English

• 0.   Introduction

  Electrochemical water splitting is a sustainable strategy for hydrogen production^[1-2]. The development of electrocatalysts for the oxygen evolution reaction (OER) is critical for reducing the high overpotential in water electrolysis due to its slow kinetics. Improving the overall energy conversion efficiency is vital to accelerate the large-scale adoption of water electrolysis for hydrogen production^[3-4]. Transition metal oxides (TMOs) are extensively studied as electrocatalysts for the OER process due to their low cost, abundant availability, and ease of integration, which have made them attractive candidates for sustainable electrochemical energy applications^[5]. Current studies show that iron, nickel, and cobalt compounds possess excellent catalytic activity in the OER owing to their unique electronic structures and electronic properties^[6-7]. Among these materials, Ni-based and Co-based systems have drawn particular interest for their superior catalytic performance. However, the relatively low electrical conductivity, limited number of active sites, and poor long-term stability in OER result in sluggish electrocatalytic efficiency.

  Bimetallic oxides (ABO_x, A and B represent metal atoms) exhibit distinct synergistic effects due to their oxygen-bridged A—O—B structural units. This unique A—O—B configuration offers several advantages, including heterointerfaces formed by the coordination geometries of A and B, as well as charge redistribution and localization induced by oxygen-bridge electron transfer^[8-12]. For example, Li et al. obtained Co₂MnO₄ by a hydrothermal-calcination process and evaluated it as an OER catalyst. The study indicates pronounced metal-oxygen electronic interaction and charge redistribution, with increased electron transfer from Mn to lattice O. It strengthened Mn—O bonding, rationalizing the synergistic stability/activity associated with oxygen-bridged Co/Mn—O frameworks^[11]. Wang et al. fabricated NiCo₂O₄ nanosheets on nickel foam (NF) via electrochemical co-deposition followed by thermal annealing. They demonstrate electron delocalization across the Ni—O—Co lattice and propose a covalent Ni→O→Co charge-transfer network that polarizes bridging oxygen and stabilizes the Co active center^[12-14]. Liu et al. constructed NiO/NiCo₂O₄ heterojunction nanoarrays on NF via a hydrothermal growth plus heat-treatment strategy. The results reveal interface-triggered charge transfer between the two oxide domains, which accelerates interfacial electron transport and enhances electrocatalytic kinetics^[13]. Within the realm of bimetallic oxides, nickel-cobalt spinel (NiCo₂O₄) has undergone extensive research in electrocatalysis. However, pure NiCo₂O₄ exhibits restricted charge transfer efficiency and slow reaction kinetics. This is because in a single Ni—O—Co structural unit, the bimetallic sites are typically in a thermodynamically stable state, lacking the ability for dynamic surface reconstruction or phase transition, thus inhibiting effective electron transport and leading to slow electron transfer kinetics^[14]. To break the inherent limitations on electronic and structural transport within rigid bimetallic frameworks, modification strategies including heterostructure construction^[15-17], morphology control^[18-20], and cationic doping^[21-22] have been widely employed to tailor the chemical microenvironments of Ni—O—Co units. As an efficient approach to achieve synergistic stability of Ni/Co bimetallic sites, through ion doping, the introduction of metal cations can alter the local coordination environment of Ni/Co sites, adjust the splitting of d-orbital energy levels, and thereby optimize the local structure and electronic environment^[23]. Furthermore, doped ions can regulate charge distribution through a charge compensation mechanism, affecting the oxidation states of Ni/Co (for instance, the introduction of low-valent cations may promote the conversion of some Co³⁺ to Co²⁺), which optimizes electron delocalization, enhances the covalency of M—O (M is the metal) bonds, and improves overall structural stability. Doping cations can also modify the formation energy of oxygen vacancies (O_V) by controlling O_V generation, thus balancing structural stability with reaction activity^[24]. Collectively, the relative positions of the d-band centers of Ni and Co can be adjusted by doping with metal cations, which can optimize the charge transfer behavior of Ni and Co to form a more stable electron pairing state, and then optimize the OER performance through the electron synergistic effect.

  Due to the arrangement and partial filling of 4f electrons, rare-earth metal ion doping provides unique advantages for improving the thermodynamic and electrochemical stability of NiCo₂O₄. First, the limited hybridization between the 4f orbitals of rare-earth elements and the 3d orbitals of transition metals enables the adjustment of the d-band center position in Ni/Co, thereby optimizing the intermediate adsorption energy and enhancing charge transfer efficiency during the OER process^[25]. Second, the high electronegativity of rare-earth ions can enhance the polarization of oxygen ligands, indirectly strengthening the Ni—O—Co bond, inhibiting oxygen loss, suppressing O_V aggregation, and enhancing M—O covalency^[26]. Moreover, rare-earth ions themselves can serve as Lewis acid sites, synergistically promoting the adsorption/activation of reactants with transition metal sites^[27].

