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Article

Co-Doped Ni@Ni(OH)2 Core–Shell Catalysts for Dual-Function Water and Urea Oxidation

by
Saba A. Aladeemy
1,*,
Maged N. Shaddad
1,
Talal F. Qahtan
2,*,
Abdulrahman I. Alharthi
1,
Kamal Shalabi
1,
Abdullah M. Al-Mayouf
3 and
Prabhakarn Arunachalam
3
1
Chemistry Department, College of Science and Humanities in Al-Kharj, Prince Sattam bin Abdulaziz University, P.O. Box 173, Al-Kharj 11942, Saudi Arabia
2
Physics Department, College of Science and Humanities in Al-Kharj, Prince Sattam bin Abdulaziz University, P.O. Box 173, Al-Kharj 11942, Saudi Arabia
3
Electrochemical Sciences Research Chair (ESRC), Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(5), 474; https://doi.org/10.3390/catal15050474
Submission received: 5 April 2025 / Revised: 30 April 2025 / Accepted: 5 May 2025 / Published: 12 May 2025
(This article belongs to the Special Issue Nanocatalysis: Integrating Sustainability and Innovation Across Diverse Applications)
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Abstract

:
Crystalline–amorphous core–shell-like heterostructures have attracted considerable attention in electrocatalysis due to their unique electronic and structural properties; however, tuning the surface composition of the amorphous shell remains a major challenge. In this work, we report a simple, low-cost, one-pot hydrazine-assisted chemical deposition method for synthesizing a series of Co-doped Ni@Ni(OH)2 catalysts with a crystalline Ni core and an amorphous Ni(OH)2 shell. Among the prepared catalysts, the sample containing 10 wt.% cobalt (denoted as b-Co-doped Ni@Ni(OH)2) exhibited the highest electrocatalytic activity toward both the oxygen evolution reaction (OER) and the urea oxidation reaction (UOR). In 1.0 M KOH, the b-Co-doped Ni@Ni(OH)2 catalyst achieved a 40 mV lower overpotential at 50 mA·cm−2 compared to undoped Ni@Ni(OH)2 for the OER. For the UOR in 0.33 M urea/1.0 M KOH, it delivered approximately twice the anodic current density relative to the undoped sample, along with improved reaction kinetics as evidenced by a Tafel slope of 70.7 mV·dec−1. This performance enhancement is attributed to the optimized core–shell-like architecture, cobalt doping-induced electronic modulation, increased electrochemically active surface area, and improved charge transfer efficiency. Overall, this study demonstrates a promising and scalable strategy for designing advanced Ni-based bifunctional catalysts for sustainable energy conversion and wastewater treatment applications.

