Recent advances in polyoxometalate-based materials and their derivatives for electrocatalysis and energy storage

Abstract

The development and implementation of sustainable clean energy have attracted more attention in response to the urgent requirements of environmental pollution and the energy crisis caused by fossil fuels. Consequently, there is a pressing demand for the advancement and implementation of diverse energy conversion and storage technologies to facilitate the generation and utilization of clean energy sources. With the characteristics of reversible redox properties, high thermodynamic and chemical stability, controllable size, and precise structure, polyoxometalates (POMs) have broad application prospects in electrocatalysis and energy storage. Herein, the significant roles of POMs in electrocatalysis and energy storage have been highlighted. Then, the recent advances of POM-based materials and their derivatives in water-splitting, CO₂ reduction reactions (CO₂RR), the N₂ reduction reaction (NRR), supercapacitors (SCs), and rechargeable batteries are systematically summarized. Furthermore, this review discusses the relationship between the catalyst structure and catalytic reaction activity by comparing different electrocatalytic or energy storage systems. Finally, the conclusions and outlooks of POM-based materials and their derivatives in electrocatalysis and energy storage are discussed to promote new progress.

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1. Introduction

The excessive consumption of fossil fuels has led to a series of problems such as environmental pollution, climate change, and the global energy crisis, making the exploration of environmentally renewable green energy and the development of clean new energy the main goals of the energy strategy.¹ While renewable energy sources such as solar, wind, nuclear, and hydrogen have been extensively explored, their geographical limitations and intermittent nature pose significant obstacles to their application and sustainable development.^2,3 If these intermittent renewable energy sources could be converted and stored, this would greatly facilitate their widespread use and enable sustainable energy utilization.

Within this context, researchers have proposed a strategy to achieve sustainable energy utilization through the mutual conversion of electrical energy and chemical energy. Electrical energy can be converted into chemical energy through energy conversion technologies, such as water-splitting to hydrogen,⁴ CO₂ reduction reactions (CO₂RRs) to chemical fuels,⁵ N₂ reduction reactions (NRR) to ammonia fuel,⁶ and so on. However, energy storage systems can realize energy conversion from stored chemical energy to electrical energy, such as supercapacitors (SCs),⁷ lithium-ion batteries (LIBs),⁸ metal–air batteries,⁹ other rechargeable batteries,¹⁰etc. In both energy conversion technologies and energy storage systems, a reliable means to improve the efficiency of energy conversion is to develop high-activity and low-cost electrocatalysts or electrode materials that reduce the energy barrier.¹¹

Polyoxometalates (POMs) are a type of metal–oxygen cluster compound, which contain abundant metal sources (Mo, W, V, etc.).¹² The basic building unit of POMs is MO_x (x = 4, 5, 6), in which M is commonly a high-valent transition metal element such as W, Mo, V, Nb, Ta, etc.¹³ These transition metal elements can be substituted by metal ions from almost all regions of the periodic table, including the d-, f-, ds-, and p-block. Additionally, the central atoms in POM anions can be substituted by various heteroatoms, including a wide range of metal and non-metal elements, such as Si, P, B, Ge, S, Se, Ga, etc. Therefore, the elemental composition, skeleton structure, valence state, and atomic number of POMs can be fine-tuned and designed,¹⁴ which can be ideal candidates for non-precious metal-based electrocatalysts and electrode materials.

Recently, researchers have reviewed the prospects of POM-based materials and their derivatives in electrocatalytic and energy storage applications. Lan et al.¹⁵ summarized advances in POM-based materials for water-splitting, CO₂ reduction, rechargeable batteries, supercapacitors, and dye-sensitized solar cells. Afterward, Lan et al.¹⁶ outlined the progress of POM-based photo/electrocatalysts for application in the HER, OER, and CO₂RR. Wei et al.¹⁷ summarized a comprehensive overview of POM-contained and POM-derived electrocatalysts for the HER. Song et al.¹⁸ reviewed the synthesis, structure modulation, and HER/OER performance of POM/MOF composites and their derivatives. Li et al.¹⁹ reviewed POM-based electrocatalysts for electrocatalytic CO₂RRs and highlighted the advantages of POMs in CO₂RRs. Zang et al.²⁰ summarized POM-based catalysts in electro/photocatalytic CO₂RRs and focused on the relationship between the structure and catalytic activity. Liu and Streb²¹ highlighted the application of POM-SAC in energy-relevant applications, including water-splitting, CO_x, and N₂ activation. Song et al.²² discussed the prepared methods of POM/nanocarbon composite materials and applied them as electrocatalysts or electrodes for LIBs, SCs, and sensors. Pang et al.²³ highlighted progress on POM-based materials in fuel cells, Li- and Na-ion batteries, and redox flow batteries. Han et al.²⁴ summarized recent research on POM derivatives for various rechargeable batteries. Zhao et al.²⁵ reviewed the advances of POM composite and POM-based composite materials in the fields of electrochemical detection, electrocatalysis, and energy. Dubal et al.²⁶ provided a review of POM-generated materials for applications in energy devices (SCs, batteries, etc.) and energy catalysts. Fabre et al.²⁷ reviewed the application of POM-functionalized (photo)electrodes in the HER, OER, and CO₂RR since the mid-2010s. It should be pointed out that POM-based materials have only been discussed in terms of electrocatalytic or energy storage. Some articles provide a comprehensive overview of POM-based materials, but a detailed overview has not received special attention in the above review. In recent years, POM-based materials and their derivatives have made significant advances in innovative preparation methods and applications. Therefore, it is essential to summarize the recent progress of POMs and their derivatives in electrocatalysis and energy storage.

In this review, the recent research progress on POM-based materials and their derivatives in electrocatalysis and energy storage applications are provided. The focus is primarily on the application of POMs in various electrocatalytic processes such as water splitting, the CO₂RR, the NRR, and energy storage including SCs and rechargeable batteries. The advantages of POMs in factors affecting catalytic performance are discussed. Finally, the challenges and outlooks of POM-based materials in electrocatalysis and energy storage are briefly discussed, and we look forward to the development of efficient and controllable POM-based electrocatalytic and electrode materials.

2. Advantages of POMs in electrocatalytic and energy-related devices

In 1826, Berzelius’ research group first discovered hetero-polyacid compound (NH₄)₃[PMo₁₂O₄₀]·nH₂O.²⁸ However, the structure of POMs could not be determined for a long time, leading to stagnation in the development of POM chemistry. It was not until 1934 that Keggin determined the crystallographic structure of H₃[PW₁₂O₄₀]·29H₂O using X-ray diffraction methods.²⁹ With the advancement of instrument technology, especially single-crystal X-ray crystallography, researchers have discovered hundreds of new POM structures.

POMs have excellent physical and chemical properties including solubility, thermal/chemical stability, acidity, reversible redox ability, high negative charges, electron/proton storage, transport capacities, etc.^16,30 Most importantly, these corresponding properties can be tuned by changing the structural dimensions, composition elements, and organic bridging components.¹⁶ Thus, the unique properties of POMs lead them to be used in applications of catalysis,^31,32 energy,^15,33 material,^34–36 sensors,^37,38 electronic,³⁹ biomedicine,⁴⁰ radionuclide capture,⁴¹ and so on. In addition, significant progress in the innovation of synthesis methods on POM-based materials and POM-derived materials has been achieved, such as an ion-exchanged method,³¹ oxidative polymerization method,⁴² supramolecule-confinement pyrolysis (SCP) strategy,⁴³ soft-template approach,⁴⁴ direct solid reaction,⁴⁵ encapsulation strategy,⁴⁶ sol–gel method,⁴⁷ magnesiothermic reduction approach,⁴⁸ electrochemical etching technology,⁴⁹ charge-directed self-assembly method,⁵⁰ solid-phase hot-pressing method,⁵¹ inkjet printing technology,⁵² interfacial engineering strategy,⁵³ single-molecule confinement strategy,⁵⁴ and so on.

2.1. Advantages of POM molecules in electrocatalytic and energy-related devices

The main characteristics of POMs are as follows (Scheme 1): (i) clear composition and diverse structures. The composition and structure of POMs are clear, which is helpful to identify the catalytic active sites and reveal the reaction mechanism.⁵⁵ A single POM molecule can contain 6 to 368 metal ions, and most elements from the periodic table can be assembled into polyoxoanion frameworks. More than 70 different elements can serve as heteroatoms, including many electrocatalytic active elements, which are conducive to regulating the electrocatalytic performance of POM-based electrocatalysts.⁵⁶ (ii) Adjustable molecular volume. The molecular volume of POMs ranges from a few nanometers to over ten nanometers, which is beneficial for combining POMs with other porous materials, such as MOFs, COFs, etc. The construction of POMOFs improves the stability of POMs in electrocatalytic and energy-related devices. Meanwhile, POMOFs can combine the advantages of POMs and MOFs, which is a promising platform for constructing electrode materials with high capacitance activity and multifunctional ion channels. (iii) Reversible redox properties. POMs can maintain high structural stability while rapidly gaining or losing electrons, which is conducive to improving the efficiency of multi-electron catalytic reactions.^57,58 (iv) Adjustable valence state. The same polyoxoanion can exist in both oxidized and reduced states, and undergo color changes during electron gain or loss, which is favorable to energy storage devices like lithium-ion batteries and pseudocapacitors where energy storage and release are based on chemical redox reactions.^23,59 (v) The modifiability of the active site. POMs can be applied as flexible scaffolds to assemble POM-based materials through host–guest, hydrogen bonding, electrostatic interaction, or organic modification. Thus, the active sites of POM materials can be regulated according to the characteristics of electrocatalytic and energy-related devices.¹⁶ (vi) Easy derivatization. POMs have oxygen-rich surfaces that are easily activated and modified to construct organic–inorganic hybrid and organic derivative materials, making POMs a promising precursor for the development of non-noble metal-based electrocatalytic materials, supercapacitors, and rechargeable battery electrode materials. The infinite molecular structures of POMs give them unparalleled physical and chemical properties, which have been extensively explored for their potential applications in catalysis, water electrolysis for hydrogen production, the CO₂RR, NRR, supercapacitors, rechargeable batteries, and other fields.

Scheme 1 The main characteristics of POMs.

2.2. Advantages of POMs as precursors for preparing electrocatalysts

The synthesis of electrocatalysts using POMs molecules as precursors has the following advantages: (i) the composition of POMs contains a variety of electrocatalytic active metal elements (such as Mo, W, V, etc.), which can be used as a metal source to improve the redox properties of electrode materials;^17,60 (ii) heteroatoms in POMs can be doped as heterogeneous elements to improve catalytic performance;⁶¹ (iii) POMs have a clear composition and crystal structure, which are easily regulated at the molecular or atomic level.⁶²

3. The research process of POMs for electrocatalytic applications

3.1. Water splitting

As a “zero-carbon” green energy carrier, hydrogen energy has the advantages of high energy density, clean and pollution-free, which is considered to be one of the most promising energy sources to replace traditional fossil energy.^63–65 Electrolyzed water hydrogen production technology with the characteristics of being clean and efficient has become the most promising large-scale hydrogen production method.⁶⁶ Water splitting includes two half-reactions, i.e., the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER).⁶⁷ At present, the high performance of HER and OER catalysts are commercial Pt/C and IrO₂/RuO₂, respectively, but their large-scale application is severely limited due to their low earth abundance, high cost, and poor stability.⁶⁸ While the efficiency of non-noble metal-based electrocatalysts is lower, which leads to the high cost of hydrogen production from electrolytic water. Therefore, the core task of developing water electrolysis technology is to develop efficient and low-cost electrocatalysts.

3.1.1. POM-based materials for water splitting. In the past few decades, POM molecular materials have attracted much attention as electrocatalysts for water splitting. Due to the superior redox ability of POMs, one or multiple electrons can be reversibly provided or accepted during the electrochemical process.⁶⁹ Besides, POMs have many advantages as catalysts for electrocatalysis, as shown in Section 2.1. Thus, POM-based materials are ideal candidates for non-noble metal-based HERs and OERs or bifunctional electrocatalysts.
POM-based HER electrocatalysts. Keita et al.⁷⁰ studied the HER activity of POM-modified electrodes in acidic electrolytes by using different POM molecular catalysts ([H₇P₈W₄₈O₁₈₄]³³⁻, [Co₆(H₂O)₃₀{Co₉Cl₂(OH)₃(H₂O)₉(β-SiW₈O₃₁)₃}]⁵⁻ and [{Co₃(B-β-SiW₉O₃₃(OH))(B-β-SiW₈O₂₉OH)₂}₂]²²⁻). The storage behavior of POMs on protons and electrons can affect the microenvironment of POM-modified electrodes, which significantly affects its HER performance. Afterward, the Cronin group⁷¹ made a significant breakthrough in the POM-based water electrolysis system. It can be observed from Fig. 1 that a water separation system was developed using the electron-coupled proton buffer (ECPB) concept. During the process of water oxidation, phosphomolybdic acid (H₃PMo₁₂O₄₀) in this system acts as a redox mediator to absorb electrons and protons generated, which results in the production of H₂ and O₂ occurring in distinct times and spaces. This method effectively avoids the problems of traditional water electrolysis processes, such as product mixing and electrolytic cell degradation. Under the action of the reduction mediator silicotungstic acid (H₄[SiW₁₂O₄₀]), the pure hydrogen produced by the platinum catalytic system at the equivalent platinum loading is more than 30 times faster than that of the proton exchange membrane electrolyzer.

