WANG et al.

Abstract

Because of features, such as adjustable structures, high porosity, and high crystallinity, metal–organic frameworks (MOFs) deservedly have received considerable attention. Nevertheless, there is still room for improvements in the electrical conductivity and chemical stability of some MOFs, because of which they cannot be utilized as electrode materials. Fortunately, MOF derivatives have received widespread attention in recent years, especially phosphide materials, which are widely used in practical applications because of their outstanding conductivity, excellent specific surface area, and standout charge mobility. In this review, the latest developments of MOF‐derived phosphides in electrocatalysis related to energy, including the excellent performance in terms of electrochemical energy storage and ingenious strategies, and diversified synthetic approaches have been emphasized and summarized. Additionally, the arduous task and feasible proposals of MOF‐derived phosphides are also discussed.

INTRODUCTION

One of the main problems that the world is facing today is the energy crisis; many researchers are seeking out and researching new clean, renewable energy technologies that can be used as substitutes. Nowadays, many advanced chemical batteries, due to their good characteristics, including high power density, high-temperature characteristics, outstanding service life, energy conservation, and environmental protection, meet people's requirements for new energy, ensure the safe use of energy, and reduce the dependence on traditional resources. Also, for better battery performance, the key lies in the selection of electrode materials.

As porous ligand materials, metal–organic frameworks (MOFs) are composed of metal ions and cluster organics, which have been widely studied. As such, various characterization methods have shown that MOF materials have excellent frameworks and pore structures,¹ which make them attractive in fields like supercapacitors (SCs), chemical sensing, gas storage and separation, adsorption, biomedical applications, proton conduction, and drug delivery.^2–35 Compared with other ligand materials, MOF materials are deemed to be one of the promising materials in the nano feild of future in view of their excellent properties such as high porosity, large surface area, controllable structure, low density, and adjustable pore size.^{20,29,36–39} Consequently, MOF materials have attracted extensive attention worldwide. Especially in electrochemical catalysis and energy storage, MOFs can provide suitable space for ion storage/transfer and electrochemical reactions according to their prominent surface area.^40–43 The once-criticized drawbacks of MOF materials of poor electrical conductivity and stability have now been shown to be solvable. The conductivity of MOF-derived phosphides is sufficient for electrochemical applications, and strategies for developing three-dimensional configurations of phosphides are available.^44–50 Nevertheless, researchers are still focusing on improving the electrical conductivity and stability of some MOF materials so that perfect electrode materials can be created.

MOF-derived materials, such as MOF‑derived nonmetal carbon materials, MOF‑derived monatomic catalysts, MOF‑derived composites consisting of metal clusters and carbon, and MOF‑derived composites consisting of metal compounds and carbon, effectively make up for the above weaknesses to some extent, and have aroused researchers' interest. In particular, MOF-derived phosphide nanomaterials can not only retain the benefits of the original MOF structure but also be useful for the practical application of energy storage, and some catalysis processes like thermal catalysis, electrocatalysis, and photocatalysis, because of the outstanding performance in active sites, physicochemical properties, and component structures.^51–61 Also, further processing and design of the above materials will help to improve the cycling durability and energy power density of MOFs, optimize the structure of carbon-based catalysts, enhance the application of active metal atoms, and promote the conductivity, stability, and the corrosion resistance of catalyst carriers.^62–65 Many researchers are still working on the development and application of MOF-derived phosphide nanomaterials. As research in MOF-derived phosphide nanomaterials is gradually becoming saturated (Figure 1A,B), it is time to comprehensively review the application of MOF-derived phosphide nanomaterials in electrochemistry.

Figure 1. (A) Trend graph of the number of studies in relation to the year. (B) Percentage of each research area on MOF-derived phosphide materials. (C) Summary of the synthesis methods of metal–organic framework-derived phosphide nanomaterials

In this overview, the advances made to date in terms of MOF-derived phosphides in energy-related electrocatalysis, including ingenious-modulated strategies, various synthetic methods to prepare NiCo bimetallic phosphide MOF-74 materials,⁶⁶ CoP/Fe₂P@mC nanocomposites,⁶⁷ NiFe phosphide nanorods derived from MIL-88-Fe₂Ni MOF (MIL = materials of Institute Lavoisier),⁶⁸ bimetallic Ni_0.4Mn_1.6P derived from Ni@Mn-MOF,⁶⁹ Ru-doped bimetallic phosphide derived from 2D MIL-53 MOF (Figure 6A,B),⁷⁰ MOF-71-derived layered Co-CoP/C,⁷¹ Co MOF-derived CoP@HPC-T (HPC = hierarchical porous carbon),⁷² M16-ZIF-67-CNT-P (ZIF = zeolitic imidazolate framework, CNT = carbon nanotube),⁷³ CuP₂@C from a Cu-MOF-derived Cu@C composite,⁷⁴ and so on are elaborated. For example, Hou and his team fabricated a superstructure consisting of ultra-fine CoP_x nanoparticles that decorated carbon nanosheets (NSs) with a porous nickel foam (NF) template, which also shows outstanding electron transport properties, mass transport, and trifunctional electrocatalytic activity.⁷⁵ The nano-sized MoP@porous carbon composites prepared by Yang et al. not only have highly exposed and accessible surface areas but also have superior catalytic properties for hydrogen precipitation reactions (Figure 11A–C).⁷⁶ Liu proposed an effective modulation strategy for the preparation of CoP/Co-MOF, and the resulting CoP/Co-MOF with a porous structure showed significant catalytic performance for hydrogen evolution reaction (HER). Besides, the strategy is scalable and is anticipated to be applied to other MOF-based materials.⁷⁷ Additionally, the diverse in situ characteristics in the catalytic course of oxygen evolution reaction (OER),^78,79 HER,^80–82 and oxygen reduction reaction (ORR),^75,83–85 as well as other reactions, the excellent performance in terms of electrochemical energy storage, and the application of SCs,⁸⁶ sodium-ion batteries (SIBs),^87–90 lithium-ion batteries (LIBs),^91–95 and other batteries^96–98 have been summarized and highlighted, which is expected to be valuable for comprehending the current advances made in MOF-derived phosphides and their utilization in various fields.

COMPOSITION OF MOF-DERIVED PHOSPHIDE NANOMATERIALS

Given the controllability of the structure and the diversity of the composition, MOF-derived materials have been considered as promising candidates for porous materials. Nevertheless, only large metal phosphides (MPs) will be acquired when fabricating MOF-derived phosphide nanomaterials in the traditional manner. In the last few years, a lot of methods (Figure 1C) have been reported to synthesize MOF-derived transition-metal phosphides (TMPs) with nanostructures, controllable morphologies, and sizes (Figure 2). When it comes to synthesis methods, the currently available strategies can be divided into several categories: the solvothermal method, the pyrolytic process, hydrothermal fabrication, the template strategy, and other methods such as the microwave-assisted method, the electrochemical method, and the sonochemical method. Multitechnology combination strategies are also used widely.

• Solvothermal/hydrothermal method: The solvothermal/hydrothermal method promotes the reaction and drives the crystallization procedure, and the obtained MOF materials usually have high thermal stability.

• Template strategy: Use of a template strategy to synthesize nanostructured MOF materials is a good option to realize efficient control of the morphology.

• Pyrolytic process: Pyrolysis is a common method for the synthesis of MOF-derived materials that are uniform in size. The acquired MOF-derived materials possess the advantage of high porosity.

• Microwave-assisted method: The microwave-assisted method facilitates the production of ultra-small hollow nanoparticles with good dispersion and high phase purity from MOF materials.

• Electrochemical method: Electrochemical methods are mainly classified into anodic synthesis, cathodic synthesis, indirect bipolar electrodeposition, electroplating substitution, and electrophoretic deposition (EPD), which allow the rapid fabrication of MOF materials with controllable morphology and excellent porosity.

• Sonochemical method: The sonochemical method helps to shape homogeneous MOF materials and requires a shorter duration of crystallization.

Therefore, as active catalysts, the TMPs are widely utilized in photocatalysis, electrocatalysis, thermocatalysis, and power storage technologies,^52,99 and their reactivities are strongly related to the properties of MPs. Furthermore, MOF-derived phosphide nanomaterials can show excellent electrochemical performances in many other widespread applications (Figure 3). Through the heterogeneous utilization of MPs, it can be revealed that in some cases, MPs themselves are significant. However, the synergistic effects of intrinsic TMPs and surface oxidation are responsible for the high reaction performances in other cases.¹⁰⁰

Monometallic phosphides

Monometallic phosphides, including phosphides of nickel, cobalt, molybdenum, and tungsten, yield phases with different compositions and structures when they are rich in metals. Moreover, monometallic phosphides have attracted considerable attention in catalysis because their performances are similar to those of metals, and they have excellent chemical resistance, high thermal stability, and good thermal conductivity.^101–105

To date, to synthesize MOF-derived monometallic phosphides, pyrolytic or template approaches have been the most widespread and efficient methods to synthesize MOF-derived monometallic phosphides with adjustable scale and morphology by reasonably controlling the environment, reaction time, temperature, and so forth.¹⁰⁶ Also, some typical strategies for fabrication are presented in the following sections.

Pyrolytic process

Using UiO-66 MOF as the host, MoN- and Mo₂C-, together with MoP-decorated carboncatalysts, can be selectively obtained without changing the morphology using three treatments: nitridation, carbonization, and phosphorization.⁷⁶ Also, using a facile low-temperature phosphorization treatment of an Fe-MOF precursor, Zhao and coworkers prepared FeP-500 nanobundles using quasi-paralleled one-dimensional nanorods (Figure 4E,F). The fabricated FeP-500 had hierarchical nanostructures accompanied by rich directional channels, ensuring abundant availability of active sites, feasible mass transfer, and convenient gas-bubble release.¹⁰⁷ Tian et al. proposed a novel, facile, controllable, low-cost, and scalable method for preparing Ni₂P and Ni₁₂P₅ via a direct phosphorization treatment of Ni-MOF (Ni-BTC, BTC = benzene-1,3,5-tricarboxylic acid) under gentle conditions, which has high potential for use for massive production.¹⁰⁸ Small-sized (about 10 nm) monodisperse Ni₂P, Ni₁₂P₅ have been fabricated and then assembled into a typical MOF, UiO-66-NH₂ (Zr₆O₄(OH)₄(BDC-NH₂)₆) (BDC = 1,4-benzenedicarboxylate), and accordingly yielded Ni₂P@UiO-66-NH₂ and Ni₁₂P₅@UiO-66-NH₂, respectively (Figure 4G,H).¹⁰⁹ Using the in situ interfacial precipitation process, a facile and effective method has been reasonably designed to directly immobilize prussian blue analogue (PBA) nanocubes (NCs) on cobalt hydroxide nanoplates (PBA@Co(OH)₂) with a subsequent pyrolytic approach in sodium hydrogen phosphide.¹¹⁰ Materials are successfully prepared by embedding Ni phosphide particles into a carbon matrix with Ni(II)PCM-101 as the single phosphorus source (Figure 4D).¹¹¹

Template strategy

Favorable intermediate binding on the Fe-doped phosphides of Ni is predicted based on the simulated results. Iron-doped Ni₂P nanoparticles inserted into CNTs are synthetized, utilizing MOF arrays that are directly cultivated on NF and presented as the template of the structure (Figure 4A). Peng et al. chose a Ni(BDC) MOF. It has an adjustable two-dimensional lamellar architecture, undergoes the treatments of carbonization and phosphorization. Through the preparation of nanoparticles of iron-doped Ni₂P implanted in CNTs, utilizing MOF arrays on NF presented as the configurable template, a large number of CNTs encapsulating nanoparticles of MPs were developed as precursors (Figure 4B,C).¹¹²

Bimetallic phosphides

Compared with monometallic phosphides, bimetal phosphides can play a synergistic role of two different metal ions, change the electronic structure of the electric catalyst, and provide more surface-active sites, thus improving the catalytic efficiency and stability of the catalyst. To further investigate the properties and derivatization of bimetallic phosphides, exploring the synthesis methods is a breakthrough point. Compared with monometallic phosphides, the synthesis methods of bimetallic phosphides are more diverse, and in the following section, some examples are presented.

