High-conductivity NiFe@Ni@Cu composite electrodes for durable and efficient industrial oxygen evolution

Abstract

The oxygen evolution reaction (OER) is a key bottleneck in water electrolysis due to its sluggish kinetics. Although nickel–iron layered double hydroxides (NiFe LDHs) are promising OER catalysts in alkaline media, their low electrical conductivity hinders industrial-scale applications. Copper mesh substrates offer excellent conductivity and low cost but suffer from poor corrosion resistance, limiting their use as electrode supports. Herein, we report a self-supported NiFe@Ni@Cu composite electrode, fabricated by sequential electrodeposition of a nickel interlayer and Ni₃Fe alloy–NiFe LDH hybrid nanosheets onto a copper mesh. This hierarchical structure synergistically combines the high conductivity of copper and the corrosion resistance of the nickel interlayer, leading to a substantial enhancement in OER activity and stability. The electrode shows a low overpotential of 283 mV at 100 mA cm⁻² and a Tafel slope of 33.4 mV dec⁻¹ in 1 M KOH. As a full water-splitting electrocatalyst, it operates stably at 500 mA cm⁻², with a 23.3% reduction in cell voltage compared to NiFe@Ni, and demonstrates outstanding durability under industrial conditions (30 wt% KOH, 80 °C) for over 80 hours. Moreover, coupled with a NiFe@Ni@Cu cathode, the integrated electrolyzer delivers 100 mA cm⁻² at 1.53 V. This substrate engineering strategy offers a scalable and cost-effective approach to efficient and durable alkaline water electrolysis.

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1. Introduction

The growing global energy demand and environmental concerns have driven the development of sustainable energy conversion and storage technologies, such as water electrolyzers and lithium–air batteries.^1,2 A key challenge in these systems is the oxygen evolution reaction (OER), which suffers from slow kinetics and high overpotentials due to its complex four-electron transfer process.^3,4 These limitations hinder the efficiency of water splitting and related electrochemical systems. As a result, the development of highly active, stable, and cost-effective electrocatalysts for the OER is crucial.⁵ While IrO₂ and RuO₂ are benchmark catalysts, their high cost and scarcity limit large-scale application. This has led to increased efforts to find earth-abundant alternatives that offer high activity, durability, and scalability for practical energy solutions.

To date, extensive research has focused on the development of novel electrocatalysts using earth-abundant 3d transition metals, with nickel–iron-based materials (such as nickel–iron (oxy)hydroxides, nickel–iron layered double hydroxides (NiFe LDHs), nickel–iron spinel oxides, and nickel–iron alloys) gaining significant attention due to their excellent catalytic performance and cost-effectiveness.^6–9 In particular, NiFe LDHs exhibit remarkable catalytic activity for the oxygen evolution reaction (OER) in alkaline media (e.g., 1 M KOH), with overpotentials typically ranging from 170 to 310 mV at a current density of 10 mA cm⁻², comparable to or even better than those of Ru-based catalysts (225–280 mV).^10–12 As such, NiFe LDHs have emerged as promising alternatives to noble metal catalysts, demonstrating substantial potential for large-scale hydrogen production in industrial water electrolysis. However, their relatively poor electrical conductivity significantly limits their practical application.¹³ To address this, recent studies have explored introducing defects, such as oxygen vacancies or cation vacancies, into the NiFe LDHs’ lattice to modulate the electronic structure and enhance conductivity.^14,15 This approach typically involves doping amphoteric metal elements into the lattice, followed by alkaline etching to create vacancy-containing structures; however, this method is complex and requires harsh conditions.¹⁶ Notably, in addition to defect engineering, the in situ growth of NiFe LDHs on conductive substrates has also proven to be an effective strategy to improve the catalytic performance.¹⁷ Studies have shown that selecting appropriate conductive substrates can significantly enhance the charge transfer dynamics at the catalyst interface, leading to a substantial improvement in the overall catalytic performance of the electrode.¹⁸

