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Article

Unlocking Synergistic Catalysis in NiP: Dual Role of Electronic Structure and Lewis Acidity for Enhanced Oxygen Evolution Reaction

by
Jiazhou Liang
1,
Jiawei Li
1,
Jiani Yan
1,
Andrew M. Rappe
2,* and
Jing Yang
1,*
1
School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China
2
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(5), 457; https://doi.org/10.3390/catal15050457
Submission received: 4 April 2025 / Revised: 1 May 2025 / Accepted: 6 May 2025 / Published: 7 May 2025
(This article belongs to the Special Issue Design and Application of Combined Catalysis)
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Abstract

:
Nickel phosphides (NixPy) are recognized as promising alternatives to noble-metal catalysts for the oxygen evolution reaction (OER). NiP, consisting of the equal stoichiometric ratio of Ni and P, could help quantify the catalytic effect of P and Ni. In this work, density functional theory (DFT) is employed to investigate the OER mechanism on NiP surfaces. We found that P atoms help stabilize O* at the adsorption sites. The rich electron donation from the Ni atom can alter the local charge distribution and enhance the interaction between O* and P atoms. Both oxygen intermediate adsorption energy and OER overpotential exhibit linear correlations with the charge of adsorption sites. Electron loss at the site induces the overall system to exhibit Lewis acidic characteristics, facilitating the OER and leading to a substantial overpotential reduction of up to 0.61 V compared to Lewis basic structures. Leveraging electronic structure theory and Lewis acid–base theory, we offer a new insight into the OER mechanism on the NiP surface, demonstrating that the catalytic activity of bulk metallic surface materials like NiP can be optimized by tailoring the local surface chemical environment.

Graphical Abstract

1. Introduction

The global surge in energy demand, coupled with the depletion of fossil fuel reserves and their severe environmental impact, has intensified the urgency for researchers to explore green energy alternatives and develop more efficient sustainable energy conversion technologies [1,2,3]. Hydrogen is widely recognized as a clean and renewable energy carrier due to its high energy density and zero carbon emissions. The most promising hydrogen production method thus far is water electrocatalytic decomposition [4,5,6,7]. In the water-splitting reaction, the OER is a critical bottleneck, as it entails a complex four-electron process that is often regarded as the rate-limiting step of the entire catalytic cycle. Consequently, the development of efficient OER electrocatalysts is of paramount importance [8,9,10].
Precious metals and their oxides, particularly Ru, Ir, RuO2, and IrO2, demonstrate superior OER electrocatalytic performance. However, their widespread adoption is severely limited by their prohibitive cost. For instance, in highly efficient acidic proton exchange membrane water electrolyzers (PEMWEs), recognized as a premier hydrogen production technology, anode OER electrocatalysts are still limited to precious Ir and Ru oxides [11]. Developing highly active and noble-metal-free electrocatalysts is critical for the large-scale application of PEMWEs [12,13,14,15]. Earth-abundant transition-metal catalysts have garnered significant attention in recent years for their potential in electrocatalytic water splitting [16,17,18]. Nickel phosphides, a representative transition-metal phosphide, have emerged as a notable contender. The favorable combination of their low preparation costs and high catalytic activity renders them promising candidates for OER catalysis [19,20,21]. It has been reported that the dissolution step in pulsed reverse electrodeposition (PRED) regulates the chemical state of phosphorus in NiP films, where increased dissolution promotes the formation of removable phosphates to enhance the OER activity of NiP films, achieving an average reduction of 55.1 mV in overpotential at a current density of 10 mA/cm2 compared to constant potential deposition (CPD) results; XPS spectra reveal that the removal of phosphates exposes more active nickel oxide sites, thereby improving OER activity [22]. Notably, NixPy with a relatively high metal content exhibits superior activity towards OER, as exemplified by Ni12P5, whose OER performance surpasses that of Ni2P due to its higher Ni content facilitating the formation of a more catalytically active NiOOH/Ni(OH)2 shell layer on the surface [23,24,25,26,27,28,29]. Phosphorus within NiP functions as an excellent supporting layer beneath the surface of nickel oxide or oxyhydroxide during the OER, enhancing the stability of the catalyst [29,30]. In addition, under actual reaction conditions, solid surfaces undergo geometric, atomic, and electronic reconstructions to minimize surface free energy. The reconstructed atomic surface stoichiometry may either mirror the bulk termination stoichiometry or differ from it [31,32,33]. Under practical conditions, nickel phosphides have been proven to undergo various surface reconstructions, including the formation of oxide layers, which significantly alter their catalytic performance compared to the bulk-terminated surfaces [34,35]. However, in our case of exploring the OER mechanism with the material stoichiometric ratio effect, we will focus more on the primitive surface and figure out the actual role of P element in the material. Therefore, it is essential to systematically investigate the NiP surface structural reconstructions and thoroughly explore the OER mechanism. Notably, transition-metal phosphides, including NiP, offer abundant metal and nonmetal adsorption sites for OER intermediates, introducing additional complexity beyond traditional metal-based systems. Most researchers believe that conventional descriptors, such as the d-band model-related features, prove insufficient in providing comprehensive insights into these complex systems [36,37,38]. Therefore, a systematic analysis of the OER reaction mechanism at different active sites is crucial for the rational design of efficient OER catalysts.
In this work, we investigate the surface reconstruction and OER activity of NiP(100), (010), and (001) using DFT. We identified the most stable surfaces among the various surface reconstructions of those low index surfaces. During the investigation of OER activity, we found that the charges at the adsorption sites are linearly correlated with the oxygen adsorption intermediates and even the OER overpotential. The results show that the charge transfer at the site can regulate the adsorption energy of the intermediate and impact the performance of the catalysts. We hope that our results can provide a reference for the design of base-metal catalysts.

