Recent advances in transition metal phosphide-based heterostructure electrocatalysts for the oxygen evolution reaction

Abstract

To rapidly develop water-splitting technology for hydrogen generation, it is essential to create efficient and cost-effective electrocatalysts. Transition metal phosphides (TMPs) have emerged as effective electrocatalysts for the oxygen evolution reaction (OER) due to their unique electronic structure and high catalytic activity. The TMP-based heterostructure catalysts exhibit superior OER activity compared to individual TMP catalysts as heterostructures not only increase the number of active sites in the catalysts but also facilitate the adsorption of intermediates. This review begins by introducing the catalytic mechanism and key parameters of the OER, followed by the preparation methods of different types of TMP-based heterostructures. Additionally, this article discusses four primary strategies to improve the OER performance of TMP-based heterostructures and the current challenges. Finally, this article provides conclusions and perspectives on utilizing TMP-based heterostructure catalysts for the OER.

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1. Introduction

Energy shortages and environmental pollution are two of the most pressing global problems facing the world today.^1–3 To address these issues, there is an urgent need to find clean and sustainable fuels to replace traditional hydrocarbons.^4,5 Hydrogen is one such fuel, offering several advantages such as being pollution-free, resource-abundant, and highly efficient, which has led to increased attention in recent years.^6,7 However, the production of hydrogen currently relies heavily on fossil fuels such as natural gas, oil, and coal, exacerbating the global energy crisis. Electrochemical water splitting is the most promising method of producing high-purity and green hydrogen, which involves two reactions: the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER).⁸ The OER process is more complex, requiring the transfer of four electrons, which makes it challenging to develop efficient electrolytic water systems.^9,10 While RuO₂/IrO₂ catalysts have traditionally been regarded as the best OER electrocatalysts, their high cost and rarity limit their widespread use.¹¹ As a result, there is a critical need to develop highly active and durable non-precious-metal catalysts to reduce energy consumption and enable the production of sustainable hydrogen fuels.

Recently, there has been a surge of interest in transition metal (Ni, Co, Fe, Cu, Mn, etc.) compounds due to their intrinsic activity, abundance, and cost-effectiveness. Among the non-precious metal electrocatalysts, transition metal phosphides (TMPs) have demonstrated exceptional OER performance. Despite their advantages, TMPs exhibit inferior performance compared to noble metal catalysts. Constructing TMP-heterostructures with phosphides, oxides, sulfides, carbides, or nitrides has been identified as an effective approach to enhance their electrocatalytic performance, such as Ni₂P–MnP@Co₂P,¹² Ni₂O₃/Ni₂P,¹³ NiCoP/MoS₂,¹⁴ MoP–Mo₂C,¹⁵ and Fe₂P–Co₂N.¹⁶ Heterostructure interfaces expose higher active sites and induce redistribution of electrons. These synergistic effects may promote the activity and efficiency of the OER process.¹⁷ Despite plentiful research on TMP-heterostructure catalysts for the OER, few papers have systematically summarized the synthesis of TMP-heterostructures and their electrocatalytic enhancement mechanisms for the OER.

In this paper, we provide a comprehensive review of the synthesis methods of various heterostructure catalysts based on transition metal phosphide/oxide (TMP/TMO), transition metal phosphide/carbide (TMP/TMC), transition metal phosphide/phosphide (TMP/TMP), transition metal phosphide/sulfide (TMP/TMS), and transition metal phosphide/nitride (TMP/TMN), and examine strategies to enhance OER catalytic activity. Firstly, we introduce the mechanism and evaluation parameters of the OER reaction. Next, we discuss the most representative synthetic methods of preparing various TMP-based heterostructures. Subsequently, we describe four strategies that can lead to highly active catalysts for the OER reaction and current challenges for TMP-based heterostructure catalysts. Finally, the paper concludes by identifying several issues and future developments in the field.

2. OER mechanism

The OER is a complex process involving a four-electron transfer.¹⁸ The kinetics of the OER is slower and more complicated than that of the HER. The mechanism of the OER revolves around three adsorption intermediates: OOH*, O*, and OH*. Under alkaline conditions, as illustrated in Fig. 1(A) and (B), the first step of the OER mechanism involves water dissociation into OH*, resulting in the release of OH⁻ ions that replace the H₂O adsorbed on the active sites, leading to the formation of an intermediate state of M–OH*. Subsequently, the intermediate product M–OH* decomposes to produce M–O*, which further generates oxygen via two separate pathways. The first route entails a direct generation of molecular oxygen by two M–O* species, and the second route is by forming the M–OOH* intermediate, which subsequently reacts with another OH⁻ to produce O₂.

Fig. 1 (A) Pathways of the OER in alkaline, neutral and acidic media. (B) OER mechanisms in alkaline, neutral and acidic media. (C) Plot of Gibbs free energy for the reactive species and intermediates of the OER versus the reaction coordinate. Blue and red lines refer to real and ideal reaction pathways,¹⁹ respectively. Copyright, 2010 Wiley-VCH.

The binding energy of the intermediate products determines the free energy associated with each elementary step of the reaction, as shown in Fig. 1(C). This figure also displays the OER scaling relationship between ideal and real electrocatalysts, along with the reaction energies of the basic steps. In more detail, the free energy of formation for reaction intermediates is denoted as ΔG, while E₀ represents the standard reversible electrode potential. E₁, E₂ and E₃ correspond to different electrode potentials. At potential E₁, the OER is unable to proceed. A theoretical ideal catalyst leads to four reaction steps, each with a free energy in equilibrium at zero (i.e., 1.23 eV).¹⁹ Nonetheless, the adsorption energy of intermediates (OH*, OOH*, and O*) is linearly correlated, making this ideal scenario unachievable.²⁰ Generally, OER reactions in alkaline solutions are more likely to occur than in acidic solutions because acidic solutions involve the breaking of H–O–H bonds.

3. Evaluation parameters for OER electrocatalysts

To evaluate the performances of OER electrocatalysts, key parameters, including overpotential, Tafel slope, voltage or current retention rate, turnover frequency (TOF), and Faraday efficiency (FE), were analysed.

