Synthetic methods and electrochemical applications for transition metal phosphide nanomaterials

Abstract

Currently, the great demand for energy has triggered the exploration of new devices for energy storage and conversion. For these devices, electrode materials with excellent performance are strongly needed. In particular, transition metal phosphide nanomaterials are emerging with excellent performances. Transition metal phosphide nanomaterials have been utilized in Li-ion batteries, Na-ion batteries, supercapacitors, solar cells, electrocatalysis, etc. This review summarizes recent developments and challenges in transition metal phosphide nanomaterials, with a focus on synthetic methods and electrochemical applications.

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Yao Lu

Yao Lu is now a student under Professor Huan Pang's supervision, at Yangzhou University of Chemistry and Chemical Engineering, China. Her research mainly focuses on the field of inorganic semiconductor nanostructures and their applications to supercapacitors.

Tianyi Wang

Tianyi Wang is currently a B.S. student under Professor Huan Pang's supervision at Yangzhou University of Chemistry and Chemical Engineering, China. His research mainly focuses on the field of electrochemical energy storage materials and their applications.

Xinran Li

Xinran Li is now a student under Professor Huan Pang's supervision, at Yangzhou University of Chemistry and Chemical Engineering, China. Her research mainly focuses on the field of micro/nanocoordination derived materials for electrochemical energy devices.

Guangxun Zhang

Guangxun Zhang is now a student under Professor Huan Pang's supervision, at Yangzhou University of Chemistry and Chemical Engineering, China. His research interests are in the field of materials for energy storage, including lithium-ion batteries, lithium–sulfur batteries, and supercapacitors.

Huaiguo Xue

Huaiguo Xue received his Ph.D. degree in polymer chemistry from Zhejiang University in 2002. He is currently a professor of physical chemistry and the dean of the College of Chemistry and Chemical Engineering at Yangzhou University. His research interests focus on electrochemistry, functional polymers and biosensors.

Huan Pang

Huan Pang received his Ph.D. degree from Nanjing University in 2011. He then founded his research group at Anyang Normal University where he was appointed as a distinguished professor in 2013. He has now joined Yangzhou University as a university distinguished professor. He has published more than 90 papers in peer-reviewed journals including Chemical Society Reviews, Advanced Materials, and Energy Environ. Sci., with 3400 citations (H-index = 31). His research interests include the development of inorganic nanostructures and their applications in flexible electronics, with a focus on energy devices.

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1. Introduction

The development of modern society largely depends on natural resources, such as oil and coal. However, at the same time, a number of problems are emerging due to the overconsumption of these resources,¹ like energy shortages, air pollution and global warming.^2–5 Thus, the demand for energy is urgent and so is the development of renewable energy sources.^6–12 Therefore, clean energy has received great attention as a promising substitute in the past few years. Two successful examples are wind turbines and solar cells.

The utilization of electrical energy can be also perceived as a special symbol of the second industrial revolution and human society stepped into the age of electricity from that time on. The great improvements in our living standards and social production depend on electrical energy to a large extent today in the 21^st century. As a consequence, the requirement for high-efficiency electrical energy storage and conversion is mandatory and urgent.¹³ Batteries and supercapacitors are excellent methods for electrochemical energy storage, while solar cells can be used for energy conversion.

Recent research regarding sustainable energy sources, as well as energy storage and conversion methods, concentrates on the exploitation of excellent materials in the fields of electrochemical energy storage and conversion. In the past, large quantities of research based on new materials focused on oxygenated chemicals,^14–22 nitrogenous compounds^23–25 and sulfocompounds,²⁶ and so forth, whereas research aimed at phosphorous compounds was rare correspondingly. In recent years, phosphorous compounds have been developed and have turned into hot spots in the field of electrochemistry. The element P belongs to the nitrogen group but holds multi-electron orbitals, so P is likely to possess superior chemical properties compared to the N atom. Due to native properties in its structure, P shows excellent performances in electrochemical applications.²⁷ Referring to recent research, a great variety of phosphorous compounds can be considered as nice choices in the electrochemistry field. Nevertheless, limited by space, we merely discuss transition metal phosphides, and other compounds containing phosphorus will be omitted.²⁸

The use of nanomaterials is regarded as a kind of excellent strategy to enhance physical properties and chemical performances.^29–31 When used in transition metal phosphides, nanomaterials can similarly display excellent properties.³² Thus, studies of transition metal phosphides are mainly based on nanomaterials. Herein, we deliver a brief account of synthesis strategies for transition metal phosphide nanomaterials (TMPNs) firstly, and ball milling methods, electrodeposition synthetic methods, phosphorization, solution-phase synthetic methods, solid-phase synthetic methods and many other synthetic methods are introduced. Secondly, applications, for instance Li-ion batteries (LIBs), Na-ion batteries (NIBs), supercapacitors (SCs), solar cells and electrocatalysis, are discussed. It should be noted that there are quantities of research on TMPN catalysis that have been performed before, especially the research of Sun et al.^{10,11,33–41} Furthermore, Wang et al. summarized recent advances regarding cobalt-based heterogeneous electrocatalysts in water splitting in detail,⁴² and we do not elaborate on applications of TMPNs in the catalytic/electro-catalytic fields. Eventually, summaries and the outlook for future research are put forward.

2. Synthetic methods

Generally, TMPNs can be synthesized through solution-phase reactions,^43–46 solid-phase reactions,^27,47–51 and so forth.⁵² However, when TMPNs are used in the field of electrochemistry, some of the synthetic strategies are different from those used in other fields. In this case, we summarize several novel methods propitious to electrochemistry as well as TMPNs, for instance ball milling methods, electrodeposition methods, phosphorization, and many other synthetic methods. These methods are recommended according to specific circumstances.

2.1 Ball milling methods

Lots of research based on such methods has been performed, while the ball milling method has been adopted in much research which is not involved with TMPNs, such as for a nanostructured Fe–8P (wt%) powder mixture.⁵³ And Brutti et al. investigated the redox activity of MgH₂ by assessing what effect would be caused as a result of ball milling pre-treatment.⁵⁴

In this review, we introduce ball milling synthetic methods for TMPNs. The ball milling method is a mechanochemical method utilizing the impact caused by the grinding medium (such as steel balls, pebbles, etc.) and the grinding effect of the grinding body and ball mill lining, and in this case, the effect of materials mixing and crushing can be reached. Several instances have been taken to illustrate its application in the synthesis of TMPNs and, tersely, we introduce these detailed applications, according to our discussion. Li et al. successfully synthesized pure GeP₅ and GeP₅/C at ambient temperature and pressure by utilizing a ball milling method.⁵⁵ And a Cu₃P sample was synthesized by Stan et al. via a ball milling method with n-dodecane existing.⁵⁶ In addition, Kim et al. prepared tin phosphide (Sn₄P₃) through ball milling.⁵⁷ A Sn_4+xP₃@(Sn–P) composite was prepared by Li et al. on a large scale, yet with low speed, through ball milling between P and Sn powders.⁵⁸ Li et al. synthesized CoP nanoparticles using a productive, gentle and simple ball-milling strategy.⁵⁹

2.2 Electrodeposition synthetic methods

Electrochemical deposition, or electrodeposition for short, is a method utilizing passing currents from external electrochemical batteries. Electrodeposition is a fascinating phenomenon. To the contrary, electroless deposition, where no external current flows, predates it by centuries. Since electricity was found and human beings put electricity into use with electrodeposition, electrodeposition has been exceptionally versatile, and valuable applications keep being invented. The electrodeposition process should be conducted under certain electrolytes as well as experimental conditions, and furthermore, the complexity of electrodeposition and the morphology of the sediment is connected with the properties of electrodeposition, relying also on the composition and pH of the electrolyte, as well as other factors.

In this review, we give a brief overview with respect to the synthesis of TMPNs. To give an example, Chandrasekar et al. prepared iron phosphide over a copper substrate through the electrodeposition technique by utilizing an aqueous acidic electrolyte.⁶⁰ Much other research concerning electrodeposition has also mushroomed. Nickel phosphide films were prepared by Lu et al. via the electrodeposition of four kinds of Ni films using choline chloride–ethylene glycol.⁶¹ Using electrodeposition, Xiang et al. fabricated porous nickel phosphide films using polystyrene sphere multi-layers as the template.⁶²

2.3 Phosphorization

Phosphorization is also perceived as a general and effective strategy. Uniform yolk–shell Sn₄P₃@C nanospheres could be successfully synthesized through a phosphatization process with yolk–shell Sn@C nanospheres.⁶³ Also, hierarchical nanostructured nickel phosphide (h-Ni₂P) spheres can be prepared through a one-pot reaction in which nickel acetylacetonate is regarded as a precursor and trioctylphosphine (TOP) is taken as a phosphorus source, and the surfactant could be tri-n-octylamine as well as oleylamine (OAm).⁶⁴ Bai et al. smoothly synthesized a Ni₂P/C composite, which is like peapods, using NiNH₄PO₄H₂O nanorods as templates.⁶⁵ Hall et al. obtained amorphous FeP₂ through the reaction of Fe(N(SiMe₃)₂)₃ with PH₃ in THF at 100 °C.⁶⁶

2.4 Solution-phase synthetic methods

Solution-phase reactions have been adopted in the synthesis of TMPNs. And much research has been done since now. For instance, Ni₂P/SiO₂ catalysts can be synthesized using a solution-phase reaction in the presence of an SiO₂ support, using Ni(acac)₂ and trioctylphosphine.⁶⁷ In addition, the novel solution-phase low temperature synthesis of a supported Ni₂P catalyst has been described. And this used nickel acetylacetonate (Ni(acac)₂) and low cost triphenylphosphine (TPP) in the presence of a coordinating solvent tri-n-octylamine (TOA) to form Ni₂P/MCM-41 catalysts.⁴⁵ Chen et al. reported the synthesis of single-crystalline Ni₂P nanowires. And they used a solution-phase approach, using nickel(II) acetylacetonate as a metal precursor and trioctylphosphine as a phosphorus source.⁶⁸ Li et al. successfully synthesized monodispersed nickel phosphide (Ni₂P) nanorods and nanoparticles via a one-step solution-phase route, in which a mixture of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) was used as the solvent, capping agent and phosphor source.⁶⁹

2.5 Solid-phase synthesis methods

TMNPs can also be synthesized through solid-phase reactions. Ni₂P nanoparticles were simply prepared through a solid phase reaction between H₂PO₂⁻ and Ni²⁺ in N₂ at relatively low temperatures. And these exhibited high activity for the hydrodesulfurization of dibenzothiophene and the hydrodenitrogenation of quinoline.²⁷ What is more, Pfeiffer et al. prepared copper phosphide (Cu₃P) thick films over copper foils via a solid-state reaction at low temperature (400 °C).⁵⁰ A solid phase reaction between nickel cations and hypophosphites might occur at 473 K in the pores of mesoporous carbon (MC), resulting in highly dispersed Ni₂P on the surface of the MC support. The Ni₂P/MC catalysts thus prepared exhibited higher activities for the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and the hydrodenitrogenation (HDN) of quinoline than Ni₂P/SiO₂ catalysts prepared through the reduction of phosphate at high temperatures.⁴⁹

2.6 Other synthetic methods

There exist a large number of other strategies to synthesize TMPNs applied in the field of electrochemistry as well. Hydrothermal reactions,^70,71 solvothermal synthetic methods,^72,73 thermal decompositions, low-temperature solid state reaction methods, and thin-film vapor–liquid–solid (TF-VLS) synthetic strategies are all efficient and undemanding methods. At 180 °C, the hydrothermal reaction of red phosphorus, nickel chloride and graphene oxide results in nickel phosphide-embedded graphene, which is in an aqueous solution of ethylene glycol.⁷⁴ Via a solvothermal reaction at 180 °C for 10 h, tin phosphide (Sn₄P₃) particles can be synthesized with different particle sizes.⁷⁵ Similarly, nickel phosphide (NiP) nanoparticles could be prepared using a facile one-pot solvothermal method.⁷⁶

