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Non-metallic element modulation of Co₃O₄/NiFe-LDH hetero-junction electrocatalysts for efficient oxygen evolution reaction†

Dipeng Sun, Yongqi Xu, Lijie Zang, Fengshuo Yu, Dong Zhao and Xiao Lyu*
School of Materials Science and Engineering, Shenyang Ligong University, Shenyang 110159, China. E-mail: lyux@sylu.edu.cn

First published on 10th July 2025

The OER, as a critical half-reaction in electrochemical water splitting, remains a bottleneck due to its sluggish four-electron transfer kinetics and high overpotential.^7–9 While noble metal-based catalysts (e.g., IrO₂ and RuO₂) exhibit superior activity, their scarcity and prohibitive costs hinder their large-scale application.^10,11 Transition metal oxides and layered double hydroxides (LDHs), such as Co₃O₄ and NiFe-LDH, have emerged as promising alternatives owing to their tunable electronic structures, abundant reserves, and cost-effectiveness. However, the intrinsic limitations of single-phase catalysts, such as poor conductivity in NiFe-LDH and insufficient active sites in Co₃O₄, often result in compromised OER performance under industrial current densities.^12,13 To address these challenges, heterostructure engineering has been widely adopted not only to synergize the advantages of different components and enhance interfacial charge transfer, but also to modulate the band structures and improve intrinsic catalytic activity.^14–16

Recent studies highlight that heterojunctions like Co₃O₄@NiFe-LDH can integrate the structural robustness of Co₃O₄ with the high intrinsic activity of NiFe-LDH, enhancing both mass transport and electronic conductivity. Nevertheless, the performance of such heterostructures is still constrained by inefficient interfacial charge redistribution and limited active-site exposure. By elemental doping, particularly with non-metallic elements like boron (B), the distinctive electronegativity and reduced atomic radius of B atoms enable lattice distortion within Co₃O₄ and the creation of electron-deficient sites, thereby driving charge redistribution across heterointerfaces. For instance, B doping in transition metal oxides induces lattice strain, optimizes d-band centers, and facilitates electron transfer, thereby improving catalytic kinetics. In NiFe-LDH systems, heteroatom incorporation (e.g., Mn or Ir) has been shown to enhance the built-in electric field at interfaces, accelerating reaction intermediates’ adsorption/desorption.^11,14 However, the existing B-doped electrocatalysts are characterized by complicated synthesis processes, high energy consumption, poor reproducibility and possible dissolution of B atoms due to changes in the coordination environment during the reconfiguration process.¹⁷

In this work, we propose a facile in situ boron doping strategy to synthesize B-doped Co₃O₄@NiFe-LDH (B-Co₃O₄@NiFe-LDH) electrocatalysts to boost the OER performance for Co₃O₄@NiFe-LDH. The B-doped Co₃O₄ was first prepared via an in situ diffusion method, followed by secondary hydrothermal growth of NiFe-LDH to construct heterojunctions (Fig. 1(a)). In the in situ diffusion approach, an amorphous Ni–B alloy was initially deposited uniformly on the NF substrate via a chemical deposition method. X-ray diffraction (XRD) analysis confirmed the formation of an amorphous Ni–B alloy on the NF substrate. As shown in Fig. S1 (ESI†), the broadened (111) diffraction peak of nickel indicates the amorphous nature of the Ni–B alloy. During the following hydrothermal method, elemental B will diffuse from the unstable amorphous alloy to participate in the growth of Co₃O₄, which induces the local disorder for the spinel Co₃O₄ structure. Thus, it facilitates the formation of abundant oxygen vacancies for spinel Co₃O₄, and the lattice distortion and formation of electron-deficient sites within Co₃O₄ induce charge redistribution across the heterointerface, which would enhance its OER performance.^18–20

	Fig. 1 (a) The process diagram for the preparation of B-Co3O4@NiFe-LDH/NF. SEM images of (b) Co3O4@NiFe-LDH/NF, and (c) B-Co3O4@NiFe-LDH/NF. (d) The corresponding XRD patterns. (e) TEM image of B-Co3O4@NiFe-LDH/NF. (f) HRTEM image of B-Co3O4@NiFe-LDH/NF. (g) GPA of B-Co3O4@NiFe-LDH/NF. (h) HAADF-STEM image and corresponding element mapping images of B-Co3O4@NiFe-LDH/NF.