2. Results and Discussions

2.1. Stability of NiP Surfaces

To identify the most stable surface, we computed the surface energies of various NiP slab models with different surface terminations. Here, we have studied the surface stability of (100)-Pris: P2, (100)-C1:Ni4P2, (100)-C2:Ni2P2, (001)-Pris: Ni2P2, (001)-C1:Ni2P2, (010)-Pris: Ni2P2, (010)-C1:P4, and (010)-C2:P2, where the symbol is defined as (facet)-Reconstructions: termination (Pris represents the pristine bulk structure termination, C1 represents the cutting of one layer of the top surface atoms, C2 represents the cutting of two layers of the top surface atoms).
Based on our calculations, we have found that P-rich surface terminations are the most energetically favorable for NiP among the low-index surfaces, which is consistent with previous theoretical and experimental studies [39,40,41,42]. The surface energy diagram of the slabs of all three lattice planes showed that the surfaces with P atom terminations have low surface energies, indicating that they are thermodynamically stable. Before the formation of bulk NiP region (−1.2 eV < Δ μ P < 0 eV), four stable surface terminations with lower surface energy ((010)-C1:P4, (010)-C2:P2, (001)-C1:Ni2P2, and (100)-C1:Ni4P2) are shown in Figure 1b. Among these surfaces, NiP(010) exhibits the lowest surface energy, indicating the most stable surface termination.