In recent years, to enhance the conductivity of catalysts, researchers have extensively explored the composite formation strategies using carbon-based materials, such as carbon nanotubes and graphene, with NiFe LDHs.^19,20 For instance, Tavar et al. reported a composite material consisting of aminated graphene and NiFe LDHs, which exhibited a 70% reduction in charge transfer resistance compared to pure NiFe LDHs and achieved an OER overpotential of only 260 mV at a current density of 10 mA cm⁻².²¹ While carbon-based materials possess excellent conductivity, they are prone to structural degradation under the highly oxidative electrochemical conditions, which significantly compromises the long-term stability of the electrodes and limits their application in industrial-scale environments.²² In contrast, metal substrates such as nickel foam and nickel mesh, owing to their superior corrosion resistance, are widely used as current collectors in practical applications.^23,24 However, in large-scale industrial systems, the relatively high series resistance of nickel-based materials, compared to copper substrates, may lead to localized voltage drops, thus increasing the overpotential required for the oxygen evolution reaction.²⁵ Additionally, the higher cost of nickel compared to copper represents a significant economic limitation for its large-scale use.

Binder-free electrocatalyst systems constructed in situ on copper-based conductive substrates have attracted increasing attention.²⁶ For example, Dhandapani et al. successfully fabricated a NiFe-LDH/CuS/Cu heterostructured electrode on copper foam via a two-step hydrothermal method, which exhibited excellent OER performance with a low overpotential of 249 mV at a current density of 50 mA cm⁻² and a small Tafel slope of 81.84 mV dec⁻¹.²⁷ However, copper substrates are prone to oxidative corrosion under industrial alkaline water electrolysis conditions (20–30 wt% KOH, 60–80 °C), which significantly deteriorates their conductivity and long-term stability.²⁸ At present, studies addressing the stability issues of copper-based electrodes under high-temperature and high-alkalinity conditions remain limited, hindering their practical deployment in large-scale water-splitting systems. Therefore, the rational design of composite substrates that combine the high conductivity of copper with the corrosion resistance of nickel holds great promise for enabling the efficient and durable operation of NiFe LDH-based OER electrocatalysts under industrial conditions.

In this work, we introduce a scalable substrate-engineering strategy to construct a self-supported NiFe@Ni@Cu hierarchical electrode for alkaline water electrolysis. A dense Ni interlayer was first electrochemically deposited onto a copper mesh to form a robust Ni@Cu composite substrate. Then, a subsequent one-step cathodic electrodeposition was performed to grow Ni₃Fe alloy–NiFe LDH nanosheets into an integrated array. The resulting NiFe@Ni@Cu electrode delivers markedly enhanced catalytic activity at large current densities, outperforming its NiFe@Ni counterpart. Crucially, the Ni interlayer imparts corrosion protection to the Cu mesh, allowing stable operation for over 80 h under industrial conditions (30 wt% KOH, 80 °C) without performance decay. This binder-free, hierarchical design combines high conductivity and durability, offering a cost-effective path toward industrial-scale alkaline electrolyzers.

2. Experimental section

2.1. Materials

Nickel(II) chloride hexahydrate (NiCl₂·6H₂O), nickel(II) sulfate heptahydrate (NiSO₄·7H₂O, ≥98.5%), ferrous sulfate heptahydrate (FeSO₄·7H₂O, ≥99%), boric acid (H₃BO₃, ≥99.5%), hydrochloric acid (HCl, 36–38%), potassium hydroxide (KOH, ≥85%), saccharin (C₇H₅NO₃S, ≥98%), 5-sulfosalicylic acid dihydrate (C₇H₁₀O₈S, ≥99%), and sodium dodecyl sulfate (C₁₂H₂₅SO₄Na, ≥97%) were all purchased from Shanghai Titan Scientific Co., Ltd. All reagents were of analytical grade and used as received without further purification. Nickel and copper meshes (200 mesh, wire diameter: 0.05 mm) were obtained from Kunshan Shengzhao New Materials Co., Ltd. Prior to use, the substrates were ultrasonically cleaned in 3 M HCl solution followed by absolute ethanol for 5 minutes to remove surface contaminants such as grease and oxides. Cleaned substrates were then stored in sealed containers filled with absolute ethanol for later use.