The morphological features of Co₃O₄, B-Co₃O₄, Co₃O₄@NiFe-LDH, and B-Co₃O₄@NiFe-LDH were systematically investigated by SEM. As shown in Fig. S1 (ESI†), Co₃O₄/NF and B-Co₃O₄/NF exhibit a sea urchin-like structure with radially aligned nanopins. Fig. 1(b) reveals the agglomeration of NiFe-LDH nanospheres on the Co₃O₄ nanoneedles for Co₃O₄@NiFe-LDH/NF. Remarkably, boron incorporation induces the homogeneous and uniform growth of NiFe-LDH nanosheets across the urchin-like backbone for B-Co₃O₄@NiFe-LDH/NF (Fig. 1(c)), which could create a hierarchical heterostructure that provides abundant accessible active sites to facilitate electrolyte infiltration and gas diffusion for the OER. Energy-dispersive X-ray spectroscopy (EDS) analysis of B-Co₃O₄ and B-Co₃O₄@NiFe-LDH confirms successful boron doping, as illustrated in Fig. S3 and S4 (ESI†). Elemental mapping further reveals the homogeneous distribution of B across the heterostructure, with no phase segregation observed.

The crystallographic structure of the catalyst was analyzed by XRD and transmission electron microscopy (TEM). As evidenced in Fig. 1(c), the distinct diffraction patterns align with Co₃O₄ (PDF#43-1003) and NiFe-LDH (PDF#51-0463) phases, unambiguously confirming the successful construction of a heterointerface. The XRD pattern (Fig. 1(c)) shows NF substrate peaks at 44.68° (111), 52.07° (200), and 76.74° (220). Co₃O₄ peaks appear at 36.9° (311), 59.3° (511), and 65.2° (440) (JCPDS 43-1003), while NiFe-LDH peaks at 34.5° (012), 39° (015), and 60.3° (0015) (JCPDS 51-0463*), confirming that both phases coexist.^21,22 As shown in Fig. S5 (ESI†), both B-Co₃O₄/NF and B-Co₃O₄@NiFe-LDH/NF exhibit a distinct shift of the (311) and (440) crystallographic planes toward lower 2θ angles compared to their undoped counterparts (Co₃O₄/NF and Co₃O₄@NiFe-LDH/NF), indicating that B doping induces lattice expansion in the Co₃O₄ structure.

TEM images clearly reveal that B-Co₃O₄@NiFe-LDH exhibits a core–shell structure with a micron acicular core surrounded by a lamellar shell, confirming the presence of a heterojunction (Fig. 1(e)). The HRTEM revealed the finer lattice structure of B-Co₃O₄@NiFe-LDH/NF (Fig. 1(f)). The spacing between the continuous lattice fringes in the nanowire core is measured to be ∼0.205 nm, coinciding with the (400) crystal plane of Co₃O₄. The distances between the clear neighbouring lattice fringes in the nanoshell region are determined to be 0.260 nm, respectively, for the NiFe-LDH planes of (012). The obvious heterointerfaces between the core Co₃O₄ wire and NiFe-LDH shell are highlighted by the red dashed line, indicating successful construction of the heterojunction. The lattice matching and interfacial strain characteristics were further investigated. As shown in Fig. S6 (ESI†), HRTEM analysis revealed well-defined lattice fringes corresponding to the (400) planes of B-Co₃O₄ (measured spacing: 0.210 nm) and the (110) planes of NiFe-LDH (measured spacing: 0.202 nm) at the heterojunction interface of the nanowire cores. The lattice mismatch was calculated to be 3.8% based on standard crystallographic parameter comparisons, indicating exceptional compatibility. This minimal mismatch demonstrates near-identical lattice parameters, atomic arrangements, and orientation relationships across the interface. Consequently, the interface exhibits few dislocations or defects, accompanied by low interfacial energy—characteristics of a stable interfacial configuration. As shown in Fig. 1(g), complementary interfacial stress analysis confirms this interpretation: the high degree of lattice coherence generates only elastic strain energy (attributable to minor lattice constant differences), with no evidence of stress concentration from dislocation-related defects. In Fig. 1(h), a homogeneous distribution of B, O, Fe, Co and Ni elements of the B-Co₃O₄@NiFe-LDH/NF is presented by the EDX elemental mapping images, suggesting successful B doping into Co₃O₄@NiFe-LDH/NF.