2.2. Stability of the Adsorption Structures

To investigate the relationship between the catalytic activity and surface active sites, we constructed multiple adsorption systems of the OER intermediates on the various adsorption sites of the surfaces. Judging by the surface density of states (DOSs), we found that many surface sites are similar in terms of geometric position and electronic structure. Taking (100)-C1:Ni4P2 as an example, the surface layer consists of four Ni atoms and two P atoms (Figure 1c). Ni1 and Ni3 are completely equivalent due to the full overlap of the DOS (see Figure 1d). The same applies to Ni2 and Ni4, as well as P1 and P2. For the NiP(001)-C1: Ni2P2 surface, both the two Ni and P sites are equivalent, leading to one set of Ni-P sites. For (010)-C1:P4 and (010)-C2:P2 surfaces, the topmost layer only contains P atoms, and they are all equivalent within the surfaces correspondingly (Figure S5).
Given the equivalency of the surface sites, we considered a one monolayer (1 ML) adsorption condition of the OER cycle species. Due to the simple structure of the adsorbates, such as O*, HO*, and HOO*, we chose the on-top adsorption site for each case. To determine stable adsorption structures, we tested four adsorption distances: 1.3 Å, 1.5 Å, 2 Å, and 2.5 Å. Our findings reveal that the diverse adsorption sites on the surfaces create a complicated atomic interaction for adsorbed OER intermediates. Notably, the available adsorption sites encompass Ni2P-hollow, Ni on-top, P on-top, as well as the bridge sites between Ni-Ni and Ni-P atoms (see the adsorption energy ( E a d s ) for various sites in Table S3). The stable oxygen intermediate adsorption sites are Ni2P-hollow, Ni on-top, P on-top, and the Ni-Ni and Ni-P bridge sites. For the HO* group, the stable sites are P on-top and the Ni-Ni bridge sites. Similar to the O* group, the HOO* can be stably adsorbed on Ni on-top, P on-top, and the Ni-Ni bridge sites.

2.3. OER Pathways on NiP Surfaces

To understand the mechanism of OER on the NiP surface, we investigated the OER process on the four stable NiP surfaces, as determined in the previous section (Figure 1b). We found that on the four stable surfaces, there are two reaction pathways for the OER, which are described as the single-site pathway and the dual-site pathway. The single-site refers to all the intermediate species adsorbing on the same site, whereas the dual-site pathway introduces a second adsorption site of the oxygen intermediate during the reaction cycle (Figure 2). A simplified version of the proposed OER mechanism as the reference pathway is provided in Figure S7. The single-site pathway occurs on all four stable surfaces, whereas the dual-site reaction can happen only on Ni4P2 and P2 surfaces. For each intermediate, the preferred adsorption location can be determined by comparing the adsorption energy at various adsorption sites. For the Ni4P2 surface, the preferred adsorption site for HO* and HOO* is the Ni-Ni bridge. The energetically favored adsorption site for O* is Ni2P-hollow instead of Ni-Ni bridge, which converts the pathway from single-site to dual-site (Figure 2a). The adsorption energy difference between O* adsorption on Ni-Ni bridge and Ni2P-hollow is 1.84 eV, according to our calculation. The dual-site pathway usually involves sublayer atoms and includes both P and Ni atoms. For the P2 surface, both HO* and HOO* are stably adsorbed on the P atom top site. O* is preferentially adsorbed on a Ni-P bridge site, forming another dual-site pathway with the adsorption energy 0.42 eV lower than the O* adsorbed onto the P atom (Figure 2b). For the P4 and Ni2P2 surface, all the OER intermediates adsorb more strongly onto the P on-top site than onto other sites, which can be described as a single-site pathway (Figure S6). A similar dual-site pathway can be found in the NiN4C4 moiety, where HO* and O* preferentially adsorb onto the C atom, whereas HOO* tends to reside on the central Ni atom, as demonstrated by Fei and coworkers [43]. However, in our dual-site case, HO* and HOO* are both adsorbed onto metal or non-metal sites (Ni-Ni bridge and P atom), while O* prefers the P-containing site (Ni2P-hollow and Ni-P bridge). We subsequently evaluated the OER catalytic activity of various sites based on the established reaction paths.