As for thermal decomposition, Keigo et al. successfully synthesized nickel phosphide through thermal decomposition, where nickel is regarded as a precursor and trioctylphosphine and trioctylphosphine oxide can be used as phosphorus sources.⁷⁷ Furthermore, thermal decomposition can be utilized to synthesize single-crystalline Ni₂P nanowires that possess a uniform diameter of about 8 nm and a length of about 100–200 nm, using syringe pumps to continuously deliver Ni–TOP (trioctylphosphine) complexes.⁷⁸ Besides the methods above, low-temperature solid-state reactions are an excellent and general strategy. By utilizing this method, An et al. successfully synthesized Ni₂P nanoparticles, which were synthesized on reduced graphene oxide (abbreviated as rGO).⁷⁹ Furthermore, Zheng et al. investigated solar cells, which were based on InP, that were synthesized via non-epitaxial thin-film vapor–liquid–solid (TF-VLS) growth strategies.⁸⁰

3. Electrochemical energy storage

3.1 Lithium storage – Li-ion batteries

Li-ion batteries (LIBs) can be defined as a means of satisfying our need for energy storage, which is key to exploiting alternative energy.^81,82 And due to their superior performance, including high voltage, high capacity, high specific energy (in other words, high energy density), excellent power density, no memory effects, long-term cycle lifetimes, low self-discharge properties and environmental benignity in practical use,^83–91 LIBs have drawn a great deal of attention. More importantly, LIBs have been widely used in portable electronic devices, and they can potentially possess wider application prospects when utilized as power sources in hybrid electric vehicles (HEVs) as well as electric vehicles (EVs).^81,92–103

Quantities of research have been performed to promote alternative electrode materials, which have specific energy densities suitable for LIBs and a higher Li⁺ utilization, in that the electrode materials determine the energy density of the batteries in significant measure. Currently, several electrode materials from TMPNs are designated as promising materials for LIBs, in addition, TMPNs with superior performance could be used as both anode materials and negative electrode materials. Some of the following mentioned materials are listed in Table 1.

Table 1 Several materials being utilized in LIBs and their basic properties

Electrode	Specific capacity/mA h g^−1	Rate	Cycles	Ref.
NiP2	750	0.13 mA cm^−2	10	77
Ni3P	557	0.2C	50	62
Ni3P–Ni	360	0.02 mA cm^−2	100	104
Ni2P	398.5	54.2 mA g^−1	50	61
Ni12P5	600	—	200	105
Ni5P4/C	644.1	0.1C	50	48
Ni2P/C	628	100 mA g^−1	200	65
Ni2P	379.8	0.1C	50	106
Ni2P	434	0.1C	50	78
Ni2P	365.3	0.5C	50	64
FeP2	906	0.1C	10	66
GeP5/C	2300	5 A g^−1	40	55
Sn4P3	315	200 mA g^−1	200	75
CoP	839.1	5 μA cm^−2	25	107

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3.1.1 Nickel phosphide materials. Via thermal decomposition, Keigo et al. successfully synthesized nickel phosphide particles; in the process of decomposition, nickel is utilized as a precursor and trioctylphosphine and trioctylphosphine oxide can be used as phosphorus sources.⁷⁷ What is more, they smoothly prepared different crystal-phase samples and these can be realized by regulating the amount of the precursor trioctylphosphine and solvents, the temperature, and the precursor nickel.

The crystal phases as well as the morphologies of the as-prepared materials were characterized utilizing XRD, as well as TEM. Utilizing nickel acetate tetrahydrate in trioctylphosphine oxide, NiP₂ particles possessing 200–500 nm diameters were acquired at 360 °C for 5 h, while spherical Ni₅P₄ particles possessing 500 nm diameters were obtained with a synthetic process with a time of 1 h, with other conditions similar to those for NiP₂. All-solid-state cells composed of NiP₂ particles and 80Li₂S center dot 20P₂S₅ (mol%) glass ceramic displayed excellent performance, with a discharge capacity of 1100 mA h g⁻¹ at a 0.13 mA cm⁻² current density, and the discharge capacity was 750 mA h g⁻¹ after 10 cycles. In the process, NiP₂ particles can be regarded as active materials and the other component is seen as a solid electrolyte.

Through electrodeposition, Xiang et al. synthesized porous nickel phosphide films. In the synthesis, polystyrene spheres could be assembled by them, and the many layers such a structure possessed were regarded as the template.⁶² When they got rid of the as-obtained template, spherical pores were reserved in the films. And the spherical pores were well-ordered as well as close-packed. Because of the existence of thin walls between the adjoining pores, a network nanostructure that was three-dimensional was obtained, and in this way, lithium ion transfer through the passivating layer is faster than before. What is more, this sort of nanostructure could stimulate charge transfer which enters into the fabricated network nanostructure. The two explanations above are based on electrochemical impedance spectra (EIS).

Hence, the network nanostructure delivers a momentous improvement in the electrochemical performance (a schematic illustration regarding the stacking of pores in films is shown in Fig. 1), particularly giving a superior rate capability. And the capacity of these kinds of triple-layer Ni₃P porous films still respectively is maintained at 557 mA h g⁻¹ and 243 mA h g⁻¹ at a charge–discharge rate of 0.2C and 5C (1C = 388 mA g⁻¹) after 50 cycles. While the discharge–charge rate rises 25 times, the reversible capacity remains 44%. These electrochemical performances are much more excellent than the electrochemical performances of single or double layer films.

Fig. 1 Schematic illustration regarding the stacking of pores in films. Adapted with permission from ref. 62, © 2011 Elsevier.

An advanced Ni₃P–Ni array, synthesized using electroless deposition, which was on a three-dimension nickel foam was utilized as an electrode in LIBs by Liu et al.¹⁰⁴ The array structure of Ni₃P–Ni can not only adapt to volume changes in the process of lithiation/de-lithiation but can also accelerate the high-rate capability, in that the interstices can serve as an ideal buffer for volume expansion in such structures. The Ni₃P–Ni arrays showed excellent electrochemical performance and were used as anodes for LIBs, possessing excellent rate capability as well as a specific capacity of 360 mA h g⁻¹ at a 0.02 mA cm⁻² current density after 100 cycles.

Nickel phosphide films were prepared by Lu et al. through electrodeposition. At room temperature, they electrodeposited four genres of Ni films by utilizing a liquid which was composed of choline chloride and ethylene glycol.⁶¹ They obtained Ni films whose morphology was sheet-like when the concentration of Ni²⁺ was 0.5 M, while they also obtained Ni films whose morphology was sphere-like when the concentration of Ni²⁺ was 1.0 M. As a consequence, the surface morphologies of these Ni–P films are related to the initial nickel films.

As for the Ni–P films, different phases, which were Ni₁₂P₅ and Ni₂P, could be obtained through phosphorization. Then they investigated the electrochemical performance of these Ni–P films, which serve as anodes in LIBs. Cyclic voltammetry as well as galvanostatic charge–discharge tests manifested that their electrochemical performance heavily relied on the surface morphology as well as the thin-film thickness. The as-prepared Ni₂P films displayed excellent electrochemical performance, including a 398.5 mA h g⁻¹ reversible discharge capacity. In addition, the samples possessed 91.4% retention after 50 cycles.

A composite with Ni₁₂P₅ nanoparticles encapsulated in carbon fiber was proposed by Zhang et al.¹⁰⁵ And it should be especially noted that the composite is peapod-like. In the synthesis, the precursors are NiNH₄PO₄ nanorods; in addition, glucose could work as a carbon source. Fig. 2 shows the whole synthetic process and TEM images display the characteristic structure of the peapod-like composite in Fig. 3a and b. Due to the hydrogen bonding which exists between carbon and the precursor, a polymer layer could be hydrothermally synthesized and later transformed into carbon fiber through the method of inert calcination. The NiNH₄PO₄ nanorods simultaneously turned into Ni₁₂P₅ nanoparticles that were encapsulated in carbon fiber. In the whole process, they suffer from decomposition as well as reduction, which is on account of the high temperature as well as the carbon fibers.

Fig. 2 Synthetic process for a peapod-like composite with Ni₁₂P₅ nanoparticles encapsulated in carbon fiber. Adapted with permission from ref. 105, © 2014 Wiley.

Fig. 3 TEM images display typical structures relating to the peapod-like composite. The resolution increases from (a) to (b), little by little. (c) An SEM image of Ni₅P₄/C spheres. (d) A TEM image of the corresponding composite of the Ni₅P₄/C spheres. (a, b) Adapted with permission from ref. 105, © 2014 Wiley; (c, d) ref. 48, © 2012 Wiley.

The synthesized composite delivered excellent performance when it was used as the anode in LIBs, compared with materials which are pure phase. To be specific, over 200 cycles, a specific capacity could be attained of 600 mA h g⁻¹. And the improved performance was attributed to synergistic effects, which are associated with the combination of Ni₁₂P₅ nanoparticles and carbon fiber to some extent.

Lu et al. synthesized a single-phase Ni₅P₄/C composite.⁴⁸ In the synthesis, they utilized a thin carbon shell, and they realized the synthesis though a two-step method which consists of a wet-chemistry reaction as well as a solid-state reaction. And the reason for the transformation from binary phase Ni₅P₄–Ni₂P to single-phase Ni₅P₄ is that phosphorus atoms diffuse in the carbon shells when the solid-state reaction is in progress. Fig. 3c and d show an SEM image of the Ni₅P₄/C spheres as well as a TEM image of the corresponding composite of the Ni₅P₄/C spheres.

And in addition, galvanostatic charge–discharge measurements manifest that the as-prepared Ni₅P₄/C composite delivers superior rate capacity as well as excellent cycling stability. And almost 76.6% of the second capacity (644.1 mA h g⁻¹) can be preserved after 50 cycles at a rate of 0.1C. Besides, at a high rate of 3C, the specific capacity of Ni₅P₄/C can reach 357.1 mA h g⁻¹. Generally, carbon shells which are amorphous can reinforce properties of the composite, such as conductivity. In addition, the aggregation of active particles can be restrained. In this way, structural stability as well as reversibility during cycling can be realized.

Bai et al. synthesized a peapod-like Ni₂P/C composite.⁶⁵ They utilized NiNH₄PO₄H₂O nanorods and these can be used as templates. Because of their enriched porosity as well as large active surface areas (113.7 m² g⁻¹), peapod-like composites deliver excellent performance when utilized as anodes in LIBs and in HER catalysts (as is shown in Fig. 4). When utilized as an anode in LIBs, the prepared nanocomposite showed 632 mA h g⁻¹ capacity at 0.1 A g⁻¹. What is more, they also delivered excellent rate capabilities when possessing a capacity of 439 mA h g⁻¹, even at 3 A g⁻¹ current density. And their capacity can reach as high as 628 mA h g⁻¹, with a pretty long cycle life, as shown in Fig. 5a.

Fig. 4 The schematic diagrams illustrate the synthetic process for peapod-like Ni₂P/C nanocomposites. Adapted with permission from ref. 65, © 2015 the Royal Society of Chemistry.

Fig. 5 (a) Electrochemical performances of peapod-like Ni₂P/C composites: cycling performance and coulombic efficiency within a voltage range of 0.01–3.0 V vs. Li/Li⁺ at 0.1 A g⁻¹; (b) SEM images of Ni₂P nanosheets; (c) comparison of Cu₃P with Li₄Ti₅O₁₂ and graphite according to charge/discharge potential, specific capacity, and specific energy density, as well as volumetric energy density. (a) Adapted with permission from ref. 65, © 2015 the Royal Society of Chemistry; (b) ref. 106, © 2012 the Royal Society of Chemistry; (c) ref. 108, © 2014 the Royal Society of Chemistry.