Pyrolytic process

For instance, through directly phosphating NiCo MOFs (NiCo-BDC) as bifunctional electrocatalysts for water splitting, NiCo bimetallic phosphoric hybrids are synthetized, which is attributed to the distinctive porous architecture together with the synergistic effect of Co₂P and Ni₂P in a bimetallic phosphide system.¹¹³ To synthesize a heterogeneous Ni₂P-Fe₂P micro-sheet, Ni foam is soaked in a solution of iron nitrate and hydrochloric acid with a subsequent phosphorization treatment. A comprehensive and commercial method for synthesizing heterogeneous P metal catalysts for water/seawater electrocatalysis is presented.¹¹⁴ After a facile one-step phosphorization reaction, a dual P-doped NiCo bimetallic material grown on NF (Sn-NiCoP_x-NF, n = 1–4) was obtained.¹¹⁵ Hollow cobalt-nickel bimetallic phosphides were prepared by Lu and coworkers by phosphating the CoNi-MOF at a low temperature, in which HL10 served as the organic linker.¹¹⁶ FeNiP/C-900 has been designed by Nie et al. using a new and safe method without adding an external phosphorous source. By embedding FeNiP/C-900 into the carbonaceous matrix, and pyrolyzing it at 900°C, a steady cage-based construction was synthesized and the hollow barrel-like structure was preserved.¹¹⁷

Hydrothermal fabrication

Through hydrothermal development of CoNi-carbonate-hydroxide nanowires (NWs) and subsequent treatment of post-phosphorization, a convenient preparation of various CoNiP NWs including compositions of cobalt and nickel is reported.¹¹⁸ Furthermore, self-supported NS arrays of iron-doped Ni₂P on the NF have been obtained using a facile hydrothermal approach and in situ phosphorization treatment. The representation of the NS arrays of iron-doped Ni₂P as bifunctional catalysts for comprehensive water splitting relies strongly on the ratio of iron doping in the Ni₂P. On Ni foam, acting as bifunctional electrocatalysts for water splitting, arrays of 3D self-supported Fe-doped Ni₂P NS are fabricated by the hydrothermal approach after in situ phosphorization.¹¹⁹

Solvothermal method

Doping Co²⁺ species into Fe-based MOF needs the following two-step reactions. A Co-doped Fe-MOF has been fabricated using a solvothermal method, and then by the oxidation and phosphorization of CoFeP, hollow nanorods are obtained.⁹³ The core–shell construction of NiCoP@MoS₂ was assembled via a solvothermal treatment, where the NiCoP substrates were randomly and uniformly coated by NSs of MoS₂, after obtaining the NiCo phosphides phosphated by the NiCo-based PBA.¹²⁰

Template strategy

A concise Ketjenblack carbon templated in situ fabrication route has been used to prepare bimetallic phosphides of NiFe that are highly dispersed as high-performance OER electrocatalysts (Figure 5C).¹²¹ Moreover, a template-assisted method proposed by Hu and his coworkers has been used to assemble 2D NSs of an oriented Ni-Co precursor onto anisotropic cuboid particles of Ag₂WO₄ 3D. After that, cuboid particles of Ag₂WO₄ are selectively eliminated by treatment of etching, heating, and phosphorization. At the same time, the Ni-Co precursor is transformed into nickel-cobalt oxide (Ni-Co-O), followed by phosphide (Ni-Co-P).¹²² (Ni_0.62Fe_0.38)₂P with a hollow nanocubic structure is a promising OER catalyst fabricated on a template of Ni-Fe PBA, with high OER activity and a large specific surface area (Figure 5D–F).¹²³

Others

Tang and coworkers fabricated highly electrocatalytically stable POMOFs/CC (POMOF = polyoxometalate-based MOFs) using a facile method of solid-phase hot-pressing as shown in Figure 5A.¹²⁴ The obtained materials are capable of showing excellent HER performance over a wide pH range. Fe-Ni-P/rGO-T (bimetallic Fe-Ni phosphide/reduced graphene oxide composite; T represents the pyrolysis temperature), the optimal material prepared by Fang, showed excellent porosity and good OER activity even better than those of commercial IrO₂, which requires methods including templates, phosphorylation, and pyrolysis (Figure 5B).¹²⁵

Trimetallic phosphides

The thermal phosphorization of the triple PBA precursor promotoes carbon mixing and results in an in situ structure with surface defects over the NCs. Concise methods have been described to devise transition-metal-based bifunctional electrocatalysts that are useful for comprehensive water splitting. The doped materials show both outstanding stability and electrocatalytic activity for HER and OER, especially trimetallic phosphide materials.

Although the catalytic activity has been significantly improved, the enhancement of catalytic activity is only vaguely attributed to the synergy of the active sites, and the specific contribution of the active sites to the synergistic effects is not clear.

The expansion from monometallic to trimetallic phosphides by incorporating more metal elements improves not only material properties, but also electrochemical properties. Although the catalytic activity was clearly improved, the specific role of the active sites in the synergistic effect is unclear. Therefore, trimetallic phosphides have also not been studied in depth, and their fabrication approaches are more limited, which are dominated by pyrolysis processes.

Pyrolysis process

For instance, through a simple phosphorization treatment, and using NCs of ternary Co-Ni-Fe (P-Co_0.9Ni_0.9Fe_1.2 NCs) of PBA as precursors, ternary cobalt-nickel-iron phosphide NCs with high conductivity, defect concentration, intrinsic activity, and optimized ratio are prepared. A simple method is provided to design efficient bifunctional electrocatalysts that is based on transition metals for comprehensive water splitting (Figure 6F,G).¹²⁷ To obtain neoteric CoFeP nanocages, a convenient phosphorization route is used to obtain a CoFe-PBA NC precursor. The porous hollow architecture and specific multivoid interior provide uncovered active sites and giant surface areas. Both Co and Fe act as active sites for the CoFeP catalyst, and the coordination between them optimizes outstanding electronic construction.¹²⁸ Polymetallic phosphide nanoparticles (NiCoFe-P-NPs) anchor in situ on the porous nanocages of NiCoFe-PBA and show excellent activity in catalysis. As a partially phosphatized host matrix, PBA porous cages can effectively further enhance the stability and conductivity of pMP-NPs (pMP = polymetallic phosphide) and overall exposure of the active sites in catalysis.¹²⁹ In addition, Zhang et al. fabricated TMP precatalysts through the reduction of the relevant TM cations with carbon nanofibers (CNFs), sodium borohydride (NaBH₄) a the solution of ethylene glycol (EG) and a subsequent treatment of postphosphorization at a temperature of 300°C with sodium hypophosphite (NaH₂PO₂) serving as the phosphorus source. To guarantee the TM NP deposition on CNFs, it is necessary to first perform pretreatment of the CNFs in acid to obtain a hydrophilic surface.¹³⁰ Moreover, after firstly phosphating the ZnCo-ZIF and then doping platinum nanoparticles, Pt-Zn₃P₂-CoP porous Pt-doping heterojunctions were synthesized. The obtained Pt-Zn₃P₂-CoP showed excellent activity of photocatalysis toward the generation of hydrogen from water splitting.¹³¹ The Fe-Co-Ni-P-1 can be synthesized through phosphorization at low temperatures, after the fabrication of a trimetallic PBA precursor covering Fe-Co PBA with Ni-Co PBA.¹³²

Hydrothermal method

A method is proposed to fabricate carbon-incorporated porous honeycomb NiCoFe phosphide nanospheres (NiCoFeP/C) utilizing NaH₂PO₂ as the source of phosphorus to react with the uniform precursors (Ni_xCo_{3 − x}[Fe(CN)₆]₂), which are prepared using a hydrothermal method at a temperature of 140°C for 6 h in a N₂ atmosphere in a tubular furnace at a temperature of 300°C for 2 h.⁸⁰

Others

Ru-doped 3D flower-like bimetallic phosphide (Ru-NiCoP/NF) on NF prepared by Ru etching and subsequent phosphorization using leaf-like cobalt ZIF (Co ZIF-L) as a precursor has been experimentally demonstrated to be a very efficient catalyst for driving HER and OER (Figure 6C–E). Also, when applied as an electrode material, it is able to achieve near 100% Faraday yields throughout the water separation process.¹²⁶

Besides the above strategies, other approaches, involving both hydrothermal/solvothermal fabrication and electrospinning, have been applied widely in one-dimensional MOFs or composites of MOF. The solvothermal strategy is a self-assembly course of metallic ions together with organic frameworks. Nevertheless, the materials prepared by electrospinning are mixtures of organic materials and MOFs, which have not received much attention from researchers.

Composite nanomaterials

In addition to the above materials, MOF-derived phosphide nanomaterials also include composite nanomaterials, which represent a novel variety of materials with two or more materials at the same time and exhibit some outstanding performances. The comprehensive properties of nanocomposites, especially the designability of their properties such as tunable material surface areas and adjustable nano-sized surface roughness, provide great convenience for further development of MOF-derived phosphide nanomaterials.

Carbon materials with MPs

Carbon materials with MPs have the combined benefits of carbon materials and MPs, and may have surprising properties on this basis.^133–135 Recently, several novel approaches have been applied to prepare carbon materials with MPs.

Pyrolysis process

For example, by the pyrolysis–oxidation–phosphorization process, after preparing the specific core–shell structure including a polydopamine shell and a Zn-Fe PBA core, nanoparticles of monodispersed iron phosphide encapsulated in a hollow carbon nano-box (denoted as FeP/HCNB) were prepared. An iron-zinc-based MOF was used to fabricate hollow carbon nanocubes coated with nanoparticles of iron phosphide (Figure 7A–C).¹³⁶ MP-CXs (CX = carbon xerogel) were prepared using a one-pot organic sol–gel strategy that included not only phosphorus precursors but also metals with subsequent pyrolysis in inert gases, similar to that for the synthesis of metal-doped CXs (M-CXs).¹³⁷ Also, an overall method for the fabrication of yolk–shell nanostructured materials has been put forward. MPs are wrapped in a hollow mesoporous carbon sphere doped with phosphorus and nitrogen (M₂P/NPC), and have the potential to be developed as effective catalysts for OER. By regulating the precursor molar ratios and annealing temperatures, Fe₂P/NPC, Co₂P/NPC, and Ni₂P/NPC can be obtained.¹³⁸ Shanmugam et al. described a simple strategy for the transformation of the nanoparticles of Co, Ni, and Fe phosphide into amorphous carbon using an organometallic precursor with a unique component under its autogenic pressure.¹³⁹ Han et al. proposed the simple fabrication of carbon confined Ni₂P through combustion of nickel nitrate and phytic acid assisted by a microwave, applying phytic acid as the P source rich in H₂PO₄ groups, and no extra fuel is needed during the microwave-assisted combustion.¹⁴⁰ Additionally, in Leonard's article, the supported MP catalysts were fabricated using a special ammonia-assisted ambient hydrolysis deposition strategy in which oxides of transition metal were deposited in a nano-sized porous carbon with subsequent phosphorization.¹⁴¹ Duan et al. reported the fabrication of CoP/C with a 2D layered construction by phosphating a Co-based MOF-71 directly and indirectly (Figure 7G).¹⁴² ZIF-67, a MOF with a high surface area, was combined with red phosphorous, and then treated with pyrolysis, promotes the formation of the composite of Co₂P/CN_x (Figure 7D–F).¹⁴³

Solvothermal method

Moreover, using a simple solvothermal approach, MoS₂ nanoparticles on rGO sheets have been synthesized.¹⁴⁴ Also, Ni₂P/C with the morphology of hollow microspheres has been prepared using a simple method based on MOF precursors, and a pure nickel phosphide with a MOF skeleton has been obtained using the solvothermal method.¹⁴⁵

Hydrothermal fabrication

The sequential procedure is to deposit Co(OH)F hydrothermally, and then convert Co-(OH)F into Co-MOF through a reaction in a vapor of imidazole, and partially phosphate it via a reaction with Na hypophosphite. In this way, a CoP/Co-MOF mixture can be prepared on a carbon fiber paper.⁷⁷

Template strategy

The scalable synthesis of NF-templated carbon NSs with a porous morphology decorated with nanoparticles of ultrafine Co phosphide has been reported. In the article of Hou et al., 2D NSs of uniform Co-MOF have been implanted on NF, followed by MOF-mediated tandem pyrolysis of carbonization or phosphorization. The as-prepared structure has a porous three-dimensional interconnected network with aligned two-dimensional carbon NSs.⁷⁵

Nitrogen-doped MPs

Due to its low price and convince, nitrogen has become an ideal doping element for phosphide materials, and nitrogen doping accelerates the advancement physically and chemically. Recently, to enhance some specific properties of phosphides, new materials have been further developed by nitrogen doping of MPs, and a few approaches have been reported for plastic application.