3.1 Overpotential

Overpotential is a crucial factor for assessing the effectiveness of OER electrocatalysts. Although the standard equilibrium potential is 1.23 V vs. RHE, the kinetic barrier means that the OER requires an additional potential to drive the electrochemical reaction. Overpotential is defined as the difference between the actual potential and the equilibrium potential. During the OER process, there are three types of overpotential, which are activation, resistance, and concentration.²¹ The activation overpotential is related to the slow rate of the electrochemical reaction. In contrast, the resistance overpotential is due to the resistance between the test electrolyte and wire, which can be compensated by iR. Finally, the concentration overpotential results from the difference between the ion concentration on the electrode surface and the actual ion concentration, which can be minimized by stirring the solution. To evaluate the OER performance of different catalysts, we typically use 10 mA cm⁻² as a benchmark for overpotential. A lower overpotential value indicates better electrocatalytic behavior of the catalyst.

3.2 Tafel slope

The Tafel slope is a parameter for assessing the reaction kinetics of an electrocatalyst. It can be obtained from the polarization curve and its value reflects the charge transfer ability and kinetics of the OER. A lower Tafel slope indicates stronger charge transfer ability and faster OER kinetics, making it a desirable property for electrocatalysts.²² In addition, a Tafel slope can help reveal the reaction mechanism, particularly the rate-determining step, which makes it essential for understanding the underlying chemical process.²³

The related equation is as follows:²⁴

η = a + b log(j) (2.1)

where a, b, η and j are the Tafel constant, the slope, the overpotential and current density, respectively.

3.3 Current or voltage retention rate

Evaluating the stability of an electrocatalyst in the OER is an important aspect of performance assessment, and can be achieved through continuous cyclic voltammetry (CV) and either chronoamperometry (CA) or chronopotentiometry (CP).²⁵ Current or voltage changes are considered during continuous CV cycles between fixed potentials to determine the catalyst's stability. Alternatively, CA measures the current density at a fixed potential, while CP tests the potential at a fixed current density. Current and voltage retention rate is used to evaluate the stability during CA and CP tests, respectively. These methods provide valuable insights into the stability of the catalyst, allowing for a more comprehensive understanding of its performance in the OER.

3.4 Faraday efficiency

The utilization efficiency of electrons in a reaction can also be determined using the Faraday efficiency (FE) index. FE is calculated based on the ratio of actual O₂ volume to theoretical gas volume.²⁶ Methods for collecting gas in the ratio method include water–gas displacement, gas chromatography, and fluorescence spectroscopy.²²

3.5 Turnover frequency

The turnover frequency (TOF) is a measure of the number of reactions per unit time and per unit active site at a specific temperature and pressure. It serves as an indicator of the intrinsic activity of a catalyst. A higher value of TOF corresponds to a larger number of available active sites and better catalytic activity.²⁷ The TOF can be calculated using the following formula:²⁸where N_A is the Avogadro constant, Γ refers to the surface concentration of the active site or the number of active sites, F represents the Faraday constant, n denotes the number of electrons transferred, and j is the current density.

4. Synthesis of TMP-based heterostructure catalysts

TMP-based heterostructures have attracted widespread attention due to their high OER activities. These heterostructures are composite structures that couple transition metal phosphides (TMPs) with other compounds such as TMPs, TMOs, TMSs, TMCs, and TMNs through interfacial interaction.^29,30 The derived heterostructures retain the inherent characteristics of the two components and benefit from enhanced performance. TMPs possess many advantages for the OER. Firstly, TMPs feature a suitable d-electron configuration and rich chemical states, as well as a unique hydrogenase-like catalytic mechanism. The phosphorous and metal sites in TMPs serve as proton acceptor and hydride acceptor sites, respectively, resulting in high OER catalytic activity.^31,32 Secondly, TMPs contribute to the formation of reactive oxy/hydroxides or phosphates and have fast charge transfer capabilities.³³ Finally, during the OER process, P is oxidized into phosphate species, which aids in absorbing water molecules and improving OER activity.^34,35

Researchers have focused on designing metal compound heterostructures using both top-down and bottom-up approaches. The former encompasses ball milling, solid-phase calcination, and exfoliation, while the latter includes hydrothermal and solvothermal methods, chemical vapor deposition (CVD), electrochemical deposition, the template method, and electrospinning method. This paper focuses on bottom-up synthesis strategies.

4.1 TMP/TMP heterostructure catalysts

Transition metal phosphides (TMPs) have unique crystal structures that consist of a tri-prism shape with a P atom located in the internal void, resulting in a spherical local structure. This unique structure promotes the exposure of unsaturated surface atoms, making TMPs highly effective catalysts.³⁶ The use of TMP/TMP heterostructures in OER electrocatalytic systems has become increasingly popular due to their improved performance when compared to single-TMPs, which is attributed to specific cooperativity. Various techniques have been employed to synthesize TMP/TMP heterostructures, including the hydrothermal/solvothermal–phosphorization method, template–phosphorization method, chemical vapor deposition–phosphorization method, pyrolysis of phosphate resin chelated metal ions, and physical mixing.

The hydrothermal method is a technique in which materials are dissolved in a water solvent and then placed in an autoclave for reaction at a specific temperature. During this process, the dissolved powder will precipitate from the solution into the reaction vessel. Hydrothermal methods have been utilized to synthesize nanostructures with distinctive shapes, sizes and morphology, which possess excellent electrochemical properties. Thanks to its low-cost, and high efficiency, the hydrothermal–phosphorization method has been widely used. For example, Zhao et al.³⁷ used the hydrothermal–phosphorization process to synthesize a sea urchin-like Co₂P/Ni₂P@NF heterostructure composed of nanowires and nanosheets, as depicted in Fig. 2(A). The process involved the preparation of transition metal hydroxides via a hydrothermal method, followed by a simple phosphating reaction that utilized NaH₂PO₂ as a P source to convert the hydroxide precursor into the corresponding TMPs. The electrocatalyst exhibited remarkable catalytic ability due to the interaction of the hierarchical heterostructure between Co₂P nanowires and Ni₂P nanosheets.

Fig. 2 (A) Synthetic protocol of Co₂P-x/Ni₂P-y@NF.³⁷ Copyright 2022, Elsevier. (B) Schematic illustration of the synthesis of the chrysanthemum-flower-like bundle structure of the hierarchical Ni–graphene–CNT–Ni₂P–CuP₂ heterostructure.²⁰ Copyright 2021, American Chemical Society.