  In this work, a series of rare-earth-doped NiCo₂O₄ electrocatalysts was synthesized via a two-step hydrothermal-annealing process, with La³⁺, Er³⁺, and Gd³⁺ introduced individually as dopants. These seagrass-shaped composites were directly grown on NF substrates without the use of binders. Among them, La³⁺ doped NiCo₂O₄ (La-NiCo₂O₄) was systematically investigated with varying La³⁺ doping concentrations to optimize its OER performance. The in situ grown seagrass-shaped NiCo₂O₄ composite facilitates electronic-structure regulation during electrolysis. Specifically, La³⁺ incorporation creates new chemical bonds and introduces O_V, and the resulting bond/defect modulation accelerates electron-transfer kinetics, thereby improving OER performance. Notably, the La-NiCo₂O₄-1 catalyst demonstrates outstanding OER activity, achieving an overpotential of 283 mV at a current density of 10 mA·cm^-2 in 1 mol·L^-1 KOH, along with excellent long-term operational stability exceeding 75 h at different current densities. This study illustrates a viable approach to constructing robust transition-metal electrocatalysts through lattice modulation and rare-earth incorporation, offering meaningful guidance for the rational development of high-efficiency OER catalytic materials.

  1.   Experimental

  1.1   Materials

  All reagents were of analytical grade and employed as received, without additional purification. Ni(NO₃)₂·6H₂O was purchased from Tianjin Damao Chemical Reagent Partnership Enterprise (Limited Partnership); Co(NO₃)₂·6H₂O, La(NO₃)₃·6H₂O, and urea were purchased from Aladdin Chemical Reagent Co., Ltd.

  1.2   Synthesis of La-NiCo₂O₄

  Rare-earth element-doped NiCo₂O₄ materials were synthesized via the hydrothermal-calcination method. A typical procedure of La-NiCo₂O₄-1 was described as follows: first, NF substrates (1 cm×1 cm) were precleaned by sequential ultrasonic treatment in absolute ethanol, deionized (DI) water, and a 3.0 mol·L^-1 hydrochloric acid (HCl) solution, each for 15 min, to remove surface oxides and contaminants. Subsequently, 1.25 mmol of Ni(NO₃)₂·6H₂O, 2.5 mmol of Co(NO₃)₂·6H₂O, 0.250 mmol of La(NO₃)₃·6H₂O, and 4.5 mmol of urea were dissolved in 13 mL of DI water to form a homogeneous precursor solution. The pretreated NF was placed in the as-prepared mixed solution, which was then sealed in a 20 mL Teflon-lined stainless-steel autoclave for hydrothermal treatment for 24 h at 120 ℃. After the vessel was cooled naturally to room temperature, the obtained solid was repeatedly washed with DI water to eliminate residual ionic species and dried at 45 ℃ overnight. The dried precursor was subsequently calcined in air at 350 ℃ for 2 h with a ramp rate of 5 ℃·min^-1, affording crystalline La-NiCo₂O₄-1. The La³⁺ content in the above precursor solution was adjusted to 0.125 and 0.375 mmol to synthesize comparative samples La-NiCo₂O₄-0 and La-NiCo₂O₄-2, respectively. The nominal La, Ni, and Co molar ratios are shown in Table S1. In addition, La was replaced by other rare-earth cations (Gd and Er: 0.25 mmol Gd(NO₃)₃·6H₂O and 0.25 mmol Er(NO₃)₃·6H₂O), and the same protocol was followed to obtain the corresponding control samples Gd-NiCo₂O₄ and Er-NiCo₂O₄. Without the addition of rare-earth cations, pure NiCo₂O₄ was prepared by the same procedure.

  1.3   Characterization

  Phase composition and crystallographic features of the as-prepared catalysts were examined by a powder X-ray diffractometer (Rigaku Ultima IV, 40 kV, 150 mA, Cu Kα, λ=0.154 056 nm, 2θ=10°-90°) together with Fourier-transform infrared (FTIR) spectroscopy (Nicolet iS50). Morphology and microstructural details were inspected using scanning electron microscopy (SEM, FEI Apreo S LoVac) and transmission electron microscopy (TEM, JEOL JEM-F200, 200 kV). Elemental chemical environments and valence information were analyzed by X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD). In situ Raman spectra were collected on a Horiba LabRAM HR Evolution confocal Raman microscope equipped with a Syncerity OE detector.

  1.4   Electrochemical measurements

  Electrochemical testing was conducted using a Zahner Zennium workstation in 1.0 mol·L^-1 KOH with a standard three-electrode configuration. In the electrochemical measurement system, a platinum electrode was adopted as the counter electrode, an Ag/AgCl electrode functioned as the reference electrode, and the sample immobilized on NF was employed as the working electrode. The potential (E′) related to the Ag/AgCl electrode was converted into the potential (E) related to the reversible hydrogen electrode (RHE) via E=E′+0.197+0.059pH. OER activity was examined by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). Kinetic parameters were extracted from Tafel plots based on η=a+blg j, where η is the OER overpotential, j denotes the current density, a represents the Tafel constant, and b represents the Tafel slope. Electrochemical impedance spectroscopy (EIS) was performed over 0.1-100 kHz to probe interfacial charge-transfer processes. The electrochemically active surface area (ECSA) of electrocatalysts was tested from capacitive CV responses, and long-term stability was assessed by chronoamperometry (i-t) test.

