures (240–640 °C) and Pt loadings (0.5 wt% and 1.5 wt%) were prepared similarly. Pt colloids were synthesized by reducing 0.53 mg of H₂PtCl₆·6H₂O in 10 mL of deionized water under H₂ at 60 °C for 1 h.

2.2. Structural characterization

The specific surface area and pore size of the Pt/SEAC catalysts were measured using Brunauer–Emmett–Teller measurements (BET, JW-HX100). The carbon carrier SEAC was characterized using a Raman Spectrometer (Renishaw/inVia Reflex). The crystal structure of Pt/SEAC was analyzed using X-Ray Diffraction (XRD, Rigaku/Mini-Flex II). The reduction process of Pt calcination was investigated via Hydrogen Temperature-Programmed-Reduction (H₂-TPR, JW-BK200C). Surface elemental composition and oxidation states of Pt/SEAC were investigated by X-ray Photoelectron Spectroscopy (XPS, Thermo Fisher Nexsa). The morphology and microstructure were analyzed using Scanning Electron Microscopy (SEM, JEOL/JSM-7610F Plus) and Transmission Electron Microscopy (TEM, JEOL/JEM-F200) coupled with High-Angle Annular Dark-Field STEM (HAADF-STEM) and Energy-Dispersive Spectroscopy (EDS). The contents of Pt were measured using an atomic absorption (AA) spectrophotometer (AA-7800, Shimadzu Corporation).

2.3. Thermochemical methanol oxidation and electrolytic hydrogen production

The thermochemical oxidation of methanol was carried out by heating PMo₁₂ (0.5 mol L⁻¹) and CH₃OH (3 mol L⁻¹) solution (20 mL) with Pt/SEAC catalysts and magnetic stirring at 100 °C for a certain reaction time. 10 μL of solution was taken out at different reaction times, and the sample was quantitatively diluted to a concentration of 1 mmol L⁻¹ PMo₁₂. The absorbance of the diluted samples was measured at the characteristic absorption peak at 700 nm using an ultraviolet–visible spectrophotometer (UV-2600i). The degree of reduction of PMo₁₂ at different times was calculated according to eqn (1):

RED = 1.0223ABS + 0.1363 (1)

where ABS refers to the absorbance value at 700 nm and RED is the degree of reduction of PMo₁₂ (the average number of electrons transferred from methanol to 1 mol of PMo₁₂ anions).

The electron transfer rate (from CH₃OH to PMo₁₂) was derived from the slope of the linear fitting of the kinetic curves. The Mass Activity (MA) of the catalyst was further calculated using eqn (2):

MA = n × k × F/m (2)

where n is the molar amount (mol) of PMo₁₂, k is the electron transfer rate, m is the mass (g) of Pt in the catalyst, and F is the Faraday constant (96 500 C mol⁻¹).

The electrolytic hydrogen production system was operated using a flow electrolytic cell. A reacted PMo₁₂–CH₃OH mixture and 1 mol L⁻¹ H₃PO₄ electrolyte were continuously pumped into the anodic and cathodic sides of the electrolytic cell, respectively, at a flow rate of 100 mL min⁻¹. Both the anode and cathode electrodes are made of graphite felt with a thickness of 5 mm, and a Nafion membrane 117 was used as the separator. A Gamry electrochemical workstation was used to characterize the electrolytic process. The linear sweep voltammetry (LSV) curves were recorded at a scan rate of 50 mV s⁻¹ at room temperature, and constant current electrolysis was carried out at a current density of 100 mA cm⁻².

The cyclic voltammetry (CV) curves were recorded at a scan rate of 50 mV s⁻¹ at room temperature. In the cathodic hydrogen production experiments, 20 mL of 0.3 mol L⁻¹ SiW₁₂ was electrolytically reduced to the 2-electron reduced form (H₆[SiW₁₂O₄₀]) by continuous electrolysis for 19.3 minutes at a current density of −50 mA cm⁻² applied using a LAND battery system (CT3002N). The entire experiment was carried out under N₂, and the volume of hydrogen was collected immediately after the addition of Pt/SEAC.

The PMo₁₂–SiW₁₂ coupled catalytic system used an anode electrolyte (0.3 mol L⁻¹ PMo₁₂ + 6 mol L⁻¹ CH₃OH) and a cathode electrolyte (0.3 mol L⁻¹ SiW₁₂). 50 mg of Pt/SEAC catalyst was added to both anodic and cathodic solutions. The anode electrolyte was pre-treated by heating at 100 °C for 12 h in the presence of 50 mg of 1 wt% Pt/SEAC catalyst. Electrochemical measurements were conducted at room temperature using LSV at a scan rate of 50 mV s⁻¹. Constant-current electrolysis was performed at 100 mA cm⁻², and the evolved hydrogen gas was quantitatively collected via the water displacement method.