2.2. Catalyst preparation

2.2.1. Preparation of Ni coating. Cleaned copper mesh was used as the cathode and nickel was used as the anode. A brief pre-nickel plating was first conducted in an acidic nickel electrolyte for 10 seconds. Subsequently, nickel was electrodeposited in a pure nickel electrolyte at 55 °C under a current density of 2 A dm⁻² for 120 s to obtain the Ni@Cu substrate. The acidic nickel electrolyte consisted of 250 g L⁻¹ of nickel sulfate and 80 mL L⁻¹ of hydrochloric acid. The pure nickel electrolyte was composed of 250 g L⁻¹ of nickel sulfate, 30 g L⁻¹ of nickel chloride, 30 g L⁻¹ of boric acid, and 0.01 g L⁻¹ of sodium dodecyl sulfate. Ni@Cu (1 cm × 100 cm) was fabricated using a roll-plating setup with a custom hanging fixture.

2.2.2. Preparation of NiFe coating. Electrodeposition of NiFe layers was performed on 1 cm × 100 cm Cu, Ni, and Ni@Cu substrates, which served as cathodes, with a Pt sheet as the anode. Fig. 1 illustrates the schematic of NiFe deposition (left) and electrochemical testing (right) on different 1 cm × 100 cm substrates. A 1 cm × 1 cm NiFe catalyst was electrodeposited under identical conditions at the front end of each substrate. During electrodeposition, the electrical contact was made 3–4 cm above the solution level, whereas during testing, the contact point was shifted to the 100 cm end while maintaining the same area for measurement. Electrodeposition was carried out in a NiFe electrolyte at a current density of 20 A dm⁻² for 120 s, yielding NiFe@Cu, NiFe@Ni, and NiFe@Ni@Cu samples, respectively. The electrolyte consisted of 43.66 g L⁻¹ of NiSO₄·6H₂O, 1.33 g L⁻¹ of anhydrous citric acid, 18.46 g L⁻¹ of FeSO₄·7H₂O, and 40 g L⁻¹ of KCl. For the preparation of NiFe@Cu, NiFe@Ni, and NiFe@Ni@Cu samples, the bottom 1 cm × 1 cm region of the 1 cm × 100 cm Cu, Ni, and Ni@Cu substrates was immersed in the electrolyte for deposition. The corresponding sample images are shown in the SI (Fig. S1).

Fig. 1 Schematic diagram of catalyst preparation (left). Schematic diagram of the oxygen evolution performance test (right).

2.3. Material characterization

The crystal structure of the samples was characterized by X-ray diffraction (XRD) using a Cu Kα radiation source (40 kV, 140 mA) at a scanning rate of 10° min⁻¹, measured using a Rigaku Ultima IV diffractometer. The surface morphology and elemental composition were analyzed using field-emission scanning electron microscopy (FE-SEM, ZEISS Sigma 300) coupled with energy-dispersive X-ray spectroscopy (EDS, ZEISS Sigma 300). The chemical states of the samples were investigated by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific) with Al Kα radiation as the excitation source. The electrical conductivity (σ) of the substrate was approximately calculated using the equation σ = L/(R·A), where L is the length of the substrate, A is the cross-sectional area (product of the substrate thickness and width), and R is the electrical resistance (direct measurement using a resistance meter).

2.4. Electrochemical measurements

Electrochemical measurements were performed using a three-electrode system on an Ivium V53522 workstation (Ivium Technologies, The Netherlands). For the oxygen evolution reaction (OER) tests, the prepared electrode (with a 1 cm × 1 cm catalytic layer immersed in electrolyte) served as the working electrode, while a platinum foil (1 cm × 1 cm) and a Hg/HgO electrode were used as the counter and reference electrodes, respectively. The electrolyte was 1 M KOH. All measured potentials were converted to the reversible hydrogen electrode (RHE) scale using the equation: E_RHE = E_Hg/HgO + 0.0592 × pH + 0.098. Stability tests were conducted in 30 wt% KOH at 80 °C. Linear sweep voltammetry (LSV) and Tafel plots were obtained at a scan rate of 5 mV s⁻¹ with 85% iR compensation. The double-layer capacitance (C_dl) was estimated from cyclic voltammetry (CV) measurements conducted at scan rates of 20, 40, 60, 80, and 100 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was carried out in the frequency range of 0.01 Hz to 10⁵ Hz with an AC amplitude of ±10 mV around the open-circuit potential. The collected EIS data were fitted using ZView software. The corrosion resistance of the substrate was evaluated using potentiodynamic polarization curves, following a prior open-circuit potential (OCP) stabilization process. The potential range for polarization measurements was set from −1.0 V to 1.0 V.

