Reusable NiCo/Cu Catalysts for Sustainable Hydrogen Generation

Highlights

What are the main findings?

• Electroless deposition provides a simple and cost-effective route to Ni–Co coatings on Cu.
• Catalytic activity strongly varies with Co content in the 4–90 wt.% range.
• Ni₁₀Co₉₀/Cu retains high stability, with only 12% activity loss after reuse.

What are the implications of the main findings?

• Synergistic Ni–Co interaction significantly enhances hydrogen generation efficiency.
• Co-rich Ni–Co coatings exhibit superior hydrogen generation activity.
• The Ni–Co/Cu system is a viable, low-cost catalyst for sustainable H₂ production.

Abstract

The generation of high-purity hydrogen via chemical reaction from hydrogen-rich materials is one of the ways in the alternative energy industry. In this approach, the utilization of catalytic materials that possess the capacity to initiate the decomposition of the starting material and the subsequent release of hydrogen is of paramount importance. In this study, nickel/cobalt-plated copper catalysts (NiCo/Cu) are presented, comprising from 4 to 90 wt.% of cobalt as catalytic materials for hydrogen generation via sodium borohydride (NaBH₄) hydrolysis reaction. The NiCo/Cu catalysts were synthesized via electroless deposition from glycine-based baths, utilizing Ni²⁺ and Co²⁺ ions as metal sources and morpholine borane (MB) as the reducing compound. The catalytic performance in alkaline NaBH₄ hydrolysis was found to correlate with the cobalt loading in the coating. The maximum rate of hydrogen production, which was determined to be 14.22 L min⁻¹ gcat⁻¹, was achieved at 343 K for a catalyst composed of 90 wt.% Co. The reaction proceeded with the activation energy of 52.5 kJ mol⁻¹, while the catalyst exhibited high durability, preserving nearly 88% of its initial activity after five successive reaction cycles. The combination of nickel and cobalt, along with their synergistic effect and high efficiency in the borohydride hydrolysis reaction, makes them promising catalysts.

1. Introduction

Hydrogen has long been recognized as an important energy carrier, with early large-scale production methods including coal gasification in the mid-19th century and water electrolysis developed in the late 19th century. Since the early 20th century, methane steam reforming has dominated industrial hydrogen production due to its efficiency and scalability, despite its significant CO₂ emissions [1,2,3]. However, growing global efforts to reduce greenhouse gas emissions have turned interest to sustainable and low-carbon hydrogen production pathways. In parallel, the use of hydrogen as an energy vector has expanded rapidly, particularly in fuel cell technologies for aerospace, transportation, and energy applications. However, challenges related to hydrogen storage, safety, and cost-effective production remain major barriers to widespread adoption [4,5,6,7,8,9,10,11,12]. As a result, research efforts have increasingly focused on the development of efficient methods for clean hydrogen generation [13,14,15,16,17,18,19,20,21]. In the domain of the alternative energy industry, the production of high-purity hydrogen through chemical reactions involving hydrogen storage materials has become a significant area of research. A number of chemical compounds, such as water (H₂O), metal hydrides (NaBH₄, KBH₄, LiBH₄, NaAlH₄, MgH₄), hydrazine hydrates (N₂H₄·H₂O), hydrazine or ammonium boranes (N₂H₄·BH₃, NH₄BH₃), formic acid (HCOOH), and methane (CH₄), have been investigated as hydrogen storage materials [22,23,24,25,26,27,28]. A critical attribute of hydrogen storage materials is their high gravimetric hydrogen capacity. As indicated by the aforementioned sources, the following materials have been identified as those with the highest hydrogen capacity. It has been determined that N₂H₄BH₃ (15.4 wt.%), LiBH₄ (18.5 wt.%), Mg(BH₄)₂ (14.9 wt.%), NH₃BH₃ (19.6 wt.%), NH₃ (17.7 wt.%), NaBH₄ (10.8 wt.%), and N₂H₄·H₂O (8 wt.%) are considered promising for hydrogen storage and use in portable fuel cells [22,23,24,25,26,27,28].

Sodium borohydride (NaBH₄) is a prominent hydride, distinguished by its numerous advantageous properties. It is inexpensive, non-flammable, environmentally friendly, stable in alkaline solutions, renewable, and can be obtained by hydrolysis from sodium borate. Additionally, it is non-toxic and does not require specialized conditions for functionality [29,30,31,32,33,34]. NaBH₄ is classified as a coordination-type hydride, the cations of which are metal ions. The substance exhibits a substantial propensity for hydrogen production, with the potential to liberate 4 moles of H₂ from a single mole of NaBH₄ through hydrolysis. The process known as borohydride hydrolysis is defined as the chemical reaction in which NaBH₄ reacts with water, resulting in the production of hydrogen gas, as depicted by the following Equation (1) [35,36,37,38,39]:

NaBH₄ + 2H₂O → NaBO₂ + 4H₂ + Q − 216.7 kJ mol⁻¹

(1)

It is worth mentioning that during this hydrolysis reaction, extremely pure hydrogen is produced. The spontaneous hydrolysis of NaBH₄ is exceedingly slow; however, the addition of a catalyst can expedite the rate of hydrogen release at room temperature. Therefore, it is imperative to identify catalysts that are selective for this reaction. This will enable the rapid generation of hydrogen and will prevent contamination of the electrode surface during the process [40,41,42].

