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Article

Hierarchical Core-Shell Cu@Cu-Ni-Co Alloy Electrocatalyst for Efficient Hydrogen Evolution in Alkaline Media

by
Hussein A. Younus
1,*,
Maimouna Al Hinai
1,
Mohammed Al Abri
1,2 and
Rashid Al-Hajri
2,*
1
Nanotechnology Research Centre, Sultan Qaboos University, Al-Khoudh, P.O. Box 17, Muscat 123, Oman
2
Department of Chemical and Petroleum Engineering, College of Engineering, Sultan Qaboos University, Al Khould, P.O. Box 33, Muscat 123, Oman
*
Authors to whom correspondence should be addressed.
Energies 2025, 18(6), 1515; https://doi.org/10.3390/en18061515
Submission received: 12 February 2025 / Revised: 10 March 2025 / Accepted: 17 March 2025 / Published: 19 March 2025
(This article belongs to the Special Issue Renewable Fuels and Chemicals)
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Abstract

:
The development of advanced electrocatalysts plays a pivotal role in enhancing hydrogen production through water electrolysis. In this study, we employed a two-step electrodeposition method to fabricate a 3D porous Cu-Co-Ni alloy with superior catalytic properties and long-term stability for hydrogen evolution reaction (HER). The resulting trimetallic alloy, Cu@Cu-Ni-Co, demonstrated significant improvements in structural integrity and catalytic performance. A comparative analysis of electrocatalysts, including Cu, Cu@Ni-Co, and Cu@Cu-Ni-Co, revealed that Cu@Cu-Ni-Co achieved the best results in alkaline media. Electrochemical tests conducted in 1.0 M NaOH showed that Cu@Cu-Ni-Co reached a current density of 10 mA cm−2 at a low overpotential of 125 mV, along with a low Tafel slope of 79.1 mV dec−1. The catalyst showed exceptional durability, retaining ~95% of its initial current density after 120 h of continuous operation at high current densities. Structural analysis confirmed that the enhanced catalytic performance arises from the synergistic interaction between Cu, Ni, and Co within the well-integrated trimetallic framework. This integration results in a large electrochemical active surface area (ECSA) of 380 cm2 and a low charge transfer resistance (15.76 Ω), facilitating efficient electron transfer and promoting superior HER activity. These findings position Cu@Cu-Ni-Co as a highly efficient and stable electrocatalyst for alkaline HER in alkaline conditions.