3. Results and discussion

3.1. Corrosion resistance of different substrates

In industrial alkaline electrolysis environments, electrodes are required to operate stably for extended periods in 30 wt% KOH solution at 80 °C, posing stringent demands on their corrosion resistance. To evaluate this property, we tested the corrosion behavior of Ni@Cu, Cu, and Ni electrodes under these harsh conditions. The potentiodynamic polarization curves are presented in Fig. 2a. The corrosion current density (I_corr) and corrosion potential (E_corr) were derived using Tafel extrapolation and are summarized in the accompanying table.²⁹ According to Table S1, Ni@Cu exhibits a corrosion current density of 8.855 × 10⁻⁶ A cm⁻², which is comparable to that of Ni (8.785 × 10⁻⁷ A cm⁻²) and significantly lower than that of Cu (1.034 × 10⁻⁴ A cm⁻²). Moreover, the corrosion potentials of Ni@Cu (0.0133 V) and Ni (−0.0313 V) are considerably more positive than that of Cu (−0.48 V), indicating a notable improvement in corrosion resistance. These results confirm that the nickel coating on the copper surface substantially enhances its corrosion resistance, rendering it comparable to that of pure nickel.

Fig. 2 (a) Polarization curves obtained in 30 wt% KOH electrolytes. (b) Stability performance of the electrode tested at 2 V in 30 wt% KOH at 80 °C. (c) XRD patterns of Ni@Cu before and after stability testing.

To evaluate the durability of Ni@Cu and Cu electrodes in a harsh industrial water splitting environment, both were used as anodes in an electrolyzer system with a Pt foil as the cathode, operated in 30 wt% KOH at 80 °C under a constant voltage of 2 V. The current–time profiles are shown in Fig. 2b. The Cu electrode dissolved completely within 4 hours, with visible copper deposition on the Pt cathode (inset of Fig. 2b). In contrast, the Ni@Cu electrode maintained stable performance for over 80 hours without any signs of dissolution (Fig. S2). Further characterization of the post-reaction Ni@Cu electrode was performed to assess morphological and structural stability. SEM images (Fig. S3a) confirmed that the nickel layer remained uniformly distributed across the copper mesh surface, with no significant morphological changes observed after the durability test. Energy-dispersive X-ray spectroscopy (EDS) (Fig. S3b) revealed a high Ni content (∼98 at%), which was nearly identical to that before the durability test, indicating strong adhesion between the Ni layer and the Cu substrate. Importantly, almost no delamination was observed under the rigorous electrolysis conditions. Additionally, X-ray diffraction (XRD) patterns before and after the reaction remained nearly unchanged (Fig. 2c), further demonstrating structural stability. These results collectively confirm that the nickel coating provides effective protection to the copper substrate, preventing corrosion in high-temperature, concentrated alkaline environments, and significantly enhancing the operational lifespan of the Cu-based electrode under industrial electrolysis conditions.

