An aqueous Zn-polyoxometalate battery for decoupled hydrogen production from alkaline water electrolysis

Abstract

Decoupled water electrolysis is a feasible technique for the large-scale production of high-purity H₂, in which heteropolyacids are attractive electron-coupled proton buffers. An aqueous Zn-phosphomolybdic acid battery is designed for the simultaneous generation of reduced phosphomolybdic acid and electricity. The reported Zn-phosphomolybdic acid battery possesses an open circuit voltage of ∼1.50 V, and the coulombic efficiency of the Zn-phosphomolybdic acid battery can remain at a high level (>90%). Furthermore, the reduced phosphomolybdic acid can be used as an active electron-coupled proton buffer for efficient decoupled hydrogen production from alkaline water electrolysis in the presence of highly dispersed ultrafine Pt nanoparticles (∼2.0 nm) on nickel–cobalt hydroxide nanosheets at a low Pt loading (0.4 mg cm⁻²) as a free-standing electrode.

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Introduction

Decoupled water electrolysis is a feasible technique for the large-scale production of high-purity H₂, in which H₂ and O₂ are produced at separate times. In reported systems, electron-coupled proton buffers (ECPBs) act as reversible electron and proton donors/acceptors with redox couples that can be energetically intermediate between the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).^1–9 Heteropolyacids (such as phosphomolybdic acid, silicotungstic acid and phosphotungstic acid) are attractive ECPBs, because they have excellent solubilities and multi-electron redox abilities in water.^1–4 However, O₂ is generally responsible for the major energy loss in these decoupled water electrolysis systems. Therefore, it is necessary to develop value-added reactions to supplant the oxygen evolution reaction for water electrolysis in these systems.¹⁰

Aqueous metal-ion batteries have received extensive attention due to their advantages of low cost, good safety and sustainability.¹¹ Among various aqueous batteries, aqueous zinc-ion batteries hold particular promise since the zinc anode displays excellent stability in water, low redox potential (E (Zn²⁺/Zn) = −0.76 V vs. standard hydrogen electrode (RHE), or E (Zn(OH)₂/Zn) = −1.25 V vs. RHE), and high theoretical capacity (820 mA h g⁻¹).¹² Transition metal compounds (such as manganese,^13,14 vanadium^15,16 and lead¹⁷), halogens^18,19 and organic compounds (such as polyaniline,²⁰ polyarylimide²¹ and quinone²²) have been reported as the cathodes for aqueous zinc-ion batteries. As mentioned above, heteropolyacids have excellent solubility and multi-electron redox ability in water, which can let electrodes be free from the limitation of work surfaces and help improve the capacity densities for aqueous zinc-ion batteries. Therefore, they may be attractive materials as candidates for the cathodes of aqueous zinc-ion batteries.

Herein, we report an aqueous Zn-phosphomolybdic acid (H₃PMo₁₂O₄₀) battery, in which the oxygen evolution reaction is replaced by the simultaneous generation of reduced phosphomolybdic acid and electricity. Furthermore, the reduced phosphomolybdic acid can be used as an active ECPB for decoupled hydrogen production from alkaline water electrolysis in the presence of a free-standing electrocatalyst with a low Pt loading.

Results and discussion

Fig. 1 shows a schematic illustration of the as-proposed aqueous Zn-phosphomolybdic acid battery. The Zn-phosphomolybdic acid battery is mainly made up of a carbon cloth cathode, Zn anode, bipolar membrane and electrolytes. In this system, a carbon cloth is used as the cathode with 0.5 M phosphomolybdic acid as the catholyte, a Zn plate is applied as the anode with 1.5 M NaOH as the anolyte, and the cathode and anode are separated by a bipolar membrane (BPM), in which the cation exchange layer (CEL) and the anion exchange layer (AEL) of the BPM are facing the catholyte and anolyte, respectively. It is noted that phosphomolybdic acid possesses excellent redox ability at an average potential of 0.3 V vs. RHE and the potential is −1.25 V vs. RHE for the Zn oxidation reaction, thus resulting in a theoretical output voltage of 1.55 V for the aqueous Zn-phosphomolybdic acid battery.^1,4,12 Fig. S1a† shows the photograph of a typical aqueous Zn-phosphomolybdic acid battery. And as shown in Fig. S2,† the reported Zn-phosphomolybdic acid battery possesses an open circuit voltage of about 1.50 V with a peak power density of 30.05 mW cm⁻², and the open circuit voltage can remain at ∼1.48 V after 6 h, indicating the good stability of the reported cell device. Fig. S1b† shows that a clock can be driven by the reported Zn-phosphomolybdic acid battery, further demonstrating the potential application for the cell device.