To prove the presence of electron transfer, X-ray photoelectron spectroscopy (XPS) was performed. The chemical states of B-Co₃O₄@NiFe-LDH/NF were investigated by XPS in comparison with that of Co₃O₄/NF, B-Co₃O₄/NF and Co₃O₄@NiFe-LDH/NF. Fig. S7 (ESI†) shows the full spectrum images of B-Co₃O₄@NiFe-LDH/NF and Co₃O₄@NiFe-LDH/NF. For Co 2p, the XPS spectra reveal two main peaks, which are assigned to Co 2p_3/2 and Co 2p_1/2 respectively, the complete spectra of the remaining samples are shown in Fig. S8 (ESI†). The binding energies at 782.6 eV of Co 2p_3/2 and 797.15 eV of Co 2p_1/2 are indicative of Co²⁺, while those at 780.98 eV of Co 2p_3/2 and 796 eV of Co 2p_1/2 are indicative of Co³⁺(Fig. 2(a)). In Fig. 2(b), the Fe 2p spectra (Fig. 2(c)) showed characteristic peaks at 713.62 eV (Fe³⁺ 2p_3/2) and 726.42 eV (Fe³⁺ 2p_1/2), 709.38 eV (Fe²⁺ 2p_3/2) and 722.18 eV (Fe²⁺ 2p_1/2), which are in agreement with the standard fitting protocols for Fe-based oxides.²³ In Fig. 2(d), for Ni species, two spin bimodal peaks in B-Co₃O₄@NiFe-LDH/NF can be divided into Ni²⁺ at 873.5/855.57 eV and Ni³⁺ at 878.76/857.6 eV, accompanied by distinct satellite peaks.²⁴ The modified electronic structure of Co and Fe is due to the high electron-donating ability of B, which changes the electron distribution.²⁵ The XPS analysis reveals that B predominantly exists in an oxidized state, as evidenced by the characteristic binding energy peaks at 191.73 eV in the high-resolution spectrum (Fig. 2(d)).²⁶ EDS mapping (Fig. 1(h)) and XPS (Fig. 2(d)) show homogeneous B distribution across the heterostructure, but the absence of B–O–Fe/Ni bonding in the XPS spectra (Fig. 2(b)–(d)) suggests negligible boron incorporation into the NiFe-LDH lattice. In Fig. 2(e), the fitting results demonstrate that the O1s peak can mainly be divided into three components, which are ascribed to lattice oxygen (529.38 eV), and OH⁻ groups (531.05 eV),²⁷ respectively. Notably, the increased intensity of the B-Co₃O₄@NiFe-LDH/NF peak at 531.05 eV in comparison to Co₃O₄@NiFe-LDH/NF indicates an abundance of hypoxic coordination sites within the material.^28–30 As shown in Fig. S9b (ESI†), it is demonstrated that B doping into Co₃O₄ generates abundant oxygen vacancies. To directly probe oxygen vacancies, EPR spectroscopy was performed. As shown in Fig. 2(f), B-Co₃O₄@NiFe-LDH/NF exhibits a prominent symmetric signal at g = 2.003, characteristic of unpaired electrons localized at oxygen vacancies.³¹ In contrast, Co₃O₄@NiFe-LDH/NF shows only negligible noise, providing conclusive evidence for B doping-induced oxygen vacancies.

	Fig. 2 XPS spectra of (a) Co 2p, (b) Fe 2p, (c) Ni 2p, (d) B 1s and (e) O 1s in B-Co3O4@NiFe-LDH/NF and Co3O4@NiFe-LDH/NF, and (f) ESR of B-Co3O4@NiFe-LDH/NF and Co3O4@NiFe-LDH/NF.