Lu et al. synthesized porous Ni₂P nanosheets through an organometallic method without a hitch.¹⁰⁶ And they succeeded in realizing a synthesis using nickel nanosheets as a template, and the morphological characterization of the Ni₂P nanosheets is displayed in Fig. 5b. These Ni₂P nanosheets possess a pretty thin thickness that is around 3 nm, as well as a high Brunauer–Emmett–Teller (BET) surface area owing to the highly porous structure, which could result in good lithium storage capacity as well as excellent rate capability. What is important is that the diffusion length regarding the lithium ions will be short due to the thin and porous Ni₂P sheets. These nanosheets, when utilized as anodes in LIBs, exhibit 379.8 mA h g⁻¹ reversible discharge capacity after 50 cycles and an excellent rate capability.

Lu et al. successfully fabricated di-nickel phosphide (Ni₂P) nanowires which are single crystalline.⁷⁸ And these nanowires possessed an 8 nm uniform diameter as well as 100–200 nm lengths. They were synthesized via thermal decomposition in which the continuous delivery of Ni–TOP complexes is necessary. Fig. 9c shows a TEM image of the Ni₂P nanowires. Compared to the hexangularly structured Ni₂P nanoparticles, these Ni₂P nanowires display specific reversible capacities of 434 mA h g⁻¹ at 0.1C and 326 mA h g⁻¹ at 0.5C after 50 cycles; in addition, the samples show excellent rate performance. Thus performances have been largely improved and the reasons for the improvement are that the size is small and the cylindrical structure is stable. And it made the interfacial contact area with electrolytes especially high; in addition, it could also relieve the strain and adapt to the volume expansion/contraction of the Ni₂P nanowires. What is more, as is shown in electrochemical impedance spectra (EIS), the enhanced electrical conductivity of these nanowires could promote the transportation of lithium ion as well as electrons in the electrode.

Lu et al. successfully synthesized hierarchical nanostructured nickel phosphide (h-Ni₂P) spheres.⁶⁴ A one-pot reaction involving an organic phase strategy is utilized by them, and in the synthetic process, the solution should be continually stirred. And the precursor they used was nickel acetylacetonate, and trioctylphosphine (TOP) can be used as the phosphorus source. Among the organic-phase mixture, a surfactant is necessary, and tri-n-octylamine as well as oleylamine (OAm) can be utilized. The specific process is shown in Fig. 6.

Fig. 6 Schematic illustration of the fabrication of h-Ni₂P. Adapted with permission from ref. 64, © 2011 American Chemical Society.

The h-Ni₂P spheres, which consist of ordered nanoparticles of 5–10 nm size as well as being filled with amorphous carbon, deliver a superior cycling performance to the Ni₂P spheres, which are synthesized by utilizing oleic acid (OA), which is regarded as the surfactant. The contact area, which is between electrolytes and Ni₂P, could increase owing to the hierarchical structure, and this is because such a structure could offer more sites to accommodate Li⁺ and shorten the length for Li^z+ diffusion, as well as enhance the reaction reactivity. Furthermore, it is the amorphous carbon as well as the hierarchical nanostructures that could adapt to expansion in volume and in this way the stability of the electrodes increases during cycling. The prepared h-Ni₂P electrode delivers excellent capacity and Coulombic efficiency. After 50 cycles, the reversible capacity of the h-Ni₂P spheres is as high as 365.3 mA h g⁻¹ at 0.5C, and 257.8 mA h g⁻¹ at 1C. And at a high rate of 3C, the specific capacity of h-Ni₂P is 167.1 mA h g⁻¹.

3.1.2 Copper phosphide materials. Porous Cu₃P was synthesized on copper foam through a corrosion strategy, which was accomplished by Ni et al.¹⁰⁸ And porous Cu₃P/Cu exhibits good electrochemical performance due to electrochemical reconstruction during cycling. Compared to commercial anodes, Cu₃P delivers a higher specific energy than Li₄Ti₅O₁₂, as well as a more excellent volumetric energy than graphite, as shown in Fig. 5c.

Stan et al. synthesized a Cu₃P sample through a mechanochemical method (ball milling) when n-dodecane was present.⁵⁶ Then the favourable electrochemical properties of the Cu₃P sample were presented, and the Cu₃P sample showed an almost theoretical specific capacity when charged to 0.02 V for the first cycle. But the capacity faded as cycling was prolonged. When the sample was cycled over the voltage range from 2.0 to 0.50 V (vs. Li/Li⁺), the performance improved. The types of formed products result in a capacity decline when cycled to a lower cut-off of 0.02 V. And the types are connected with the energy barrier, which can be used to reconvert to primary materials. However, when the potential is 0.50 V, the energy barrier can be much smaller relatively, and what is more, capacity retention can also reveal it.

3.1.3 Iron phosphide materials. Hall et al. obtained amorphous FeP₂ through the reaction of Fe(N(SiMe₃)₂)₃ with PH₃ in THF at 100 °C.⁶⁶ Fig. 7a shows an SEM image of amorphous FeP₂, which is synthesized at low temperature. Amorphous FeP₂, when utilized as the anode in LIBs, delivers excellent performance in electrochemical lithiation/de-lithiation. The gravimetric discharge capacity is 1258 mA h g⁻¹, and the charge capacity is 766 mA h g⁻¹ (Fig. 7b). What is more, when cycled for the first time, the reversibility is 61% and the discharge capacity is 906 mA h g⁻¹ after 10 cycles. This translates to a 66% retention of the theoretical full conversion capacity of FeP₂ (1365 mA h g⁻¹).

Fig. 7 (a) SEM image of amorphous FeP₂ synthesized at low temperature; (b) discharge and charge capacities for the first 10 cycles of amorphous FeP₂ at 0.1C. Adapted with permission from ref. 66, © 2012 American Chemical Society.

At room temperature, Chandrasekar et al. prepared iron phosphide over a Cu substrate at high temperature.⁶⁰ A cost-effective electrodeposition method was utilized by them to synthesize samples, and aqueous acidic electrolyte was utilized in the synthetic process. The phosphorus content of the alloys differed and varied with the composition of the source. And as-prepared deposits were annealed at 400 °C for 3 h. Such a process goes on under a constant inert gas flow rate in a tubular furnace. Fig. 9d shows an FEGSEM image of electrodeposited Fe₂P annealed samples. Then they characterized the phase composition as well as the morphology by utilizing X-ray diffraction and scanning electron microscopy. They tested samples when the samples were used as anode materials between 0.01 and 2.5 V, at a constant rate of 10 μA cm⁻².

Fig. 8 Crystal structures of orthorhombic black P, rhombohedral black P and rhombohedral GeP₅. Adapted with permission from ref. 55, © 2015 the Royal Society of Chemistry.

Fig. 9 (a) and (b) SEM images of 10–10 h milled GeP₅/C nanocomposites; (c) TEM image of Ni₂P nanowires; (d) FEGSEM image of electrodeposited Fe₂P annealed samples. (a, b) Adapted with permission from ref. 55, © 2015 the Royal Society of Chemistry; (c) ref. 78, © 2012 the Royal Society of Chemistry; (d) ref. 60, © 2014 Springer.

3.1.4 Germanium phosphide materials. Li et al. successfully synthesized pure GeP₅ and a GeP₅/C nanocomposite by utilizing a facile ball milling strategy.⁵⁵ Fig. 9a and b exhibit SEM images of GeP₅/C gained using ball milling. And GeP₅ was tested as the anode in LIBs. Referring to XRD Rietveld refinement as well as first principle calculations, GeP₅ has a layered structure which is two-dimensional and analogous to the structure of graphite and black P (shown in Fig. 8). In addition, a high conductivity is revealed, which is respectively 10 000 and 10 times that of the conductivity of black P and graphite.

When used as an anode, GeP₅ and the carbon composite both show an excellent reversible capacity of 2300 mA h g⁻¹, as well as a good initial coulombic efficiency of about 95%. What is more, ex situ XRD as well as CV tests show GeP₅ goes through conversion as well as a lithium storage mechanism, which is connected with the alloy. Furthermore, both germanium and phosphorus make a contribution to the capacity. What is more, GeP₅/C delivers excellent cycle stability, as well as possessing a high rate capacity of 2127 mA h g⁻¹ at 5 A g⁻¹.

3.1.5 Tin phosphide materials. Liu et al. successfully synthesized different-sized Sn₄P₃ nanoparticles through a gentle solvothermal strategy for 10 h at 180 °C.⁷⁵ The mean size of the as-obtained Sn₄P₃ nanoparticles is around 15 nm. What is more, the size of the targets can be controlled by regulating the ratio of the solvent with ease.

Besides, their excellent cycling stability as well as rate performance have been characterized acting as anodes in LIBs. Results display that Sn₄P₃ nanoparticles of the smallest size could deliver the greatest cycling as well as rate performances. These nanoparticles exhibited a discharge capacity that was about 612 mA h g⁻¹ after 10 cycles, in addition, they could retain 442 mA h g⁻¹ after 320 cycles at 100 mA g⁻¹ within a voltage range of 0.01–3.0 V. Because of their small size, the performances of the Sn₄P₃ electrodes might be improved. What is more, the as-obtained Sn₄P₃ nanoparticles have been characterized as anodes for Na-ion batteries (NIBs) as well, and could exhibit a 305 mA h g⁻¹ reversible capacity after 10 cycles at a 50 mA g⁻¹ current density.

3.1.6 Cobalt phosphide materials. Through pulsed laser deposition, Cui et al. successfully synthesized nanocrystalline CoP films.¹⁰⁷ The physical and electrochemical performances of CoP as the anode in LIBs were investigated. Over a potential range of 0.1–3.0 V, the reversible discharge capacity concerning a CoP/Li cell was designated in the range of 788.0–1055.7 mA h g⁻¹. Furthermore, galvanostatic cycling showed an 839.1 mA h g⁻¹ reversible capacity, which was achieved from 25 cycles. A reversible conversion reaction mechanism between CoP and Co/Li₃P was proposed and revealed using ex situ TEM and SAED. The specific reaction includes decomposition from CoP to a Co/Li₃P composite, as well as nanocrystalline structural re-formation.

3.2 Sodium storage – Na-ion batteries

LIBs have received much attention owing to their cycling stability, excellent energy density and many other advantages; these have made LIBs universally utilized as rechargeable power sources in portable electronic devices, HEVs and EVs.^109–111 What is more, there are lots of new electrode materials emerging so as to improve the performance of LIBs. Nevertheless, the high expense and insufficient abundance of lithium resources hinder the development of LIBs. It is difficult to satisfy need for large-scale application, especially in industry.^112–115

Na-ion batteries (NIBs) have received considerable attention as a potentially alternative to LIBs, with readily available and geographically diverse reserves of the metal.^116–123 Hence, the cost of sodium is lower than that of lithium and there is also the potential for cost reduction via electrodes.^124–129 However, it is a challenging task to develop a suitable and excellent host electrode material for accommodating sodium ions which allows reversible ion insertion and rapid extraction in its structure, owing to the around 55% larger radius of Na⁺ compared with Li⁺, which causes slower kinetics with Na⁺.^130–134

Although there exists not too much research based on TMPNs in the field of NIBs, in recent times, with research into electrode materials for NIBs, TMPNs have gradually emerged and turned into research hot points. Below are recent developments in NIBs, with specific electrode materials and their electrochemical performances. For example, the performances of some materials are summarized in Table 2.

Table 2 Summary of representative materials for NIBs

Electrode	Specific capacity/mA h g^−1	Rate	Cycles	Ref.
Sn4P3@C	360	1.5C	400	63
Sn4P3/C	850	500 mA g^−1	—	135
Sn4P3	718	100 mA g^−1	100	57
Sn4+xP3	165	5000 mA g^−1	100	58
CuP2/C	2000	—	—	136
NiP3	900	10C	15	137

---

3.2.1 Tin phosphide materials. Through a top-down phosphorized route, Liu et al. successfully synthesized uniform yolk–shell Sn₄P₃@C nanospheres.⁶³ And in the synthesis, yolk–shell Sn@C nanospheres could be regarded as the precursor. Fig. 10a illustrates the fabrication procedure concerning yolk–shell Sn₄P₃@C nanospheres. In addition, it should be noted that such structures possess some remarkable features for anode use, as displayed in Fig. 10b.

