Multimetallic Layered Double Hydroxides as OER Catalysts for High-Performance Water Electrolysis

Abstract

Water electrolysis represents a viable and scalable green hydrogen production technology, which mitigates carbon emissions and contributes to environmental sustainability. Transition metal-based layered double hydroxides (LDHs) exhibit excellent oxygen evolution reaction (OER) efficiency, attributed to their adjustable interlayer spacing combined with abundant active sites. Here, we report a uniform multimetallic catalyst, demonstrating robust and efficient OER performance for high-performance water splitting. SEM and TEM confirmed its ultrathin hierarchical nanosheet structure. The characteristic peaks of LDH in XRD and Raman spectra further verified the successful synthesis of the LDH material. Fe-CoZn LDH delivers exceptional OER performance in 1 M KOH, requiring overpotentials of just 209, 238, and 267 mV to reach 10, 100, and 400 mA cm⁻², respectively. The catalyst also demonstrates exceptional hydrogen evolution reaction (HER) performance, achieving 10 mA cm⁻² at 119 mV. It also has excellent stability, with stable operation for up to 100 h under 100 mA cm⁻² in 1 M KOH electrolyte solution.

1. Introduction

Against the backdrop of global energy transition, advancing clean energy technologies has become an urgent scientific imperative, driven by both the non-renewable nature and depletion of fossil fuel reserves, as well as their associated climate change and pollution challenges. Advancing clean and efficient energy technologies is critical to addressing energy crises and achieving low-carbon development. Recently, hydrogen production from electrolytic water has attracted a lot of attention as an economical, clean, efficient and long-lasting method of energy production [1,2,3]. It is crucial for researchers to design innovative and unique materials for efficient OER and to optimise the reaction process [4]. Transition metal-based LDHs have been extensively investigated as highly efficient electrocatalysts for the OER [5,6,7]. Due to their exceptional electrocatalytic properties, transition metal-containing LDHs represent a top-tier choice for scalable and affordable OER applications [8].

LDHs exhibit unique structural tunability, including the adjustable composition and ratio of metal cations in the layers, as well as replaceable intercalated anions, controllable crystal size and morphology, and tunable interlayer spacing. The large surface area provides numerous catalytic sites, significantly enhancing catalytic efficiency. Moreover, since most LDHs in OER catalysts are Fe-, Co-, and Ni-based transition metals, which are abundant and widely available, they offer significant cost advantages. Thus, LDHs exhibit outstanding performance and promising potential in catalytic applications [9,10,11,12]. Owing to the distinctive structure of LDHs, their OER performance can be modulated via metal cation incorporation, and interlayer ion exchange [13,14]. The strategic introduction of cationic dopants into transition metal LDHs significantly boosts their electrocatalytic performance. Beyond creating new active sites, these dopants tailor the host’s electronic properties, leading to optimized binding strengths for OER intermediates [15]. Incorporating metal ions with varying electronegativities represents an effective strategy to activate Ni/Fe-LDHs and boost their OER activity. Recent studies demonstrate that improving electrical conductivity to promote charge transfer and tailoring nanostructures to maximize active site exposure represent effective strategies for enhancing OER kinetics [16,17,18]. Similar catalysts, as exemplified by W-doped NiFe LDH [19] or Fe-doped CoNi LDH [20], have been reported, indicating that transition metal incorporation promotes charge transfer kinetics, consequently improving OER activity.

In the present work, a homogeneous multimetallic NiFe-CoZn LDH catalyst was synthesized by transforming CoZn-based metal–organic frameworks (MOFs), which demonstrates outstanding OER catalytic performance in alkaline electrolytes. The bimetallic co-doped LDH was obtained via an etching and co-deposition strategy, reconstructing the original MOF’s large sheet-like array into ultrathin hierarchical LDH nanosheet arrays. This tailored microstructure promotes extensive exposure of active sites while enabling efficient mass transfer via optimized electrolyte diffusion. The phase composition and electronic structure of the synthesized catalysts were characterized by XRD, Raman spectroscopy, and XPS. Their electrocatalytic performance for the OER, HER, and overall water splitting was systematically evaluated, along with an assessment of their long-term stability.

