Impact of bimetallic synergies on Mo-doping NiFeOOH: Insights into enhanced OER activity and reconstructed electronic structure

Abstract

NiFe (oxy)hydroxide (NiFeOOH) is recognized as a highly active non-precious metal catalyst in alkaline water electrolysis due to its exceptional catalytic properties. In this work, high valence molybdenum (Mo) is introduced to improve the electronic structure and enhance the electrical conductivity of NiFeOOH for oxygen evolution reaction (OER). The introduction of Mo results in a Mo-doped NiFeOOH catalyst with a significantly reduced overpotential of 205 mV at 10 mA/cm² and a Tafel slope of 31.7 mV/dec, enabling stable operation for up to 170 h. Both empirical experiment and theory simulations are employed to gain insight into the 3d-electron interactions between molybdenum and nickel (Ni), iron (Fe) in Mo-doped NiFeOOH. The results indicate that Mo-doping enhances the valence states of Ni and Fe, leading to a shift in the d-band center of the bimetallic active sites. This modification affects the transformation of Mo-doped NiFeOOH into the γ-NiFeOOH active phase. This potent combination lends credence to its potential suitability and utility in OER applications.

Graphical abstract

Keywords

Mo doping / NiFe (oxy)hydroxide (NiFeOOH) / bimetallic synergies / oxygen evolution reaction (OER) / electronic structure

1 Introduction

Efficiently converting and utilizing renewable energy is a pressing scientific challenge. Electrochemical water splitting has emerged as a potential method for renewable energy production, comprising alkaline electrolytic water, proton exchange membrane electrolytic water, and solid oxide electrolytic water technologies [1]. Alkaline electrolytic water involves two distinct half-reactions, the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode [2–4]. However, the OER process is hindered by the need for a four-electron transfer, leading to slow reaction kinetics and the formation of oxygen intermediates (*OH, *O, *OOH, *OO) [5–8], resulting in a high overpotential [9]. Currently, noble metal-based oxides such as RuO₂ and IrO₂ are the most efficient electrocatalysts for water splitting [10]. However, their scarcity in nature and limited stability in alkaline electrolytes obstruct their widespread applications [11]. Transition metal hydroxides or (oxy)hydroxides have shown promise as alternative electrocatalysts, offering higher activity, lower costs, and improved stability [12]. The metal sites on the surfaces of these materials act as active sites for surface reactions [13,14]. Transition metal sulfides, phosphides, and nitrides have also been investigated for their high catalytic activity [15–18].

NiFe (oxy)hydroxide (NiFeOOH) nanosheets, among a variety of electrocatalysts, have gained significant attention due to their cost-effectiveness, plentiful resources, and high efficiency [19–21]. The distinct electronic configurations of Ni and Fe result in moderate bond strengths with electrocatalytic intermediates, leading to a superior performance in catalytic reactions such as OER. Given the crucial role of bond strength between active sites and intermediates in the OER activity, optimizing the structure and composition of NiFe-based catalysts can enhance the electrocatalytic process [22–24]. Currently, a variety of metallic cation dopants have been used to enhance the OER efficiency of electrocatalysts [25]. Among these dopants, molybdenum (Mo) is particularly noteworthy. Mo, as an n-type dopant, could be utilized to increase the number of free electrons in active sites. It has also been demonstrated that Mo doping can improve the conductivity and durability of the underlying materials [26]. As a result, Mo is considered a key heteroatom dopant for electrode components, enabling superior electrochemical properties. He et al. [27] reported that the introduction of Mo activates the lattice oxygen in NiFeOOH, thereby enhancing the OER catalytic activity. Tamboli et al. [28] introduced an electrodeposition approach for Mo-doped NiFe LDH, achieving a low overpotential of 230 mV at 30 mA/cm². Yin et al. [29] proposed a self-sacrificial template method for MoNiFe-LDH nanotubes with an overpotential of 317 mV at 20 mA/cm².

Although advancements have been achieved in comprehending Mo-doped NiFeOOH, obstacles still exist in establishing a simple synthetic method for MoNiFeOOH catalysts. It is crucial to achieve uniform Mo doping, regulate the surface area, and ensure stability and longevity in operating conditions. Overcoming these hurdles is essential to unlock the complete capabilities of MoNiFeOOH catalysts. Additionally, current experimental analysis and the density functional theory (DFT) computations focusing on Mo-doping NiFeOOH for improved OER reveal only the changes in the valence states of Ni/Fe caused by Mo doping [28,29], while scarcely uncover how Mo changes the valence states of Ni and Fe, the adsorption state of OER intermediates, and interactions between metals [30].