Compared to the hydrothermal method, the CVD method offers several advantages including speed and environmental safety. In a recent study, the Ni₂P–CuP₂ heterostructure was synthesized using the CVD-phosphorization method.²⁰ The process involved depositing Ni–graphene–CNTs on Ni foam, followed by the deposition of Ni and Cu by an electrochemical chronoamperometry method and phosphorization. Fig. 2(B) illustrates the deposition of Ni–graphene–CNTs on Ni foam.

The template–phosphorization method has become a commonly used approach for synthesizing TMP/TMP heterostructures because of its precise controllability. The template method includes four steps: (i) template synthesis, (ii) using various methods to grow the required materials on the surface of the template, (iii) selective removal of templates, with the common methods being calcination or etching with solvents,³⁸ and (iv) obtaining the final product. A Ni₂P–Co₂P heterostructure was synthesized by Zhou et al.³⁹ through a facile template-phosphorization strategy. In their work, nickel foam (NF) was used as the template. First, 2D ZIF-67 was grown on porous NF by a simple liquid deposition process. Subsequently, NiCo(OH)_x/NF was prepared by an ion-exchange and etching reaction. Finally, the NiCo(OH)_x@glucose was converted to the target product Ni₂P–Co₂P@C/NF through phosphorization. This approach shows promising potential for the synthesis of other TMP/TMP heterostructures with tailored properties.

Our group synthesized a Mn₂P–MnP heterostructure by heating Mn²⁺-chelated phosphate resin in the presence of potassium hydroxide.⁴⁰ This resulted in accurate regulation of the Mn : P ratio through the potassium hydroxide content. The study showed that the growth mechanism of manganese phosphide involved an intermediate process of Mn₂P₂O₇. This is a general and effective technique for synthesizing metal phosphide heterogeneous nanoparticles with easy, precision, and control. Other techniques have also been reported, such as the FeP–CoP heterostructure prepared by physical mixing of nanostructures.^41,42 Although this method is easily manageable, it may be challenging to achieve a complete heterogeneous structure and product diversity.

4.2 TMP/TMO heterostructure catalysts

Transition metal oxides (TMOs) have become one of the most promising OER catalysts due to their low cost, adjustable structure, and high electrocatalytic activity. However, they have low electric conductivities. Transition metal phosphides exhibit higher conductivity and fast charge transport. Constructing TMP/TMO heterostructures is an effective way to integrate each other's advantages for the OER reaction. The synthesis strategies that have been used for the design of TMP/TMO heterostructures mainly include thermal decomposition of metal–phosphine complexes, phosphating of metal oxides, and oxidation of metal phosphides.

Our group synthesized Mn₂P–Mn₂O₃ heterogeneous nanoparticles on a porous framework through one-step heat treatment of Mn²⁺-exchanged phosphonic resin mixed with KOH under a N₂ atmosphere (Fig. 3(A)).⁴³ The phosphonic acid group reacted with Mn²⁺ to produce Mn₂P and Mn₂O₃ while KOH acted as an activating agent to yield numerous pores. Our group also synthesized Co₂P–Co_xO_y heterostructure particles using a simultaneous phosphating/oxidation strategy (Fig. 3(B)).⁴⁴ By adjusting the amount of O₂ during heat treatment, Co₂P partly oxidized into cobalt oxides with controlled stoichiometric ratios of Co and O to obtain Co₂P–Co_xO_y (Co_xO_y = CoO or Co₃O₄) heterogeneous nanoparticles. This facile and low-cost strategy is scalable and is used to synthesize TMP/TMO heterogeneous nanoparticles in large quantities, including Cu₃P–Cu₂O Janus nanoparticles for overall water splitting. Our group reported producing a hundred grams of Cu₃P–Cu₂O Janus nanoparticles using this strategy.⁴⁵

Fig. 3 (A) Illustration exhibiting the preparation process of Mn₂P–Mn₂O₃/PNCF.⁴³ Copyright 2022, Elsevier. (B) Schematic illustration of the preparation process of Co₂P–Co₃O₄/C.⁴⁴ Copyright 2023, Wiley-VCH. (C) Illustration of the synthetic procedure of CoP/MnO.⁴⁷ Copyright 2022, Elsevier. (D) The synthetic procedure of NiO@Ni₂P.⁴⁸ Copyright 2022, Wiley-VCH.

The process of phosphating metal oxides to TMP/TMO heterostructures involves synthesizing metals or metal oxides using hydrothermal/solvothermal etc. techniques followed by partially converting the metal oxides into phosphides by reacting them with a phosphorus source. For instance, Yu et al.⁴⁶ synthesized a CoP@a-CoO_x heterostructure by generating the CoCo-LDH plate precursor through a one-step solvothermal process followed by converting it to CoP@a-CoO_x through phosphating at 300 °C. Similarly, Chang et al.⁴⁷ reported synthesizing a CoP/MnO electrocatalyst through synthesis of MnCo₂O₄ with hollow nanofibers through electrospinning technology followed by a low-temperature phosphating process with NaH₂PO₂ as the phosphorus source (Fig. 3(C)). Phosphating first and then oxidizing metal oxides/hydroxides can also result in TMP/TMO heterostructures. Yan et al.⁴⁸ synthesized NiO@Ni₂P heterogeneous microspheres by first hydrothermally treating nickel hydroxide to form Ni(OH)₂ nanosheets, partially phosphatizing them to form Ni(OH)₂@Ni₂P, and then oxidizing them at 300 °C to form NiO@Ni₂P heterostructures (Fig. 3(D)). This multi-step method produced hydrangea-like NiO@Ni₂P heterogeneous microspheres that demonstrated high activity in the OER due to interfacial electron redistribution. However, while conversion of metal oxides/hydroxides to TMP/TMO heterostructures via step-by-step phosphating and oxidation offers flexibility in morphological diversity adjustment, multi-step reactions increase the difficulty of the reaction control, and strong corrosive or flammable phosphorus sources can pose safety hazards.