2.4. Investigation of Electrocatalytic OER Mechanism

To investigate the relationship between the nature of surface active sites and catalytic activity, we calculated the Gibbs free energy of the OER intermediates at U = 0 V and U = 1.23 V according to the proposed reaction pathways (Figure 3a–c). One important parameter used to evaluate catalytic activity is the OER overpotential, which is determined by the free energy of the rate-determining step (RDS). For the single-site OER pathway of P4 and Ni2P2 surfaces, the RDS is O* oxidation to HOO* with overpotentials as large as 1.54 V and 1.57 V (U = 0 V) (Figure 3c). Moreover, the adsorption energies of the O* on P4 and Ni2P2 surfaces are very similar (Figure 3d), which leads to the overpotential being pretty similar to each other. For the P2 surface, the RDS is the oxidation of O* to HOO* for both the single-site and dual-site reaction pathways (Figure 3b). Notably, the overpotential for the single-site pathway is 1.68 V, whereas the overpotential is significantly lower, at 1.07 V, for the dual-site pathway. Therefore, the OER prefers the dual-site pathway on the P2 surface. Furthermore, the involvement of the Ni atom enhances the adsorption of O* on the P2 surface ( E a d s ( O * ) = − 4.56   eV ) compared to the single-site pathway with the P atom-only interaction. However, the joint interaction between Ni and P can cause too strong adsorption of the O*, hindering the OER cycle. In the case of the Ni4P2 dual-site pathway, O* adsorbs onto the Ni2P-hollow site with a strong adsorption energy of −5.16 eV and an overpotential of 1.30 V, hindering the participation of O* in subsequent reactions. By contrast, Ni4P2 with the single-site pathway shows the lowest overpotential of 0.78 V amongst all the surfaces, featuring the oxidation of HOO* to O2 as the RDS (Figure 3a) rather than the more common RDS of O* to HOO*. The metal atom site (Ni-Ni bridge site) provides moderate adsorption to the intermediates and activates the reaction cycle (Figure 3d). According to the Sabatier principle, the adsorption energy of the intermediate needs to fall within an appropriate range to ensure the equilibrium between the adsorption and desorption of the intermediate [44]. Given that O* exhibits a strong adsorption affinity to the Ni2P-hollow site, more energy is required to desorb O*, resulting in a larger overpotential and shifting the RDS to the oxidation of O* to HOO*; therefore, it hinders subsequent reactions. Consequently, for the Ni4P2 surface, the OER prefers a single-site pathway (Figure 2a) rather than a dual-site one.
While energetic analysis is helpful for explaining the OER catalytic activity at various active sites, we present an analysis grounded in electronic structure theory to elucidate the relationship between the catalytic activity and the adsorption site characteristics.
Focusing on the RDS investigations, we found that the interaction between O* and the sites encompassing Ni and P atoms is usually stronger than in other cases. In the case of the P2 surface, the differential charge density depicts that O* adsorption reveals a significant redistribution of charge between O* and various surface sites (Figure 4a–d). The adsorbed O* induces electron accumulation at both the Ni-P bridge site and the P site. Due to the high electronegativity of oxygen, O* tends to obtain electrons from Ni and P atoms within the adsorption sites. In the single-site pathway, O* adsorbs onto the P site and obtains electrons from the P atoms in the adsorption site. In the dual-site pathway, O* interacts with the Ni-P bridge site, and the electronic interaction between the O* and the P atoms on the Ni-P bridge site is stronger than that on the P site. Meanwhile, O* exhibits a larger charge accumulation at the Ni-P bridge site compared to that at the P site. All these phenomena can be attributed to the additional electron donation from the Ni atom, which alters the local charge distribution and enhances the interaction between O* and P atoms. The electronic interaction of O* on the Ni4P2 surface is similar to that on the P2 surface (Figure S8). In the single-site pathway, strong charge accumulation and depletion can be observed between O* and Ni atoms at the Ni-Ni bridge site. In the dual-site pathway, O* acquires electrons from both the Ni atoms and the P atom residing within the Ni2P-hollow site. Notably, on the Ni2P-hollow site, a relatively stronger electron dissipation occurs between O* and P atoms as compared to that between Ni atoms. The strong electronic interaction between O* and P engenders a more negative adsorption energy of O* at the Ni2P site ( E a d s ( O * ) = − 5.16   eV ) in comparison to that of the Ni-Ni bridge site composed solely of Ni atoms ( E a d s ( O * ) = − 3.32   eV ), thereby conferring enhanced adsorption stability.