2. Material Synthesis

Initially, NF pieces (2 × 4 cm²) were immersed in 3 M HCl during sonication lasting 8 min, after which they were rinsed using ethanol followed by deionized water (12 min). Two precursor solutions were then prepared: one comprising 32 mL of 0.04 M Co(NO₃)₂·6H₂O and 0.01 M Zn(NO₃)₂·6H₂O, and the second solution prepared with 32 mL of 0.4 M 2-methylimidazole (C₄H₆N₂). These solutions were rapidly combined, and the pre-treated NF was immediately soaked in the resulting mixture. After incubation at 25 °C for 4 h, the NF was retrieved, rinsed using DI water, then dried under vacuum at 60 °C for a duration of 12 h. In the subsequent step, the CoZn MOF-coated NF (1 cm × 2 cm) was immersed in a mixed solution of 4 mL containing 0.21 M Ni(NO₃)₂·6H₂O and 0.09 M FeSO₄·7H₂O for 1 h. The product was then washed using DI water and air-dried.

3. Results and Discussion

3.1. Structural and Morphological Characterisation

Firstly, nickel foam-supported CoZn-MOF nanosheet arrays were fabricated through a solution-based approach, exhibiting vertical alignment [21]. CoZn MOFs were grown on NF with high density and uniformity, as confirmed by the nanosheet array morphology in (Figure 1a), and the magnified SEM micrograph demonstrates the smooth surface of MOF nanosheets, with a thickness of about 150 nm. After immersion in a Ni²⁺ and Fe³⁺ solution at room temperature, numerous ultrathin nanosheets with lengths of approximately 8 μm and thicknesses around 150 nanometers were directly constructed on the surface of CoZn MOF nanosheets, resulting in a hierarchical nanosheet architecture (Figure 1b,c). Protons generated from Ni²⁺/Fe²⁺ hydrolysis etched the CoZn MOF, releasing Co²⁺ and Zn²⁺ ions during this process. These liberated ions co-precipitated with Ni²⁺ and Fe²⁺ to form NiFe LDH and CoZn LDH, which subsequently integrated with the remaining organic framework of the MOF, resulting in the hierarchical nanosheet structure [22,23]. Transmission electron microscopy (TEM) characterization (Figure 1d) provides definitive evidence of the hierarchical nanosheet architecture, revealing the uniform vertical alignment of LDH nanosheets on MOF microsheet substrates. High-resolution diffraction rings belonging to the CoZn LDH (−332) plane and NiFe LDH (012) plane are shown by selective area electron diffraction (SAED) in (Figure 1e). HRTEM analysis showed clear lattice fringes corresponding to d-spacings of 0.152 nm (CoZn LDH, (−332) plane) and 0.259 nm (NiFe LDH, (012) plane), as seen in (Figure 1f) [24]. Elemental mapping further confirmed the homogeneous distribution of all constituent elements (Figure 1g).

To identify the precise crystalline phases of the catalysts, we performed XRD on various samples. The XRD diffractograms of the CoZn MOF presented in (Figure 2a) demonstrate display characteristic diffraction peaks in the range of 5 to 45 degrees, which can be attributed to the zeolitic imidazolate framework structure (ZIF-67), along with two prominent peaks originating from the underlying Ni substrate [25]. The XRD pattern exhibited two characteristic peaks at 11.5° and 39.3°, corresponding to the (003) and (012) planes of the nickel hydroxide phase (JCPDS No. 38-0715). As evidenced by the Raman spectra (Figure 2b), the LDH material is successfully generated, as representative stretching modes belonging to LDH were detected for this sample at 456, 533, and 1050 cm⁻¹ [26]. Then we employed XPS analysis to probe the chemical state and electronic structure of the catalyst. In the Ni 2p region (Figure 2c), the peaks located at 856.12 eV and 873.71 eV are assigned to Ni³⁺ 2p_3/2 and 2p_1/2 states, respectively. The binding energy at 854.77 eV is attributed to Ni²⁺ 2p_3/2 in Ni-O, which has oscillating satellite features at 862.63 and 880.34 eV (denoted by “Sat”). Consequently, Ni exists in a composite chemical state of Ni³⁺ and Ni²⁺ [27]. The XPS analysis of NiFe-CoZn LDH reveals distinct oxidation states for each metal: Fe³⁺ is identified by its characteristic Fe 2p_3/2 (713.33 eV) and Fe 2p_1/2 (727.13 eV) peaks with satellite features at 862.07 and 879.94 eV (Figure 2d), while Co 2p spectra show mixed Co²⁺/Co³⁺ states through primary peaks at 782.28 eV (Co 2p_3/2) and 797.67 eV (Co 2p_1/2) and satellite peaks at 786.61 and 804.2 eV (Figure 2e). Additionally, Zn 2p peaks at 1021.97 eV (Zn 2p_3/2) and 1044.94 eV (Zn 2p_1/2) confirm the presence of Zn-O species (Figure 2f) [28,29].