1. Introduction

Among various hydrogen production methods, water electrolysis stands out as the most promising and mature technology, capable of generating hydrogen without relying on fossil inputs or producing harmful by-products. It has achieved significant advancements at the commercial scale, demonstrating its viability for sustainable hydrogen production [1]. However, for large-scale implementation, hydrogen production efficiency is influenced by several factors, including electrocatalyst activity, electrolyte composition, electrode architecture, cell design, operating temperature, pressure, and system integration with renewable energy sources [2,3]. As a key part of the electrolyzer, the electrode engineering and optimizing the electrocatalyst composition and structure are crucial for improving not only the stability and lifespan of the electrolyzers, but it directly impacts the electricity consumption during the electrolysis process, which is the main parameter affecting the hydrogen price. For instance, a slight decrease in the catalyst overpotential by 200 mV would reduce the electricity consumption by ~10% and lead to a 5–7.5% reduction in the total hydrogen production cost, considering that electricity is the dominant contributor to the overall production expenses. Consequently, developing electrocatalysts with lower overpotentials, high durability, and enhanced mass transport properties is essential to minimizing energy losses and driving down the cost of green hydrogen production, ultimately making it more competitive with conventional fossil-based hydrogen sources [4,5,6,7].
Typically, the water-splitting process involves two essential half-reactions: the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. Extensive research is underway to optimize electrode compositions on both sides, aiming to achieve high catalytic activity, lower operational potential for high current densities, and long-term durability for thousands of hours. While Pt is widely regarded as the benchmark HER catalyst in acidic media [8,9], its stability under prolonged operation remains a concern due to gradual dissolution at high current densities [10]. Additionally, surface poisoning by adsorbed hydroxyl species in alkaline media further reduces its HER efficiency [11,12,13,14]. The sluggish Volmer step in alkaline conditions also limits its overall activity, making it less effective than in acidic environments [15]. Similarly, IrO2 and RuO2 are among the most active OER catalysts, but their high cost, poor long-term stability, and susceptibility to oxidative degradation in alkaline environments pose significant barriers to commercialization [16,17,18,19]. At high voltages, these catalysts tend to undergo dissolution and surface restructuring, further affecting their durability [20,21,22]. Additionally, in alkaline OER, the formation of peroxide species can compete with oxygen evolution, reducing overall efficiency [23,24]. Thus, despite their outstanding catalytic performance, noble metal-based catalysts (Pt for HER and IrO2/RuO2 for OER) remain limited by high costs, scarcity, and instability at industrially relevant current densities, restricting their large-scale implementation.
To address these challenges, research efforts have increasingly focused on earth-abundant transition metal-based catalysts such as Ni, Co, Fe, Cu, and Mn-based materials, which offer promising activity, stability, and cost-effectiveness [25,26,27,28,29,30,31,32,33]. Various strategies have been explored to enhance the catalytic activity, stability, and efficiency of electrocatalysts for HER and OER. These include composition tuning, structural and morphological engineering, defect modulation, phase control, interface engineering, strain engineering, electronic structure modulation, as well as the incorporation of high-entropy materials (HEMs), metal-support interactions (MSI), and single-atom catalysts (SACs). These modifications collectively optimize electronic properties, charge transport, active site exposure, adsorption energy, and corrosion resistance, thereby improving overall catalytic efficiency and durability [34,35,36,37,38,39,40,41]. While these methods have shown promise, alloying has emerged as a particularly powerful approach for developing high-performance electrocatalysts [42,43,44,45]. Unlike monometallic catalysts, alloys allow for fine-tuning of adsorption properties, optimizing the balance between reactant binding and product desorption. Additionally, alloying introduces new active sites that facilitate charge transfer and reduce overpotential, leading to superior catalytic performance. Furthermore, structural reinforcement and self-healing properties in certain alloy systems enable long-term durability under harsh electrochemical conditions [46,47,48,49,50].
In addition to their compositional versatility, multimetallic catalysts exhibit a range of physical and electronic properties that significantly enhance electrocatalytic performance. The incorporation of multiple metals modifies the electronic structure, tunes the d-band center, and optimizes reaction intermediate binding energies, leading to more efficient hydrogen and oxygen evolution reactions [51,52]. Additionally, engineered nanostructures, such as porous or hierarchical architectures, increase active surface area, improve electron conductivity, and enhance mass and charge transport, collectively boosting catalytic activity and durability. Unlike monometallic catalysts, multimetallic alloys enable precise tuning of adsorption energies, achieving an optimal balance between reactant binding and product desorption [53,54,55]. This volcano-plot optimization ensures highly active catalytic sites without excessive adsorption, thereby improving turnover frequencies and reaction kinetics. Synergistic metal interactions introduce active sites, lattice strain, and defects, further improving selectivity and reactivity. Additionally, electronic redistribution stabilizes less durable components, reducing dissolution and degradation [56]. While protective oxides depend on intrinsic metal properties, alloying influences their stability and adherence, enhancing corrosion resistance. Furthermore, some alloy systems exhibit dynamic surface reconstruction and self-healing properties, regenerating active sites for extended operation. Charge transfer efficiency is improved through electronic state hybridization, facilitating faster electron transport and minimizing energy losses.
Among multimetallic catalysts, cobalt-based and nickel-based alloys have garnered significant attention as efficient electrocatalysts for HER. Their high intrinsic catalytic activity, structural stability, and electrochemical robustness make them promising candidates for large-scale hydrogen production [57,58]. In particular, the incorporation of cobalt into nickel-based alloys has been shown to substantially enhance HER performance, as Co effectively modulates the electronic structure of Ni, reducing hydrogen adsorption energy and accelerating reaction kinetics. Studies on amorphous Ni-Co alloys have demonstrated lower overpotentials and enhanced reaction rates compared to their monometallic counterparts, attributed to synergistic electronic interactions and improved charge transfer efficiency [59]. Electrodeposited Ni-Co alloys further exhibit remarkable HER performance, benefiting from their high corrosion resistance, mechanical strength, and moderate thermal conductivity, which contribute to their industrial viability [60]. Additionally, copper incorporation into Ni-based alloys, particularly in nanoporous Ni-Cu films, have also demonstrated superior HER performance in alkaline media. This enhancement is primarily attributed to optimized electrodeposition conditions and precise control over alloy composition, leading to increased surface area, enhanced electrochemical activity, and improved electron transport properties, effectively mitigating the morphological limitations observed in Ni-Co alloys [61,62]. However, a key limitation of these alloys is that electrodeposition often yields compact structures with low surface area, restricting active site exposure and necessitating higher overpotentials to drive hydrogen evolution efficiently [63,64,65]. Recent investigations into Cu-Ni-Co alloy electrocatalysts have further demonstrated promising activity, achieving an overpotential of 257 mV at 10 mA cm−2 in acidic conditions. However, their stability remains a concern, with performance degradation occurring after 500 electrochemical cycles, highlighting the need for structural reinforcement and improved durability [66].
Building on our previous work, where we developed a two-step electrodeposition process to fabricate a 3D porous Cu-Co-Ni alloy that demonstrated reasonable catalytic activity and excellent stability in acidic and neutral conditions [67], we have now extended our investigation to evaluate its catalytic performance for HER in alkaline media. The electrodeposited Cu@Cu-Ni-Co trimetallic alloy exhibited superior structural integrity and electrocatalytic performance compared to both Cu@Ni-Co and Cu-only catalysts. Electrochemical testing in 1.0 M NaOH confirmed its outstanding activity, achieving a current density of 10 mA cm−2 at an exceptionally low overpotential of 125 mV, along with a favorable Tafel slope of 79.1 mV dec−1, indicative of rapid HER kinetics. Furthermore, the catalyst exhibited remarkable stability, maintaining high performance over five continuous days under elevated potentials without significant loss in current density.

2. Materials and Methods

2.1. Materials

The following chemicals were used in this study: anhydrous copper (II) sulfate (CuSO4) (Sigma-Aldrich, St. Louis, MO, USA), nickel (II) chloride hexahydrate (NiCl2·6H2O) (Merck KGaA, Darmstadt, Germany), cobalt (II) chloride hexahydrate (CoCl2·6H2O) (Merck KGaA), potassium dihydrogen phosphate (KH2PO4) (Sigma-Aldrich), and sodium hydroxide (NaOH) (Merck KGaA). High-purity argon gas (99.99%) was supplied by Air Products. All reagents were of analytical grade and used without further purification. Deionized water (resistivity: 18 MΩ·cm) from a Purite water purification system (Purite Ltd., Oxon, UK) was used throughout the experiments.

2.2. Electrocatalysts’ Synthesis

Prior to electrodeposition, glassy carbon electrodes (GCEs), Ni-foam, and indium tin oxide (ITO) substrates underwent surface preparation to ensure uniform deposition and optimal adhesion. GCEs were mechanically polished using a 50:50 nm alumina mixture, followed by sequential ultrasonication in deionized (DI) water, acetone, and DI water before being dried at 50° C. Ni-foam substrates (1 cm2) were pretreated by ultrasonication in 1.0 M HCl for 10 min to remove surface oxides, rinsed thoroughly with DI water and ethanol, and then dried at room temperature. ITO electrodes (1.0 cm2) were cleaned by ultrasonication in surfactant solution, ethanol, and DI water for 5 min each, followed by drying. Electrodeposition was carried out using a CHI760E Electrochemical Workstation (CH Instruments, Austin, TX, USA), with GCE (0.071 cm2), Ni-foam (1 cm2), or ITO (1.0 cm2) as the working electrode, platinum wire as the counter electrode, and Ag/AgCl (KCl-saturated) as the reference electrode. The Cu-based electrocatalysts were synthesized from a 25 mM KH2PO4 electrolyte solution, with specific deposition parameters and electrolyte compositions provided in Table S1.