Fig. 1 Schematic illustration of the aqueous Zn-phosphomolybdic acid battery.

Chronopotentiometric potential-time curves at different discharging current densities for the aqueous Zn-phosphomolybdic acid battery are shown in Fig. 2a. The cell voltage of the reported battery decreases slowly with the increase of the discharging current density (i.e., 0.99 V @ 10 mA cm⁻², 0.86 V @ 20 mA cm⁻² and 0.75 V @ 30 mA cm⁻²). Furthermore, the specific capacities are 755, 765 and 740 mA h g⁻¹ at the discharging current density of 10, 20 and 30 mA cm⁻² for the zinc anode, respectively, when normalized to the mass of the consumed zinc plate (see Fig. 2b). The results indicate that the coulombic efficiency for the Zn-phosphomolybdic acid battery is at a high level (>90%). As shown in Fig. S3,† there are no obvious changes in the discharging current density and open circuit voltage of the reported battery after discharging at 10, 20 and 30 mA cm⁻², in which the Zn anode (including the anolyte) is refreshed and the phosphomolybdic acid is oxidized at 50 mA cm⁻² per 6 h, further confirming the good stability of the reported cell device.

Fig. 2 (a) Chronopotentiometric potential-time and (b) chronopotentiometric potential-specific capacity curves at different discharging current densities. Phosphomolybdic acid was pre-reduced for 2 h at 20 mA cm⁻².

Phosphomolybdic acid, as an important part of the reported aqueous battery, has been identified by IR and ³¹P NMR techniques after the discharging test. As shown in Fig. S4a and b,† the characteristic peaks of H₃PMo₁₂O₄₀ in the IR spectra are similar before and after the test. And the ³¹P NMR chemical shift of H₃PMo₁₂O₄₀ (Fig. S5a and b†) has no distinct change in its position. Moreover, the polyoxometalate cluster (PMo₁₂O₄₀³⁻, m/z = 607.5) can be found in the mass spectrum (see Fig. S6a and b†). Therefore, the original Keggin structure is retained for phosphomolybdic acid. These results suggest that the present battery exhibits good stability in the simultaneous generation of reduced phosphomolybdic acid and electricity.

As mentioned above, the reduced phosphomolybdic acid is an active ECPB for decoupled hydrogen production from water electrolysis.^1,4 During the reoxidation of the reduced phosphomolybdic into phosphomolybdic acid, H₂O is reduced to give H₂ that contains no O₂. However, given the adequate reversibility of the heteropolyacids only at low pH values, the reported ECPB systems are implemented in acid aqueous solutions. This requirement limits the optimized design of HER electrocatalysts. A bipolar membrane-based decoupled electrolysis system has been constructed for alkaline water in our previous work.⁴ Therefore, highly dispersed ultrafine Pt nanoparticles on a nickel-cobalt layered double hydroxide nanoarray is prepared by an in situ redox reaction without any external agent and is used as a free-standing electrocatalyst for the HER in alkaline water.