Pyrolytic process

For example, the octahedron of MoP@PPC (PPC = phosphorus-doped porous carbon) is prepared using a MOF-assisted method with the help of tunable phosphorization and carbonization.¹⁴⁶ Lu and coworkers have designed an effective three-dimensional bead string-like NeCoO@CoP on NF using the self-sacrificial MOF-templated strategy, a simple and tunable fabrication method (Figure 8F).¹⁴⁷ CoP/NC NC materials have been appropriately designed by Dong et al. through a strategy of pyrolysis derived from a Co-Co/PB MOF-based precursor.¹⁴⁸ Wei and coworkers have proposed a self-templated method to prepare an Fe-Ni-based single-crystal open capsular MOF by a pyrolysis process (Figure 8A).¹⁴⁹

Hydrothermal fabrication

Bian et al. prepared ultra-thin NiFe LDH (LDH = layered double hydroxide) NSs grown on nanoflake arrays of Ni-doped carbon using a simple hydrothermal strategy, and then treated it with phosphorization to obtain CC-NC-NiFeP, which is a dual function and durable high activity catalyst.¹⁵⁰ The synthesis of SO₄²⁻ anions bridged MOFs derived Cu₃N-Cu₃P/NPSCNWs@NF (NPSCNWs = N, P, S-tri-doped carbon nanowireis) fabricated through a strategy of interface reformation including the treatments of hydrothermal method and a subsequent pyrolysis process.¹⁵¹ An electrode of a MOF-derived Ni-doped porous carbon/Co/CoP/carbon paper (NC/Co/CoP/CP) composite was assembled through EPD and postprocessing reactions. Sha et al. fabricated a nitrogen-doped carbon material with stable properties doped with CoP₃/FeP nanoarrays (N-C@CoP₃/FeP) by a low-temperature phosphorylation reaction (Figure 8A).¹⁵² Sun and her group successfully prepared materials of Ni₂P nanoparticles embedded on MOF-derived Co,N-doped porous carbon polyhedra (Ni₂P/CoN-PCP) using the pyrolysis method (Figure 8C–E).¹⁵³

Template strategy

The composites of CoP@NC/GO (GO = graphene oxide) have been synthesized by Sun et al., and MOF-Co encapsulated by GO was used as a self-template to prepare CoP@NC/GO by low-temperature phosphorization treatment.⁹¹

Others

ZIF-67 has been prefabricated and assembled on CP using the EPD method.¹⁵⁴

MOFs with MPs

MOF, a porous crystalized material consisting of organic ligands and metal atoms, is utilized to form microporous constructions adjustably with large pore volume and abundant specific surface area as a precursor. Phosphides of most metals synthesized by the phosphorization of MOF materials usually have porous morphologies with superior conductivity and large specific surface area. As a result, transformation of MOF into MP provides a means of fabrication of hollow structures.¹⁴⁵ Herein, good strategies to prepare MOFs with MPs are initially summarized.

Pyrolytic process

For instance, ultrasmall Rh₂P nanoparticles, which not only have high dispersion but also low loading on NPC, are prepared by supramolecular starch-assisted confinement-assembly-pyrolysis (SCAP).¹⁵⁵ Hu et al. designed and synthesized a new Fe-doped CoP hollow triangle plate array (Fe-CoP HTPAs) nanostructure applying the monometallic Co-based zeolitic imidazolate framework (ZIF-67) by exchange reaction of a postsynthetic ligand at room temperature, and then carried out simple phosphorization (Figure 9D).¹⁵⁶ Moreover, a simple method for the preparation of a cathode of a biphasic Ni₅P₄-Ni₂P NS array that is 3D self-supported is proposed. The mentioned cathode is fabricated by straightforward phosphorization of commercial NF in phosphorus vapor.¹⁵⁷ In the next approach, phosphorus-modified tungsten nitride/reduced graphene oxide (P-WN/rGO) was designed for application in HER. Yan et al. first synthesized WN on rGO as an efficient, low-cost electrocatalyst by utilizing a cluster of H₃[PO₄(W₃O₉)₄] as the source of W with subsequent phosphorization.¹⁵⁸ Yan et al. reported a method for mass production of in situ growth of nanoscale hybrids of Ni₂P/Fe₂P on NF (Ni₂P/Fe₂P/NF). As a state-of-the-art electrode for comprehensive urea electrolysis, (Ni₂P/Fe₂P/NF was synthesized through a 30 s manual quaking reaction of pretreated NF and FeCl₃·6H₂O, K₃[Fe(CN)₆], followed by a simple phosphorization treatment, which could be a potentially useful industrializable method for further studying precious-free materials that act as bifunctional catalysts. The synthesized Ni₂P/Fe₂P/NF electrode shows outstanding activity of urea oxidation reaction (UOR) and HER, and a low cell voltage.¹⁵⁹ Also, Ai et al., using a chemical inversion procedure with a new precursor of nickel oxalate, prepared nanocrystalline nickel phosphide (Ni₂P), which revealed that it has high activity towards peroxymonosulfate (PMS). The Ni₂P of nanocrystalline was fabricated by a solid chemical formation of Ni oxalate when sodium hypophosphite was present in a thermal reaction.¹⁶⁰ Moreover, Huang et al. prepared a mesoporous FeP self-supported electrode (meso-FeP/CC) using a simple approach, which was mild and facile, and did not need to be strictly treated in a corrosive environment or high temperatures.¹⁶¹ Through an electrochemical activation approach, cobalt phosphides (CoPs) were successfully prepared to improve the behavior of MP-based catalysts for OER.¹⁶² Ghosh et al. synthesized nickel phosphide phases, Ni₂P, Ni₁₂P₅, and Ni₅P_4, through colloidal thermal decomposition. For all these preparations, trioctylphosphine (TOP) and oleylamine (OLAM) were utilized as capping agents, with the former utilized as a P resource.¹⁶³ Molybdenum phosphide doped with titanium (Ti-MoP) was synthesized by Chen and his colleagues as a catalyst of the HER that shows stable and highly active performances in an acidic solution. By doping titanium into phosphide, the electronic constructions of titanium, molybdenum, and phosphorus were entirely rebuilt, dramatically optimizing the activity of HER and the stability of the MoP, so as to solve problems like spontaneous oxidation during the HER reaction in an acidic solution or in air.¹⁶⁴ Successfully making direct preoxidation that endows Co foam to serve as the sole cobalt source and a conductive collector, Liu et al. first prepared a Fe₂P-Co₂P/CF electrode, by straightforward peroxidation, in which Co, Co₂P, and Fe₂P foam can be closely linked to develop a durable catalyst active for overall water splitting in alkaline media (Figure 9A–C).¹⁶⁵

Hydrothermal fabrication

Furthermore, to prepare cobalt phosphide nanoparticles (Co₂P NPs), Yin et al. proposed a simple and mild hydrothermal approach using red P and Co acetate instead of a pernicious phosphorous source as raw materials using a simple hydrothermal method.¹⁶⁶

Solvothermal method

Hollow Ni₂P/C microspheres were fabricated using a convenient and straightforward solvothermal strategy assisted by MOF precursors.¹⁴⁵

Template strategy

By an intercalation reaction method with a layer that yielded α-Co(OH)₂ NCs as self-sacrificing templates, Guo et al. have successfully prepared nanocones of Fe-Co PBA. After phosphorization and calcination, Fe-Co PBA NCs can be transferred to iron-doped Co_xP NCs without obvious shrinkage (Figure 9F–H).¹⁶⁷

MOF-DERIVED PHOSPHIDES FOR ELECTROCHEMICAL ENERGY CONVERSION

A major challenge for reproducible energy generation has emerged, that is, to construct efficiently precious-free metal bifunctional electrocatalysts not only for OER but also for HER. As novel kinds of electrocatalysts that can catalyze both OER and HER, MPs have good stability and high efficiency.

OER

With a number of advantages such as absence of pollution gases, especially greenhouse gases, highly productive hydrogen, and the high purity of the product in electrochemical water splitting, OER is particularly important in sustainable energy technologies. MPs have drawn increasingly more attention as catalysts active for OER because of their outstanding physical and chemical performances. Also, in the past few years, we have made considerable efforts to prepare more active and stable phosphide materials.¹⁶⁸