2.3. Characterization Techniques

2.3.1. Structural and Chemical Analysis

The crystallographic structure of the electrocatalysts was analyzed using X-ray diffraction (XRD) on a Rigaku MiniFlex diffractometer (Tokyo, Japan). Surface elemental composition and oxidation states were examined via X-ray photoelectron spectroscopy (XPS, Scienta Omicron Instrument (Taunusstein, Germany)). All binding energy values in XPS spectra were referenced to the C 1s peak at 284.8 eV for calibration. For XRD and XPS measurements, a large-area Ni-foam electrode (~1.0 cm2) was used for catalyst deposition. After drying, the catalyst was carefully removed from the electrode for subsequent characterization.

2.3.2. Morphological and Elemental Analysis

The surface morphology of the Cu-based electrocatalysts was investigated using field emission scanning electron microscopy (FESEM) (JEOL JSM-7600F, 15 kV, Tokyo, Japan) and transmission electron microscopy (TEM) (JEOL JEM-2100F, 200 kV). Elemental composition analysis was conducted using energy-dispersive X-ray spectroscopy (EDS) coupled with FESEM, while a high-resolution JEOL JEM-2100F transmission electron microscope (TEM) operated at 200 kV was employed for nanoscale structural evaluation.

2.3.3. Electrochemical Measurements

Electrochemical measurements were performed using a three-electrode setup on a CHI760E Electrochemical Workstation in 1.0 M NaOH using GCE. Current densities were normalized to the geometric area of the working electrode, and potentials were referenced to the reversible hydrogen electrode (RHE) using the equation: ERHE = EAg/AgCl + E°Ag/AgCl + 0.0591 × pH, where E°Ag/AgCl = 0.197 V.
HER performance was evaluated via linear sweep voltammetry (LSV) at room temperature with a 100 mV·s−1 scan rate. Electrochemical impedance spectroscopy (EIS) was conducted from 0.1 MHz to 1 Hz at −1.0 V. Cyclic voltammetry (CV) was used to determine the double-layer capacitance (Cdl) and electrochemical surface area (ECSA) by scanning the non-Faradaic region at 20–200 mV·s−1.
Durability testing of the best electrocatalysts was performed using chronoamperometry (CA) at −0.2 V and −0.4 V for 48 h, and at −0.6 V (vs. RHE) for five consecutive days. Stability was further confirmed through an Accelerated Degradation Test (ADT) involving 1000 CV cycles at 100 mV·s−1, followed by LSV. The remaining catalysts underwent either short-duration stability testing or ADT.