In this study, a straightforward one-step hydrothermal technique was utilized to synthesize Mo-doped NiFeOOH ternary metal nanosheet catalysts supported on nickel foam (NF), and the distinctive configuration could offer ample active sites for the catalytic process. The interplay among Mo, Ni, and Fe was evidenced from experimental analysis, and the Mo incorporation also supports the reconstruction of the γ-NiFeOOH active phase. Particularly, DFT-based theoretical investigation was performed to analyze the most effective OER pathway through two lattice oxygen mechanism (LOM) routes. The oxygen evolution mechanism with synergistic effects in Mo-doped NiFeOOH was uncovered, resulting in an enhanced OER catalytic activity due to optimized binding energies of the reaction intermediates.

2 Experimental section

2.1 Chemical and materials

All of the materials, i.e., NiCl₂·6H₂O (98%, Sinopharm), Na₂MoO₄·2H₂O (99%, Sinopharm), FeCl₃ (97%, Sinopharm), anhydrous ethanol (99.7%, Sinopharm), n-butylamine (99%, Sinopharm), and KOH (85%, Sinopharm) were of analytical grade and used as received without further purification. The water required for the experiments was ultrapure water (18.25 MΩ·cm).

2.2 Synthesis of MoNiFe/NF

The NF (10 mm × 20 mm) underwent pretreatment in diluted hydrochloric acid (HCl, 3 mol/L) for 30 min to remove surface nickel oxides. This was followed by sonication with anhydrous ethanol and deionized water for 20 min. Subsequently, a solution containing 0.8 mmol of NiCl₂·6H₂O, 0.1 mmol of Na₂MoO₄·2H₂O, and 0.2 mmol of FeCl₃ was dissolved in 9 mL of anhydrous ethanol, and stirred at room temperature with the addition of 0.5 mL n-butylamine. The pre-treated NF was then immersed in the precursor solution and heated in a Teflonlined stainless-steel autoclave at 160 °C for 15 h. Upon cooling to room temperature, the NF was rinsed several times with deionized water and anhydrous ethanol, and dried under vacuum at 60 °C for 12 h. Furthermore, MoNiFe/NF catalysts with varied doping concentrations (1 : 8 : 2, 3 : 16 : 3, 2 : 8 : 1, 3 : 40 : 12) were synthesized by adjusting the Mo to Fe ratio in the solution while maintaining total cation and Ni content constant. For comparison, NiFe/NF, NiMo/NF, and Ni/NF structures were prepared using a similar method as that of MoNiFe/NF, except that the molybdenum, iron, molybdenum, and iron precursors were added to the Teflonlined stainless-steel autoclave separately.

3 Results and discussion

3.1 Structural characterization of catalysts

The Mo-doped NiFeOOH (MoNiFe/NF) electrocatalyst was synthesized via a single-step hydrothermal process. It is important to mention that the X-ray diffraction (XRD) peaks associated with NiFeOOH in NiFe/NF or MoNiFeOOH in MoNiFe/NF were undetectable, as shown in Fig. S1 in Electronic Supplementary Material. Consequently, the XRD peaks mainly corresponded to the Ni substrate. During subsequent cyclic voltammetry (CV) experiments, it is expected that the NiFe(OH)₂/NiFeOOH redox couple will emerge, leading to the conversion of a portion of Ni0 in the electrode into Ni(OH)₂ and NiOOH [31]. The morphological and structural characteristics of MoNiFe/NF and NiFe/NF were analyzed using scanning electron microscopy (SEM). The SEM image at low magnification illustrates the uniform growth of the MoNiFe/NF and NiFe/NF catalysts on the NF as depicted in Fig.1(a) and S2(a). Upon further magnification, the ultra-thin nanosheet structure of the MoNiFe/NF and NiFe/NF catalysts is more visible (Fig.1(b) and S2(b)). This unique structure enhances the specific surface area, exposing a greater number of active sites for OER. The morphology of the MoNiFe/NF electrocatalyst is further characterized in Fig. S3. Transmission electron microscopy (TEM) images of MoNiFe/NF and NiFe/NF flakes are presented in Fig.1(c) and 1(d), Figs. S2(d) and S2(e), respectively. The measured spacing between adjacent lattice planes is 0.21 and 0.27 nm, allowing for the identification of the (105) plane of oxyhydroxide and the (101) plane of hydroxide, respectively. The selected area electron diffraction (SAED) pattern of the (oxy)hydroxide reveals distinct diffraction rings of the (101) and (1010) planes for NiFe(OH)₂ and the (105) plane for NiFeOOH (Fig.1(e)). Additionally, the energy-dispersive X-ray spectroscopy (EDS) mapping analysis shown in Fig.1(f) demonstrates the spatial distribution of Mo, Ni, and Fe elements within the MoNiFe/NF, while the spatial distribution of O, Ni, and Fe elements within the NiFe/NF is illustrated in Fig. S2(f). It corroborated the successful synthesis of the composites by the distributed presence of these elements in their corresponding positions. Inductively the coupled plasma mass spectroscopy (ICP-MS) was recorded, with the results further demonstrating the Mo doping in NiFeOOH (Table S3).