Fig. 10 (a) A schematic illustration providing information on uniform yolk–shell Sn₄P₃@C nanosphere anodes; (b) schematic illustration of the sodiation process in various Sn₄P₃-based anodes including bare Sn₄P₃ nanoparticles and core–shell Sn₄P₃@C nanoparticles, as well as the current yolk–shell Sn₄P₃@C nanoparticles. Adapted with permission from ref. 63, © 2015 the Royal Society of Chemistry.

Fig. 11a and b exhibit high-magnification SEM and low-magnification TEM images of the final products. The fabricated structure of the Sn₄P₃ electrodes is very crucial. A carbon shell that is thin and conformal, as well as self-supporting, can thoroughly protect the yolk–shell Sn₄P₃ nanoparticles. Because of the void space between the shells and nanoparticles, the expansion of Sn₄P₃ can be carried out without deforming the shells or disrupting the stable solid-electrolyte interphase (SEI) which is located on the outside surface. Because of the unique structure, the as-prepared nanospheres, when used as anodes in NIBs, deliver an excellent reversible capacity of 790 mA h g⁻¹ and great rate capability, as well as cycling stability (a highly stable capacity of 360 mA h g⁻¹ at 1.5C after a long-term 400 cycles).

Fig. 11 (a) High-magnification SEM and (b) low-magnification TEM images of uniform yolk–shell Sn₄P₃@C nanosphere products; (c) SEM image of CoP particles; (d) HRTEM image of a Sn₄P₃/C composite. (a, b) Adapted with permission from ref. 63, © 2015 the Royal Society of Chemistry; (c) ref. 59, © 2015 Elsevier; (d) ref. 135, © 2014 American Chemical Society.

Qian et al. utilized a synthetic approach for a Sn₄P₃/C nanocomposite with good performance that can serve as the anode in NIBs.¹³⁵ A HRTEM image of Sn₄P₃/C is shown in Fig. 11d. The as-obtained anodes show an excellent reversible capacity of 850 mA h g⁻¹ as well as superior rate capability, with 50% capacity output at 500 mA g⁻¹. Besides, good cycling stability, with 86% capacity retention over 150 cycles, is demonstrated. The remarkable performance derives from a synergistic Na-storage mechanism in the Sn₄P₃ anodes, as shown in Fig. 12.

Fig. 12 Schematic illustration of the Na-storage mechanism in Sn₄P₃ electrodes. Adapted with permission from ref. 135, © 2014 American Chemical Society.

Kim et al. prepared tin phosphide (Sn₄P₃) through ball milling.⁵⁷ When tested as anodes in NIBs, Sn₄P₃ delivered excellent electrochemical performances, for example, a reversible capacity of 718 mA h g⁻¹. In addition, the cycling stability is excellent and the decay in capacity is so negligible that it might be neglected over 100 cycles. As is shown in Fig. 13a, Sn₄P₃ delivered a superior cycling performance to Sn, although both electrodes showed a similar reversible capacity of around 718 mA h g⁻¹ when a fluoroethylene carbonate (FEC) additive was utilized. The sodiation profile of Sn₄P₃ exhibits two steps situated at ∼0.2 and 0.01 V vs. Na/Na⁺, and this can be further supported through differential capacity (dQ/dV) plots (Fig. 13b).

Fig. 13 (a) Cycling performances of Sn₄P₃ and Sn electrodes obtained with and without FEC additive. (b) Differential capacity (dQ/dV) plots for Sn₄P₃ electrodes over the first cycle, with and without FEC additive. Adapted with permission from ref. 57, © 2014 Wiley.

Through ball milling between elemental phosphorus and tin powder, a Sn_4+xP₃@(Sn–P) composite was prepared by Li et al. in large quantities.⁵⁸ Fig. 14a and b show SEM and HRTEM images of the Sn_4+xP₃@(Sn–P) composite. The electrochemical performance of the composite was examined through use as an anode in NIBs. The results showed that a Sn_4+xP₃@(Sn–P) electrode with a CMC binder showed a high capacity of 502 mA h g⁻¹ at a current density of 100 mA g⁻¹. Besides, the cycling stability of the Sn_4+xP₃@(Sn–P) electrode was improved because of the addition of 5% fluoroethylene carbonate (FEC) to the electrolyte. What is more, the as-prepared anode exhibited a 465 mA h g⁻¹ stable capacity, whose retention was 92.6% at a current density of 100 mA g⁻¹ over 100 cycles. This capacity is excellent for anodes in NIBs. Furthermore, the as-obtained anode delivered great rate capability.

Fig. 14 (a) SEM and (b) HRTEM images of a Sn_4+xP₃@(Sn–P) composite. The inset in (b) is the SAED pattern from lower magnification TEM; (c) SEM image; (d) TEM image. (a, b) Adapted with permission from ref. 58, © 2014 Wiley; (b) ref. 31, © 2015 Elsevier; (c, d) ref. 136, © 2015 the Royal Society of Chemistry.

3.2.2 Cobalt phosphide materials. Li et al. synthesized CoP nanoparticles using a productive, gentle and simple ball-milling strategy.⁵⁹ An SEM image of the CoP nanoparticles is displayed in Fig. 11c. In addition, when utilized as an anode in NIBs, the sample was examined in sodium half-cells. The as-prepared electrode can deliver a 770 mA h g⁻¹ initial capacity. The good capacity provided a good alternative to conventional anodes in NIBs. Lower polarization and great rate capability are other significant advantages. Owing to these advantages, CoP could be a potentially promising anode in NIBs.

3.2.3 Copper phosphide materials. A CuP₂/C composite was successfully prepared by Zhao et al.¹³⁶ The composite was composed of copper phosphide as well as Super P carbon black. Fig. 15 shows a schematic illustration of the fabrication process for the CuP₂/C composite nanostructure, and Fig. 15 also displays the evolution of the structure when charging and discharging. The products are made of crystalline CuP₂ cores, whose surfaces are coated with carbon black.

Fig. 15 A schematic illustration of the formation process for a CuP₂/C composite nanostructure and the structural evolution during electrochemical charging and discharging. Adapted with permission from ref. 136, © 2015 the Royal Society of Chemistry.

Several studies of the CuP₂/C composites, including an SEM image and TEM image, are shown in Fig. 14c and d. As a potential anode candidate, the as-obtained samples could reach the goal of fast reversible sodiation as well as desodiation. And the realization of this function relies on the mechanism of conversion. In addition, they exhibited a great capacity of more than 500 mA h g⁻¹. The excellent rate capability as well as the short-term cycling stability are other advantages for these samples.

3.2.4 Nickle phosphide materials. In NIBs, searching for appropriate negative electrodes is an essential issue for future advances in batteries. However, really efficient as well as excellent negative electrode materials have not yet been developed in NIBs, as far as we are concerned. However, Fullenwarth et al. demonstrated that NiP₃ could be an excellent candidate for this purpose.¹³⁷

NiP₃-based electrodes were utilized as negative electrodes for NIBs. The fabrication of a composition close to Li₃P as well as Na₃P was revealed during research regarding the reaction mechanism. In the fabrication, they embedded nickel nanoparticles to synthesize the target samples after discharge. Moreover, owing to the fabrication of a carboxymethyl cellulose/carbon black (CMC/CB) electrode, NiP₃ electrodes deliver pretty excellent capacity. And for NIBs, the reversible capacity can reach in excess of 900 mA h g⁻¹ after 15 cycles.

3.3 Supercapacitors

Among numerous ways of energy storage, supercapacitors (SCs) have been recognized as a promising kind of energy storage device and have attracted more and more attention owing to higher power densities, faster charging/discharging rates, and longer cyclic performances, as well as better safety compared to any other energy storage strategies, such as LIBs and NIBs.^138–152

SCs can be generally divided into two sorts, (i) electrical double-layer capacitors (EDLCs) and (ii) pseudocapacitors (PDCs), based on the energy storage mechanism. The former stores energy in a non-faradic way through the accumulation of charge at the interface between the electrode and electrolyte, while the latter is based on reversible faradic reactions that lead to pseudocapacitive behavior.^153–161

In general, an ideal electrode material for SCs should possess high conductivity, high energy and power delivery, a pretty large surface area, and suitable pore size distribution, as well as excellent cycling ability and electrochemical reproducibility.^162,163 However, the practical application of SCs is restricted by the lack of electrode materials with facile synthetic methods as well as low cost. Thus, searching for excellent electrode materials was the focus in past decades. Inchoate studies of SCs primarily concentrated on carbonaceous materials, and carbonaceous materials possessed relatively low specific capacitance as well as instability when at a relatively high charge–discharge rate.¹⁶⁴ And their energy density is lower than that of rechargeable batteries, which hinders the utilization of SCs.^165–168

Thanks to recent remarkable progress, TMPNs are noted to have metalloid properties as well as excellent electrical conductivity, so they are fit to be electrode materials.^169–173 Among all the TMPNs, nickel phosphide nanomaterials have drawn extensive interest as they include a number of phases and excellent properties, as well as potential electrochemical applications.¹⁷⁴ The following examples are detailed statements of recent research based on nickel phosphide nanomaterials.

An effective phosphorization synthesis was demonstrated to strengthen supercapacitor performance, based on transition metal oxides, as well as hydroxides, in a study operated by Zhou et al.¹⁷⁵ Fig. 16a and b show SEM images of Ni₂P NS/NF. Through the phosphorization of Ni(OH)₂ nanosheets, a three-dimensional networked Ni₂P nanosheet array can be grown on a surface which consists of Ni foam. According to the voltammetric measurements, Ni foam-supported Ni₂P nanosheets (Ni₂P NS/NF) could be used as electrodes and they were recognized to show a specific capacitance of 2141 F g⁻¹ at 50 mV s⁻¹ (Fig. 16c) and could remain at 1109 F g⁻¹ even at a current density of 83.3 A g⁻¹. In addition, the specific capacitance could remain almost steady at 1437 F g⁻¹ between 2000 and 5000 charging/discharging cycles (Fig. 16d). They found that similar performance can be observed in Ni₂P powder, because it has eliminated the effect of the Ni foam.

Fig. 16 (a) and (b) SEM images of Ni₂P NS/NF; (c) CV curves for Ni₂P NS/NF at different scan rates. (d) Cycling stability of Ni₂P NS/NF and Ni(OH)₂ NS/NF at a current density of 10 A g⁻¹. Adapted with permission from ref. 175, © 2015 Wiley.

Lu et al. successfully coated Ni₂P particles via an electroless plating method homogenously with amorphous Ni.¹⁷⁶ Fig. 17a and b show an SEM image and TEM image of the Ni₂P/Ni composite, which would be created through an electroless plating process for 10 min. The Ni₂P nanoparticles are completely coated with a 20–30 nm thick layer. The pseudocapacitive behavior regarding the as-prepared samples can be characterized using cyclic voltammograms as well as galvanostatic charge–discharge tests in 2 M LiOH.

Fig. 17 (a) SEM image of Ni₂P/Ni; (b) TEM image of Ni₂P/Ni; (c) TEM image and (d) HR-TEM of a Ni₂P/rGO composite. (a, b) Adapted with permission from ref. 176, © 2013 the Royal Society of Chemistry; (c, d) ref. 79, © 2013 the Royal Society of Chemistry.

It could be designated that the specific capacitance of the Ni₂P/Ni composite decreased by 19%, from 581 F g⁻¹ at 1 A g⁻¹ to 464 F g⁻¹ at 40 A g⁻¹. What is more, the as-prepared samples also show excellent cycling capacity. The capacitance of the as-prepared samples is excellent. The specific capacitance can reach 1115 F g⁻¹ at 2 A g⁻¹, and it can be retained at 1029 F g⁻¹ (92.3% capacity retention) after 3000 cycles. Importantly, the great improvement in pseudocapacitive performance could be attributed to the flask-like Ni coating. The flake-like coating could offer the fast transfer of ions and electrons, as well as a number of active sites.

