Amorphous Co-NiB@NF as an Efficient Electrocatalyst for Urea Oxidation Reaction

Abstract

Transition metal-based catalysts designed for efficient urea oxidation reactions (UOR) are essential for hydrogen production via urea-assisted water electrolysis. A series of amorphous nickel–cobalt boride catalysts supported on nickel foam were in situ synthesized via a stepwise chemical deposition method (SCDM). The systematic investigation focused on the relationships between synthesis parameters (deposition cycles, reactant feed ratio), morphological characteristics, and UOR performance. Notably, the optimized Co-NiB@NF catalyst exhibits a porous hierarchical architecture composed of metallic nanoparticles encapsulated by surface-wrinkled nanosheets, forming abundant exposed active sites. Electrochemical measurements demonstrate that this catalyst requires a low cell potential of 1.29 V to achieve a current density of 10 mA cm⁻². Moreover, it maintains 83% of the initial current density after 10 h of continuous electrolysis, highlighting its superior durability. The structural-property relationship revealed here provides valuable insights into the rational design of efficient amorphous boride catalysts for urea-assisted hydrogen production.

1. Introduction

Hydrogen, as a clean energy carrier, demonstrates significant potential for mitigating large-scale greenhouse gas emissions and environmental pollution associated with fossil fuels [1]. Among diverse hydrogen production technologies, electrochemical water splitting is considered the most promising pathway for “green” hydrogen generation, primarily due to its direct integration with intermittent renewable energy sources (e.g., wind and solar power). Nevertheless, the high thermodynamic potential of the anodic oxygen evolution reaction (OER, 1.23 V vs. RHE) and its sluggish kinetics remain critical bottlenecks impeding its industrial implementation [2].

Using urea as an anodic sacrificial reagent and replacing the OER with the more thermodynamically favorable urea oxidation reaction (UOR, 0.37 V vs. RHE) not only reduces energy consumption but also provides a synergistic solution for nitrogen-containing wastewater treatment [3]. Nevertheless, UOR involves a complex six-electron transfer mechanism and suffers from sluggish kinetics, underscoring the urgent need for highly active and robust electrocatalysts. In alkaline electrolyte, the overall urea electrolysis reaction is as follows:

$$\mathrm{A}\mathrm{n}\mathrm{o}\mathrm{d}\mathrm{e}:\mathrm{C}\mathrm{O}{\left(\mathrm{N}{\mathrm{H}}_2\right)}_2+6\mathrm{O}{\mathrm{H}}^{-}\to {\mathrm{N}}_2+{\mathrm{C}\mathrm{O}}_2+5{\mathrm{H}}_2\mathrm{O}+6{\mathrm{e}}^{-}$$

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$$\mathrm{A}\mathrm{a}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{d}\mathrm{e}:6{\mathrm{H}}_2\mathrm{O}+6{\mathrm{e}}^{-}\to 3{\mathrm{H}}_2+6{\mathrm{O}\mathrm{H}}^{-}$$

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$$\mathrm{O}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{l}\mathrm{l}:\mathrm{C}\mathrm{O}{\left(\mathrm{N}{\mathrm{H}}_2\right)}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{N}}_2+{\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2$$

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To date, various binary and ternary transition-metal boride (TMB) catalysts-such as Co-Zn-B and W-Fe-Ni-B have demonstrated exceptional UOR performance. This stems from the tunable combinations of transition metal ions, the metalloid character of boron, and its electron-deficient structure, which collectively enable remarkable flexibility in electronic structure engineering [4,5]. Most transition metal boride-based catalysts synthesized currently are in amorphous form; compared to their crystalline counterparts, they exhibit more flexible structures and higher defect density [6]. Existing literature has shown that amorphous nickel–cobalt borides possess high catalytic activity toward OER, yet their performance in UOR remains largely unexplored. Additionally, conventional chemical deposition methods for synthesizing amorphous borides often suffer from issues like low efficiency, uneven loading, and difficulty in controlling loading amounts. To address these challenges, this work modifies the chemical deposition approach and elucidates the correlations among synthesis conditions (deposition cycles, feed ratio), product morphology, and UOR performance.

In this study, an amorphous cobalt-doped nickel boride catalyst (Co-NiB@NF) supported on nickel foam was in situ synthesized via the stepwise chemical deposition method (SCDM). The as-prepared catalyst features metal boride nanoparticles encapsulated by layered oxides/hydroxides. It delivers a potential of only 1.29 V at a current density of 10 mA·cm⁻² and 1.41 V at 100 mA·cm⁻², with 83% current retention over 10 h, demonstrating exceptional catalytic activity and stability. This facile synthesis strategy for amorphous TMB catalysts provides an effective pathway for the development of high-performance UOR electrocatalysts.