Fig. 1 Schematic of the ECPB-based approach to water splitting.⁷¹ Copyright 2013, Springer Nature.

Although reduced POMs have been proven to be good HER electrocatalysts, their catalytic performance will be limited due to their small specific surface area and poor conductivity.⁷² To increase the specific surface area and conductivity of POMs, researchers have combined POMs with larger specific surface area materials including metal–organic framework (MOFs),⁷³ graphene,⁷⁴ carbon nanotubes (CNTs),⁷⁵ covalent organic frameworks (COFs),⁷⁶etc. Nohra et al.⁷⁷ first reported a POM-based MOF compound (POMOF) for the HER (Fig. 2a) with a turnover frequency (TOF) of 6.7 s⁻¹ at an overpotential of 200 mV. Lan's group⁷⁸ synthesized NENU-500 and NENU-501, which showed excellent HER performance in acidic electrolytes, as plotted in Fig. 2b. Liu et al.⁷⁹ prepared a new type of P₈W₄₈/rGO nanocomposites by anchoring POM [H₇P₈W₄₈O₁₈₄]³³⁻ (P₈W₄₈) on the 3D structure of reduced graphene oxide (rGO) nanosheets using an electroreduction method (Fig. 2c), which showed excellent HER performance and nearly 100% Faraday efficiency (FE) under acidic conditions. Kortz's group⁸⁰ prepared nanocomposites of POMs (P₂W₁₈, P₅W₃₀, and P₈W₄₈) and rGO through one-step electroreduction synthesis, and studied the relationship between POMs size, shape, composition, charge, and HER activity. Wei's group⁸¹ co-deposited Cu and Anderson-type POM (NiMo₆O₂₄) on TiO₂ arrays to prepare NiMo₆O₂₄@Cu/TNA, and investigated its structure and valence state. It was proven that the POM can regulate and modify the surface morphology of Cu dendrites. In acidic HER, NiMo₆O₂₄ as a Brønsted acid, can improve H⁺ transfer, further adsorb H atoms, make up for the weak H atom adsorption capacity of Cu dendrites, and accelerate the HER rate by combining the strong electron transfer effect of POM molecules.

Fig. 2 (a) Comparison of the HER activity of ε(trim)_4/3, ε₂(trim)₂, and [ε(trim)]_∞.⁷⁷ Copyright 2011, American Chemical Society (ACS). (b) Comparison of the HER activity of the NENU-500, NENU-501, and other POMOF catalysts.⁷⁸ Copyright 2015, American Chemical Society (ACS). (c) Scheme of the synthesis of the P₈W₄₈/rGO nanocomposite.⁷⁹ Copyright 2016, Royal Society of Chemistry (RSC).

POM-based OER electrocatalysts. POM-based OER electrocatalysts also face the problem of poor stability and conductivity of POMs. Thus, the preparation of high-efficient and stable OER electrocatalysts by combining POMs with conductive substrates (such as graphene, CNT, nickel foam, conductive polymers, etc.) has been widely studied.^82,83 The effective electron transfer between POMs and conductive substrates is crucial for the OER process of multi-electron transfer. POMs modified by highly redox-active metals (Ru and Co) are the most promising catalysts for OER electrocatalysis.⁸⁴

Howells et al.⁸⁵ reported the first POM molecular electrocatalyst Na₁₄[Ru^III₂Zn₂(H₂O)₂(ZnW₉O₃₄)₂] for the OER. Toma et al.⁸⁶ assembled Ru-containing POM clusters (Ru₄POM: M₁₀[Ru₄(H₂O)₄(μ-O)₄(μ-OH)₂(γ-SiW₁₀O₃₆)₂], M = Cs, Li) and multi-walled carbon nanotubes (MWCNT) into an efficient and stable OER catalyst (POM/MWCNT). As shown in Fig. 3a, the interaction between the MWCNT carrier and POMs increased the specific surface area and accelerated the electron transfer rate to the electrode, which is beneficial for the improvement of OER performance. Song's group⁸⁷ deposited POM ([Co_6.8Ni_1.2W₁₂O₄₂(OH)₄(H₂O)₈]) molecular catalyst microcrystalline on the nickel foam electrode to prepare a NiCo-POM/Ni electrode (Fig. 3b), which has high OER performance and near 100% FE. The NiCo-POM/Ni electrode can work for a long time under alkaline conditions, and the crystal structure of POM remains unchanged (Fig. 3b). Blasco-Ahicart et al.⁸⁸ reported two water-insoluble cobalt-based POM catalysts: Ba[Co-POM]/CP (Ba₈[Co₉(H₂O)₆(OH)₃(HPO₄)₂(PW₉O₃₄)₃]·55H₂O) and Cs[Co-POM]/CP (Cs₁₅K[Co₉(H₂O)₆(OH)₃(HPO₄)₂(PW₉O₃₄)₃]·28H₂O), which have better OER performance and good stability (Fig. 3c), indicating that this type of non-noble metal POMs is expected to replace precious metal catalysts. Cronin's group⁸⁹ synthesized Co₄Mo_xW_y with a sandwich structure via Mo-doped into [Co₄(H₂O)₂(PW₉O₃₄)₂]¹⁰⁻ (Co₄W₁₈). As illustrated in Fig. 3c, water can be oxidized at a lower initial potential by adjusting the amount of Mo doping in POMs and controlling the doping sites of Mo.

Fig. 3 (a) Scheme for a water-splitting electrocatalytic cell with the integrated nanostructured oxygen-evolving anode of POM/MWCNT.⁸⁶ Copyright 2010, Macmillan Publishers. (b) Deposition of Dexter–Silverton polyoxometalate microcrystals on nickel foam electrodes with their OER activity.⁸⁷ Copyright 2017, Wiley-VCH. (c) Molecular structure of the Co-POM cluster and electrochemical behavior of Co-POM/CP electrodes.⁸⁸ Copyright 2017, Springer Nature.

POM-based HER and OER bifunctional electrocatalysts. Wei's group⁹⁰ anchored POM molecules (PW₁₂) on NF-based zinc–cobalt sulfide nanowires to prepare a bifunctional self-supporting electrode (POM@ZnCoS/NF), which displayed high HER and OER activity (Fig. 4a and b). The anchored POM nanoparticles increased the number of electrochemically active sites and electrochemically active surface area. Their work showed that the incorporation of POMs has obvious advantages in the design of bifunctional catalysts with low cost and high performance. In addition, Talib et al.⁹¹ investigated the effect of single transition metal (M₁) loaded on phosphomolybdic acid (PMA) clusters on the HER and OER electrocatalytic performance by density functional theory (DFT) calculation. The volcano diagram calculation showed that V₁/PMA, Ti₁/PMA, Ru₁/PMA, and Pt₁/PMA catalysts have good HER performance (Fig. 4c), while Co₁/PMA and Pt₁/PMA can be used as highly active OER catalysts (Fig. 4d). This study showed that PMA clusters are a promising single-atom support for the HER and OER, providing high-efficiency electrocatalytic activity and low cost.

Fig. 4 (a) The synthesis diagram of POM@ZnCoS NWs. (b) Half-cell LSV curves for the OER and HER of the as-prepared electrodes at a scan rate 10 mV s⁻¹.⁹⁰ Copyright 2021, Wiley-VCH. (c) HER volcano plot and (d) OER volcano plot of the M₁/PMA cluster.⁹¹ Copyright 2021, American Chemical Society (ACS).

3.1.2. POM-derived materials for water splitting. Although POMs molecular catalysts have made some progress in the field of HER and OER, the application of POMs electrocatalysis is still limited by the solubility and poor stability of POMs in the reaction medium.¹⁶ In recent years, the preparation of POM-derived materials via thermal conversion technology using POM molecules as precursors has attracted wide attention. Pyrolysis of POMs (such as carbonization, nitridation and phosphating) in an inert atmosphere is an effective method to enhance the activity and stability of POM-based electrocatalysts.¹⁵ Heteroatom-doped and carbon-coated composites can be prepared via annealing the mixture of heteroatom-doped POMs and carbon sources. During the pyrolysis treatment process, the carbon source is converted into a graphitized carbon layer wrapped on the surface of the catalyst, which can improve the surface area and conductivity of the catalyst. Carbon-coating can protect the metal species from corrosion in a harsh electrolyte environment.⁹² Additionally, doping heteroatoms (such as N, P, S, etc.) can adjust the surface properties of the carbon layer and increase the electron density of the carbon layer.⁹³ Zou's group⁹⁴ used soluble small-sized POMs (ammonium molybdate) as the Mo source and dicyandiamide (DCA) as the small-molecule organic carbon source. After mixing and calcining them under an Ar atmosphere, ultra-small molybdenum carbide nanoparticles (Mo₂C@NC) with N-doped and C-coated were prepared. The nanoparticles have high HER activity in all-pH electrolytes. Similarly, Li's group prepared nano-sized HER electrocatalysts P-W₂C@NC,⁹⁵ MoP/Mo₂C@C,⁹⁶ CoMoP@C,⁹⁷ TM-Mo₂C@C,⁹⁸ and Co₂P/WC@NC⁹⁹ by mixing Mo-based or W-based POMs with DCA and carbonization, as well as preparing POM-derived HER electrocatalysts by pyrolysis of POMs with other carbon materials, such as MoC_x@C,¹⁰⁰ Ni/WC@NC,¹⁰¹ P-Mo₂C@NC,¹⁰² and MoPS/NC.¹⁰³

In addition, Lan's group used phosphomolybdic acid and phosphotungstic acid as POM sources assembling with polypyrrole (PPy) and rGO into nanocomposites, namely PMo₁₂-PPy/rGO and PW₁₂-PPy/rGO. Then, further calcined PMo₁₂-PPy/rGO and PW₁₂-PPy/rGO under a N₂ atmosphere to synthesize N, P co-doped and C-coated carbide catalysts: Mo₂C@NPC/NPRGO¹⁰⁴ (Fig. 5a) and NC@W_xC/NRGO¹⁰⁵ (Fig. 5c). It can be seen from Fig. 5b and d that both of these materials exhibit a HER performance comparable to 20% Pt/C. Wei's group¹⁰⁶ designed and synthesized the N-doped molybdenum phosphide and molybdenum carbide composite catalyst N@MoPC_x using benzene imine-derivatized POMs (Mo₄O(ArPO₃)₂(NAr)₄(μ₂-NAr)₅, Ar = phenyl, Mo₄-CNP) as the precursor under the high-temperature pyrolysis in inert gas (Fig. 5e). Benzenimine and phenyl phosphonic acid ligands as N, C, and P sources can be doped, carbonized and phosphatized at the same time. The organic ligand can in situ generate substrate carbon during the pyrolysis process, which was beneficial to enhance the conductivity and stability of the catalyst. The composite material exhibits excellent HER activity and stability. Afterward, the N-implanted Mo₂C (N@Mo₂C-n/CFP) was prepared by pyrolysis organoimido-derivatized POM that can directly adjust the amounts of N atoms in N@Mo₂C-n/CFP¹⁰⁷ (Fig. 5f). This design of the molecular structure of POMs to achieve in situ doping is another effective method for preparing high-efficiency POM-based composite catalysts. Wang's group¹⁰⁸ through a high-temperature calcination process synthesized the Co/WC@NC electrocatalyst using bimetallic POMs ([Co₉(OH)₃(H₂O)₆(HPO₄)₂(PW₉O₃₄)₃]¹⁶⁻) as soft templates and polyethyleneimine (PEI) as a carbon source. The Co/WC@NC displayed high HER activity and stability in both acidic and alkaline electrolytes mainly attributed to the synergistic effect of Co nanoparticles with WC, and the protective effect of the graphitized carbon layer.