X-ray photoelectron spectroscopy (XPS) was employed to further probe the valence states and elemental composition of the MoNiFe/NF and NiFe/NF. The XPS full spectrum (Fig.2(a)) shows the existence of Ni, Fe, Mo, O, and C elements in MoNiFe/NF, and relatively a small number of Ni, Fe, O, and C elements in NiFe/NF. According to the Ni 2p XPS spectra (Fig.2(b)), the MoNiFe/NF manifests distinct peaks at 855.15 and 872.4 eV, which are attributable to the Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2 states, respectively. Concurrently, the peaks at 856.7 and 874.2 eV are associated with the Ni³⁺ 2p_3/2 and Ni³⁺ 2p_1/2 states, respectively. Moreover, the satellite peak (denoted as “Sat.”) composite observed in the spectra was identified as spin-orbit doublets [32], characterized by peak positions at 861.3 and 878.6 eV. The existence of Ni³⁺ is likely associated with the Ni 2p_3/2 spectrum of the in-situ formation of NiOOH on the surface of the catalyst. In addition, two distinct sets of spin-orbit peaks in the Ni 2p spectra of NiFe/NF catalysts imply the presence of Ni²⁺ (855.5/872.8 eV) and Ni³⁺ (857.1/874.6 eV) valence states. A crucial observation is the 0.4 eV positive shift in the Binding Energy (BE) of the Ni 2p in MoNiFe/NF facilitated by the introduction of Mo in comparison to that of NiFe/NF, which indicates that there is a transfer of electrons from Ni to O, resulting in enriching the Mo site with a higher charge concentration. The reason for the redistribution of charge might be the strong reduction property of Mo⁶⁺. Distinct peaks were detected at 711.3 eV (Fe 2p_3/2), 724.4 eV (Fe 2p_1/2) of NiFe/NF, and 711.58 eV (Fe 2p_3/2), 724.68 eV (Fe 2p_1/2) of MoNiFe/NF, which reveal the characteristic valence state of Fe³⁺, and the 0.28 eV positive shift in the BE of the Fe 2p (Fig.2(c)). In the Mo 3d spectrum, two peaks situated at 231.9 eV (Mo 3d_5/2) and 235.0 eV (Mo 3d_3/2) are indicative of the Mo⁶⁺ state of MoNiFe/NF (Fig.2(d)). The O 1s XPS spectrums of MoNiFe/NF and NiFe/NF (Fig.2(e)) show two distinct peaks at 531.2 and 532.6 eV. The peak with the lower BE (531.2V) is attributed to M-OH, while the one at 532.6 eV belongs to O_vac [33]. Furthermore, the narrowing of the valence band spectra provides evidence for the role of Mo in tuning the electronic structure of NiFe/NF (Fig.2(f)). The inclusion of Mo in NiFe/NF instigates a slight shift of the valence band maximum (VBM) toward lower binding energies compared to NiFe/NF, which effectively draws the d-band center nearer to the Fermi level [34]. This shift in the d-band center impacts the adsorption energy of OER intermediates, thereby enhancing the electrocatalytic activity. Therefore, the substantial shifts observed in the Ni 2p and Fe 2p BE and VBM of MoNiFe/NF compared to NiFe/NF highlight the strong interelectronic interactions between Mo and NiFe/NF, leading to significant changes in the electronic state of MoNiFe/NF which could substantially affect activity and stability. Moreover, the Bader charge results were obtained for the optimized structures (Fig.2(g) and (h)) show that Ni and Fe have higher charge states with the incorporation of Mo, which is due to the fact that the electronegativity of Fe and Ni is weaker than that of Mo. The Ni³⁺ and Fe³⁺ around the Ni and Fe sites in the MoNiFe system have more empty orbitals compared to the NiFe control, which would result in the Ni and Fe sites being potentially active adsorption sites (Fig.2(i)). Therefore, the lack of NiFe/NF signals in the XRD pattern and TEM observations, coupled with the clear detection of NiFe/NF in the Raman spectra and XPS spectrum, strongly suggests that the NiFe/NF synthesized has either a poor crystallinity or a low amount, or both.