An et al. successfully synthesized Ni₂P nanoparticles on reduced graphene oxide (rGO) via a solid state reaction at low-temperature.⁷⁹ TEM and HR-TEM images of the Ni₂P/rGO composite are shown in Fig. 17c and d. And the Ni₂P/rGO nanoparticles were utilized as a pseudocapacitive material, which is associated with energy storage. In addition, the specific capacitance of the as-prepared Ni₂P/rGO is 2266 F g⁻¹ and it also displays excellent cycling performance when utilized as a capacitor material. After 2500 cycles, the loss in specific capacitance is rather small and even negligible, as shown in Fig. 5d. This great result suggests that a Ni₂P/rGO electrode is a candidate for a pseudocapacitor with excellent performance, such as superior specific capacitance as well as a stable cycling capacity.

Duan et al. proposed a novel synthetic method for the fabrication of core/shell nanostructures which are composed of two parts: noble metal cores, as well as semiconductor shells which are single-crystal and possess disparate crystal systems.¹⁷⁷ And the synthetic route involves a simple phosphorization process, in which triphenylphosphine served as a capping agent as well as the phosphorous source which reacts with Au/Ni₁₂P₅ core/shell nanoparticles (NPs), and this process makes Au/Ni₁₂P₅ core/shell nanoparticles (NPs) convert from Au–Ni bimetallic heterodimers. The transformation process is illustrated in Fig. 18.

Fig. 18 Schematic illustration of the fabrication of Au/Ni₁₂P₅ core/shell NPs. Adapted with permission from ref. 177, © 2014 Nature.

In addition, they further investigated the SC performance of the Au-modified Ni₁₂P₅ nanostructures. When serving as supercapacitor electrodes, the metal/semiconductor nanostructure with synergistic effects is superior to the oligomer-like counterpart. The specific capacitance of the electrode composed of core/shell nanoparticles (NPs) is up to 806.1 F g⁻¹ (91.1% capacity retention after 500 charge–discharge cycles).

A one-step strategy has been utilized to prepare Ni₂P nanoparticles. And these nanoparticles could be further embellished with graphene, and in this way graphene-modified Ni₂P nanocomposites could come into formation. Du et al. tested the electrochemical performance of Ni₂P samples.⁷⁶ This showed that supercapacitor electrodes composed of the as-prepared samples delivered excellent electrochemical performance.

The as-obtained electrodes deliver a larger specific capacitance, better cycling stabilization and superior rate capability, as well as greater energy density. With only 5% (wt%) graphene loaded, the electrochemical performance could be greatly improved. Fig. 19b displays CV curves at different scan rates from supercapacitor electrodes composed of a Ni₂P@5% GR(graphene) nanocomposite and a TEM image of the as-prepared Ni₂P@5% GR (graphene) nanocomposite is shown in Fig. 19a. The improvement in the electrochemical performance is derived from the network structure. And such a structure allows for efficient charge migration as well as electrolyte diffusion, and furthermore it holds back the expansion/contraction of the volume as well as the corrosion of the Ni₂P particles.

Fig. 19 (a) An SEM image of a Ni₂P@5% GR nanocomposite; (b) CV curves at different scan rates recorded for supercapacitor electrodes consisting of a Ni₂P@5% GR (graphene) nanocomposite. Adapted with permission from ref. 76, © 2014 Elsevier.

Du et al. successfully synthesized nickel phosphide (NiP) nanoparticles using a one-pot solvothermal strategy that has been applied extensively.¹⁷⁸ The obtained nickel phosphide was further assembled into two kinds of SCs: one is asymmetric and another is symmetric. They tested the electrochemical performance of these as-obtained assembled SCs in an electrolyte composed of 3.0 mol L⁻¹ KOH. It turned out that the as-prepared asymmetric supercapacitors made of Ni₂P nanoparticles as well as activated carbon (Ni₂P//AC ASCs) delivered excellent electrochemical performance, such as a stable electrochemical window of 0–1.5 V, a superior energy density of 64.6 W h kg⁻¹ at a power density of 1029 W kg⁻¹, and great cycling stability (95.1% capacitance retention). And the specific results are shown in Fig. 20.

Fig. 20 (a) CV curves for Ni₂P//AC ASCs at different scan rates; (b) discharge curves for the Ni₂P//AC ASCs; (c) Ragone plot relating to the energy and power densities of Ni₂P//AC ASCs; (d) cycling performance curves for Ni₂P//AC ASCs and Ni₂P//Ni₂P SSCs. Adapted with permission from ref. 178, © 2015 Elsevier.

On the contrary, the symmetric supercapacitors composed of Ni₂P nanoparticles delivered an electrochemical performance that was not so good. It should be noted that Ni₂P//AC ASCs could drive a number of electronic units by charging and then discharging. In addition, two asymmetric supercapacitors can be linked in series to drive a red light-emitting diode and a rotating motor.

4. Electrochemical energy conversion – solar cells

Solar energy conversion can be recognized as the most dependable and feasible approach to satisfy growing energy demands in an environmentally friendly way, and optimization regarding solar energy resources is momentous and indispensable.¹⁷⁹ Dye-sensitized solar cells (DSSCs), as a 3^rd generation photovoltaic (PV) conversion method, have been demonstrated to achieve conversion from solar energy to electricity,^180,181 and what is more, DSSCs possess the strengths of low-cost, high conversion efficiency and easy fabrication.^182–187 Since the pioneering work that was reported by O'Regan and Grätzel, quantities of papers have been published to put forward techniques and novel materials for the implementation of DSSCs.^188–192

In DSSCs, the counter electrode (CE) is the pivotal component affecting the photovoltaic properties of DSSCs, and it can be utilized as a mediator which can collect electrons to reduce I³⁻ to I⁻ to regenerate the redox couple.^193,194 And it generally consists of platinum because of its favourable electrocatalytic activity for the iodide–triiodide redox couple as well as its great conductivity.^195–197

But the mass production of DSSCs is restricted by the high cost and corrosion of platinum.¹⁹⁸ The known reserves of platinum all over the world are rather limited. Meanwhile, in recent years, the development of proton exchange membrane fuel cells (PEMFCs) has aggravated the shortage of platinum resources, as in PEMFCs platinum can be utilized as an indispensable electrocatalyst.⁷¹

Hence, there is a desperate need to exploit low-cost as well as high-efficiency counter electrodes possessing high electrocatalytic activity that are analogous to platinum and, in this way, the traditional and exorbitant Pt counter electrode can be replaced in DSSCs. And it should be noted that TMPNs are practical alternatives owing to their excellent electrocatalytic activity, superior electrical conductivity, and great stability.

4.1 Nickel phosphide materials

At 180 °C, a hydrothermal reaction gives access to the synthesis of nickel phosphide-embedded graphene, which is in an aqueous solution of ethylene glycol.⁷⁴ The hydrothermal synthesis involves red phosphorus, nickel chloride and graphene oxide. And the prepared samples were studied as counter electrodes in dye-sensitized solar cells. It is emphasized that nickel phosphide-embedded graphene delivers superior catalytic activity and weak diffusion impedance. Fig. 21a displays I–V curves of DSSCs which possess different counter electrodes.

Fig. 21 (a) I–V characteristic curves of DSSCs with different counter electrodes consisting of graphene, Ni₁₂P₅, a graphene–Ni₁₂P₅ composite and FTO-Pt, measured under simulated sunlight of 100 mW cm⁻² (AM 1.5); (b) CVs of Pt, Ni₁₂P₅, and Ni₂P electrodes at a scan rate of 10 mV s⁻¹ in acetonitrile containing 0.5 M LiClO₄, 50 mM LiI and 10 mM I₂. The inset shows consecutive CVs and changes in the redox peak currents with the cycle number for the Ni₂P (PR) electrode; (c) the photocurrent–voltage (J–V) characteristics of DSSCs employing Pt, Ni₂P (PR), Ni₂P (PS), and Ni₁₂P₅ (PS) counter electrodes; (d) J–V curves for T₂/T⁻ based DSSCs using mesoporous carbon (MC), Ni₅P₄, Ni₅P₄/C and Pt counter electrodes. (a) Adapted with permission from ref. 74, © 2012 the Owner Societies; (b, c) ref. 199, © 2013 the Royal Society of Chemistry; (d) ref. 200, © 2012 the Royal Society of Chemistry.

Through pulse-reverse deposition, Wu et al. directly coated a Ni₂P nanolayer, which possesses porous nanospheres, on tin oxide glass doped with fluorine and this was investigated for use as a counter electrode catalyst in DSSCs.¹⁹⁹ Periodic pulse-reverse (PR) deposition was fulfilled with a voltage of −0.8 V for 6 s and a reverse voltage of 0.1 V for 24 s, and the periodic voltage was repeated for 500 cycles. Compared with PR deposition, potentiostatic (PS) deposition was accomplished using a voltage of −0.8 V for 50 min.

The preparation strategy impacts on the morphology as well as the microstructure of the nickel phosphide electrodes. In fact, the differences in the as-obtained particle sizes are owing to the voltage which is applied in the process. And PS deposition results in the sequential growth of the grain, which forms bigger nanoparticles. On the contrary, PR deposition constrains the growth of the grain, owing to the anodic dissolution of regions which are rich in nickel during the anodic process, and is inclined to prepare smaller nanoparticles upon the surface made up of fluorine-doped tin oxide (FTO).

A nanolayer of nickel phosphide possessing porous nanospheres was synthesized through PR deposition. Fig. 22a displays an SEM image of the nickel phosphide serving as an electrode which is prepared through PR deposition. Fig. 22b displays a TEM image of the nickel phosphide nanoparticles which could be obtained using PR deposition after annealing at 500 °C for 1 h.

Fig. 22 (a) Top-view as well as side-view SEM images of nickel phosphide electrodes prepared using PR deposition; (b) TEM image of nickel phosphide nanoparticles obtained from PR deposition, scale bar = 50 nm; (c) SEM morphology image of Ni₅P₄/C; (d) SEM image of nickel phosphide/FTO. (a, b) Adapted with permission from ref. 199, © 2013 the Royal Society of Chemistry; (c) ref. 200, © 2012 the Royal Society of Chemistry; (d) ref. 201, © 2015 the Royal Society of Chemistry.

The anodic dissolution in Ni-rich regions that follows the deposition of the cathode resulted in the formation of porous Ni₂P nanospheres on the surface made up of fluorine-doped tin oxide (FTO) for PR deposition, while PS deposition could form a Ni₁₂P₅ layer with compact nanospheres. The transparency and catalytic activity of the Ni₂P electrodes obtained using PR deposition are greater than those of the PS-deposited Ni₁₂P₅ electrodes.

Fig. 21c shows the J–V characteristics of DSSCs that employ platinum as well as nickel phosphide counter electrodes (CEs). The photovoltaic properties of DSSCs with Ni₂P obtained using PR deposition can be comparable to the photovoltaic properties of DSSCs with platinum and, in addition, they are superior to the photovoltaic properties of DSSCs with Ni₂P (PS) and Ni₁₂P₅ (PS). Fig. 21b shows cyclic voltammograms (CVs) of Ni₂P (PR), Ni₁₂P₅ (PS), and platinum electrodes. The nickel phosphide nanoparticles display a distinctive electrocatalytic performance but resemble the Pt electrodes.

Wu et al. proposed that carbon-supported Ni₅P₄ (Ni₅P₄/C), molybdenum phosphide (MoP) and nickel phosphide (Ni₅P₄) can be used in DSSCs as counter electrode (CE) catalysts and they can be used for the reconstruction of the traditional I³⁻/I⁻ redox couple and the novel organic T₂/T⁻ redox couple.²⁰⁰ With regard to the I³⁻/I⁻ redox couple, MoP and Ni₅P₄ showed decent catalytic activity and DSSCs using MoP and Ni₅P₄ CEs produced power conversion efficiencies (PCEs) of 4.92 and 5.71%.