Generally, noble metals such as Pt, Ru, Pd, Rh, Ir have been established as excellent and efficient catalysts for the hydrolysis of sodium borohydride. There is ample reference to support the assertion that the noble metal-based catalysts are indeed active [43,44,45,46,47,48]. For instance, Doherty’s research examined platinum (Pt) nanoparticles that were stabilized by a polymer-immobilized ionic liquid. This investigation revealed the activation energy of 23.9 kJ mol⁻¹ for PtNP@PPh2-PEGIILS and 35.6 kJ mol⁻¹ for PtNP@PPh2-NDEYLIILS, as evidenced by borohydride hydrolysis [43]. Ru immobilized Al₂O₃ pellets demonstrated 41.8 kJ mol⁻¹ of activation energy for borohydride hydrolysis [44]. Ecer et al. successfully synthesized cross-linked pumice–polymer gel brush–Pd nanoparticle catalysts with the activation energy of 26.85 kJ mol⁻¹ [45]. However, due to the extremely high price of Pt, Rh, Ir, Ru, and Pd, as well as the rapid surface poisoning of bulk metal, attention has shifted to the transition metals such as Ni, Co, Mn, Cu, Fe, Cr, Mo and W [49,50,51,52,53,54,55,56,57,58,59,60]. Cobalt has been identified as a highly effective and environmentally friendly metal for the borohydride hydrolysis reaction, exhibiting an activation energy range of 35–66 kJ mol⁻¹ [61,62,63,64,65,66]. However, concerns related to the environmental footprint and supply-chain sustainability of cobalt have encouraged the development of catalytic systems with reduced cobalt content [67,68]. Nickel has been investigated as a potentially more affordable and less hazardous alternative to cobalt. Its health risks are generally lower than those of cobalt; however, its efficiency in the borohydride hydrolysis reaction is slightly lower, exhibiting an activation energy range of 53–72 kJ mol⁻¹ [69,70,71,72]. Consequently, research efforts have been directed towards the development of catalysts that combine two, three, or four metals. The objective of this research is twofold: first, to minimize the amount of cobalt required, and second, to enhance the catalytic activity of the reaction [73,74,75,76,77]. Lakhali et al. presented a Co-Mo-B catalyst deposited on an itaconic acid (IA)-modified carbon nanotube matrix exhibiting HGR of 5.3 L g_metal⁻¹ min⁻¹ and E_a equal 24.58 kJ mol⁻¹ [73] A high-entropy alloy Fe₁₀Co₁₀Ni₁₀Cr₁₀Mn₆₀ ribbon catalyst with a face-centered cubic structure achieved 55.75 kJ mol⁻¹ activation energy to catalyze the hydrolysis of sodium borohydride to generate 18.46 L m⁻² min⁻¹ hydrogen [77]. The Fe₃O₄@SiO₂/Co-Mo-B nanocatalyst exhibited remarcable catalytic efficacy, attaining a hydrogen generation rate of 16.5 L g_metal⁻¹ min⁻¹ with an activation energy of 32.18 kJ mol⁻¹ for NaBH₄ hydrolysis at 50 °C [74]. A comprehensive review of the extant literature reveals that combinations of two, three, or four metals have been demonstrated to be an effective method for reducing the activation energy of the borohydride reaction and enhancing hydrogen generation.

Various methods for obtaining coatings of pure metals or their alloys have been recently described in the literature. Z. Liang et al. succeeded in synthesizing a NiB/NiFe₂O₄ catalyst by the impregnation–chemical reduction method. By using this catalyst, the activation energy for the sodium borohydride hydrolysis reaction was equal to 72.52 kJ mol⁻¹ [71]. Ch. H. Liu et al. presented a wet chemical reduction method for the preparation of stable and environmentally friendly Co/IR-120 catalysts with high residual magnetism and activation energy of 66.67 kJ mol⁻¹ for hydrogen generation from alkaline NaBH₄ solution [78]. S. U. Jeong et al. prepared a CoB catalyst by the chemical reduction method. The activation energy of this amorphous material for hydrogen generation from alkaline NaBH₄ solution was equal to 64.87 kJ mol⁻¹ [66]. Ch. Yue et al. reported a bimetallic Ni₉Co₁ catalyst produced by electrospinning exhibiting both a high specific surface area and excellent stability. Catalyst exhibited of 64.2 kJ mol⁻¹ activation energy of borohydride hydrolysis reaction [79], as well as the above-mentioned Fe₃O₄@SiO₂/Co-Mo-B nanocatalyst synthesized by the hydrothermal method [74]. Moreover, electroless metal deposition is frequently referenced as one of the most elementary and economical techniques for acquiring pure metals [80,81,82,83,84,85]. Various electrolessly prepared catalysts exhibit reliable activation energy of NaBH₄ hydrolysis and hydrogen generation. For example, Wang et al., using the electroless-deposited method, synthesized Co–W–P alloy catalysts supported on γ-Al₂O₃ exhibiting a hydrogen generation rate of 11.82 L min⁻¹ g_cat⁻¹ and the activation energy of 49.58 kJ mol⁻¹ [80]. In turn, the Ni–Fe–B sample prepared via galvanic replacement for 60 min (Ni–Fe–B-60) exhibited good catalytic performance at 313 K, achieving a hydrogen generation rate of 4.3 L min⁻¹ gcat⁻¹ and the apparent activation energy of 33.7 kJ mol⁻¹ for NaBH₄ hydrolysis [83]. Nie et al. present Ni–Fe–B catalysts chemically reduced with NaBH_4, demonstrating 5.14 L min⁻¹ g_cat⁻¹ and the activation energy of 57 kJ mol⁻¹ at the same 313 K temperature [84]. The electroless-deposited Co–Ni–P/Pd-TiO₂ alloy catalyst demonstrated the activation energy of 57 kJ mol⁻¹ in the hydrolysis of alkaline NaBH₄ and preserved 81.4% of its initial activity following the fifth run [82]. In turn, Co–Ni–P catalysts deposited on a Cu sheet by the electroless plating method, having a mock strawberry-like shape, exhibited the hydrogen generation rate of 2.17 L min⁻¹ g⁻¹ and the activation energy of 53.5 kJ mol⁻¹ [85]. A review of the collected data indicates that the method of catalyst preparation can influence the activity of the prepared catalysts.