  2.   Results and discussion

  2.1   Structure analysis and morphology characterization

  The procedures for the catalyst preparation are illustrated in Scheme 1. To comprehensively understand the structural evolution induced by rare-earth element doping in NiCo₂O₄, X-ray diffraction (XRD) analysis was performed on both La-doped NiCo₂O₄ samples with varying La doping contents and NiCo₂O₄ samples doped with different rare-earth ions (La³⁺, Er³⁺, and Gd³⁺). As shown in Fig. 1a, the XRD pattern of NiCo₂O₄ exhibited well-defined diffraction peaks corresponding to the cubic spinel structure of NiCo₂O₄ (PDF No.73-1702), with major reflections located at 2θ=31.1°, 36.7°, 59.1°, and 65.0°, indexed to the (220), (311), (511), and (440) planes, respectively^[28-29]. After introducing La³⁺, the absence of impurity peaks in any of the La-containing compounds confirmed the successful formation of single-phase spinel structures and the effective incorporation of La³⁺ ions into the host lattice. Notably, a slight but consistent leftward shift in the diffraction peaks was observed. This systematic shift can be attributed to the partial substitution of smaller Ni²⁺/Co³⁺ ions by larger La³⁺ ions, which would lead to expansion of the lattice parameters. In spinel NiCo₂O₄, the spinel oxide (AB₂O₄) framework with Ni/Co mainly occupying octahedral B sites provides sufficient compositional and valence tolerance for partial substitution by La³⁺. Owing to its large ionic radius and high valence, La³⁺ can be incorporated into or near the octahedral sublattice, generating La—O—M (M=Ni, Co) units that induce lattice strain and charge redistribution^[30]. Moreover, the La-NiCo₂O₄-1 samples also displayed slight peak broadening and intensity attenuation, suggesting a reduction in crystallinity and an increase in structural disorder. As shown in Fig.S1-S3, the peak shift and broadening results could also be detected in other samples. With increasing doping concentration and the ionic radius enlarging (La³⁺ > Gd³⁺ > Er³⁺ in ionic radius), the peak shift was more significantly obvious. As a result, the lattice strain and charge redistribution might promote the creation of O_V as an efficient charge-compensation pathway to accelerate electron transfer. FTIR spectra of different catalysts were employed to probe the surface functional groups and coordination molecules (Fig. 1b). For NiCo₂O₄, the peaks at about 640 and 550 cm^-1 can be related to the M—O bond of NiCo₂O₄. After La³⁺ incorporation, the peaks of La-NiCo₂O₄-1 at approximately 640 and 550 cm^-1 exhibited slight red-shifting and broadening. This observation was consistent with the XRD results, indicating that the incorporation of La³⁺ causes lattice distortion and may promote O_V formation. The spectral signature of functional groups introduced by the introduction of La³⁺ was not observed. This absence can be attributed to the closed-shell structure of La³⁺, which requires high-energy electronic transitions. The —OH/H—O—H features were weak for both samples, implying limited surface hydroxyls/adsorbed water after heat treatment^[31]. As evidenced by the peak shifts in both techniques, the results demonstrated that the incorporation of La³⁺ preserved the spinel skeleton while tuning M—O bonding and defect states, which is expected to benefit electrochemical activity.

  Scheme 1

  

  Scheme 1.  Synthesis process of La-NiCo₂O₄

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  Figure 1

  

  Figure 1.  (a) XRD patterns and (b) FTIR spectra of NiCo₂O₄ and La-NiCo₂O₄-1; (c) SEM image, (d) TEM image (inset: the corresponding SAED pattern), (e) HRTEM image, and (f) EDS elemental mappings (Scale bar: 100 nm) of La-NiCo₂O₄-1