Similarly, when Ni₅P₄ and mesoporous carbon (MC) were integrated into one composite (Ni₅P₄/C), the catalytic activity could be enhanced significantly. Fig. 22c displays an SEM morphology image of Ni₅P₄/C, and DSSCs with Ni₅P₄/C delivered a 7.54% PCE, which verged on that of DSSCs utilizing platinum counter electrodes (7.76%). As is shown in Fig. 21d, the advantages of Ni₅P₄/C towards the T₂/T⁻ redox couple were more important than those towards the I³⁻/I⁻ redox couple. Ni₅P₄/C showed much better performance than Pt and DSSCs with Ni₅P₄/C CE showed a PCE of 4.75%, much greater than the photovoltaic performance of DSSCs with a platinum CE (3.38%).

A monolayer of nickel phosphide clusters possessing mesoporous nanoparticles could be immediately synthesized on fluorine-doped tin oxide (FTO) glass by Wu et al. through cyclic voltammetric deposition.²⁰¹ They used the prepared samples as CEs in DSSCs. Fig. 23 clearly illustrates the formation process of the nickel phosphide clusters, which are discrete on fluorine-doped tin oxide (FTO) glass, by methods of cyclic voltammetric deposition and an SEM image of nickel phosphide/FTO is shown in Fig. 22d. Cyclic voltammetry incorporated anodic dissolution in regions rich in nickel, which follows the deposition of the cathode, and it could give rise to the fabrication of discrete clusters possessing mesoporous nanoparticles.

Fig. 23 Schematic diagram illustrating the formation of discrete nickel phosphide clusters on fluorine-doped tin oxide (FTO) glass through cyclic voltammetric deposition. Adapted with permission from ref. 201, © 2015 the Royal Society of Chemistry.

Nickel phosphide can be characterized as a specific substance, Ni₅P₄, through annealing at 500 °C, and furthermore, its electrocatalytic features could be assessed using cyclic voltammetry as well as the electrochemical impedance existing in the iodide/triiodide system. The mesoporous Ni₅P₄ catalyst synthesized using cyclic voltammetric methods displays excellent electrocatalytic ability towards the iodide/triiodide redox couple because of the low diffusion impedance and resistance to charge transfer. The PCE of DSSCs with a Ni₅P₄ counter electrode can reach 7.6%, which is higher than that of DSSCs using platinum nanocluster counter electrodes (7.2%).

4.2 Indium phosphide materials

Wang et al. proposed that a graphene based van der Waals heterostructure has superior performance, especially for a graphene/semiconductor Schottky junction.²⁰² And it offers a new platform for solar cell applications.^203,204 Compared with Si, which is the dominant material in current photovoltaic applications,^205–207 indium phosphide (InP) has a bandgap of 1.34 eV, which is located in an optimum energy range for solar energy conversion. The band diagram of a graphene/p-InP Schottky junction is shown in Fig. 24a.

Fig. 24 (a) Band diagram of the graphene/p-InP Schottky junction interface showing the tunable graphene Fermi level E_F-gr; (b) schematic structure of the graphene/p-InP Schottky junction device. Adapted with permission from ref. 202, © 2015 Elsevier.

They achieved graphene/p-InP solar cells possessing a power conversion efficiency (PCE) of 3.3% under AM 1.5 G illumination through delicately designing and engineering a van der Waals heterostructure between graphene and indium phosphide (InP), which had a suitable bandgap of 1.34 eV for solar energy conversion. Fig. 24b shows the schematic structure of the graphene/p-InP Schottky junction device. Chemical doping or electrical field modulation was used to tune the Fermi level of graphene, which led to a PCE of 5.6% for the device under gating effects. Separation and recombination processes in the graphene/InP heterojunction, without and with graphene doping, are displayed in Fig. 25a and b, respectively. Furthermore, according to transient photoluminescence measurements, the interface recombination rate could be reduced while graphene is doped or gated.

Fig. 25 Schematic diagrams of the carrier separation and recombination processes in graphene/InP (a) and doped graphene/InP (b) heterojunctions after excitation from a light source. Adapted with permission from ref. 202, © 2015 Elsevier.

Zheng et al. investigated solar cells that were based on InP, which were synthesized via non epitaxial thin-film vapor–liquid–solid (TF-VLS) synthetic strategies.⁸⁰ There exist a molybdenum back contact, p-InP absorber layer, n-TiO₂ electron selective contact, and indium tin oxide transparent top electrode in the cell structure.

In addition, they introduced an ex situ p-doping process for TF-VLS grown InP and examined the performance of the solar cells, for example, the optoelectronic uniformity as well as the electrical behavior of grain boundaries. Under simulated 1 sun illumination, the PCE of first generation solar cells can reach 12.1% with an open-circuit voltage (V_OC) of 692 mV, a short-circuit current (J_SC) of 26.9 mA cm⁻², and a fill factor (FF) of 65%. Although limited by series resistances, including the top contact, the FF of cells can be alleviated in the future via an optimization of the devices. Under 1 sun, the highest measured V_OC was 692 mV, and this approached the optically implied V_OC of around 795 mV, which can be obtained from the luminescence yield of p-InP.

Ko et al. fabricated indium phosphide (InP) nanopillars which were single-crystalline and grown on a substrate composed of silicon.²⁰⁸ They synthesized the samples for solar cells and they provided an excellent performance, with a 0.534 V V_OC under Air Mass 1.5 Global (AM 1.5 G) illumination. What is more, the PCE can reach 19.6%.

The samples were utilized for solar cells and then they were tested using a solar simulator. The I–V characteristic curves which could be tested in the dark are shown in Fig. 26. Illuminated using a AM 1.5 G solar spectrum, the as-prepared solar cells delivered a V_OC of 0.534 V and an I_SC of 96.0 pA, as well as a fill factor of 48.2%. Normalizing I_SC to the projected exposed area yields a J_SC of 76.3 mA cm⁻². This is more than a factor of 2 higher than the J_SC of 32.2 mA cm⁻² predicted using the Shockley–Queisser limit for a planar solar cell.

Fig. 26 Single nanopillar solar cell electrical characteristics. (a, b) Room-temperature dark and 1 sun (AM 1.5 G) IV characteristics of a single InP nanopillar solar cell on the linear (a) and log (b) scale. Adapted with permission from ref. 208, © 2015 American Chemical Society.

4.3 Gallium phosphide materials

Feifel et al. investigated n-Si/p-Si homojunction solar cells in which GaP served as a passivator as well as a contact layer on the n-Si emitter, with different approaches for backside passivation as shown in Fig. 27b.²⁰⁹ What's more, the backside structure of the final solar cell is pivotal to the structure of GaP/Si solar cells. They discussed disparate silicon solar cells possessing a GaP window, as well as contact layers which were grown using metal-organic vapor phase epitaxy (MOVPE). In addition, they established a process which was propitious for offering low resistance contacts because it could retain the quality of the bulk material silicon as well as offer backside passivation.

Fig. 27 (a) Illuminated measurements which are under an AM 1.5 G spectrum and an irradiation intensity of 1000 W m⁻² and the dark I–V characteristics of an average type II solar cell. (b) The structure of the two different species of solar cells (left: type I, right: type II) that were produced and characterized. Adapted with permission from ref. 209, © 2015 IEEE.

The two backside passivation approaches that could be applied prior to the metal-organic vapor phase epitaxy (MOVPE) process are characterized as type I and type II. Type I, which is based on the strength of a highly doped BSF layer, and passivation of type II, composed of an Al₂O₃ layer, were compared. The cells with Al₂O₃ backside passivation deliver higher V_OC values compared with the BSF cells, and type II solar cells deliver a V_OC of 633 mV and a J_SC of 24.7 mA cm⁻² without an antireflective coating, as well as a FF of 79.9%. In addition, the I–V curve under illumination and a dark I–V of an average type II cell is shown in Fig. 27a.

Hybrid organic–inorganic solar cells were fabricated by Wang et al.²¹⁰ They utilized macroporous n-type GaP and poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS) to synthesize such solar cells. The high aspect ratio structure of macroporous GaP led to higher photocurrents as well as an external quantum yield according to the wavelength.

Hybrid macroporous GaP/PEDOT:PSS devices displayed a J_SC of 2.34 mA cm⁻², a V_OC of 0.95 V, a FF of 0.54, and an overall efficiency of 1.21% under 1.0 sun illumination. By using current density–voltage measurements, the extent of the effect of the dopant density of GaP on the performance of the hybrid devices can be probed. Gold-coated macroporous GaP, prior to PEDOT:PSS coating, exhibited increased device performance, with an efficiency of 1.81%. On the contrary, planar GaP/PEDOT:PSS modified with gold displayed a lower external quantum yield over all wavelengths and reduced J_SC and V_OC values.

4.4. Zinc phosphide materials

Graphene has become an attractive material for optoelectronic and photovoltaic devices, with extraordinary properties such as high carrier mobility and high optical transparency.^211,212 The barrier tuning of a graphene–Zn₃P₂ field-effect solar cell with a semitransparent top-gate electrode was studied by Oscar et al. and they demonstrated that this tuning could embellish the photovoltaic efficiency of solar cells.²¹³

Adding a semitransparent top electrostatic gate allows for the tuning of the graphene Fermi level, and as a consequence, the energy barrier at the graphene–Zn₃P₂ joint goes from an ohmic contact at negative gate voltages to a rectifying barrier at positive gate voltages. As photovoltaic measurements show, the efficiency conversion increased 2-fold when they increased the junction barrier and gate voltages to maximize the photovoltaic response. The photovoltaic current and voltage are shown in Fig. 28. An open-circuit voltage V_OC = 0.53 V, as well as an efficiency of 1.9% under AM 1.5 1 sun solar illumination, were obtained at a first-rank gate voltage of +2 V.

Fig. 28 The photovoltaic response of a cell under AM 1.5 illumination showing the photocurrent (top) and power (bottom) as a function of VB for different gate voltages. Adapted with permission from ref. 213, © 2014 American Chemical Society.

5. Electrocatalysis

5.1 Hydrogen evolution reaction

5.1.1 Cobalt phosphide. Huang et al. successfully synthesized Co₂P nanorods which show efficient catalytic activity for the hydrogen evolution reaction (HER), with the overpotential required for a current density of 20 mA cm⁻² being as small as 167 mV in acidic solution and 171 mV in basic solution.²¹⁴ The morphology is rod-like, which is shown in Fig. 29a, and in addition, the Co₂P nanorods can work stably in both acidic and basic solutions for the HER. And the performances which the Co₂P nanorods deliver manifest that Co₂P nanorods are promising materials for the HER. During the HER process, the rates of the discharge step and desorption step appear to be comparable because the HER process follows a Volmer–Heyrovsky mechanism. Due to the similarity of the charged nature of Co and P in the Co₂P nanorods to those of the hydride-acceptor and proton-acceptor in highly efficient Ni₂P catalysts, [NiFe] hydrogenase, and its analogues, the HER catalytic activity of the Co₂P nanorods might be related to the charged nature of Co and P (Fig. 30 and 31).

Fig. 29 (a) TEM image of Co₂P nanorods; (b) SEM image of CoP NPAs; (c) TEM image of CoP nanoparticles; (d) low-magnification SEM image of as-prepared CoP NCs. (a) Adapted with permission from ref. 214, © 2014 Elsevier; (b) ref. 215, © 2015 the Royal Society of Chemistry; (c) ref. 216, © 2014 Wiley; (d) ref. 217, © 2015 American Chemical Society.