3.2 Electrocatalytic performance

A series of examinations were conducted on the OER of the MoNiFe/NF within 1 mol/L KOH alkaline medium with a standard three-electrode cell. Bare NF, Ni/NF, NiFe/NF, and NiMo/NF were tested under the same conditions for comparison. The iR-corrected CV curves (Fig.3(a)) at 5 mV/s were used to reveal the OER activity of those samples. The overpotential of the MoNiFe/NF is 205 mV at a current density of 10 mA/cm² which is noticeably lower than those of NiFe/NF, NiMo/NF, Ni/NF, and bare NF (measuring 1.48, 1.52, 1.6, and 1.62 V vs. RHE, respectively). A hypothesis is drawn that the superior OER performance of MoNiFe/NF may stem from a meticulously optimized redistribution of charge density through bridging O²⁻ and favorable adsorption energies of oxygen-containing intermediates following the Sabatier principle [35]. The catalytic efficiency of MoNiFe/NF were simultaneously modified by changing the proportion of Mo to Fe in the composition. The OER polarization curves shown in Fig. S4 illustrate the impact of different Mo/Fe ratios on the performance of MoNiFe/NF samples. The optimal electrocatalytic activity for the OER was found when the molar ratio of the precursor materials for synthesizing MoNiFe/NF on NF substrate was set at 1 : 8 : 2 for the ratio of amount-of-substance concentration of Mo, Ni, and Fe. This specific ratio resulted in the highest efficiency. The electrocatalytic activity of MoNiFe/NF for OER was then evaluated by measuring the Tafel slope (Fig.3(b)), which is a measure of the reaction kinetics. The Tafel slope of MoNiFe/NF was 31.7 mV/dec, which was significantly smaller than that of NiFe/NF (51.7 mV/dec), Ni−Mo/NF (66.3 mV/dec), Ni/NF (93.3 mV/dec), and bare NF (116.2 mV/dec). It underscores the rapid reaction kinetics exhibited by MoNiFe/NF in the context of OER, thereby testifying to its exceptional electrocatalytic efficiency. Furthermore, electrochemical impedance spectroscopy (EIS) was deployed to scrutinize the charge transfer kinetics of the catalysts during the electrochemical process. The semicircle corresponding to MoNiFe/NF was noticeably smaller than those associated with the other samples under control implying superior conductivity and accelerated charge transfer for MoNiFe/NF (Fig.3(c)). The double-layer capacitance (C_dl) was gauged via CV testing which is conducted outside the Faraday region to quantify the electrochemical surface area (ECSA) and roughness factor (RF).

The results of calculated C_dl for those samples further reveal that MoNiFe/NF outshine others by presenting the highest C_dl value (9.39 mF/cm²) (Fig.3(d) and Fig.3(e) and S5). In addition, MoNiFe/NF has the highest ECSA-normalized current density compared to other electrodes (Fig.3(f)). The above results indicate the prevalence of more active sites within the synthesized MoNiFe/NF lead to its superior performance.