Thoroughly analyzing the density of states (DOSs) of the various adsorption structures among different surface terminations, we found that P atoms tend to assist in stabilizing O* at the adsorption sites. When Ni and P combine at the site, such a P involvement can further activate the Ni sites in the catalytic cycle, driving Ni as an electron donor. The interaction of O* at P sites of various surfaces presents a similar behavior in that the major oxygen p orbital peak lies around −5 eV and overlaps with the phosphorus p orbital, indicating a strong and stable chemical bond formation between O* and P. The non-bonding oxygen p orbital peak can be found around −2.5 eV with an almost half-full occupation, making it difficult for the O* to propagate further reactions (Figure 5b,d). This phenomenon is attributed to relatively low adsorption energy and high overpotential compared to other cases. When O* adsorbs onto the Ni-Ni bridge site found on the Ni4P2 surface (Figure 5a), the oxygen p orbital shifts to the higher energy region in general compared to O*-P case (Figure 5b), and the previous non-bonding part hybridizes with the Ni d orbital crossing the Fermi level, forming an active state to proceed the further reaction along the OER cycle. When a P atom presents within the adsorption site, as is the case for the Ni-P bridge site (Figure 5c), the major p contribution of the O* falls back to around −5 eV, and the reduced overlap between the oxygen p orbital and Ni d orbital suggests a weakened O*-Ni interaction. However, the d band center of Ni shifts higher to be close to the Fermi level, which activates the Ni atom in the Ni-P bridge site for the next reaction step. In this case, the Ni atom provides its excess d electrons to O* (Figure 4a), leaving the unsaturated site for the opportunity of a second water molecule to break to form HOO* (SI Equation (S3)). Hence, the dual-site pathway is preferred on the P2 surface.
Except for the Ni4P2 surface, the RDS of the OER reaction on P4, P2, and Ni2P2 consistently involves the oxidation of O* to HOO*. Therefore, we performed Bader charge analysis on adsorption sites and oxygen intermediates to investigate the charge transfer (Table S4). Previous studies have demonstrated that substantial charge transfer between active sites and adsorbates typically correlates with lower adsorption energy [45,46,47,48]. To investigate the potential mechanism of the RDS on various NiP surfaces, we calculated the adsorption energy of O* and the amount of charge transferred at the adsorption site. We observed a strong linear correlation between the amount of charge transferred at the adsorption site and the adsorption energy of O* (Figure 6a). Within the charge range of 1 to 2 | e | for the adsorption site, the adsorption energy of O* exhibits a decreasing trend with increasing charge, indicating that charge transfer occurring at the adsorption sites can effectively modulate the adsorption energy of O*, which provides insights for optimizing intermediate adsorption to regulate the performance of OER catalysts [49,50]. In addition, we calculated the Bader charge of the sites before adsorption. We found that the charge of the adsorption site and the OER overpotential are linearly related. The overpotential decreases as the charge lost from the adsorption site increases (Figure 6b). It is accepted that the adsorption properties of reaction intermediates are closely linked to the catalytic activity of the reaction [51,52]. Thus, the binding energy differences of intermediates ( Δ G O * − Δ G HO * ) are widely used to evaluate the OER activity. However, directly regulating the value of Δ G O * − Δ G HO * to improve catalytic performance is impractical. Our discovery establishes a direct linear relationship between the charge of the adsorption site and OER overpotential, indicating that catalytic performance can be directly optimized by regulating the charge of the adsorption site, thus providing a more precise electronic regulation strategy for catalyst design. Moreover, we observed that the adsorption sites containing Ni atoms tend to form a Lewis acid system (Figure 6b). Previous studies have shown that OER intermediates exhibit behaviors similar to Lewis bases with lone electron pairs [53,54,55]. When the adsorption site loses electronic charge, the electron cloud density around the site atoms decreases relatively, enhancing its electron-accepting ability, making the overall structure exhibit Lewis acid characteristics, which is conducive to the formation and adsorption of oxygen-containing intermediates, thereby promoting the progress of OER. In this study, compared to structures with Lewis basic characteristics, those exhibiting Lewis acidity show a remarkable decrease in overpotential by up to 0.61 V. Moreover, the charge of the adsorption sites can directly influence the performance of the NiP catalysts, indicating that even though NiP exhibits metallic properties, the local chemical environment can still determine the catalytic activity. In other words, the catalytic activity of bulk metallic surface material, such as NiP, can be optimized by tailoring the local chemical environment on the surface.