For instance, for the growth of promising energy transformation technologies including devices which can do fundamental solar water-splitting, Li-air batteries, and water electrolyzers, the objective appraisal of the electrocatalysts activity for the oxidation of wateris of great significance. Nevertheless, the current methods applied to assess catalysts for oxygen evolution are atypical, and discourage the comparision of the stability, as well as activity. The evaluation protocol for the stability, activity, and faradaic productiveness of electrodeposited electrocatalysts for oxygen evolution is reported. Moreover, attention has been paid to strategies for evaluating electrochemically active surface areas (ECSAs) and electrocatalytic stability and activity under conditions related to a complete solar water-splitting device. McCrory et al.'s chief advantage is that the overpotential to reach an electricity concentration of 10 mA cm⁻² per geometric field, needs about the current concentration supposed for a device which possesses 10% effective solar-to-fuels transformation. By using the aforementioned methods to measure the surface area, electrocatalyst turnover frequencies (TOFs) can be determined. The reported protocol was utilized in alkaline and acidic solutions to analyze the activity of oxygen in systems like CoO_x, CoFeO_x, CoPi, NiO_x, NiFeO_x, NiLaO_x,NiCeO_x, NiCoO_x, and NiCuO_x. The activity of oxygen evolution of a catalyst of IrO_x that is electrodeposited was examined for contrast as well. By comparing the catalytic performance of the studied OER catalysts, two general results have been observed: (1) every precious-free metal system reached 10 mA cm⁻² electricity concentrations at homologous operating overpotentials in the range of 0.35–0.43 V in an alkaline solution, and (2) in acidic solutions, every system except for IrO_x was volatile under oxidative conditions.¹⁶⁹ Moreover, Suntivich et al. found that Ba_0.5Sr_0.5Co_0.8Fe_0.2O_{3 − δ} (BSCF) catalyzes the OER with original activity in alkaline media, which is much higher than that of the advanced iridium oxide catalyst. From a design principle established by systematic examination of more than 10 transition-metal oxides, the high activity of BSCF was forecasted, which indicated that the original activity of OER relies on the occupancy of the 3D electron with a surface TM cation e_g symmetry in an oxide. It is forecasted that the top value of the activity OER was to occour at an e_g possession near unity, with super covalency of transition metal-oxygen links.¹⁷⁰ In Suen's article, ongoing efforts are still under way to search for clean, sustainable, and effective power generation to meet practical power requirements. Among heterogeneous optimized techniques, electrocatalysis, especially for OER, is of great importance for many new electrocatalysts that have been developed to improve gas evolution efficiency. To obtain high-performance electrocatalysts, immense efforts have been made. As a result, the development of new technologies has been promoted to study the fundamental mechanism of OER or the features of materials. It is proved that not only the basis of the OER mechanism, but also the key role electrocatalyst plays. Although it may be difficult to resolve all issues, the purpose is to provide an overall insight into the OER and follow the progress achieved. First, some measurement standardsand the theoretical principles of electrode kinetics regarding the catalysts are examined. Then, several kinds of materials to determine the activity of OER are studied, where the materials of metal oxide formed the basis of the OER mechanism while materials of nonoxide are hopeful to realize comprehensive water splitting. During OER, in situ methods of electrocatalytic representation are vital and able to provide highly effective methods for the preparation of superior electrocatalysts for OER. Eventually, the OER mechanism is discussed both theoretically and experimentally, and possible strategies to achieve better OER behavior are put forward in consideration of future developments.¹⁷¹ He et al. proposed that hollow nanostructures have received more attention from researchers in electrochemical energy conversion and storage owing to the distinctive structural features. However, the synthesis of hollow MPs that are nanostructured, especially nonspherical hollow nanostructures, has rarely been reported. As a result, a MOF-based strategy has been put forward to prepare carbon-incorporated nickel-cobalt hybridized MP nanosized boxes NiCoP/C. The OER is selected to exhibits the performance of the NiCoP/C nanosized boxes in electrochemistry. The nanosized boxes of Ni-Co mixed-MP, namely, NiCoP and Ni-Co LDH have been prepared for comparison. Thanks to the compositional and structural advantages, the fabricated nanosized boxes of NiCoP/C show long-term stability and optimized electrocatalytic activity of OER (Figure 10G–I).¹⁷² A feasible method is provided by Zhang et al. to surmount the bottlenecks of long-term stability of potassium-ion batteries (PIBs) by formation of a solid electrolyte interphase and regulating dendrite growth, promoting intense research on anode materials based on P in PIBs by manipulation of the salt/additive chemistry of the electrolytes and materials design.¹⁷³ Furthermore, a bottom-up one-pot solvothermal approach and a subsequent phosphorization treatment at a low temperature were studied to fabricate three-dimensional hierarchical materials. The material is flower-like and used to form ultrathin Co based on the NSs of bimetallic phosphide (CoM-P-3DHFLMs, 3DHFLMs = 3D hierarchical flower-like materials), which is a durable and efficient electrocatalyst for OER for commercial applications. Because of their advantages of unique structures and reasonable compositions, it is easier for CoM-P-3DHFLMs to achieve the required electrocatalytic efficiencies than RuO₂ or IrO₂ counterparts. Overpotentials to reach 10 mA cm⁻² is only 292 mV for CoNi⁻, 307 mV for CoCu-P-3DHFLM, and 318 mV for CoMn⁻, respectively, and have almost no decay after 10 h. Factors including good stability, nonprecious metal, and high efficiency make the fabricated 3DHFLMs promising candidates for OER (Figure 10A–F).¹⁷⁴ TMPs recently have been considered to be hopeful Earth-abundant electrocatalysts for the OER and the HER. A scalable and general method has been studied for the fabrication of MP electrodes in view of the reaction of commercial metal foils (ferrum, cobalt, nickel, cuprum, and nickel-ferrum) with different organo-phosphine reagents. Read et al. proposed that the obtained phosphide electrodes can show perfect electrocatalytic OER and HER characters. To achieve the most active electrodes for the production of 10 mA cm⁻² current densities corresponding to the operation, the overpotentials of only −128, −183, and 277 mV are needed for the HER in acid (Ni₂P), the HER in base (Ni₂P), and the OER in base (NiFeP), respectively. Such OER and HER behaviors are better than synthesized samples, which use more expensive and elaborate procedures.¹⁷⁵ Dutta argues that a distinctive core-shell nanostructured OER catalyst is composed of an active amorphous phosphide of Ni shell and a chemically inactive crystalline Fe oxide core. And this catalyst leads to the excellent activity of OER. It is even reported that activators promote the OER activity by advancing the redox reactions but the exclusive position of Fe in the nanoscale architectures surely improved the effectivity because of ideal placement.¹⁷⁶ It is the elevation of electrocatalytic OER activity which also served as a rate-determining step that is of great value to achieve effective hydrogen production by water electrolysis. Ji et al. revealed that the overpotential of the OER can be dramatically decreased by ~123 mV during electronic chemistry activation of the cobalt phosphide nanoflower supported on carbon cloth (CoP/CC). The aforementioned process requires a very low overpotential (176 mV) at the 10 mA cm⁻² current density. The voltage of the electrolyzer of CoP/CC||CoP/CC is correspondingly reduced from 1.64 to 1.49 V at 10 mA cm⁻². What's more, the structures of active intermediate crystal, downsizing particles, porous surfaces generation, as well as new interfaces in the process of OER activation can reduce the barrier of energy and further speed up the process of OER.¹⁶² The MP nanoparticles, which Shanmugam et al. prepared in a carbon matrix, have been reported as electrocatalysts for OER, and reach 10 mA cm⁻² current density for OER with a 370 mV overpotential in 1 M KOH media (Table 1).¹³⁹

Figure 2. Summary of different metal phosphide phases for metal–organic frameworks-derived phosphide nanomaterials and a radar chart showing the amount of articles of common metal phosphides (WP, MoP, Cu3P, Fe4P, Fe3P, Fe2P, FeP, Co2P, CoP, Ni3P, Ni12P5, Ni2P, Ni5P4, NiP, NiP2, NiP3)

Figure 3. Summary of the main attractive developments and representations in the exploration of metal–organic frameworks-derived phosphide nanomaterials for electrochemical applications. HER, hydrogen evolution reaction; OER, oxygen evolution reaction; ORR, oxygen reduction reaction; SCs, supercapacitors; SIB, sodium-ion battery

Figure 4. (A) Schematic illustration of catalyst preparation. (B) Scanning electron microscopy (SEM) images of NF@Fe2-Ni2P/C with different magnifications. (C) Transmission electron microscopy (TEM) image of Fe2-Ni2P/C. Reproduced with permission: Copyright 2019, American Chemical Society.112 (D) (I) Conventional synthesis of a metal phosphide by combining phosphorus and metal–organic framework (MOF) precursors and (II) the novel synthesis of a metal phosphide from a single-source crystalline MOF precursor. Reproduced with permission: Copyright 2019, American Chemical Society.111 (E) Schematic illustration of synthetic processors for nanobundles (FeP-500). (F) SEM images of FeP-500 at a magnification of 200 nm. Reproduced with permission: Copyright 2019, Wiley-VCH.107 (G) Schematic illustration for the synthesis of transition-metal phosphides (TMPs) and TMPs@UiO-66-NH2. (H) TEM image of Ni2P@UiO-66-NH2 (inset: size distribution of Ni2P nanoparticle). Reproduced with permission: Copyright 2020, Wiley-VCH109

Figure 5. (A) Schematic representation of the solid-phase hot-pressing synthesis of polyoxometalate-based metal–organic framework on CC and subsequent preparation of Cu-M-P/CC (M = Mo or W, CC = carbon cloth). Reproduced with permission: Copyright 2018, The Royal Society of Chemistry.124 (B) Schematic illustration showing the stepwise fabrication of Fe-Ni-P/rGO-T composite. Reproduced with permission: Copyright 2017, American Chemical Society.125 (C) Schematic illustration of the fabrication of (Ni1-xFex)2P/C-KB-T (KB = Ketjenblack carbon). Reproduced with permission: Copyright 2020, The Royal Society of Chemistry.121 (D) Scanning electron microscopy (SEM) image of the Ni-Fe PBA-3 precursor. (E) SEM image, (F) transmission electron microscopy image of the obtained (Ni0.62Fe0.38)2P. Reproduced with permission: Copyright 2017, The Royal Society of Chemistry123

Figure 6. (A) Schematic showing the synthetic route of the Ru-NiFeP/NF nanosheets. (B) Scanning electron microscopy (SEM) images of Ru-NiFeP/NF. Reproduced with permission: Copyright 2020, Elsevier.70 (C) Schematic illustration of the fabrication and reaction of Ru-NiCoP/NF catalysts. (D,E) SEM image of Ru-NiCoP/NF. Reproduced with permission: Copyright 2020, Elsevier.126 (F) Schematic of consecutive processes for synthesizing ternary metal phosphides of P-Co0.9Ni0.9Fe1.2 NCs. (G) SEM image of P-Co0.9Ni0.9Fe1.2 NCs. Reproduced with permission: Copyright 2019, Springer.127

Figure 7. (A) Schematic preparation of iron phosphide encapsulated in a hollow carbon nano-box (FeP/HCNB). (B) Transmission electron microscopy (TEM) images of FeP/HCNB. (C) TEM image of FeP/C. Reproduced with permission: Copyright 2019, Elsevier.136 (D) Graphical presentation of the synthesis of Co2P/CNx nanocubes. Bright-field TEM images of (E) the ZIF-67 and (F) Co2P/CNx nanocubes. Scanning electron microscopy images are shown in the inset (scale bar, 500 nm). (G) Schematic of the formation of di-CoP/C and In-CoP/C derived from Co-MOF-71. Reproduced with permission: Copyright 2021, Elsevier.142

Figure 8. (A) Schematic illustration of the preparation process of nitrogen-doped carbon hollow (NCH) framework with iron-nickel phosphide (FeNiP) nanoparticles. Reproduced with permission: Copyright 2019, American Chemical Society.149 (B) Synthesis schematic of in situ growth process of MOFs in polyaniline derivatives as precursors to construct N-C@CoP3/FeP nanoarrays. Reproduced with permission: Copyright 2021, Elsevier.152 (C) Schematic illustration for the preparation of Ni2P/CoN-PCP (PCP = porous carbon polyhedrons). (D) Scanning electron microscopy and (E) transmission electron microscopy images of Ni2P/CoN-PCP. Reproduced with permission: Copyright 2018, The Royal Society of Chemistry.153 (F) Schematic illustration of the synthesis process of bead string-like N-CoO@CoP arrays. Reproduced with permission: Copyright 2019, Elsevier.147

Figure 9. (A) Schematic illustration to fabricate a seamless integrated electrode of Fe2P-Co2P/CF (CF = Co foam). (B,C) Scanning electron microscopy (SEM) images of the cathode and the anode for overall water splitting after a 120 h test at 65°C. Reproduced with permission: Copyright 2020, American Chemical Society.165 (D) Schematic illustration of the construction of mesoporous Fe-CoP hollow triangle plate array. Reproduced with permission: Copyright 2018, Wiley.156 (E) Schematic showing the mechanism for the transformation of Co(OH)2 nanocubes (NCs) into Fe-Co PBA NCs. SEM images of (F) Fe-Co3O4 NCs and (G,H) Co3O4 NCs. Reproduced with permission: Copyright 2018, American Chemical Society.167