The OER overpotentials of different electrocatalysts reveal the low overpotential of MoNiFe/NF at a robust current density of 10 and 100 mA/cm², peaking at approximately 205 and 270 mV, respectively, which can be comparable to that of the commercial standard RuO₂ [36] (Fig.3(g)). The MoNiFe/NF catalyst showcases a superior performance when juxtaposed with recently reported NiFe-based catalytic electrodes (Fig.3(h) and Fig.3(i) and Table S4). Rationalized doping of Mo⁶⁺ contributes to an improved electrocatalytic activity, which can be assessed through turnover frequency (TOF). The TOF in the MoNiFe/NF stand at 0.2 s^–1 at an overpotential of 270 mV, which nearly quintuples NiFe (0.045 s^–1, Fig.4(a)). Furthermore, the current density of MoNiFe/NF (100 mA/cm²) is approximately 3.6 times that of NiFe/NF (28 mA/cm²). The increasing trend of TOF with overpotential shows that MoNiFe/NF boasts a substantially higher activity relative to NiFe/NF (Fig.4(b)), which demonstrates that the MoNiFe/NF catalyst possesses an elevated intrinsic activity toward OER that could achieve more efficient energy conversion.

To examine the durability of OER on MoNiFe/NF electrodes over extended durations, chronopotentiometry experiments were conducted at 10 and 100 mA/cm² respectively. Following 44 h of oxygen evolution at 10 mA/cm², the overpotential of the MoNiFe/NF electrode retained its stability (Fig.4(c) and 4(e)). Subsequently, continuous oxygen evolution was maintained for approximately 126 h at 100 mA/cm² accompanied by a negligible decrement in current density of 2%. Following 170 h of continuous operation, the catalytic activity of the MoNiFe/NF electrode remains largely unchanged. It is illustrated that the emergence of voltage spikes in the chronopotentiometry plot which could be attributed to temperature fluctuations. Specifically, the temperature-dependent CV curves demonstrate that an increase in temperature leads to an increase in OER activity, and vice versa, a decrease in temperature results in a corresponding decrease in activity (Fig. S7).

SEM, TEM, EDS, and XPS were deployed to delineate the morphology, elemental distribution, valence states, and elemental composition of MoNiFe/NF post-OER stability testing as illustrated. The results reveal a uniform distribution of Ni, Fe, and Mo across the surface (Fig. S8) and the remained chemical state of Mo (Fig. S9), indicating the superior stability of the MoNiFe/NF within the alkaline media. An H-type sealed electrolytic cell for drainage method (Fig. S10) was used to calculate the FE. The experimental O₂ yield at the anode is close to the theoretical O₂ yield, which can be considered that the FE is close to 100% (Fig.4(d)). The result of FE shows a good selectivity of the MoNiFe/NF for the OER.

3.3 Mechanism investigation of bimetallic synergies on MoNiFeOOH

A thorough investigation was conducted to explore the intricate relationship between Ni, Fe, and Mo in advanced MoNiFe/NF, which enhanced OER performance. Operando Raman spectroscopy meticulously recorded from the open circuit potential (OCP) to 0.7 V (vs. Ag/AgCl) was employed for analysis. Detailed operando Raman spectra for NiFe/NF are presented in Fig.5(a) and 5(c). At OCP, a distinct spectral peak identified as α-NiFe(OH)₂ is observed at approximately 461 and 528 cm^–1. Two resonant bands around 478 and 558 cm^–1 may be attributed to Ni-O vibrations present in NiFeOOH [53]. While both β-NiFeOOH and γ-NiFeOOH exhibit dual-band patterns in these wavenumbers, variations in their intensity ratios can be observed. As the potential increases to 0.1 and 0.2 V, the contrast in intensity between the band at 478 cm^–1 and the band at 558 cm^–1 (associated with γ-NiFeOOH) reduces in comparison to that of β-NiFeOOH. When surpassing an applied potential of 0.3 V and beyond, the intensity difference between the peaks at 478 and 558 cm^–1 declines. (I₄₇₈/I₅₅₈) indicative of the transition from β-NiOOH to γ-NiOOH phase changes.