3.2. Morphological and structural characterization of NiFe coatings on different substrates

X-ray diffraction (XRD) analysis was conducted to characterize the phase composition of the electrodeposited coatings. Fig. 3a presents the XRD patterns of the Ni@Cu, NiFe@Cu, NiFe@Ni, and NiFe@Ni@Cu electrodes. For Ni@Cu, three diffraction peaks were observed at 43.3°, 50.6°, and 74.0°, corresponding to the (111), (200), and (220) planes of metallic Cu (PDF#85-1326),³⁰ respectively. In addition, diffraction peaks at 44.8°, 52.2°, and 76.5° were assigned to the (111), (200), and (220) planes of metallic Ni (JCPDS#87-0712).³¹ In the case of NiFe@Cu, besides the Cu-related peaks, additional peaks appeared at 44.3°, 51.6°, and 76.0°, which can be indexed to the (111), (200), and (220) planes of the Ni₃Fe alloy (PDF#38-0419). Upon magnification of the NiFe@Cu XRD pattern, a series of weak diffraction peaks were detected at 11.5°, 23.3°, 34.6°, 39.0°, 46.4°, 60.3°, 61.2°, and 65.2°, corresponding to the (003), (006), (012), (015), (018), (110), (113), and (116) planes of the NiFe layered double hydroxide (NiFe LDHs, PDF#51-0463).³² The presence of these weak peaks may be attributed to the low crystallinity of NiFe LDHs. Due to the close proximity of the diffraction angles for Ni and Ni₃Fe,³³ peak overlap occurs in the XRD patterns. For example, the broad peak at around 44° observed in both NiFe@Ni and NiFe@Ni@Cu can be deconvoluted into contributions from both Ni₃Fe and Ni (Fig. 3b). Additionally, weak peaks corresponding to NiFe LDHs were also detected in the XRD patterns of NiFe@Ni and NiFe@Ni@Cu. Due to the strong interference from the metallic substrate, which overshadowed the catalyst coating peaks, we prepared powder samples by scraping the catalyst coating from the substrate for XRD characterization (Fig. S4). Compared to the substrate-containing electrodes, the powder samples exhibit significantly enhanced peak intensities for both Ni₃Fe and NiFe-LDH phases. These results demonstrate that the NiFe coatings electrodeposited on different substrates primarily consist of Ni₃Fe alloy and NiFe LDHs.

Fig. 3 (a) The XRD patterns of Ni@Cu, NiFe@Cu, NiFe@Ni, and NiFe@Ni@Cu. (b) The locally magnified XRD patterns of NiFe@Cu, NiFe@Ni, and NiFe@Ni@Cu.

To investigate the morphology of NiFe coatings on different substrates, scanning electron microscopy (SEM) analysis was performed, as shown in Fig. 4. The bare copper mesh exhibits a relatively smooth surface (Fig. S5a and b). As shown in the SEM images, the low-magnification image of Ni@Cu reveals a surface morphology that is nearly identical to that of the pristine copper mesh (Fig. 4a). Higher-magnification SEM images indicate that the coating consists of densely packed nanoparticles (Fig. 4b). Cross-sectional metallographic images of Ni@Cu reveal that the nickel coating is uniformly adhered to the copper surface, with a thickness of approximately 1–1.5 µm (Fig. S5c). Energy-dispersive X-ray spectroscopy (EDS) analysis of Ni@Cu shows that the atomic composition of the coating consists of Ni (99.0 at%), Cu (0.8 at%), and O (0.2 at%), indicating a uniform Ni layer over the copper substrate (Fig. S6).

Fig. 4 SEM images of (a) and (b) Ni@Cu and (c) NiFe@Ni@Cu. (d) EDS elemental mapping images of O, Ni, and Fe for NiFe@Ni@Cu.

The morphology of NiFe@Ni@Cu (Fig. 4c) reveals vertically aligned nanosheets uniformly grown on the Ni@Cu substrate. These nanosheets overlap with one another, forming a porous network structure that facilitates electrolyte infiltration and oxygen release, thereby enhancing the oxygen evolution reaction (OER) activity.³⁴ Elemental mapping (Fig. 4d) confirms the presence and uniform distribution of Ni, Fe, and O elements in the sample. Additionally, the morphologies of NiFe@Cu (Fig. S7a) and NiFe@Ni (Fig. S7b) are consistent with that of NiFe@Ni@Cu.

To further investigate the surface composition and chemical valence states of the NiFe coatings, X-ray photoelectron spectroscopy (XPS) analysis was conducted. The wide-scan XPS spectra of NiFe@Cu, NiFe@Ni, and NiFe@Ni@Cu confirm the presence of Ni, Fe, and O elements (Fig. S8), consistent with the energy-dispersive X-ray spectroscopy (EDS) results. As shown in Fig. 5a, the high-resolution Ni 2p spectrum exhibits peaks at 853.0 eV and 868.6 eV, corresponding to Ni 2p_3/2 and Ni 2p_1/2 of metallic Ni species,³⁵ while peaks at 856.3 eV and 874.0 eV are assigned to Ni 2p_3/2 and Ni 2p_1/2 of Ni²⁺.³⁶ The corresponding satellite peaks are observed at 862.1 eV and 880.1 eV. Fig. 5b presents the high-resolution Fe 2p spectrum, with peaks at 706.8 eV and 719.9 eV attributed to Fe 2p_3/2 and Fe 2p_1/2 of metallic Fe.³⁷ Deconvoluted peaks at 711.6 eV and 724.3 eV, along with their corresponding satellite peaks, are associated with Fe 2p_3/2 and Fe 2p_1/2 of Fe³⁺.³⁸ In the O 1s spectrum (Fig. 5c), the peak at 529.9 eV corresponds to metal–oxygen bonds, the peak at 531.7 eV is related to oxygen in hydroxides, and the 533.0 eV peak is assigned to chemisorbed water molecules.³⁹ Notably, the XPS spectra of NiFe@Cu and NiFe@Ni closely resemble that of NiFe@Ni@Cu, further confirming that NiFe coatings with similar chemical states were successfully formed on different substrates via electrodeposition (Fig. S9), in agreement with the XRD results.

