A Review of the Structure–Property Relationship of Nickel Phosphides in Hydrogen Production

Abstract

Hydrogen, one of the most promising forms of new energy sources, due to its high energy density, low emissions, and potential to decarbonize various sectors, has attracted significant research attention. It is known that electrocatalytic hydrogen production is one of the most widely investigated research directions due to its high efficiency in the conversion of electricity to H₂ gas. However, given the limited reserves and high cost of precious metals, the search for non-precious metal-based catalysts has been widely explored, for example, transition metal phosphides, oxides, and sulfides. Despite this interest, a detailed survey unveils that the surface and internal structures of the alternative catalysts, including their surface reconstruction, composition, and electronic structure, are poorly studied. As a result, a disconnection in the structure–property relationship severely hinders the rational design of efficient and reliable non-precious metal-based catalysts. In this review, by focusing on Ni₅P₄, a bifunctional catalyst for water splitting, we systematically summarize the material motifs pertaining to the different synthetic methods, surface characteristics, and hydrolysis properties. It is believed that a cascaded correlation may provide insights toward understanding the fundamental catalytic mechanism and design of robust alternative catalysts for hydrogen production.

1. Introduction

Benefitting from the advantages of sustainable and renewable attributes, low cost, and environmental friendliness, the hydrogen production reaction via water splitting has become one of the most promising strategies to resolve the potential energy crisis [1]. The electrochemical water splitting reaction, which is usually divided into the separate processes of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), requires only electrical energy and water as the input and can generate a considerable amount of H₂ and O₂. The catalysts play a key role in reducing the activation energy of the thermodynamic climbing process of water splitting [2]. However, the lack of crustal reserves of precious metals such as platinum (Pt) and ruthenium (Ru), commonly used in water-splitting reactions, has led to high costs in hydrogen production and has hindered sustainable development. Therefore, cheap and abundant non-precious metal nitrides, sulfides, phosphides, carbides, and other materials have been considered as possible alternatives to precious metal catalysts.

Transition metal phosphides (TMPs) exhibited excellent catalytic performance when used in the hydrodesulfurization (HDS) reaction in the 1980s, surpassing even the mainstream MoS₂ catalyst used at that time [3]. A similar reversible adsorption/desorption process for HDS suggests that TMPs may possess desirable HER catalytic properties as well. This hypothesis was confirmed in subsequent theoretical and experimental studies [4,5]. With advancements in synthesis technology and the enhancement of physical/chemical properties, TMPs with various structures and morphologies have been demonstrated to be catalytically active in the HER. As a typical example, it has been reported that nickel phosphides can be used for overall water splitting. It is worth noting that nickel phosphides not only exhibit excellent catalytic performance in water splitting but also play a catalytic role in the oxygen reduction reaction [6], CO₂ reduction [7], the water–gas shift [8], and hydrodeoxygenation [9]. Therefore, exploring the surface structures of nickel phosphides has become quite important in understanding their catalytic performance.

Nickel phosphides with a wide range of Ni:P ratios allow for the formation of various structural phases. Therefore, the multiphase mixing of structural phases may lead to changes in catalytic performance. For instance, it has been demonstrated that Ni₂P exhibits excellent catalytic activity for the HER [4]. Single-phase Ni₅P₄ micron particles, synthesized using a solvothermal method, exhibit comparable rate-limiting kinetics to Pt in a 1M H₂SO₄ environment at low current densities [10]. However, most studies show that the catalytic performance of Ni₅P₄ is passivated in most cases. High-resolution scanning transmission electron microscopy (STEM) has revealed that this passivation occurs due to the self-epitaxial growth of NiP_x (0 < x ≤ 0.5) and Ni₂P nanolayers on top of the (001)-oriented Ni₅P₄ phase. Additionally, bi-directional chemical gradients create a deficiency of nickel in the bulk Ni₅P₄ phase [11]. These findings highlight the importance of considering the multi-dimensional structural features of a catalyst to gain a deeper insight into its catalytic performance.

It is known that catalysts usually respond to the environment by changing their local and extended structures. Meanwhile, the deviation of surface composition and structure from the ideal bulk termination planes is usually referred to as surface reconstruction [12]. Since the surface structure plays a key role in catalysis, understanding the catalytic sites of the reconstructed surfaces by performing surface-sensitive characterization is essential for establishing the structure−property relationship. However, this is a challenging task, especially under the conditions of a real catalytic reaction environment, where mesoscale Raman spectroscopy, atomic force microscopy, and computational modeling are usually applied [13,14]. For this reason, ex situ characterization techniques are usually implemented to probe the surface structures of the catalysts, e.g., scanning electron microscopy (SEM), X-ray diffraction (XRD), surface-sensitive Brunauer–Emmett–Teller (BET) techniques, X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and the extended X-ray absorption fine structure (EXAFS) technique. From the reported literature [15,16,17,18], one can see that the surface atomic structures, under either ex situ or in situ conditions, are far from being clarified in terms of understanding their catalytic performance.