3. Computational Methods

For the DFT calculations, we use the Quantum ESPRESSO software package (version 7.1) with a plane-wave basis set [56,57]. The exchange-correlation energy was calculated using the Perdew–Burke–Ernzerhof (PBE) formulation of the generalized gradient approximation (GGA) [58]. Non-local norm-conserving pseudopotentials constructed with the OPIUM code are used [59,60]. The van der Waals interaction (vdW) was approximated using the DFT-D3 method [61]. During NiP slab and the adsorption structure optimization, only the top half of the atoms are relaxed, keeping the lower half fixed. The convergence accuracy of the total energy of the system is 10−5 eV, and the convergence accuracy of the forces is set to 10−4 eV/Å. The k-point sampling in the first Brillouin zone for the supercells is shown in Table S2.
The NiP cell structure in the international crystal structure database (ICSD; collection code 27159, space group Pbca) is selected as the bulk structure for NiP calculation [62]. The lattice parameters of completely relaxed NiP are a = 6.103 Å, b = 4.929 Å, and c = 6.905 Å, which are within 1% of the experimental values (a = 6.050 Å, b = 4.881 Å, and c = 6.890 Å). We construct a slab model to simulate the surface structures. The ( 2 × 1 × 1 ) supercell with two formula units of NiP along the z-direction ensures the decoupling of the continuous plates. Adsorption was allowed on one side of the slab, and to ensure the decoupling of the consecutive slabs, a 15 Å-thick vacuum region is employed, with an artificial dipole correction inserted into the center of the vacuum layer [63]. This work studies three surface directions of NiP: (001), (010), and (100), and uses a fully relaxed unit cell to construct the supercell structure to construct the slab model. Interface tailoring can effectively improve the activity and stability of the catalyst [64,65,66]. Judging by the pristine surface slab geometry directly generated from the bulk, the slab layer geometry repeats itself within two layers. Therefore, our search for low-energy reconstructions starts with one or the other layer of the original crystal structure to ensure that the search covers the possible reconstructions (see the Supporting Information (SI) for more details).
The catalytic activity of OER catalysts depends on the Gibbs free energies of intermediates ( Δ G H O * , Δ G O * , and Δ G H O O * ) [67]. For an ideal OER cycle, the Gibbs free energy change between two adjacent intermediates should be 1.23 eV (U = 0 V). By calculating the free energy of each OER reaction step, we can use the calculated OER overpotential to evaluate the catalyst’s OER performance. More computational details are provided in the Supporting Information.