2. Results and Discussion

2.1. Morphology and Structure

The amorphous Co-NiB@NF catalyst was synthesized via a stepwise chemical deposition method under ambient conditions (room temperature, non-oxygen-isolated atmosphere). Compared with conventional single-step deposition, this stepwise strategy exhibits higher efficiency and better control over catalyst loading.

As shown in Figure 1a, Co-NiB@NF exhibits a hierarchical architecture, where primary particles (~100 nm in diameter) are decorated with irregularly grown crumpled nanosheets (~100 nm in depth). High-resolution Transmission Electron Microscopy (TEM) (Figure 1b) reveals discontinuous lattice fringes with short-range ordered but long-range disordered atomic arrangements, characteristic of amorphous materials. Fourier transform analysis of locally ordered regions yielded a lattice spacing of 2.64 Å, indexed to the (101) plane of nickel hydroxide (PDF#01-089-7111). EDS elemental mapping (Figure 1c) confirmed the coexistence of Ni, Co, B, and O, evidencing successful multi-element doping while indicating partial oxidation caused by oxygen exposure during synthesis and storage. The X-ray diffraction (XRD) pattern is shown in Figure 2, displaying significantly broadened diffraction peaks at around 2θ = 20° and 45°. These broad peaks are characteristic of metal borides, indicating the amorphous structure of the Co-NiB@NF catalyst. The weak diffraction peaks of Ni/Co oxides appearing at approximately 2θ = 36° and 60° suggest that surface oxidation of the catalyst occurred during synthesis and storage [7]. During synthesis, Co-NiB@NF initially deposited as reduced metallic boride nanoparticles on the NF substrate. Subsequent self-reconstruction under aerobic conditions generated an amorphous boride-containing oxide/hydroxide overlayer encapsulating the nanocrystalline metallic cores. These boron-derived species act as stabilizing agents, protecting the metallic cores from oxidative degradation during UOR catalysis [8].

X-ray photoelectron spectroscopy (XPS) was used to analyze the surface chemical states of the Ni-B@NF and Co-NiB@NF catalysts (Figure 3). In the XPS spectrum of Co-NiB@NF, elements such as Ni, Co, B, and O were detected, consistent with the EDS results. The Ni/Co ratio was found to be 8.5:1.5, slightly higher than the 9:1 molar ratio used during synthesis. In the Ni2p spectrum, two prominent peaks at 855.33 eV and 856.33 eV were assigned to Ni²⁺ and Ni³⁺ oxidation states, respectively, while a weaker peak at 852.53 eV was assigned to Ni⁰ (Figure 3a). Similarly, in the Co2p spectrum (Figure 3b), characteristic peaks of Co²⁺ and Co³⁺ were detected, along with a weaker Co⁰ peak [9]. The B1s spectrum shows a small amount of M-B bonds and a large amount of B–O species (Figure 3c). The high-valent nickel and cobalt originate from atmospheric exposure during the preparation and storage process, leading to the formation of Ni/Co oxides and hydroxides on the surface of Co-NiB@NF, although a small amount of unoxidized Ni/Co metals remain [10]. In the B1s spectrum, the B⁰ characteristic peak of Co-NiB@NF at 189.59 eV shifts negatively by 0.5 eV compared to the B⁰ peak of Ni-B@NF at 189.09 eV, confirming the successful synthesis of the Co-NiB ternary boride [11]. The O1s spectrum is shown in Figure 3d, indicating that oxygen elements on the surface of Co-NiB@NF are predominantly hydroxyl oxygen, with a small amount of lattice oxygen. The high content of oxygen vacancy species suggests that the amorphous boride possesses a rich defect structure. After electrochemical testing, the B1s signal in Co-NiB@NF completely disappeared, accompanied by a significant increase in M³⁺ content, indicating that the catalyst underwent further surface reconstruction during the test. The changes in XPS spectra demonstrate that the newly formed oxide layer masked the signals of boron–oxygen species [12,13].

The ubiquitous presence of oxide/hydroxide layers, structural defects, and zero-valent metal signatures on Co-NiB@NF are consistent with literature descriptions of amorphous metal borides possessing a “oxide/hydroxide shell encapsulating a metal-boride core” [14].