Fig. 5 (a) The synthetic process and (b) HER activity of Mo₂C@NPC/NPRGO.¹⁰⁴ Copyright 2016, Nature. (c) The preparation process and (d) HER activity of NC@W_xC/NRGO.¹⁰⁵ Copyright 2017, Wiley-VCH. (e) The synthetic procedure for the preparation of N@MoPC_x.¹⁰⁶ Copyright 2017, Wiley-VCH. (f) A schematic illustration of the synthesis of N@Mo₂C-n/CFP.¹⁰⁷ Copyright 2017, Wiley-VCH.

Furthermore, POM-derived catalysts can also be prepared by pyrolysis of POM precursors in a reducing atmosphere. Fu's group¹⁰⁹ prepared a P-doped WN/rGO composite catalyst by further phosphating the composite of phosphotungstic acid and graphene after annealing in an NH₃ atmosphere (Fig. 6a and b). Owing to the P doping and the interaction between rGO and WN, the catalyst has high HER performance, as plotted in Fig. 6c and d. In addition, Tan's group¹¹⁰ adopted a molecular-to-cluster strategy to calcine the mixture of POM molecules ([{Co₄(OH)₃PO₄}₄(SiW₉O₃₄)₄]³²⁻, [{Fe₂Co₂(OH)₃PO₄}₄(SiW₉O₃₄)₄]²⁴⁻, and [{FeCo₃(OH)₃PO₄}₄(SiW₉O₃₄)₄]²⁸⁻) and EDA-C60 under a H₂/Ar atmosphere to prepare ultra-small transition metal clusters (TMC: 1-CoW, 2-CoFeW, and 3-CoFeW) with controllable chemical composition (Fig. 6e–g). The catalytic performance of TMC was fine-tuned by surface doping, which can solve the adsorption performance of the reaction intermediate on the surface of the transition metal clusters (Fig. 6h). As shown in Fig. 6i, the prepared 3-CoFeW exhibited high OER performance and stability in alkaline media.

Fig. 6 (a) SEM image and (b) TEM image of P-WN/rGO; (c) LSV curves and (d) Tafel plots for the as-prepared electrodes.¹⁰⁹ Copyright 2015, Wiley-VCH. (e) Schematic illustration of synthesizing the TMCs; (f) SEM image, (g) TEM image, and (h) HAADF-STEM image of 1-CoW; (i) OER curves of catalysts loaded on carbon cloth.¹¹⁰ Copyright 2018, American Chemical Society (ACS).

Although the above methods can effectively improve the activity of POM-derived electrocatalysts, there still exist some problems. For example, POM molecules, especially transition metal-substituted POMs molecules, tend to form multiphase mixed structures after annealing treatment, which may lead to unclear active sites. In addition, high-temperature pyrolysis treatment requires large energy consumption, which is contrary to the concept of green sustainability. At present, the preparation of POM-derived materials using a mild hydrothermal synthesis method can take advantage of the solubility of most POMs and meanwhile can save energy. In addition, researchers have further improved the performance of POM-derived materials through various regulation methods, such as a surface reconstruction strategy, interface engineering strategy, defect construction strategy, and atom doping strategy, single-atom catalysts (SACs) strategy, etc.

Surface reconstruction strategy. During the alkaline electrocatalytic process, the oxides, sulfides, nitrides, and phosphides of the later transition metal (Fe, Co, Ni, etc.) are prone to surface reconstruction, and the corresponding hydroxides or hydroxyl oxides are derived and participate in the catalytic reaction as true active species.¹¹¹ While the existence of high valence states former transition metals (Mo, W, etc.) can accelerate the occurrence of surface reconstruction. Hence, exploring the surface reconstruction of electrocatalysts has important significance for understanding the structure–activity relationship of catalysts and developing high-performance catalysts. Lan's group¹¹² reported a P-NiMo₄N₅@Ni self-supporting electrode with controllable morphology through hydrothermal and phosphating treatment (Fig. 7a). It is worth noting that PMo₁₂ with water solubility and acidity can moderately dissolve a small amount of Ni on the NF surface, which was conducive to the uniform growth of a nano-array structure on NF. Meanwhile, heteroatom P in PMo₁₂ was introduced into the electrode material. After the OER stability test, the surface of the P-NiMo₄N₅@Ni electrode oxidized to OER active species such as metal hydroxides or hydroxyl oxides. As displayed in Fig. 7b, the P-NiMo₄N₅@Ni showed high HER, OER, and water-splitting performance. Zhang et al.⁴⁹ prepared a CoMo_xO₄@CP self-supporting electrode by depositing PMo₁₂ and Co salt on carbon paper (CP). During the electrochemical activation process (Fig. 7c), the electrode surface undergoes continuous structural evolution due to the etching effect of MoO₄²⁻, resulting in a significant increase in OER activity (Fig. 7a and e). Kou et al.¹¹³ synthesized a Co–Mo₂C material as an OER pre-catalyst by anion exchange reaction of Co in Co-MOF with MoO₄²⁻ in POMs. During the OER process, the metal Co phase rapidly reconstructed into γ-CoOOH enriched with Mo, which significantly improved the catalytic activity (Fig. 7f). Through various in situ characterization analyses, the process of electron interaction was summarized as shown in Fig. 7g.

Fig. 7 (a) The synthetic process of P-NiMo₄N₅@Ni composite. (b) Chronoamperometric curves of P-NiMo₄N₅@Ni-1 for the HER, OER, and water splitting.¹¹² Copyright 2018, Elsevier B.V. (c) The synthetic process of CoMo_xO₄@CP (x = 0.3, 1 or 3). (d) In situ Raman spectra with the increase of the chronoamperometry test time. (e) LSV measured before and after chronoamperometry test.⁴⁹ Copyright 2019, Elsevier B.V. (f) The process of phase transition of Co–Mo₂C to Mo-enriching γ-CoOOH. (g) Schematic representation of the electronic coupling between Co and Mo in the post-OER products of Co, Mo₂C, and Co–Mo₂C heterostructure.¹¹³ Copyright 2020, American Chemical Society (ACS).

Interface engineering strategy. Interface engineering has important significance for improving the performance of heterostructure electrocatalysts, especially for electrocatalytic reactions involving various reaction intermediates such as the alkaline HER and OER.¹¹⁴ Heterostructure electrocatalysts with rich interfaces exhibit obvious kinetic acceleration behavior for alkaline HER or OER owing to the synergistic effect of different components, then balance the adsorption or desorption behavior of intermediates at the interface.^115,116 Interface engineering can effectively adjust the electronic structure of active sites by means of electronic interaction,¹¹⁷ interface bonding,¹¹⁸ and lattice strain,¹¹⁹ thus optimizing the binding energy of target intermediates. Furthermore, the confinement effect of the composite interface to maintain the high density of active sites of the catalyst is essential for improving stability.¹²⁰

Heteropolyoxometalates are composed of fixed-proportion heteroatoms and coordination atoms. Thus, applying POMs as precursors is beneficial to the construction of multi-interface catalysts. Sun's group¹²¹ successively prepared bimetallic sulfide and carbon cloth composite electrocatalysts (oxygenated–CoS₂–MoS₂) using the hydrothermal method to sulfurize Anderson-type POMs ((NH₄)₄[Co^IIMo₆O₂₄H₆]·6H₂O). Owing to the exposure of active heterointerfaces, and the facilitated charge transport, the overall water-splitting performance of oxygenated–CoS₂–MoS₂ arrays delivered 10 mA cm⁻² at 1.6 V (Fig. 8a). Lu et al.¹²² constructed multiple interface-regulated CoS₂–MoS₂ heterojunction catalysts (Co₂Mo₁₀S_x/CC) at the atomic level using Evans–Showell-type POM (Co₃[Co₂Mo₁₀O₃₈H₄]) as a molecular pre-assembly platform using a hydrothermal method. Due to the electronic interaction between the active component CoS₂ and MoS₂, the active center reconstructed into the CoMoS site through the interface effect (Fig. 8b). Compared with the single synthesized Co₂Mo₁₀S_x/CC, the Co₂Mo₁₀S_x/CC electrode derived from POMs showed higher over water-splitting activity (Fig. 8c). Experiments combined with theoretical calculations showed that its excellent electrocatalytic activity was attributed to the improved intrinsic catalytic activity of CoMoS sites, the synergistic effect of heterostructures, and the heterogeneous interface with defect-rich sites. Hou et al.¹²³ used a POM-based functional material ([(1,2,4-triazole)₃Co₂·6H₂O][H₃GeMo₁₂O₄₀]·9H₂O) as a bimetallic precursor for hydrothermal sulfidation, and finally generated a CoS₂@MoS₂ nanoarray catalyst with a multiple interface structure. Due to the high crystallinity of POMs and the fixed-proportion of bimetallic sources, the synthesized CoS₂@MoS₂@CC-30 h electrode material displayed high electrocatalytic HER activity in a wide pH range (Fig. 8d and e). Based on the advantages of POMs and multi-interface component catalysts, Zhang et al.⁵³ used an Anderson-type POM (NH₄)₄[FeMo₆O₂₄H₆]·6H₂O (FeMo₆) as a bimetallic precursor to prepare the Fe–Mo–S/Ni₃S₂@NF electrode with a multi-interface using a one-step hydrothermal method (Fig. 8f). DFT calculations showed that HER activity was mainly provided by the Fe₃S₄/Ni₃S₂(003) and Ni₃S₂(003)/Mo₃S₁₃ interfaces (Fig. 8g), while the Fe₃S₄/Ni₃S₂(110) interface provided a high OER performance (Fig. 8h). Thereby, the Fe–Mo–S/Ni₃S₂@NF electrode showed high HER, OER, and overall water-splitting performance, as shown in Fig. 8i.

Fig. 8 (a) Overall water splitting performance through the use of O–CoMoS array as a perspective anode and cathode.¹²¹ Copyright 2018, American Chemical Society (ACS). (b) HRTEM images of Co₅Mo₁₀S_x@CC. (c) Overall water-splitting performance of Co₅Mo₁₀S_x@CC.¹²² Copyright 2020, Elsevier B.V. A comparison of overpotentials in CoS₂@MoS₂@CC-30 h and the reported MoS₂-based electrocatalysts at 10 mA cm⁻² in (d) 0.5 M H₂SO₄ and (e) 1.0 M KOH electrolyte.¹²³ Copyright 2020, Wiley-VCH. (f) HRTEM images with different magnifications of Fe–Mo–S/Ni₃S₂@NF. (g) HER free energy changes on reaction surfaces. (h) Free energy diagrams for the OER process at the Fe₃S₄/Ni₃S₂(110) interface. (i) Overall water-splitting performance of Fe–Mo–S/Ni₃S₂@NF.⁵³ Copyright 2020, Elsevier B.V.

Defect construction strategy. Defects can affect the electronic structure, surface and interface properties, thereby changing the original distribution to adjust the adsorption energy of the intermediate.¹²⁴ In addition, some defects can act as electrocatalytic active sites, such as vacancies, unsaturated coordination sites, and lattice defects to enhance the catalytic activity of the materials.¹²⁵ Thus, the reasonable construction of defects is helpful to adjust and optimize the electrocatalytic performance of POM-based materials. Xie's group¹²⁶ synthesized MoS₂ ultrathin nanosheets with defect-rich structures by using ammonium molybdate as a Mo source with excessive thiourea under hydrothermal conditions (Fig. 9a). The MoS₂ nanosheets with a rich defect structure could expose more edge active sites, which exhibited excellent HER performance, as plotted in Fig. 9b. Lan's group¹²⁷ combined PMo₁₂ that has good water solubility and strong redox properties with GO to prepare MoS₂/N-RGO composite with defects after the sulfurization process (Fig. 9c and d). Due to electrostatic repulsion, PMo₁₂ particles can be uniformly dispersed between GO layers. Meanwhile, GO can be reduced to RGO owing to the strong reducibility of PMo₁₂. The MoS₂/N-RGO composite displayed excellent HER performance under acidic conditions. Tang et al.¹²⁸in situ prepared a defective CoMoS/KB nanocluster catalyst (Fig. 9e) by using small and uniform sizes of reduced POMs ((NH₄)₃PMo₁₂O₄₀) as the template and anchor sites of metal salts, which was beneficial to expose more edge active sites. Therefore, the CoMoS/KB catalyst has excellent activity and stability for the HER. In summary, the construction of surface defects provides a solution to further improve the catalytic activity.