In this study, we present Ni coatings enhanced by varying amounts of Co deposited on the surface of a Cu sheet using a fast, straightforward, and cost-effective electroless metal deposition technique. The composition of the obtained two-component (NiCo) coatings was characterized using inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The established Co content in the composition was found to be equal to 4, 10, 20, 80, and 90 wt.%. The obtained coatings were thus designated Ni₉₆Co₄/Cu, Ni₉₀Co₁₀/Cu, Ni₈₀Co₂₀/Cu, Ni₂₀Co₈₀/Cu, and Ni₁₀Co₉₀/Cu. The catalytic activity of the prepared catalysts was investigated for the hydrogen generation reaction from the hydrolysis of an alkaline sodium borohydride solution.

2. Materials and Methods

2.1. Chemical Reagents

Nickel sulfate heptahydrate (NiSO₄·7H₂O, 98%), cobalt sulfate heptahydrate (CoSO₄·7H₂O, 99.5%), morpholine borane (MB, C₄H₈ONH·BH₃, 97%), sodium hydroxide (NaOH, 98.8%), palladium chloride (PdCl₂, 99.95%), hydrochloric acid (HCl, 35–38%), copper sheet (Cu, 99.8% purity), glycine (NH₂CH₂COOH, 99.5%) and sulfuric acid (H₂SO₄, 96%) were purchased from Chempur Company (Piekary Śląskie, Poland). Calcium magnesium oxide (50–100%), known as “Vienna Lime”, was purchased from Kremer Pigmente GmbH & Co. KG supplier (Aichstetten, Germany). All reagents were of analytical grade and utilised directly without additional purification.

2.2. Catalyst Preparation and Processing

The NiCo coatings were deposited from an electroless plating solution that contained Ni²⁺ and Co²⁺ precursors, glycine as a ligand for Ni²⁺ and Co²⁺ ions, and MB as a reducing agent.

Table 1. The composition of the electroless plating solutions and their corresponding catalyst deposition parameters.

Catalyst	Composition of Solution (mol L−1)				Deposition Conditions
	NiSO4	Glycine	MB	CoSO4	pH	T, °C	t, min
Ni96Co4/Cu	0.1	0.2	0.2	0.0025	7	50	5
Ni90Co10/Cu				0.005
Ni80Co20/Cu				0.01
Ni20Co80/Cu				0.1
Ni10Co90/Cu	0.05			0.2

presents the chemical composition of the electroless plating solution and the deposition conditions for each catalyst. The optimal pH value of 7 was achieved by adding a NaOH solution during the preparation of the plating solution at room temperature.

The Cu sheet as the substrate was predicated on its stability and the ease with which identical Ni coatings can be deposited on it. Figure 1 demonstrates the preparation process of coatings. The initial step involved subjecting Cu sheets (1 cm × 1 cm) to a mechanical pretreatment with “Vienna Lime” to clean the surface. The sheets were then rinsed with deionized water, held in a solution of 10% HCl for one minute in order to eliminate any remaining inorganic impurities, and then cleaned again. Following the cleaning procedure, the Cu surfaces underwent activation by immersing them in a solution of 0.5 g L⁻¹ PdCl₂ for 10 s, facilitating the deposition of Pd(II) species onto the substrate. Further, the activated and rinsed with deionized water Cu sheets of 2 cm² geometrical area were placed into freshly prepared electroless plating solutions (Table 1) at a temperature of 50 °C for 5 min to deposit Ni₉₆Co₄, Ni₉₀Co₁₀, Ni₈₀Co₂₀, Ni₂₀Co₈₀, and Ni₁₀Co₉₀ coatings. Furthermore, Ni/Cu and Co/Cu coatings were prepared under identical conditions for the purpose of comparison.

2.3. Structural and Surface Characterization Methods

The surface topography of the NiCo/Cu catalysts was characterized by means of scanning electron microscopy (SEM) using a TM4000 Plus (HITACHI, Tokyo, Japan) microscope. The samples were mounted on aluminum stubs using conductive carbon tape. SEM imaging was performed in high-vacuum mode at an accelerating voltage of 15 kV. Elemental analysis was conducted using an Oxford Instruments NanoAnalysis EDS system equipped with AZtecOne software (version 3.3; Oxford Instruments plc, High Wycombe, UK) at an accelerating voltage of 15 kV and a live time of 60 s. The composition of the catalysts was analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. Measurements were carried out on a Perkin Elmer Optima 7000DV spectrometer (PerkinElmer, Waltham, MA, USA) at the characteristic wavelengths of 231.604 nm for Ni and 228.616 nm for Co.

X-ray photoelectron spectroscopy (XPS) was utilized to investigate the surface composition of different NiCo catalysts. This was achieved by employing a Kratos Analytical spectrometer (Kratos AXIS Supra+, Manchester, UK) with monochromatic Al Kα radiation (1486.6 eV) at a power of 225 W. The analysis chamber maintained a base pressure below 1 × 10⁻⁹ mbar, and a low-energy electron flood gun was applied for charge compensation. Survey spectra were acquired with a pass energy of 160 eV and an energy step of 1 eV, while high-resolution spectra of individual elemental peaks were acquired at a pass energy of 20 eV with 0.1 eV steps. The scale of binding energy was referenced to the adventitious carbon peak at 284.8 eV. The high-resolution Ni 2p and Co 2p spectra were fitted after Shirley background subtraction, and the Ni 2p analysis was performed considering only the 2p₃/₂ region. Metallic Ni⁰ and Co⁰ components were modeled using asymmetric Lorentzian–Gaussian line shapes, LA(1.2, 2.2, 10) for Ni⁰ and LA(1.33, 2.44, 69) for Co⁰, while all oxidized species (Ni²⁺, Co²⁺, Co³⁺) and their satellite features were fitted using symmetric GL(30) line shapes. All XPS data were first converted to VAMAS format, then processed using CasaXPS software version 2.3.26PR1.0 (Casa Software Ltd., Teignmouth, Devon, UK).