In this review, we consider the recent research progress about nickel phosphides, from material synthesis to structure characterization (down to the atomic level) and catalytic performance, so as to establish a clear structure–property relationship and offer an effective strategy toward improving HER activity. Specifically, the case of Ni₅P₄ is highlighted by elucidating the effects of phase, composition, morphology, and surface structure in influencing HER activity [10,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47]. It is hoped that this review may provide references to the study of other transition-metal-based catalysts and that researchers may pay attention to detailed structural characterization in future studies.

2. Structure Variation in Ni_xP_y: Phase, Morphology, and Composition

The chemical properties of phosphides are very complex because different constituent elements and stoichiometric ratios can give rise to different structures [21]. Surface element enrichment has always been a focal point in controlling the physical and chemical properties of transition-metal phosphides and regulating elemental proportions is considered the key method to mediate the surface structure. During synthesis, phosphorus may spontaneously form extended chains or crystals, introducing possible sources of secondary phases [48,49]. This provides many possibilities to create different surface modifications. Correspondingly, determining the structure and composition of the as-grown catalyst is crucial for evaluating the structural evolution during operation. Since the surface only accounts for a small proportion with respect to its bulk, characterizing the surface structure of the catalysts turns out to be quite challenging.

2.1. Crystal Structures of Ni_xP_y

In total, there are more than nine kinds of nickel phosphide polymorphs, depending on the Ni:P stoichiometric ratio [50]. For example, Ni₁₂P₅ and Ni₃P possess a tetragonal structure, whereas Ni₅P₄, Ni₅P₂, and Ni₂P possess a hexagonal structure. Ni₇P₃ and NiP₃ have a cubic structure, whereas Ni₈P₃, NiP, and NiP₂ retain trigonal, orthorhombic, and monoclinic structures, respectively [51] (shown in Figure 1). One should note that the NiP₂ also has a cubic phase [52]. As can be seen, the Ni-P system covers six major crystal systems. Despite only simple changes appearing in the Ni:P ratio, e.g., from the quadripartite Ni₁₂P₅ (dominated by Ni-Ni bonds) to the cubic NiP₃ (dominated by P-P bonds), they bring significant changes in the crystal and electronic structures. Since the Ni-Ni bonds are stronger than the P-P bonds, the increase in the P-Ni ratio naturally leads to a decrease in phase stability. Conversely, taking Ni₂P and Ni₅P₄ as examples, which have the same crystal system, the dominant Ni-Ni bonds give rise to similar metallic conductivity [53]. One additional point worth highlighting is that Ni₂P, with the non-centrosymmetric space group P$\overline{6}$2m, has recently been identified to be a topological ferroelectric metal, where the in-plane polyhedral polarity couples with the elemental valence state and spin polarization [54]. For Ni₅P₄ (space group P6₃mc), its crystallographic symmetry is broken along the c-axis. These structural features, which are pertinent to the stacking of different phases, should definitely be considered while probing their catalytic mechanism.

2.2. Phase, Size, and Morphology Variations in Ni_xP_y Synthesis

As shown in Figure 2, different synthetic methods have been adopted to synthesize nanostructured nickel phosphides with various phases and morphologies, including thermal decomposition, solid–gas reaction methods, and electrochemistry methods. The most common synthetic methods can be categorized into the wet chemical method and solid–gas method, which are both widely used in the synthesis of nanoparticles.

Wet chemical methods typically involve the reaction of metal salts or metallic nickel with a phosphorus source in a hot solvent under an inert atmosphere. According to the different sources of phosphorus used, wet chemical methods can be divided into two categories: thermal decomposition and hydrothermal methods. Hydrothermal methods are more cost-effective and are readily accessible compared to the organic phosphorus sources used in thermal decomposition. Therefore, hydrothermal methods offer an advantageous approach due to their lower cost and ease of access to inorganic phosphorus sources.

It is generally accepted that thermodynamic and kinetic effects control the synthesis products [59]. According to the Arrhenius formula, temperature is the key to controlling the reaction rate. To obtain nanomaterials with better crystallinity, the most approximate thermal decomposition temperature is around 300 °C for 2 h, while the solvothermal method only needs about 180 °C to obtain nanocrystals, which usually takes a longer time. In comparison, the solid–gas reaction method is more convenient, although the morphology of the synthesized nanomaterials is not that well defined, and a higher reaction temperature is required. Ray, A. et al. systematically summarized the mechanism of morphology control by using the thermal decomposition method [57]. They listed factors such as the Ni and P precursors, (P/Ni) ratio, solvent, reaction temperature, and time as key variables in the synthesis of phase pure nickel phosphides with a particular size and morphology [60]. However, it remains inconclusive as to how these factors affect the nucleation and growth processes of nanocrystals and the structural and compositional discrepancy between the surface and the bulk interior.