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  The surface morphologies of the as-prepared electrodes were systematically examined by SEM. As shown in Fig. 1c, La-NiCo₂O₄-1 grew uniformly on the NF (Fig.S4) substrate, forming a vertically aligned, seagrass-like nanoarray with a relatively loose packing density. In contrast, as shown in Fig.S5, pristine NiCo₂O₄ exhibited a denser and more disordered seagrass architecture. Notably, the incorporation of La³⁺ does not induce an obvious morphological transition at the microscale (Fig.S6 and S7), suggesting that the La³⁺ effect is primarily associated with lattice/electronic modulation rather than a drastic reshaping of the framework. For the other rare-earth doped samples, Gd-NiCo₂O₄ retained a needle-like morphology, where densely packed nanoneedles radiated outward from the NF surface (Fig.S8), while Er-NiCo₂O₄ also presented a seagrass-like structure (Fig.S9). In comparison, the more loosely packed seagrass array of La-NiCo₂O₄ is expected to expose abundant edge sites and to facilitate electrolyte permeation, thereby improving reactant accessibility and ion/electron transport, which is favorable for electrochemical reactions^[32]. TEM further confirmed the seagrass-like morphology of La-NiCo₂O₄-1 (Fig. 1d). The corresponding selected area electron diffraction (SAED) pattern displayed distinct diffraction spots that can be indexed to the spinel NiCo₂O₄ phase, further confirming the crystalline nature of La-NiCo₂O₄-1. Specifically, the SAED pattern of La-NiCo₂O₄-1 exhibited diffraction rings indexed to the cubic spinel NiCo₂O₄, with the (220), (422), and (442) planes clearly shown. Compared with the undoped NiCo₂O₄, the diffraction spots of La-doped samples were slightly shifted outward relative to those of NiCo₂O₄, which can be attributed to lattice expansion induced by La incorporation. High-resolution transmission electron microscopy (HRTEM) revealed well-resolved lattice fringes with an interplanar spacing of 0.289 nm, which can be indexed to the (220) planes of the spinel NiCo₂O₄ phase (Fig. 1e)^[33-34]. The elemental composition was evaluated by energy-dispersive X-ray spectroscopy (EDS). As summarized in Fig.S10 and Table S2, the content of La was 8.37% relative to that of Co, in good agreement with the nominal precursor ratio. Elemental mappings of Ni, Co, La, and O (Fig. 1f) indicated a spatially uniform distribution across the nanoarray. Likewise, the mapping results for Gd-NiCo₂O₄ and Er-NiCo₂O₄ (Figs.S8 and S9) confirmed the homogeneous distribution of all constituent elements within the corresponding architectures.

  2.2   XPS characterization

  The elemental composition and electronic structure of NiCo₂O₄ and La-NiCo₂O₄-1 were systematically tested by XPS. Compared with NiCo₂O₄ (Fig.S11), the survey spectra confirm the presence of Ni, Co, O, and La elements in La-NiCo₂O₄-1 (Fig.S12). In the high-resolution Ni2p spectrum (Fig. 2a), the peaks of Ni2p_3/2 and Ni2p_1/2 were detected at 854.2 and 872.1 eV, respectively, together with the detection of two shake-up satellite peaks at 861.2 and 879.1 eV, indicating the existence of Ni²⁺. At 856.1 and 873.8 eV, the peaks for Ni³⁺ were also observed^[35]. It was noted that the Ni2p envelope of La-NiCo₂O₄-1 showed a slight shift toward high binding energy relative to NiCo₂O₄. Considering the partial overlap between Ni²⁺ and Ni³⁺ components, this result was taken as semi-quantitative evidence of electronic modulation rather than definitive proof of exact valence-state shifts. The result suggests the electronic coupling between the 4f orbitals of La³⁺ and the 3d orbitals of Ni²⁺, which can adjust the d-band center of Ni²⁺, consequently accelerating the electron transfer kinetics. Similarly, the Co2p spectra (Fig. 2b) showed the coexistence of Co²⁺ and Co³⁺, with spin-orbit doublets at 779.7 eV/781.6 eV and 795.2 eV/797.1 eV, corresponding to Co³⁺ and Co²⁺, respectively^[36]. In addition, the weak shake-up satellite features appearing at 785.1 and 802.6 eV further supported the presence of Co²⁺. A positive shift of the Co³⁺2p_3/2 peak was also observed in the spectrum of La-NiCo₂O₄-1. These shifts in binding energy suggest electron redistribution resulting from the incorporation of La³⁺, leading to strong electronic interactions between La and the Ni/Co centers. The interactions can modulate the d-band structure and promote the formation of La—O—M bonds, thereby enhancing electron transfer kinetics during electrocatalysis. In the O1s spectra (Fig. 2c), the O1s peaks were deconvoluted into three peaks. The first one located at 529.4 eV can be attributed to lattice oxygen (M—O). And the peak at 531.2 eV is the characteristic peak of O_V or defective oxygen (O′_v). The peak at 533.5 eV represents the H—O—H bond of the surface-adsorbed species. Compared with pristine NiCo₂O₄, the increased peak area of defective oxygen indicates that the incorporation of La³⁺ induces lattice distortion. This distortion improves electron delocalization, which suppresses the aggregation of O_V. Consequently, it indirectly strengthens the Ni—O—Co bonds and promotes the formation of La—O—M bonds. The enhanced covalency of these La—O—M bonds subsequently facilitates electron transport. The substantial shifts in Ni2p, Co2p, and O1s spectra indirectly confirm that the incorporation of La³⁺ not only alters the electronic environment of Ni²⁺ and Co³⁺ to construct La—O—M units but also effectively induces abundant O_V, thereby enhancing electron transfer behavior and promoting the intrinsic OER catalytic performance. In addition, as shown in Figs.S13-S16, the Gd- and Er-doped samples also exhibited discernible binding-energy shifts in the XPS spectra. However, the magnitudes of these shifts were smaller than those observed for the La³⁺-doped sample. These findings indicate that the incorporation of Gd³⁺ and Er³⁺ results in weaker structural-electronic coupling compared to that induced by La³⁺, leading to a more limited enhancement in reaction kinetics and interfacial charge-transfer processes.