An X-ray diffractometer D2 PHASER, equipped with a LYNXEYE XE-T detector and Cu Kα radiation (λ = 1.54060 Å) (Bruker, Karlsruhe, Germany), was used to obtain the XRD patterns of the studied catalysts. The diffractometer was operated at a voltage of 30 kV and a current of 10 mA. Measurements were carried out in step scan mode within the 2θ range of 10–90°, with a step size of 0.041° (on the 2θ scale) and a counting time of 2 s per step. The phase was identified using the software package DIFFRAC.EVA version V5.0 (Bruker, Karlsruhe, Germany). Furthermore, the lattice parameter (a) was estimated from the peak positions at the (111) reflections using Bragg’s law (Equation (2)):

nλ = 2d sinθ

(2)

where n is an integer (the order of reflection, n = 1), λ is the wavelength of the incident X-ray (Cu Kα = 1.5406 Å), d is the distance between the atomic planes in the crystal, and θ is the angle of incidence (and reflection). The lattice spacing of a cubic system can be determined through the following relation (Equation (3)):

$$d= {\displaystyle \frac{a}{\sqrt{h^2+k^2+l^2}}}$$

(3)

where a is the lattice spacing of the cubic crystal, and h, k, and l are the Miller indices of the Bragg plane: h + k + l = 3. The lattice spacing can be calculated using Equations (4) and (5):

$$a= {\displaystyle \frac{\lambda }{2 sin\theta }}\sqrt{3}$$

(4)

where

$$\theta ={\displaystyle \frac{2\theta }{2}}$$

(5)

2.4. Hydrolysis Measurements of NaBH₄

Hydrogen generation experiments were conducted using a thermostatically controlled, hermetically sealed reaction flask connected to a MilliGascounter (type MGC-1 V3.2 PMMA, Ritter, Bochum, Germany) for accurate volume measurement. The reaction mixture consisted of 15 mL of an alkaline solution containing 0.4 wt.% NaOH and 5 wt.% NaBH₄. The hydrogen evolved, catalyzed by the synthesized NiCo/Cu catalysts, was quantified in real time using a MilliGascounter, which was interfaced with a personal computer. During each measurement, the catalysts were immersed in the NaBH₄ solution, which was maintained at the target temperature, and stirred continuously using a magnetic stirrer. The hydrogen generation rate (HGR) was recorded across a temperature range of 303–343 K in order to ascertain the activation energy (E_a) of the hydrolysis reaction. E_a is defined as the minimum energy required for reactants to undergo transformation and is inversely related to catalytic activity—lower E_a values generally indicate higher reactivity. The Arrhenius equation was used to calculate the E_a, with the linear relationship between ln(k) and 1/T presented in Equation (2):

$$lnk =lnA-{\displaystyle \frac{E_a}{RT}}$$

(6)

where A—the pre-exponential factor, E_a—the activation energy (J), R—the general gas constant (8.314 J mol⁻¹ K⁻¹), and k—the reaction coefficient.

3. Results and Discussion

3.1. Morphology and Composition of Coatings

As was described above, Co-containing catalysts exhibit efficient catalytic activity towards the hydrogen generation reaction; therefore, in this work, we investigated Co as an additive for Ni coatings to enhance the activity of Ni coatings and diminish the use of cobalt due to the above-mentioned reasons. The aim of this work was to prepare efficient and industrially attractive catalysts using a simple, fast, cost-effective electroless nickel plating process. During the 5 min deposition process, NiCo coatings with varying cobalt mass percentages ranging from 4 to 90 wt.% were deposited on Cu sheets and labeled as Ni₉₆Co₄/Cu, Ni₉₀Co₁₀/Cu, Ni₈₀Co₂₀/Cu, Ni₂₀Co₈₀/Cu, and Ni₁₀Co₉₀/Cu. These coatings were deposited using chemical deposition baths, with MB acting as a reductant. Figure 2 presents the surface morphology of the prepared Ni₉₆Co₄/Cu (a), Ni₉₀Co₁₀/Cu (b), Ni₈₀Co₂₀/Cu (c), Ni₂₀Co₈₀/Cu (d), and Ni₁₀Co₉₀/Cu (e) catalysts as well as particles distribution (Figure 2a′–e′) and elemental composition (Figure 2a″–e″) evaluated by SEM and EDX techniques. The images indicate that the catalyst surfaces exhibit distinctive multi-layered cauliflower-like architectures, with particles densely and uniformly distributed showing no visible cracks or voids. Additionally, SEM analysis reveals that the coatings consist of particles of different sizes, which merge to form oval-shaped agglomerates (Figure 2a–e), thereby enhancing the exposed surface area of the catalysts. The average size of the surface agglomerates was determined to range between 0.20 and 2.5 µm. As illustrated in Figure 2a′–e′, the predominant particle size ranges from approximately 0.4 to 0.7 µm. Figure 2a″–e″ show the EDX spectra of the same catalysts. EDX indicates the presence of nickel and cobalt in all the coatings. Additionally, an increase in cobalt and a decrease in nickel are observed with the corresponding percentage of metal in the coating of a given catalyst. Furthermore, the percentage of Ni and Co metals in the coating obtained by EDX analysis correlates to that obtained by ICP-OES (

Table 2. The elemental composition and metal loading of the prepared NiCo/Cu coatings analyzed via ICP-OES.