The synthesis method chosen plays an important role in controlling the morphology of the material. Taking Ni₅P₄ as an example, in one study, a 3D nanoflower with exposed (001) surfaces has been synthesized with the solid–gas reaction method; the schematic diagram is shown in Figure 2c, and the Ni-Ni bonds are proposed as being responsible for the catalytic activity [34]. In the synthesis of ultrathin nickel–phosphide nanosheets, wherein Ni hydroxide precursors and NaH₂PO₂ are used, it has been confirmed that the synthesis temperature plays a key role in controlling the Ni:P ratio of the target product [59]. When using the solid–gas reaction method, wherein the red phosphorus is deposited directly on the surface of the NF substrate, the Ni₅P₄ may form nanodisks with a thickness of around 170 nm [32,61]. One should note that although the diffraction pattern can be indexed to a single phase of Ni₅P₄ along the (001) direction, the coexistence of Ni₅P₄ and Ni₂P phases has been unveiled in several studies [46,47]. Apart from the morphology change, one can also foresee that their special morphology may also tune their catalytic properties, due to the varying surface chemistry and adsorption sites.

Laurence et al. studied the formation energy of each phase using density functional theory calculations; the change in enthalpy as a function of the P:Ni ratio is also presented (Figure 3a). The ΔfH° is the average value after dividing the number of atoms in each formula unit, so as to make a direct comparison between the different bulk phases. The larger the negative value of enthalpy in molar formation, the higher the bond energy of the product will be. This indicates that more heat is released as a more stable system is generated. It can be seen that with the increase in the P:Ni ratio, the phase stability does not change much in the Ni-rich phases. As it increases to 4:5, the formation enthalpy of the Ni₅P₄ phase is 22% lower than that of the Ni₂P phase. However, the bonding energy indicates that the stability of the Ni₅P₄ phase is less than that of the most stable Ni₂P phase. This tells us that the Ni₅P₄ and Ni₂P phases tend to coexist in the phosphides. From the chemical potential plotted as a function of the Ni:P ratio (Figure 3b), one can see an evolution from Ni-rich (Ni₁₂P₅, Ni₂P, Ni₅P₄) to P-rich NiP₂ phases. Since a smaller change in chemical potential corresponds to a smaller change in free energy, the system with the lowest energy should be in the diagonal position in the phase diagram, which is Ni₂P. It is worth noting that the calculation at 0 K ignores the contribution of entropy. Therefore, it can only predict stable products at low temperatures.

What is more, when comparing the effects of the different synthesis methods on the structure and morphology of catalysts, we noticed that the yields of various methods serve as crucial data with significant implications for the application of catalysts. However, we observed that this key data point is missing in a considerable number of articles. This absence could potentially impact readers’ decisions when selecting and referencing these synthesis methods for comparison and application.

Although a single phase is deemed to be more efficient in catalysis than multiple phases of nickel phosphide in electrode kinetics, synthesizing a pure and single Ni₅P₄ phase is not as easy as expected, as reflected in the XRD data. It is claimed that a single-phase ultrathin holey Ni₅P₄ nanoflower has been synthesized by Yao et al. (Figure 3d) [34]. By comparing the PDF cards of Ni₂P and Ni₅P₄ (Ni₅P₄: blue; Ni₂P: red; Ni₂P-Ni₅P₄: green), it can be seen that a lot of the XRD peaks of Ni₅P₄ overlap with those of Ni₂P. Since the XRD reflects more phase information from the angle, and the intensity information is more a question of orientation as two phases with approximate crystal-plane spacing coexist, it becomes quite difficult to analyze this qualitatively from XRD analysis.

Apart from surface structural and chemical reconstruction [62], chemical doping can also modify the catalytic properties of the catalysts [63]. For example, in Fe-doped Ni₅P₄, which has rich P vacancies, Qi et al. showed that the XPS peaks of Ni 2p and P 2p shifted to higher energies [64]. With partial contribution from surface oxidization [65], this also suggests possible surface reconstruction induced by Fe doping. Because of the chemical modification, the electron transfer process is accelerated during the catalytic reactions [64]. The XPS spectra of Ni 2p_3/2 for the Ni_xP_y nanosheets before and after HER testing provide further insight into the surface properties of the material (Figure 3e). Upon analysis, the Ni valence (indicated by the green arrows) is found to range from 0 to 1, suggesting the bonding of Ni atoms with P and other relevant atoms on the phosphide surfaces. This finding implies that a layer of NiP_x (0 < x < 0.5) may form on the phosphide surfaces due to the surface reconstruction of Ni and P atoms.