1. Introduction

The advancement of sustainable hydrogen production technologies is critical to achieving a carbon-neutral future and addressing the growing global energy demand [1,2]. Hydrogen (H2) is considered a clean energy carrier due to its high energy density and zero-carbon emission combustion, making it pivotal for decarbonizing various sectors [3,4,5]. Hydrogen fuel (H2), with its superior energy density and environmentally favorable properties, has attracted attention as a promising alternative to conventional renewable energy vectors [6,7]. However, the overall efficiency of water electrolysis is significantly limited by the sluggish kinetics of the oxygen evolution reaction (OER), which involves a complex four-electron transfer process and multiple proton-coupled electron transfer steps, leading to a high overpotential requirement [8,9,10,11,12]. Therefore, developing efficient, low-cost electrocatalysts to accelerate the OER is vital for advancing hydrogen production technologies.
To reduce the high energy input required by conventional water electrolysis, the anodic urea oxidation reaction (UOR) has been proposed as a promising alternative strategy [12,13]. Unlike the OER, which demands a high thermodynamic potential of 1.23 V versus the reversible hydrogen electrode (RHE), the UOR proceeds at a much lower theoretical oxidation potential of 0.37 V versus RHE [14].
This substantial reduction in required potential translates into significant energy savings, thereby improving the overall efficiency of hydrogen production systems. Additionally, the UOR offers the unique advantage of simultaneously addressing environmental concerns, as it enables the electrochemical treatment of urea-rich wastewater streams, which are generated from agricultural runoff, municipal sewage, and industrial processes [15,16,17]. Urea, a nitrogen-rich organic compound, is abundant in such wastewaters, and its electrooxidation not only aids in water purification but also contributes to sustainable energy generation [18,19,20,21]. The fundamental anodic and cathodic half-reactions for urea-assisted electrolysis are described by the following equations:
Anodic reaction: CO(NH2)2 + 6OH− → N2 + CO2 + 5H2O + 6e−
Cathodic reaction: 6H2O + 6e− → 3H2 + 6OH−
Through this reaction pathway, hydrogen gas evolves at the cathode while nitrogen gas (N2) and carbon dioxide (CO2) are generated at the anode, minimizing the release of harmful byproducts. Furthermore, compared to conventional water splitting, urea electrolysis can reduce the total energy consumption for hydrogen production by approximately 70%, making it a highly energy-efficient and cost-effective process [21,22,23].
Although noble metal catalysts, such as IrO2, and RuO2, exhibit outstanding catalytic activity for both the OER and UOR, their high cost and scarcity severely limit their large-scale applications. Consequently, efforts have focused on developing cost-effective, non-noble metal-based alternatives [4,24,25,26]. Ni(OH)2 exists in different structural forms, including the α-phase, β-phase, and an amorphous state, each with distinct electrochemical characteristics [27,28,29,30]. Ni(OH)2’s two-dimensional layered structure enhances ionic conductivity and electrochemical performance in many electrocatalytic applications by intercalating various anions [31,32,33]. While amorphous Ni(OH)2 typically provides more abundant active sites and faster ion diffusion pathways [26,34,35,36], it suffers from poor intrinsic electrical conductivity and limited structural stability, which hinder its practical application in electrocatalysis.
To overcome these limitations, two main strategies have been explored. The first involves the design of crystalline–amorphous core–shell nanostructures, such as Ni@Ni(OH)2, where a conductive crystalline Ni core improves electron transport, while the amorphous Ni(OH)2 shell enhances catalytic activity [34,35,37]. This approach effectively combines the electrical advantages of metallic Ni with the chemical activity of hydroxide shells. However, fabricating such architectures often involves complicated multi-step processes, including template-assisted synthesis and post-treatment [14,37,38]. The second strategy focuses on heteroatom doping by incorporating elements, such as Fe [11], Al [39], Zn [40], and Co [41], into Ni(OH)2 lattices to modulate the electronic structure and improve conductivity and stability. Among these, cobalt (Co) doping is particularly attractive because Co2+ ions have a similar ionic radius to Ni2+, allowing substitution with minimal lattice distortion while effectively enhancing electronic conductivity and structural robustness [12,42,43,44].
Previous studies have demonstrated the potential benefits of cobalt incorporation. For example, Zhang et al. [45] synthesized a Ni-Co@CC heterostructure that exhibited excellent UOR performance with a low overpotential of 1.362 V at η10 in 0.33 M urea solution. Similarly, Wang et al. [46] reported that 5% Co-doped Ni(OH)2 catalysts deposited on nickel foam significantly improved urea adsorption and oxidation kinetics. Liu et al. [47] developed Ni/Mo2C@CN nanocomposites with enhanced catalytic synergy between Ni and Mo2C cores, achieving a current density of 10 mA/cm2 in urea electrolysis. However, despite these advances, there remains a notable gap: few studies have successfully combined both strategies—core–shell-like architecture engineering and cobalt doping—into a simple, one-step synthesis for bifunctional electrocatalysts. Existing methods often involve complex, multi-step procedures or separate doping and shell deposition stages, limiting scalability and practical application.
In this work, we report a facile, one-pot hydrazine-assisted chemical reduction strategy to synthesize Co-doped Ni@Ni(OH)2 nanostructures with a crystalline–amorphous architecture. This scalable method simultaneously builds the core–shell-like architecture and introduces cobalt doping in a single step, optimizing both electron transport and catalytic reactivity. The resulting catalysts exhibit enhanced surface area, lower charge-transfer resistance, and superior bifunctional activity toward both the OER and UOR in alkaline media. Compared to undoped Ni@Ni(OH)2, the 10 wt.% Co-doped sample achieves a 40 mV lower overpotential at 50 mA·cm−2 for the OER and approximately double the anodic current for the UOR. Our approach provides significant advancement by integrating structural and electronic optimization in a cost-effective, scalable manner, offering a new strategy for developing efficient non-noble metal electrocatalysts for sustainable energy conversion and environmental remediation.

2. Experimental Section

2.1. Materials

Cobalt (II) chloride hydrate (CoCl2·6H2O, 97.5%) and nickel (II) chloride hexahydrate (NiCl2·6H2O, 98%) were procured from BDH Chemicals Ltd. (Poole Dorset, UK). Sodium hydroxide pellets (NaOH, 98%) were obtained from Daejung Chemical & Metals Co., Ltd. (Gyeonggi, Republic of Korea) Potassium hydroxide pellets (KOH, 85.0%) were sourced from the AnalaR group. Hydrazine monohydrate (N2H4·H2O, 99%) was acquired from Panreac, while urea was supplied by AVONCHEM Corp. Deionized water was obtained using a Milli-Q water purification system for all experimental procedures. All reagents were of analytical grade and were utilized without additional purification.

2.2. Chemical Deposition of Co-Doped Ni@Ni(OH)2 Core-Shell Catalysts

A series of catalysts, including Co-doped Ni@Ni(OH)2 with varying cobalt contents and pure Ni@Ni(OH)2, was synthesized via a facile chemical reduction method using hydrazine monohydrate. To prepare the b-Co-doped Ni@Ni(OH)2 catalyst (with 10 wt.% Co), an aqueous solution containing NiCl2·6H2O and CoCl2·6H2O was prepared in a beaker (100 mL) and stirred for 30 min. Subsequently, 10 mL of hydrazine monohydrate (N2H4·H2O) was added dropwise to the previous solution, leading to a purple complex formation. The mixture was stirred vigorously for 2 h, followed by the slow addition of 50 mL of NaOH solution. The obtained solution was kept under continuous stirring for 18 h until a black precipitate formed. The precipitate was then collected by filtration, washed several times with ethanol and deionized water, and dried at room temperature. To obtain catalysts with different Co doping levels of 5%, 10%, and 20% CoCl2·6H2O were used, yielding samples referred to as a-Co-doped, b-Co-doped, and c-Co-doped Ni@Ni(OH)2, respectively. For comparison, a pure Co@Co(OH)2 sample was also synthesized following the same procedure but without the addition of nickel precursors.