2.4. In situ SERS tests

In situ EC-SERS measurements were performed in a custom-built spectroelectrochemical cell using a confocal Raman microscopy system (Renishaw/inVia Reflex). The Raman spectrum was obtained using a 533 nm laser and a 50× with a nominal aperture of 0.5. In SERS measurements, the potential of the working electrode is controlled by the electrochemical workstation (CORRTEST/Bi-Potentiostat). A single-sided gold-coated silicon wafer coated with a layer of platinum by the potential deposition method was used as the Raman testing substrate.

2.5. Theoretical calculations

All DFT calculations in this work were performed using the Vienna ab initio simulation program (VASP). Electronic structures were computed utilizing the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional, while the ionic cores were represented by the projector-augmented wave (PAW) potentials. A cut-off energy of 450 eV for the plane-wave basis expansion and the spin-polarized method was selected. We build a three-layer Pt(1 1 1) slab with 7 × 7 atoms at each layer and a heterojunction structure where PMo₁₂ was adsorbed on the three-layer Pt(111) slab. The top two layers were allowed to fully relax, while the remaining layers were held fixed. A vacuum layer of ∼20 Å was set up to eliminate interaction between adjacent slabs. A 1 × 1 × 1 Monkhorst–Pack k-point mesh sampling was used for all optimizations. The van der Waals (vdW) interaction was described by the DFT-D3 method. Equilibrium was reached when the forces on relaxed atoms and the energies in self-consistent iterations became less than 0.05 eV Å⁻¹ and 10⁻⁵ eV, respectively. The adsorption energies of adsorbed molecules were defined by

−ΔE = E_ads+Pt(111) − E_Pt(111) − E_ads (3)

where E_ads+Pt(111), E_Pt(111), and E_ads represent the energy of a Pt (111) slab with an adsorbed molecule, the energy of the empty Pt (111) slab, and the energy of the adsorbate (guest molecule), respectively. Zero-point energy and thermal contributions are neglected in this definition.

The reaction energies of methanol and CO oxidation along the proposed reaction pathways were calculated as the energy difference between reaction products and reactants. The reaction energies (ΔE_rxn) were defined by

ΔE_rxn = E_product − E_reactant (4)

where E_product and E_reactant represent the total energies of the product and the reactant bound to the Pt slab. Positive and negative reaction formation energies correspond to an energetically unfavorable process and an energetically favorable process, respectively.

3. Results and discussion

3.1. Construction and structural characterization of the Pt/SEAC synergistic PMo₁₂/SiW₁₂ catalytic system

Fig. 1a shows the rationally designed PMo₁₂–SiW₁₂ co-catalytic system coupled with a bifunctional Pt/SEAC catalyst to enable low-energy, high-efficiency hydrogen production from methanol oxidation. Methanol, which acts as a portable organic storage material of hydrogen, can be obtained from multiple renewable energy-based syntheses, such as CO₂ reduction and biomass conversion into methanol. The key to applying methanol as a hydrogen carrier is the quick and effective decomposition of methanol for hydrogen production under mild conditions. In this work, Pt/SEAC coupled with PMo₁₂ was used for rapid methanol oxidation. PMo₁₂ serves as a synergetic electron/proton carrier harvesting electrons from methanol and transferring them to the anode (Fig. 1b). In contrast, conventional electro-oxidation of methanol directly transfers the electrons from methanol to the anode electrode, which is usually blocked by Pt catalyst detachment and poisoning (Fig. 1c). At the cathode, SiW₁₂ functions as a proton–electron shuttle in solution, reversibly accepting and protons/electrons from the cathodic electrode and releasing them to Pt/SEAC for hydrogen evolution. The decoupled Pt catalyst from the electrode mediated by the redox reaction of SiW₁₂ solves the issues of the growth and coverage of hydrogen gas bubbles, improving the efficiency of hydrogen evolution. Based on the combination of the Pt/SEAC catalyst with PMo₁₂–SiW₁₂ co-catalysts at the anode and cathode sides, respectively, the catalytic efficiency of methanol-to-hydrogen conversion can be significantly improved, and the Pt loading on the catalyst is expected to be reduced.

Fig. 1 (a) Schematic of the Pt/SEAC–PMo₁₂/SiW₁₂ system for coupling methanol oxidation and hydrogen evolution. (b) Electron transfer pathways in this system and (c) comparison with conventional electrocatalysis methanol oxidation.