3. Catalytic Performance of a Nanostructured Metal-Rich Nickel Phosphide (Ni₅P₄, Ni₂P) Electrocatalyst in the HER

3.1. The Principle of Electrocatalytic Reactions

The HER is a cathodic semi-reaction process involved in water splitting and is one of the most widely studied electrochemical reactions. In contrast to the slow kinetics of OER and the oxygen reduction reaction (ORR), the kinetics of the HER on precious metal electrodes are much faster. During the hydrolysis process, the HER begins with the Volmer reaction, i.e., electrons migrate to the cathode that is coated with a hydrogen evolution catalyst, which then couple with the protons provided by H₂O (under alkaline conditions) and H₃O⁺ (under acidic conditions) in the electrolytic cell. In this process, the protons are reduced to intermediate H* at the catalyst adsorption sites. There are two reaction mechanisms for the subsequent reaction, namely, the Tafel and Heyrovsky reactions. In the Tafel reaction, two H* at adjacent sites directly combine and desorb from the surface to form H₂ gas. In the Heyrovsky reaction, the H* atoms and protons (under acidic conditions) or H₂O molecules (under alkaline conditions) directly interact to form H₂, completing the entire hydrogen evolution process.

During the reaction, the choice of the above two reaction mechanisms, either Volmer–Tafel or Volmer–Heyrovsky, depends on various operating parameters, such as pH, electrode potential, and the electrode’s structure and properties.

Volmer step: H⁺ (aq) + e⁻ ⇌ H*(1)

Heyrovsky step: H⁺ (aq) + e⁻ + H* ⇌ H₂(g)(2)

Tafel step: 2H* ⇌ H₂(g)(3)

In the HER mechanism, it is not difficult to conclude that the strength of metal M-H* bonds has a greater impact on the catalytic activity. On the one hand, in the first step (Volmer reaction), the M sites need to have a strong enough ability to adsorb H* and form strong enough M-H* bonds, which is also the premise of the entire HER process. On the other hand, the presence of a catalyst can not only promote the formation of intermediate M-H* but can also realize the easy desorption of this intermediate to generate H₂.

For the exploration of effective catalysts, previous research has generally focused on the volcanic principal trend of HER, which can be explained by the Sabatier principle. The Sabatier principle reveals, to a certain extent, the relationship between material bonding energy and catalytic reaction activity (e.g., for the HER, the one showing neither the strongest nor the weakest M-H bonding is the best). Two reversible processes in catalytic reactions need to be considered, namely, adsorption and desorption. In the Ni-P system (shown in Figure 3c), the diverse structural phases make the bonding energy and H-adsorption capacity different. Therefore, their catalytic performance is different as well.

Although theoretical calculations can help to confirm the reliability of the experimental data, the acquisition of binding energy data is limited by finding out the intermediate reaction pathways, which, in principle, can be infinite on the surface of the nanoparticles. Therefore, it is very difficult to disclose the fundamental mechanism of catalysis by a consideration of the change in free energy in simplified models and the limited intermediate reaction steps.

Recently, some profound theoretical and experimental reasoning has been proposed. He et al. reported a universal self-gating phenomenon in semiconductors, which may explain the electron conduction modulation of ultrathin semiconductor catalysts in electrocatalytic reactions [66]. Firstly, they constructed micro-electric cells to perform in situ electronic/electrochemical measurements for the various types of semiconductors. The electronic signals from the upper panel of the n-type MoS₂, p-type WSe_1.8Te_0.2, and bipolar WSe₂ showed their respective threshold voltages. This illustrates a strong correlation between the semiconducting type and electrocatalytic behavior. This tells us that the n-type/p-type semiconductor catalyst tends to be efficient in cathodic/anodic reactions such as hydrogen/oxygen evolution, respectively. Correspondingly, a bipolar semiconductor catalyst tends to be bifunctional in these reactions. This finding broadens our understanding of HER and OER catalysis to a large extent.

3.2. Electrocatalytic Activity of the Ni₅P₄ Electrocatalyst in the HER

Figure 4a,b shows the HER performance metrics of multiple transition-metal phosphides and sulfides, which are widely used for the HER in comparison to Ni₅P₄ at pH 0 and 14, using the data presented in Table 1. One can see that in both acidic and alkaline electrolytes, the Tafel slope of Ni₅P₄ (both as nc-MPs and MPs) is smaller than that of other transition-metal phosphides and is comparable to that of Pt. Interestingly, in an alkaline electrolyte, the Tafel slope of Ni₅P₄ in different morphologies shows the same trend of being smaller than Pt, especially the one with a rice shape [23]. In acid, these values of the Tafel slope can be associated with idealized HER reactions at the low H coverage limit. The Tafel slope of Ni₅P₄ MPs and Pt in acid (∼30 mV/dec) indicates that the Tafel step is rate-determining on Ni₅P₄. Industrial electrolyzes always operate at much greater current density. Consequently, higher overpotential should be considered as an important factor as well. As can be seen, the exchange current density of Ni₅P₄ is more than one order of magnitude higher than other top transition-metal phosphides in acid electrolytes, as reported in the literature listed in Table 1, and is about ten times higher than that of Pt. One reason for the increased exchange current density of Ni₅P₄ compared to that of Pt could be attributed to its unique intrinsic properties and structural advantages. That is why we decided to conduct this work to explore the relationship between structure and catalytic performance. Due to the structure tuning of Ni₅P₄, it may offer more active sites for the HER and exhibit an enhanced electronic structure that facilitates efficient electron transfer. Although the higher exchange current density of Ni₅P₄ shows great potential, this performance could vary, depending on synthesis methods, reaction conditions, stability, and so on. Therefore, further studies are needed to fully understand its behavior and optimize its properties for practical applications.