3. Results and Discussion

The catalysts were synthesized following our previous report [67], and are described here concisely. The electrocatalysts were prepared via a two-step electrodeposition process. First, Cu nanowires were deposited onto the working electrode (glassy carbon electrode (GCE), Ni-foam, or indium tin oxide (ITO)) from a 0.02 M CuSO4/0.025 M KH2PO4 (pH 4.0) solution at +0.011 V vs. RHE, forming a 3D dendritic structure. In the second step, Ni and Co were co-deposited over the Cu support at −0.466 V vs. RHE to obtain Cu@Ni-Co, while simultaneous deposition of Cu, Ni, and Co produced Cu@Cu–Ni–Co. The deposition time was fixed at 900 s for all catalysts except for Cu, which was deposited for 600 s. Current densities initially stabilized at 1.2–1.3 mA cm−2 for Cu, 2.0–3.0 mA cm−2 for Ni-Co, and 2.5 mA cm−2 for the Cu-Ni-Co alloy in the first 300 s, before increasing to 15 mA cm−2 by the end of the experiment (Figure S1).
The electrodeposited catalysts were thoroughly analyzed to determine their composition and structural characteristics using X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray photoelectron spectroscopy (XPS). The XRD patterns were processed using the Integrated X-ray Powder Diffraction PDXL2 software, as shown in Figure 1. XRD analysis revealed prominent diffraction peaks at approximately 43.30°, 50.43°, and 74.11°, corresponding to the (111), (200), and (220) planes of face-centered cubic (fcc) metallic Cu (ICSD 98-005-3756). In the Cu@Ni-Co catalyst, Cu diffraction peaks remained dominant, while additional peaks at 51.7° (Co (200)) and 44.43° (Ni (111)) confirmed the incorporation of cobalt and nickel into the alloy structure. A comparison between Cu@Cu-Ni-Co and Cu@Ni-Co reveals distinct differences in peak intensities and crystallinity. While both catalysts exhibit characteristic Cu peaks, the Cu@Cu-Ni-Co alloy shows more pronounced Cu reflections, indicating a higher Cu content due to the second Cu deposition step. This suggests that a Cu-Co-Ni alloy shell formed around the Cu core, which was electrodeposited in the first step, enhancing structural stability and electrical conductivity. Furthermore, the sharper diffraction peaks in Cu@Cu-Ni-Co compared to Cu@Ni-Co indicate higher crystallinity, suggesting that the second Cu deposition step controlled the ion diffusion and deposition kinetics, leading to more ordered crystal growth and reduced structural disorder. Additionally, the Cu@Cu-Ni-Co diffraction peaks exhibit slight shifts compared to pure Cu, particularly in the (111) and (200) planes. These shifts likely result from lattice strain induced by Ni and Co incorporation, as their smaller atomic radii may cause contraction within the Cu lattice, altering its crystallographic parameters. Peak broadening observed in Cu@Cu-Ni-Co suggests the presence of smaller crystallites or increased structural disorder, which can enhance electrocatalytic performance by introducing more active sites for the HER. The highly crystalline structure of Cu@Cu-Ni-Co, as indicated by well-defined diffraction peaks, suggests strong structural integrity, which is essential for long-term electrochemical durability. The presence of Ni and Co is expected to modify the electronic structure, optimizing adsorption energies for hydrogen intermediates and enhancing HER kinetics. Furthermore, the observed lattice strain and broadened peaks suggest defects and strain-induced active sites, which can improve catalytic efficiency. The reinforced Cu content in Cu@Cu-Ni-Co also indicates higher electrical conductivity, which is critical for efficient electron transfer during electrochemical reactions [65,68].
The SEM images presented in Figure 2 and Figure S2 illustrate the structural evolution of the electrodeposited catalysts, revealing significant morphological transformations from Cu to Cu@Ni-Co and Cu@Cu-Ni-Co. The Cu sample exhibits a well-defined dendritic nanostructure, with elongated branches extending from a central growth point. These nanostructures are highly branched and uniformly distributed, suggesting a controlled growth mechanism during electrodeposition. The high-magnification image further confirms the formation of sharp, crystalline Cu nanowires, which provide an excellent framework for subsequent metal deposition. Upon the incorporation of Ni and Co, the morphology of Cu@Ni-Co undergoes a notable transformation, shifting from the highly branched dendritic structure of Cu to a more compact and clustered arrangement. The deposition of Ni and Co appears to distort the original framework, leading to the formation of densely packed, irregularly shaped aggregates rather than maintaining the well-defined branches observed in Cu. The transformation from a branched Cu morphology to a more compact Ni-Co structure might be due to the intrinsic electrodeposition behavior of Ni and Co, where differences in nucleation kinetics and ion diffusion lead to a shift from elongated dendritic growth to a denser, more granular morphology. This transition is characteristic of Ni-Co alloy deposition and occurs due to the complex interplay of surface energy, deposition rates, and metal ion interactions during co-electrodeposition [69,70].
The structural evolution further progresses in the Cu@Cu-Ni-Co catalyst (Figure 2a–c), which exhibits a more interconnected and porous network compared to Cu@Ni-Co. The introduction of copper in the second deposition step appears to influence crystal growth and metal distribution, potentially modulating ion diffusion and deposition kinetics, thereby stabilizing the morphology and enhancing structural coherence. Unlike the more compact and granular structure of Cu@Ni-Co, Cu@Cu-Ni-Co retains a higher degree of order, as seen in the densely packed and uniformly arranged branches. This improved structural integration suggests that the Cu-rich environment in the second deposition stage plays a crucial role in directing grain growth, likely reducing structural disorder while preserving a well-defined 3D hierarchical architecture. The increased porosity and interconnected morphology may contribute to enhanced charge and mass transfer, while the structural stability provided by the Cu-Co-Ni alloy shell ensures long-term durability under electrochemical conditions.
The elemental mapping confirms the successful incorporation and distribution of Cu, Ni, and Co across the catalysts. The pure Cu sample exhibits a uniform Cu distribution with no detectable impurities, indicating that the initial electrodeposition yielded a clean Cu framework. In Cu@Ni-Co, a homogeneous distribution of Ni and Co over the Cu framework is observed, suggesting successful alloy formation. The Cu@Cu-Ni-Co catalyst shows an even more uniform distribution of Cu, Ni, and Co, further supporting the formation of a Cu-Co-Ni alloy shell surrounding the Cu core. The presence of Cu in the second deposition step appears to play a role in controlling electrodeposit growth and preserving the 3D structure. While Cu deposition typically enhances crystallinity, its role in alloy formation is governed by its interaction with Co and Ni during electrodeposition. Given Cu has a relatively low nucleation overpotential compared to Co and Ni, its deposition is likely to be more uniform, facilitating a smoother and more continuous metal distribution. This effect may reduce phase separation between the deposited metals and enhance electrical connectivity across the alloy, improving overall catalytic performance. The presence of oxygen in all three samples suggests surface oxidation, which could influence the electronic structure and stability of the catalysts. Overall, SEM and EDS results provide strong evidence that Cu@Cu-Ni-Co features a structurally optimized, highly interconnected nanostructure with uniform elemental distribution, properties that are essential for high-performance electrocatalysis. The combination of enhanced crystallinity, increased surface area with branched structures, and a well-integrated Cu-Co-Ni alloy shell is expected to provide synergistic electronic and catalytic effects, reinforcing its superior performance for alkaline HER applications.