Fig. 5 High-resolution XPS spectra of (a) Ni 2p, (b) Fe 2p, and (c) O 1s.

3.3. OER performance of NiFe coatings on different substrates

The oxygen evolution reaction (OER) performance of NiFe@Ni@Cu, NiFe@Cu, and NiFe@Ni electrodes was evaluated using a standard three-electrode system in 1 M KOH at 25 °C. To determine the optimal alloy/LDH composition, we systematically investigated the effect of different Ni²⁺/Fe²⁺ molar ratios (1 : 1, 2 : 1, 3 : 1 and 4 : 1) on OER performance (Fig. S4). Our results demonstrated that a Ni²⁺/Fe²⁺ molar ratio of 3 : 1 yielded the highest OER activity (Fig. S10). This superior performance can be attributed to the higher content of catalytically active NiFe LDH in the 3 : 1 sample. XRD analysis (Fig. S11) revealed that the 3 : 1 sample displayed the most intense diffraction peaks corresponding to NiFe LDH, confirming its enriched active phase content. This optimized ratio was consistently employed throughout all subsequent investigations to ensure the best catalytic performance of the coating. The LSV curves in Fig. 6a reveal two distinct operational regimes. At low current densities (<100 mA cm⁻²), all three electrodes exhibit similar kinetic behavior. At 10 mA cm⁻², the overpotentials are comparable: 208 mV (NiFe@Ni@Cu), 210 mV (NiFe@Cu), and 213 mV (NiFe@Ni). The corresponding Tafel slopes are also closely matched at 33–40 mV dec⁻¹ (Fig. 6b). This similarity reflects the dominant role of the NiFe coating's intrinsic properties in the kinetically controlled regime. However, under industrially relevant conditions (≥100 mA cm⁻²), NiFe@Ni@Cu demonstrates significantly enhanced OER activity. At 100 mA cm⁻², NiFe@Ni@Cu achieves an overpotential of 283 mV, outperforming NiFe@Cu (292 mV) and NiFe@Ni (312 mV) by 9 mV and 29 mV, respectively. This superior performance at high current densities can be attributed to the efficient electron transport facilitated by the Cu substrate, which becomes critical when mass transport limitations begin to dominate. Compared to recently reported NiFe-based catalysts, the OER performance of the NiFe@Ni@Cu is superior to that of most of the reported catalysts (Table S2). The electrochemically active surface area (ECSA) was estimated by measuring the double-layer capacitance (C_dl) via cyclic voltammetry (CV),⁴⁰ as shown in Fig. S12. Typically, a higher C_dl value suggests more electrochemically active sites. The C_dl value for NiFe@Ni@Cu is 22.91 mF cm⁻², which is comparable to those of NiFe@Cu (21.76 mF cm⁻²) and NiFe@Ni (19.34 mF cm⁻²), as shown in Fig. 6c.

Fig. 6 (a) LSV curves, (b) Tafel plots, (c) C_dl values, (d) EIS plots, and (e) constant current of NiFe@Cu, NiFe@Ni, and NiFe@Ni@Cu.