Catalyst	Element, wt.%		Element Loadings, µg cm−2			Deposition Rate, µm h−1
	Ni	Co	Ni	Co	Ni + Co
Ni96Co4/Cu	96.45	3.55	1047.0	38.5	1085.5	14.6
Ni90Co10/Cu	89.95	10.05	789.0	88.2	877.2	11.8
Ni80Co20/Cu	79.58	20.42	732.9	188.1	921.0	12.4
Ni20Co80/Cu	22.61	77.39	179.4	614.0	793.4	10.7
Ni10Co90/Cu	9.56	90.44	52.5	496.3	548.8	7.4

). As demonstrated in the experimental findings, the metal content, as determined by ICP-OES analysis, exhibits a decline in the following sequence for the catalysts designated Ni₉₆Co₄/Cu, Ni₉₀Co₁₀/Cu, Ni₈₀Co₂₀/Cu, Ni₂₀Co₈₀/Cu, and Ni₁₀Co₉₀/Cu: from an initial value of 1047 µg cm⁻² for Ni to a final value of 52.5 µg cm⁻², and from an initial value of 38.5 µg cm⁻² for Co to a final value of 496.3 µg cm⁻² (Table 2). As illustrated in Table 2, the deposition rate of NiCo coatings exhibits a decline with an increase in the amount of Co present in the coatings. The predominance of Co²⁺ in the bath results in a reduction in the overall deposition rate. In general, Ni and Co co-deposit due to their analogous standard reduction potentials of Ni²⁺/Ni⁰ (−0.25 V) and Co²⁺/Co⁰ (−0.28 V). However, Co²⁺ typically exhibits lower reduction kinetics compared to Ni²⁺ when complexed with glycine and reduced by MB [86,87,88,89]. A comparison of the deposition bath at the lowest and highest concentrations of Co²⁺ reveals a twofold decrease in deposition rate (Table 2).

3.2. Structural Characterization of the Catalysts (XRD)

X-ray diffraction (XRD) analysis revealed that all synthesized catalysts exhibited highly similar crystallographic patterns (Figure 3). All profiles displayed three sharp diffraction peaks at 2θ = ~44°, 51°, and 74°, corresponding to the (111), (200), and (220) planes of crystalline Cu from the underlying Cu sheet (JCPDS No. 04-0836) [90] as a result of the penetration depth of X-rays through the coating.

These reflections are also consistent with the positions of the characteristic planes of face-centered cubic (FCC) Ni (JCPDS No. 04-0850) and Co (JCPDS No. 15-0806), suggesting the formation of a NiCo alloy with analogous (111), (200), and (220) planes [91,92]. The relative intensities of these peaks varied with Co content, indicating that cobalt incorporation influences the Ni–Co alloy structure. In the case of the Co-free sample (X = 0), the reflections match well with standard FCC Ni. A pronounced (111) peak indicates a preferred orientation along this plane, which is typical of electrodeposited Ni coatings (Figure 3). Upon incorporation of Co (X = 4–20 wt.%), the diffraction peaks systematically shift relative to pure Ni, without the appearance of additional phases. This continuous peak displacement confirms the formation of a substitutional Ni–Co solid solution, wherein Co atoms replace Ni atoms in the lattice. This substitution leads to a change in the lattice parameter, in accordance with Vegard’s law. At higher Co contents (X = 80 and 90 wt.%), the patterns still show only FCC-type reflections and no evidence of hexagonal close-packed (HCP) Co or intermetallic compounds. This suggests that the coatings retain a single-phase FCC structure despite the high Co concentration. Furthermore, the lattice constants for the investigated NiCo coatings were calculated from the peak positions at the (111) reflections using Bragg’s law (see 2.3 Characterization of Catalysts), and the results are shown in

Table 3. The calculated lattice constants of NiCo coatings with varying amounts of Co.

Co (wt.%)	2θ (111)	a, Å
0	44.50	3.5235
10	44.65	3.5123
20	44.80	3.5012
80	45.05	3.4828
90	45.15	3.4754

.

As evident from the obtained data, the lattice parameter (a) calculated from the (111) reflection decreases progressively as the Co content increases. It decreases from ~3.5235 Å for pure Ni to ~3.4754 Å at 90 wt.% Co, indicating lattice contraction due to the substitution of smaller Co atoms into the Ni FCC lattice. The linear variation in the lattice parameter with composition confirms the formation of a continuous Ni–Co solid solution in accordance with Vegard’s law. The progressive decrease in peak intensity with increasing Co content implies enhanced amorphization of the bimetallic phase [93]. Additionally, the absence of distinct Ni and Co reflections indicates lower crystallinity of the samples [94].

3.3. Surface Chemistry and Oxidation States (XPS)

The XPS analysis was used to investigate the elemental composition of the surface of the prepared Ni₉₆Co₄/Cu, Ni₉₀Co₁₀/Cu, Ni₈₀Co₂₀/Cu, Ni₂₀Co₈₀/Cu, and Ni₁₀Co₉₀/Cu coatings. Figure 4 presents the high-resolution spectra of Ni 2p, Co 2p, and O 1s for each coating. In general, typical XPS spectra with clearly resolved spin–orbit doublets characteristic of Ni and Co were observed for all investigated coatings (Figure 4).

The most prominent Ni 2p₃/₂ binding energy peak, located at 852.6–852.8 eV, corresponds to metallic Ni⁰ (Figure 4a–e) [95]. In addition to the metallic component, a noticeable contribution at 855.4–856.4 eV is observed, which is attributed to Ni²⁺ species. Furthermore, the shake-up satellite features located at approximately 861–864.6 eV confirm the presence of oxidized nickel species [96,97,98]. Peaks in the range of 853.1–856.4 eV can be attributed to NiO, while satellite peaks at higher binding energies (857.3–864.6 eV) correspond to Ni(OH)₂ compounds [99]. The relative intensity of the Ni²⁺ component indicates partial surface oxidation of nickel in all coatings.

The analysis of the Co 2p XPS spectra reveals the presence of characteristic Co 2p₃/₂ and 2p₁/₂ peaks. The Co 2p₃/₂ binding energy peak at 777.8–778.3 eV and the Co 2p₁/₂ peak at 792.8–793.3 eV correspond to metallic Co⁰ for all investigated coatings (Figure 4f–j) [99]. Furthermore, the high-intensity Co 2p₃/₂ peaks at 781.5 ± 0.5 eV and Co 2p₁/₂ peaks at 797.3 ± 0.5 eV, accompanied by satellite peaks at 785.3 ± 0.6 eV (2p₃/₂) and 802.4 ± 0.5 eV (2p₁/₂), are associated with Co²⁺/Co³⁺ species, indicating the presence of CoO, Co(OH)₂, and Co₃O₄ compounds in the coatings [100,101]. The relatively strong satellite structure suggests a significant contribution of oxidized cobalt species at the surface.