During the in situ synthesis of amorphous Co-NiB@NF catalysts via the SCDM, deposition parameters-specifically the number of deposition cycles and Co precursor ratio-directly governed the morphological evolution of the catalyst layer on the NF substrate. Figure 4 shows SEM images of Co-NiB@NF samples prepared with varying numbers of deposition cycles. As the number of deposition cycles increases, Co-NiB@NF-9 (Figure 4a), Co-NiB@NF-11 (Figure 4b), and Co-NiB@NF-13 (Figure 4c) exhibit significant morphological differences. These morphological changes can be attributed to the layer-by-layer assembly of catalyst particles during the SCDM process. Tests of catalyst loading and performance (Figure S1) indicate that increasing the number of deposition cycles leads to higher catalyst loading (When the number of deposition cycles is 11, the loading amount of Co-NiB@NF is 2.75 mg·cm⁻²). As a result, Co-NiB@NF-11 exhibits higher performance than Co-NiB@NF-9; however, Co-NiB@NF-13, despite having the highest loading, shows a decline in performance. These results suggest that when the number of deposition cycles is relatively low, the catalyst loading is insufficient. Conversely, when the number of deposition cycles is excessively high, the deposited layer becomes too thick, which adversely affects mass transfer. Thus, an optimal number of deposition cycles yields a porous deposited layer with an appropriate catalyst loading on the NF surface.

The experimental results indicate that varying the Co precursor ratio has a pronounced influence on the morphology of these amorphous Ni-Co-B catalysts. This effect that has been rarely reported in the literature. Figure 5 presents SEM images corresponding to different Co precursor ratios, revealing a strong correlation between the precursor ratio and the resulting catalyst morphology. As the proportion of Co²⁺ increases (from Ni-B@NF in Figure 5a to Co-B@NF in Figure 5d), the catalyst morphology changes significantly and systematically. These observations suggest that the presence of Co²⁺ during the deposition process promotes aggregation of the deposited material, with this tendency becoming stronger at higher Co²⁺ ratios. Such aggregation directly reduces the number of electrochemically active sites on the catalyst surface, thereby impairing its performance in the UOR (Figure S2).

2.2. UOR Performance

Figure 6a shows that the Co-NiB@NF catalyst exhibits optimal UOR catalytic performance, achieving a current density of 10 mA cm⁻² at only 1.29 V, and reaching 100 mA cm⁻² at 1.41 V. Under identical test parameters, when evaluating the OER performance of this catalyst in 1 M KOH, a potential of 1.77 V is required to achieve a current density of 100 mA cm⁻² (Figure 7a). Furthermore, in a 10 h chronoamperometry test, the current density shows only a small decrease (Figure 7c), indicating excellent stability. This performance is comparable to most previously reported transition metal-based catalysts (

Table 1. Comparison of catalyst performance.

Material/Substrate	Electrolyte	Potential (V vs. RHE) @ (mA cm−2)	Reference
Co-NiB@NF	1 M KOH + 0.33 M urea	1.29@10	This work
aFe-NiB	1 M KOH + 0.5 M urea	1.29@10	[8]
Mn-Ni(OH)2/CP	1 M KOH + 0.33 M urea	1.33@10	[15]
NiMoS2/NF	1 M KOH + 0.33 M urea	1.39@10	[16]
Ru-Ni3N@NC	1 M KOH + 0.33 M urea	1.36@10	[17]
O-NiMoP/NF	1 M KOH + 0.5 M urea	1.41@100	[18]
Ni/NiMoN	1 M KOH + 0.5 M urea	1.39@100	[19]
W-NT@NF-3	1 M KOH + 0.33 M urea	1.43@100	[20]

).

When compared with the monometallic catalysts Ni-B@NF and Co-B@NF, as well as the bimetallic hydroxide Ni₉Co₁-LDH@NF, Co-NiB@NF demonstrates superior performance. Its Tafel slope is 63.07 mV dec⁻¹ (Figure 6b), which is higher than that of Ni-B@NF (43.97 mV dec⁻¹), indicating that Co doping does not fully exert a positive effect. The electrochemical impedance spectroscopy (EIS) in Figure 6c and the double-layer capacitance measurements in Figure S3 and Figure 6d show that the introduction of a small amount of Co not only significantly reduces the charge transfer resistance of Co-NiB@NF but also decreases its electrochemical active surface area.

Consequently, Co-NiB@NF achieves higher current densities and lower overpotentials than Ni-B@NF. This can be attributed to the Co-induced improvement in charge transfer, which enhances the catalyst’s intrinsic activity. The higher Tafel slope, however, results from Co doping altering the catalyst morphology and reducing the number of active sites. Compared with Ni₉Co₁-LDH@NF, Co-NiB@NF exhibits a higher current density and a lower Tafel slope, highlighting the advantages brought about by its amorphous structure and B-element doping.