Figure 10. (A,B) Scanning electron microscopy image with elemental mapping data of the CoNiP-3DHHS (3DHHS = 3D hierarchical structure). (C) Tafel plots of the mass activity versus overpotential. (D,E) Transmission electron microscopy image with elemental mapping data of the CoNiP-3DHHS. (F) Polarization curves. Reproduced with permission: Copyright 2019, Elsevier.174 (G) Field-emission scanning electron microscopy (FESEM) and (H) Transmission electron microscopy images of ZIF-67@LDH nanoboxes synthesized by allowing ZIF-67 nanocubes to react with Ni-(NO3)2 at 258°C for 90 min. (I) Polarization curves of Ni-Co LDH, NiCoP, and NiCoP/C nanoboxes in an O2-saturated 1.0 M KOH solution. Reproduced with permission: Copyright 2017, Wiley-VCH172

Table 1 Benchmarking the metal-phosphide-based electrocatalysts with respect to the OER overpotential at 10 mA cm⁻² (η¹⁰)

Catalyst	Current density (j, mA cm−2)	Overpotentials (η) at corresponding (mV)	Tafel slope (mV dec−1)	Electrolytes	References
CoP/CC	10	176		1 M KOH	[162]
CoP hollow polyhedron	10	400	57	1 M KOH	[177]
Co-P foam	10	300	74	1 M KOH	[178]
Flower-like NiFe-P	10	233	42.5	1 M KOH	[179]
CoP-based nanoneedle	10	281	/	1 M KOH	[180]
CoP NWs	10	248	78	1 M KOH	[181]
Co2PNWs	10	~270	62	1 M KOH	[182]
CoP-MNA	10	290	65	1 M KOH	[183]
(Ni0.5Fe0.5)2P microflower	10	203	57	1 M KOH	[184]
CP@Ni-P	10	~270	73	0.1 M NaOH	[185]
CoP-CNT	10	330	50	1 M KOH	[186]
Co2P/Co-foil	10	319	79	0.1 M PBS	[187]
Co-Pi NA/Ti	10	450	187	1 M KOH	[188]
Cu3P/CF	10	412	63	1 M KOH	[189]
FeP@CNT	10	300	53	1 M KOH	[190]
NiCoP/rGO	10	270	65.7	1 M KOH	[191]
Fe1.1Mn0.9P nanorod	10	440	39	1 M KOH	[192]
Ni1.85Fe0.15P NSAs/NF	10	270	96	1 M KOH	[193]
Co0.7Fe0.3P/CNT	10	243	35	1 M KOH	[194]
Co0.68Fe0.32P polyhedrons	10	289	66	1 M KOH	[195]
Al-CoP/CC	10	330	67	1 M KOH	[196]
Fe-Co-P alloy	10	252	33	1 M KOH	[197]
Ni2P/NiOx nanoparticles	10	290	59	1 M KOH	[198]
O-incorporated Ni2P nanosheets	10	347	63	1 M KOH	[199]
CoP NR/C	10	320	71	1 M KOH	[200]
Needle-shaped Co2P	10	310	50	1 M KOH	[201]
Mn-Co oxyphosphide particles	10	320	52	1 M KOH	[202]
Co3FePxO	10	291	85	1 M KOH	[203]
α-Co2P nanoparticles	10	592	94.4	1 M KOH	[204]
O-Ni1 − xFexP2 (x = 0.25)	10	155	55	1 M KOH	[205]

Abbreviations: CC, carbon cloth; OER, oxygen evolution reaction.

HER

TMPs have been previously found active toward HER and now, emerge as alternative electrocatalysts among numerous candidates because of their good HER catalytic characters. This class of electrocatalyst with excellent electrocatalytic activities has thus opened up a new strategy in the search of precious-metal-free catalysts for HER and there has been much research that employs phosphide as catalysts for HER.^206,207

Continuable hydrogen production is a necessary prerequisite of the approaching hydrogen economy. Water electrolysis, driven by reproducible resources together with direct solar-to-hydrogen transformation based on photochemical water splitting and photoelectrochemistry, is a promising way for sustainable hydrogen manufacture. All these processes require precious nonmetal HER catalysts with high activity to make the water splitting procedure more effective and economical. Current study efforts devoted to the preparation of precious nonmetal electrocatalysts, especially the research on the catalytic properties of nanoscale for HER, are highlighted.  This paper reviews a variety of significant nonprecious metal electrocatalysts, including MPs, metal nitrides, metal carbides, metal selenides, metal sulfides, and nanocarbons mixed with heteroatom. In this research, stress is laid on the synthesis pathways of the electrocatalysts for HER, the projects of capability enhancement, and the relationship between architecture/composition and catalytic activity. Various vital examples are summarized to show that Pt-free HER electrocatalysts, when mixed with appropriate semiconductor photocatalysts, can be used as effective cocatalysts for boosting facile solar-to-hydrogen adaptation not only in photoelectrochemical water splitting systems but also in photochemical water splitting systems.²⁰⁸ In the research of Sun's group, HER is key for the equipment of renewable power storage. To enhance the intrinsic activity of the electrocatalysts for HER, which is based on earth-rich metals, is still a big challenge. Sn-NiCoP_x-NF, a dual P-doped NiCo bimetallic material developed on Ni foam through a convenient phosphorization treatment, is suggested. The catalysts perform a new class of electrocatalysts with enormously modified activity for HER. In an alkaline medium, unprecedentedly effective catalytic activity for HER is achieved because of the S₂-NiCoP_x/NF with an optimized P/S ratio. At 50 mA cm⁻² current density, the overpotential is very low (144 mV), lower than that of pure NiCo bimetallic phosphide catalyst (NiCoP_x−NF) (190 mV). Additionally, S₂-NiCoP_x/NF also has fast reaction dynamics, the lowest Tafel slope (66 mV dec⁻¹), and good HER stability. Therefore, the benefits of the dual-anion-improvement approach to modifying catalytic activity are experimentally exhibited, and the pathway is offered for the excellent design of effective and inexpensive electrocatalysts for conversion devices and power storage.¹¹⁵ In addition, from the perspective of Cho et al., TMPs were emerging as promising electrocatalysts for HER due to their low consumption and good activity while compared with the conventional HER electrocatalysts such as Pt. Nevertheless, the dependency of HER activity on different crystal phases has not been well comprehended. Phosphide of Fe nanoparticles with two unique phases was compounded through the electrochemical formation from metallic Fe to phosphides of Fe (Figure 11D–F).²⁰⁶ Nanoparticles of co-promoted Mo phosphide were developed for the first time and successfully researched as a commercial electrocatalyst for HER. The prepared catalyst shows the perfect activity of HER with a Tafel slope of only 50 mV dec⁻¹. The addition of cobalt reduces the particle size of Mo phosphide-based catalysts, improves the charge transfer, and heightens the intrinsic activity of each active site, which paves the way for improving the representation of HER of ternary or multiple TMP catalysts.²¹⁰ Tang et al. argued that the synthesis and the performance of mixed catalyst (Ni₂P NSs@CoP NW on a carbon cloth) for the reaction have been described. The synergistic effects and heterostructure of the CoP and Ni₂P components lead to a very low overpotential of only 55 mV so as to reach a catalytic current density of 10 mA cm⁻², meeting the requirements of TMP electrocatalysts. This process could be easily expanded to the fabrication of corresponding heterostructured bimetallic sulfides or selenides for HER (Figure 11G–I).²⁰⁹ As Liu et al. pointed out, the Rh₂P/NPC catalyst performs an excellent activity for HER. The Rh₂P/NPC catalyst possesses a superior performance for electrocatalytic HER activity than the catalyst of 20% Pt/C, especially when under large current densities. Compared with RHE, it requires a low overpotential of only 160, 341, and 411 mV to achieve the current densities of 10, 100, and 300 mA cm⁻², respectively. The prepared Rh₂P/NPC material is expected to be a promising electrocatalyst for seawater electrolysis. Furthermore, the Rh₂P/NPC catalyst possesses outstanding stability over current densities in a wide range. As a result, the Rh₂P/NPC catalyst is able to be utilized as a strong catalyst for hydrogen production during seawater electrolysis.¹⁵⁵ Also, Wu's group found that in preliminary research, the porous Ni-Cr-Fe electrode possesses outstanding catalytic activity and long-term stability for HER in seawater. Good stability and outstanding effectivity of the porous nickel-chromium-iron electrode catalyst has opened up a broad prospect for its commercial applications in the hydrogen revolution.²¹¹ In addition, the self-supported mesoporous FeP electrode performs a great activity for HER and needs low overpotentials (57 and 84 mV to drive 10 mA cm⁻² in an acidic and alkaline medium, respectively). What's more, the prepared electrode displays fast kinetics and a small Tafel slope of 42 and 60 mV dec⁻¹ in the acidic and alkaline solution, respectively. The superior catalytic behavior of the electrode induced by the mesoporous framework is accompanied by a large surface area (74.1 m² g⁻¹) and self-supported highly conductive architecture. The former equips the catalyst with an outstanding active-site density (3.5 × 10¹⁸ site per mg) and massively available mesopores, and the latter optimizes reaction kinetics.¹⁶¹ To clarify Gosh's view, here is another analogy. Charge separation between the P^δ− and Ni^δ+ sites in different nickel-phosphorus phases deserve to be emphasized so as to obtain satisfactory catalytic reaction results. Ni₂P showed a remarkable advancement to the HER, with an overpotential of 126 and 180 mV to reach 10 mA cm⁻² in acid and in alkaline solution, respectively. Ni₁₂P₅ now serves as the highly effective catalyst, accompanying a TOF = 23.0 min⁻¹ for hydrogen evolution from ammonia-borane, and TOF = 17.3 min⁻¹ from NaBH₄ according to noble metal nanoparticles.¹⁶³ What's more, various porous nano-architecture carbocatalysts, such as MoN, MoP, and Mo₂C, have been fused and decorated to shape a mixed architecture by use of Mo-based composites. By using the open porosity of MOFs during pyrolysis, highly dispersed MoO₂ small nanoparticles can be deposited in porous carbon by chemical vapor deposition. The carbocatalysts decorated by MoP-, MoN-, and Mo₂C- can be selectively obtained without changing the morphology after undergoing several treatments of phosphorization, nitridation, and carbonization. Among these composites based on molybdenum, the composites of MoP@porous carbon (MoP@PC) performed extraordinary HER activity in 0.5 M H₂SO₄ aqueous media compared to MoN@PC, Mo₂C@PC, and MoO₂@PC. A hopeful system of multifunctional lab-on-a-particle structures which would help a lot on fuel-cell catalysis and energy conversion is offered.⁷⁶

After subsequent etching and treatments of phosphorization, a new template-engaged method is found by Hu et al., which is proven to be applicable in synthesizing open and hierarchical nickel-cobalt-phosphorus hollow nanobricks (HNBs). Thanks to the distinctive nanostructures with rich pathways for mass diffusion and ample electrolyte-accessible surface, the synthesized nickel-cobalt-phosphorus HNBs show high electrocatalytic activity, which only needs 270 and 107 mV for OERs and HERs to achieve the current density of 10 mA cm⁻², respectively, and show superior stability in the alkaline electrolyte (Figure 11J–L) (Table 2).¹²²

Table 2 Benchmarking metal-phosphide-based electrocatalysts with respect to the HER overpotential at 10 mA cm⁻² (η¹⁰)

Catalyst	Current density (j, mA cm−2)	Overpotentials (η) at corresponding (mV)	Tafel slope (mV dec−1)	Electrolytes	References
Ni2P	10	180	/	in 0.5 M KOH	[163]
NiCo-precursor/NF	50	144	66	in 1.0 M KOH	[115]
Rh2P/NPC composite	10	17	/	in 1.0 M KOH	[155]
Ti-MoP	10	81.5	44.5	in a 0.5 M H2SO4(aq)	[164]
Ni-Co-P HNB	10	107	53	in 1.0 M KOH	[122]
Ni2P NS@CoP NW-CC	10	55	48	in 0.5 M H2SO4	[210]
Meso-FeP/CC	10	57	60	in alkaline media	[161]

Abbreviations: CC, carbon cloth; HER, hydrogen evolution reaction; HNB, hollow nanobrick; NF, nickel foam.