2.3. Characterization

X-ray diffraction (XRD) patterns of the as-deposited catalyst were collected utilizing a MiniFlex-600 (Rigaku) instrument with Cu Kα radiation (40 kV, 15 mA). Fourier transform infrared (FT−IR) spectroscopic analysis was conducted employing a Bruker Tensor 27 FT-IR spectrometer. Morphological characterization was performed via field-emission scanning electron microscopy (FE−SEM) using a JSM-7610F operated at an accelerating voltage of 15 kV, equipped with an energy-dispersive X-ray spectroscopy (EDX) system for elemental analysis. Surface chemical composition was investigated through X-ray Photoelectron Spectroscopy (XPS) utilizing a Thermo Fisher Scientific K-Alpha+ system, with high-resolution spectra acquired under vacuum conditions. Textural properties were determined through nitrogen (N2) adsorption–desorption isotherms measured on a NOVA 2200e instrument

3. Results and Discussion

3.1. Catalysts Characterization

Using a hydrazine-assisted chemical reduction method, a series of pure Ni@Ni(OH)2, pure Co@Co(OH)2, and different ratios of Co-doped Ni@Ni(OH)2 catalytic materials with a core–shell architecture were synthesized. Figure 1 illustrates the preparation of the core–shell b-Co-doped Ni@Ni(OH)2 catalyst, delineating each step from initial precursor preparation via one-step chemical deposition.
X-ray diffraction (XRD) analysis was conducted to elucidate the crystallographic structure and phase composition of the synthesized catalysts. As shown in Figure 2a, the diffractogram of pure Ni@Ni(OH)2 displays three sharp peaks at 2θ values of 44.62°, 51.66°, and 76.23°, corresponding to the (111), (200), and (220) planes of face-centered cubic (fcc) metallic Ni (JCPDS No. 01-077-3085) [37]. Additionally, broader peaks observed at 19.05°, 32.82°, 38.29°, and 51.94° match the β-Ni(OH)2 phase (JCPDS No. 01-076-8989), confirming the partial oxidation of nickel during synthesis [48]. This coexistence of a conductive metallic core and an electroactive hydroxide shell is critical for enhancing charge transport and providing abundant catalytic sites.
Notably, the investigated Ni@Ni(OH)2 samples exhibit heterogeneous crystallographic phases with stacking fault defects, which can act as additional catalytically active centers and facilitate electron transfer during electrochemical reactions [49]. Upon cobalt incorporation, a slight shift in diffraction peaks was observed, indicative of lattice parameter expansion due to cobalt ion substitution within the Ni(OH)2 lattice. Simultaneously, the intensity of the β-Ni(OH)2 peaks decreases with increasing cobalt content, suggesting a decrease in crystallinity. This structural distortion likely introduces more defect sites and enhances the amorphous character of the shell, which are known to improve catalytic activity by providing more accessible active sites [34,46]. Furthermore, the XRD pattern of the pure Co@Co(OH)2 sample (Figure S1) shows peaks at 2θ values of 41.78°, 44.51°, 47.43°, and 75.89°, corresponding to hexagonal close-packed (hcp) metallic cobalt (JCPDS No. 089-4308), confirming the successful deposition of cobalt-based phases.
Fourier transform infrared (FT−IR) spectra (Figure 2b) further support the structural features observed in XRD. Broad absorption bands at 3426.76 and 3544.35 cm−1 correspond to O–H stretching vibrations of hydroxyl groups and adsorbed water, confirming the hydroxylated nature of the Ni(OH)2 shell. Bands at 1458.48 and 1392.14 cm−1 are assigned to water bending modes, while peaks at 402.11, 549.54, and 729.68 cm−1 correspond to metal–oxygen (Ni–O and Co–O) stretching vibrations. The presence of abundant hydroxyl groups and metal–oxygen bonds suggests a high density of electrochemically active sites, facilitating adsorption and subsequent oxidation of water or urea molecules.
The surface area and porosity of the catalysts were analyzed by N2-physisorption measurements (Figure S2a,b). The Brunauer–Emmett–Teller (BET) surface areas were found to be 17.83 m2/g for pure Ni@Ni(OH)2, 1.49 m2/g for pure Co@Co(OH)2, and 24.55 m2/g for b-Co-doped Ni@Ni(OH)2. Importantly, cobalt doping led to a significant increase in surface area compared to undoped Ni@Ni(OH)2. This increase in surface area is attributed to the structural distortion and inhibited crystal growth induced by cobalt incorporation. The increased porosity and enlarged active surface enhance electrolyte accessibility and facilitate faster mass transport, both of which are beneficial for improving electrocatalytic activity. Additionally, the pore size distribution (PSD) analysis using the Barrett–Joyner–Halenda (BJH) method revealed mesoporous structures with pore sizes ranging from 15.74 to 23.04 nm across the samples. Such mesoporosity is advantageous for electrocatalytic processes, as it provides efficient pathways for ion diffusion and gas evolution, particularly important for multi-electron reactions, such as the OER and UOR.
The composition and morphology of the synthesized materials were further examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) high-resolution TEM (HR−TEM) and energy-dispersive X-ray spectroscopy (EDS). As shown in Figure 3a–c, the SEM images of the pure Ni@Ni(OH)2 sample exhibits a relatively uniform structure composed of interconnected, irregularly shaped particles with an average diameter of approximately 100 nm. In contrast, the b-Co-doped Ni@Ni(OH)2 catalyst (Figure 3d–f) displays smaller particle sizes, approximately 80 nm in diameter, and features thin, sheet-like structures distributed over the particle surfaces. This morphological change upon cobalt doping suggests a modulation of crystal growth and surface reconstruction, likely contributing to the increase in electrochemical surface area observed in subsequent measurements.