The Pt/SEAC catalysts were synthesized via H₂ thermal reduction at different calcination temperatures (240–640 °C) using H₂PtCl₆·6H₂O as the precursor. The N₂ adsorption–desorption isotherm analysis revealed that the average pore size of the catalyst ranged from 2.24 to 2.27 nm and a slight increase in specific surface area with increasing calcination temperature (400–640 °C), attributed to H₂ etching of amorphous carbon (Fig. 2a and Fig. S1). Raman spectra (Fig. 2b) showed an increasing I_D/I_G ratio from 240 to 560 °C, which further indicates an increase in porous defects due to H₂ etching at high temperature. The oxygen-containing functional groups (C–O and C=O) of amorphous carbon were also reduced with elevated calcination temperature, corroborated by C 1s XPS analysis (Table S1). As shown in Fig. 2c, the broad diffraction peaks of SEAC in the XRD spectrum at 2θ = 23.84° and 44.14° indicate its low crystallinity, which is consistent with the Raman spectroscopy results, showing a high I_D/I_G ratio and a high specific surface area. The weak diffraction peaks at 39.76° and 46.24° corresponding to Pt nanoparticles of Pt/SEAC calcined at 640 °C indicate that high temperatures lead to aggregation of Pt nanoparticles and a decrease of Pt dispersion. As shown in Fig. 2d, H₂-TPR analysis revealed that Pt⁴⁺ was gradually reduced to Pt⁰ during the H₂ calcination process at about 500 °C, with a total hydrogen consumption of 1468 μmol g⁻¹ (100–620 °C). Although high calcination temperatures (e.g., 640 °C) increased the Pt⁰ ratio, XRD confirmed that high calcination temperature leads to aggregation of Pt particles, which may compromise catalytic activity.

Fig. 2 Structural characterization of Pt/SEAC catalysts at different calcination temperatures. (a) BET isotherms and pore size distribution, (b) Raman spectra, (c) XRD patterns, (d) H₂-TPR profile, and (e) Pt 4f XPS spectra of Pt/SEAC catalysts at different calcination temperatures.

The surface composition and Pt oxidation state of Pt/SEAC at different calcination temperatures (240–640 °C) were studied by XPS. The full spectrum (Fig. S2–S7) confirms the presence of C, O, and Pt. The Pt 4f spectrum (Fig. 2e) shows double peaks (Pt 4f_7/2 and Pt 4f_5/2) that can be fitted to the Pt⁰ and Pt²⁺ components. The rightward shift of the main peaks of Pt 4f_7/2 and Pt 4f_5/2 with increasing calcination temperature indicates that the high temperature promotes the reduction of Pt⁴⁺ (from H₂PtCl₆) and partially reduced Pt²⁺ to the metallic state Pt⁰. At low temperatures, the oxygen-containing functional groups (e.g., –COOH, –OH) of the SEAC support have a charge transfer effect with Pt⁰, resulting in a partial positive charge on Pt and a leftward shift of these peaks. High-temperature calcination reduced these functional groups, thereby increasing the electron density of Pt⁰. These results suggest that metallic Pt⁰ is the primary active species, and high-temperature calcination enhances the Pt⁰ proportion, which may improve catalytic activity.

The morphology of the catalysts at different calcination temperatures was characterized by Scanning Electron Microscopy (SEM). As shown in Fig. 3a and S8, the Pt/SEAC catalysts exhibited irregular particles with smooth surfaces. The particle size and distribution of Pt nanoparticles on the SEAC carrier were further investigated by Transmission Electron Microscopy (TEM), High-Angle Annular Dark-Field STEM (HAADF-STEM), and Energy-Dispersive Spectroscopy (EDS). Fig. 3b and c show that Pt nanoparticles were highly dispersed on the SEAC surface, with an average particle size of 3.27 nm calculated from the particle size distribution. As shown in Fig. 3d, the lattice spacing of the Pt nanoparticles is approximately 0.21 nm, corresponding to the (111) plane of metallic Pt⁰. The corresponding EDS analysis further confirmed the uniform dispersion of Pt on the SEAC carrier (Fig. 3e–g). Additionally, Pt/SEAC catalysts with different Pt loadings of 0.5 wt%, 1 wt%, and 1.5 wt% were prepared. The actual Pt loadings determined by atomic absorption (AA) tests were 0.47 wt%, 0.98 wt%, and 1.52 wt%, respectively, which are very close to the expected values (Table S2), and their standard curves are shown in Fig. S9.

Fig. 3 (a) SEM image, (b–d) TEM images with particle size distribution (inset), (e) HAADF-STEM image, and EDS elemental maps of (f) C and (g) Pt for the 560 °C-Pt/SEAC catalyst.