Fig. 30 (a) TEM image of Ni₂P nanoparticles; (b) high-magnification SEM image of a Ni₅P₄–Ni₂P-NS array cathode; (c) FE-SEM image of MoP; (d) high-magnification SEM image of Cu₃P NW/CF. (a) Adapted with permission from ref. 219, © 2013 American Chemical Society; (b) ref. 220, © 2015 Wiley; (c) ref. 223, © 2014 the Royal Society of Chemistry; (d) ref. 224, © 2014 Wiley.

Fig. 31 (a) The morphology of Ni₅P₄ after 500 cycles through SEM imaging; (b) SEM image of CoP₂/RGO; (c) TEM image of BNC/Co₂P-2; (d) SEM image of CoP; (a) adapted with permission from ref. 13, © 2015 Wiley; (b) ref. 226, © 2016 the Royal Society of Chemistry; (c) ref. 231, © 2015 the Royal Society of Chemistry; (d) ref. 59, © 2015 Elsevier.

Li et al. demonstrated that various nanostructured cobalt phosphide assemblies can be synthesized through the direct chemical transformation of a Co-containing metal–organic framework (ZIF-67-Co) under mild phosphorization conditions.²¹⁵ Furthermore, Fig. 29b shows a typical low-magnification SEM image of CoP nanoparticle assemblies (NPAs). And this suggests that the as-prepared CoP is composed of a large number of irregular polyhedron-like particles. In addition, the resulting CoP nanoparticle assemblies (NPAs) and nanorod assemblies (NRAs) can be selectively synthesized by rationally tuning the calcination atmosphere. And for the HER, CoP NRAs were more highly active than CoP NPAs, which can be proved by their relatively low onset overpotential (similar to 86 mV), small Tafel slope (similar to 69 mV per decade) and long-term stability in acidic media.

Popczun et al. successfully synthesized cobalt phosphide nanoparticles via the reaction between Co nanoparticles and trioctylphosphine.²¹⁶ And then the as-obtained nanoparticles were utilized as electrocatalysts for the HER under strongly acidic conditions (0.50 M H₂SO₄, pH 0.3). Fig. 29c displays a TEM image of the CoP nanoparticles. In addition, electrodes based on CoP nanoparticles supported by Ti (2 mg cm⁻² mass loading) produced a cathodic current density of 20 mA cm⁻² at an overpotential of −85 mV. Furthermore, the CoP/Ti electrodes could be stable over 24 h for the HER in 0.50 M H₂SO₄. To be specific, the activity can remain unchanged after 400 cyclic voltammetry sweeps, which indicates long-term viability under the operating conditions.

Yang et al. fabricated urchin-like CoP nanocrystals (NCs) and these nanocrystals can be utilized as catalysts for the HER.²¹⁷ The as-prepared CoP NCs held excellent electrocatalytic activity and long-term stability. A low-magnification SEM image of the as-prepared CoP NCs is displayed in Fig. 29d. In addition, the urchin-like CoP NCs, with a diameter of 5 μm, could be synthesized via a hydrothermal reaction following phosphidation treatment under an N₂ atmosphere. CoP NCs deliver an excellent HER catalytic performance, with a low onset overpotential of 50 mV, a small Tafel slope of 46 mV per decade, and an exceptional low overpotential of similar to 180 mV at a current density of 100 mA cm⁻² with a mass loading density of 0.28 mg cm⁻². More importantly, the urchin-like CoP NCs present superior stability and keep their catalytic activity for at least 10 000 CV cycles for the HER in 0.5 M H₂SO₄, which is attributed to their three-dimensional structure.

5.1.2 Nickel phosphide. Ni₂P nanoparticles were synthesized by Moon et al. through a ligand stabilization method and these nanoparticles can be applied to the HER.²¹⁸ And the electrocatalytic HER activity and stability for the Ni₂P nanocatalyst were tested in 0.5M H₂SO₄, and the Ni₂P electrocatalyst delivered a low onset potential for the HER of around −0.02 V vs. RHE, a little more negative compared to a Pt catalyst, which shows almost 0 V vs. reversible hydrogen electrode (RHE), and a Tafel slope of 75 mV per decade, i.e., following a Volmer step as a rate-determining step. Density functional theory (DFT) calculations for hydrogen adsorption over the Ni₂P surfaces (001) and (002) revealed that hydrogen adsorption might occur via two reaction pathways, consecutive or simultaneous hydrogen adsorption. Consecutive hydrogen adsorption on threefold hollow (TFH)-Ni sites followed by on P(II) sites on the Ni₂P (001) surface led to a lower reaction barrier than for simultaneous hydrogen adsorption. These results thus demonstrated that a Volmer step might follow the consecutive adsorption mechanism over a Ni₂P surface.

Ni₂P nanoparticles have been investigated by Popczun et al. for the HER in acidic solutions, under which proton exchange membrane-based electrolysis is operational.²¹⁹ A TEM image of the Ni₂P nanoparticles is displayed in Fig. 30a. What is more, the Ni₂P nanoparticles which were catalytically active were hollow and faceted to expose a high density of the Ni₂P (001) surface. And these have been proved to be active HER catalysts.

Wang et al. presented an approach for the synthesis of a three-dimensional self-supported biphasic Ni₅P₄–Ni₂P nanosheet (NS) array cathode.²²⁰ The target can be obtained through the direct phosphorization of commercially available nickel foam using phosphorus vapor. Fig. 30b shows a high-magnification SEM image of the Ni₅P₄–Ni₂P-NS array cathode. What is more, the as-prepared three-dimensional Ni₅P₄–Ni₂P-NS array cathode delivers excellent electrocatalytic activity and long-term durability for the HER in an acidic medium. In addition, the fabrication procedure is scalable, which predicts the prospect for utilization in water electrolysis and other self-supported TMNP HER cathodes.

5.1.3 Iron phosphide. Callejas successfully synthesized FeP nanoparticles.²²¹ And as an electrocatalyst (operating at a current density of −10 mA cm⁻²), FeP nanoparticles deposited at a mass loading of approximately 1 mg cm⁻² on a Ti substrate exhibited overpotentials of −50 mV in 0.50 M H₂SO₄ and −102 mV in 1.0 M phosphate buffered saline. In addition, the FeP nanoparticles supported sustained hydrogen production, with essentially quantitative faradaic yields for extended time periods under galvanostatic control. Under UV illumination in both acidic and neutral-pH solutions, FeP nanoparticles deposited on TiO₂ produced H₂ at rates and amounts that begin to approach those of Pt/TiO₂.

Liang et al. prepared self-supported FeP nanorod arrays on carbon cloth (FeP NAs/CC) via the low-temperature phosphidation of Fe₂O₃ NAs/CC.³⁷ As a novel 3D hydrogen evolution cathode in acidic media, FeP NAs/CC delivers high catalytic activity. What is more, FeP NAs/CC only needs an overpotential of 58 mV to afford a current density of 10 mA cm⁻². This electrode also works efficiently in both neutral and alkaline solutions.

5.1.4 Molybdenum phosphide. McEnaney successfully synthesized amorphous MoP nanoparticles, and then these nanoparticles were characterized as electrocatalysts for the HER in 0.50 M H₂SO₄ (pH 0.3).²²² Amorphous MoP nanoparticles formed upon the heating of Mo(CO)₆ and trioctylphosphine in squalane at 320 °C, and the nanoparticles remained amorphous after heating at 450 °C in H₂ (5%)/Ar (95%) to remove surface ligands. At mass loadings of 1 mg cm⁻², the MoP/Ti electrodes exhibited overpotentials of −90 and −105 mV (−110 and −140 mV without iR correction) at current densities of 10 and 20 mA cm⁻², respectively. These HER overpotentials remained nearly constant over 500 cyclic voltammetry sweeps and 18 h of galvanostatic testing, indicating stability in acidic media under the operating conditions.

Xiao et al. proposed that molybdenum phosphide was a novel cost-effective catalyst (a FE-SEM image of MoP is shown in Fig. 30c), which displays high activity for the HER in both acid and alkaline media, even in bulk form.²²³ Comparative analysis of Mo, Mo₃P and MoP as catalysts towards the HER manifests that the phosphorization process can modify the properties of the metal, and in addition, different degrees of phosphorization lead to distinct activities and stabilities. What is more, theoretical calculations via density functional theory also indicate the simple phosphorization of molybdenum to synthesize MoP. And the theoretical calculations introduce an excellent H-delivery system which attains nearly zero binding to H at a certain H coverage.

5.1.5 Other phosphides. Xing et al. proposed utilizing tungsten phosphide submicroparticles as a high-efficiency HER catalyst.⁴¹ And the as-prepared particles delivered high durability for the HER in acidic water. In addition, this catalyst displayed a low onset overpotential of 54 mV, a Tafel slope of 57 mV per decade, an exchange current density of 0.017 mA cm⁻², and nearly 100% faradaic efficiency. And it needs overpotentials of 115 and 161 mV to afford current densities of 2 and 10 mA cm⁻², respectively. Furthermore, this catalyst also performs well in activity and durability under neutral and basic conditions.

Tian et al. proposed that self-supported Cu₃P nanowire arrays grown on commercial porous copper foam (Cu₃P NW/CF) could be synthesized via a low-temperature phosphidation reaction.²²⁴ Fig. 30d displays a high-magnification SEM image of Cu₃P NW/CF. The reaction begins from a Cu(OH)₂ NW/CF precursor. Furthermore, when used as a three-dimensional HER cathode in acidic electrolyte, Cu₃P NW/CF can maintain its activity for at least 25 hours and deliver an onset overpotential of 62 mV, a Tafel slope of 67 mV per decade, and a faradaic efficiency close to 100%. The catalytic current density can approach 10 mA cm⁻² at an overpotential of 143 mV.

5.2 Photoelectrochemical water splitting

5.2.1 Nickel phosphide. The investigation into nickel phosphide (Ni₅P₄) as a catalyst for the HER in strong acidic and alkaline environments is described by Ledendecker et al.¹³ And the catalyst can be grown in a three-dimensional hierarchical structure directly on a nickel substrate. The morphology of the as-obtained samples after 500 cycles by SEM imaging is shown in Fig. 31a. As a result, the as-prepared Ni₅P₄ can become a promising candidate for water splitting devices. What is more, the as-prepared Ni₅P₄ held high catalytic activity towards the HER, as well as high stability especially in acidic solution. And Ni₅P₄ was utilized for the OER and showed activity. When utilized as a bifunctional catalyst, the practical relevance of Ni₅P₄ for the overall water splitting reaction was demonstrated, with 10 mA cm⁻² achieved below 1.7 V.

Wu et al. fabricated nanocrystalline nickel phosphide (Ni₁₂P₅) through a simple hydrothermal method, using NiCl₂ and red phosphorus as raw materials.²²⁵ And then they characterized the crystal structure, morphology and surface chemical composition of the as-prepared sample via XRD, SEM and XPS, respectively. The catalytic activity for the HER from water was investigated under visible light irradiation with fluorescein sodium as the photosensitizer and triethanolamine as the sacrificial electron donor. And the Ni₁₂P₅ sample showed high catalytic activity (10 760 mmol h⁻¹ g⁻¹, TOF = 9.3 h⁻¹) and good stability (15 h). The results indicated that Ni₁₂P₅ held a high cathodic current and small charge transfer resistance, which further proved that Ni₁₂P₅ could efficiently catalyze the HER.

5.2.2 Cobalt phosphide. Wang et al. described the first preparation of CoP₂/RGO from GO and cobalt salts through a low-temperature phosphidation reaction and a SEM image of CoP₂/RGO is shown in Fig. 31b.²²⁶ As a novel non noble-metal HER and OER catalyst, CoP₂/RGO delivered ultrahigh catalytic activity in both acidic and alkaline electrolyte. What is more, this bifunctional catalyst only needed a cell voltage of 1.56 V to drive 10 mA cm⁻² for full water splitting in an alkaline solution.