The intense binding energy peak at 531.9 eV in the O 1s spectra for all coatings can be attributed to surface hydroxyl groups and defect-related oxygen species (Figure 4k–o) [102]. A lower binding energy component at 530.6–530.8 eV, clearly visible in the Co-rich coatings (Ni₂₀Co₈₀/Cu and Ni₁₀Co₉₀/Cu), corresponds to lattice oxygen (M–O), indicating the formation of cobalt oxide species. Additionally, weak binding energy peaks at approximately 534.0–535.2 eV in the O 1s spectra can be attributed to adsorbed H₂O on the coating surfaces (Figure 4k–o) [102].

In summary, all coatings contain metallic Ni and Co; however, both elements exhibit partial surface oxidation. The Co 2p spectra show more pronounced oxidized components and satellite features, indicating that cobalt has a higher tendency toward oxidation compared to nickel under the investigated conditions.

3.4. Catalytic Activity of NiCo/C Toward NaBH₄ Hydrolysis

The catalytic performance of the synthesized NiCo/Cu coatings toward alkaline sodium borohydride hydrolysis was investigated using an aqueous solution containing 5 wt.% NaBH₄ and 0.4 wt.% NaOH within the temperature interval of 303–343 K. The effect of temperature on hydrogen evolution is summarized in Figure 5, while the corresponding Arrhenius plots used to determine the apparent activation energies (E_a) are shown in Figure 5a′–e′. For all investigated catalysts, an increase in temperature resulted in a pronounced enhancement of the hydrogen generation rate (HGR) (Figure 5). Specifically, when the temperature was raised from 303 to 343 K, the HGR values of the Ni₉₆Co₄/Cu catalyst range from 0.31 to 5.09 mL min⁻¹, the Ni₉₀Co₁₀/Cu catalyst ranges from 0.42 to 5.73 mL min⁻¹, the Ni₈₀Co₂₀/Cu catalyst ranges from 0.56 to 8.30 mL min⁻¹, the Ni₂₀Co₈₀/Cu catalyst ranges from 1.24 to 14.59 mL min⁻¹, and the Ni₁₀Co₉₀/Cu catalyst ranges from 1.46 to 15.61 mL min⁻¹ (

Table 4. Hydrogen generation characteristics and activation energies of the prepared catalysts.

Catalyst	Ea, kJ mol−1	T, K	v, mL min−1	v, L min−1 gcat−1
Ni96Co4/Cu	63.81	303	0.31	0.14
		313	0.46	0.21
		323	1.10	0.51
		333	2.94	1.35
		343	5.09	2.34
Ni90Co10/Cu	59.55	303	0.42	0.24
		313	0.66	0.38
		323	1.38	0.79
		333	3.56	2.03
		343	5.73	3.27
Ni80Co20/Cu	57.82	303	0.56	0.30
		313	1.09	0.59
		323	1.76	0.96
		333	3.77	2.05
		343	8.30	4.51
Ni20Co80/Cu	56.4	303	1.24	0.78
		313	2.13	1.34
		323	5.77	3.64
		333	10.66	6.72
		343	14.59	9.19
Ni10Co90/Cu	52.5	303	1.46	1.33
		313	2.84	2.59
		323	6.93	6.31
		333	10.24	9.33
		343	15.61	14.22

). Table 4 presents the volume of HGR normalized by the mass (L min⁻¹ g_cat⁻¹) of the Ni and Co components as well. These values have been achieved by dividing the volumetric rate by the mass of the NiCo amount determined by ICP-OES. Beyond the temperature effect, the catalytic activity was strongly influenced by the chemical composition of the coatings. As evidenced in Table 4, increasing the cobalt content in the catalyst led to a systematic rise in HGR. At 303 K, the hydrogen evolution rate of the Ni₁₀Co₉₀/Cu catalyst was approximately five times higher than that of the Ni₉₆Co₄/Cu coating, highlighting the beneficial role of cobalt incorporation. To further clarify the contribution of cobalt, the performance of NiCo/Cu coatings was compared with that of monometallic Ni/Cu and Co/Cu catalysts prepared under identical conditions (Figure 6 and

Table 5. Hydrogen generation characteristics and activation energies of the prepared pure Ni/Cu and Co/Cu catalysts.

Catalyst	Ea, kJ mol−1	T, K	v, mL min−1	v, L min−1 gcat−1
Ni/Cu	67.9	303 K	0.18	0.24
		313 K	0.39	0.52
		323 K	0.77	1.03
		333 K	1.64	2.19
		343 K	3.99	5.34
Co/Cu	55.5	303 K	0.94	0.75
		313 K	1.89	1.51
		323 K	3.71	2.97
		333 K	7.13	5.71
		343 K	11.92	9.55

). The introduction of a small amount of cobalt (Ni₉₆Co₄/Cu) resulted in an approximately 1.5-fold increase in HGR across the entire temperature range relative to the Ni/Cu catalyst. This enhancement can be attributed to the increased density of catalytically active sites and synergistic interactions between Ni and Co species [103]. Notably, coatings with high cobalt content (80–90 wt.%) exhibited superior activity, achieving up to a twofold increase in HGR compared to Co/Cu and up to a sixfold improvement relative to Ni/Cu catalysts. However, this performance was achieved with a reduced overall cobalt demand compared to monometallic Co/Cu catalysts. The observed result suggests a correlation between the activity of two-component coatings and not only the increased cobalt content, but also the synergistic effect.