4.3 TMP/TMS heterostructure catalysts

The introduction of TMPs creates a TMP/TMS heterostructure that can optimize the innate activity and exhibit higher activities for the OER.^49,50 The synthesis of TMP/TMS heterostructures typically involves a two-step process that includes sulfurization–phosphorization or phosphorization–sulfurization of metal/oxide/hydroxide precursors, or a one-step process that involves phosphorization and sulfurization of metal/oxide/hydroxides. For example, Pan et al.⁵¹ synthesized an electrocatalytic CoS₂CoP/CC via a hydrothermal–phosphorization–sulfurization method, as shown in Fig. 4(A). In this method, Co(OH)F precursor nanoclusters were first grown on carbon cloth (CC) through a hydrothermal reaction, followed by the growth of CoP on the nanoclusters during the phosphorization process. Finally, the reactant CoP/Co(OH)F was converted into CoS₂/CoP through sulfurization heating with S powders at 150 °C. Khan et al.⁵² also synthesized a Ni₃S₂@Ni₅P₄ nanosheet electrocatalyst using a phosphorization process of a Ni₃S₂ precursor, as shown in Fig. 4(B). The Ni₃S₂-precursor was first grown on Ni foam (NF) by hydrothermal synthesis with thiourea, and it was then phosphorized with NaH₂PO₂ as a phosphorous source.

Fig. 4 (A) The preparation process of CoS₂/CoP/CC.⁵¹ Copyright 2019, Springer. (B) The synthetic route for Ni₃S₂@Ni₅P₄/NF.⁵² Copyright 2022, Elsevier. (C) Schematic illustration of the preparation process of Ni–S–P/NF.⁵⁴ Copyright 2020, Elsevier.

By accurately selecting the current and time, controlled morphology electrodeposits can be obtained.⁵³ A NiS/Ni₂P (Ni–S–P) electrocatalyst was synthesized using a simple two-step cyclic voltammetry electrodeposition approach.⁵⁴ To obtain Ni–S, sulfurization and electrodeposition were performed using Ni(NO₃)₂ and CH₄N₂S solutions. Ni–S was subsequently transformed into the target Ni–S–P product through electrodeposition and phosphorization, as shown in Fig. 4(C). The distinctive NiS/Ni₂P structure possesses numerous active sites and gas passages. The electrodeposition method also allows for convenient operations with low energy consumption and easy control of the TMP/TMS heterostructures.

4.4 TMP/TMN heterostructure catalysts

Transition metal nitrides (TMNs) are highly suitable electrocatalysts for electrocatalytic reactions owing to their excellent electrical conductivity and corrosion resistance.^55,56 Recently, TMP/TMN heterostructures have displayed remarkable electrocatalytic activity for the OER. Synthesis methods for TMP/TMN heterostructures include synchronous phosphating and nitriding, as well as phosphating of TMN and nitriding of TMP. Typically, preparation of the metal precursor involves techniques such as hydrothermal/solvothermal methods, followed by transformation of the precursor into corresponding TMP and TMN via phosphating and nitriding through heat treatment. Fan et al.⁵⁷ developed a 3D covalent porphyrin polymer (CPF) grafted with triphenylphosphine by special carbonization (see Fig. 5(A)). The Fe–TPP–CPF precursor was synthesized using the Suzuki-coupling reactions and exhibited a large pore volume and narrow pore size distribution. Heating the Fe–TPP–CPF product under a nitrogen atmosphere yielded Fe₂P/Fe₄N@C. Guo et al.⁵⁸ fabricated Co₂P/CoN using a two-step pyrolysis (see Fig. 5(B)). The pluronic123(P123)–melamine–triphenylphosphine dry gel was immersed in Co(NO₃)₂·6H₂O, with P123 serving as the P source. Pyrolyzing the absorbed Co precursors at 550 °C under an Ar atmosphere resulted in the Co/g-C₃N₄ nanosheet. Finally, the Co/g-C₃N₄ was pyrolyzed at 1000 °C and carbonized in NCNT to obtain Co₂P/CoN. Yang et al.⁵⁹ produced VN–Co–P nanowires using a hydrothermal reaction followed by annealing in ammonia (NH₃). Hu et al.⁶⁰ reported nitrogen and phosphorus dual-doped carbon (Co/CoN/Co₂P-NPC) through a solvothermal and phosphating process.

Fig. 5 (A) Schematic diagram of the experimental process of Fe₂P/Fe₄N@C.⁵⁷ Copyright 2017, American Chemical Society. (B) The synthetic route for Co₂P/CoN-in-NCNTs.⁵⁸ Copyright 2018, Wiley-VCH.

4.5 TMP/TMC heterostructure catalysts

Transition metal carbides (TMCs) exhibit similar characteristics to Pt, which make them promising catalysts for the OER.⁶¹ The construction of TMP/TMC heterostructures can enhance the activity of the OER. Two commonly used approaches for synthesizing TMP/TMC heterostructures are synchronous phosphating and carbonization, as well as stepwise phosphating and carbonization. Wu et al.⁶² successfully synthesized V₈C₇/CoP by using hydrothermal, carbonization, and phosphoration techniques (Fig. 6(A)). Similarly, Dutta et al.⁶³ prepared CoP–Mo₂C@NC using this strategy. Li et al.⁶⁴ developed a facile solid-phase polymerization and pyrolysis method to prepare the Fe_xP/Fe₃C heterostructure (Fig. 6(B)). However, high-temperature carbonization typically leads to low yields and aggregation of TMP/TMC heterostructures. Our group has successfully produced MoP–Mo₂C quantum dot heterostructures uniformly hosted on a 3D hierarchically porous thin N, P-doped carbon sheet network through a one-pot simultaneous phosphating–carbonization–activation approach.¹⁵ Chelate confinement can prevent aggregation of MoP/Mo₂C quantum dots. An overview of various synthesis strategies of TMP-based heterostructure catalysts and their OER performances is provided in Table 1.

Fig. 6 (A) Schematic illustration of the synthesis of the V₈C₇/CoP-0.18 microspheres.⁶² Copyright 2020, Elsevier. (B) Synthesis scheme of the Fe_xP/Fe₃C-based heterostructure with controllable iron phosphide crystal phases.⁶⁴ Copyright 2022, American Chemical Society.