  Figure 2

  

  Figure 2.  (a) Ni2p, (b) Co2p, and (c) O1s XPS spectra of NiCo₂O₄ and La-NiCo₂O₄-1; (d) La3d XPS spectrum of La-NiCo₂O₄-1

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  2.3   OER activity and stability tests

  The OER electrocatalytic activities of NiCo₂O₄ and a series of La-NiCo₂O₄ samples with varying La contents were systematically evaluated using LSV in 1 mol·L^-1 KOH. As illustrated in Fig. 3a, pristine NF and NiCo₂O₄ required relatively high overpotentials of 455 and 331 mV at a current density of 10 mA·cm^-2, respectively. Notably, the La-NiCo₂O₄-1 achieved the lowest overpotential of 283 mV, indicating an optimal balance between defect engineering and electronic structure modulation. Samples with lower (La-NiCo₂O₄-0) or higher (La-NiCo₂O₄-2) doping levels showed comparatively diminished activity in Fig. 3a, which should be attributable to an insufficient number of active defect sites or excessive La-induced aggregation that blocked active surface area. Moreover, as the potential became more positive, La-NiCo₂O₄-1 exhibited a markedly steeper rise in current density than the remaining catalysts. This behavior was further corroborated by the Tafel slopes presented in Fig. 3b. La-NiCo₂O₄-1 exhibited a relatively small Tafel slope. The Tafel slopes of NF, NiCo₂O₄, La-NiCo₂O₄-0, La-NiCo₂O₄-1, and La-NiCo₂O₄-2 were 196, 139, 65, 59, and 80 mV·dec^-1, respectively. The summary of overpotential and Tafel slope is shown in Fig. 3c. The results indicated a rapid increase in current density and rapid OER kinetics of La-NiCo₂O₄-1. In addition, to explore the inherent capacity of the catalyst in alkaline media, CV curves were recorded within the non-faradaic window of 0-0.1 V (vs RHE) at scan rates ranging from 20 to 80 mV·s^-1 (Fig.S17). As shown in Fig. 3d and 3e, the double-layer capacitance (C_dl=33.93 mF·cm^-2) of La-NiCo₂O₄-1 was bigger than that of La-NiCo₂O₄-0 (31.49 mF·cm^-2) and La-NiCo₂O₄-2 (31.34 mF·cm^-2). A bigger C_dl value indicates a higher electrochemical double-layer capacitance with larger ECSA. As demonstrated in Fig. 3e and Table 1, the semicircular diameter of La-NiCo₂O₄-1 was much lower than that of La-NiCo₂O₄-0 (5.88 Ω) and La-NiCo₂O₄-2 (6.84 Ω), indicating La-NiCo₂O₄-1 possessed the fastest electron transport. Fig. 3f shows the fitting circuit diagram. To further optimize the catalytic performance, we systematically investigated the effect of rare-earth element doping concentration. Rare-earth-doped NiCo₂O₄ (Gd-NiCo₂O₄ and Er-NiCo₂O₄) were synthesized and evaluated in 1 mol·L^-1 KOH using a standard three-electrode system. In contrast, as shown in Fig.S18, all rare-earth-doped samples exhibited markedly improved OER performance, indicating the significant catalytic enhancement imparted by rare-earth ion incorporation. In terms of overpotential (in ascending order), the catalysts ranked as La-NiCo₂O₄-1 < Gd-NiCo₂O₄ < Er-NiCo₂O₄ < NiCo₂O₄, confirming the superior activity of the rare-earth-doped samples over the pristine NiCo₂O₄. Fig.S19 presents the Tafel slope, which was obtained through a linear fitting process. La-NiCo₂O₄-1 exhibited a notably low Tafel slope of 59 mV·dec^-1, while the corresponding values were 139, 88, and 129 mV·dec^-1 for NiCo₂O₄, Gd-NiCo₂O₄, and Er-NiCo₂O₄, respectively. This substantial decrease clearly signified enhanced reaction kinetics and improved electron transfer processes, suggesting that the incorporation of La³⁺ effectively reduces the OER energy barrier and accelerates water dissociation^[37]. As shown in Fig.S20 and S21, La-NiCo₂O₄-1 revealed the highest C_dl value (33.93 mF·cm^-2), followed by Gd-NiCo₂O₄ (21.38 mF·cm^-2), Er-NiCo₂O₄ (19.44 mF·cm^-2), and NiCo₂O₄ (17.11 mF·cm^-2), indicating that La-NiCo₂O₄-1 possessed the highest electrochemically active surface area. To further assess the site-normalized OER activity, the apparent turnover frequency (TOF) of La-NiCo₂O₄-1 was estimated. The apparent TOF was 0.030 6 s^-1 at 10 mA·cm^-2, indicating favorable OER kinetics. Detailed calculation procedures, assumptions, and discussions regarding the estimation of active site density were provided in the Supporting information. According to the results of the EIS analysis, La-NiCo₂O₄-1 had a lower charge transfer resistance (R_ct=2.60 Ω) than that of NF (46.91 Ω), NiCo₂O₄ (11.61 Ω), Gd-NiCo₂O₄ (6.84 Ω), and Er-NiCo₂O₄ (13.92 Ω), as shown in Fig. 3e and Fig.S22. The impedance response suggests a small interfacial charge-transfer barrier. This observation aligned well with the polarization trends obtained from LSV. Combined with XRD, TEM, and XPS analyses, the superior performance of La-NiCo₂O₄ can be mainly attributed to more efficient charge redistribution. Under the same coordination environment, La³⁺ possesses a substantially larger effective ionic radius compared to Gd³⁺ and Er³⁺, resulting in greater local size mismatch. This mismatch induces stronger lattice distortion and promotes defect formation, which in turn leads to the strongest structure-electronic coupling within this electronic environment. As a result, the electronic states of Ni/Co are optimized, facilitating faster interfacial electron transfer and ultimately accelerating OER kinetic process^[38].