Fig. 9 (a) Structural models and synthetic path diagrams of defect-free and defect-rich structures; (b) LSV curves of various samples as indicated.¹²⁶ Copyright 2013, Wiley-VCH. (c) The schematic preparation process of MoS₂/N-RGO-T nanocomposite; (d) HRTEM of MoS₂/N-RGO-180.¹²⁷ Copyright 2016, Wiley-VCH. (e) Schematic illustration of the synthesis of the CoMoS/KB catalyst.¹²⁸ Copyright 2021, American Chemical Society (ACS).

Doping control strategy. Heteroatom doping can evidently increase the intrinsic conductivity of the catalyst, regulate the electronic structure, and optimize the adsorption and desorption of intermediates on the catalyst surface, which play a key role in improving the electrocatalytic performance.¹²⁹ Heteroatom doping can be roughly divided into non-metal atom doping (such as N, P, S, O, etc.) and metal atom doping (such as Fe, Co, Ni, Zn, etc.). Among them, non-metallic atom doping can enhance the electronic structure at the catalytic active sites to effectively adjust the work function and increase the adsorption of the reactants at a specific position, thereby improving their electrocatalytic activity.¹³⁰ Metal doping can accelerate the electron transfer in different sites through the synergistic effect between metal atoms and other atoms, then reduce the energy barrier of the reaction intermediate.¹³¹

Heteropolyoxometalates have inherent heteroatoms, which can provide various doped heteroatoms (such as P, W, Mo, Ni, Co, Fe, etc.) for the final electrocatalyst. These heteroatoms can enhance the electrochemical performance through empty orbits or lone pair electrons.¹³² Cronin's group⁸⁹ synthesized Co₄Mo_xW_y containing a Mo and W mixed sandwich structure through doping Mo into Co₄W₁₈, as shown in Fig. 10a. Adjusting the amount of Mo doping in POMs could control the doping sites of Mo, which oxidized water at a lower onset potential without degradation of the catalyst. The OER overpotential of 3b[Cs] was 188 mV lower than that of the pure tungsten skeleton (Fig. 10b). Their work provided a strategy for preparing molecular heterogeneous catalysts by using Mo-doped POM clusters to improve water oxidation performance, and also opens up new ideas for the development of POM-based electrolyzed water catalysts via similar design principles. Zhang et al.⁶¹ synthesized a Fe–S–NiMoO₄/MoO₃@NF self-supporting electrode using FeMo₆ as a precursor through a one-step solvothermal method, as illustrated in Fig. 10c. The FeMo₆ precursor could accurately control the doping ratio of Fe in Fe–S–NiMoO₄/MoO₃@NF. The in situ Raman experiments (Fig. 10d–f) on the optimal Fe–S–NiMoO₄/MoO₃@NF sample in the electrolyte temperature range of 25–80 °C showed that the improvement of OER activity was due to the formation of more OER active phases, and finally achieved a quasi-industrial OER performance (Fig. 10g).

Fig. 10 (a) Crystallographic data for the six compounds of Co₄Mo_xW_y; (b) water oxidation activity of 1b[Cs]–3b[Cs] compounds.⁸⁹ Copyright 2018, Springer Nature. (c) Schematic map of the synthesis of Fe–S–NiMoO₄/MoO₃@NF; In situ Raman spectra of Fe–S–NiMoO₄/MoO₃@NF during the OER process at different electrolysis temperatures: (d) 25 °C, (e) 50 °C, and (f) 80 °C; (g) Comparison of the requirement overpotential of the prepared electrodes at 25–80 °C at 500 mA cm⁻².⁶¹ Copyright 2021, Elsevier B.V.

In addition to single-atom doping, multi-atom doping is also an effective way to enhance catalytic performance. Wei's group¹³³ prepared 1T-MoS₂ co-doped with different TMs and O atoms, namely NiO@1T-MoS₂, CoO@1T-MoS₂, and FeO@1T-MoS₂ using Anderson-type POMs that contain different heteroatoms as precursors for electrochemical HERs, as shown in Fig. 11a. It can be observed from the HAADF-STEM image in Fig. 11b that NiO@1T-MoS₂ showed a hexagonal intensity change, which was consistent with the single-layer of 1T-MoS₂ reported by Eda, G. et al.¹³⁴ While the strength of Ni centers and Mo atoms in all regions of the sample have changed (Fig. 11c), indicating that the hexagonal units of NiMo₆ were well retained in the final NiO@1T-MoS₂ nanosheets, which avoided the agglomeration problem that may be caused by direct doping of metal atoms. Experimental and theoretical studies have shown that Ni and O atoms were simultaneously doped into ultrathin 1T-MoS₂ nanosheets, which could effectively regulate the electronic structure, and also provided more catalytic active sites, reducing the energy barrier required for hydrolysis. Zhang et al.¹³⁵ prepared P and Mo co-doped Ni₃S₂ (P–Mo–Ni₃S₂@NF) using PMo₁₂ as the dopants through a theory-guided strategy (Fig. 11d). As presented in Fig. 11e, the P–Mo–Ni₃S₂@NF electrode showed a decent performance of urea assisted electrolytic water.

Fig. 11 (a) The preparation process of XO@1T-MoS₂/CFP; (b) aberration-corrected atomic resolution HAADF-STEM image (scale bar: 0.5 nm); (c) intensity profiles along the lines indicated in (b).¹³³ Copyright 2019, Springer Nature. (d) A comparison of E_ads(urea), ε_d, and ICOHP values. (e) A comparison of the UOR and OER activity of P–Mo–Ni₃S₂@NF.¹³⁵ Copyright 2023, Royal Society of Chemistry (RSC).

Single-atom catalyst (SAC) strategy. A single-atom catalyst can be used to stabilize a single independent atom on the carrier, which has the advantage of clear active sites of homogeneous catalysts and recyclable heterogeneous catalysts.¹³⁶ POMs are ideal molecular systems for exploring SAC anchoring,²¹ which can greatly help to avoid the agglomeration of metal parts during the synthesis process.¹⁷ Li's group¹³⁷ used POMs (PtW₆, PtMo₆, PtV₉ and PtSiW₁₁) as Pt-based SAC models, and the prepared {PtW₆}/C has excellent electrocatalytic HER activity. This work clearly explains the structure–activity relationship of the electrocatalytic HER of SAC Pt for the first-time using POMs as a model. Specifically, the catalytic activity of Pt SACs electrocatalysts is affected by both the metal–oxide support and the coordination environment of Pt single atom active sites (Fig. 12a). The coordination environments of Pt single atoms were similar in the models of {PtW₆}, {PtMo₆} and {PtV₉}, and the electron acquisition of the Pt single atom was affected due to the different electron affinity of the metal–oxide support carrier, which led to the different effective potential and catalytic activity of the Pt center. Whereas in the POM molecular models of {PtSiW₁₁} and {PtW₆}, the Pt single atoms exhibited similar metal–oxide supports, the different coordination environment of single atom Pt in the two POM models resulted in different catalytic reaction pathways, thus affecting the final HER activity (Fig. 12b). Zhang's group¹³⁸ fabricated Pt₁–Mo₂C–C catalyst derived Pt-POMs (Fig. 12c), which exhibited high activity and stability in the HER. Talib et al.¹³⁹ investigated the HER, OER, and ORR performance of M₁/PW₁₂O₄₀ single-atom electrocatalysts via DFT calculation. The above studies demonstrated that POM-based SACs display the advantages of performance improvement in electrocatalysis. The following points can be summarised. (i) Due to the high stability and electron-rich properties of POM clusters, the POM-based SACs can effectively adjust the electronic properties of SAC active sites, promote electron transfer between SAC active sites and existing support, which can be the key factor in improving the catalytic efficiency. (ii) The overall catalytic activity of the POM-based SACs can be attributed to the synergy between all the metal sites in the framework. (iii) The interaction between SACs sites and POM clusters is usually stabilized by coordination bonds or covalent coordination bonds, which will prevent the aggregation and leaching of SAC active sites in the catalytic reaction process, thus enhancing the stability. The HER and OER performances of the POM-based materials and POM-derived materials in this review are shown in Table 1 and Table 2, respectively.

Fig. 12 (a) Schematic illustration of Pt-based SAC models and HER performance. (b) Different hydrogen evolution sites responsible for the {PtW₆} and {PtSiW₁₁}.¹³⁷ Copyright 2021, American Chemical Society (ACS). (c) Schematic illustration of the formation of Pt₁–Mo₂C–C.¹³⁸ Copyright 2021, ELSEVIER B.V.

Table 1 Comparison of the POM-based and POM-derived materials in this review for HER performance

Electrocatalysts	Electrolyte	j (mA cm^−2)	η (mV)	Tafel slope (mV dec^−1)	Ref.
NENU-500	0.5 M H2SO4	10	237	96	78
NENU-501	0.5 M H2SO4	10	392	137	78
P8W48/rGO	0.5 M H2SO4	10	28	38	79
P2W18@rGF_ox	0.5 M H2SO4	10	35	37	80
P5W30@rGF_ox	0.5 M H2SO4	10	33	33	80
P8W48@rGF_ox	0.5 M H2SO4	10	44	41	80
NiMo6O24@Cu/TNA	0.5 M H2SO4	10	215	89.2	81
POM@ZnCoS/NF	1 M KOH	10	170	53.5	90
Mo2C@NC	0.5 M H2SO4	10	124	60	94
P-W2C@NC	0.5 M H2SO4	10	89	53	95
MoP/Mo2C@C	0.5 M H2SO4	10	89	45	96
CoMoP@C	0.5 M H2SO4	10	41	50	97
Ni–Mo2C@C	0.5 M H2SO4	10	72	65.8	98
Co2P/WC@NC	0.5 M H2SO4	10	91	40	99
MoCx@C	0.5 M H2SO4	10	79	56	100
Ni/WC@NC	0.5 M H2SO4	10	53	43.5	101
P-Mo2C@NC	0.5 M H2SO4	10	109	76	102
MoPS/NC	0.5 M H2SO4	10	120	52	103
Mo2C@NPC/NPRGO	0.5 M H2SO4	10	34	33.6	104
WxC@NC/NRGO	0.5 M H2SO4	10	100	58.4	105
N@MoPCx-800	0.5 M H2SO4	10	108	69.4	106
N@Mo2C-3/CFP	0.5 M H2SO4	10	56	51	107
Co/WC@NC	0.5 M H2SO4	10	129	93	108
Co/WC@NC	1 M KOH	10	142	91	108
P-WN/rGO	0.5 M H2SO4	10	85	54	109
O-CoMoS	1 M KOH	10	97	70	121
CoS2@MoS2@CC-30 h	0.5 M H2SO4	10	65	122	123
Fe–Mo–S/Ni3S2@NF	1 M KOH	10	141	123	53
defect-rich MoS2	0.5 M H2SO4	13	200	50	126
MoS2/N-RGO-180	0.5 M H2SO4	10	56	41.3	127
CoMoS/KB	1 M KOH	10	81	64	128
NiO@1T-MoS2	1 M KOH	10	46	52	133
{PtW6}/C	0.5 M H2SO4	10	22	29.8	137
Pt1–Mo2C–C	1 M KOH	10	155	64	138

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Table 2 Comparison of the POM-based and POM-derived materials in this review for OER performance

Electrocatalysts	Electrolyte	j (mA cm^−2)	η (mV)	Tafel slope (mV dec^−1)	Ref.
NiCo-POM/Ni	0.1 M KOH	10	360	126	87
Cs[Co-POM]	1 M H2SO4	10	466	98	88
Ba[Co-POM]	1 M H2SO4	10	361	97	88
POM@ZnCoS/NF	0.5 M H2SO4	10	200	67.7	90
3-CoFeW	1 M KOH	10	192	36	110
P-NiMo4N5@Ni-1	1 M KOH	100	297	33.6	112
Mo1–CoOOH@CP	1 M KOH	10	274	66	49
Co–Mo2C	1 M KOH	10	190	94.3	113
O-CoMoS	1 M KOH	10	272	45	121
Co5Mo10Sx/CC	1 M KOH	10	153	57.8	122
Fe–Mo–S/Ni3S2@NF	1 M KOH	100	300	98	53
Fe–S–NiMoO4/MoO3@NF	1 M KOH	10	212	41	61

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3.2 CO₂ reduction reaction

Recently, the excessive consumption of fossil fuels has led to a large release of CO₂ into the atmosphere, exacerbating the greenhouse effect and disrupting the natural carbon balance.¹⁴⁰ In this context, there is an urgent demand to explore effective technologies for reducing the concentration of CO₂ in the atmosphere. Considering that CO₂ is an essential C1 resource for various industrial processes and plant growth, the rational conversion of CO₂ into high-value chemicals has emerged as a clean carbon-neutral strategy.¹⁴¹ This approach not only addresses environmental issues caused by CO₂ but also offers a prospective means to meet future sustainable energy development. Structurally, CO₂ molecules are linear and centrally symmetric, characterized by high stability, a large energy gap, and low electron affinity,^142,143 making the activation of CO₂ a challenging process that requires significant energy input and stringent reaction conditions. Due to the environmental compatibility and mild reaction conditions, CO₂ reduction reactions (CO₂RRs) have gained widespread attention, demonstrating the enormous potential for future industrial applications.^5,144 Nevertheless, the unregulated interaction between electrons and protons during the CO₂RR has resulted in the emergence of multiple coupling processes, leading to a diverse range of reaction pathways. Consequently, the CO₂RR often yields numerous distinct products, ultimately resulting in low FE and selectivity.¹⁴⁵ To date, various types of catalyst materials have been developed and applied in the CO₂RR.^146–148 Although these catalysts have shown good CO₂RR activity and selectivity, the sources of catalyst activity and the mechanisms for controlling selectivity are still unclear. A deeper understanding of the reaction mechanism and process of the CO₂RR requires further research. Therefore, constructing well-defined heterogeneous CO₂RR model catalysts is an important approach to elucidating the reaction mechanism of the CO₂RR and designing efficient heterogeneous catalysts for the CO₂RR.