Table 1 Different TMP-based heterostructure catalysts for the OER in alkaline media

Catalyst	Synthesis method	η 10 (mV)	Tafel slope (mV dec^−1)	Ref.
Ni2P/Cu3P	Hydrothermal–phosphorization	262	78.1	69
Co2P/V3P	Hydrothermal–coprecipitation–phosphorization	261	67.3	70
CoP3/Fe2P@NF	Hydrothermal–phosphorization	236	50.9	36
Sn4P3/Co2P	Hydrothermal–phosphorization	280.4	52.7	71
CoMoP/MoP@C	Hydrothermal–calcination–CVD	287	74.4	72
Co2P–Ni2P/NF	Hydrothermal–calcination	230	73	73
CoMnP/Ni2P	Hydrothermal–phosphorization	130	49	74
FeP/Ni2P	Solvothermal–phosphorization	240	69.95	75
CoP4/FeP4	Coprecipitation–phosphorization	270	42.4	76
FeP–CoP	Coprecipitation–phosphorization	270	49	77
CoP/Co2P/NCNT	Carbonization–phosphorization	289	94.02	78
Ni2P–Co2P	Etching–phosphorization	283	56.4	79
Ni3S2@Ni2P/MoS2	Hydrothermal–phosphorization/sulfurization	175	46	80
Ni2P–MoS2	Hydrothermal–phosphorization	258	83.7	81
Ni2P–Ni3S2	Hydrothermal–phosphorization–sulfurization	210	62	82
CoP/CN@MoS2	Solvothermal–phosphorization/sulfurization	289	69	83
Ni3S2–Ni3P/NF	Hydrothermal–phosphorization–sulfurization	200	16.1	84
Mn3O4/CoP	Hydrothermal–phosphorization	306	51.8	85
NiCo2O4@Ni2P	Hydrothermal–phosphorization/oxidation	350	59	86
CoWO4/CoP2	Hydrothermal–phosphorization	252	86.9	87
CoP–CoO/CC	Hydrothermal–phosphorization	210	90	88
CoP/CeO2	Solvothermal–phosphorization	224	90.3	89
CuO@Cu3P/CF	Oxidation–phosphorization	267	151	56
S-CoP/Co3O4/CP	Annealing–electrodeposition	211	58.4	90
NiFeP–MnO2	Phosphorization	149	29	91
Fe3O4–CoPx	Annealing–electrodeposition	331	122	92
FeP/Fe3O4	Phosphorization	229	27.6	93
Co2P–Co3O4	Electrodeposition–oxidation	265	57	94
CoP/CeO2–FeOxH	Electrodeposition	300	75.7	95
Cu–Cu3P/CuO	Electrochemical–oxidation	315	74.8	96
Cu3P–Cu2O	Thermal–decomposition	286	79.02	45
Co2P–Co3O4	Thermal–decomposition	246	69.5	44
Co3W3C/CoP	Electrospinning–calcination–phosphorization	200	72	97
Co2P/Mo3Co3C@C	Hydrothermal–phosphorization	362	82	98
Co2P/Co2N	Electrospinning–phosphorization	360	67.61	16

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TMPs have a significant impact on their OER performance due to the influence of their atomic structure on their electronic structure. Firstly, phosphides possess inherent metallic properties, as demonstrated by their electronic density of states near the Fermi level. This electronic structure allows for efficient electron transfer through the coupling effect between d–p–d orbitals and regulates the strength of adsorption intermediates.⁶⁵ Additionally, the presence of phosphorus in TMPs leads to an increase in atomic spacing, which in turn weakens the interactions between metals and causes d-band contraction. As a result, TMPs exhibit higher electron density and good conductivity.^66–68 Consequently, TMPs possess unique characteristics and atomic and electronic structures that enable them to achieve higher charge carrier transfer efficiency, fast electron transfer, and enhanced catalytic activity.

5. Improvements of TMP-based heterostructure catalysts

Electrocatalysts for the OER must be highly efficient and meet certain conditions: (1) rapid electron transmission, (2) sufficient contact area to expose active sites, and (3) excellent intrinsic activity. To achieve highly active electrocatalysts, we can use strategies such as metal-doping, vacancy engineering, morphology regulation, and multicomponent heterostructures to improve TMP-based heterostructure materials.

5.1 Metal-doping strategy

Metal-doping involves introducing another metal atom to the catalyst, which can adjust its adsorption capacity, expose active sites, and modulate the electronic structure.^99,100 Doped metals can also serve as active sites to influence the electronic structure or react directly.¹⁰¹ For example, the doping of Mn caused the main diffraction peak in the XRD pattern to slightly shift to the right compared to that of Ni₅P₄ (Fig. 7(A)).^102,103 A Ce-doped CoMoP/MoP@C heterogeneous catalyst was found to exhibit high OER performance due to shifts in Co 2p, Mo 3d, and P 2p XPS spectra (Fig. 7(B)).⁷² Strong evidence of Mn doping in Ni₂P/Fe₂P was obtained from extended synchrotron X-ray absorption fine structure measurements (Fig. 7(C)).¹⁰³ Hu et al.¹⁰⁴ synthesized a Fe-doped Fe–CoP/CoO heterostructure (Fig. 7(D)), which exhibited superior OER activity with a η₁₀ of 219 mV, a Tafel slope of 52 mV dec⁻¹ and a C_dl value of 4.53 mF cm⁻² due to the increased carrier density of CoP resulting from Fe doping. The valence edge of Fe–CoP was observed to be closer to the Fermi level than that of CoP.¹⁰⁵ Furthermore, the adsorption of Fe on oxygen-containing intermediates was enhanced, resulting in a reduced Gibbs free energy of Fe–CoP/CoO required for O* → OOH* (1.71 eV → 1.64 eV) (Fig. 7(E)).^106,107 Similarly, doping with Ru and B was shown to modulate the electronic structure of the Ni₂P/Ni₅P₄ catalyst, resulting in improved OER performance (η = 270 mV, at 10 mA cm⁻² in Fig. 7(F)).¹⁰⁸ Xu et al.¹⁰² designed a Mn-doped Ni₂P–Ni₅P₄/NF hybrid catalyst, where the doping of Mn exposed a number of active sites, leading to excellent OER performance. Metal doping mainly alters the electronic structure and local binding environment, affecting the adsorption energy of intermediates and promoting charge transfer. However, introducing metal catalysts can be unstable and undergo oxidation and dissolution, or surface reconstruction during the OER process.¹⁰⁹

Fig. 7 (A) XRD pattern of Mn–Ni–P/NF.¹⁰² Copyright 2020, Elsevier. (B) XPS of Mo 3d regions of Ce-doped CoMoP/MoP@C and CoMoP/MoP@C.⁷² Copyright 2021, American Chemical Society. (C) Mn toward fabricated electrocatalysts.¹⁰³ Copyright 2023, Elsevier. (D) Calculated total and partial electronic density of states (TDOS and PDOS) for CoP and Fe–CoP.¹⁰⁴ Copyright 2019, Elsevier. (E) The DFT calculated Gibbs free energy of the OER process on the CoPO and Fe–CoPO surfaces, respectively.¹⁰⁴ Copyright 2019, Elsevier. Purple, blue, green and dark green represent cobalt atom, phosphorus atom, iron atom and oxygen atom in the structural model, respectively. (F) LSV curves of the prepared catalysts and RuO₂ for the OER in 1 M KOH.¹⁰⁸ Copyright 2022, Royal Society of Chemistry.