4. Conclusions

In summary, we systematically investigated the OER mechanism on NiP surfaces using DFT calculations. Electronic structure theory analysis reveals that the surface P atoms work as stabilizers to stabilize O* at the adsorption sites. The excessive electrons from Ni atoms can alter the local charge distribution and enhance the interaction between O* and P atoms. Moreover, we found that the charge of the adsorption sites can be linearly correlated with both the adsorption energy of oxygen intermediates and the OER overpotential. When the site undergoes charge loss, the entire system demonstrates Lewis acid characteristics, making it favorable for OER. Compared to structures with Lewis basic characteristics, those exhibiting Lewis acidity show a remarkable decrease in overpotential by up to 0.61 V. This work elucidates the OER mechanism on the NiP surface, demonstrating that the catalytic activity of bulk metallic surface materials like NiP can be optimized by tailoring the local chemical environment through strategies such as doping-induced Lewis acid site engineering. These findings establish a valuable framework for guiding base metal catalyst design. Future work could explore doping strategies to introduce Lewis acidic sites and investigate surface oxidative reconstruction under high anodic potentials and low-pH conditions to better evaluate and enhance catalyst performance.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15050457/s1, Figure S1: (a) (100)-Pris; (b) (100)-C1; (c) (100)-C2. Pris represents the original surface, C1 represents the cutting of one layer of surface termination atoms, and C2 represents the cutting of two layers of surface termination atoms. The pink sphere represents P atoms and the gray sphere represents Ni atoms. Figure S2: Side view of NiP(100) terminations. (a) (100)-Pris terminations P2 (b) (100)-C1 terminations Ni4P2. (c) (100)-C2 terminations Ni2P2. (d) Surface energy of different terminations. Figure S3: Side view of NiP(010) terminations. (a) (010)-Pris terminations Ni2P2. (b) (010)-C1 terminations P4. (c) (010)-C2 terminations P2. (d) Surface energy of different terminations. Figure S4: Side view of NiP(001) terminations. (a) (001)-Pris terminations Ni2P2. (b) (001)-C1 terminations Ni2P2. (c) Surface energy of different terminations. Figure S5: surface termination atoms distribution of (a) (001)-C1: Ni2P2, (c) (010)-C2: P2, and (e) (010)-C1: P4. DOS analyses of (b) (001)-C1: Ni2P2, (d) (010)-C2:P2, and (f) (010)-C1: P4 surface atoms. Figure S6: The single-site pathway towards OER of (a) Ni2P2 surface and (b) P4 surface. In the ball-and-stick images, Ni atoms are in light gray, P atoms are in pink, O atoms are in purple, and H atoms are in white. Figure S7: (a) Surface site schematic. (b) A schematic depiction of single-site (L) and dual-site (R) mechanism. Figure S8: Differential charge density distribution of O* adsorbed onto the (a) Ni-Ni bridge site and (c) Ni2P-hollow site of the Ni4P2 surface (Isosurface value = 0.007 eÅ−3). (b) and (d) Cross-sectional views of the differential charge density in Figure S8a and S8c, respectively. The scale bar represents the magnitude of the charge density difference; the blue ends stand for charge depletion, whereas the red ends represent charge accumulation. Figure S9: Differential charge density distribution of O* adsorbed onto the P site of the (a) Ni2P2 and (b) P4 surface. Table S1: Magnetic moments of the system. Table S2: k-points setting for different structures. Table S3: O*, HO*, and HOO* adsorption energies for the available adsorption sites. Table S4: Calculated Bader charge of the adsorption systems formed by O* at different adsorption sites. Refs. [68,69].

Author Contributions

J.L. (Jiazhou Liang): investigation, conceptualization, methodology, validation, and writing—original draft; J.L. (Jiawei Li): investigation; J.Y. (Jiani Yan): data curation; A.M.R.: review and editing; J.Y. (Jing Yang): conceptualization, supervision, writing—review and editing, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research is funded by the National Key R&D Program of China (No. 2023YFB2405800).

Data Availability Statement

The data that support the findings of this study are available upon request.