However, the interference from oxygen bubbles gradually reduces peak intensity, disrupting the resonant signal of NiFeOOH. The transformation process observed in operando Raman spectra of MoNiFe/NF mirrors that of NiFe/NF. (Fig.5(b) and Fig.5(d)). Remarkably, the intermediate active phase in MoNiFe/NF remains stable with increasing bias potential, suggesting that MoNiFe/NF shares similar structural evolution and catalytic sites with NiFe/NF. The I₄₇₈/I₅₅₈ ratio shows a significant increase at a potential of 0.2 V or higher, indicating a prevalence of γ-NiFeOOH over β-NiFeOOH. The key advantage of the crystal structure of γ-NiFeOOH lies in its large inter-layer spacing of around 7 Å, permitting the absorption of intercalated species such as water or ions between the layers [34]. This spacing facilitates the ionic intercalation of OH^- anions (from the electrolyte), exposing more active sites for OER (Fig.5). This finding aligns with previous research on the impact of inter-layer anions in NiFe^‒ or Ni-based layered double hydroxides (LDHs), highlighting the enhanced OER activity in the presence of OH^‒/CO₃^2‒ species in LDH interlayers [54,55]. Overall, the superior performance of in-situ produced γ-NiFeOOH from MoNiFe/NF, compared to conventional NiFe/NF electrocatalysts, is attributed to its capability to modify defective active site structures through the ionic intercalation of OH^‒ within the expansive layered spacing of γ-NiFeOOH. This enhancement leads to an increased ECSA, an improved electronic conductivity, a superior OER kinetics, and a reduced charge transfer resistance, collectively promoting the OER process.

DFT calculations were harnessed to theoretically elucidate the mechanism underpinning the elevated electrocatalytic OER activity of MoNiFe/NF. The Sabatier principle [34] in electrocatalysis suggests that the optimal adsorption energy of reaction intermediates on a catalyst surface should be balanced to achieve the highest activity, which applies to OER where materials with an intermediate adsorption strength for intermediates exhibit the highest activity. DFT calculations were deployed to discern the OER mechanism on the NiFe/NF and MoNiFe/NF. The O 2p band center and the Mott-Hubbard splitting in d orbitals were calculated including electron-filled lower Hubbard band (LHB) and empty upper Hubbard band (UHB) as shown in Fig. S14 and Table S5, the oxygen vacancy formation energy (E_f_vac, Fig. S15), the Gibbs free energy map of the AEM pathway (Fig. S16), and the activation free energy (ΔG‡) of the rate-determining step (RDS) (Figs. S17 and S18 and Table S6). The results reveal that the OER pathway of Mo-doped NiFe/NF is dominated by the LOM. The OER pathway calculations for two distinct LOMs were undertaken as portrayed in Fig.6(a), Fig.6(b) and S19. The LOM-1 pathway commences by generating oxyhydroxide via deprotonation (Step 1) within MoNiFe/NF, then proceeds with OH⁻ nucleophilic attack on exposed lattice oxygen to create *OOH (Step 2). The *OOH is deprotonated next (Step 3), resulting in the release of gaseous O₂ from the lattice and leaving a surface oxygen vacancy (Step 4). OH⁻ replenishes these oxygen vacancies, facilitating surface regeneration (Step 5). Similar to LOM-1, the LOM-2 pathway involves the generation of *OOH from exposed lattice oxygen through the nucleophilic attack by OH⁻. Energies and configurations of significant intermediates (*OH, *O, *OOH, and *OO) in the OER were analyzed on NiFe/NF and MoNiFe/NF surfaces. Free energy diagrams depicting the reaction pathway were created for each catalyst (Fig.6(c) and Fig.6(d)). The results show that the deprotonation of *OH and formation of *OOH has the highest energy barriers for NiFe/NF, particularly at the RDS with energy barriers of 1.27 and 1.4 eV on OH and O sites, respectively (Table S7). In contrast, the deprotonation of *OH has an energy barrier of 0.58 eV on the OH site for MoNiFe/NF, while the generation of *OOH on the O site becomes the RDS with a significant energy barrier of 1.12 eV. Consequently, further calculations were conducted utilizing the LOM-1 pathway. The influence of Mo⁶⁺ ion doping on the electronic configuration of the catalyst was also explored. The charge density difference analysis indicates that the electron transfers from O to Mo post-Mo⁶⁺ ions integration with an apparent accumulation of electrons on the Mo⁶⁺ and depletion on the O²⁻ (Fig.7(a) and Fig.7(b)). This suggests that a part of the electron transfer from Ni³⁺ and Fe³⁺ to Mo⁶⁺ via the O²⁻ bridges in MoNiFe/NF (Fig.7(c)). The Ni³⁺ possesses partially filled eg orbital which results in a weak Π-donation with the O²⁻ bridges. Contrastingly, the half-full filled t_2g orbital of Fe³⁺ can overlap with the p-orbital of the O²⁻ bridges. Furthermore, the vacant 4d orbital of Mo⁶⁺ leads to a strong π-donation from the O²⁻ bridges to Mo⁶⁺.