5.2 Vacancy engineering strategy

Vacancies are significant for enhancing the catalyst activity and are categorized as cation, anion, or mixed vacancies. While cation vacancies have not been extensively researched due to their high formation energy, P, S, and O anion vacancies have gained widespread attention. These vacancies can be obtained through various methods, including wet-chemistry, plasma etching,¹¹⁰ doping,¹¹¹ and heat treatment,¹¹² which can accurately adjust their concentration by controlling the reaction conditions.¹¹³ To detect vacancies, one can use techniques such as STEM, XPS, EPR, XAFS, Raman spectroscopy, and DFT. Zhu et al.¹¹⁴ used TEM to study the structure of F–FeCoPv@IF and found vacancies on its surface (Fig. 8(A)). Vacancies can be detected by observing the changes in peak position or intensity of the material using XPS, compared to materials without defects.¹¹⁵ In a heterostructure NiCo/NiCoP (Fig. 8(B)), oxygen vacancies were combined to accelerate the electrical conductivity and surface redox kinetics.¹¹⁶ EPR is a sensitive technique used to detect and characterize vacancies in materials, providing fingerprint information (g values) about unpaired electrons of the material.¹¹⁷ Yan et al.¹¹⁸ estimated the P vacancies of Ar–NiCoP|V using EPR (Fig. 8(C)). XAFS provides information about vacancies, such as bond length, oxidation states, atomic coordination number, and unoccupied electron states.¹¹⁵ Li et al.⁹⁰ used XAFS to analyze the vacancies of (s-CoP/Co₃O₄) which indicated the successful formation of P vacancies (Fig. 8(D)).

Fig. 8 (A) HRTEM images of F–FeCoPv@IF.¹¹⁴ Copyright 2023, Elsevier. (B) XPS spectra of various elements in NiCo/NiCoP hybrid: O 1s.¹¹⁶ Copyright 2019, Elsevier. (C) EPR spectra of Ar–NiCoP|V.¹¹⁸ Copyright 2019, Royal Society of Chemistry. (D) Fourier transforms of k²-weighted EXAFS data at R space for the Co Kedge of CoP and s-CoP of s-CoP/Co₃O₄.⁹⁰ Copyright 2021, Royal Society of Chemistry. (E) Free-energy diagrams for the OER pathway of different samples.¹²² Copyright 2022, American Chemical Society. (F) The OER polarization curves of Fe₂P–WO_2.92/NF.¹²⁴ Copyright 2022, Elsevier.

Vacancy engineering enhances the intrinsic activity and improves the electrical conductivity by tailoring the electronic structure and optimizing the adsorption energy of intermediates.^119–121 Vacancies in TMPs, mainly P vacancies, induce the formation of O vacancies. A CeO₂–Ni₂P heterostructure nanosheet was constructed,¹²² and the enriched O_v on NiOOH was indicated by increased O_ad/O_L values after CV activation.¹²³ More oxygen vacancies were found in Ni₂P-derived NiOOH through EPR. The RDS of CeO₂–NiOOH is OOH* → O* with a low ΔG of 1.75 eV. But, the RDS of CeO₂–NiOOH–O_v is the deprotonation of OH* intermediates, and the energy barrier is 1.70 eV (Fig. 8(E)). Obviously, the incorporation of O_v further improved the adsorption strength of O*, OH* and OOH* intermediates, resulting in the boosted OER activity. Yang et al.¹²⁴ fabricated an oxygen-vacancy-rich Fe₂P–WO_2.92/NF electrocatalyst observed through XPS spectrum of O 1s and lattice rearrangement causing the formation of WO_2.92. The presence of oxygen vacancies reduced lattice stress and improved hydrophilicity. The Fe₂P–WO_2.92 catalyst displayed a lower overpotential (η₁₀ = 215 mV), smaller Tafel slope (46.3 mV dec⁻¹), and largest TOF value (0.05599 s⁻¹ at 280 mV) compared to those of Fe₂P/NF and WO_2.92/NF, indicating that oxygen vacancies further enhance the OER activity (Fig. 8(F)). Additionally, Fe₂P–WO_2.92 has excellent long-term durability that only increased by about 8.4% after 100 h of continuous operation at 10 mA cm⁻².

5.3 Morphology regulation strategy

Metal phosphide-based heterostructure catalysts can produce different nanostructures based on morphology. Zhao et al.³⁷ created a Co₂P–Ni₂P heterostructure with various morphologies to enhance the OER performance. Controlling the amount of Co²⁺ and Ni²⁺ in the synthesis reaction can alter the morphology and microstructure of Co₂P–Ni₂P. Co₂P-1.6/Ni₂P-0.4 has a hierarchical nanostructure composed of mostly nanowires and fewer nanosheets. Meanwhile, Co₂P-1/Ni₂P-1 is made up of a nest-like hierarchical nanostructure, made of microspheres and nanowires that grow in various directions. Co₂P-1/Ni₂P-1 has better electrocatalytic properties, with only 310 mV at 10 mA cm⁻² overpotential compared to Co₂P-1.6/Ni₂P-0.4 (η₁₀ = 350 mV). Additionally, Co₂P-1/Ni₂P-1 has a smaller Tafel slope (69.9 mV dec⁻¹) than Co₂P-1.6/Ni₂P-0.4, illustrating its proficient OER kinetics. Co₂P-1/Ni₂P-1 also has a C_dl value of 34.4 mF cm⁻², almost 1.7 times higher than that of Co₂P-1.6/Ni₂P-0.4. Due to the hierarchical nanostructure assembled by slender and flexible nanowires and microspheres, a more significant active surface area and electron transport channels become available during the electrocatalytic process.