  Figure 3

  

  Figure 3.  (a) LSV curves, (b) Tafel plots, (c) overpotentials and Tafel slope histograms, (d) estimation of C_dl by plotting of current densities as a function of scanning rates, and (e) Nyquist plots of the samples (inset: high frequency region magnified image); (f) Fitting circuit diagram of EIS

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  Table 1

  

  Table 1.  Fitting results of the EIS

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  Material	Rs / Ωa	Rct / Ω
  NF	1.70	46.91
  NiCo2O4	1.68	11.61
  La-NiCo2O4-1	1.57	2.60
  La-NiCo2O4-0	1.31	5.88
  La-NiCo2O4-2	1.21	6.84
  aRs: solution resistance.

  As shown in Fig. 4a, the durability of La-NiCo₂O₄-1 was evaluated in 1.0 mol·L^-1 KOH at a constant potential with an initial current density of 10 mA·cm^-2 for 42 h by an i-t test, during which only a slight decay was observed. Moreover, the multiple current steps of i-t tests showed that the current density rapidly leveled out and remained stable during every 15 h test, at constant potentials with a current density change of 50→100→150→100→50 mA·cm^-2 in Fig. 4b. The finding demonstrates that La-NiCo₂O₄-1 possessed a definite period of durability. As shown in Fig.S23, compared with the fresh electrode, the post-OER sample still showed a relatively small charge-transfer resistance, although a slight increase in R_ct was observed after the long-term test, which can be attributed to partial surface deactivation and catalyst detachment during prolonged anodic polarization. As shown in Fig. 4c, the catalyst retained approximately 87.5% of its initial activity after 42 h of OER operation, indicating moderate stability. The superior OER durability of La-NiCo₂O₄-1 is mainly attributed to the formation of robust La—O—M coordination units within the spinel framework. These stable oxygen-bridged linkages reinforce the local metal-oxygen network and provide an "electronic anchoring" effect, enabling a sustained charge redistribution that stabilizes the Ni/Co redox couple and maintains a more reliable electron-transport pathway during prolonged anodic polarization. To benchmark the OER performance of La-NiCo₂O₄-1, a comparative analysis was conducted against recently reported electrocatalysts^[39-48]. As summarized in Fig. 4d, our catalyst exhibits superior OER activity with a low overpotential at a current density of 10 mA·cm^-2, underscoring its potential for practical OER applications.

  Figure 4

  

  Figure 4.  (a) i-t curves at current densities of 10 mA·cm^-2 for 42 h, (b) multi-step i-t plots for 75 h, and (c) LSV curves before and after the 42 h OER test of La-NiCo₂O₄-1; (d) Comparison of the overpotentials of La-NiCo₂O₄-1 and reported NiCo-based catalysts toward OER

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  Beyond outstanding OER activity, durability is essential for the electrode materials. Therefore, XRD, FTIR, SEM, TEM, and XPS measurements were employed to explore the structure of La-NiCo₂O₄-1 after 75 h of OER to investigate the stability of La-NiCo₂O₄-1 in 1 mol·L^-1 KOH electrolyte under operational conditions. As shown in Fig. 5a, compared with fresh samples, the used La-NiCo₂O₄-1 showed no pronounced shift in peak positions after the OER process, whereas the diffraction intensities decreased, which can be attributed to partial catalyst detachment during electrochemical operation. Meanwhile, the observed broadening of diffraction peaks for the used La-NiCo₂O₄-1 indicates lattice expansion and the introduction of strain. This is consistent with the partial substitution of smaller octahedral Ni/Co ions by larger La³⁺ while maintaining the spinel structure. Similar behavior was reported in La/Mn-doped cobalt spinels, in which a distorted yet structurally stable lattice under OER conditions is directly associated with enhanced long-term durability^[30].