Elsewhere, to investigate why nickel phosphides perform worse in alkaline than in acidic electrolytes, Laursen et al. explored the surface chemical state of the catalyst and found an additional chemical state of nickel phosphide both before and after catalysis, as well as the presence of a layer (≲1 nm thick) of nickel phosphate, Ni_x(PO_y)_z, formed during air exposure. In alkaline environments, the nickel phosphorus oxide is insoluble and will be fully reduced before reaching a catalytic steady state. In contrast, the phosphate partially dissolves in the acidic electrolyte [22], which could be an important reason for the enhanced catalytic activity seen in the alkaline solution.

Despite the above discussions about the surface structures, a clear structure–activity relationship is yet to be determined. Fortunately, some efforts have been made regarding DFT calculations and microscopy characterizations.

3.3. Stability of the Ni₅P₄ Electrocatalyst in the HER

Durability and stability are very critical aspects when evaluating the performance of a catalyst in practical applications. Stability is indeed a crucial parameter for assessing catalyst activity and performance.

Based on the data presented in Table 1, it can be seen that Ni₂P exhibits significantly longer stable durations at relatively high overpotentials (175 mV and 130 mV), lasting for 65 h and 57 h, respectively. However, the experimental results for Ni₅P₄ indicate durations of 16 h, 8.33 h, 22 h, and 11.11 h at different overpotentials (21 mV, 150 mV, 147 mV, and 120 mV). In comparison, as previously reported, CoP and FeP perform better under similar conditions, with CoP maintaining stability for 16 h at 52 mV overpotential, and FeP lasting for 24 h at 100 mV overpotential.

From these data, it can be concluded that the stability of Ni₅P₄ at high overpotentials is notably inferior to Ni₂P and even falls short of CoP and FeP. Therefore, stability emerges as a significant concern limiting the industrial application of Ni₅P₄. This underscores the importance of focusing on structural design and modification strategies for Ni₅P₄ in future research to enhance its stability and propel its practical utilization.

4. Structure Characterization of Ni_xP_y

4.1. Results of DFT

By using density functional theory (DFT) calculations, Wexler et al. have demonstrated that surface nonmetal modification can significantly improve the HER activity of Ni₂P. They found that the Ni-Ni bond length is an effective descriptor for the hydrogen evolution activity of Ni₃P₂ (0001)-terminated Ni₂P. As shown in Figure 5a, structural diagrams of several H-binding sites on the surface of Ni₂P and Ni₅P₄ have been revealed. Meanwhile, their calculation results suggest that the induced local geometry of the Ni₃-hollow site, rather than the electronic character of the dopants, accounts for the improved HER activity. This finding provides an insight that increases our understanding of the HER mechanism of the nonmetal-doped surfaces of transition metal phosphides [67].

Furthermore, they investigated the reconstructed Ni₂P (0001) and Ni₅P₄ (0001)/(000$\overline{1}$) surfaces in an aqueous solution using DFT and thermodynamics theory. The reason for considering the (0001) and (000$\overline{1}$) surfaces lies in the fact that the inversion symmetry along the c axis is broken in Ni₅P₄. Given that the ensemble of electric dipoles in structural domains can regulate the spatial distribution of electrons and holes [54,69,70], the non-equivalent crystal planes with different atomic terminations may behave differently in the catalytic reaction. It is known that the surface P content of Ni₂P (0001) is influenced by the applied potential. Specifically, at −0.21 V ≥ U ≥ −0.36 V versus the standard hydrogen electrode at pH = 0, a stable PH_x-enriched Ni₃P₂ termination was identified, which is consistent with P-rich reconstruction in ultrahigh vacuum conditions. Beyond this potential range, the stoichiometric Ni₃P₂ surface becomes passivated by H coverage at the Ni₃-hollow sites. In contrast, Ni₅P₄ (000$\overline{1}$) does not exhibit the preferential adsorption of additional P. Furthermore, the Ni₄P₃ bulk termination of Ni₅P₄ (000$\overline{1}$) is passivated by H at both the Ni₃ and P₃-hollow sites [71].