3.2. Thermal decomposition and electrochemical characterization of CH₃OH oxidation by Pt/SEAC-coordinated PMo₁₂

Different polyoxometalates (POMs) were utilized as CH₃OH oxidation catalysts and electron carriers, including typical Keggin structured PMo₁₂, PW₁₂, and SiW₁₂, as well as mixed metal-addenda structures such as PW₆Mo₆, PMo₁₁V, etc. (Fig. 4a). For example, PMo₁₂ consists of a central P–O tetrahedron surrounded by 12 Mo–O octahedra linked through shared oxygen atoms. Metal ions are coordinated within metal–oxygen octahedra, exposing only oxygen atoms at the surface in bridging or terminal configurations. The measured electrode potentials of different POMs are shown in Fig. 4b. Vanadium-doped POMs exhibit higher electrode potentials, whereas tungsten-only POMs possess lower electrode potentials; thus the order of redox abilities is POM(V) > POM(Mo) > POM(W). Upon addition of the Pt-SEAC catalyst and methanol, the POMs oxidize methanol and convert to the reduced states, resulting in decreases in electrode potentials. However, the reduced vanadium-containing POM still maintains a relatively high electrode potential (0.53 V), indicating that it requires a higher oxidation potential to regenerate through electro-oxidation. Tungsten-containing POMs exhibit diminished oxidizing capabilities, with silicotungstic acid in particular demonstrating negligible redox activity toward methanol. Therefore, PMo₁₂ is preferred as the POM for methanol oxidation, as it can effectively oxidize methanol and subsequently transform into a reduced state with a lower electrode potential (0.46 V), meaning that only a smaller voltage is needed to achieve catalyst regeneration and cyclic reactions.

Fig. 4 (a) The structures of different kinds of POMs used as electron carriers in this study. (b) The initial electrode potentials of different POMs and the time-dependent changes in electrode potential after adding Pt/SEAC and CH₃OH at 60 °C. (c) The kinetic curves of the reactions of 0.5 wt%, 1 wt%, and 1.5 wt% Pt/SEAC catalysts, Pt colloids, the commercial Sinero 40%-Pt/C catalyst, the Premetek 40%-Pt/C catalyst, and the Sigma 5 wt%-Pt/C catalyst in a solution of 0.05 mol L⁻¹ PMo₁₂ and 3 mol L⁻¹ CH₃OH, respectively (at 100 °C and the amount of Pt is 0.5 mg). (d) UV-Vis spectra of PMo₁₂ solution after thermal oxidation of CH₃OH by 1 wt% Pt/SEAC. (e) Electrochemical impedance spectra of different Pt/C catalysts (25 °C). (f) The mass activity of the catalysts prepared at different calcination temperatures measured at varying CH₃OH concentrations (0.05 mol L⁻¹ PMo₁₂ at 100 °C). (g) Linear sweep voltammetry (LSV) curves of the PMo₁₂–CH₃OH reaction system after heating with the 1 wt% Pt/SEAC catalyst for 1 h, 6 h, and 12 h. (h) Constant-current electrolytic curves of PMo₁₂–CH₃OH solutions with different reduction degrees of PMo₁₂ (current density: 100 mA cm⁻² and total electrode area: 1 cm²). (i) Decomposition kinetics and (j) mass activity plots of the 1 wt% Pt/SEAC catalyst in the oxidation of ethanol, ethylene glycol, isopropanol, glycerol, n-butanol, and benzyl alcohol (0.05 mol L⁻¹ PMo₁₂, 100 °C).

Meanwhile, the low potential of tungsten-containing POMs can be utilized to design them as electron carriers for cathodic hydrogen evolution.

Thermal decomposition of CH₃OH with Pt/SEAC and PMo₁₂ was evaluated at 100 °C (Fig. 4c). The performance of the Pt/SEAC catalysts with Pt loadings of 0.5 wt%, 1 wt%, and 1.5 wt% was compared with that of unloaded Pt colloids and three commercial Pt/C catalysts (Sinero 40%-Pt/C, Premetek 40%-Pt/C, and Sigma 5 wt%-Pt/C). Mass activity was determined by spectrophotometry measurement of the reaction solution at 700 nm using a UV-Vis spectrophotometer. As shown in Fig. 4d, the absorbance of PMo₁₂ increased with reaction time, indicating an increase in the reduction degree, which is defined as the average number of electrons received by each PMo₁₂, and also a decrease in the electrode potential verified by the electrode potential measurement. Reaction kinetic measurements revealed that the 1 wt% Pt/SEAC-catalyzed PMo₁₂–CH₃OH reaction exhibited the highest electron transfer rate, as it exhibits low resistance (Fig. 4e and Table S3). However, unloaded Pt colloids showed a very low reaction rate due to the severe aggregation (Fig. S10). The calculated mass activity for 1 wt% Pt/SEAC reached 829.6 mA mg_Pt⁻¹, 4 times higher than that of the commercial Sinero 40%-Pt/C catalyst (231.6 mA mg_Pt⁻¹; Fig. S11), demonstrating its superior catalytic performance.