ORR

Applied as effective catalysts for ORR in the alkaline media, MPs, whose TMP crystalline phase construction is the principal element to determine their catalytic activity, indicate metallic performance accompanying good electrical conductivity when they are rich in metals. The key for catalysts based on TMP to achieve excellent stability without dramatically impairing the conductivity is to balance the ratio of transition metal and phosphorous, and this manipulation will bring about the growth of catalytic activity.²¹²

As Razmjooei pointed out, considering the difficulty of developing an inexpensive catalyst showing high activity for the ORR, MPs are a burgeoning alternative to noble metal-based electrocatalysts. Besides reasonable costs, MPs have substantial advantages, such as good chemical stability and high conductivity. Since the catalytic activity of MPs highly relies on the metal/phosphorous ratio, the Ni_xP_y/C with tractable NP phases is reported to be a hopeful ORR electrocatalyst in both alkaline and acidic electrolytes. The composites of Ni_xP_y/C have been prepared by carboning an Ni-struvite (NiNH₄PO₄·H₂O). It is as forecasted that the peak of the ORR catalytic activity in both alkaline and acidic solutions was reached when the composite of Ni_xP_y/C, accompanied with the highest proportion of Ni₂P as a predominant phase, was prepared at 800°C.²¹² Also, Chen et al. argued that facilely synthesizing mixtures based on Co₂P, Mn₂P, and Ni₂P nanoparticles and heteroatom-doped CNTs, can be used as efficient ORR catalysts in alkaline medium. TMP nanoparticles and graphite carbon form the core/shell framework, and the catalytic activity of the carbon shell for ORR is dramatically influenced by the nanoparticles (core). In the process dominated by 4e⁻, Mn₂P- and Co₂P-based mixtures on decorated heteroatom-doped carbon exhibited outstanding catalytic activity for ORR, lower overpotential, and optimized methanol durability and tolerance, outperforming Ni₂P-decorated heteroatom-doped carbon in terms of the catalytic activity for ORR. The change in ORR behavior is mainly due to the joint effects of the binding energy shift and electronegativity of the TMPs, which probably lead to a diverse surface electro-architecture of the heteroatom-doped carbon. Low binding energy shift and low electronegativity of the TMPs can bring about robust electron-donating capacity for the TMP nanoparticles, thus improving the ORR catalytic activity of the mixed materials. It is of remarkable importance to study the developed ORR catalysts on the basis of heteroatom-doped carbon by the way of reasonably designing the structure of mixture materials.²¹³ To clarify the view of Aijaz et al., it should be highlighted that highly performed and reversible oxygen electrodes are critical not only for the OER and ORR but also for different power conversion equipment, such as regenerative fuel cells and metal-air batteries. Yet, the application of the abovementioned electrodes is hindered by instability and insufficient activity of electrocatalysts for oxygen reduction and water splitting. It is reported that electrocatalysts that possess highly active bifunctionality for electrodes of oxygen contain nanoparticles of core-shell Co@Co₃O₄ implanted within CNT-grafted nitrogen-doped carbon-polyhedra, which is received by the pyrolysis of Co metal–organic structure (ZIF-67) in an H₂ atmosphere and followed by tunable oxidative calcination. The catalysts provide a reversible overvoltage of 0.85 V in the alkaline solution, outperforming IrO₂, RuO₂, and Pt/C. Thus they are arranged among the excellent noble-free metal electrocatalysts for invertible oxygen electrodes.²¹⁴ In 0.1 M KOH media, the yolk-shell structure and nickel, phosphorus co-doped carbon layer was conducted with rotating disk electrode to evaluate its electrochemical catalytic properties. With a scan rate of 20 mV s⁻¹, the cyclic voltammetric was exhibited in O₂- and N₂-saturated 0.1 M KOH media. The highest current potential of the cathode reduction of Fe₂P/NPC, Co₂P/NPC, and Ni₂P/NPC is 0.90, 0.81, and 0.79 V versus RHE, respectively. The NPC without MPs shows a poorer activity for ORR, whose highest and the half-wave potentials are  0.75 and 0.60 V, respectively, compared with RHE. Remarkably, a much more aggressive ORR start-up potential and highest potential are supplied by the M₂P/NPC compared with nonmetal products, which allows the M₂P to act more critical in developing the catalytic presentation for ORR.¹³⁸

Others

Besides the aforementioned ORR, HER, and OER, there are other electrocatalytic reactions, such as hydrogen oxidation reaction (HOR), carbon dioxide reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), UOR, and methanol oxidation reaction (MOR). Research on catalysts of MOF-derived MPs were conducted as well.⁵²

Electrochemical CO₂RR

MoP@In-PC was obtained by Han's group utilizing MP as the catalyst for CO₂ electroreduction. The high catalytic activity and higher current density make MoP nanoparticles supported on porous carbon more promising for effective CO₂ adsorption than those proposed novel with very high Faradaic efficiency (FE) (Figure 12A,B).²¹⁵

Figure 11. (A) Transmission electron microscopy (TEM) image of MoP@PC. (B,C) Polarization curves for six electrocatalysts. Reproduced with permission: Copyright 2016, Wiley.76 (D) TEM image of NP samples during the phase transformation from Fe into FeP. (E) Cyclic voltammetric (CV) curves for iron phosphide NPs in acid (0.5 M H2SO4) with a loading amount of 105 μg cm−2geo, as well as Pt/C and Fe NPs for comparison. (F) Tafel plots of iron phosphide NP samples according to the reaction time and temperature. Reproduced with permission: Copyright 2018, Elsevier.206 (G) Scanning electron microscopy image and X-ray diffraction patterns for Ni3S4 NS@Co3S4. (H) Linear sweep voltammetry curves for Ni3S4 NS@Co3S4 NW-CC and Ni0.85Se [email protected] NW-CC at  2 mV s−1, with insets showing Tafel plots. (I) Electrolysis curves for Ni3S4 NS@Co3S4 NW-CC and Ni0.85Se [email protected] NW-CC at overpotentials of 150 mV for 12 h in 0.5 M H2SO4. Reproduced with permission: Copyright 2017, American Chemical Society.209 (J) FESEM image of the hierarchical N-Co-P hollow nanobricks (HNBs). (K) Polarization curves and (L) Tafel plots of the hierarchical Ni-Co-P HNBs and Ni-Co-P nanosheets for hydrogen evolution reaction. Reproduced with permission: Copyright 2018, The Royal Society of Chemistry.122

Figure 12. (A) Linear sweep voltammetry curves for five electrodes. (B) Tafel plots for formic acid production over the five electrodes. Reproduced with permission: Copyright 2018, Wiley.215 (C) Linear sweep voltammetric curves in N2-saturated (red line) or Ar-saturated (blue line) electrolytes. (D) Transmission electron microscopy image of Co phosphide hierarchical hollow nanocage. Reproduced with permission: Copyright 2018, Wiley.216

NRR

Xu and coworkers fabricated an electrocatalyst of nonprecious metal Co phosphide hierarchical hollow nanocage (CoP HNC) from a ZIF-67 precursor via a layered double-hydroxide intermediate, and the well-designed catalyst of CoP has a new morphology of three-leveled nanoparticle-NS-nanocage. The CoP HNC possesses the architectures of high Faraday efficiency, a featured exponentially increased NH₃ yield, and a high selectivity for ammonia while employed as the catalyst in electrochemical NRR (Figure 12C,D).²¹⁶

MOF-DERIVED PHOSPHIDES FOR ELECTROCHEMICAL ENERGY STORAGE

Nanostructured materials have been widely researched in energy-related fields, and MPs are promising candidates owing to their benefits of high Li⁺, Na⁺, and electrons transport rates, as well as high surface areas and short charge diffusion paths.

SCs

SCs have become one of the most powerful candidates in the area of new energy-related applications owing to their outstanding performance, and green and environment-friendly advantages. In this section, we will overview the properties and applications of some typical MPs in SCs.

With high stability, security and flexibility, some of the energy storage devices are ideal. Rationally designing and fabricating the NS arrays of moss-like NiCo bimetallic phosphides on carbon fabrics helps them be facilely disposed as the electrodes to make highly flexible and all-solid-state mixed SCs. Due to structured merits and synergistic effects of the composition theoretically proved by calculations with density functional theory, the prepared NS arrays of Ni-Co-P showed excellent electrochemical representation and outstanding stability. The assembled hybrid SCs exhibited an energy density as high as 48.4 Wh kg⁻¹ at 811.2 W kg⁻¹ with desirable cyclic stability. Moreover, under different bending states, the capacitance did not see a remarkable decrease after 1000 times bending, indicating mechanical stability and outstanding flexibility of these NS arrays inspired by biology. The mixture SC, as the equipment for power storage, possesses electrochemical performance and optimum mechanical stability, illustrating its promising potential (Figure 13A–E).²¹⁸ In another instance, in the fields of power storage and conversion, TMPs/phosphides, a new kind of prominent electroactive materials, have drawn extensive attention because they possess good electrical conductivity and their properties are similar to metal. The fabrication of NSs of hierarchical Ni-Co-P/PO_x/C is provided in a tunable manner utilizing NiCo-MOF as the precursor, and a subsequent phosphorization pathway at low temperature. The NSs of Ni-Co-P/PO_x/C showed outstanding rate capability and excellent reversible capacity and delivered capacities of 583 and 365.7 C g⁻¹ at 1 and 30 A g⁻¹, respectively. What's more, dual electrode equipment brought together by Ni-Co-P/PO_x/C and decreased oxide of graphene exhibited the highest energy concentration of 37.59 Wh kg⁻¹ at an energy concentration of 800 W kg⁻¹ and a steady capacitive behavior, indicating that the prepared bimetallic Ni-Co-P/C is hopeful for being used as an anode material in capacitive energy storage devices.²²⁰ Also, as Zhou argued, because of the coexistence and synergism of various transition metals, binary metal compounds possess richer redox reactions and higher conductivity. Moreover, carbon, when used for composite materials, possesses the benefits of promoting electronic conductivity to enhance fast electron transmissions. Various Co₂P/C, Ni₂P/C, Ni_mP_n/C, and Ni_xCo_{2 − x}P/C nanohybrids with diverse Ni/Co molar ratios were prepared through in situ calcination/phosphorization of MOF precursors. X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive spectrometer, scanning electron microscopy (SEM), and electrochemical tests are used to characterize these nanohybrid materials. The structure of NiCoP/C material is granular and NiCoP nanoparticles are anchored in abundant carbons. Using both the carbon anchoring effect and bimetallic synergism, the obtained NiCoP/C performed extraordinary specific capacities of 775.7 and 582.4 C g⁻¹ at 1 and 20 A g⁻¹ (20-fold) accompanying excellent rate capability (75.1% retention), much better than the related Co₂P/C, Ni₂P/C, Ni_mP_n/C, other samples of Ni_xCo_{2 − x}P/C and heterogeneous reported MP nanosized composites/structures. Moreover, the NiCoP/C//activated carbon asymmetric SC was prepared, which showed a better energy concentration of 47.6 Wh kg⁻¹ at a power concentration of 798.9 W kg⁻¹, better than those nickel/cobalt phosphide nanomaterials (Figure 13F–I).²¹⁷ What's more, Gayathri et al. proposed that it is hopeful yet challenging to make interpenetrated heterostructures prepared from desirable power materials to develop efficient SCs. A cobalt phosphide/cobalt oxide heterostructure that is leaf-shaped, (CoP_x)_{1 − y}/CoO_y (0.44 > y > 0.06), was fabricated from the molecular precursor of two-dimensional ZIF-Co-L through the Co₃O₄ intermediate phosphorization. The effective heterostructure construction formed by surface/bulk composition transformation remarkably changes the electronic structure and interfacial performances and restrains the improvement of the performance of SCs. What's more, phosphorization of gas-phase requires a core-shell construction mechanism through the diffusion of the gas, and the control of the Kirkendall effect. The excellent heterostructure shows pronounced interfacial performances derived from the interface of CoO/Co⁰/CoP when y = 0.10. Therefore, it promotes an outstanding specific capacitance of 467 F g⁻¹ at 5 A g⁻¹ and significant cycling stability of about 91% even after 10,000 cycles at 30 A g⁻¹. An optimized raise in the ~107% cyclability has been realized by using a mixture of graphene. Furthermore, an asymmetric SC device is prepared, which implies that cycling stability is about 93% after 10,000 cycles and energy concentration is rationally as high as 12.7 Wh kg⁻¹ at a power concentration of 370 W kg⁻¹. This study exhibits the CoP_x/CoO heterostructure interfacial performances which improve power storage behavior through bulk/surface compositional variation, thus affording a method for improving electrodes of heterostructure for SC with high-performance (Figure 13J–L).²¹⁹