  Figure 5

  

  Figure 5.  (a) XRD patterns of La-NiCo₂O₄-1 before and after 75 h OER test; (b) SEM, (c) TEM, and (d) HRTEM images of La-NiCo₂O₄-1 after OER test

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  Compared with fresh La-NiCo₂O₄-1, in the FTIR spectra (Fig.S24), the absorption peaks corresponding to the stretching vibrations of Ni—O and Co—O remained clearly visible at 620 and 705 cm^-1 of used La-NiCo₂O₄-1, indicating that the spinel structure is well preserved during 75 h of electrochemical operation. There were weak absorptions in the range of 1 044-1 384 cm^-1, which can be attributed to A—O—B/M—O—H bridging bonds and residual nitrate or carbonate species, and the band at ca. 1 609 cm^-1 corresponding to O—H stretching vibrations was significantly enhanced, confirming higher surface hydroxyl coverage after electrochemical cycling^[31]. After the 75 h OER test, it could be found that the morphology of La-NiCo₂O₄-1 was not obviously changed (Fig. 5b-5d and S25). Besides, as seen in Fig.S26, Ni, Co, O, and La elements were still detected in the XPS spectra, further suggesting their good stability. Furthermore, compared with fresh La-NiCo₂O₄-1 in Fig. 2, the Ni2p and Co2p XPS spectra of used La-NiCo₂O₄-1 collected after 75 h of OER revealed that the relative fractions of Ni³⁺ and Co²⁺ increased markedly after the reaction. As shown in Fig.S26, the observed shifts in Ni2p and Co2p binding energies further evidenced a redistribution of electron density and an adjustment of the Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ ratios, where optimized high-valence 3d centers and controlled La-O-Ni/Co species simultaneously boosted OER activity^[26]. According to the above characterization results, La-NiCo₂O₄-1 is an efficient OER electrode material accompanied by credible durability in an alkaline environment.

  2.4   OER mechanism

  To provide direct experimental evidence for the potential-induced surface evolution of La-NiCo₂O₄-1, in situ Raman spectroscopy was performed under different applied potentials in 1.0 mol·L^-1 KOH. As shown in Fig. 6a, La-NiCo₂O₄-1 exhibited broad Raman signals in the range of 400-560 cm^-1 at open circuit potential (OCP) and low applied potentials, which can be assigned to the M—O lattice vibrations of the spinel NiCo₂O₄ framework. With increasing applied potential, the Raman response in the 450-560 cm^-1 region gradually became stronger and broader, indicating the oxidation of surface Ni/Co sites and the formation of Ni/Co oxyhydroxide-like species under anodic polarization. This potential-dependent spectral evolution demonstrates that the initial spinel surface undergoes dynamic reconstruction into an oxyhydroxide-like active layer during OER. Combining the HRTEM and XPS data, it can be concluded that La³⁺ mainly modulates the local coordination and electronic environment of Ni/Co sites rather than forming a separate La oxide phase. Thus, there was no La—O Raman band observed. The weak broad feature around 780-830 cm^-1 might be associated with high-valence M—O related vibrations. Therefore, the in situ Raman results suggest the formation of reconstructed Ni/Co oxyhydroxide-like active species, which is consistent with the proposed adsorbate evolution mechanism (AEM) pathway. Based on the in situ Raman results and the above electrochemical-structural characterizations, the OER process of La-NiCo₂O₄-1 is proposed within the conventional AEM^[49-50]. First, the induced La³⁺ endows the La-NiCo₂O₄-1 system with redistribution and defect-related oxygen environments, increasing the local polarity. The interfacial charge transfer is facilitated, thereby providing many more electronically tuned adsorption sites for oxygenated intermediates^[51]. As shown in Fig. 6b, the stable La—O—M bonding and charge transport existed in La-NiCo₂O₄-1, which were proved by the above characterizations. It can be concluded that La-containing spinel oxides can retain long-term anodic durability during which the dopant-stabilized lattice chemistry can suppress degradation under harsh OER conditions. The robustness against over-oxidation/dissolution during prolonged polarization can also be improved. Then, the La-NiCo₂O₄ undergoes dynamic surface reconstruction into an (oxy)hydroxide-like catalytic layer (Eq.1)^[14]. Second, the high-valence La-doped Ni/Co (La-Ni/Co) sites in octahedral coordination within this reconstructed surface region are the catalytically relevant centers. On the reconstructed La-Ni/Co sites, the OER typically begins via hydroxide adsorption (Eq.2, where * is the radical species)^[52]. This step generates surface-bound hydroxyl intermediates (*OH) and constitutes the first electron-transfer event. The adsorbed *OH undergoes deprotonation to yield surface oxo species (*O). This step is kinetically important because it involves strengthening the M—O bond and reorganizing the local coordination environment. An increased fraction of defect-related oxygen can tune M—O covalency and lower the energetic cost for *OH→*O conversion (Eq. 3)^[52]. Fourth, O—O coupling proceeds by nucleophilic attack of hydroxide on the surface *O. And the hydroperoxo intermediate (*OOH) is produced (Eq.4). Finally, oxygen is released, and the catalytic site is regenerated (Eq.5)^[52].