3.2. Electrochemical Properties of Alkaline Electrolyte Solutions

The OER and HER catalytic activity of NiFe-CoZn LDH was evaluated in 1.0 M KOH using LSV. As demonstrated in (Figure 3a,b), the catalyst delivers outstanding performance, achieving remarkably low overpotentials of 209, 238, and 267 mV at 10, 100, and 400 mA cm⁻², respectively, all of which are significantly lower than those of the CoZn MOF counterpart (302 mV, 379 mV, and 425 mV, respectively). The overpotential at 100 mA⁻² compares favourably with recently reported layered hydroxide catalysts, and NiFe-CoZn LDH outperforms the majority of reported bifunctional electrocatalysts in terms of electrochemical activity (Figure 3e) [30,31,32]. The NiFe-CoZn LDH displays a reduced Tafel slope (36.8 mV dec⁻¹) relative to CoZn MOFs (77.2 mV dec⁻¹), indicating enhanced reaction kinetics (Figure 3c). EIS measurements show that the R_ct value of NiFe-CoZn LDH is very small at 0.43 Ω, much lower than that of CoZn MOFs at 8.8 Ω. It is well established that Tafel slopes and charge transfer resistance (Rct) are closely associated with electron and mass transport, and catalytic kinetics. The enhanced OER kinetics observed here can be attributed to the layered nanosheet architecture and the self-supported nature of the NiFe-CoZn LDH arrays, which collectively help to minimize contact resistance (Figure 3d). Owing to their favorable electrical conductivity, the hierarchical ultrathin nanosheets function as effective charge transport channels, which significantly enhances electron transfer rates [33,34].

The NiFe-CoZn LDH exhibited exceptional OER stability under 100 mA cm⁻², demonstrating only a 32 mV overpotential increase after 100 h of continuous testing (Figure 4a). The remarkable stability can be attributed to the structural stability of the LDH introduced by etching, and the morphology of the samples remained essentially unchanged after the stability tests, still maintaining the hierarchical nanostructure, and can be confirmed in the post-test SEM images (Figure 4b).

NiFe-CoZn LDH showed outstanding catalytic performance, achieving overpotentials of 119, 251, and 285 mV at 10, 50, and 100 mA cm⁻², respectively (Figure 5a), significantly lower than those of CoZn MOFs (198, 282, and 332 mV at corresponding current densities (Figure 5b)). To assess the HER kinetics, we performed a corresponding linear fit to the Tafel plot (Figure 5c). NiFe-CoZn LDH displays a lower Tafel slope (105.6 mV dec⁻¹) than CoZn MOFs (162.5 mV dec⁻¹), indicating more favorable HER kinetics and a faster reaction rate. EIS characterizations were performed to investigate the charge transfer characteristics. As shown in the Nyquist plots (Figure 5d), all catalysts display clear semicircular arcs, representing interfacial charge transfer resistance (Rct). The Rct value for NiFe-CoZn LDH is notably low at 8.2 Ω, compared to 18.6 Ω for CoZn MOFs. This reduced resistance suggests enhanced electron transfer efficiency and improved catalytic kinetics, consistent with the Tafel slope analysis, which corroborates the superior charge transport capability and catalytic activity [35].

An alkaline water electrolyzer with NiFe-CoZn LDH catalyst in 1 M KOH was assembled and tested for overall water splitting. As evident from the chronopotentiometric data (Figure 6a), the cell demonstrated exceptional operational stability over 50 h, exhibiting minimal voltage fluctuation with only 3.9% performance decay. Post-stability LSV analysis confirms the electrode’s stable performance, sustaining 100 mA cm⁻² at 1.64 V with negligible degradation (Figure 6b).