Figure 13. Scanning electron microscopy (SEM) images of (A) Ni2P/C, (B) Co2P/C, and (C) NiCoP/C. (D) Cyclic voltammetric at different scan rates for the NiCoP/C electrode, respectively. (E) Specific capacity of the NiCoP/C, Ni2P/C, and Co2P/C electrodes as a function of current density. Reproduced with permission: Copyright 2020, Elsevier.217 SEM images of the Ni-Co-P nanosheet arrays on carbon fabrics: (F) pure carbon; (G) fabrics Ni-Co-P-5. (H) Cycling performance of the assembled supercapacitor at 10 A g−1; (I) specific capacities of Ni-Co-P nanosheet arrays. Reproduced with permission: Copyright 2020, Elsevier.218 (J) Transmission electron microscopy image and elemental mapping (Co, P, O) of CPx/CO-20. (K) Specific capacitance of (CoPx)1 − y/CoOy at different current densities. (L) Cycling stability of the (CoPx)1 − y/CoOy at 30 A g−1, where the cyclability is calculated from the 600th cycle. Reproduced with permission: Copyright 2020, Elsevier.219

LIBs

Because of the superior theoretical power concentration and the abundance of P in nature, lithium-sulfur batteries have been recognized as a hopeful novel battery system. Nevertheless, the sluggish activity and active polysulfide materials significantly impede the massive employment of lithium-sulfur batteries, and usual LIBs are not able to satisfy the demand for high-power and high energy density. And then MOF-derived phosphide became a kind of promising candidate material for the LIB batteries.

By using the MOF-derived strategy, the MoP@PC, in which MoP nanocrystallites are highly dispersed and assembled in phosphorus-doped porous carbon, is first prepared. By developing a cooperative and well-designed interface and adsorption conversion to improve kinetics, MoP@PC is able to convert and capture polysulfides effectively as a polysulfide reservoir. In general, the cell of Li-S, which possesses a MoP@PC reservoir shows that the original specific capacity at 0.5 C was 1158 mA h g⁻¹ and the use of S has been improved by 17%. The current strategy affords suitable guidelines to improve the interface reaction of electrocatalytic materials for the battery of LiS and to fabricate compounds of other transition metals that are highly dispersed (Figure 14A–E).²²¹ It is no longer a secret that silicon phosphide (SiP) has been a hotspot in the power storage area because it possesses an ideal theoretical volume for LIBs anodes of 3120 mA h g⁻¹ and special performances of 2D structure, which is semiconducting and steady. In particular, as the thickness is cut down to the scale of nanometer, the chemical and physical performances will be consumedly converted. Electrochemically, nanoflakes that contain Si phosphide and possess a few layers have been separated successfully by a mechanical way and their properties are investigated. These few-layer SiP nanoflakes perform stronger charge transfer energy, better electrical conductivity, and lower resistance compared to bulk SiP, resulting in the improvement of cyclability and chargeability of LIBs. In situ TEM implies that invertible phase change occurs dynamically in the course of either lithiation or delithiation; various novel phases come into being in order, that is, Li_xSi (x ≤ 3.75), amorphous Li₃P, and polycrystal SiP, with an area amplification of about 233% for nanoflake of single SiP during the process of lithiation. At the same time, a good original capacity of 862 mA h g⁻¹ during discharge and 737 mA h g⁻¹ during charge is able to be realized in experiments via accommodating the ratios of electrode components. What's more, capacity retention of ~460 mA h g⁻¹ is still kept even after 100 cycles. Efforts have been devoted to developing new few-layers of Si phosphide materials, which are considered promising anodes for LIBs (Figure 14F–G).²²² Also, as is widely known, batteries of lithium-oxygen (Li-O₂) have been considered ideal for electric vehicles owing to their high power concentration. However, for the realistic utilization of these abovementioned batteries, the growth of lithium dendrite, and the severe decomposition of electrolytes on lithium metals remains a tough question. In this regard, an active 2D phosphorene-derived LiP is shown as a metallic Li preserved layer, where the nanoscale preserved layer on metallic lithium restrain the growth of lithium dendrite and the decomposition of the electrolyte. This restraint is ascribed to thermodynamic performances of the active LiP preserved layer in electrochemistry. The electrolyte decomposition is restrained on the preserved layer as the redox potential of the LiP layer is better than that of electrolyte decomposition. Lithium electroplating has a thermodynamical disadvantage on LiP layers, which impedes the development of Li dendrite during cycling. Therefore, the nanoscale LiP preserved layer polishes up the cycle behavior of dual lithium cells and batteries of Li-O₂. Various theoretical calculations and ex situ analyses uphold these performances of the phosphorene-derived LiP preserved layer (Figure 14H–L).²²³ In addition, thanks to Wang et al.'s efforts, metallic lithium, as the anode for updated lithium metal batteries with high energy concentration, is synthesized. The product possesses the features of low redox potential and high theoretical specific capacity. However, its coulombic efficiency decreases badly and this will bring security risks. In this effort, nickel phosphides are proven to be advantageous for uniform lithium growth and nucleation for the first time. They exhibit high affinity to Li and standout electron conductivity and provide excellent conductivity of Li ion when transformed into the vertical nickel phosphides NSs. At 1 mA cm⁻², the optimized coulombic efficiency is as high as 98.5% for 280 cycles and in a symmetric cell, outstanding cycling stability over 3000 h is achieved as well. Consequently, excellent stability, superior coulombic efficiency, standout cycling stability, and extraordinary performance of rate can be fulfilled when made pairs with LiFePO₄ cathode in a full cell.²²⁴

Figure 14. (A) Scanning electron microscopy (SEM) image of [Cu2(BTC)4/3(H2O)2]6[H3PMo12O40] (NENU-5). (B) SEM image of MoP@porous carbon (MoP@PC). (C,D) Transmission electron microscopy images of the MoP@PC. (E) Long rate-cycling performance of the L-S cells with various separators at 0.5 C. MoP@PC, highly dispersed MoP nanocrystallites encapsulated in phosphorus-doped porous carbon. Reproduced with permission: Copyright 2020, Elsevier.221 (F) Cycling stability of SiP/CNT/CMC. (G) SEM image of the bulk SiP (BSIP), showing effect of the phosphorene-derived Li3P protective layers on Li dendrite growth. Reproduced with permission: Copyright 2020, American Chemical Society.222 Ex situ FESEM images of (H) bare and (I) phosphorene-coated Li metal electrodes with 1 M lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI) in ethylene carbonate:diethyl carbonate (EC:DEC, 1:1 volume ratio) after Li plating at 0.1 mA cm−2 for 20 h (areal capacity: 2 mA h cm−2). (J–L) Electrochemical performance of Li metal symmetric cells with various electrolytes. (L) Galvanostatic charge–discharge voltage profiles of Li metal symmetric cells for the plating/stripping of bare (black) and phosphorene-coated (red) Li metal electrodes. Electrolytes in Li/electrolyte/Li symmetric cells: (J) 1 M LiNO3 in DMA (DMA = N,N-dimethylacetamide) and (K,L) 1.3 M LiPF6 in EC:DEC. Current pulse: (J) ±0.1 mA cm−2 for each 0.5 h cycle, (K) ±0.1 mA cm−2 for each 0.5 h cycle, and (L) ±2 mA cm−2 for each 0.5 h cycle. Reproduced with permission: Copyright 2018, American Chemical Society223

SIBs

With the enormously growing requirement arising from the field of energy storage and due to the widespread application and feasibility of sodium, SIBs have been regarded as a promising candidate for the recent commercially available LIBs.