  $ \mathrm{M}^{2+}-\mathrm{O}+\mathrm{OH}^{-} \rightarrow \mathrm{M}^{3+}-\mathrm{OOH}+\mathrm{e}^{-} $

  (1)

  $ ^{*}+\mathrm{OH}^{-} \rightarrow {}^*{\rm{OH}}+\mathrm{e}^{-} $

  (2)

  $ ^{*}\mathrm{OH}+\mathrm{OH}^{-} \rightarrow {}^*{\rm{O}+\mathrm{H}_2 \mathrm{O}}+\mathrm{e}^{-} $

  (3)

  $ ^{*} \mathrm{O}+\mathrm{OH}^{-} \rightarrow {}^*{\rm{OOH}}+\mathrm{e}^{-} $

  (4)

  $ ^{*} \mathrm{OOH}+\mathrm{OH}^{-} \rightarrow ^{*}+\mathrm{O}_2+\mathrm{H}_2 \mathrm{O}+\mathrm{e}^{-} $

  (5)

  Figure 6

  

  Figure 6.  (a) In situ Raman spectra of La-NiCo₂O₄-1 under different applied potentials; (b) Proposed OER mechanism of La-NiCo₂O₄-1

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  3.   Conclusions

  In summary, the OER performance of NiCo₂O₄ electrocatalysts modified with rare-earth element dopants through a hydrothermal calcination process was systematically investigated. Among the tested rare-earth ions doped NiCo₂O₄ catalysts, La-NiCo₂O₄-1 demonstrated the highest effectiveness by delivering superior electrocatalytic activity with an overpotential of 283 mV at 10 mA·cm^-2, a low Tafel slope of 59 mV·dec^-1, and a minimal R_ct of 2.60 Ω. Furthermore, La-NiCo₂O₄ demonstrated excellent electrochemical durability, maintaining stable activity for 75 h in 1 mol·L^-1 KOH solution at different voltage levels. The excellent OER performance is attributed to the altered Ni/Co electronic configuration resulting from La³⁺ incorporation, which induces lattice distortion and facilitates charge transfer behaviour in OER. These results highlight the critical role of both dopant identity and concentration in tuning the catalytic activity of spinel oxides, offering valuable insights for the rational design of high-performance OER electrocatalysts.

  
  Acknowledgment: The work was supported by the National Natural Science Foundation of China (Grant No.22366034), the Natural Science Foundation of Jiangxi Province (Grant No.20242BAB20102), Natural Science Foundation of Tianjin City, China (Grant No.21JCYBJC00330), the National Key R&D Plan of China (Grant No.2024YFB3614400), the Natural Science Foundation of Shangrao University (Grant No.2025SRKJZB004), and Ministry of Education Industry-Academia Collaboration Project [Large-Scale Instrument Laboratory Safety Faculty Training Program (Grant No.231000287255403)]. Supporting information is available at http://www.wjhxxb.cn
  Notes: The authors declare no competing financial interest.
  
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• 

  Scheme 1  Synthesis process of La-NiCo₂O₄

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  Figure 1  (a) XRD patterns and (b) FTIR spectra of NiCo₂O₄ and La-NiCo₂O₄-1; (c) SEM image, (d) TEM image (inset: the corresponding SAED pattern), (e) HRTEM image, and (f) EDS elemental mappings (Scale bar: 100 nm) of La-NiCo₂O₄-1

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  Figure 2  (a) Ni2p, (b) Co2p, and (c) O1s XPS spectra of NiCo₂O₄ and La-NiCo₂O₄-1; (d) La3d XPS spectrum of La-NiCo₂O₄-1

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  Figure 3  (a) LSV curves, (b) Tafel plots, (c) overpotentials and Tafel slope histograms, (d) estimation of C_dl by plotting of current densities as a function of scanning rates, and (e) Nyquist plots of the samples (inset: high frequency region magnified image); (f) Fitting circuit diagram of EIS

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  Figure 4  (a) i-t curves at current densities of 10 mA·cm^-2 for 42 h, (b) multi-step i-t plots for 75 h, and (c) LSV curves before and after the 42 h OER test of La-NiCo₂O₄-1; (d) Comparison of the overpotentials of La-NiCo₂O₄-1 and reported NiCo-based catalysts toward OER

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  Figure 5  (a) XRD patterns of La-NiCo₂O₄-1 before and after 75 h OER test; (b) SEM, (c) TEM, and (d) HRTEM images of La-NiCo₂O₄-1 after OER test

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  Figure 6  (a) In situ Raman spectra of La-NiCo₂O₄-1 under different applied potentials; (b) Proposed OER mechanism of La-NiCo₂O₄-1

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  Table 1.  Fitting results of the EIS

  Material	Rs / Ωa	Rct / Ω
  NF	1.70	46.91
  NiCo2O4	1.68	11.61
  La-NiCo2O4-1	1.57	2.60
  La-NiCo2O4-0	1.31	5.88
  La-NiCo2O4-2	1.21	6.84
  aRs: solution resistance.

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