Transition metal hydroxide nanosheets are first deposited on porous nickel foam (sample Ni/Co–Ni foam) in nitrate aqueous solutions by an electrodeposition method.^23–25 After the deposition, as shown in Fig. 3a, the silver-gray nickel foam changes to green for sample Ni/Co–Ni foam. The loading of Pt nanoparticles on sample Ni/Co–Ni foam is performed by soaking the Ni/Co–Ni foam electrode in Na₂PtCl₆ aqueous solution at room temperature, in which a redox reaction occurs between transition metal ions in the nickel–cobalt layered double hydroxide and PtCl₆²⁻ species.^26,27 As a result, a black porous electrode (sample Pt–Ni/Co–Ni foam) is obtained (see Fig. 3a). Fig. 3b shows the XRD patterns of the as-prepared samples. It is noted that only Ni foam peaks are observed in the XRD pattern. This may be ascribed to the very thin nickel–cobalt hydroxide nanosheets and highly dispersed ultrafine Pt nanoparticles on the Ni substrate. Moreover, the morphologies of the as-prepared porous electrodes are obtained by the SEM technique. As shown in Fig. 3c, well-aligned nickel–cobalt layered double hydroxide nanosheet arrays are deposited on Ni foam for sample Ni/Co–Ni foam. The nanoarray structure can also be observed from the SEM image of sample Pt–Ni/Co–Ni foam in Fig. 3d, which suggests that the transition metal double hydroxide nanosheet arrays are well-maintained after loading of Pt nanoparticles. The TEM technique is introduced to further investigate the morphology of sample Pt–Ni/Co–Ni foam. The TEM image (see Fig. 4a and b) confirms the double hydroxide nanosheet arrays with highly dispersed Pt nanoparticles. Moreover, the high-resolution TEM image (see Fig. 4c) shows clear lattice fringes for the as-prepared sample. Note that the d = 0.226 nm of the lattice fringe matches that of the (111) crystallographic plane of Pt (PDF # 04-0802). As shown in Fig. 4d, the average particle diameter of Pt nanoparticles is ∼2.0 nm for the as-prepared electrode. Furthermore, the content of metallic Pt measured by the ICP-AES technique is ∼0.4 mg cm⁻² for sample Pt–Ni/Co–Ni foam.

Fig. 3 (a) Photographs, (b) XRD patterns and (c and d) SEM images of the as-prepared porous electrodes.

Fig. 4 (a and b) TEM images, (c) high-resolution TEM image and (d) size distribution of Pt nanoparticles for sample Pt–Ni/Co–Ni foam.

The electrocatalytic activity of the free-standing Pt based electrocatalyst for the HER from water electrolysis is first evaluated by using a classical three-electrode in 1 M NaOH aqueous solution. Polarization curves from LSV tests display the geometric current density plotted against the potential with respect to the RHE for sample Pt–Ni/Co–Ni foam (Fig. 5a), as well as those of commercial Pt mesh, Ni foam and sample Ni/Co–Ni foam for the purpose of comparison. The polarization curves reveal that sample Pt–Ni/Co–Ni foam exhibits a superior current density to the other samples in NaOH aqueous solution. The obtained electrode is able to provide a current density of 100 mA cm⁻² at an overpotential of 105 mV, which is extremely lower than the values of commercial Pt mesh (315 mV), Ni foam (370 mV) and sample Ni/Co–Ni foam (325 mV). Further, sample Pt–Ni/Co–Ni foam is an active HER electrode without additional binders and extra substrates, which can completely vie with the commercial Pt/C catalyst (sample Pt/C–Ni foam, see Fig. 5a). A long-term electrocatalytic process has been performed at a current density of 50 mA cm⁻² to evaluate the catalytic durability of sample Pt–Ni/Co–Ni foam. As shown in Fig. 5b, there is no obvious change in the potential curve of the HER for sample Pt–Ni/Co–Ni foam over a period of 20 h. Moreover, the morphology of sample Pt–Ni/Co–Ni foam shows no significant variation after the durability test (see Fig. S7a†). The results suggest that sample Pt–Ni/Co–Ni foam is an efficient porous electrode with an outstanding stability for the HER.

Fig. 5 (a) Polarization curves of the as-prepared electrodes in 1 M NaOH aqueous solution, (b) chronopotentiometric potential-time curves for sample Pt–Ni/Co–Ni foam, (c) electrolytic voltages for alkaline water electrolysis in a two-electrode configuration and (d) electrolytic voltage–time curves for decoupled alkaline water electrolysis with phosphomolybdic acid (0.5 M) as an electron-coupled proton buffer.