The most active surfaces for the HER were found to be Ni₃P₂+P+(7/3)H for Ni₂P (0001) and Ni₄P₃+4H for Ni₅P₄ (000$\overline{1}$), which were attributed to weak H adsorption at the P catalytic sites. This contradicts other computational results, wherein Ni₃-hollow is suggested as the active catalytic site [71]. An analysis of the viable catalytic cycles and the calculation of reaction-free energies for the associated elementary steps showed that the overpotential on the Ni₄P₃+4H surface of Ni₅P₄ (000$\overline{1}$) (−0.16 V) is lower than that of the Ni₃P₂+P+(7/3)H surface of Ni₂P (0001) (−0.21 V) [71]. This discrepancy is explained by the abundance of P₃-hollow sites on Ni₅P₄ and the limited surface stability of the P-enriched Ni₂P (0001) surface phase. These trends in the calculated catalytic overpotentials, coupled with potential-dependent bulk and surface stabilities, may explain why the nickel phosphides exhibit a performance comparable to that of Pt. Furthermore, their study explains why Ni₅P₄ outperforms Ni₂P in the HER, aligning with the experimental findings.

Consistent with the research findings, Yang et al. demonstrated that the metallic nature of Ni₅P₄, which is primarily due to the Ni atoms, facilitates electron transfer in HER processes [68]. The energy barrier for water dissociation at the P₃-hollow site was analyzed, showing that the process is thermodynamically favorable. The energy barrier for water dissociation at the Ni₃-hollow sites was found to be significantly higher than that of the P₃-hollow sites. This can be attributed to the immediate capture of dissociated H and OH species by the P₃-hollow site.

4.2. Electron Microscopy Characterization

To better understand catalytic performance and establish a structure–property relationship, electron microscopy has been applied not only to characterize the morphology, elemental ratio, and valence state information but also atomic-level information, using atomic-resolution scanning/transmission electron microscopy (S/TEM) techniques. This helps to determine the surface reconstruction feature of materials, qualitatively and even quantitatively [72,73].

Some years ago, the low-energy electron diffraction (LEED) technique was first used to complement the shortcomings of TEM in the exploration of surface atomic structure. The LEED image of Ni₂P studied by Moula et al. showed a long-range ordered structure on the surface of Ni₂P, in addition to the typical (1 × 1) structure (Figure 6a). The (2/3 × 2/3) reconstructed LEED patterns are observed at 71.2 and 91.1 eV of beam energy, respectively. However, the scanning tunneling microscopy (STM) measurements showed only a well-ordered (1 × 1) hexagonal structure, which is explained as a rearrangement of Ni on the P (1 × 1) structure. Thus, the surface will have a minute but finite number of dipoles, to make this surface structure more stable than that of the original one [74]. In other words, the density of Ni atoms on the sample surface can adjust the stability of the Ni₂P surface structure, and the absence of Ni makes the Ni₂P surface more stable. In terms of DFT calculations, the P-rich surfaces are more active and provide more active sites.

A similar phenomenon has been unveiled using S/TEM. By using the surface-sensitive high-angle annular-dark field (HAADF) imaging technique, an atomic-scale study carried out by Wei et al. found that a Ni₅P₄ nanosheet with preferential (001) orientation is covered in Ni-rich NiP_x/Ni₂P (0 < x ≤ 0.5) nanolayers. They found that during the phosphorization process, the Ni atoms diffused quickly toward the outer surfaces, and the inverse diffusion of P atoms toward the inner surfaces took place [11]. In a thermodynamic non-equilibrium state, this explains why the Ni tends to be enriched on the surfaces of the phosphides. Furthermore, an image simulation-based structural study showed the potential existence of Ni vacancies in the Ni₅P₄. By investigating the atomic-scale structure of the Ni₅P₄ (100) surface (Figure 6e), model-based image simulations further reveal that there may indeed be many Ni vacancies in the Ni₅P₄ nanosheet, but there is less Ni atom epitaxy on the (100) surface.

The characterization results show that even for nanosheet-like materials, as may be applicable to other transition-metal compounds, e.g., oxides, phosphides, nitrides, and carbides, complex facet-dependent surface structural reconstruction may exist. Furthermore, given the discrepancy between real surface structures and constructed DFT models, one should seriously consider the rationality of the DFT results in interpretations of the catalytic reaction pathway and mechanism. This highlights the fact that apart from controlling the sample morphology and shape, exploring the facet-dependent structural and chemical features, both for the surface and the interior of the catalyst, down to the atomic level is crucial for establishing a clearer structure–property relationship.

5. Conclusions

Overall, transition metal (Ni, Co, Fe, Mo, and W) phosphides are deemed to be a promising family of catalysts for the HER, given their high catalytic activity and stability in acidic aqueous electrolytes. From recent reports, it appears that anion substitution [75,76] or using composites with hydroxide species can further enhance their catalytic performance. Despite these findings and continued research efforts, a mechanistic study relating to the role of the surface chemical-potential gradient, spin polarization, and crystallographic polarity is also worth further exploration [77]. The combination of in situ characterization techniques, e.g., XANES, Raman, XPS, XRD, and S/TEM, with computational support should be particularly valuable to gain an in-depth understanding of the evolution process of hydrogen production during operational conditions [78,79,80]. It is believed that implementation of these in situ techniques may help to optimize the performance of the catalysts.