The TEM images of the Cu@Cu-Ni-Co electrocatalyst at various magnifications (Figure 2d–f) provide comprehensive insights into its morphology, nanostructure, and crystallinity, revealing essential details about its structural evolution and potential impact on electrocatalytic performance. At lower magnifications, the electrocatalyst displays a 3D, dendritic structure, with branches forming an interconnected network. As the magnification increases, these structures appear more compact and uniform, suggesting that the second Cu deposition step contributed to a more integrated and coherent morphology. The branched and hierarchical structure, combined with the uniform coverage of Cu, Ni, and Co, indicates successful deposition of a Cu-Co-Ni alloy shell over the Cu core. Higher-resolution TEM images (Figure 2g) reveal that the branches exhibit a core–shell configuration, with a Cu-enriched core and a Cu-Ni-Co alloy shell. This configuration is particularly evident in the HRTEM images, where the well-defined lattice fringes provide direct evidence of crystallinity. The measured d-spacing values confirm the presence of multiple phases within the trimetallic alloy. The inner region of the Cu@Cu-Ni-Co catalyst exhibits a spacing of 0.217 nm, which corresponds to the (111) plane of metallic Cu or metallic Co, indicating the dominant presence of these elements in the more compact inner region [71,72]. Additionally, a spacing of 0.274 nm, assigned to the (002) plane of β-Ni(OH)2, suggests the partial incorporation of nickel hydroxide species within this region [73]. The presence of 0.195 nm, which corresponds to the (−112) plane of CuO, suggests localized oxidation of Cu [74]. Moving toward the outer region, the spacing of 0.234 nm, assigned to the (111) plane of NixCo3-xO, indicates the formation of mixed nickel–cobalt oxides, suggesting a gradual transition from metallic-rich to oxidized components [75]. A spacing of 0.204 nm, corresponding to the (111) plane of metallic Co [76], metallic Ni [77], or Ni–Co alloys [78], confirms the retention of some metallic cobalt species within this layer, likely improving conductivity. Furthermore, the spacing of 0.265 nm, attributed to Co(OH)2 or Ni(OH)2, suggests the presence of hydroxides in the outermost region [79]. This structural transition from a more metallic inner region to an oxide/hydroxide-enriched outer region supports the hypothesis that the electrodeposition process led to a gradient composition, ensuring a conductive core while incorporating hydroxides and oxides that may enhance catalytic performance.
The elemental mapping (Figure 2h) further validates the core–shell structure by showing the uniform distribution of Cu, Ni, and Co across the catalyst. The Cu-rich core and the Ni-Co-rich shell are clearly visible, indicating successful co-deposition of these metals in the second electrodeposition step. Unlike conventional core–shell configurations where metals are distinctly layered, the presence of Cu in both the core and shell regions suggests a trimetallic alloy shell, formed through co-deposition of Cu, Ni, and Co, ensuring better structural stability and enhanced electronic connectivity. The presence of oxygen in the shell region implies partial surface oxidation, which may alter surface properties and contribute to the formation of additional active sites. The unique 3D hierarchical structure and core–shell configuration of Cu@Cu-Ni-Co play a pivotal role in maintaining structural integrity and ensuring consistent catalytic performance. The interconnected network facilitates efficient mass transport of reactants to active sites, while the high surface area enhances active site exposure, both of which are critical for improving reaction kinetics. The heterogeneity in electronic properties between the Cu-rich core and the Cu-Ni-Co alloy shell may introduce synergistic effects, optimizing hydrogen adsorption energies and enhancing overall HER efficiency.
It is noteworthy to mention that the SEM-EDS analysis (Figure S3, Table S2) reveals that Cu@Cu-Ni-Co contains 55.3 wt.% Cu, 28.1 wt.% Ni, 10.7 wt.% Co, and 5.8 wt.% oxygen, indicating that copper is the dominant element, with significant contributions from nickel and cobalt, likely concentrated in the shell. In contrast, the TEM-EDS analysis (Figure S4, Table S3) of the same sample shows a higher Cu content of 73.77 wt.% and lower Ni and Co contents of 6.47 wt.% each, along with only 2.06 wt.% oxygen. The differences between the SEM-EDS and TEM-EDS compositions arise from their sampling depths and resolution: SEM-EDS provides an average composition of the surface and subsurface regions, capturing higher oxygen content due to surface oxidation, while TEM-EDS focuses on localized, thinner regions, highlighting the core–shell structure with a higher Cu content and minimal surface oxidation.
The XPS spectra of the Cu@Cu-Ni-Co catalyst (Figure 3 and Figure S5) confirms the presence of copper, cobalt, nickel, oxygen, and adventitious carbon. These elements are consistent with the expected composition of the trimetallic alloy, indicating a successful electrodeposition process. The high-resolution XPS spectra provide more detailed insights into the oxidation states and chemical environments of the individual elements. Basically, the high-resolution spectra indicated an atomic ratio of Cu/Ni/Co/O of approximately 3:1.2:1:1. In the high-resolution spectrum for copper, the Cu 2p3/2 region shows three peaks at 932.07 eV, 934.17 eV, and 936.39 eV, corresponding to metallic copper (Cu0) or cuprous ions (Cu+), cupric oxide (CuO), and copper hydroxide (Cu(OH)2), respectively. Additionally, the broad satellite peaks between 939.0 eV and 947.7 eV are characteristic of Cu2+ species. The predominant intensity of the CuO peak suggests that the surface is highly enriched with oxidized copper, while the presence of Cu0/Cu+ and Cu(OH)2 indicates partial surface oxidation and hydroxylation during the preparation or exposure to air [80]. For cobalt, the Co 2p spectrum displays two distinct peaks at 781.8 eV (Co 2p3/2) and 797.5 eV (Co 2p1/2), which are characteristic of Co2+ species, specifically from Co(OH)2 [81]. These peaks are accompanied by satellite features at 786.5 eV and 803.4 eV, further confirming the hydroxide phase. No evidence of metallic cobalt (Co0) was observed, indicating that surface cobalt on the catalyst surface exists primarily in an oxidized form. The nickel XPS spectrum reveals similar surface chemistry, with the dominant peaks at 856.4 eV (Ni 2p3/2) and 874.2 eV (Ni 2p1/2), consistent with Ni(OH)2. Satellite peaks at 862.3 eV and 880.7 eV are also typical of Ni2+ species [82]. The O 1s spectrum further confirms the surface oxidation states of the sample, with three distinct peaks at 531.3 eV, 532.3 eV, and 533.6 eV, corresponding to oxide ions (O2−), hydroxyl groups (-OH), and adsorbed water (H-O-H), respectively. The high intensity of the oxygen signal reflects the significant surface oxidation of Cu, Co, and Ni, forming a complex mixture of oxides and hydroxides. This result aligns with the XPS findings for the metal species, confirming that the surface of the Cu@Cu-Ni-Co catalyst is predominantly covered with oxide and hydroxide phases. The absence of these oxide and hydroxide phases in the XRD patterns suggests that they are surface-localized and present in concentrations too low to be detected by bulk-sensitive XRD analysis. This is a typical phenomenon for transition metal-based electrocatalysts, where surface oxidation occurs during exposure to ambient conditions. Such surface oxides and hydroxides can play an essential role in enhancing the catalyst’s performance by providing additional active sites for hydrogen evolution in alkaline media [67,83,84].