Kim argued that P-abundant nanoparticles of 6-MnP₄ have been prepared through high energy mechanical milling (HEMM) and their electrochemical performances as an anode not only for LIBs but also for SIBs have been researched with a particular emphasis on the activity in electrochemistry and mechanism of the reaction. The nanoparticles of 6-MnP₄ with a triclinic structure (P-1), which consist of crystallites sized from 5 to 20 nm, are first prepared by HEMM. The electrode of MnP₄ exhibits high initial discharge and charge capacity of 1876 and 1615 mA h g⁻¹ for LIBs and 1234 and 1028 mA h g⁻¹ for SIBs and its high initial Coulombic efficiency is 86% for LIBs and 83% for SIBs, exhibiting the great potential of high-capacity anodes. In addition, the reasonably good rate capability and constant cyclability of MnP₄ can be advanced via the production of MnP₄/graphene nanocomposites and vanadium-substituted Mn_0.75V_0.25P₄ solid solutions (Figure 15A–E).²²⁵ Furthermore, Park et al. have successfully synthesized a carbon-coated Na₃V_1.96Fe_0.04(PO₄)₃-Fe₂P composite (NVFP-FP/C) by using a one-pot pyro-synthesis method. As a cathode material, its excellent Na storage property for SIB is demonstrated. The composite of Na₃V_1.96Fe_0.04(PO₄)₃-Fe₂P is composed of electroconductive ultra-fine Fe₂P crystals and specific nanoclusters are randomly scattered among the particles of NVP. The existence of Fe₂P in the cathode of the aforementioned composite greatly advances its electroconductivity and the diffusion coefficient of Na⁺-ion. As a result, the cathode of NVFP-FP/C shows a good discharge capacity at the temperature of 0.1°C (103.2 mA h g⁻¹), distinct stability, and no significant blemish even after 2000 cycles at the temperature of 20°C and superior power capability at as high as 40°C.²²⁷ Additionally, a green method for preparing nanocomposite of Sn₄P₃/C will be described and its superior sodium storage property as a new anode of SIBs will be revealed. This anode of Sn₄P₃/C is able to show a reversible capacity up to 850 mA h g⁻¹ and a prominent rate capability of 50% capacity output at 500 mA g⁻¹. Besides, in the anode of Sn₄P_3, the Sn nanosized particles serve as electrochannels to afford electroactivation of the phosphorus component; the phosphorus and P-derived materials Na₃P act as a host matrix to remit the grouping of the nanoparticles of stannum over the insertion reaction of sodium. Due to the synergistic mechanism of sodium storage in the anode of Sn₄P₃, it can circulate 86% capacity retention even after 150 cycles.²²⁸ As Anantharaj et al. pointed out in the article, the validity and use of ten important parameters are discussed: (1) exchange current density (j₀), (2) ECSA, (3) FE, (4) iR-corrected overpotential at a defined current density, (5) mass activity, (6) measurement of double layer capacitance (C_dl) for different electrocatalytic materials, (7) specific activity, (8) overpotential at a defined current density, (9) Tafel slope, and (10) TOF, which are often applied not only in OER but also in HER.²²⁹ To settle three questions of SIB, involving poor cycle life, inferior power density, and outstanding energy density, Li et al. made great efforts over the past few years, and via improving the electrolyte composition or the electrode structure, both energy density and cycling lifespans are optimized.²³⁰ Also, it is known that TMPs have been widely studied because their intercalation potentials compared with Li/Li⁺ are relatively low, while their theoretical capacities are high. However, its commercial applications have been impeded by low electrical conductivity and remarkable volume variation during cycling. A convenient method for preparing graphene (GR) assembled with a hollow FeP@carbon nanocomposite (H-FeP@C@GR) is proposed, which is to treat a hybrid with a hot method, a C coating procedure, a phosphorization treatment, and a carbothermic reaction. The nanospheres of hollow iron phosphide shelled with thin carbon layers are splendidly incorporated into the GR matrix, co-connecting to transform a three-dimensional hierarchical structure. Utilized as anode materials for both LIBs and SIBs, H-FeP@C@GR showed superior electrochemical representations (Figure 15F–I).²²⁶ Chang et al. argued that SIBs deserve to become hopeful low-cost alternatives to LIBs in energy-related commercialization induced by the natural resource of sodium, which is far more abundant than lithium. Unfortunately, sodium ions have a radius 1.5 times as large as that of lithium ions, which results in tough sodiation/desodiation at SIB anodes and decreased behaviors. As a result, the research on the latest materials of SIB anode aimed at promoting sodiation/desodiation has turned into a leading exploration field.²³¹ Faced with the challenges in tuning the microarchitectures (particle size, crystalline state, and components) of MP anodes for SIBs, it is extra difficult and very critical to put forward a reasonable approach to developing MP anodes possessing hierarchically porous structure for better-performed SIBs. A distinctive core-shell porous FeP@CoP phosphide microcubes co-connected through decreased rGO NSs (rGO@CoP@FeP) are successfully prepared by a treatment of phosphorization with PB as template at a low temperature. The SIB anodes of rGO@CoP@FeP, which have hierarchical architecture, show greatly enhanced reversible capacity, long-term cycling stability, and superior rate capability. The improved rGO@CoP@FeP electrochemical property is attributed specifically to the porous microstructure of core-shell and the synergistic effect between the components of phosphide (Figure 15J–L).⁸⁸

Figure 15. (A) Scanning electron microscopy (SEM) image and (B) transmission electron microscopy (TEM) image in low magnification of as-synthesized MnP4 nanoparticles. Cycle performance of MnP4 and MnP4/graphene (MnP4/G) electrodes for sodium-ion batteries (SIBs) at a current density of (C) 50 mA g−1 and (D) 500 mA g−1 with activation of initial three cycles at 50 mA g−1 and (E) rate capability. Reproduced with permission: Copyright 2021, Wiley.225 TEM images of (F) hollow FeP@carbon nanocomposite (H-FeP@C) nanospheres and (G) the H-FeP@C@GR nanocomposite. (H) Cycling performance of all the samples. (I) Long cycling performance of the graphene assembled with a hollow FeP@carbon nanocomposite (H-FeP@C@GR) at a current density of 0.5 A g−1. Reproduced with permission: Copyright 2017, American Chemical Society.226 (J) SEM image of Prussian blue-derived porous C-FeP microcubes. (K) Rate capability and (L) cycling performance of the rGO@CoP@C-FeP, CoP@C-FeP, and C-FeP electrodes. Reproduced with permission: Copyright 2017, Elsevier.88

Other batteries

Certainly, besides the batteries mentioned above, MOF-derived metal phosphates are also prevalent in many other batteries, and with the efforts of many researchers, a great number of gratifying results have been shown.

Due to the relatively low operation voltage and high theoretical capacity, MPs are expected to be PIBs, which possess high energy density for the anode. The immense volume variation during cycling will cause sudden capacity attenuation and result in the failure of the MP mechanism. The concentrated electrolyte is utilized during cycling to realize outstanding steadiness for PIBs and potassium-metal batteries while other electrolytes are more dilute, mainly because of the anion-derived solid-electrolyte interphase layer with robust and uniform structure, which helps keep the electrode integrity, restrain electrolyte decomposition, and refrain from excessive side reactions. Further research also demonstrates that compared with traditional dilute electrolytes, enriched electrolyte and metallic K demonstrate synergistic benefits of excellent compatibility, enhanced safety, and pronounced electrochemical stability. A doable strategy is provided to allay the remission of MPs to develop high-energy-density PIBs.²³² Moreover, Wu et al., through utilizing porous Ni-Fe-C, Ni-Cr-Fe, Ni-Fe-Mo, and Ni-Ti electrodes fabricated by preparation ways containing elemental powder reactions, studied the HER happened in seawater. After measuring four types of electrode makings, the open porosities are 25.27%, 20.47%, 23.05%, and 29.05%, respectively. The electrochemical exhibition of these electrodes has been investigated by electrochemical impedance spectroscopy, cyclic voltammetry, and polarization measurement. The primary outcomes show that the porous Ni-Cr-Fe electrode has relatively good and lasting stability and excellent catalytic activity for HER in seawater. The reasonable stability and high efficiency of porous Ni-Cr-Fe as electrode catalyst mirror its promising utilizations in the rising revolution of hydrogen.²¹¹ In Liu's opinion, it is pivotal for renewable H₂ production to exploit stable, economical, and effective electrocatalysts to realize effective water splitting commercially. Co foams are used as the only source of cobalt and conductive collectors. Facile preoxidation helps the integration of a seamless electrode, in which a highly active, durable, and versatile catalyst aimed for overall water splitting in alkaline electrolyte has been firstly fabricated and Fe₂P, Co₂P, and Co foam are tightly linked through chemical bonds. The as-synthesized electrode of Fe₂P-Co₂P/CF for HER exhibits a low overpotential of only 145, 208, and 254 mV, while for OER it exhibits an overpotential of 243, 291, and 317 mV, indicating current 100, 500, and 1000 mA cm⁻² concentrations, respectively. Remarkably, using Fe₂P-Co₂P/CF as either a cathode or an anode at the room temperature and quasi-industrial temperature (25 and 65°C), the operating voltages show only 1.87 and 1.71 V to achieve 500 mA cm⁻², respectively, and over 300 h superior long durability. Because of the strong in situ growth as well as the robustly coupled heterojunction formation between Co₂P and Fe₂P, Fe₂P-Co₂P/CF has become a wonderful potential candidate to satisfy the large-scale employment of commercial water electrolysis. Its performance is more than prominent.¹⁶⁵

SUMMARY AND OUTLOOK

In summary, this review supplies an overview of the up-to-date developments in the exploration of MOF-derived phosphide nanomaterials for electrochemical applications. Due to rich active sites, unique physicochemical properties, controllable component structure, and a moderate price over noble metals, nanomaterials of MOF-derived phosphide have been extensively studied as promising electrode materials in the past few years. The structure and composition of the assembled complex nanomaterials can be manipulated by the change of experimental conditions. Furthermore, the fabricated MOF-derived phosphide nanomaterials retain the advantages of the original MOFs in architecture. As a result, diversified synthetic methods and ingenious-modulated strategies for preparations of monometallic, bimetallic, trimetallic phosphides, and composite nanomaterials with MPs have been highlighted and the as-prepared synthetic products exhibited ideal electrochemical performances, making MOF-derived phosphide nanomaterials favorable not only for fundamental utilization but also for potential practical applications in energy fields, such as electrode materials for LIBs, SIBs, SCs, and other batteries. Despite great efforts devoted and tremendous headway made in this field, it should be noticed that the straightforward utilization of MOF-derived phosphide nanomaterials as electrode materials is still challenging.

One of the challenges for MOF-derived phosphide nanomaterials is that the controlled preparation methods of MOF-derived phosphide nanomaterials remain very limited and, as a result, with the continuous improvement of preparation strategies, it is believed that in the near future the requirements of the practical application can be satisfied. What's more, initial coulombic efficiency is an important factor for the application of commercial batteries. However, MOF-derived phosphide nanomaterials, which exhibit low initial coulombic efficiency, make it suffer a large loss of irreversible capacity at the first cycle. In terms of the application in energy storage, reducing energy loss and optimizing other performances deserves attention, and at the same time, there remain some obstacles in catalysis. Even though MOF-derived phosphide nanomaterials perform standout catalytic activities, their stabilities are still below the requirement of practical applications and there is much room for optimization. In addition, in some of the currently available methods of synthesizing the nanostructured phosphorus-based anodes, several key improvements have been made, including safety concerns, sophisticated synthetic methodologies, and the absence of practical applications for low-cost commercial production. With a comprehensive understanding of the potentialities of MOF-derived nanomaterials, we further discuss the probable research directions to promote their characterizations.

• (1)

  More preparation methods. Further research should focus on the innovation in synthetic strategies of MOF-derived phosphide nanomaterials with controllable structure and composition. It is urgently needed to break the limitation of the existing controlled preparation methods of MOF-derived phosphide nanomaterials. More synthetic strategies, especially the reproducible, addressable, and large-area fabrication techniques, are supposed to be composed to further expand the application range for material, structure, and function. The innovative combination of multiple preparation techniques may just as well deserve a try.

• (2)

  The elemental doping. The representation of unstable surface may be induced by the under-coordinated phosphorus site and elemental doping. The elemental doping, as a highly efficient way to optimize the stability, is recommended. Most of the recent research on MOF-derived phosphides are within the range of iron, cobalt, nickel, cuprum, molybdenum, and tungsten, and doping elements are restricted within the aforementioned boundary as well. Through the discussion in this review, it is not hard to find that with the increase of elements, some phosphides will show outstanding performance. Considering this, if we research more in element doping and try to synthesize some high-entropy phosphates, there may be a breakthrough.

• (3)

  Electronic structure engineering. Although various strategies have been developed for tuning the electronic structure of MP, including heteroatom-doping, interface-coupling, and so forth, there still exists an urgent challenge for the accurate regulation of the electronic structure of MP at the atomic level toward target applications. Therefore, from the right beginning of the MP, the atomic-level engineering of the electronic structure of relative MOF is crucial for the derived phosphide nanomaterials.

As discussed above, even though considerable developments have been made in recent years, it is still necessary to develop feasible power storage equipment based on MOF-derived phosphide nanomaterials due to the excellent properties of their active sites, physicochemical properties, and component structures. It is believed that with the rapid development of electrochemical studies from the aspect of nanoscale, MOF-derived phosphide nanomaterials will bring fundamental and technical breakthroughs for electronic devices in the future and will be employed in wider practical applications, including energy storage and conversion applications.

ACKNOWLEDGMENTS

This study was supported by the National Natural Science Foundation of China (NSFC-U1904215), Natural Science Foundation of Jiangsu Province (BK20200044), and Program for Young Changjiang Scholars of the Ministry of Education, China (Q2018270). We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions and the technical support we received at the Testing Center of Yangzhou University. Jingqi Tian is grateful for the support of the Specially-Appointed Professor Plan in Jiangsu Province.

CONFLICT OF INTEREST

The authors declare no conflict of interest.