As a result, partial electron transfer from Fe³⁺ and Ni³⁺ to Mo⁶⁺ via the O²⁻ bridges transpires, aligning with the XPS results where the oxidation states of Fe³⁺ and Ni³⁺ slightly increased. The Bader charge analysis unveil that Ni³⁺ experiences an increase from approximately 0.98 to 1.28, whereas the Fe rises from around 1.38 to 1.54 following Mo doping. The results demonstrate a strong electronic interaction between Mo and Ni, as well as between Mo and Fe. This interaction has the effect of increasing the valence state of both Ni and Fe (Fig. S20). Although the precise valence of each metal in multi-metal materials could not be pinpointed through DFT calculations, the heightened valence of Ni and Fe atoms renders the vacant orbit more amenable to the adsorption of OER intermediates. The density of states (DOSs) and projected density of states (PDOSs) of NiFe/NF and MoNiFe/NF are showcased in Fig.8, and S21 and S22. Notably, Mo doping induces changes in the DOS of the electronic states in MoNiFe/NF particularly in proximity to the Fermi level, thereby indicating that MoNiFe/NF exhibits a relatively elevated conductivity and facilitated expeditious electron transfer during catalysis. Evidence of p-d hybridization is seen through the overlap of respective orbitals between Fe and Ni d-orbitals and O p-orbitals, resulting in the creation of O-Fe and O-Ni bonds. The DOS near the Fermi level is greatly influenced by the Fe 3d, Ni 3d, and O 2p orbitals, as shown in Fig.8(a).

Conversely, the PDOS of Fe 3d and Ni 3d undergoes significant changes due to Mo doping in NiFe/NF leading to a decrease in the occupancy of Fe 3d and Ni 3d orbitals and the O 2p band center is determined to be −3.21 and −2.90 eV for NiFe/NF and MoNiFe/NF, respectively (Fig.8(b) and 8(d)). The d-band center of Ni and Fe in both catalysts was calculated and found to shift to −2.73 and −3.48 eV from −2.87 and −4.58 eV for MoNiFe/NF and NiFe/NF, respectively, which places it closer to the Fermi level. This shift implies that the incorporation of Mo causes the anti-bonding states to move above the Fermi level and becomes unoccupied, leading to an increased bond strength [55]. In the case of transition metals, there is a general trend where the binding strength increases as the d-band center energy increases relative to the Fermi level. However, if the d-band center energy is excessively low, subsequent intermediates may form weak bonds and exhibit difficulty in adsorbing, thereby resulting in the poisoning of active sites. In contrast, if the d-band center energy is excessively high, subsequent intermediates may become strongly bound and difficult to desorb, making it arduous to activate said intermediates and diminishing the likelihood of subsequent reactions [56]. Hence, the optimal scenario arises when the d-band center energy attains a moderate level, thereby optimizing the adsorption energy of intermediates and enhancing the alkaline OER activity.

4 Conclusions

In conclusion, a ternary Mo-doped NiFe/NF catalyst is constructed by a simple, one-pot hydrothermal approach exhibiting a remarkable performance in the alkaline medium for OER. The MoNiFe/NF catalyst exhibits a low overpotential of 205 mV at 10 mA/cm² and only experiences a 2% decrease in performance after continuous stability testing for 170 h at 10 mA/cm² (44 h) and 100 mA/cm² (126 h). Experimental analysis and DFT calculations reveal that the presence of Mo⁶⁺ ions lead to a negative shift in the transformation potential of β-NiFeOOH to γ-NiFeOOH, resulting in a lower OER overpotential. Besides, the introduction of Mo⁶⁺ increases the active electrons near the Fermi level in the MoNiFe/NF, and the transfer of electrons from eg orbits of Ni³⁺, t_2g orbits of Fe³⁺ to Mo⁶⁺ via O²⁻ bridges can boost OER activity, while the strong electronic interactions between Ni³⁺, Fe³⁺, and Mo⁶⁺ ensures high stability of the bimetallic sites. The high catalytic activity results from a fast cationic electrochemical oxidation process. This study introduces a promising and versatile approach for developing more efficient and durable OER electrocatalysts in the future, utilizing trimetallic or more complex (oxy)hydroxide catalyst systems.

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