3.3. Investigation of Electrocatalytic Performance and Stability

To evaluate the catalytic enhancement of NiFe-CoZn LDH, CV was conducted at scan rates ranging from 20 to 100 mV s⁻¹ (Figure 7a,b), enabling the determination of Cdl and ECSA. The current density variations were correlated with scan rate and analyzed via linear regression (Figure 7c,d). NiFe-CoZn LDH exhibited a significantly higher double-layer capacitance (2.90 mF cm⁻²) compared to CoZn MOFs (0.55 mF cm⁻²), confirming its larger ECSA thereby consequently more exposed OER-active sites.

3.4. Advantages over Conventional Electrodes

Compared to conventional precious metal catalysts (e.g., Pt/C, IrO₂), our Ni/Fe-CoZn LDH significantly reduces material costs by utilizing earth-abundant transition metals, while maintaining competitive bifunctional activity (OER: 209 mV; HER: 119 mV @ 10 mA cm⁻²). Unlike traditional systems that require physically separated HER and OER catalysts, our single-phase design eliminates interfacial incompatibility issues while maintaining performance comparable to benchmark Pt/C-IrO₂ couples in a simplified two-electrode configuration. This synergy of cost-effectiveness and integrated functionality addresses critical bottlenecks in scalable water electrolysis. Notably, our catalyst exhibits a lower overpotential and Tafel slope than most LDH-based benchmarks (

Table 1. Comparison of the catalytic performance between the as-prepared catalyst and previously reported LDH-based catalysts.

Electrocatalyst	Tafel Slope	Overpotential at 10 mA cm−2	References
Co@NiFe-LDH	44 mV dec−1	253 mV	[36]
Co3O4@NiFe-LDH/NF	40.4 mV dec−1	215 mV	[37]
Co-NiFe LDH	38.4 mV dec−1	254 mV	[38]
CoP@NiCo-LDH	46.6 mV dec−1	225 mV	[39]
FeCo-LDH@Co(OH)2	75.8 mV dec−1	230 mV	[40]
def-Ru-NiFe LDH/NCO	52 mV dec−1	225 mV	[41]
NiFe@LDH	76 mV dec−1	260 mV	[42]
MoS2/NiFeCr LDH	61 mV dec−1	224 mV	[43]
This work	36.8 mV dec−1	209 mV

), demonstrating superior kinetics. Moreover, our catalyst demonstrates excellent reproducibility and scalability. Three catalysts were synthesized under identical preparation conditions and their OER performance was evaluated. As shown in (Figures S1–S3), the LSV curves of the three samples exhibit nearly identical OER activity, with an overpotential of 208 (±2) mV at 10 mA cm⁻². A large-area catalyst was also fabricated using this method (Figure S4). A 1 cm² sample demonstrated an OER overpotential of 210 mV at 10 mA cm⁻² (Figure S5), confirming the catalyst’s excellent reproducibility and scalability.

4. Conclusions

This study demonstrates that ultrathin Ni/Fe-CoZn LDH nanosheets serve as an exceptional bifunctional electrocatalyst exhibiting both superior activity and stability toward overall water splitting, achieving exceptional performance with 209 mV (OER) and 119 mV (HER) at 10 mA cm⁻², surpassing most reported noble-metal-free catalysts to date. The improved catalytic performance arises from the cooperative interaction of Ni/Fe co-doping, which optimizes electronic structure for favorable reaction intermediate adsorption, and the hierarchical ultrathin nanosheet morphology, facilitating maximal exposure of active sites while enabling efficient charge transfer and mass transport. Remarkably, the catalyst demonstrates exceptional stability, sustaining 100 h of continuous electrolysis at 100 mA cm⁻² with minimal degradation. In a two-electrode system, the electrolyzer delivers an ultralow cell voltage of 1.64 V at 100 mA cm⁻², highlighting its potential for industrial-scale hydrogen production. This work advances the design of cost-effective, high-performance water-splitting catalysts and provides critical insights into achieving both high activity and stability under practical operating conditions. Despite the promising performance, further efforts are needed to advance the practical application of Ni/Fe-CoZn LDH.

(1)

Long-term stability tests under industrial operating conditions (current density > 500 mA cm⁻²) are necessary to assess practical viability.

(2)

The as-prepared catalyst will be integrated into an anion exchange membrane water electrolyzer (AEMWE) for industrial-scale overall water splitting tests under high current densities.