The effects of PMo₁₂ concentration (0.05–0.1 mol L⁻¹), CH₃OH concentration (1.5–4.5 mol L⁻¹), reaction temperature (60–120 °C), and catalyst dosage (0.025–0.1 g) on methanol oxidation performance were systematically investigated by adjusting the calcination temperature of 1 wt% Pt/SEAC. The results revealed that 0.05 mol L⁻¹ is the optimized concentration of PMo₁₂ for methanol oxidation (Fig. S12–S14). As shown in Fig. 4f and Fig. S15 and S16, 560 °C-Pt/SEAC exhibited the best performance under the optimized conditions (0.05 mol L⁻¹ PMo₁₂, 3 mol L⁻¹ CH₃OH, 0.5 g catalyst, and 100 °C), achieving a mass activity of 829.6 mA mg_Pt⁻¹, which is higher than most reported Pt-based electrocatalysts (Table S4).

However, this catalyst showed lower activity in conventional electrocatalytic methanol oxidation (Fig. S17), highlighting its thermocatalytic specificity. After reacting with methanol, the PMo₁₂ was reduced, and the solution turned deep blue (Fig. S18). Different reduction degrees of PMo₁₂ solutions were obtained by extending the reaction time to 1 h, 6 h, and 12 h. The reduced PMo₁₂ solution was pumped into the anode side of the electrolytic cell for PMo₁₂ regeneration. As shown in Fig. 4g, higher PMo₁₂ reduction degrees resulted in increased current densities at the same applied voltage. At a current density of 100 mA cm⁻², the working voltage required was around 0.656 V vs. Ag/AgCl, which is significantly lower than the standard water electrolysis potential (1.23 V). Constant-current electrolysis (100 mA cm⁻²) revealed that the Faraday efficiency of reduced PMo₁₂ regenerated on the anode electrode was 96%, while the working voltage decreased with increasing reduction degree (Fig. 4h). These results demonstrate that the highly reduced PMo₁₂ can store a high number of electrons/protons, enabling low-energy consumption oxidation regeneration.

After reacting with methanol, the reduced PMo₁₂ was characterized by IR spectra, which showed a significant weakness of the characteristic vibration of the Mo=O_d bond peaks (∼960 cm⁻¹), indicating the reduction of Mo⁶⁺ to Mo⁵⁺/Mo⁴⁺ (Fig. S19). The methanol thermal decomposition products were further analyzed by ¹H NMR and ¹³C NMR. There are no characteristic peaks for formaldehyde or formic acid that were detected (Fig. S20). ¹³C NMR further confirmed that only a small amount of methanol remained in the reacted solution (Fig. S21). Emission gas analysis detected a high concentration of CO₂ and N₂ (Fig. S22). Therefore, CH₃OH was completely oxidized to CO₂ and H₂O, with protons and electrons stored in PMo₁₂ throughout the process.

Different alcohol-based liquid organic hydrogen carriers (LOHCs), including ethanol, ethylene glycol, isopropanol, glycerol, n-butanol, and benzyl alcohol, were systematically investigated in the thermal decomposition reactions using Pt/SEAC. As shown in Fig. 4i and j, the mass activity of the catalyst in the alcohol and polyol substrates was in the range of 463.2–733.4 mA mg_Pt⁻¹. The isopropanol system exhibited the highest MA (733.4 mA mg_Pt⁻¹), indicating favorable oxidative kinetics of the Pt/SEAC and PMo₁₂ catalytic systems. The versatility can be attributed to the well-dispersed Pt nanoparticles on the SEAC support and the high oxidative properties of PMo₁₂.

3.3. Mechanism study of CH₃OH oxidation by Pt/SEAC-coordinated PMo₁₂

Combining density functional theory (DFT) calculations with in situ surface-enhanced Raman spectroscopy (SERS), the structure of the PMo₁₂/Pt heterojunction interface and the promoting effects of CH₃OH decomposition and CO oxidation were systematically investigated. PMo₁₂ exhibits two adsorption configurations on the Pt surface (Fig. S23), with structure (b) possessing lower energy and thus representing the dominant adsorption mode. Differential charge density calculation further reveals electron transfer from Pt to PMo₁₂, resulting in positive charge accumulation in the Pt catalyst, thereby enhancing the adsorption of CH₃OH and H₂O (Fig. S24). Fig. 5a shows the energies for O–H bond cleavage of CH₃OH molecules on the PMo₁₂/Pt and Pt surfaces, respectively. With the only Pt surface, the reaction is an endergonic process with the ΔE reaching +0.51 eV, whereas on the PMo₁₂/Pt surface, it decreases to +0.089 eV, indicating that CH₃OH undergoes hydroxyl bond dissociation more readily on the PMo₁₂/Pt surface (Fig. 5b and c). This is attributed to PMo₁₂ serving as an effective acceptor for electrons and protons, whereby the cleaved H atom binds to an O atom of PMo₁₂, forming a Mo–O–H structure. This specific structure also contributes to lowering the energy barrier for subsequent C–H bond dissociation (Fig. S25). Among the possible configurations of protonated PMo₁₂, proton binding at the interface between PMo₁₂ and Pt is energetically most favorable, owing to its lowest potential energy (Fig. S26). Computational analysis of the possible configurations for protonated PMo₁₂ reveals that the proton binding site at the interface in contact with Pt is energetically most favorable, attributable to its lowest location and potential energy.