Acknowledgments

The authors acknowledge the Hefei Advanced Computing Center for providing high-performance computing resources.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Ball-and-stick diagram of the crystal structure of NiP. (b) Surface energies of surfaces (010)-C1:P4, (010)-C2:P2, (001)-C1:Ni2P2, and (100)-C1:Ni4P2 as a function of P chemical potential ( Δ μ P ). (c) Ball-and-stick representation of (100)-C1: Ni4P2 surface termination atoms distribution. (d) DOS analyses of (100)-C1: Ni4P2 surface termination atoms. In the ball-and-stick images, Ni atoms are light gray, and P atoms are pink.
Figure 1. (a) Ball-and-stick diagram of the crystal structure of NiP. (b) Surface energies of surfaces (010)-C1:P4, (010)-C2:P2, (001)-C1:Ni2P2, and (100)-C1:Ni4P2 as a function of P chemical potential ( Δ μ P ). (c) Ball-and-stick representation of (100)-C1: Ni4P2 surface termination atoms distribution. (d) DOS analyses of (100)-C1: Ni4P2 surface termination atoms. In the ball-and-stick images, Ni atoms are light gray, and P atoms are pink.
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Figure 2. Proposed mechanism of the OER as our reference pathway. The single-site and dual-site pathways towards OER of (a) Ni4P2 surface and (b) P2 surface. In the ball-and-stick images, Ni atoms are light gray, P atoms are pink, O atoms are purple, and H atoms are white.
Figure 2. Proposed mechanism of the OER as our reference pathway. The single-site and dual-site pathways towards OER of (a) Ni4P2 surface and (b) P2 surface. In the ball-and-stick images, Ni atoms are light gray, P atoms are pink, O atoms are purple, and H atoms are white.
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Figure 3. Free energy diagrams of OER for (a) Ni4P2 and (b) P2 with single-site and dual-site pathways, and (c) Ni2P2 and P4 with single-site pathway at U = 0 V and 1.23 V. (d) O* adsorption energy for the available adsorption sites.
Figure 3. Free energy diagrams of OER for (a) Ni4P2 and (b) P2 with single-site and dual-site pathways, and (c) Ni2P2 and P4 with single-site pathway at U = 0 V and 1.23 V. (d) O* adsorption energy for the available adsorption sites.
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Figure 4. Differential charge density distribution of O* adsorbed onto the (a) Ni-P bridge site and (c) P site of the P2 surface (isosurface value = 0.007 eÅ−3). The yellow regions represent electron charge accumulation, whereas the blue regions indicate charge depletion. (b,d) Cross-sectional views of the differential charge density in Figure 4a,c, respectively. The scale bar represents the magnitude of the electronic charge density difference; blue represents electronic charge depletion, and red represents electronic charge accumulation.
Figure 4. Differential charge density distribution of O* adsorbed onto the (a) Ni-P bridge site and (c) P site of the P2 surface (isosurface value = 0.007 eÅ−3). The yellow regions represent electron charge accumulation, whereas the blue regions indicate charge depletion. (b,d) Cross-sectional views of the differential charge density in Figure 4a,c, respectively. The scale bar represents the magnitude of the electronic charge density difference; blue represents electronic charge depletion, and red represents electronic charge accumulation.
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Figure 5. PDOS analyses of O* adsorbed onto (a) Ni-Ni bridge site of the Ni4P2 surface and (b) P site of the P4 surface. PDOS analyses of O* adsorbed onto (c) Ni-P bridge site in the dual-site pathway and (d) P site in the single-site pathway of the P2 surface.
Figure 5. PDOS analyses of O* adsorbed onto (a) Ni-Ni bridge site of the Ni4P2 surface and (b) P site of the P4 surface. PDOS analyses of O* adsorbed onto (c) Ni-P bridge site in the dual-site pathway and (d) P site in the single-site pathway of the P2 surface.
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Figure 6. (a) The relationship between adsorption energy (O*) and Bader charge of the adsorption atoms (Ni and P). (b) The relationship between the charge of the sites before adsorption and OER overpotential η O E R . The gray dashed lines are the linearly fitted lines.
Figure 6. (a) The relationship between adsorption energy (O*) and Bader charge of the adsorption atoms (Ni and P). (b) The relationship between the charge of the sites before adsorption and OER overpotential η O E R . The gray dashed lines are the linearly fitted lines.
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Liang, J.; Li, J.; Yan, J.; Rappe, A.M.; Yang, J. Unlocking Synergistic Catalysis in NiP: Dual Role of Electronic Structure and Lewis Acidity for Enhanced Oxygen Evolution Reaction. Catalysts 2025, 15, 457. https://doi.org/10.3390/catal15050457

AMA Style

Liang J, Li J, Yan J, Rappe AM, Yang J. Unlocking Synergistic Catalysis in NiP: Dual Role of Electronic Structure and Lewis Acidity for Enhanced Oxygen Evolution Reaction. Catalysts. 2025; 15(5):457. https://doi.org/10.3390/catal15050457

Chicago/Turabian Style

Liang, Jiazhou, Jiawei Li, Jiani Yan, Andrew M. Rappe, and Jing Yang. 2025. "Unlocking Synergistic Catalysis in NiP: Dual Role of Electronic Structure and Lewis Acidity for Enhanced Oxygen Evolution Reaction" Catalysts 15, no. 5: 457. https://doi.org/10.3390/catal15050457

APA Style

Liang, J., Li, J., Yan, J., Rappe, A. M., & Yang, J. (2025). Unlocking Synergistic Catalysis in NiP: Dual Role of Electronic Structure and Lewis Acidity for Enhanced Oxygen Evolution Reaction. Catalysts, 15(5), 457. https://doi.org/10.3390/catal15050457

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