Fig. 5c shows the electrolytic voltages for alkaline water electrolysis in a two-electrode configuration with different electrodes. It is found that the electrolytic voltage can be kept at a relatively lower value, when sample Pt–Ni/Co–Ni foam with a low Pt loading and noble-metal-free NiFe-layered double hydroxide on Ni foam (prepared by a reported hydrothermal method,²⁸ denoted as samples NiFe LDH) replace the Pt mesh as the free-standing HER and OER electrodes, respectively. As shown in Fig. S8,† a bipolar membrane based decoupled electrolysis system can also be constructed for alkaline water with phosphomolybdic acid as an electron-coupled proton buffer.⁴ The decoupled electrolysis system exhibits high efficiency and good stability for multiple oxygen and hydrogen evolution cycles (see Fig. 5d). Moreover, the evolving O₂ and H₂ gases have been measured by gas chromatography headspace analysis for the decoupled OER and HER. The volume contents of the O₂ and H₂ gases are 1.3% and 2.5%, respectively, indicating that the evolutions of O₂ and H₂ proceed in a near-stoichiometric ratio (1 : 2) for the reported electrolysis system.

After establishing the feasibility of Zn-phosphomolybdic acid battery discharging for the generation of reduced phosphomolybdic acid and electricity, decoupled hydrogen production from alkaline water electrolysis with the oxidation of the reduced phosphomolybdic acid in a two-electrode configuration was further investigated in the presence of the as-prepared porous electrode as the HER catalyst. Fig. 6 shows the recycles of Zn-phosphomolybdic acid battery discharging and decoupled hydrogen production from alkaline water electrolysis at 20 mA cm⁻². During an operation time of 60 h, the average discharge voltage of the reported battery is ∼0.85 V for the simultaneous generation of reduced phosphomolybdic acid and electricity, and the electrolytic voltage can be kept at a low value (∼1.0 V) for the HER over the free-standing Pt based electrode from alkaline water electrolysis. The results of IR (Fig. S4c†), ³¹P NMR (Fig. S5c†) and mass spectra (Fig. S6c†) suggest that no detectable change of the Keggin-typed structure has been observed for H₃PMo₁₂O₄₀. And the nanoarray structure can also be observed from the SEM image of sample Pt–Ni/Co–Ni foam in Fig. S7b† after the durability test. Therefore, reduced phosphomolybdic acid can be obtained with electricity during the discharging processes of the Zn-phosphomolybdic acid battery, and decoupled hydrogen production from alkaline water electrolysis is implemented by reoxidizing reduced phosphomolybdic acid into phosphomolybdic acid.

Fig. 6 Recycles of Zn-phosphomolybdic acid battery discharging and decoupled hydrogen production from alkaline water electrolysis at 20 mA cm⁻². Phosphomolybdic acid was pre-reduced for 2 h.

Conclusions

An aqueous Zn-polyoxometalate battery with an open circuit voltage of about 1.50 V is constructed for decoupled water electrolysis, in which the oxygen evolution reaction is replaced by the simultaneous generation of reduced phosphomolybdic acid and electricity. The coulombic efficiency of the Zn-phosphomolybdic acid battery is at a high level (>90%), and the present battery exhibits good stability in the simultaneous generation of reduced phosphomolybdic acid and electricity. Furthermore, efficient decoupled hydrogen production from alkaline water electrolysis with good stability can be implemented by reoxidizing reduced phosphomolybdic acid into phosphomolybdic acid. For the decoupled hydrogen production, nickel–cobalt layered double hydroxide loading highly dispersed ultrafine Pt nanoparticles (∼2.0 nm) at a low Pt loading (0.4 mg cm⁻²) is an efficient and stable free-standing HER electrode, which can completely vie with the commercial Pt/C catalyst.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22275185 and 52073286), the Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZR132 and 2021ZZ115), and the Science and Technology Service Network Initiative from the Chinese Academy of Science (STS2021T3046).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details for the preparation of H₃PMo₁₂O₄₀, loading of Pt nanoparticles on nickel–cobalt layered double hydroxide nanosheets, assembly of a Zn-phosphomolybdic acid battery and characterization; photographs of the aqueous Zn-phosphomolybdic acid battery and a clock driven by the aqueous Zn-phosphomolybdic acid battery; open circuit voltage plots, polarization curve and power density curve of the aqueous Zn-phosphomolybdic acid battery; polarization curves and open circuit voltage plots of the aqueous Zn-phosphomolybdic acid battery at various times; IR spectra, ³¹P NMR and mass spectra of H₃PMo₁₂O₄₀ before and after the tests; SEM images of sample Pt–Ni/Co–Ni foam after the durability test; schematic illustration of phosphomolybdic acid-mediated alkaline water electrolysis with the decoupled OER and HER. See DOI: https://doi.org/10.1039/d4se00331d

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