Fig. 5 (a) Energy diagrams of CH₃OH cleavage on PMo₁₂/Pt and Pt. (b) Structure diagrams of the Pt–CH₃OH and (c) Pt–CH₃O* steps on the PMo₁₂/Pt surface. (d) Energy diagrams of CO oxidation on PMo₁₂/Pt and Pt. (e) Potential-dependent SERS spectra recorded on Pt/Au in aqueous and PMo₁₂ solutions under a CO atmosphere at room temperature (25 °C). Potential-dependent SERS spectra recorded on Pt/Au in (f) 0.033 mol L⁻¹ PMo₁₂ + 0.1 mol L⁻¹ CH₃OH and (g) 0.1 mol L⁻¹ HClO₄ + 0.1 mol L⁻¹ CH₃OH solutions at room temperature (25 °C).

The PMo₁₂/Pt interface also significantly facilitates water dissociation. As presented in Fig. S27, the reaction energy for H–OH bond cleavage is 0.8 eV on the only Pt surface, but it decreases to 0.23 eV on the PMo₁₂/Pt surface. The dissociation energies for water molecules adsorbed at different sites (sites A and B, as shown in Fig. S28) are similar, which demonstrates that water molecules dissociate more readily on the PMo₁₂/Pt surface. The water dissociation generates Pt–OH species that serve as a key reactive intermediate for the subsequent CO oxidation process. It is well known that CO is a typical poison for Pt catalysts, and the CO oxidation constitutes a critical step in methanol oxidation. Fig. 5d depicts the adsorption structures of CO, the intermediate states, and produced CO₂ on the PMo₁₂/Pt and Pt surfaces. The reaction energy for the initial CO oxidation step is −0.12 eV on pure Pt, compared to −0.71 eV on the PMo₁₂/Pt surface, demonstrating that the introduction of PMo₁₂ significantly enhances the CO oxidation. This is possibly because the active Pt–OH species formed at the interface efficiently promote the CO + OH → CO₂ + H⁺ reaction (Fig. S29). As a result, the degradation of Pt catalytic activity due to CO adsorption is effectively mitigated. In summary, the PMo₁₂/Pt heterointerface modulates the electronic structure, facilitates the dissociation of both CH₃OH and H₂O, and enables an efficient CO oxidation pathway, thereby synergistically improving the overall methanol oxidation performance and suppressing CO poisoning of Pt.

To validate the CO oxidation process, the adsorption behavior of CO on the Pt surface under different solution environments was investigated using in situ surface-enhanced Raman spectroscopy. As shown in Fig. 5e, in aqueous solution, characteristic CO vibration peaks were clearly visible on the Pt surface: the band at 463 cm⁻¹ corresponds to the Pt–C stretching vibration, while the bands at 1921 cm⁻¹ and 2065 cm⁻¹ are assigned to the C–O stretching vibrations in the bridge and top adsorption configurations, respectively. In contrast, in the PMo₁₂ solution, no characteristic CO peaks were detected. This finding demonstrates that PMo₁₂ effectively promotes the oxidation of CO to CO₂, thereby substantially alleviating Pt catalyst poisoning induced by strong CO adsorption, which is consistent with the DFT computational results. Subsequent investigation into the reaction behavior of CO produced during methanol oxidation was conducted using potential-step in situ surface-enhanced Raman spectroscopy (−0.2 V → 1.2 V). The results presented in Fig. 5f demonstrate that the electrochemical oxidation of methanol on the Pt surface generates CO intermediates. As the electrode potential increases, the characteristic adsorption peaks of CO decrease, indicating progressive oxidation of CO. However, even upon application of higher potentials, notable CO characteristic peaks still exist on the Pt surface, indicating incomplete oxidative removal of CO, which consequently induces catalyst poisoning and diminishes catalytic activity. In contrast, in the PMo₁₂ solution, no characteristic peaks attributable to Pt-adsorbed CO are observed at any applied potential, further demonstrating that, during the methanol oxidation process, PMo₁₂ facilitates the complete and expeditious oxidation of adsorbed CO intermediates on the Pt surface to CO₂, thereby ensuring the continuous and efficient methanol oxidation on the active sites of the Pt catalyst (Fig. 5g).