To further verify the internal structural configuration, TEM and HR−TEM analyses were conducted on the b-Co-doped Ni@Ni(OH)2 catalyst (Figure 4a–d). The TEM images reveal a distinct contrast between the denser inner region and the surrounding lighter outer layer, suggesting the formation of a heterogeneous structure. HR−TEM micrographs (Figure 4b,c) provide more detailed evidence, clearly showing an ultrathin peripheral layer, approximately 5 nm thick, which lacks ordered lattice fringes. This indicates that the outer layer is structurally disordered and can be attributed to an amorphous Ni(OH)2 phase encapsulating a crystalline Co-Ni core. Enhanced magnification images (Figure 4d) of the inner region exhibit well-defined lattice fringes with uniform spacing and multidirectional orientation, consistent with the presence of a crystalline metallic Ni-based core.
The formation of this crystalline–amorphous heterostructure is expected to play a key role in the electrocatalytic performance enhancement. The conductive crystalline core facilitates efficient electron transport, while the amorphous Ni(OH)2 shell provides abundant active sites and improved electrolyte accessibility. This synergistic architecture is anticipated to reduce charge-transfer resistance and enhance the kinetics of both oxygen evolution and urea oxidation reactions, as confirmed by subsequent electrochemical evaluations.
The elemental composition of the pure Ni@Ni(OH)2 and b-Co-doped Ni@Ni(OH)2 deposits was determined through EDS analysis, with results presented in Figure 5a,b. We found that the ratio of metals in the final deposits differed from the initial metal ion concentrations used in the deposition solutions. This discrepancy is a common phenomenon, attributed to various resistance factors in the solution, including surface, concentration, and ohmic overpotentials [50].
X-ray photoelectron spectroscopy (XPS) analysis was conducted to elucidate the surface chemical composition of pure Ni@Ni(OH)2, Co@Co(OH)2, and various compositions of b-Co-doped Ni@Ni(OH)2 NPs catalysts. Overall survey spectra shown in Figure S3a and Figure 6a confirmed the presence of Co, Ni, C (as a reference), and O signals for all as-made materials, corroborating the EDS findings. Moreover, the contents of these elements of all as-made catalysts in atomic and weight percents were shown in Table 1 and Table S1. As can be seen in Table 1, the Co, Ni and O contents in b-Co-doped Ni@Ni(OH)2 NP catalysts were estimated to be 3.86, 15.22, and 66.41 at%, respectively. High-resolution Ni 2p spectra (Figure 6b) of pure Ni@Ni(OH)2 and b-Co-doped Ni@Ni(OH)2 NP catalysts revealed four distinct peaks, comprising two spin–orbit doublets (Ni2+/3+2p3/2 and Ni2+/3+2p1/2) and a characteristic shake-up satellite at 867.34 eV. In pure Ni@Ni(OH)2, the peaks at 859.34 eV and 879.03 eV correspond to Ni2+, while signals at 862.18 eV and 880.30 eV are attributed to Ni3+ species arising from surface oxidation. The b-Co-doped Ni@Ni(OH)2 sample exhibited a noticeable shift in Ni2+/3+ dominant peaks compared to pure Ni@Ni(OH)2, indicating electron withdrawal from nickel atoms. Additionally, there are peaks related to Ni 2p in the xps of a-Co-doped Ni@Ni(OH)2 and c-Co-doped Ni@Ni(OH)2 NP catalysts, as illustrated in Figure S3b. The Co 2p spectrum of b-Co-doped Ni@Ni(OH)2 sample shown in Figure 6c displayed characteristic peaks at 778.81 eV and 803.74 eV with a spin–energy separation of 24.93 eV, corresponding to the characteristic of Co2+ in hydroxide environments [34,46]. This finding confirmed the Co ion incorporation of pure Ni@Ni(OH)2 and provided evidence that cobalt incorporation modulates the electronic structure of pure Ni@Ni(OH)2. In Figure S3c, the Co 2p spectra of the pure Co@Co(OH)2 catalyst also exhibit the peaks at binding energy associated with Co2+. The O 1s spectrum in Figure 6d displays two distinct peaks, where the higher binding energy region (~531–532 eV) is generally attributed to adsorbed oxygen species (Oads), such as OH− or O2−, and the lower binding energy range (~529–530 eV) corresponds to lattice oxygen (O2−) typically found in metal oxides or hydroxides. A noticeable positive shift in these O 1s peaks upon cobalt incorporation indicates a modification of the electronic structure. When comparing the two samples, the b-Co–doped Ni@Ni(OH)2 shows a more intense and broader peak near 531 eV, pointing to a greater abundance of surface hydroxyls or chemisorbed oxygen species. In contrast, the Ni@Ni(OH)2 sample demonstrates lower peak intensity, reflecting a reduced number of active oxygen species. These observations confirm that Co doping in Ni(OH)2 enhances surface reactivity and modifies the electronic structure of Ni, thereby improving its catalytic potential. Supporting data from Figure S3d further illustrate the O 1s spectral features of other related catalysts, reinforcing the successful synthesis of Co-doped Ni@Ni(OH)2 core–shell-like architecture.