To elucidate the impact of B doping on the OER performance, a series of electrochemical measurements were performed on B-Co₃O₄@NiFe-LDH/NF, Co₃O₄@NiFe-LDH/NF, and commercial IrO₂ electrocatalysts via a three-electrode system in 1 M KOH. All electrochemical tests were activated prior to testing (Fig. S10, ESI†). Fig. 3(a) shows that B-Co₃O₄@NiFe-LDH/NF requires the lowest overpotential at 10 mA cm⁻², with even better performance at higher current densities. The optimal boron deposition time was found to be 30 min (Fig. S14, ESI†). The results showed that boron doping improved the OER performance of the Co₃O₄@NiFe-LDH electrocatalysts. At 10 mA cm⁻², B-Co₃O₄@NiFe-LDH/NF shows a low overpotential of 115 mV, outperforming Co₃O₄@NiFe-LDH/NF (140 mV) and IrO₂ (320 mV) (Fig. 3(b)). B-Co₃O₄@NiFe-LDH/NF shows superior OER activity, evidenced by its lowest Tafel slope (53.6 mV dec⁻¹ vs. 160.2 and 215.8 mV dec⁻¹ for Co₃O₄@NiFe-LDH/NF and IrO₂, respectively) indicating faster kinetics (Fig. 3(c)). CV measurements (Fig. S11, ESI†) further reveal its double layer capacitance. In Fig. 3(d), the C_dl values of B-Co₃O₄@NiFe-LDH/NF, Co₃O₄@NiFe-LDH/NF and IrO₂ are 39.1, 30.3, 19.8 mF cm⁻², respectively. It is evident that the C_dl values of B-Co₃O₄@NiFe-LDH/NF are larger than those of Co₃O₄@NiFe-LDH/NF and IrO₂, indicating their larger active area. EIS analysis (Fig. 3(e)) reveals that B-Co₃O₄@NiFe-LDH/NF exhibits the lowest charge-transfer resistance, demonstrating enhanced conductivity and faster kinetics. This confirms that boron doping effectively improves both charge transfer and OER performance. The boron content significantly influences activity (Fig. S12–S14, ESI†), with B-Co₃O₄@NiFe-LDH/NF showing superior OER performance compared to other catalysts (Fig. 3(g)).^14,32–39 In addition, the stability of B-Co₃O₄@NiFe-LDH/NF was evaluated by testing its multi-current step curve at a current density range of 10–250 mA cm⁻². As shown in Fig. 3(f), when the current density increases from 10 to 250 mA cm⁻² in increments of 50 mA cm⁻², the potential increases sharply and remains stable for 2 h. As shown in Fig. S15 (ESI†), the excellent electrochemical stability of B-Co₃O₄@NiFe-LDH/NF is well illustrated by the fact that the current density remains stable after 40 h of stable operation at a current density of about 10 mA cm⁻².

	Fig. 3 (a) LSV curves of B-Co3O4@NiFe-LDH/NF, Co3O4@NiFe-LDH/NF and IrO2. (b) Overpotential of the catalysts at a current density of 10 and 50 mA cm−2. (c) Tafel slopes. (d) Double-layer capacitance. (e) EIS chart (inset: equivalent circuit). (f) Multi-current step curve of B-Co3O4@NiFe-LDH/NF at a current density of 10–250 mA cm−2. (g) Comparison of overpotential and Tafel slope of B-Co3O4@NiFe-LDH/NF with other electrocatalysts at 50 mA cm−2 current density.

The water splitting performance of B-Co₃O₄@NiFe-LDH/NF was evaluated in an anion exchange membrane (AEM) electrolyzer at a flow rate of 50 mL min⁻¹ in this work. Fig. 4(a) shows that B-Co₃O₄@NiFe-LDH/NF achieves higher current density than IrO₂‖Pt/Ti. The system maintains stable potential during 12 h operation (Fig. 4(b)) and shows nearly identical post-test LSV profiles (Fig. 4(c)), confirming exceptional stability in AEM electrolysis. Building on its excellent OER performance, B-Co₃O₄@NiFe-LDH/NF was paired with Pt/Ti for water splitting. The system achieved 1.57 V at 10 mA cm⁻² (Fig. 4(d)), outperforming the IrO₂‖Pt/Ti benchmark (1.75 V). A symmetric B-Co₃O₄@NiFe-LDH/NF electrolyzer showed ideal water splitting performance in Hoffman apparatus tests (Fig. 4(e) and (f)), producing H₂/O₂ at the theoretical 2:1 ratio with ∼100% faradaic efficiency.

	Fig. 4 (a) LSV curves comparing B-Co3O4@NiFe-LDH/NF‖Pt/Ti and IrO2‖Pt/Ti AEM electrolyzers. (b) 12 h stability test at 10 mA cm−2 (inset: photos and pre/post-test LSV). (c) Pre/post-test LSV comparison. (d) Overall water splitting performance. (e) Gas collection data. (f) H2/O2 production rates at 10 mA cm−2 (Hoffman apparatus shown).

The overpotential of B-Co₃O₄@NiFe-LDH/NF in 1 M KOH electrolyte was 115 mV at 10 mA cm⁻² with a Tafel slope of 53.6 mV dec⁻¹. The addition of B caused lattice distortions and the creation of electron-deficient sites within Co₃O₄, which in turn drove the redistribution of charge at the heterointerfaces, and thus, exhibits significantly enhanced OER activity. This study provides a favorable solution for the design of high-performance Co₃O₄@NiFe-LDH heterostructured OER catalysts, addressing the inefficient interfacial charge redistribution and limited active site exposure of the original heterostructured OER catalysts.

Conflicts of interest

Data availability

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