The electrocatalytic performance of Cu, Cu@Ni-Co, and Cu@Cu-Ni-Co catalysts for HER was evaluated in 1.0 M NaOH using a standard three-electrode setup (Figure 4). The LSV curves (Figure 4a and Figure S6) offer a clear comparison of the HER activity of these catalysts, along with a commercial 10% Pt/C catalyst as a benchmark. While the bare glassy carbon (GC) electrode displayed negligible HER activity, the Cu@Cu-Ni-Co catalyst demonstrated the best performance among the tested materials, achieving a significantly lower overpotential at 10 mA cm−2 compared to Cu and Cu@Ni-Co. Although the Pt/C catalyst required only 80 mV to reach 10 mA cm−2, Cu@Cu-Ni-Co exhibited a much higher catalytic current density at −0.5 V, reaching −275 mA cm−2, compared to −220 mA cm−2 for Pt/C, underscoring its remarkable activity. The corresponding Tafel plots (Figure 4b) further reveal the superior kinetics of Cu@Cu-Ni-Co, which exhibits the lowest Tafel slope (76.2 mV dec−1) compared to Cu@Ni-Co (88.2 mV dec−1) and Cu (166.7 mV dec−1). This result highlights the significant role of Ni, Cu, and Co incorporation into the electrocatalyst in enhancing the electronic conductivity and reducing the energy barrier for HER under alkaline conditions. The overpotential required to achieve a current density of 10 mA cm−2 was only 125 mV for Cu@Cu-Ni-Co, which is significantly lower than 162 mV for Cu@Ni-Co and 386 mV for Cu (Figure 4c). This improvement is directly correlated with the electrochemical surface area (ECSA), which was calculated from double-layer capacitance (Cdl) measurements (Figure S7 and Figure S8). The ECSA of Cu@Cu-Ni-Co reached 380 cm2, far surpassing that of Cu@Ni-Co (135 cm2) and Cu (30 cm2), as shown in Figure 4d (Table S4). This increase in ECSA is attributed to the well-structured 3D hierarchical trimetallic shell, which provides a higher density of active sites for HER. The enhanced surface area contributes significantly to the catalyst’s superior performance, promoting better mass transport and electron transfer efficiency. Electrochemical impedance spectroscopy (EIS) provided additional insights into the charge transfer characteristics of the catalysts. The Nyquist plots (Figure S9 and Figure S10) show that Cu@Cu-Ni-Co exhibited the lowest charge transfer resistance (Rct) of 15.76 Ω, significantly lower than those of Cu and Cu@Ni-Co (Table S5). This reduced Rct indicates superior conductivity at the electrode/electrolyte interface, correlating with the high ECSA and enhanced HER kinetics. The improved charge transfer efficiency contributes to the overall catalytic performance of Cu@Cu-Ni-Co. It is noteworthy to mention that that the Cu@Cu-Ni-Co electrocatalyst demonstrated enhanced catalytic activity in NaOH compared to KOH under identical conditions, as evidenced by its lower overpotential and higher catalytic current density (Figure S11). This aligns with studies showing that alkali metal cations influence electrocatalytic performance by modulating reaction kinetics and interfacial interactions. In alkaline media, HER activity correlates with cation hydration energy (Li+ > Na+ > K+ > Rb+ > Cs+), supporting our observation that Na+ enhances catalytic efficiency relative to K+ [85,86].
To evaluate the long-term durability of the catalysts, chronoamperometric stability tests were conducted at multiple potentials (−0.2 V, −0.4 V, and −0.6 V vs. RHE) over two to five days (Figure 4e). The Cu@Cu-Ni-Co catalyst exhibited excellent stability, maintaining a steady current density of approximately 270 mA cm−2 at −0.6 V without significant degradation. After 120 h of continuous bulk electrolysis at such a high current density, it retained ~95% of its initial value, stabilizing at 255 mA cm−2. In contrast, Cu and Cu@Ni-Co showed significantly lower current densities at −0.6 V, reaching only 15 mA cm−2 and 130 mA cm−2, respectively (Figure S12 and Figure S13). Further accelerated degradation tests (ADT) confirmed the durability of Cu@Cu-Ni-Co. Even after 1000 cycles of CV, the LSV curve of Cu@Cu-Ni-Co remained unchanged, with a slight improvement in onset potential. This stability can be attributed to the robust hierarchical structure and strong interaction between Cu, Ni, and Co within the trimetallic framework, which resists surface reconstruction and degradation during prolonged operation. The performance of Cu@Cu-Ni-Co was benchmarked against other reported HER catalysts in alkaline media (Figure 4f). Compared to a wide range of state-of-the-art materials, Cu@Cu-Ni-Co demonstrated a competitive balance of low overpotential, favorable Tafel slope, and excellent stability, positioning it as a promising candidate for practical alkaline hydrogen evolution applications. The synergy between Cu, Ni, and Co, along with the optimized morphology, plays a crucial role in achieving this exceptional performance.
Post-electrolysis characterization was conducted using XRD and HRTEM to assess potential changes in the structural integrity, composition, and morphology of the Cu@Cu-Ni-Co catalyst. For this evaluation, catalysts were electrodeposited and subjected to chronoamperometry tests at −0.6 V vs. RHE for 3 h in 1.0 M NaOH. Following electrolysis, the samples were collected, dried under vacuum, and subsequently analyzed. The post-electrolysis XRD patterns (Figure 5) indicate that the primary crystalline phases remain largely intact, with characteristic peaks for Cu, Ni, and Co still clearly visible. This retention of the original crystalline structure highlights the robustness of the trimetallic catalyst. Notably, there is no evidence of new crystalline phases or major phase transformations, as no additional peaks emerge post-electrolysis. However, a slight reduction in the intensity of the peaks suggests minor surface restructuring, likely due to surface oxidation or slight amorphization, rather than bulk phase degradation. This observation aligns with the catalyst’s stable electrochemical performance, confirming that its crystalline framework remains preserved even after bulk electrolysis. The post-electrolysis TEM analysis (Figure 5) further validates the structural stability of the Cu@Cu-Ni-Co catalyst. The dendritic 3D morphology remains well-preserved, with no signs of structural collapse or significant aggregation. Elemental mapping demonstrates the uniform distribution of Cu, Ni, and Co across the catalyst surface, indicating minimal elemental leaching or phase segregation. Additionally, post-electrolysis TEM-EDX analysis (Figure S14, Table S6) reveals that the elemental composition remains largely unchanged compared to the as-prepared catalyst, with Cu, Ni, and Co contents showing minimal variation. The slight increase in oxygen content (from 2.43 to 6.54 wt.%) suggests surface hydroxide/oxide enrichment, a common phenomenon in transition metal-based electrocatalysts under alkaline conditions. HRTEM images confirm that the crystalline structure is retained, with well-defined lattice fringes. The measured d-spacings of 0.195 nm, 0.204 nm, 0.234 nm, 0.265 nm, and 0.274 nm remain consistent with those in the as-prepared catalyst, reinforcing that no significant amorphization or structural deterioration has occurred. Collectively, XRD and TEM analyses confirm that the Cu@Cu-Ni-Co catalyst retains its structural integrity and composition after prolonged HER operation. The absence of significant degradation, phase transformation, or elemental dissolution further underscores its long-term durability and effectiveness as an HER electrocatalyst.