3.2. Electrochemical Properties of Co-Doped Ni@Ni(OH)2 Catalysts

3.2.1. OER Characteristics

Linear sweep voltammetry (LSV) was employed to evaluate the electrocatalytic performance of the synthesized materials loaded on carbon paper (CP) substrates for the OER in alkaline medium (1.0M KOH), as illustrated in Figure 7a. All electrochemical measurements were conducted utilizing a three-electrode configuration incorporating working electrodes composed of pure Ni@Ni(OH)2, a-Co-doped Ni@Ni(OH)2, b-Co-doped Ni@Ni(OH)2, c-Co-doped Ni@Ni(OH)2, and Co@Co(OH)2 catalysts. Following the incorporation of Co nanoparticles into pure Ni@Ni(OH)2 catalysts via the deposition methodology, the LSV presented in Figure 7a demonstrates that all deposited materials exhibit well-defined redox peaks attributable to Ni2+/Ni3+ species. This observation confirms the formation of catalytically active NiOOH forms within the synthesized electrocatalysts through reversible transformations of the Ni2+/Ni3+ redox couple for the OER in 1.0 M KOH. At a current density of 50 mA·cm−2, the overpotential of the synthesized b-Co-doped Ni@Ni(OH)2 nanoparticles for the OER exhibited a 40-mV anodic shift relative to pure Ni@Ni(OH)2 nanoparticles. Furthermore, the OER current density demonstrated an approximate two-fold enhancement compared to pure Ni@Ni(OH)2 nanoparticles, indicating a significantly increased density of catalytically active sites in the b-Co-doped Ni@Ni(OH)2 nanostructures. The experimental results indicate that optimal electrocatalytic performance is achieved with cobalt doping at 10 wt.% at 1.6 V vs. RHE, representing the ideal compositional parameters for Co-doped Ni@Ni(OH)2 electrodes. The incorporation of cobalt effectively promotes OER activity through complementary mechanisms: the enhancement of the electrochemical surface area and the improvement of electrical conductivity properties. Furthermore, Tafel slope analysis (Figure 7b) was conducted to evaluate the reaction kinetics across all synthesized catalysts. The results demonstrate that a-Co-doped Ni@Ni(OH)2 catalysts exhibited superior electrocatalytic performance for the OER compared with other catalysts, attributable to enhanced charge-transfer rates and accelerated reaction kinetics throughout the electrochemical OER process. This kinetic advantage, quantified through Tafel slope determination, confirms the beneficial effect of specific cobalt doping concentrations on the catalytic mechanism of oxygen evolution at the electrode–electrolyte interface. Electrochemical impedance spectroscopy (EIS) was employed to investigate electrode reaction kinetics across the synthesized catalysts. As evidenced in Figure 7c and Table 2, the charge transfer resistance (R1) values for pure Ni@Ni(OH)2 and b-Co-doped Ni@Ni(OH)2 were determined to be 10.05 Ω and 7.23 Ω, respectively, whereas c-Co-doped Ni@Ni(OH)2 and a-Co-doped Ni@Ni(OH)2 exhibited significantly higher values of 132.9 Ω and 11.5 Ω. The markedly reduced R1 observed for pure Ni@Ni(OH)2 and particularly b-Co-doped Ni@Ni(OH)2 nanoparticles suggests that strategic cobalt doping effectively facilitates electron transfer processes during the OER. In this investigation, a comparative analysis was conducted between the electrocatalytic performance metrics of pure Ni@Ni(OH)2 and b-Co-doped Ni@Ni(OH)2-based catalysts and previously reported Ni-containing electrocatalytic materials (Table S2), as well as related electrocatalytic systems, with specific emphasis on electrocatalytic current density values and standardized analytical conditions. Figure 7d shows the stability test of b-Co-doped Ni@Ni(OH)2 NPs catalysts via chronoamperometry measurements (CA) at 1.50 V vs. RHE. The results reveal that the as-made b-Co-doped Ni@Ni(OH)2 NPs catalyst shows better OER stability.
Electrochemical active surface area (ECSA) is recognized as a critical determinant of electrocatalytic performance. ECSA exhibits direct proportionality to electrochemical double-layer capacitance (Cdl), where elevated Cdl values indicate increased accessible electrode surface area. Cyclic voltammetry measurements of the pure Ni@Ni(OH)2, a-Co-doped Ni@Ni(OH)2, b-Co-doped Ni@Ni(OH)2, c-Co-doped Ni@Ni(OH)2, and Co@Co(OH)2 nanoparticle catalysts were conducted within a non-faradaic potential region in 1.0 M KOH electrolyte at variable scan rates ranging from 10 to 50 mV/s, as illustrated in Figure 8a–f. The Cdl values were quantitatively determined from the slope of the linear relationship between the charging-discharging current density differential and scan rate. As demonstrated in Figure 8f, the calculated Cdl values for pure Ni@Ni(OH)2, a-Co-doped Ni@Ni(OH)2, b-Co-doped Ni@Ni(OH)2, c-Co-doped Ni@Ni(OH)2, and Co@Co(OH)2 nanoparticle catalysts were determined to be 64.5, 2.89, 99.5, 1.12, and 1.51 mF, respectively. These results demonstrate that the b-Co-doped Ni@Ni(OH)2 nanoparticles exhibit significantly higher active site density (Cdl = 99.5 mF) compared to other catalysts, which facilitates enhanced urea diffusion into the interior surface area of the core–shell structure. Additionally, the incorporation of Co within the Ni@Ni(OH)2 catalyst matrix promotes improved charge transfer kinetics for the UOR through synergistic effects between the metal components.