In terms of a perspective regarding Ni₅P₄ and beyond, the shape of future research can be outlined as follows:

• Although most mixed phases of Ni₅P₄, as synthesized by different methods, are reported to have good electrocatalytic activity for HER, they also have shortcomings like uncontrolled agglomeration, low hydrophilicity, and poor contact resistance, which hinder their mechanistic study and future application.

• So far, the Ni₅P₄ still has a large probability of intergrowth with Ni₂P and other phosphide phases, which is linked closely with the thermodynamic non-equilibrium conditions. Thus, the preparation of a pure single-phase Ni₅P₄ phase, e.g., by using molecular beam epitaxy, remains very important for in-depth structural and catalytic mechanism study.

• The current techniques can help to correlate structural changes in the catalytic performance. However, there is still a lack of direct and in situ imaging and spectroscopy techniques that can deliver the temporal-scale crystal and electronic structural changes and interpret the micro-scale catalytic mechanism.

• Although the development of phosphide-based materials has achieved significant progress in the electrolysis of water, the long-term stability and overpotential, which are significantly lower than that of Pt above the exchange current density of 1000 mA cm², are far from meeting industrial requirements. Therefore, improving their activity/stability via rational surface structural and compositional design is still needed to optimize their catalytic performance.

• According to current research, various sample preparation methods, such as impregnation techniques, precipitation [81], and deposition [82], have been employed to modify Ni_xP_y materials through single-atom doping or morphology control. These modifications have endowed Ni₅P₄ catalysts with diverse catalytic properties, including bifunctional catalysis [83,84,85,86,87], thus opening new avenues for the application of Ni_xP_y materials in hydrolysis reactions. Future investigations should prioritize understanding the underlying mechanisms behind these structural modifications and guide further in-depth studies on performance enhancement.

Acknowledgments

The authors thank colleagues at the Ernst Ruska-Centre (ER-C) of the Research Centre Juelich in Germany for providing technical support and helpful discussion.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Table

Figure 1. Crystal structures of different NixPy phases, from Ni-rich Ni3P to P-rich NiP3. (a–h) The schematic diagram of the Ni3P, Ni12P5, NiP3, NiP2, Ni2P, Ni5P4, Ni8P3, and NiP2 structure belongs to the tetragonal, cubic, hexagonal, trigonal, and monoclinic crystal systems, respectively. The P atom is colored purple, and the Ni atom is colored grey.

Figure 2. The formation mechanism of the as–synthesized nickel phosphide with different phases and morphologies. (a) Thermal decomposition. Ref. [55]. Copyright © 2020 Elsevier Ltd. (b) electrochemistry methods. Ref. [56]. Copyright © 2019, Royal Society of Chemistry. (c) The solid–gas reaction method. Ref. [57]. Copyright © 2020, Royal Society of Chemistry. (d) Schematic representation of the nickel foam–based solid–gas method for Ni5P4 nanosheets with SEM images. Ref. [58]. Copyright © 2017, Royal Society of Chemistry. Ref. [46]. Copyright © 2015 WILEY-VCH Verlag GmbH & Co., KGaA, Weinheim.

Figure 3. (a) Change in enthalpy as a function of the Ni/P ratio. Ref. [28]. Copyright © 2018, American Chemical Society. (b) Change of chemical potential as a function of the Ni/P ratio. Ref. [48]. Copyright © 2016, American Chemical Society. (c) The dependence of the HER turnover frequency (vs. RHE/[H2/s]) on the calculated differential free energy of H adsorption (ΔGH) for various kinds of transition metal phosphide and Pt. Ref. [28]. Copyright © 2018, American Chemical Society. (d) XRD characterization of the process of Ni-P single-phase transformation. Ref. [34]. Copyright © 2023, Royal Society of Chemistry. (e) XPS signals of the Ni5P4 nanosheet, characterized before and after the HER test. The peaks are fitted based on simple models. For ease of comparison, typical binding energies about Ni-containing species are marked out. The Ni valence (green arrows) in the range of 0~1 suggests the reconstruction of Ni atoms with P and other relevant atoms on the phosphide surfaces. Ref. [11]. Copyright © 2020, American Chemical Society.

Figure 4. Comparison of three electrocatalyst performance metrics (Tafel slope, the HER overpotential, and exchange current density) for the Ni5P4 catalyst and the other state of the HER electrocatalysts at pH 0 (a) and 14 (b); the whole set of data is based on the literature report detailed in Table 1. The color scale ranges from red to blue, corresponding to the exchange current density values from maximum to minimum, while the white squares indicate data points for which the exchange current density value was not found in the literature.