4. Conclusions

In this study, we have successfully developed a hierarchical Cu@Cu-Ni-Co trimetallic electrocatalyst with a core–shell structure through a two-step electrodeposition process. Detailed morphological and structural analyses confirmed the formation of a well-integrated core–shell structure, where the initial copper deposition served as a stable core for the subsequent deposition of a trimetallic alloy shell. TEM and XPS analyses revealed the presence of metallic Cu in the core and a Ni-Co-Cu alloy shell with partially oxidized surface species, suggesting a highly complex and synergistic structure beneficial for catalysis. The catalyst exhibited a significantly increased ECSA, reflecting its abundant active sites and optimized 3D architecture. The electrocatalytic performance of Cu@Cu-Ni-Co for hydrogen evolution in alkaline media demonstrated remarkable improvements compared to Cu and Cu@Ni-Co. The Cu@Cu-Ni-Co catalyst achieved a current density of 10 mA cm−2 at an overpotential of only 125 mV, with a favorable Tafel slope of 76.2 mV dec−1. Its catalytic current density at −0.5 V reached −275 mA cm−2, surpassing that of commercial 10% Pt/C. Electrochemical impedance spectroscopy confirmed the superior conductivity and reduced charge transfer resistance at the catalyst/electrolyte interface, further contributing to its outstanding catalytic activity. Stability tests conducted at multiple potentials confirmed the exceptional durability of the Cu@Cu-Ni-Co catalyst, retaining ~95% of its initial current density even after 120 h of continuous operation at high current densities. Moreover, post-electrolysis characterization revealed minimal structural degradation or elemental leaching, reinforcing its long-term stability and suitability for sustained HER applications. When benchmarked against other recently reported electrocatalysts, Cu@Cu-Ni-Co demonstrated competitive performance, with its low overpotential, favorable kinetics, and excellent long-term stability positioning it as a promising candidate for alkaline hydrogen evolution reactions. These findings underscore the importance of rational design and precise tuning of the composition and morphology of trimetallic electrocatalysts for achieving high catalytic efficiency and durability.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en18061515/s1, Figure S1. Amperometric i-t curves for electrodeposition: (a) Cu, (b) Cu@Ni-Co, (c) Cu@Cu-Ni-Co. Figure S2. SEM images and EDS mapping of different electrocatalysts:(a) Cu, (b) Cu@Ni-Co, and (c) Cu@Cu-Ni-Co. Figure S3. SEM-EDS spectra showing the elemental composition of (a) Cu, (b) Cu@Ni-Co, and (c) Cu@Cu-Ni-Co electrocatalysts. Figure S4. TEM-EDS spectrum of the Cu@Cu-Ni-Co electrocatalyst. Figure S5. XPS survey of the as-synthesized Cu@Co-Ni-Cu electrocatalyst. Figure S6. Full range LSV curves of Cu, Cu@Ni-Co, Cu@Cu-Ni-Co, and Pt/C electrocatalysts in 1.0 M NaOH. Figure S7. CV curves at different scan rates and their related plots of j vs. v1/2 in 1.0 M NaOH for (a) Cu, (b) Cu@Ni-Co and (c) Cu@Cu-Ni-Co electrocatalysts. Figure S8. Double layer capacitance (Cdl) for Cu, Cu@Ni-Co and Cu@Cu-Ni-Co electrocatalysts in 1.0 M NaOH. Figure S9. Nyquist plots of Cu, Cu@Ni-Co, and Cu@Cu-Ni-Co electrocatalysts in 1.0 M NaOH, the inset depicts Nyquist Plots for GC. Figure S10. Experimental and simulated Nyquist plots of (a) Cu, (b) Cu@Ni-Co and (c) Cu@Cu-Ni-Co, electrocatalysts, the insets show electrical equivalent circuit. Figure S11. LSV curves of the Cu@Cu-Ni-Co electrocatalyst recorded in 1.0 M NaOH and 1.0 M KOH, demonstrating enhanced catalytic activity in NaOH, as evidenced by a lower overpotential and higher current density. Figure S12. it-amperometric curves at −0.6 V over 3 h and inset: LSV curves before and after 1000 CV cycles in 1 M NaOH for Cu. Figure S13. It-amperometric curves at −0.6 V over 3 h and inset: LSV curves before and after 1000 CV cycles in 1 M NaOH for Cu@Ni-Co. Figure S14. TEM-EDS spectrum of the post-electrolysis Cu@Cu-Ni-Co electrocatalyst. Table S1. Electrodeposition parameters of various Cu-based electrocatalysts. Table S2. SEM-EDS elemental compositions analysis of Cu, Cu@Ni-Co and Cu@Cu-Ni-Co electrocatalysts. Table S3. Elemental composition analysis of the Cu@Cu-Ni-Co electrocatalyst based on TEM-EDS measurements. Table S4. The double layer capacitance (Cdl), specific capacitance ( C s ) and electrochemical activity surface area (ECSA) of different Cu-based electrocatalysts in 1.0 M NaOH. Table S5. The resistance of solution and the charge transfer resistance of various Cu-based electrocatalysts in 1.0 M NaOH. Table S6. Elemental composition analysis of the post-electrolysis Cu@Cu-Ni-Co electrocatalyst based on TEM-EDS measurements. Table S7. Comparison of the HER activity of our Cu-based electrocatalysts and previous reported Cu-based electrocatalysts in alkaline electrolyte. References [87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, H.A.Y. and R.A.