3.2.1. POM-based molecular materials for the CO₂RR. POMs have unique redox properties, excellent electron/proton storage and transport capabilities, and show significant potential in the photocatalytic and electrocatalytic reduction of CO₂.¹⁹ In addition, POMs possess abundant surface oxygen atoms, which exhibit Lewis basicity and can serve as potential active centers for CO₂ molecule activation and coupling.¹⁴⁹ Xu's group¹⁵⁰ demonstrated that metal atom-substituted heteropolyanions ((C₃H₅N₂)₃(C₃H₄N₂)[PMo₁₁CoO₃₈(CO₂)]·4H₂O and (C₃H₅N₂)₄[SiMo₁₁CoO₃₈(CO₂)]·4H₂O) can undergo covalent interactions with CO₂ molecules through X-ray single-crystal diffraction. The bond angle of the covalently bonded CO₂ molecule showed a slight deviation, bending from 180° to 158.7°. This indicates that the metal atoms in the POMs clusters can immobilize and activate CO₂ molecules. Accordingly, POMs possess a well-defined structure and the potential to adsorb and activate CO₂, making them suitable as a structural model for comprehending the crucial stages involved in the molecular-scale CO₂RR reaction process.

The initial investigation into the catalytic activity of POMs in the CO₂RR was studied by Kozik et al.¹⁵¹ As illustrated in Fig. 13a–d, a series of spectroscopic experiments were applied to demonstrate the formation of POM–CO₂ complexes. Kozik et al.¹⁵² prepared the first POMs catalyst, α-[SiW₁₁O₃₉Co]⁶⁻ (SiW₁₁Co), capable of electrochemically catalyzing the CO₂RR. Compared to the CV curve of SiW₁₁Co in dichloromethane (CH₂Cl₂) electrolyte bubbled without CO₂, the CV curve of SiW₁₁Co in CH₂Cl₂ electrolyte with CO₂ exhibited a higher reduction in current signal, suggesting that SiW₁₁Co possesses high CO₂RR activity. The CV curve of α-[P₂W₁₈O₆₂]⁶⁻ (P₂W₁₈) without TM substitution remained largely unchanged in the CH₂Cl₂ electrolyte before and after CO₂ bubbling, indicating that TM-substituted POMs play a key role in the CO₂RR (Fig. 13f). Therefore, the future direction for constructing efficient and highly selective CO₂RR catalysts based on POMs in aqueous electrolytes lies in the creation of stable, multiphase CO₂RR active sites. This work provides design principles and guidance for the development of novel heterogeneous CO₂RR electrocatalysts.

Fig. 13 (a) Visible spectra of α-[SiW₁₁O₃₉Co(OH₂)]⁶⁻, α-[SiW₁₁O₃₉Co(_)]⁶⁻, and α-[SiW₁₁O₃₉Co(CO₂)]⁶⁻ in toluene. (b) Infrared spectrum of α-[SiW₁₁O₃₉Co(CO₂)]⁶⁻ in CCl₄. (c) Difference spectrum between α-[SiW₁₁O₃₉Co(CO₂)]⁶⁻ and α-[SiW₁₁O₃₉Co(_)]⁶⁻. (d) ¹³C NMR spectra of α-[SiW₁₁O₃₉Co(CO₂)]⁶⁻ in toluene at different temperatures.¹⁵¹ Copyright 1998, American Chemical Society (ACS). (e) CV curves of SiW₁₁Co in CH₂Cl₂ before and after bubbling with CO₂ gas. (f) CV curves of P₂W₁₈ in CH₂Cl₂ before and after bubbling with CO₂ gas.¹⁵² Copyright 2002, Springer.

3.2.2. POM-involved materials for the CO₂RR. Designing POM-based heterogeneous electrocatalysts has emerged as a challenging approach for designing CO₂RR catalysts owing to the solubility of various POMs in aqueous solutions. Constructing POM-involved inorganic–organic hybrid materials can significantly reduce the solubility of POMs, and integrate organic molecular catalysts with CO₂RR activity into POMs. Therefore, designing POM-involved inorganic–organic hybrid materials becomes an ideal strategy for constructing novel CO₂RR electrocatalysts.¹⁶ Du et al.¹⁴⁵ prepared a series of POM-carbonyl manganese compounds (abbreviated as POM–MnL) and used them as heterogeneous electrocatalysts for the CO₂RR (Fig. 14a). Among them, SiW₁₂–MnL displayed the most significant electrocatalytic activity, selectively reducing CO₂ to CO. Various characterizations combined with DFT results indicated that the electron transfer process between the POMs and MnL units can suppress the competing HER, enabling efficient CO₂ reduction to CO (Fig. 14b). Furthermore, by changing the type of POMs in the POM–MnL composite catalyst, the electron transfer pathway can be regulated (Fig. 14c), allowing for control of the FE in the CO₂RR. Wang's group¹⁵³ developed a method to incorporate PMo₁₂ into Zn–CoTAPc double molecular layer sandwich nanosheets (MLSs). Various characterizations confirmed that the nanosheet had a thickness of ∼1.2 nm and consisted of two reticular layers of Zn–CoTAPc with interlayered PMo₁₂ anions (Fig. 14d). This unique structure provided highly exposed Co active sites and structural stability against stacking and deactivation. The MLSs showed exceptional catalytic performance in the CO₂RR, and the performance of MLSs was further enhanced with external light irradiation (Fig. 14e and f).

Fig. 14 (a) The CO₂RR reaction mechanism of POM–MnL. (b) Potential energy surfaces for H⁺ and CO₂ addition to the respective active CO₂ and H adducts. (c) The electronic configurations for the different reduction degree states.¹⁴⁵ Copyright 2020, Royal Society of Chemistry (RSC). (d) The synthetic procedure toward Zn–CoTAPc/PMo₁₂ MLSs. (e) FE_CO-potential plots and (f) j_CO and TOF_CO plots of Zn–CoTAPc/PMo₁₂ MLSs.¹⁵³ Copyright 2021, American Chemical Society (ACS).

Furthermore, Lan's group has previously reported various series of POMOF materials as heterogeneous catalysts for electrocatalytic CO₂RRs.^154–156 They conducted a hydrothermal method to prepare a series of stable POM-metalloporphyrin organic framework (PMOF) materials for electrocatalytic CO₂RRs (Fig. 15a).¹⁵⁴ The research findings revealed that the TM in PMOFs plays a crucial role in the electrocatalytic CO₂RR. Among them, Co-modified PMOFs exhibited the highest CO₂RR activity (Fig. 15b). Although Co-PMOF exhibited higher CO₂RR activity than Co-TCPP, the electrocatalytic CO₂RR activity of PMOFs was still derived from the M-TCPP component, and the regulation of different types of POMs in this composite system needs to be further studied. Afterward, they synthesized a series of mixed-valence POM-based MOF composite catalysts (POM@PCN-222(Co), POM = [Co^IIICo^II(H₂O)W₁₁O₃₉]⁷⁻, M = Co, Fe, Mn, and Ni) using a post-modification method¹⁵⁵ (Fig. 15c). The POM@PCN-222(Co) electrocatalyst combines the advantages of POMs and metalloporphyrin single metal sites, exhibiting highly efficient activity for electrocatalytic CO₂ reduction to CO. DFT calculations indicated that POMs could facilitate the directed transfer of electrons from the electrode to the single metal sites, enriching the electron density of the Co center (Fig. 15d), which reduced the reaction energy barrier for the key rate-determining steps in the CO₂RR and enhanced the activity of electrocatalytic CO₂RR. In addition, metalloporphyrins (M-TCPPs) were used to construct a series of POM-based MOF (M-POMOFs: [PMo^V₈Mo^VI₄O₃₅(OH)₅Zn^II₄]₂[Fe^III-TCPP-Cl] Guest; [PMo^V₈Mo^VI₄O₃₅(OH)₅Zn^II₄]₂[M^II-TCPP][H₂O] Guest; M = Zn, Ni, Cu, Co, and Mn) for photo- and electrocatalytic CO₂RR (PCR and ECR) by Lan's group¹⁵⁶ (Fig. 15e). This study aimed to uncover the role of single metal sites and clusters in catalysis. In the ECR (Fig. 15f), the Fe-POMOF catalyst exhibited a superior FE in converting CO₂ to CO. Their research contributed to understanding the relationships between single metal sites and cluster catalysis.

Fig. 15 (a) Schematic illustration of the structures of M-PMOFs. (b) Electrocatalytic performances of M-PMOFs.¹⁵⁴ Copyright 2018, Springer Nature. (c) A schematic diagram of the synthesis steps and reaction pathways of POM@PCN-222(Co). (d) DFT calculations of the CO₂RR in H-POM@PCN-222(Co).¹⁵⁵ Copyright 2021, Wiley-VCH. (e) The crystal structures of M-POMOFs. (f) Electrocatalytic performances of M-POMOFs.¹⁵⁶ Copyright 2022, Science Publishing Group.

In addition, POMs can also be used as reducing agents and stabilizers, which are widely applied in the synthesis of nanomaterials. Guo et al. synthesized a series of silver nanocluster catalysts reduced by electrodeposition with PMo₁₂ (Ag-PMo)¹⁵⁷ or bovine serum albumin (BSA), subsequently modified by SiW₁₂ (AgNC@BSA–SiW).¹⁵⁸ The incorporation of POMs as electron transfer mediators resulted in significantly improved efficiency of the silver nanocluster catalysts in the reduction of CO₂ to CO, surpassing the performance of bulk Ag. Wei's group¹⁵⁹ utilized Mo₈-type POM-modified copper nanocubes to obtain nano-catalysts with abundant Cu–O–Mo interfaces for electrocatalytic CO₂ reduction to produce acetic acid. The Mo₈-modified Cu nano-catalyst can efficiently convert CO₂ to acetic acid with a high FE. DFT calculations revealed that the POMs modification facilitates the formation of abundant Cu–O–Mo active sites, promoting the generation of *CH₃ intermediates and facilitating their coupling with CO₂ molecules, ultimately leading to the production of acetic acid. This work demonstrated the control of the Cu nano-catalyst interface via POM modification, enabling deep reduction of CO₂ and providing guidance for the regulation of CO₂ reduction depth using POMs. The summary of POM-involved materials for electrocatalytic CO₂RR in this review is shown in Table 3.