A comparative analysis of the activation energy values obtained for the Ni₉₆Co₄/Cu, Ni₉₀Co₁₀/Cu, Ni₈₀Co₂₀/Cu, Ni₂₀Co₈₀/Cu, and Ni₁₀Co₉₀/Cu catalysts reveals a systematic decrease in E_a from 63.81 to 52.5 kJ mol⁻¹ with increasing cobalt content in the coatings (63.81 > 59.66 > 57.82 > 56.4 > 52.5 kJ mol⁻¹, respectively). This trend clearly indicates that cobalt incorporation facilitates the hydrolysis of NaBH₄. A similar tendency is observed when comparing monometallic Ni/Cu and Co/Cu catalysts with their bimetallic NiCo/Cu counterparts. The activation energy of the Ni/Cu coating (67.9 kJ mol⁻¹) decreases from the lowest 4 wt.% content of cobalt (63.81 kJ mol⁻¹). It is noteworthy that the Ni₁₀Co₉₀/Cu catalyst exhibits the E_a value of 52.5 kJ mol⁻¹, which is lower than that of the bare Co/Cu catalyst (55.5 kJ mol⁻¹). This result indicates the beneficial effect of combining both metals. While cobalt has been demonstrated to have higher activity than nickel in terms of NaBH₄ hydrolysis, it is characterized by diminished stability in alkaline environments. In contrast, nickel has been demonstrated to enhance the stability of catalysts and modify the electronic environment of cobalt atoms, thereby augmenting the overall catalytic performance [104,105]. The obtained results confirm that the bimetallic NiCo/Cu coatings exhibit superior catalytic activity and hydrogen generation compared to monometallic systems, which can be attributed to synergistic interactions between Ni and Co [13,105]. Furthermore, the incorporation of transition metals has been documented to enhance critical catalytic parameters, such as activity, durability, and stability, while concomitantly reducing the activation energy. Metals of this nature, which are frequently present in oxidized form, have been shown to act as promoters by modifying the structural and electronic environment of the catalyst [59,106]. In accordance with the aforementioned observations, XPS analysis has confirmed the presence of oxidized cobalt species, most likely cobalt trioxide (Co₃O₄). This finding suggests that the oxidized cobalt species may further influence the catalytic behavior by promoting interfacial interactions and contributing to the overall catalytic process.

As discussed in the preceding section, a substantial body of research has been conducted on the hydrolysis of borohydride, encompassing a wide range of catalysts, including both noble and non-precious metals, as well as single metals, compounds, and alloys. The outcomes of these studies have been documented in the extant literature.

Table 6. Comparison of activation energies obtained in this work with those recently reported in the literature.

Catalyst	Electrolyte	Ea, kJ mol−1	Reference
Co–Ni–P/Pd-TiO2	0.30 M NaBH4 + 10 wt.% NaOH	57.0	[82]
Ni-B	10 wt.% NaBH4 + 5 wt.% NaOH	61.84	[72]
NiCoB	2.7 wt.% NaBH4 + 15 wt.% NaOH	62.0	[107]
Ni9Co1	5 wt.% NaBH4 + 2 M NaOH	64.2	[79]
Co/Al2O3–Ni foam	20 wt.% NaBH4 + 5 wt.% NaOH	64.3	[108]
Co-B	20 wt.% NaBH4 + 5 wt.% NaOH	64.87	[66]
Co	5 wt.% NaBH4 + 5 wt.% NaOH	66.67	[78]
Ni/Cu	5 wt.% NaBH4 + 0.4 wt.% NaOH	67.9	[109]
Nickel–Cobalt (Alloy)	5 wt.% NaBH4 + 1 wt.% NaOH	68.84	[110]
NiB/NiFe2O4	5 wt.% NaBH4 + 10 wt.% NaOH	72.52	[71]
Ni96Co4/Cu	5 wt.% NaBH4 + 0.4 wt.% NaOH	63.81	this study
Ni90Co10/Cu	5 wt.% NaBH4 + 0.4 wt.% NaOH	59.55	this study
Ni80Co20/Cu	5 wt.% NaBH4 + 0.4 wt.% NaOH	57.82	this study
Ni20Co80/Cu	5 wt.% NaBH4 + 0.4 wt.% NaOH	56.4	this study
Ni10Co90/Cu	5 wt.% NaBH4 + 0.4 wt.% NaOH	52.5	this study

presents a comparison of the results obtained from our prepared Ni₉₆Co₄/Cu, Ni₉₀Co₁₀/Cu, Ni₈₀Co₂₀/Cu, Ni₂₀Co₈₀/Cu, and Ni₁₀Co₉₀/Cu catalysts with the research data previously presented in the literature on the activation energies (Table 6). It is seen that our prepared catalysts exhibit the activation energy values in the range of 52.5 to 63.81 kJ mol⁻¹, which are comparable or even lower if compare with various catalyst described, such as Co-Ni-P/Cu (53.5 kJ mol⁻¹) [85], Co-Ni-P/Pd-TiO₂ (57 kJ mol⁻¹) [82], Ni-B (61.84 kJ mol⁻¹) [72], NiCoB (62 kJ mol⁻¹) [107], Ni₉Co₁ (64.2 kJ mol⁻¹) [79], Co/Al₂O₃-Ni foam (64.3 kJ mol⁻¹) [108], Co-B (64.87 kJ mol⁻¹) [66], Co (66.67 kJ mol⁻¹) [78], Ni/Cu (67.9 kJ mol⁻¹) [109], nickel-cobalt (alloys) (68.84 kJ mol⁻¹) [110] or NiB/NiFe₂O₄ (72.52 kJ mol⁻¹) [71]. Moreover, taking into account the simplicity and cost-effectiveness of the preparation method, our prepared catalysts are competitive with other catalysts described recently for the application of hydrogen generation from an alkaline sodium borohydride solution. The investigation demonstrated that the Ni₁₀Co₉₀/Cu catalyst exhibited the highest efficiency in catalyzing the NaBH₄ hydrolysis reaction, resulting in the greatest H₂ generation volume. As a result, the stability and reusability tests were carried out on the Ni₁₀Co₉₀/Cu catalyst, given its key role in the practical application of the hydrogen generation systems.