Electrochemical impedance spectroscopy (EIS) was further conducted to evaluate the charge transport behavior and conductivity of the electrodes (Fig. 6d). An equivalent circuit model, shown in the inset of Fig. 6d, was used to fit the impedance spectra. In this model, R_s represents the series resistance, CPE is a constant phase element, and R_ct corresponds to the charge transfer resistance.⁴¹R_s primarily reflects the resistance of the solution, conductive substrate, electrode, and connecting wires.⁴² The fitted R_s values indicate substrate-dependent conductivity: 1.416 Ω for NiFe@Ni@Cu and 1.423 Ω for NiFe@Cu, both approximately 50% lower than that of NiFe@Ni (2.103 Ω). This disparity underscores the superior electrical conductivity of copper-based substrates over nickel, which mitigates ohmic losses and facilitates efficient charge transport—particularly advantageous at elevated current densities where IR drops can exacerbate overpotentials. Direct current (DC) resistance measurements corroborated these findings, revealing conductivities of 7.35 × 10⁶ S m⁻¹ for Cu and 6.79 × 10⁶ S m⁻¹ for Ni@Cu (approximately 4.7 times higher than that of Ni at 1.568 × 10⁶ S m⁻¹). In contrast, the R_ct values are comparable across the electrodes: 0.303 Ω for NiFe@Ni@Cu, 0.339 Ω for NiFe@Cu, and 0.323 Ω for NiFe@Ni. This similarity aligns with the fact that R_ct is primarily governed by the electrode/electrolyte interface characteristics,⁴³ and the NiFe coatings were synthesized under identical deposition and electrochemical conditions on all substrates. Consequently, the exceptional electrocatalytic activity of NiFe@Ni@Cu stems from the high conductivity of the Cu substrate, which minimizes ohmic losses.

In industrial alkaline electrolyzers, current densities are operated at several hundred mA cm².⁴⁴ In these demanding high-current-density regimes, the electrical conductivity of the substrate plays a pivotal role in mitigating ohmic losses, thereby directly impacting the overall energy efficiency of the water-splitting process. To elucidate this substrate-dependent effect, we systematically evaluated the oxygen evolution reaction (OER) performance of NiFe@Ni@Cu, NiFe@Cu, and NiFe@Ni at a constant current density of 500 mA cm⁻² using chronopotentiometry in a two-electrode full-cell setup. As illustrated in Fig. 6e, the cell voltages required to sustain 500 mA cm⁻² followed the order: NiFe@Ni@Cu (4.35 V) < NiFe@Cu (4.6 V on average) < NiFe@Ni (5.37 V). Notably, the NiFe@Ni electrode demanded a 23.44% higher cell voltage compared to NiFe@Ni@Cu, which can be primarily attributed to the inferior electrical conductivity of the Ni substrate relative to Cu. Furthermore, long-term stability assessments revealed substrate-specific degradation behaviors. The NiFe@Cu electrode exhibited a gradual voltage increase from 4.38 V to 4.63 V after 30 h of operation. In contrast, the NiFe@Ni@Cu and NiFe@Ni electrodes maintained stable voltages with no discernible escalation over the same timeframe, underscoring their superior durability. The observed differences in oxygen evolution activity among various substrates can be primarily attributed to the intrinsic properties of the substrates themselves, as XPS, EDS, and XRD analyses confirm that the deposited NiFe catalysts exhibit identical chemical compositions across all substrates.

In addition, the application of NiFe@Ni@Cu under industrial conditions (30 wt% KOH, 80 °C) for long-term oxygen evolution reactions showed stable performance for more than 80 hours (Fig. S13); this stability in the NiFe@Ni@Cu configuration likely stems from the synergistic effects of the Cu core's high conductivity and the Ni interlayer's protective role against oxidation, highlighting its promise for scalable, high-performance alkaline water electrolysis applications.

To validate the structural and compositional integrity of the catalyst after long-term operation, we performed comprehensive characterization of NiFe@Ni@Cu following the OER stability test (Fig. S14). SEM images (Fig. S14a) reveal that the interconnected nanosheet morphology is well preserved. XRD patterns (Fig. S14b) show diffraction peaks attributable to Ni₃Fe alloy and NiFe LDH phases with no additional peaks, confirming excellent structural stability. XPS analysis of the Ni 2p_3/2 region confirms the emergence of Ni³⁺ species, which can be attributed to the formation of NiOOH during the OER (Fig. S14c). Correspondingly, the Fe 2p_3/2 spectra exhibit a positive shift in binding energy (Fig. S14d), indicating an increased proportion of Fe³⁺ species. These observations collectively suggest that NiFe LDH evolves toward higher-valence metal species under OER conditions, in agreement with previous reports.³³ These results collectively demonstrate that NiFe@Ni@Cu maintains robust structural, morphological, and chemical stability under prolonged OER conditions.