3.2.2. UOR Characteristics

Electrochemical characterization was conducted on the Co-doped Ni@Ni(OH)2 catalysts to assess their electrocatalytic performance for the urea oxidation reaction (UOR) as depicted in Figure 9. Figure 9a demonstrates that despite similarities between the UOR and OER electrochemical profiles, b-Co-doped Ni@Ni(OH)2 nanoparticles exhibited superior UOR activity under alkaline conditions. The UOR catalytic properties of all synthesized materials were evaluated in a 0.33 M urea/1.0 M KOH. The b-Co-doped Ni@Ni(OH)2 catalyst demonstrated enhanced current density at potentials exceeding 1.30 V, indicating superior electrocatalytic efficiency toward urea oxidation. The electrochemical behavior observed suggests competitive adsorption between urea and Ni(OH)2 electrooxidation to NiOOH during the positive potential scan [19,21,51]. At 1.6 V, b-Co-doped Ni@Ni(OH)2 generated approximately twice the current density compared to undoped Ni@Ni(OH)2, underscoring the critical role of enhanced electronic conductivity. This significant performance differential provides compelling evidence for the importance of conductivity optimization in these electrochemical systems. Figure 9b presents the Tafel slope analysis performed to elucidate reaction kinetics among all fabricated catalysts. The data indicate that both pure Ni@Ni(OH)2 and b-Co-doped Ni@Ni(OH)2 catalysts demonstrated superior electrocatalytic efficiency toward the UOR relative to alternative catalytic materials, a phenomenon attributable to enhanced electron transfer kinetics and accelerated reaction pathways during the electrochemical UOR process. This kinetic enhancement, quantitatively established through Tafel slope determination, provides evidence for the favorable influence of specific cobalt dopant concentrations on the mechanistic pathways governing urea oxidation at the electrode–electrolyte interface. Electrochemical impedance spectroscopy (EIS) was utilized to characterize the electrode reaction kinetics among the synthesized catalytic materials. As illustrated in Figure 9c and quantified in Table 3, the charge transfer resistance (R1) values were determined to be 1.59 Ω and 1.09 Ω for pure Ni@Ni(OH)2 and b-Co-doped Ni@Ni(OH)2, respectively, which are lower than that observed in other catalysts. The notably reduced R1 observed for pure Ni@Ni(OH)2 and particularly b-Co-doped Ni@Ni(OH)2 nanostructures indicates that strategic cobalt incorporation effectively enhances electron transfer processes during the UOR. Throughout this investigation, pure Ni@Ni(OH)2, b-Co-doped Ni@Ni(OH)2, and alternative electrode compositions demonstrated variable solution resistance values, attributable to specific electrostatic interactions between the b-Co-doped Ni@Ni(OH)2 interface and hydroxide ions proximal to the electrode surface. Consequently, cobalt incorporation enhances the electronic conductivity properties of pure Ni@Ni(OH)2 via two primary mechanisms: (1) increasing the density of electrochemically active sites available for catalytic reactions, and (2) facilitating more efficient charge transfer kinetics at the electrode–electrolyte interface. The derived kinetic parameters demonstrate significant correlation with those determined through CV and EIS measurements. Supplementary Table S3 presents a comparative analysis of alkaline UOR performance between pure Ni@Ni(OH)2 and b-Co-doped Ni@Ni(OH)2 and previously documented noble-metal-free electrocatalysts, summarizing key kinetic parameters including overpotentials required to achieve a current density of 10 mA cm−2 (η10) and corresponding Tafel slopes. Figure 9d illustrates the chronoamperometric profile of the b-Co-doped Ni@Ni(OH)2 catalyst synthesized through chemical deposition, revealing sustained electrochemical performance that indicates robust catalytic activity and stability for the UOR under alkaline conditions. The observed current oscillations during electrochemical measurements can be attributed to the evolution and subsequent desorption of gaseous reaction products from the electrode surface [38]. Regarding long-term catalytic performance, the initial degradation in electrocatalytic activity was primarily caused by carbon monoxide adsorption on the active sites of both nickel-based catalytic materials during the electrooxidation process [21,52,53,54].

4. Conclusions

In summary, crystalline–amorphous Ni@Ni(OH)2 and cobalt-doped Ni@Ni(OH)2 (b-Co-doped Ni@Ni(OH)2) catalysts were successfully synthesized via a simple, one-pot hydrazine-assisted chemical deposition method. Comprehensive structural characterization using XRD, FT−IR, SEM, EDS, and HR−TEM confirmed the formation of a core–shell-like architecture, comprising a conductive crystalline Ni core encapsulated by an amorphous Ni(OH)2 shell. High-resolution TEM analysis further validated the presence of a ~5 nm thick amorphous shell, critical for enhancing catalytic activity. Cobalt incorporation effectively modulated the shell structure, suppressed crystallinity, introduced beneficial defects, and significantly increased the BET surface area to 24.55 m2/g, compared to the undoped sample. The optimized b-Co-doped catalyst demonstrated superior bifunctional electrocatalytic performance. In alkaline OER (1.0 M KOH), the b-Co-doped Ni@Ni(OH)2 catalyst achieved a 40 mV lower overpotential at 50 mA·cm−2 compared to pure Ni@Ni(OH)2, confirming enhanced intrinsic activity. For the UOR in 0.33 M urea/1.0 M KOH, the catalyst reached 10 mA·cm−2 at a low potential of 1.39 V versus RHE, along with an improved Tafel slope of 70.7 mV·dec−1, indicating accelerated reaction kinetics. Electrochemical impedance spectroscopy (EIS) further demonstrated reduced charge transfer resistance, underscoring the critical role of the conductive core, defective amorphous shell, and cobalt doping in facilitating efficient electron transport and electrolyte diffusion. This work highlights a significant advancement by simultaneously integrating core–shell engineering and cobalt-induced electronic modulation in a scalable, low-cost process. The developed strategy offers a versatile and efficient platform for designing next-generation non-noble metal bifunctional electrocatalysts for sustainable hydrogen production and wastewater treatment applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15050474/s1, Figure S1: XRD pattern; Figure S2: N2 sorption isotherms (a), and corresponding BJH desorption pore size distributions (b) of pure Ni@Ni(OH)2, pure Co@Co(OH)2, and b-Co-doped Ni@Ni(OH)2 catalysts; Figure S3: overall survey scan (a), Ni 2p spectra (b), Co 2p spectra (c), and O 1s spectra (d) for pure Co@Co(OH)2, a-Co-doped Ni@Ni(OH)2, and c-Co-doped Ni@Ni(OH)2 catalysts; Table S1: XPS surface elements analysis of the pure Co@Co(OH)2, a-Co-doped Ni@Ni(OH)2, and c-Co-doped Ni@Ni(OH)2 catalysts; Table S2: electrochemical parameters of b-Co-doped Ni@Ni(OH)2 nanoparticle electrocatalyst compared with that reported in the literature for OER in alkaline condition; Table S3: electrochemical parameters of b-Co-doped Ni@Ni(OH)2 NPs electrocatalyst compared with that previously reported in the literature for the UOR using different substrates in alkaline condition. References [55,56,57,58,59,60,61,62] are citied in the Supplementary Materials.