Table 3 The summary of the POM-involved materials for electrocatalytic CO₂RR in this review

Electrocatalyst	Electrolyte	Potential	Principal product	FE (%)	Ref.
SiW12–MnL/KB	0.5 M KHCO3	−0.72 V vs. RHE	CO	95	145
PW12–MnL/KB	0.5 M KHCO3	−0.72 V vs. RHE	CO	80	145
PMo12–MnL/KB	0.5 M KHCO3	−0.52 V vs. RHE	CO	65	145
MnL/KB	0.5 M KHCO3	−0.72 V vs. RHE	CO	65	145
Zn–CoTAPc/PMo12 MLSs (dark)	0.5 M KHCO3	−0.7 V vs. RHE	CO	96.1	153
Zn–CoTAPc/PMo12 MLSs (light)	0.5 M KHCO3	−0.75 V vs. RHE	CO	96.2	153
Co-PMOF	0.5 M KHCO3	−0.8 V vs. RHE	CO	98.7	154
Fe-PMOF	0.5 M KHCO3	−0.7 V vs. RHE	CO	28.8	154
Ni-PMOF	0.5 M KHCO3	−0.8 V vs. RHE	CO	18.5	154
Zn-PMOF	0.5 M KHCO3	−0.9 V vs. RHE	CO	0.95	154
H-POM@PCN-222(Co)	0.5 M KHCO3	−0.8 V vs. RHE	CO	96.2	155
M-POM@PCN-222(Co)	0.5 M KHCO3	−0.8 V vs. RHE	CO	91.2	155
L-POM@PCN-222(Co)	0.5 M KHCO3	−0.8 V vs. RHE	CO	71.2	155
Fe-POMOF	0.5 M KHCO3	−0.7 V vs. RHE	CO	92.1	156
Mo8@Cu/TNA	NaHCO3	−1.13 V vs. RHE	CH3COOH	48.68	159

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The above research demonstrates the application of a series of POM-involved catalysts in electrocatalytic CO₂RRs. These results indicate that when POMs are combined with organic molecular catalysts possessing CO₂RR activity through electrostatic, covalent, or coordination interactions, the solubility of POM catalysts in the electrolyte can be significantly reduced, enabling the construction of heterogeneous POMs catalysts. These findings provide an explanation for the origin of activity in multi-component, multi-center composite electrocatalysts, thereby offering a deeper understanding of the design of heterogeneous CO₂RR electrocatalysis.

3.3. N₂ reduction reaction

Ammonia (NH₃), as a hydrogen-rich carrier with great potential, is an important next-generation green energy source for fuel cells.¹⁶⁰ However, industrial ammonia synthesis still relies on the traditional Haber–Bosch method, which requires high temperature (400–600 °C) and high pressure (20–40 MPa), accounting for 1–2% of global energy consumption, as well as large CO₂ emissions.^161,162 Therefore, exploring the green environmental protection ammonia synthesis technology with normal temperature and pressure is essential for converting intermittent energy sources (such as solar energy and electricity) into continuous ammonia fuel that can be stored and transported anytime and anywhere.

Electrocatalytic nitrogen reduction reaction (NRR) has low energy consumption that can be carried out at room temperature and pressure, which is the most likely method to replace the traditional Hubble method for the production of NH₃.^163–165 However, the Faraday efficiency of the NRR is still relatively low due to the limitation of thermodynamics and kinetics.¹⁶⁶ In terms of thermodynamics, N₂ molecules are extremely chemically stable under natural conditions and are difficult to activate.¹⁶⁷ From a kinetic point of view, the main challenge of the NRR is the high dissociation energy of nitrogen and the serious side reaction of hydrogen evolution.^163,168 Thus, it is necessary to demand the development of highly active electrocatalysts that can effectively reduce the activation energy of N₂ reduction and inhibit the side reaction of hydrogen evolution.

3.3.1. POM-based materials for the NRR. In the NRR process, POMs can be defined as an electron reservoir to provide electrons directly to N₂ molecules, which is beneficial to the activation of N₂ molecules.^57,58 Besides, POMs have been proved by DFT calculations to be an electron source pointing to the active site in the NRR process.⁵⁷ Hence, POM-based materials can be used as an ideal candidate catalyst for the NRR in both experimental and theoretical studies.

Recently, Liao et al.¹⁶⁹ synthesized Mo-PTA@CNT catalyst by immobilizing Mo-PTA (PTA = H₃PW₁₂O₄₀) on multi-walled CNT (Fig. 16a), in which Mo-PTA could efficiently adsorb and activate N₂ molecules, and CNT could enrich the N₂ concentration and accelerate NRR electron transfer. Thus, the as-prepared Mo-PTA@CNT exhibited high NRR activity with an NH₃ yield rate and FE value (Fig. 16b). Yang et al.⁵⁷ integrated POMs into stable MOFs to construct POMOFs (Fe_xCo_yMOF-P₂W₁₈) for the NRR (Fig. 16c). In the Fe_xCo_yMOF-P₂W₁₈ catalyst, P₂W₁₈ provided electron-rich centers and increased the electron transfer capacity, whereas the Fe_xCo_yMOF skeletons increased the stability of the reduced electron-rich POMs. Benefiting from the synergistic effect between POMs and MOFs, the Fe_xCo_yMOF-P₂W₁₈ catalyst showed excellent electrocatalytic NRR activity. It can be observed from Fig. 16d that the FeCoMOF-P₂W₁₈ catalyst exhibited the highest NH₃ yield rate (47.04 μg h⁻¹) and FE (31.76%) as compared to other Fe_xCo_yMOF-P₂W₁₈. Feng et al.¹⁷⁰ fabricated an Ag/PW₁₂/Zr–mTiO₂ composite with a mesoporous structure through one-pot synthesis (Fig. 16e), which showed superior activity in photo- and electro-driven NRR. In the Ag/PW₁₂/Zr–mTiO₂ composite, Ag NPs and PW₁₂ facilitated charge transfer efficiency due to interface engineering, whereas doping Zr in mTiO₂ significantly promoted chemisorption of N₂, together with enhanced active sites and the production rate of NH₃/NH⁴⁺. As a result, the Ag/PW₁₂/Zr–mTiO₂ composite could reach an NH₃/NH⁴⁺ production rate of 324.2 μmol g_cat⁻¹ h⁻¹ in a photo-driven NRR process and 55 μg g_cat⁻¹ h⁻¹ in an electro-driven NRR process.

Fig. 16 (a) Schematic diagrams of Mo-PTA and Mo-PTA@CNT. (b) NH₃ yield rates and faradaic efficiencies of Mo-PTA@CNT samples with different Mo loadings.¹⁶⁹ Copyright 2021, Wiley-VCH. (c) Proposed electron-transfer channel of Fe_xCo_yMOF and Fe_xCo_yMOF-P₂W₁₈ for e-NRR. (d) j–t curve for 12 h of a continuous e-NRR process (inset: the NH₃ yields and FEs of before and after electrolysis for 12 h).⁵⁷ Copyright 2023, Wiley-VCH. (e) Schematic representation of synthesizing Ag/PW₁₂/Zr–mTiO₂.¹⁷⁰ Copyright 2021, Wiley-VCH.

In addition, some researchers also proved the superiority of POM-based materials as NRR catalysts by DFT calculations. Gao et al.¹⁷¹ calculated phosphotungstic acid (PTA)-supported single metal and proved that the M-PTA showed high NRR activity (Fig. 17a and b). Lin et al.¹⁷² developed a series of POM (SiMA, SiTA, PMA, and PTA) supported Ru single atoms as NRR electrocatalysts (Fig. 17c). Among them, Ru-PMA and Ru-PTA displayed excellent NRR activity in the entire reactions. Talib et al.¹⁷³ proved that the NRR mechanisms of Mo₁/PMA SAC as an NRR catalyst for activating N₂ to convert NH₃ were the enzymatic pathway via DFT calculations. The Mo₁/PMA catalyst could impede the competitive HER process, thus enhancing the selectivity of NRR (Fig. 17d). Wang et al.¹⁷⁴ anchored Ta atoms on a Ta-substituted POM support to form dual-atom catalysts (DACs) for the NRR. The DFT calculations suggested that the interaction between Ta and Ta-POM was the bonding interaction, and the POM carrier surface structure was sensitive to the Ta substituted effects (Fig. 17e). Lin et al.¹⁷⁵ manifested that the central heteroatoms (X = Fe, Co, Ni, Cu, Zn, Ga, Ge, As) of POMs could adjust the coordination environment of a single Mo atom (Fig. 17f), and Mo atoms supported the POM catalyst with the central heteroatoms of Co and As has higher NRR activity and a lower limiting potential.

Fig. 17 (a) The calculated ΔG(*N₂) and ΔG(*H) on M-PTA. (b) The Gibbs free energy of *N₂ and *H adsorbed on M-PTA.¹⁷¹ Copyright 2019, Royal Society of Chemistry (RSC). (c) Optimized structures of POM and Ru-POM.¹⁷² Copyright 2020, Royal Society of Chemistry (RSC). (d) Calculated Gibbs free energies of N₂ and H₂ adsorbed on the M₁/PMA cluster.¹⁷³ Copyright 2022, Royal Society of Chemistry (RSC). (e) The structure of [N(CH₃)₄⁺]₃[Ta^II₂/POM] (POM = [PW₁₁O₃₉]⁷⁻).¹⁷⁴ Copyright 2022, Royal Society of Chemistry (RSC). (f) Optimized structures of POM.¹⁷⁵ Copyright 2023, Springer, Tsinghua University Press 2022.

3.3.2 POM-derived materials for the NRR. According to previous studies, Mo-based and W-based electrocatalysts exhibit highly active NRR catalysts.^176,177 POMs contain the highest oxidation state of early transition metals (Mo, W, V, etc.), which are considered as excellent platforms to construct Mo- and W-derived electrocatalysts.^12,61 Wang et al.¹⁷⁸ fabricated Fe_1.89Mo_4.11O₇/FeS₂@C and FeMoO₄/FeS₂@C using PMo₁₂@MIL-100(Fe)@PVP as the precursor (Fig. 18a). As presented in Fig. 18b, the Fe_1.89Mo_4.11O₇/FeS₂@C showed a high NH₃ yield rate and FE in acidic solution, higher than most reported NRR catalysts. Wang et al.¹⁷⁹ prepared a POM-derived CoS₂/MoS₂ electrocatalyst with POMs (H₃PMo₁₂O₄₀) as the precursor for the NRR, as shown in Fig. 18c. Compared with individual CoS₂ or MoS₂, the CoS₂/MoS₂ catalyst displayed an enhanced NRR activity (Fig. 18d), which was due to the bimetals synergistic effect improving the activation of nitrogen molecules and speeding up the charge transfer at the interface. Yang et al.¹⁸⁰ synthesized ternary oxide materials (MoFeO@X, X = 650, 750, 850) utilizing POMs (FeMo₆) as the precursor. The FeMo₆ precursor was conducive to the formation of controllable multi-interfaces, and could also control the uniform dispersion of ternary oxide composite at the molecular level. The NH₃ yield rate and FE of the synthesized MoFeO@750 reached 16.57 μg h⁻¹ mg_cat.⁻¹ and 12.33% at −0.4 V vs. RHE in 0.1 mol·L⁻¹ Na₂SO₄ solution. Yin et al.¹⁷⁷ fabricated Pt–W/Mo@rGO-6 with PW₆Mo₆ as the W/Mo platform (Fig. 18e), in which the Mo and W species exhibited a synergetic effect playing a critical role in the activation and dissociation of N₂. As a result, the NH₃ yield of the Pt–W/Mo@rGO-6 electrocatalyst was reaching 79.2 μg h⁻¹ mg_cat.⁻¹ at −0.3 V vs. RHE. A summary of the POM-based materials and POM-derived materials for electrocatalytic NRR in this review is shown in Table 4.

Fig. 18 (a) An illustration of the preparation of Fe_1.89Mo_4.11O₇/FeS₂@C and FeMoO₄/FeS₂@C composites. (b) Comparison of the NH₃ yield rate and FE of the prepared electrodes.¹⁷⁸ Copyright 2020, Royal Society of Chemistry (RSC). (c) Schematic illustration for the one-step hydrothermal synthesis of CoS₂/MoS₂ catalyst. (d) Comparison of NH₃ yield rate and FE of CoS₂, MoS₂, CoMoS-4, CoMoS-6, and CoMoS-8.¹⁷⁹ Copyright 2021, Elsevier Inc. (e) Schematic synthesis of Pt–W/Mo@rGO-X (X means the electroplating time).¹⁷⁷ Copyright 2023, Elsevier B.V.