3.4. PMo₁₂/SiW₁₂ coupled methanol decomposition and hydrogen evolution

A cathodic decoupled hydrogen evolution system was also constructed using the redox mediator SiW₁₂. The mediator H₄[SiW₁₂O₄₀] was electrolyzed at a constant current of 50 mA cm⁻² to convert it to the 2-electron reduced form (H₆[SiW₁₂O₄₀]). This solution is stable under a nitrogen-protected atmosphere and can release hydrogen gas catalyzed by the 1 wt% Pt/SEAC catalyst (Fig. S30). As shown in Fig. 6a and b, the 1 wt% Pt/SEAC catalyst generated >73.5 mL of H₂ within 12 min, achieving complete conversion from the 2-electron reduced form to the 1-electron reduced form. The initial hydrogen evolution rate catalyzed by the 1 wt% Pt/SEAC catalyst reached 1862.5 mmol h⁻¹ mg_Pt⁻¹ (Fig. S31 and Table S5), surpassing the performance of commercial catalysts, such as Sinoer 40% Pt/C (146 mmol h⁻¹ mg_Pt⁻¹), Premetek 40% Pt/C (133 mmol h⁻¹ mg_Pt⁻¹), and Sigma 5 wt% Pt/C (262.3 mmol h⁻¹ mg_Pt⁻¹). According to previous reports,⁷ the Pt-catalyzed hydrogenolysis reaction can rapidly and completely convert the 2-electron reduced form to the 1-electron reduced form, but only 10–36% of the 1-electron reduced form can be partially converted to H₄[SiW₁₂O₄₀]. UV-Vis spectral analysis (Fig. S32) further confirmed that using the 1 wt% Pt/SEAC catalyst, the conversion of the 1-electron reduced SiW₁₂ to H₄[SiW₁₂O₄₀] reached 51%. Therefore, the 1 wt% Pt/SEAC catalyst exhibits significantly superior hydrogen evolution performance compared to commercial Pt/C catalysts, providing an efficient catalyst for SiW₁₂-mediated hydrogen production systems.

Fig. 6 (a) Comparison of hydrogen production rates from the H₆[SiW₁₂O₄₀] solution with various types of catalysts. (b) Linear fitting plots of initial hydrogen production rates. (c) The polarization curves of the coupled electrolytic system with the anodic solution of 0.3 mol L⁻¹ PMo₁₂ and 6 mol L⁻¹ CH₃OH (pretreated at 100 °C for 12 h with 50 mg of the 1 wt% Pt/SEAC catalyst) and the cathodic solution of 1 mol L⁻¹ H₃PO₄ and 0.3 mol L⁻¹ SiW₁₂, respectively. (d) Working voltage–time profiles during constant-current electrolysis (100 mA cm⁻²). (e) The working voltage–time curves of the coupled electrolytic system with an anode solution (30 mL of a 0.3 mol L⁻¹ solution of PMo₁₂ and a 6 mol L⁻¹ solution of CH₃OH, pre-treated at 100 °C for 12 hours first, catalyzed by 50 mg of 1% mass fraction of Pt/SEAC) and a cathode solution (30 mL of a 0.3 mol L⁻¹ solution of SiW₁₂) under different conditions. (f) CV curves of initial PMo₁₂ and PMo₁₂ regenerated by electrolysis.

Based on the performances of Pt/SEAC for anodic methanol oxidation with PMo₁₂ and cathodic hydrogen evolution with SiW₁₂, we constructed a PMo₁₂–SiW₁₂ coupled catalytic system in this work.

Polarization curves revealed that the PMo₁₂–SiW₁₂ system for methanol oxidation and hydrogen evolution required a low working voltage of only 0.89 V at 1 A cm⁻² (Fig. 6c). The working voltage is 54.6% lower than that of the PMo₁₂–H₃PO₄ system (1.96 V at 1 A cm⁻²), which is coupled with conventional acidic hydrogen evolution in H₃PO₄ solution. Fig. 6d shows the constant-current electrolysis performance of the PMo₁₂–SiW₁₂ and PMo₁₂–H₃PO₄ systems at 100 mA cm⁻² for 3.2 h. The PMo₁₂–SiW₁₂ system required an applied potential of 0.5–0.6 V, whereas the PMo₁₂–H₃PO₄ system required an applied potential of 0.8–0.9 V. In addition, cathodic hydrogen evolution kinetic analysis (Fig. S33) revealed that the initial hydrogen production rate of 1 wt% Pt/SEAC in the PMo₁₂–SiW₁₂ system was 454.82 mmol h⁻¹, and 14 mL of hydrogen could be produced in 5 s. The hydrogen production rate is 243 times higher compared to the PMo₁₂–H₃PO₄ system (1.87 mmol h⁻¹).