Author Contributions

S.A.A.: conceptualization, data curation, formal analysis, investigation, writing—review and editing, supervision, funding acquisition, project administration. M.N.S.: data curation, formal analysis, investigation. T.F.Q.: data curation, review and editing, formal analysis, investigation. A.I.A.: data curation, formal analysis. K.S.: investigation. A.M.A.-M.: supervision. P.A.: data curation, formal analysis. All authors have read and agreed to the published version of the manuscript.

Funding

This project is sponsored by Prince Sattam bin Abdulaziz University (PSAU) for funding this research work through the project number PSAU/2024/01/31222.

Data Availability Statement

All data generated or analyzed during this study are included in this published article (and its Supplementary Information Files).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic illustration of the preparation of core-shell Co-doped Ni@Ni(OH)2 catalyst via one-step hydrazine-assisted chemical deposition approach.
Figure 1. Schematic illustration of the preparation of core-shell Co-doped Ni@Ni(OH)2 catalyst via one-step hydrazine-assisted chemical deposition approach.
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Figure 2. X-ray diffraction profiles (a), FT-IR spectra (b) of as-deposited pure Ni@Ni(OH)2 and various ratios of Co-doped Ni@Ni(OH)2 catalysts.
Figure 2. X-ray diffraction profiles (a), FT-IR spectra (b) of as-deposited pure Ni@Ni(OH)2 and various ratios of Co-doped Ni@Ni(OH)2 catalysts.
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Figure 3. FE−SEM images of as-deposited pure Ni@Ni(OH)2 catalysts (a–c), and b-Co-doped Ni@Ni(OH)2 catalysts (d–f) prepared by one-step hydrazine-assisted deposition approach.
Figure 3. FE−SEM images of as-deposited pure Ni@Ni(OH)2 catalysts (a–c), and b-Co-doped Ni@Ni(OH)2 catalysts (d–f) prepared by one-step hydrazine-assisted deposition approach.
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Figure 4. TEM and HR−TEM images of b-Co-doped Ni@Ni(OH)2 catalysts with different magnifications (a–d).
Figure 4. TEM and HR−TEM images of b-Co-doped Ni@Ni(OH)2 catalysts with different magnifications (a–d).
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Catalysts 15 00474 g005
Figure 5. EDX profile of Co, Ni, and O elements for pure Ni@Ni(OH)2 (a), and b-Co-doped Ni@Ni(OH)2 (b) catalysts.
Figure 5. EDX profile of Co, Ni, and O elements for pure Ni@Ni(OH)2 (a), and b-Co-doped Ni@Ni(OH)2 (b) catalysts.
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Figure 6. Overall survey spectra of pure Ni@Ni(OH)2 and b-Co-doped Ni@Ni(OH)2 catalysts (a). High-resolution XPS profiles of Ni 2p (b), Co 2p (c), O 1s (d) in pure Ni@Ni(OH)2 and b-Co-doped Ni@Ni(OH)2 catalysts.
Figure 6. Overall survey spectra of pure Ni@Ni(OH)2 and b-Co-doped Ni@Ni(OH)2 catalysts (a). High-resolution XPS profiles of Ni 2p (b), Co 2p (c), O 1s (d) in pure Ni@Ni(OH)2 and b-Co-doped Ni@Ni(OH)2 catalysts.
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Figure 7. LSV at 20 mVs−1 in 1.0 M KOH (a), corresponding Tafel plots (b), EIS measurements (c) of as-deposited pure Ni@Ni(OH)2, Co@Co(OH)2 and various ratios of Co-doped Ni@Ni(OH)2 catalysts, and chronoamperometry analysis (d) of b- Co-doped Ni@Ni(OH)2 catalysts in 1.0 M KOH at 1.5 V.
Figure 7. LSV at 20 mVs−1 in 1.0 M KOH (a), corresponding Tafel plots (b), EIS measurements (c) of as-deposited pure Ni@Ni(OH)2, Co@Co(OH)2 and various ratios of Co-doped Ni@Ni(OH)2 catalysts, and chronoamperometry analysis (d) of b- Co-doped Ni@Ni(OH)2 catalysts in 1.0 M KOH at 1.5 V.
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Figure 8. Capacitive currents in 1.0 M KOH for pure Ni@Ni(OH)2 (a), a-Co-doped Ni@Ni(OH)2 (b), b-Co-doped Ni@Ni(OH)2 (c), c-Co-doped Ni@Ni(OH)2 (d), pure Co@Co(OH)2 (e), and relationship between double layer capacitances and different scan rates for the as-made catalysts (f).