3D nanomaterials offer several advantages, including higher specific areas, better contact with electrolytes, higher electronic diffusion efficiency, rapid release of gas bubbles, and intermediate adsorption and transformation.¹²⁵ Li et al.¹²⁶ developed 3D flower-like CoP₃/CeO₂ heterostructures, in which the concentration of Co²⁺ and Ce³⁺ is adjusted to achieve optimal OER performance. The morphology of CoP₃/CeO₂-2 and CoP₃/CeO₂-3 exhibited irregular ultrathin nanosheets and a flower-like structure. While CoP₃/CeO₂-2 demonstrated good OER activity with low overpotential (η₁₀ = 339.2 mV), smaller Tafel slope of 126 mV dec⁻¹, and smaller charge transfer resistance (1.57 Ω), CoP₃/CeO₂-3 exhibited less activity. The performance difference can be attributed to the sparse pore structure composed of nanosheets in CoP₃/CeO₂-2, which provides a large surface area and interface between the catalyst and electrolyte, as compared to the more compact pores of CoP₃/CeO₂-3. Furthermore, the 3D flower-like architecture contributes to the abundant accessible active sites and fast electron/mass transport, resulting in superior OER activity for CoP₃/CeO₂-2.

Despite significant advancements in morphology regulation of electrocatalysts, there are still challenges that need to be addressed. One of these challenges is the tendency of 1D and 2D morphologies to stack on top of each other, which can block active sites and reduce catalytic activity.¹²⁷ Additionally, 3D morphologies are prone to collapsing and breaking, which diminishes the available surface area. Therefore, there is a need to tackle the issues of stacking and collapsing of nanostructures during the OER process.

5.4 Multicomponent heterostructure strategy

A useful strategy for improving the OER performance is to construct a multicomponent heterostructure. The multicomponent structure effectively adjusts the electronic structure of the active center, reducing the adsorption capacity for intermediates and thereby improving the OER activity.¹²⁸ For example, Song et al.¹²⁹ created Co/CeO₂/Co₂P/CoP multicomponent heterostructures for OER electrocatalysts. HRTEM images revealed that Co/CeO₂/Co₂P/CoP was successfully composed (Fig. 9(A)). The Co/CeO₂/Co₂P/CoP electrocatalyst exhibited low overpotential (307 mV at 10 mA cm⁻²) compared to Co/Co₂P/CoP and CeO₂/CoP (which required 331 mV and 341 mV, respectively, to achieve the same current density) (Fig. 9(B)). Co/CeO₂/Co₂P/CoP also showed faster OER kinetics with a smaller Tafel slope (99.6 mV dec⁻¹) compared to Co/Co₂P/CoP and CeO₂/CoP (which had Tafel slopes of 104 and 114.8 mV dec⁻¹). The charge transfer resistance (R_ct) of Co/CeO₂/Co₂P/CoP was 18.78 Ω, significantly smaller than that of Co/Co₂P/CoP (20.82 Ω) and CeO₂/CoP (27.79 Ω). Co/CeO₂/Co₂P/CoP showed excellent OER activity, with a durability of 24 h at a current density of 10 mA cm⁻² (Fig. 9(C)). The rich heterostructure interfaces and synergistic effects between various components made Co/CeO₂/Co₂P/CoP the most effective OER electrocatalyst. Du et al.¹³⁰ synthesized a ternary TMP composite Fe₂P/CoP/Ni₅P₄ electrocatalyst (Fig. 9(D)) which exhibited smaller overpotential compared to Fe₂P/CoP at the same current density, indicating the best activity. The synergistic effects among ternary TMPs and between them made Fe₂P/CoP/Ni₅P₄ a highly active OER electrocatalyst. Other multicomponent heterostructures reported include those of Fe₂P/CoP/Ni₅P₄ and Ni₃S₂@Ni₂P/MoS₂. Constructing multicomponent heterostructures generates more catalytically active centers, thereby improving the intrinsic activity of the catalyst. Multiple active sites also exhibit strong synergistic effects, enhancing electrocatalytic performance.¹³¹

Fig. 9 (A) HRTEM images of the Co/CeO₂/Co₂P/CoP@NC.¹²⁹ Copyright 2020, Elsevier. (B) Polarization curves of Co/CeO₂/Co₂P/CoP@NC.¹²⁹ Copyright 2020, Elsevier. (C) The chronoamperometry curve of Co/CeO₂/Co₂P/CoP@NC.¹²⁹ Copyright 2020, Elsevier. (D) HRTEM image and SAED pattern of the Fe₂P/CoP/Ni₅P₄.¹³⁰ Copyright 2019, American Chemical Society.

6. Challenges of TMP-based heterostructure catalysts

6.1 Interface structure and electron transfer

Heterostructures not only can generate electron redistribution at the interface to achieve synergy but also change the crystal phase of the compound to generate a new interface structure. The interface interaction can optimize the electronic structure and enhance active site density. The electrocatalytic performance could also be improved by the interface structure due to the OER reaction typically occurring on the surface of catalysts. The interface of the heterostructure optimizes the adsorption of OH for the OER. Two effects, the electron effect and ensemble effect, play an important role of heterointerfaces in catalysts. The electron effect refers to the electronic interaction between multi-components across the heterointerface, leading to energy band difference. The difference induces electron-directed transfer and creates a charge distribution gradient at the interface. The electron effect balances the adsorption energy of reaction intermediates by coupling their electronic configurations. Moreover, the ensemble effect typically serves as bridges or adsorption sites with different coordination environments to accelerate the adsorption process of intermediates.¹³²

Interface charge transfer is the result of modifying the interface properties through heterostructures by combining different components. An example of this is the work done by Liu et al. who reported bimetallic phosphide nanosheets containing abundant Cu₃P/Ni₂P heterogenous interfaces on Cu foam.¹³³ The HRTEM image of Cu₃P/Ni₂P displays an obvious heterogenous interface between Cu₃P and Ni₂P. As shown in Fig. 10(A), the strong charge redistribution occurred at the interface as the average valence of Cu decreases, while the average valence of P in Ni₂P portion increases. Therefore, the valence electron state of active sites could be regulated and adsorption of intermediates, thereby lowering the energy barrier. The heterointerfaces will lead to changed electronic structures, as electrons transfer from one component to another, resulting in modified adsorption and desorption free energies of the intermediates. The Fe-doped Ni₃S₂/Ni₂P heterostructure with an abundant interface was constructed,¹³⁴ as shown in Fig. 10(B), and the values of ΔG decreased after the formation of the interface between Ni₂P and Ni₃S₂, which suggests that the heterointerfaces can promote chemisorption of the oxygen-containing intermediates. Furthermore, the interface can effectively regulate the electronic structure and d-band center, and boost the conductivity to improve the performance of catalysts, as shown in Fig. 10(C). For ensuring the activity and stability of catalysts in the OER, maintaining close connections between interface components is crucial. Additionally, although the interface structure has been widely used, the decisive factors of the interface effects between different phases are still unclear.