Table 4 A summary of POM-based and POM-derived materials for electrocatalytic NRR in this review

Electrocatalyst	Electrolyte	Potential	NH3 yield rate (μg h^−1 mgcat.^−1)	FE (%)	Ref.
Mo-PTA@CNT	0.1 M K2SO4	−0.1 V vs. RHE	51 ± 1	81 ± 1	169
FeCoMOF-P2W18	0.1 M HCl	−0.4 V vs. RHE	47.04	31.56	57
Ag/PW12/Zr–mTiO2	0.05 M H2SO4	−0.6 V vs. RHE	55	2.2	170
Fe1.89Mo4.11O7/FeS2@C	0.5 M H2SO4	−0.4 V vs. RHE	105.3	54.7	178
Fe1.89Mo4.11O7/FeS2@C	0.1 M KOH	−0.4 V vs. RHE	86.3	53.6	178
CoS2/MoS2	1 M K2SO4	−0.25 V vs. RHE	38.61	34.66	179
MoFeO@750	0.1 M Na2SO4	−0.4 V vs. RHE	16.57	12.33	180
Pt–W/Mo@rGO-6	0.05 M H2SO4	−0.3 V vs. RHE	79.2	20	177

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4. The research process of POM-based materials for energy storage devices

4.1. Supercapacitors

Supercapacitors (SCs) have the advantages of large power, long cycles, and simple operation, which have become an ideal choice for electrochemical energy storage.^181–183 SCs contain two main types, i.e., an electrochemical double-layer capacitor that stores charge by reversibly adsorbing ions in the electrolyte and pseudocapacitors that work by relying on Faraday reactions.¹⁸⁴ POMs can achieve fast and reversible electron transfer, making them very suitable for use as pseudocapacitors.¹⁸⁵ However, the inherent low conductivity and high solubility of POMs in many solvents hinder their direct application as electrode materials of SCs.¹⁸⁶ Therefore, researchers immobilized POMs on chemically stable, high-surface-area carbon carriers to form mixed or nanocomposite electrochemical supercapacitor electrodes.²² The carbon carrier can improve the conductivity, and simultaneously provide an electrochemical double-layer capacitor to achieve a complementary effect with the Faraday charge storage of POM molecules.^187–189

At present, researchers have prepared a variety of POM-based electrode materials, such as POM-CNT, POM-activated carbon (POM-AC), POM-porous carbon (POM-PC), POM-graphene (POM-GO), and POM-conductive polymer (POM-COP), etc. for SCs. Chen et al.¹⁹⁰ reported a novel nanocomposite material of SWCNT-TBA-PV₂Mo₁₀ for the first time, which exhibited improved power density and energy density due to the redox activity of TBA-PV₂Mo₁₀ in the material and the high conductivity and double-layer capacitance of SWCNTs. Nevertheless, the electrostatic bonding approach, while effective in introducing non-conductive molecules, often leads to undesired electron transfer and compromises the structural stability of POMs, thereby hindering their practical use in electrochemistry. To address the issue of leaching, Wang et al.¹⁹¹ fabricated a 3D hybrid hydrogel (PMo₁₂/PPy/CNT) employing an electrostatic co-assembly strategy. As illustrated in Fig. 19a, the PMo₁₂/PPy/CNT consisted of a highly uniform and tightly distributed arrangement of individual PMo₁₂ molecules on PPy hydrogel-functionalized CNT. Compared with conventional composite systems, the PPy hydrogel linker, resembling a “fishnet” serving as the dual purpose of precisely confining the PMo₁₂ clusters in a supramolecular manner and ensuring efficient electrical contact between PMo₁₂ and CNTs (Fig. 19b and c). As a result, the PMo₁₂/PPy/CNT hybrid hydrogel exhibited enhanced supercapacitor performance compared to PMo₁₂/PPy/CNT composite systems. Furthermore, the specific capacitance of the solid device remained stable even when subjected to various bending or twisting conditions (Fig. 19d). Yang et al.¹⁹² developed a coaxial cable-like hybrid material (CNT/PMo/PANI) composed of CNTs, polyoxometalate (PMo), and polyaniline (PANI) (Fig. 19e and f). The PMo clusters and PANI played crucial roles as active centers for rapid and reversible redox reactions, contributing to pseudocapacitance.

Fig. 19 (a) Fabrication process of PMo₁₂/PPy/CNT; TEM images of (b) PMo₁₂/PPy/CNT hybrid hydrogel and (c) PMo₁₂/PPy/CNT composite. (d) Schematic drawing of the bent SSC and photographs of the flexible SSC under various flexible conditions.¹⁹¹ Copyright 2020, Royal Society of Chemistry (RSC). (e) Schematic diagram of the synthesis mechanism for 1D coaxial cable-like CNT/PMo/PANI. (f) SEM image of CNT/PMo/PANI.¹⁹² Copyright 2020, Wiley-VCH.

The term “carbon material” specifically pertains to well-defined nano-structures of sp²-hybridized carbon. It has been observed that there exists a significant chemisorption interaction between POMs and carbon materials,^22,193 which is beneficial to the stability of the composite SCs electrode. Guevara et al.¹⁹⁴ applied PW₁₂ adsorbed onto AC as electrode materials (AC-PW₁₂) (Fig. 20a), resulting in an expanded voltage window of 1.6 V in acidic liquid electrolyte and a high capacitance of 254 F g⁻¹ (Fig. 20b). However, the full potential of the pore structure in commercial AC has not been utilized, and the distribution of supported POMs remains uneven. Subsequently, Palomino et al.¹⁹⁵ made efforts to modify the pore size of AC to enhance the capacitance of PMo₁₂-AC (Fig. 20c). While this study successfully adjusted the pore size to enhance PMo₁₂ dispersion, a significant issue remains with the occurrence of leaching or aggregation of PMo₁₂ due to the presence of numerous mesopores (Fig. 20d). Consequently, precise control over the substrate hole size is crucial in addressing this challenge. Hu et al.¹⁹⁶ reported an AC@PMo₁₂ composite material in which PMo₁₂ was grown in microporous carbon for SCs. As shown in Fig. 20e, the AC@PMo₁₂ composite material exhibited a uniform distribution of PMo₁₂ within the carbon pores. When AC@PMo₁₂ was assembled into an asymmetric supercapacitor and subjected to constant current charging/discharging cycles in the P-IL electrolyte for 10 000 cycles, the capacitance of the device showed almost no decrease (Fig. 20f).

Fig. 20 (a) SEM image of the AC–PW₁₂ hybrid material. (b) CV curves for AC and AC–PW₁₂ at 10 mV s⁻¹.¹⁹⁴ Copyright 2014, Royal Society of Chemistry (RSC). (c) Adsorption–desorption isotherms of the carbon support with different activation levels. (d) TEM image of C250-1-PMo₁₂-5.¹⁹⁵ Copyright 2016, Elsevier Ltd. (e) SEM image of AC@PMo₁₂. (f) Long-term cycling performance of the asymmetric supercapacitor (AC@PMo₁₂//AC1) over 10 000 cycles at a current density of 10 A g⁻¹.¹⁹⁶ Copyright 2016, Elsevier B.V.

Researchers have discovered that POMs are more prone to adsorb within the pore size range of 1–2 nm in carbon-based substrates, enabling the achievement of exceptionally high POM loading.¹⁹⁷ Genovese et al.¹⁸⁵ used pine cones as a biotemplate for synthesizing a PC-PMo₁₂ hybrid with a high specific surface area. Yang et al.⁵⁴ designed a unique type of porous carbon (referred to as micro-PC) with a monodisperse super-micropore size of ∼1.09 nm. Yang et al.¹⁹⁸ designed PMo₁₂-coupled RGO through a simple anion exchange process (Fig. 21a), which showed a high density of POMs distributed individually on the surface of PIL-functionalized RGOs. As shown in Fig. 21b, PMo₁₂-RGO displayed a high SCs performance. Hou et al.¹⁹⁹ reported two cases of POMs ([SiMo₁₂O₄₀]⁴⁻/[SiW₁₂O₄₀]⁴⁻) combined with metal cup aromatic compounds ([Ag₅(C₂H₂N₃)₆][H₅⊂SiMo₁₂O₄₀] and [Ag₅(C₂H₂N₃)₆][H₅⊂SiW₁₂O₄₀], [C₂H₂N₃ = 1H-1,2,4-triazole]) (Fig. 21c). They proposed that the oxidation ability of Keggin ions was the main influencing factor on the electrochemical performance of these POMs combined with metal cup aromatic compounds. Due to the synergistic effect of GO and POMs, the composite electrode achieved a maximum specific capacitance of 230.2 F g⁻¹ (Fig. 21d), which is higher than most reported POM electrode materials. Pakulski et al.²⁰⁰ prepared a novel hybrid material of Mo₁₃₂-DTAB-EEG with a porous 3D superstructure (Fig. 21e). Combining the redox activity of Mo₁₃₂ and the high conductivity of EEG, the Mo₁₃₂-DTAB-EEG enabled a specific capacitance as high as 65 F g⁻¹, as presented in Fig. 21f.

Fig. 21 (a) The synthetic process for the POM/PIL/RGO nanohybrids. (b) Capacitance retention of the POM/PIL/RGO and POM/RGO electrodes evaluated.¹⁹⁸ Copyright 2014, Wiley-VCH. (c) The synthetic route of compound 1@GO composite materials. (d) Comparison of CVs for 1- and 1@nGO-based electrodes.¹⁹⁹ Copyright 2019, American Chemical Society (ACS). (e) The formation process of Mo₁₃₂-DTAB-EEG hybrid material. (f) Comparison of specific capacitance vs. scan rate for Mo₁₃₂-DTAB-EEG.²⁰⁰ Copyright 2018, Elsevier B.V.

The polymer exhibits electrical conductivity primarily via the delocalized electrons in the main chain and utilizes the abundant functional groups or positive properties to support POMs by physical cross-linking and electrostatic interaction.^201,202 Liu et al.²⁰³ prepared PMo₁₂/PPy composite via anchored PMo₁₂ onto a highly interconnected 3D conductive polypyrrole (PPy) hydrogel scaffold (Fig. 22a). In a three-electrode system, the specific capacitance of the PMo₁₂/PPy composite hydrogel was measured to be 776 F g⁻¹, nearly 2.5 times higher than that of traditional PMo₁₂/PPy composite materials (Fig. 22b). The solid-state supercapacitor achieved a maximum specific capacitance of 162.1 F g⁻¹ and an energy density of 50.66 W h kg⁻¹ at a power density of 750 W kg⁻¹, surpassing the electrochemical performance of other POMs or metal oxides (Fig. 22c). The summary of POM-based electrode materials for SCs in this review is displayed in Table 5.

Fig. 22 (a) The preparation process for the PMo₁₂/PPy hybrid hydrogel for SCs. (b) Specific capacitances of the prepared samples at various current densities. (c) Ragone plots of the P₂-PPy gel//P₂-PPy gel device compared with the values reported for other devices.²⁰³ Copyright 2019, Elsevier Ltd.

Table 5 The summary of POM-based electrode materials for SCs in this review

Electrode materials	Electrolyte	Specific capacitance	Energy density	Capacitance retention	Ref.
SWCNT-TBA-PV2Mo10	1 M H2SO4	444 F g^−1@2 A g^−1	15.4 W h kg^−1	95%@6500 cycles	190
PMo12/PPy/CNT	0.5 M H2SO4	1170 F g^−1@1 A g^−1	67.5 μW h cm^−2	86%@3000 cycles	191
CNT/PMo/PANI	1 M H2SO4	825 F g^−1@5 A g^−1	18.7 W h kg^−1	85%@6000 cycles	192
AC–PW12	1 M H2SO4	254 F g^−1	2.32 W h cm^−3	98%@30000 cycles	194
C250-3-PMo12-2	1 M H2SO4	285 F g^−1@6 mA cm^−2	10.2 W h kg^−1	—	195
AC@PMo12	acidic P-IL	223 F g^−1@1 mV s^−1	—	∼100%@10 000 cycles	196
Pinecone AC/PMo12O40	1 M H2SO4	361 F g^−1@10 mV s^−1	—	70%@5000 cycles	185
PMo12@micro-PC	0.5 M H2SO4	443 mF cm^−2@3 mA cm^−2	0.12 mW h cm^−2	81%@7500 cycles	54
POM/PIL/RGO	0.5 M H2SO4	408 F g^−1@0.5 A g^−1	56 W h kg^−1	98%@2000 cycles	198
Compound 1@15% GO	0.5 M H2SO4	230.2 F g^−1@0.5 A g^−1	—	93%@1000 cycles	199
Mo132-DTAB-EEG	1 M H2SO4	65 F g ^−1@1 mV s^−1	—	99%@5000 cycles	200
PMo12/PPy	1 M H2SO4	300 F g^−1@5 A g^−1	50.66 W h kg^−1	80%@1000 cycles	203

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4.2. Rechargeable batteries

Rechargeable batteries drive the global energy transition to renewable energy but are faced with resource constraints.²⁰⁴ At present, the common rechargeable batteries include lithium-ion batteries (LIBs), lithium–sulfur batteries (LSBs), metal-ion batteries, metal–air batteries, etc.²⁰⁵ The development of rechargeable batteries with high energy and power density, safety, long cycle life, fast response, and low cost is the goal of researchers.²⁰⁶ Therefore, the design of high-performance electrode materials has important significance for improving the performance of rechargeable batteries.