Jin et al. prepared uniform Co₂P ultrathin nanowires via a facile and rapid microwave-assisted method.²²⁷ And Co₂P can be utilized as a catalyst for the water splitting reaction, and the results show that the as-prepared samples are excellent bifunctional electrocatalysts. In acidic and alkaline electrolyte, both Co₂P and CoP NWs show a low overpotential and small Tafel slope for the HER, close to those of platinum. In addition, Co₂P nanowires display the best performance among CoP nanowires and RuO₂ towards the OER. According to mechanistic studies, cobalt oxo/hydroxide with phosphate units are formed in situ during the catalysis of water oxidation. Therefore, an alkaline water electrolysis cell was constructed based on the Co₂P NWs as both the anode and cathode. The electricity-to-hydrogen conversion efficiency was ∼84%, which is higher than for Pt/RuO₂.

5.2.3 Iron phosphides. Yan et al. successfully fabricated a flexible electrode, which was based on a carbon cloth substrate and iron phosphide nanotubes coated with an iron oxide/phosphate layer.²²⁸ And the as-prepared electrode can be utilized to catalyze water splitting and the as-prepared electrode shows excellent electrocatalytic activity for both the HER and the oxygen evolution reaction (OER) at modest overpotentials. The surface iron oxide/phosphate, which is formed in situ, is proposed to improve the HER activity by facilitating the water-dissociation step and serves directly as the catalytically-active component for the OER process.

5.2.4 Other phosphides. Zhu et al. successfully synthesized large-scale, triangular pore arrays on GaP via the simple and highly-efficient approach of electrochemical etching under a high field.²²⁹ And the as-prepared ordered porous GaP delivers high performance in its photoelectrochemical (PEC) properties compared with bulk GaP. What is more, the photocurrent of the porous GaP exceeded that of the bulk material by one order of magnitude under 0.1 V compared to a reversible hydrogen electrode (RHE). And this suggests that the porous structure could enhance the photo-response and facilitate the separation of photo-induced carrier charges and their collection. And it should be pointed out that the structure of the triangular pore arrays cooperating with their depth determined the PEC performance of GaP. Besides, by testing the PEC performance, the optimal etching depth could be obtained. Because of the porous structure, hydrogen production significantly enhanced, which suggested that the porous GaP should have potential for photocatalytic water splitting.

Semimetallic MoP₂ nanoparticles were found to be a novel photocatalyst to efficiently degenerate methyl orange and produce H₂ from water under visible light irradiation.²³⁰ And MoP₂ nanoparticles were synthesized by utilizing a solid-state reaction route via a vacuum encapsulation technique, followed by acid washing. Both first-principle band-structure calculations and experimental measurements revealed typical semimetallic characteristics for MoP₂. The obtained MoP₂ nanoparticles displayed superior photocatalytic performance for the degradation of methyl orange with good stability and the reduction of water assisted by the sacrificial elemental Pt under visible light. The detection of hydroxyl radicals in solution in the presence of MoP₂ with fluorescence spectroscopy confirmed the photodegradable activity.

5.3 Oxygen reduction reaction

Dicobalt phosphide nanoparticles encased in boron and nitrogen co-doped graphitic layers (BNC/Co₂P) were fabricated by Han et al.²³¹ A TEM image of BNC/Co₂P-2 is shown in Fig. 31c. The Co₂P nanoparticle core played a positive role in activating the B, N co-doped graphitic shell towards the ORR. And they utilized a one-step pyrolysis method, and BNC/Co₂P exhibited excellent electrocatalytic oxygen reduction reaction (ORR) performance compared with commercial Pt/C towards the ORR via a four-electron pathway in alkaline solution, with long-term stability and excellent methanol tolerance.

Carbon supported PtP (PtP/C) catalysts were synthesized by Ma et al. from Pt(NO₃)₂ and phosphorus yellow at room temperature.²³² And the content of P in the as-prepared PtP/C catalyst was high, in addition, as the content of P increased, the average size of the PtP particles decreased. The electrocatalytic performance of the PtP/C catalysts prepared with this method for the ORR was better than that of a commercial Pt/C catalyst. And the reason why P can promote the enhancement in the electrocatalytic performance of the PtP/C catalyst for the ORR is that Pt and P can form an alloy and then the electron density of Pt is decreased.

5.4 Other electrocatalysis

5.4.1 Methanol oxidation reaction. Zhu et al. developed an ion-exchange method to synthesize Co₂P on graphitized carbon and further deposit Pt nanoparticles.²³³ And then they characterized the properties of the Pt deposited on Co₂P/C (Pt–Co₂P/C) via a large number of methods and the results displayed the improved catalytic activity as well as stability for methanol electro-oxidation. Moreover, the electrochemical testing indicates that the Pt–Co₂P/C catalyst holds almost four times the catalytic activity and 120 mV more negative onset potential compared with commercial Pt/C.

A carbon nanotube (CNT) supported Pt–cobalt phosphide (CoP) electrocatalyst (Pt/CoP/CNTs) was designed and prepared by Li et al. for methanol oxidation (MOR).²³⁴ The modification of CoP decreases the Pt particle size significantly and increases the electrochemical surface area due to the interaction between Pt and CoP, which is evidenced by transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Among all these catalysts, the Pt/4% CoP/CNTs catalyst exhibits the best MOR activity of 1600 mA mg⁻¹ Pt, which is six times that of Pt/CNTs. Moreover, this catalyst also exhibits a higher onset current density and more steady current density than other Pt-based catalysts. The work provides a promising method to develop highly active and stable Pt-based catalysts for direct methanol fuel cells.

5.4.2 Lithium/sodium storage. Wang et al. synthesized a new kind of unique hybrid nanostructure (Ni₁₂P₅/CNT nanohybrids) by designing a one-pot hot-solution colloidal synthetic route.²³⁵ The synthetic method is the direct in situ growth of Ni₁₂P₅ nanocrystals onto oxidized multiwall carbon nanotubes (CNTs). In addition, the CNTs can not only improve the conductivity of the hybrids, but also effectively prevent the aggregation of Ni₁₂P₅ nanoparticles in the cycling process. When utilized as a HER catalyst operating in acidic electrolyte, the as-prepared Ni₁₂P₅/CNT nanohybrids deliver an onset overpotential as low as 52 mV and a Tafel slope of 56 mV per decade and only require overpotentials of 65 and 129 mV to attain current densities of 2 and 10 mA cm⁻², respectively. What is more, the Ni₁₂P₅/CNT nanohybrids display a high capacity, excellent cycling stability and good rate performance. And this performance derives from a synergetic effect between Ni₁₂P₅ and CNTs.

Through a productive and simple ball-milling method, Li et al. synthesized CoP. A SEM image of the as-prepared sample is displayed in Fig. 31d. And then it was tested as an anode material for NIBs. Furthermore, the as-obtained CoP can exhibit a large initial capacity of 770 mA h g⁻¹ and deliver lower voltage polarization. Via ex situ XPS and STEM characterization, it was concluded that the sodium storage mechanism of CoP involves a reaction of P with Na.

6. Conclusion and outlook

In summary, we have summarized the synthetic strategies that are suitable for TMPNs and stated several applications to electrochemical storage and conversion, including Li-ion batteries, Na-ion batteries, supercapacitors, solar cells and electrocatalysis. In recent years, TMPNs have been proven to be one of the most promising and excellent materials in the field of electrochemistry. Several points in regard to the developing trends in TMPNs in electrochemical energy storage and conversion are listed as follows:

(1) Although a good deal of effort has been made and great advancement has been performed in Li-ion batteries and Na-ion batteries recently, there are still some problems that require solving. What should be further established are synthetic methods, experimental methods and relevant theory, coupled with the exploration of new and applied materials.²³⁶ And Na-ion batteries are both similar and different from Li-ion batteries, and therefore, research based on Na-ion batteries is faced with the same challenges as for Li-ion batteries, for example, capacity fading on cycling, and electrolyte decomposition when there is a high voltage. The exploration of new materials for Na-ion batteries and Li-ion batteries is imperative, including solvents, active electrode materials, carbon additives, binders, electrolyte salts, and so on.

(2) As for supercapacitors, limited energy density is the primary challenge impeding their extensive energy storage application. And it is the discovery of new electrode materials that can help us surmount this primary challenge. Especially, the fabrication of composite materials is a significant strategy for boosting the performance of supercapacitors. Wang et al. put forward four favored properties of supercapacitor electrode materials: (i) a high specific surface area, (ii) suitable pore-size distribution, pore networks, and pore length, (iii) low internal electrical resistance, and (iv) better cycling stability.²³⁷

(3) The main challenge in DSSCs is to overcome the loss of potential drop in the process of regeneration, and hence, trying to construct a system in which the regeneration of the oxidized dye can come true is necessary. It is a fact that components with superior performance have been developed. When combined with other substances forming composites, TMPNs could turn into excellent materials, and this is a promising field worth exploring.

Abbreviation index

TMPNs	Transition metal phosphide nanomaterials
h-Ni2P	Hierarchical nanostructured nickel phosphide
TOP	Trioctylphosphine
OAm	Oleylamine
TF-VLS	Thin-film vapor–liquid–solid
rGO	Reduced graphene oxide
LIBs	Li-ion batteries
HEVs	Hybrid electric vehicles
EVs	Electric vehicles
EIS	Electrochemical impedance spectrum
SEM	Scanning electronic microscopy
FEGSEM	Field emission gun scanning electronic microscopy
TEM	Transmission electron microscopy
BET	Brunauer–Emmett–Teller
OA	Oleic acid
THF	Tetrahydrofuran
XRD	X-ray diffraction
CV	Cyclic voltammetry
SAED	Selected-area electron diffraction
NIBs	Na-ion batteries
FEC	Fluoroethylene carbonate
SEI	Stable solid-electrolyte interphase
HRTEM	High resolution transmission electron microscopy
CMC	Carboxymethyl cellulose
CMC/CB	Carboxymethyl cellulose/carbon black
SCs	Supercapacitors
EDLCs	Electrical double-layer capacitors
PDCs	Pseudocapacitors
Ni2P NS/NF	Ni foam-supported Ni2P nanosheet
NPs	Nanoparticles
GR	Graphene
Ni2P//AC ASCs	Ni2P nanoparticles and activated carbon
DSSCs	Dye-sensitized solar cells
PV	Photovoltaic
PEMFCs	Proton exchange membrane fuel cells
PR	Pulse-reverse
PS	Potentiostatic
FTO	Fluorine-doped tin oxide
PCE	Power conversion efficiency
J–V	Photocurrent–voltage
CE	Counter electrode
MC	Mesoporous carbon:
VOC	Open-circuit voltage
FF	Fill factor
JSC	Short-circuit current
MOVPE	Metal-organic vapor phase epitaxy
HER	Hydrogen evolution reaction
ORR	Oxygen reduction reaction
NCs	Nanocrystals
XAFS	X-ray absorption fine structure
OER	Oxygen evolution reaction
MOR	Methanol oxidation
NPAs	Nanoparticle assemblies
NRAs	Nanorod assemblies
NCs	Nanocrystals
RHE	Reversible hydrogen electrode
DFT	Density functional theory
NS	Nanosheet
PEC	Photoelectrochemical
RHE	Reversible hydrogen electrode
CNTs	Carbon nanotubes

Acknowledgements

This work was supported by the Program for New Century Excellent Talents of the University in China (grant no. NCET-13-0645) and the National Natural Science Foundation of China (NSFC-21201010,21671170 21173183, 21505118 and 51202106), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (164200510018), the Plan for Scientific Innovation Talent of Henan Province, the Program for Innovative Research Team (in Science and Technology) in the University of Henan Province (14IRTSTHN004, 16IRTSTHN003), the Science & Technology Foundation of Henan Province (122102210253 and 13A150019), the Science & Technology Foundation of Jiangsu Province (BK20150438), the Six Talent Plan (2015-XCL-030), Innovation and Entrepreneurship Training Program for College Students in Jiangsu province (No. 201611117047Y), and the China Postdoctoral Science Foundation (2012M521115). 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.

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