A reusability test was carried out by measuring the Ni₁₀Co₉₀/Cu catalyst in five cycles of hydrogen generation in a 5 wt.% NaBH₄ and 0.4 wt.% NaOH solution at 323 K (Figure 7). At the end of each cycle, the catalyst was meticulously washed multiple times with deionized water to eliminate any sodium metaborate that might have accumulated, as this could potentially result in clogging of the active centers. Following this thorough cleansing, the catalyst was dried and reused. As illustrated in Figure 7, the catalytic performance of the Ni₁₀Co₉₀/Cu catalyst displays negligible variation. Subsequent to five cycles, the Ni₁₀Co₉₀/Cu catalyst maintained 87.9% of its catalytic activity toward NaBH₄ hydrolysis compared to its initial value (Figure 7). Figure 7 indicates that during the test, after the third cycle, the catalytic activity of Ni₁₀Co₉₀/Cu stabilized in comparison to the level observed at the end of the first cycle. A survey of analogous catalytic systems suggests that the modest decline in activity observed during the first three cycles may be attributed to the accumulation of trace amounts of sodium metaborate by-products on the catalyst surface. The deposition of these species on active sites can partially occlude them, thereby impeding hydrogen evolution and resulting in a gradual decline in catalytic activity. It is noteworthy that the SEM analysis of the Ni₁₀Co₉₀/Cu catalyst, conducted after five catalytic cycles, reveals no substantial alterations in morphology (Figure 7b), suggesting that the overall surface structure remains unaltered. Furthermore, surface oxidation of nickel-containing catalysts may also contribute to the observed activity loss during the initial cycles [111]. Moreover, ICP-OES measurements performed after the third cycle demonstrated a negligible alteration in the mass of the coating catalysts. Nonetheless, following the fifth cycle, the ICP-OES results demonstrate a lack of further alterations. It is reasonable to hypothesize that the combined effect of these factors is responsible for the moderate decrease in catalyst surface activity that is observed during the initial stages of cycling. These observations suggest that the initial activity loss is more likely associated with surface-related chemical changes rather than structural degradation of the catalyst. Although a slight decrease in the catalytic activity was observed over five cycles, the obtained result is significant if compared with other similar catalysts reported in the literature, such as Co-Ni-P/Cu (81.4%) [85], Co-Mn-B (73%) [106], Co–Ni–P/Pd-TiO₂ (86.4%) [82], Ni–Fe–B-60 (90%) [83] or Co/IR-120 (53.3%) [78].

In summary, the proposed Ni₉₆Co₄/Cu, Ni₉₀Co₁₀/Cu, Ni₈₀Co₂₀/Cu, Ni₂₀Co₈₀/Cu, and Ni₁₀Co₉₀/Cu catalysts prepared by a rapid, straightforward, and cost-effective electroless deposition method exhibited relatively low activation energies and confirmed adequate catalytic efficiency for borohydride hydrolysis. This phenomenon can be attributed to the unique surface properties of the substrate and the synergistic effect of Ni and Co.

4. Conclusions

In this work, NiCo/Cu coatings with varying cobalt contents ranging from 4 to 90 wt.% were successfully deposited by employing the rapid, simple, and cost-effective electroless metal deposition method. In this instance, morpholine borane functions as a reductant in the glycine-based plating solution. The NiCo/Cu coatings that were deposited were found to have compact, multi-layered cauliflower structures, with no evidence of cracks or defects. These coatings demonstrated active catalytic behavior in the sodium borohydride hydrolysis reaction.

The rate of hydrogen evolution is contingent upon two factors: the cobalt concentration in the catalyst and the temperature of the reaction solution. In both cases, an increase in the cobalt content and temperature results in an increase in the hydrogen evolution rate. Even the lowest amount of cobalt in the Ni₉₆Co₄/Cu catalysts diminishes the activation energy (63.81 kJ mol⁻¹) and almost twofold enhances the hydrogen generation rate (0.31 mL min⁻¹) compared to the bare Ni/Cu catalyst, which demonstrated the activation energy of 67.9 kJ mol⁻¹ and a hydrogen generation rate of 0.18 mL min⁻¹ at a temperature of 303 K for the investigated NaBH₄ hydrolysis reaction. The Ni₁₀Co₉₀/Cu catalyst was observed to be predominantly effective in generating hydrogen at rates ranging from 1.33 to 14.22 L min⁻¹ g_cat⁻¹ within the temperature range from 303 to 343 K, respectively. Conversely, the activation energy of the borohydride hydrolysis reaction exhibits a decrease from 63.81 to 52.5 kJ mol⁻¹ when the cobalt content in the NiCo/Cu catalysts is augmented from 4 to 90 wt.%. This observation signifies that Ni₁₀Co₉₀/Cu manifests the lowest activation energy and the most substantial H₂ generation for the examined alkaline borohydride hydrolysis reaction. It is noteworthy that all the investigated Ni₉₆Co₄/Cu, Ni₉₀Co₁₀/Cu, Ni₈₀Co₂₀/Cu, Ni₂₀Co₈₀/Cu, and Ni₁₀Co₉₀/Cu catalysts exhibited superior performance in comparison to the results obtained for the bare Ni/Cu and Co/Cu catalysts.

The incorporation of Co into Ni/Cu coatings has been demonstrated to enhance the catalytic activity of two-component catalysts, particularly with respect to the hydrogen generation rate. This process has been shown to reduce the activation energy of the reaction and to exhibit remarkable reusable stability of catalytic activity for NaBH₄ hydrolysis. Consequently, these catalysts are a promising option for hydrogen production from alkaline NaBH₄ solutions.