POM-based electrode materials for LIBs. The high energy density of LIBs has always been a forefront topic in battery research. The performance of LIBs mainly depends on the active materials used for Li⁺ storage electrodes.²⁰⁷ The basic requirements for electrode materials include high reversible capacity, good cycling stability, high safety performance, low cost, and environmental friendliness.^208,209 It is necessary to use negative electrode materials with larger capacities and a faster Li⁺ diffusion rate to enhance the energy and power density of LIBs.²¹⁰ Although different microscale materials such as metal oxides,²¹¹ alloys,²¹² and pyrolytic carbon²¹³ have been developed as negative electrode materials for LIBs, the morphological and structural changes associated with these microscale materials during battery operation pose limitations such as high electronic resistance, slow solid-state Li⁺ diffusion rate, small surface area, and poor shape flexibility. Therefore, current microscale structured negative electrode materials for LIBs, especially under high current density, still face issues such as poor cycling stability and insufficient charge/discharge capacity, which hinder the further development of current LIBs.

POMs, known for the high stability of thermodynamic and chemical, controllable size, precise structure, and the ability of high-valent metal ions in POMs to undergo multi-electron redox reactions during battery cycling, are considered potential electrode materials for LIBs.^23,59 Wang et al.²¹⁴ conducted a study using X-ray absorption fine structure analysis to study the structure and electronic states of PMo₁₂, which indicated that when PMo₁₂ was applied as a positive electrode material for LIBs, the Mo⁶⁺ ions in PMo₁₂ were reduced to Mo⁴⁺ during the discharge process, accompanied by the transfer of 24 electrons (Fig. 23a). Their study revealed the behavior characteristics of PMo₁₂ as an “electron sponge” and demonstrated its significant potential as a high-performance electrode material for LIBs. Sonoyama et al.²¹⁵ utilized Anderson-type POMs Na₃[AlMo₆O₂₄H₆] (NAM) as a novel cathode material for high capacity and excellent stability in LIBs (Fig. 23b). The stability of the [AlMo₆O₂₄H₆]³⁻ during the charge–discharge process was investigated by in situ polycrystalline X-ray diffraction and Raman spectroscopy. This significant finding provided experimental and theoretical evidence for the application of POMs and POM-based composite materials as electrode materials in LIBs.

Fig. 23 (a) A schematic diagram of electron gain and loss during charge and discharge of PMo₁₂ cathode materials.²¹⁴ Copyright 2012, American Chemical Society (ACS). (b) A schematic image of the reaction mechanism for NAM.²¹⁵ Copyright 2014, Elsevier B.V. (c) Crystal structure of Li₇[V₁₅O₃₆(CO₃)] and its ratio of power to mass-energy density as the cathode material for LIBs.²¹⁶ Copyright 2015, Wiley-VCH. (d) View of polyanion, macrocation, and crystal structure of NiV₁₄ along the [001] axis in polyhedron mode. (e) Rate performance at various current densities.²¹⁷ Copyright 2023, Tsinghua University Press.

V (50.96 g mol⁻¹) has a lower relative molecular mass and higher redox capability as compared to Mo (95.96 g mol⁻¹) and W (183.84 g mol⁻¹). Thus, V-based POMs may exhibit a higher mass-energy density and specific capacity when used as battery electrodes. Subsequently, researchers have extensively explored the application of polyoxovanadates (POVs) as electrode materials in LIBs. Cronin's group²¹⁶ discovered that Li₇[V₁₅O₃₆(CO₃)] showed promising potential as a positive electrode material for LIBs (Fig. 23c). The V^IV and V^V in Li₇[V₁₅O₃₆(CO₃)] could reversibly gain and lose electrons during the charge–discharge process. Therefore, the pouch cell using Li₇[V₁₅O₃₆(CO₃)] as a positive electrode material exhibited excellent rate performance and cycling stability. Wang et al.²¹⁷ introduced a novel V-POM ([Ni(Phen)₃][ClV₁₄O₃₄]Cl, NiV₁₄) with a large-sized polycation containing multiple charges for studying its application as a negative electrode material in LIBs. The presence of the multi-charged large-sized polycation [Ni(Phen)₃]²⁺ in NiV₁₄ provided more stable open channels (Fig. 23d). The LIBs assembled with NiV₁₄ exhibited a high reversible capacity, high Coulombic efficiency and good stability (Fig. 23e). Electrochemical analysis indicated that capacitance-controlled charge storage processes and fast Li⁺ ion diffusion kinetics were the main factors contributing to the improved rate performance of the NiV₁₄ electrode.

Although POMs possess precise structures, tunable sizes, and excellent redox performance, their dissolvable nature in the electrolyte and low specific surface area of POMs lead to low capacity, poor stability, and shorter lifespan.²¹⁸ Additionally, the uneven dispersion and aggregation of POM clusters hinder electron transfer between the material and the electrode, further impacting its cycling and rate performance. These drawbacks still limit the advance of POMs as electrode materials for LIBs. To address these issues, researchers have proposed combining POMs with other materials that have high specific surface areas or porous structures, such as conductive matrices, MOFs, composite materials, etc. By forming composites with these materials, it is possible to achieve uniform loading of POMs while maintaining a high specific surface area and porosity, thereby enhancing the functionality in conjunction with the conductive matrix or MOFs. The POM-based electrode materials for LIBs reported in recent years are summarized in Table 6.

Table 6 A summary of the POM-based electrode materials for LIBs reported in recent years

Electrode materials	Electrode	Discharge capacities	Rate performance	Ref.
POM/CNT	Anode	850 mA h·g^−1@0.5 mA cm^−2	1532 mA h·g^−1@0.05 mA cm^−2	219
			565 mA h·g^−1@1 mA cm^−2
Py-Anderson-CNT	Anode	1899 mA h g^−1@0.5 mA cm^−2	1072 mA h g^−1@0.05 mA cm^−2	220
			480 mA h g^−1@1 mA cm^−2
CNTs-SiW11	Anode	1189 mA h g^−1@0.5 mA cm^−2	802 mA h g^−1@0.05 mA cm^−2	221
			584 mA h g^−1@1 mA cm^−2
POM/RGO–MCB	Cathode	67 A h kg^−1@1.5 V	—	222
NAM−EDAG	Anode	1835 mA h g^−1@100 mA g^−1	1100 mA h g^−1@100 mA g^−1	223
			400 mA h g^−1@10 A g^−1
Mn3V19-HIL/RGO	Cathode	214 mA h g^−1@100 mA g^−1	121 mA h g^−1@5 A g^−1	224
			73 mA h g^−1@2 A g^−1
Co-SiW@GO	Anode	452 mA h g^−1@2 A g^−1	373 mA h g^−1@0.1 A g^−1	225
			242 mA h g^−1@5 A g^−1
MF POMs/MXenes	Anode	297 mA h g^−1@1.0 A g^−1	301 mA h g^−1@0.1 A g^−1	226
			115 mA h g^−1@4 A g^−1
VM POMs/MXenes	Anode	196 W h kg^−1	627 mA h g^−1@0.1 A g^−1	227
			244 mA h g^−1@4 A g^−1
POMOF	Anode	1525 mA h g^−1@0.25C	1525 mA h g^−1@0.25 C	228
			1421 mA h g^−1@1.25 C
NNU-11	Anode	1322 mA h g^−1@50 mA g^−1	—	229
NAU3	Anode	1406 mA h g^−1@100 mA g^−1	488 mA h g^−1@0.1 A g^−1	230
			283 mA h g^−1@1 A g^−1
POMOFs/RGO	Anode	2368 mA h g^−1@50 mA g^−1	409 mA h g^−1@2 A g^−1	231
			365 mA h g^−1@3 A g^−1
PMo10V2-ILs@MIL-100	Anode	1666 mA h g^−1@100 mA g^−1	1033 mA h g^−1@0.1 A g^−1	232
			348 mA h g^−1@3 A g^−1

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POM-based electrode materials for other rechargeable batteries. POM-based materials are also applied as the electrode material for studies in lithium–sulfur batteries, metal-ion batteries and metal–air batteries. Wei's group²³³ synthesized rGO–CNT/PW₁₂ as the cathodes in LSBs (Fig. 24a), which displayed 1425 mA h g⁻¹ of specific capacities, and 573 mA h g⁻¹ of rate capability. Their work confirmed the POM catalyst as a key structural element in the charging/discharging process of LSBs (Fig. 24b). In view of the shuttle effect of lithium polysulfides in LSBs, Chen's group²³⁴ prepared {Co₄W₁₈}/rGO materials (Fig. 24c), which can not only adsorb lithium polysulfides but also improve battery capacity and cycle performance. By using PMo₁₂ as a crystal growth regulator, Ren et al.²³⁵ induced the oriented growth of Na₃V₂(PO₄)₂F₃ crystals along the [110] crystal axis, and constructed a Na₃V₂(PO₄)₂F₃ cathode material with continuous sodium ion diffusion channels for sodium ion batteries (Fig. 24d). Shen et al.²³⁶ used an electron/ion sponge POM (Na₂H₈[MnV₁₃O₃₈], NMV) as the cathode material for sodium-ion batteries (Fig. 24e). In NMV, it can accommodate 11 electron/ion pairs per mole content, which has a great contribution to the high capacitance of the material. Mandal et al.²³⁷ fabricated a PVIM–CoPOM/NCNT composite with a high ORR and OER performance, as shown in Fig. 24f. As a result, the Zn–air batter performance using PVIM–CoPOM/NCNT as the air cathode exhibited a low charge–discharge potential gap and high energy density.

Fig. 24 (a) A schematic diagram of rGO–CNT/PW₁₂ as the cathode in LSBs. (b) The proposed catalytic scheme mediated by a Keggin-type {PW₁₂} catalyst in a LSB system.²³³ Copyright 2021, American Chemical Society (ACS). (c) A schematic diagram of the synthesis of {Co₄W₁₈}/rGO materials.²³⁴ Copyright 2022, Springer Nature. (d) NVPF synthesis process.²³⁵ Copyright 2023, Wiley-VCH GmbH. (e) Schematic illustration of Na ion storage in a notion of electron/ion sponge and the calculated Na-ion chemical diffusion coefficients.²³⁶ Copyright 2017, American Chemical Society (ACS). (f) Scheme representing the bifunctional activity of the PVIM–CoPOM/NCNT composite towards both the OER and ORR.²³⁷ Copyright 2021, Royal Society of Chemistry (RSC).

5 Conclusions and outlooks

In this review, recently developed POM-based and POM-derived materials in the field of water-splitting, CO₂RR, NRR, SCs, and LIBs are systematically summarized. The fundamental roles of POMs in clean energy conversion and storage systems are introduced. The material design principles have been emphasized, ranging from single clusters, assemblies, organic–inorganic hybrids to derivatives. The relationship of the structure and performance of POM-based nanostructures in electrocatalytic applications is also discussed. There still exist challenges and opportunities to meet the requirements of electrocatalytic and energy applications, including but not limited to the following items.

(1) In the context of catalysis and energy-related applications of POMs, one of the key challenges to continue research is anchoring POMs to suitable substrates or constructing POM-based composites. Stability is a crucial factor in the application of POMs as electrode materials. Most POMs are water-soluble and have relatively large molecular sizes. How to control the conditions to make POM particles well dispersed and fixed on the support bracket, or coated POMs with other materials, will effectively improve the stability, which deserves further study.

(2) The unique physical and chemical properties of POMs make them show significant advantages in electrocatalytic applications. To fully understand the changes in fine dynamic structure, the evolution of active sites, and key reaction intermediates during the reaction of POM-based materials, the challenge is developing more accurate in situ electrochemical characterization techniques.

(3) Due to the complexity of POM-based catalysts, exploring the combination of multivariate analysis and machine learning for data analysis to achieve the redesign and optimization of catalysts can accelerate the development of POMs in the field of electrocatalytic and energy.

(4) Although many POM-based materials and their derivatives have been reported as catalysts for electrocatalytic and energy fields, most of the synthesis methods are limited to the laboratory scale due to the complex synthesis process. Therefore, it is very important to develop simple and scalable synthetic routes. At the same time, to achieve industrial applications, it is also necessary to rationally design POM-based materials with high mechanical strength and stability.

Conflicts of interest

The authors declare no conflict of interest in this manuscript.

Acknowledgements

The authors greatly acknowledge the financial support from the National Natural Science Foundation of China (22301257, 21072221, 21875247, 21172252).

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