3.4. HER and overall water splitting performance

Inspired by the remarkable HER activity of the reported NiFe alloy–NiFe-LDH hybrid,³³ we investigated the HER performance of NiFe@Ni@Cu to evaluate its bifunctional electrocatalytic capability toward overall water splitting. As shown in Fig. 7a, NiFe@Ni@Cu requires overpotentials of only 80 mV and 229 mV to achieve current densities of 10 mA cm⁻² and 100 mA cm⁻², respectively, significantly outperforming commercial Pt foil (η₁₀ = 111 mV, η₁₀₀ = 275 mV). Tafel slope analysis reveals a value of 60.89 mV dec⁻¹ for NiFe@Ni@Cu (Fig. 7b), substantially lower than that of Pt foil (87.09 mV dec⁻¹), indicating faster HER kinetics. For overall water splitting (Fig. 7c), a two-electrode configuration employing NiFe@Ni@Cu as both the anode and cathode NiFe@Ni@Cu(−)||NiFe@Ni@Cu(+) requires only 1.53 V to deliver 100 mA cm⁻², outperforming most reported bifunctional catalysts.^45–48 These results demonstrate the excellent bifunctional electrocatalytic activity of NiFe@Ni@Cu for practical water electrolysis applications.

Fig. 7 (a) LSV curves and (b) Tafel plots of NiFe@Ni@Cu and Pt foil. (c) The LSV curve of overall water-splitting for electrolyzer.

4. Conclusions

In summary, this study presents a high-conductivity, corrosion-resistant NiFe@Ni@Cu composite electrode fabricated through sequential electrodeposition of a dense Ni interlayer and Ni₃Fe–NiFe LDH hybrid nanosheet arrays on a Cu mesh. This electrode demonstrates enhanced OER performance in 1 M KOH, achieving an overpotential of 283 mV at 100 mA cm⁻² and a Tafel slope of 33.4 mV dec⁻¹. At high current densities (500 mA cm⁻²), it exhibits a 23.3% reduction in cell voltage compared to NiFe@Ni, attributable to the enhanced conductivity of the copper core that mitigates ohmic losses. Long-term stability tests under industrial water electrolysis conditions (30 wt% KOH at 80 °C) confirm robust operation for over 80 h, enabled by the protective barrier of the nickel coating against corrosion. The NiFe@Ni@Cu cathode showcases superior HER performance with low overpotentials (η₁₀ = 80 mV, η₁₀₀ = 229 mV). Notably, the bifunctional NiFe@Ni@Cu(−)||NiFe@Ni@Cu(+) electrode pair enables exceptionally efficient overall water splitting at a cell voltage of merely 1.53 V for 100 mA cm⁻². In the development of electrolytic water-splitting catalysts, long-term stability is of paramount importance. While selecting highly active catalysts is essential, the stability and conductivity of the substrate must also be carefully considered; this binder-free, hierarchical substrate-engineering approach effectively integrates the superior conductivity of copper with the corrosion resistance of nickel, providing a scalable and cost-effective pathway for industrial alkaline electrolyzers and guiding future advancements in sustainable hydrogen production.

Author contributions

Yequan Zhu: experimental method exploration, validation, data collation, data integration, and writing – manuscript; Xiaoman Zheng: investigation, data collation, and visualization; Shuo Ming: investigation; Huaizi Li: investigation; Xinya Han: investigation; Yu Wang: supervision and writing – review and editing; Huiying Li: scientific theory support, conceptualization, methodology, and formal analysis; Zhenwei Wang: resources, supervision, project management, and funding.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data generated or analyzed during this study are included in this published article (and its supplementary information (SI)). The supplementary information contains 14 supplementary figures (digital photographs, SEM/EDS analysis, XRD patterns, XPS spectra, electrochemical measurements, and stability test) and 2 supplementary tables (Tafel fitting results and OER electrocatalyst comparisons). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nj03318g.

The outcome data supporting this study can be requested from the corresponding authors; please note that the original data have not been processed in any way. Most of the icons in this article will involve the conversion of the original data, instead of using raw data directly.

Acknowledgements

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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