Fig. 10 (A) Schematic illustration of interfacial charge transfer of Cu₃P/Ni₂P.¹³³ Copyright 2022, Elsevier. (B) The calculated free-energy profiles of the OER at 0 V of Ni₃S₂/Ni₂P.¹³⁴ Copyright 2021, Elsevier. (C) Charge density difference plots of the interface and carbon-coating of Ni₃S₂/Ni₂P.¹³⁴ Copyright 2021, Elsevier. (D) Schematic diagram of band structures, energy band diagram of Ni₂P and CoCH, and charge density difference plot at the Ni₂P–CoCH interface.¹³⁸ Copyright 2023, Wiley-VCH. (E) Raman spectra of all samples during the OER of Ni₅P₄@FeP.¹⁴² Copyright 2022, Elsevier. (F) Schematic diagram of the dynamic surface reconstruction of Ni₅P₄@FeP.¹⁴² Copyright 2022, Elsevier.

6.2 Built-in electric field

The built-in electric field (BIEF) is then generated by combining two hetero components with different Fermi levels. BIEF at the interface of a heterojunction can effectively adjust the electronic structure and optimize the adsorption capacity of reaction intermediates to reduce the overpotential of the OER.^135–137 Zhang et al.¹³⁸ employed a continuous hydrothermal–phosphorization process to construct a Ni₂P–CoCH heterostructure. Fig. 10(D) shows that Ni₂P has a greater work function difference compared to CoCH. Therefore, the internal electric field drives electrons to transfer from CoCH to Ni₂P, inducing local charge redistribution at the interface and fabricating the deprotonation process from OH* to O*. Similarly, Niu et al.¹³⁹ prepared CoP–FeP heterostructures by taking advantage of the built-in electric field at the interface to form intimate contact between CoP and FeP and optimize the electron density, resulting in increased electrocatalytic activity. However, the complicated chemical environments of heterointerfaces pose difficulties in investigating the action mechanism of the BIEF.¹³⁶

6.3 Component reconstruction

During the OER process, the surface of phosphides transforms into corresponding oxides/hydroxides given their thermodynamic instability in strongly oxidative environments. These oxides/hydroxides are capable of acting as the actual active species towards the OER.¹⁴⁰ Recently, a phenomenon known as component reconstruction of electrocatalysts was recognized as a mechanism leading to the formation of reactive sites. For instance, Cheng et al.¹⁴¹ found that in Fe₂O₃@Ni₂P/Ni(PO₃)₂, NiOOH and FeOOH structures were formed on the surface and they were considered the real catalytic species. Li et al.¹⁴² utilized in situ Raman spectroscopy along with ex situ characterization to analyze the surface adaptive reconstruction on Ni₅P₄@FeP hybrids after the OER. The results showed that the hybrids are rapidly reconstructed to NiFe₂O₄ and are further oxidized to amorphous Ni/FeOOH@NiFe₂O₄ intermediates at high potentials (Fig. 10(E) and (F)). Consequently, the plentiful interfaces between NiOOH and NiFe₂O₄ synergistically contributed to superior OER activity with the overpotential of the current density of 10 mA cm⁻² being only 205 mV and the Tafel slope being 42.3 mV dec⁻¹.

The complex composition of the reconstruction has made it difficult for researchers to fully understand the mechanism of electrochemical oxidation. Therefore, more accurate characterization is needed to understand each component involved in the reconstruction process, which would aid in understanding the OER mechanism. In addition, for catalysts with weak stability, the reconstruction process may disrupt the internal structure due to the dissolution of components.

7. Conclusions and perspectives

The low-cost, highly-efficient TMP-based heterostructure electrocatalysts like TMP/TMP, TMP/TMO, TMP/TMS, TMP/TMN, and TMP/TMC heterostructures have been summarized as the base of potential practical electrocatalysts for the OER. To further explore the OER performances of TMP-based heterostructure electrocatalysts, several key parameters such as overpotential, Tafel slope, Faraday efficiency, turnover frequency, and voltage or current retention rate have been reviewed to evaluate the electrocatalytic properties. Surface regulation strategies such as metal-doping, vacancy engineering, morphology regulation and multicomponent heterostructures have been studied to improve OER performances. While these tools have been proven to be effective, practical TMP-based heterostructure electrocatalysts need additional attention. Firstly, synthesizing TMP-based heterostructures on a larger scale is necessary to lower production costs. Secondly, precise control of the composition and morphology of TMP-heterostructures is difficult to attain. Thirdly, despite their effectiveness, the electro-catalytic activities of TMP-based heterostructures still fall short of commercial precious metals. Fourthly, sustaining long-term preservation of the catalyst activity remains a difficult problem, especially with the loss of P that seriously affects activity in alkaline solutions. Fifthly, interface structure and electron transfer, built-in electric field and component reconstruction are still unclear and more research in this area is needed and deserves attention especially the processes in industrial water electrolysis. Lastly, there is still a need to elaborate on the functional mechanism between heterostructures and H₂O. In situ evaluation of the functional mechanisms of TMP-based heterostructures for the OER, including an in-depth understanding and exploration of the structural evolution of heterostructures and dynamic interaction mechanisms between catalysts and H₂O above 1.23 V vs. RHE, is currently lacking and needs immediate attention.

Although TMP-based heterostructures exhibit excellent activity for the OER in laboratory research, there are still significant gaps to be addressed in terms of meeting the requirements for efficient and stable operation in industrial practical applications. Rarely have studies focused on the industrial practical applications of TMP-based heterostructures for the OER. In addition, due to the complex synthesis required for TMP-based heterostructures, their production at larger scales remains limited to laboratory settings. To overcome this obstacle, future heterostructure design should prioritize scalability and compatibility with industrial manufacturing processes. Despite these challenges, there is substantial room for improvement, making the design of TMP-based heterostructures for the OER a valuable area of research. Overall, we believe that TMP-based heterostructures hold great promise as electrocatalysts for achieving highly efficient oxygen evolution and water splitting.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51962002) and the Natural Science Foundation of Guangxi (2022GXNSFAA035463).

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