The long-term stability of the PMo₁₂–SiW₁₂ coupled catalytic system was evaluated in a coupled methanol oxidation (anode)–hydrogen evolution (cathode) system. The anolyte was a 0.3 mol L⁻¹ PMo₁₂ solution with 6 mol L⁻¹ CH₃OH (preheated at 100 °C with the 1 wt% Pt/SEAC catalyst). During the electrolysis, the anolyte and catholyte solutions were maintained at 60 °C and a constant current density of 100 mA cm⁻². As shown in Fig. 6e (red line), the working voltage was maintained at around 0.5 V, which is far below the theoretical water splitting potential (1.23 V), demonstrating the high catalytic stability of Pt/SEAC. After 8 hours of electrolysis, the working voltage dramatically increased over 1.2 V, which means the running out of electroactive reduced PMo₁₂ at the anodic side and the completion of electrolysis. After the subsequent addition of methanol fuel and its reaction with PMo₁₂, the anolyte could be recycled in the electrolysis. Cyclic runs were performed and showed stable electrolytic performances at a high current density of 200 mA cm⁻² (Fig. S34). In addition, cyclic voltammetry tests of the anodic and cathodic solutions confirmed the structural stability of PMo₁₂ and SiW₁₂ after long-time running of electrolysis, compared to the initial solution (Fig. 6f and Fig. S35).

The PMo₁₂–SiW₁₂ coupled system exhibited excellent electrolytic hydrogen production performance. With the working current of 100 mA cm⁻², the system is maintained at a stable working potential of around 0.5 V. Assuming that the electrode is scaled to 1 m², continuous operation for 1 h will produce 0.42 Nm³ H₂. The energy consumption for methanol electrolysis to hydrogen conversion in this work includes the heating and electrolytic energy. Based on the electrolytic experiments, the electric energy is calculated as 1.19 kW h Nm⁻³ H₂. However, thermodynamic analysis shows that the total reaction of methanol to hydrogen is endothermic: CH₃OH(l) + H₂O(l) → CO₂(g) + 3H₂(g) ΔH ≈ +131 kJ mol⁻¹. This energy requirement can be approximately incorporated into the energy consumption for heating. Therefore, the total energy consumption for hydrogen production from methanol in this work could be 1.73 kW h Nm⁻³ H₂, which is only 38.02% of conventional alkaline water electrolysis (4.55 kW h Nm⁻³ H₂), as summarized in Fig. S36 and Table S6.

Furthermore, a life cycle energy analysis for hydrogen production from CH₃OH electrolysis was conducted. The total energy consumption is 277.3–359.3 MJ kg⁻¹ H₂ in this electrolytic route (Table S7). Therefore, the total cost of hydrogen production through the PMo₁₂–SiW₁₂ system is proposed to be $ 3.72–6.12 kg⁻¹ H₂, with a profit of approximately $ 0.28–4.28 kg⁻¹ H₂ (Table S8). In summary, the cost advantage of the coupled PMo₁₂–SiW₁₂ system can be ascribed to: (1) the PMo₁₂–Pt synergy significantly boosts catalytic efficiency while reducing Pt loading, saving more cost on Pt than spent on the acid. Its stable Keggin structure also ensures reliable operation with lower maintenance; (2) synergistic electron transfer between SiW₁₂ and PMo₁₂, which lowered the electrolytic working voltage; and (3) bifunctional catalysis by Pt/SEAC, simultaneously promoting anodic methanol decomposition and cathodic proton reduction, thereby enabling redox coupling of POMs and energy saving. This work provides an energy-efficient and scalable technological pathway for methanol conversion and green hydrogen production.

4. Conclusion

In summary, we have engineered a bifunctional Pt/SEAC catalyst integrated with a PMo₁₂–SiW₁₂ system for efficient methanol-to-hydrogen conversion. The catalyst demonstrates a mass activity of 829.6 mA mg_Pt⁻¹ for methanol oxidation and a hydrogen evolution rate of 1862.50 mmol h⁻¹ mg_Pt⁻¹, outperforming commercial Pt/C catalysts. The coupled system operates at 0.89 V under a current density of 1 A cm⁻² and maintains stable performance at 100 mA cm⁻² for over 10 hours. The energy consumption can be reduced to as low as 1.73 kW h Nm⁻³ H₂, representing only 38.02% of that required for traditional water electrolysis. This synergistic PMo₁₂–SiW₁₂ coupled system effectively addresses issues related to catalyst detachment inherent in conventional systems and can be adapted for use with other organic hydrogen carriers, thereby offering an energy-efficient and cost-effective approach for sustainable hydrogen production.

Author contributions

W. L. conceived and designed the project. X. H. mainly conducted experiments and analyzed the results. W. L. and X. H. drafted and revised the manuscript. W. X., G. X., and W. W. provided experimental assistance and helped collect the data. X. B., B. Z., and J. S. helped with experimental characterization. All authors contributed to the discussion and revision of the paper.

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, including comprehensive characterizations, electrochemical performance tests and theoretical calculations of the catalysts, is available. See DOI: https://doi.org/10.1039/d5gc06993a.

Further inquiries can be directed to the corresponding author.

Acknowledgements

The authors acknowledge financial support from the National Natural Science Foundation of China (22278446), the Science and Technology Innovation Program of Hunan Province (2024RC1010), and the National Natural Science Foundation of China (U24A20490). This work was supported in part by the High Performance Computing Center of Central South University.

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