Figure 8. Capacitive currents in 1.0 M KOH for pure Ni@Ni(OH)2 (a), a-Co-doped Ni@Ni(OH)2 (b), b-Co-doped Ni@Ni(OH)2 (c), c-Co-doped Ni@Ni(OH)2 (d), pure Co@Co(OH)2 (e), and relationship between double layer capacitances and different scan rates for the as-made catalysts (f).
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Figure 9. LSV in 0.33 M urea/1.0 M KOH at 20 mVs−1 (a), corresponding Tafel plots (b), EIS measurements (c) of as-deposited pure Ni@Ni(OH)2, Co@Co(OH)2 and various ratios of Co-doped Ni@Ni(OH)2 catalysts, and chronoamperometry analysis (d) of b- Co-doped Ni@Ni(OH)2 catalysts in 0.33 M urea/1.0 M KOH at 1.5 V.
Figure 9. LSV in 0.33 M urea/1.0 M KOH at 20 mVs−1 (a), corresponding Tafel plots (b), EIS measurements (c) of as-deposited pure Ni@Ni(OH)2, Co@Co(OH)2 and various ratios of Co-doped Ni@Ni(OH)2 catalysts, and chronoamperometry analysis (d) of b- Co-doped Ni@Ni(OH)2 catalysts in 0.33 M urea/1.0 M KOH at 1.5 V.
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Table 1. XPS surface elements analysis elements of the pure Ni@Ni(OH)2 and b-Co-Doped Ni@Ni(OH)2 catalysts.
Table 1. XPS surface elements analysis elements of the pure Ni@Ni(OH)2 and b-Co-Doped Ni@Ni(OH)2 catalysts.
Materials, ElementsPure Ni@Ni(OH)2
(Atomic, %)		b-Co-doped Ni@Ni(OH)2
(Atomic, %)
Ni18.7115.22
O65.8866.41
Co-3.86
C15.4114.51
Total100
Table 2. EIS parameters of Ni@Ni(OH)2, Co@Co(OH)2, and different Co-doped Ni@Ni(OH)2 catalysts at 1.6 V vs. RHE for OER in 1.0 M KOH.
Table 2. EIS parameters of Ni@Ni(OH)2, Co@Co(OH)2, and different Co-doped Ni@Ni(OH)2 catalysts at 1.6 V vs. RHE for OER in 1.0 M KOH.
Anodic MaterialsEIS Parameters: Rs + Q1/R1 + C2/R3
Rs, ohmQ1, FR1, ohmC2, FR3, ohm
Ni@Ni(OH)22.674.44 × 10−1110.050.020513.33
a-Co-doped Ni@Ni(OH)24.491.5 6 × 10−411.500.008210.32
b-Co-doped Ni@Ni(OH)23.460.01247.230.04759.77
c-Co-doped Ni@Ni(OH)23.754.39 × 10−5132.910.000995
Co@Co(OH)24.951.65 × 10−50.4670.000522.21
Table 3. EIS parameters of Ni@Ni(OH)2, Co@Co(OH)2, and different Co-doped Ni@Ni(OH)2 catalysts recorded through fitting EIS spectra measured at 1.6 V vs. RHE for UOR in 0.33 M urea.
Table 3. EIS parameters of Ni@Ni(OH)2, Co@Co(OH)2, and different Co-doped Ni@Ni(OH)2 catalysts recorded through fitting EIS spectra measured at 1.6 V vs. RHE for UOR in 0.33 M urea.
Anodic MaterialsEIS Parameters:
Rs + Q1/R1 + C2/R3+Ws
Rs, ohmQ1, FR1, ohmC2, FR3, ohmWs
Ni@Ni(OH)24.69488.6547 × 10−51.59180.006951.05216.398
a-Co-doped Ni@Ni(OH)24.77380.0002324.5280.0105153.575.372
b-Co-doped Ni@Ni(OH)23.01420.002841.09380.016953.41410.146
c-Co-doped Ni@Ni(OH)23.53890.0001269.2420.00027423.52.7949
Co@Co(OH)24.96384.5011 × 10−563.8220.00014450.80.5679
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Aladeemy, S.A.; Shaddad, M.N.; Qahtan, T.F.; Alharthi, A.I.; Shalabi, K.; Al-Mayouf, A.M.; Arunachalam, P. Co-Doped Ni@Ni(OH)2 Core–Shell Catalysts for Dual-Function Water and Urea Oxidation. Catalysts 2025, 15, 474. https://doi.org/10.3390/catal15050474

AMA Style

Aladeemy SA, Shaddad MN, Qahtan TF, Alharthi AI, Shalabi K, Al-Mayouf AM, Arunachalam P. Co-Doped Ni@Ni(OH)2 Core–Shell Catalysts for Dual-Function Water and Urea Oxidation. Catalysts. 2025; 15(5):474. https://doi.org/10.3390/catal15050474

Chicago/Turabian Style

Aladeemy, Saba A., Maged N. Shaddad, Talal F. Qahtan, Abdulrahman I. Alharthi, Kamal Shalabi, Abdullah M. Al-Mayouf, and Prabhakarn Arunachalam. 2025. "Co-Doped Ni@Ni(OH)2 Core–Shell Catalysts for Dual-Function Water and Urea Oxidation" Catalysts 15, no. 5: 474. https://doi.org/10.3390/catal15050474

APA Style

Aladeemy, S. A., Shaddad, M. N., Qahtan, T. F., Alharthi, A. I., Shalabi, K., Al-Mayouf, A. M., & Arunachalam, P. (2025). Co-Doped Ni@Ni(OH)2 Core–Shell Catalysts for Dual-Function Water and Urea Oxidation. Catalysts, 15(5), 474. https://doi.org/10.3390/catal15050474

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