Figure 5. (a) Stable geometries of H adsorption at the Ni2P (001) and Ni5P4 (0001) surfaces. Ref. [67]. Copyright © 2017, American Chemical Society. (b) Gibbs free energy of H adsorption (ΔGads), as a function of H coverage. Ref. [68]. (c) The DFT result of bulk Ni5P4 and the P3-hollow site. Ref. [68].

Figure 6. (a) LEED characterization of Ni2P. Ref. [74] Copyright © 2006 John Wiley & Sons, Ltd. (b–d) Surface atom reconstruction of Ni5P4 along [001], unveiled by comparing experimental (b) and simulated (c), along with the corresponding structural model of NiPx/Ni2P/Ni5P4 (d). The red circle denotes the surface Ni atom that has been reconstructed at the point where the P atom is located in the structure. The green circle represents the surface Ni atom that has been reconstructed at the position of the original Ni atom in Ni5P4. Lastly, the blue circle indicates the position of the Ni-P column in the original Ni5P4 unit cell without Ni atom surface remodeling. Ref. [11]. Copyright © 2020, American Chemical Society. (e) Surface atom reconstruction of Ni5P4 along (100) by STEM, overlaid with a simulated image and a corresponding Ni/Ni5P4 structural model. (f) Atomic resolution TEM image of NiPx/Ni2P/Ni5P4, recorded along the [001] direction. The insert shows a simulated image of Ni5P4 (thickness t = 6.2 nm). The yellow-dashed lines denote the atomic terraces resulting from surface reconstruction. Ref. [11]. Copyright © 2020, American Chemical Society. (g) The lattice match relationship between Ni5P4 and Ni2P, along their common [001] direction. Ref. [11]. Copyright © 2020, American Chemical Society. (h) Cross-sectional view of the NiPx/Ni2P/Ni5P4 layered structure along the [010]54 direction (right), showing the bi-directional chemical potential. In the schematic diagram of the structure, the gray, green, and blue spheres represent the Ni atoms in Ni5P4, Ni2P, and NiPx, respectively. On the other hand, the yellow and purple spheres represent the P atoms in Ni5P4 and the reconstructed layer, respectively. Ref. [11]. Copyright © 2020, American Chemical Society.

Table 1

Comparison of HER activity in the different nickel phosphide systems.

Catalyst, Morphology	Tafel Slope/mVdec−1 (pH)	Overpotential (η)/mV(j/mAcm−2; pH)	Exchange Current Density/A cm−2	Duration/h(Overpotential/mV)	Ref.
Ni2P NPs/Ni	~46	116 (10; 0)	3.3 × 10−5	-	[5]
Ni2P NS/Ni foam	68	115 (10; 0)	-	65 (175)	[29]
NiP2 NS/CC	51	75 (10; 0)	2.60 × 10−4	57 (130)	[30]
Ni5P4 MP	30	43 (10; 0)	5.8 × 10−4	-	[28]
Ni5P4 nc-MP	33	33 (10; −0.5)	2.1 × 10−3	16 (21)	[10]
Ni5P4 NS/NF	53	108 (10; 0)	-	-	[11]
Ni5P4 NPs	42	118 (10; 0)	-	8.33 (150)	[33]
Ni5P4 3D Nanoflower	79.38	106 (10; 14)	-	22 (147)	[34]
Ni5P4 Rice-shape/NF	64	64 (10; 14)	-	11.11 (120)	[31]
Ni3P PHNs	50	85 (10; 0)	-	-	[35]
Ni3P MP	41	66 (10; 0)	3 × 10−3	-	[28]
Ni12P5	63	107 (10; 0)	2 × 10−4	-	[36]
Ni3S2/AT-NF	107	200 (10; 0)	-	-	[37]
Pt	30	23 (10; 0)	2.5 × 10−3	-	[38]
FeP	37	48 (10; 0)	4.3 × 10−4	16 (52)	[38]
CoP NP	50	74 (10; 0)	1.4 × 10−4	24 (100)	[39]
MoSx thinfilm	40	202 (10; −0.5)	1.3 × 10−7	-	[40]
MoS2	43	187 (10; 0)	4.2 × 10−7	-	[41]
MoS2/rGO	41	154 (10; 0)	2.5 × 10−5	-	[42]
NiMo Alloy	132	288 (10; 14)	7.9 × 10−5	-	[43]
NiMo NP	-	82 (10; 14)	-	-	[44]
Ni3P MP	119	291 (10; 14)	5.2 ± 0.9 × 10−5	-	[28]
Ni5P4 MP	-	193 (10; 14)	-	-	[28]
Ni5P4 nc-MP	98	49 (10; 14)	4.2 × 10−3	-	[10]
Pt	120	155 (10; 13)	6.9 × 10−4	-	[45]

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