-H.; Methodology, H.A.Y. and M.A.H.; Investigation, H.A.Y. and M.A.H.; Resources, H.A.Y. and M.A.A.; Writing—original draft, H.A.Y.; Writing—review & editing, H.A.Y., M.A.A. and R.A.-H.; Visualization, H.A.Y. and M.A.H.; Supervision, H.A.Y., M.A.A. and R.A.-H.; Project administration, M.A.A. and R.A.-H.; Funding acquisition, M.A.A. and R.A.-H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Deanship of Research Fund at Sultan Qaboos University under grant number RF/DVC/NRC/24/01 for financial support. The APC was covered by the Central Journal Publication Fees Fund Program, provided by the Deanship of Research at Sultan Qaboos University.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors gratefully acknowledge the Surface Science laboratory, Physics Department, College of Science, Sultan Qaboos University for X-ray photoelectron spectroscopy measurement.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-ray diffraction patterns of pristine Cu, Cu@Ni-Co, and Cu@Cu-Ni-Co electrocatalysts.
Figure 1. X-ray diffraction patterns of pristine Cu, Cu@Ni-Co, and Cu@Cu-Ni-Co electrocatalysts.
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Figure 2. SEM images (a–c), TEM images (d–f), high-resolution TEM (HR-TEM) images (g), and elemental mapping via EDS (h) of the Cu@Cu-Ni-Co electrocatalyst.
Figure 2. SEM images (a–c), TEM images (d–f), high-resolution TEM (HR-TEM) images (g), and elemental mapping via EDS (h) of the Cu@Cu-Ni-Co electrocatalyst.
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Figure 3. High-resolution X-ray photoelectron spectroscopy (XPS) spectra of Cu@Cu-Ni-Co, showcasing (a) Cu 2p, (b) Co 2p, (c) Ni 2p, and (d) O 1s regions. The green dotted line represents the raw experimental XPS spectrum, while the brown solid line corresponds to the fitted spectrum obtained through peak deconvolution.
Figure 3. High-resolution X-ray photoelectron spectroscopy (XPS) spectra of Cu@Cu-Ni-Co, showcasing (a) Cu 2p, (b) Co 2p, (c) Ni 2p, and (d) O 1s regions. The green dotted line represents the raw experimental XPS spectrum, while the brown solid line corresponds to the fitted spectrum obtained through peak deconvolution.
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Figure 4. Electrocatalytic HER performance of Cu, Cu@Ni-Co, and Cu@Cu-Ni-Co in 1.0 M NaOH: (a) LSV curves comparing HER activity, (b) Tafel plots, (c) comparison of overpotentials at 10 mA cm−2 and catalytic current densities at −0.5 V vs. RHE, (d) electrochemical surface area (ECSA) calculated from double-layer capacitance measurements, (e) chronoamperometric stability tests of Cu@Cu-Ni-Co at −0.2, −0.4, and −0.6 V vs. RHE over multiple days (the inset shows accelerated durability testing (ADT) before and after 1000 cyclic voltammetry cycles), and (f) benchmarking of Cu@Cu-Ni-Co with recently reported earth-abundant HER electrocatalysts in terms of overpotential and Tafel slope.
Figure 4. Electrocatalytic HER performance of Cu, Cu@Ni-Co, and Cu@Cu-Ni-Co in 1.0 M NaOH: (a) LSV curves comparing HER activity, (b) Tafel plots, (c) comparison of overpotentials at 10 mA cm−2 and catalytic current densities at −0.5 V vs. RHE, (d) electrochemical surface area (ECSA) calculated from double-layer capacitance measurements, (e) chronoamperometric stability tests of Cu@Cu-Ni-Co at −0.2, −0.4, and −0.6 V vs. RHE over multiple days (the inset shows accelerated durability testing (ADT) before and after 1000 cyclic voltammetry cycles), and (f) benchmarking of Cu@Cu-Ni-Co with recently reported earth-abundant HER electrocatalysts in terms of overpotential and Tafel slope.
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Figure 5. XRD (a), TEM (b,c), HRTEM (d), and TEM-EDS (e) images of the Cu@Cu–Ni–Co electrocatalyst after 3 h of bulk electrolysis at −0.6 V vs. RHE in 1.0 M NaOH.
Figure 5. XRD (a), TEM (b,c), HRTEM (d), and TEM-EDS (e) images of the Cu@Cu–Ni–Co electrocatalyst after 3 h of bulk electrolysis at −0.6 V vs. RHE in 1.0 M NaOH.
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Younus, H.A.; Al Hinai, M.; Al Abri, M.; Al-Hajri, R. Hierarchical Core-Shell Cu@Cu-Ni-Co Alloy Electrocatalyst for Efficient Hydrogen Evolution in Alkaline Media. Energies 2025, 18, 1515. https://doi.org/10.3390/en18061515

AMA Style

Younus HA, Al Hinai M, Al Abri M, Al-Hajri R. Hierarchical Core-Shell Cu@Cu-Ni-Co Alloy Electrocatalyst for Efficient Hydrogen Evolution in Alkaline Media. Energies. 2025; 18(6):1515. https://doi.org/10.3390/en18061515

Chicago/Turabian Style

Younus, Hussein A., Maimouna Al Hinai, Mohammed Al Abri, and Rashid Al-Hajri. 2025. "Hierarchical Core-Shell Cu@Cu-Ni-Co Alloy Electrocatalyst for Efficient Hydrogen Evolution in Alkaline Media" Energies 18, no. 6: 1515. https://doi.org/10.3390/en18061515

APA Style

Younus, H. A., Al Hinai, M., Al Abri, M., & Al-Hajri, R. (2025). Hierarchical Core-Shell Cu@Cu-Ni-Co Alloy Electrocatalyst for Efficient Hydrogen Evolution in Alkaline Media. Energies, 18(6), 1515. https://doi.org/10.3390/en18061515

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