Recent theoretical studies on Ni-B electrocatalysts indicate that the original Ni-BOx species undergo self-reconstruction into γ-NiOOH to catalyze UOR. This γ-NiOOH phase is thermodynamically favored under oxidative conditions and has been identified as the primary active species for urea oxidation reaction (UOR) catalysis. However, the catalytic cycle on NiOOH involves energetically demanding steps, such as the formation of oxygenated intermediates (*OOH or *OH) on its surface, which kinetically limits the overall UOR kinetics [21]. In other words, while NiOOH represents the stable oxidized Ni phase, the adsorption and conversion of urea-derived intermediates on this surface may encounter kinetic barriers.

The electrochemical behavior of the catalyst was observed via CV measurements (Figure 7b): compared with Ni-B@NF, the generation potential of Ni³⁺/Ni²⁺ in Co-NiB@NF exhibited a negative shift of 25 mV, indicating that amorphous Co-NiB@NF accelerates structural self-reconstruction during UOR. This can be attributed to doped Co cations reducing the structural self-transformation energy, thereby lowering the energy barrier for UOR. Additionally, d-d orbital delocalization between Ni and Co decreases the d-band center of Ni, balancing the adsorption energy of UOR intermediates on the NiOOH surface, which endows Co-NiB@NF with higher UOR activity than Ni-B@NF. The experimental results are consistent with these theoretical predictions: introducing a small amount of Co into the Ni-B@NF catalyst significantly enhances its electrocatalytic performance [8,14].

Based on the above analysis, we conclude that the hydroxide form of Ni³⁺ serves as the main active center for OER, while Co reduces the energy barrier for forming Ni³⁺ intermediates and optimizes the adsorption energy of UOR intermediate products.

3. Materials and Methods

3.1. Preparation of Ni-Co Boride

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Preparation of Deposition Solutions

Solution A was prepared by dissolving NiSO₄·6H₂O and CoSO₄·7H₂O in deionized water such that the total metal ion concentration was 0.20 M. Sodium succinate (0.20 M) was added as a complexing agent.

Solution B was prepared by dissolving NaOH (0.10 M) and NaBH₄ (0.60 M) in deionized water.

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Stepwise Deposition Procedure

In each deposition cycle, 10 mL of Solution A and 10 mL of Solution B were placed in separate beakers. Nickel foam (NF) substrates were first immersed in Solution A for 1 min, then immediately transferred to Solution B for 1 min, and finally rinsed thoroughly with deionized water. This sequence constitutes one deposition cycle.

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Tailoring Ni:Co Ratio and Deposition Number

By varying the Ni:Co feed ratio in Solution A and the number of deposition cycles, a series of Ni-Co boride catalysts were synthesized. For example, when the Ni: Co molar ratio was set to 9:1 and the deposition was repeated 11 times, the resulting sample was denoted as Co-NiB@NF.

3.2. Preparation of Ni₉Co₁-LDH@NF

The total concentration of NiSO₄·6H₂O and CoSO₄·7H₂O in the solution was set to 0.20 M with a Ni: Co molar ratio of 9:1, and 0.09 M urea was used as the precipitating agent. Forty milliliters of this solution was placed together with NF into a 50 mL beaker and heated in an 80 °C water bath for 7 h to obtain Ni₉Co₁-LDH@NF.

The working electrode was prepared by cutting the as-synthesized catalyst into a 1 × 1.5 cm² piece, which was then fixed with a platinum plate electrode clip to ensure an immersed area of 1 cm² in the electrolyte.

3.3. Catalytic Characterization

The catalyst samples were characterized by powder X-ray diffraction (XRD, Tongda TD-3700, Cu Ka radiation, Liaoning, China), scanning electron microscopy (SEM, TM4000 Hitachi, Tokyo, Japan), transmission electron microscopy (TEM, Tecnai F20, FEI, Waltham, MA, USA), and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Al Ka-rays source, Thermo Fisher, Waltham, MA, USA). In the XPS measurements, all the binding energies (BE) were calibrated using the C 1s peak (at 284.8 eV) of the adventitious carbon as an internal standard. The curve fitting was performed using Thermo Avantage software (Version: 5.9921, Thermo Fisher, Waltham, MA, USA). The samples required for XRD, XPS, and TEM characterizations were powders obtained by ultrasonically treating the catalyst to peel it off from the NF substrate.

3.4. Electrochemical Measurement

Electrochemical measurements were performed in a standard three-electrode configuration using a CHI 660E (CH Instruments, Inc., Austin, TX, USA) electrochemical analyzer. The electrochemical cell comprised a catalytic electrode as the working electrode, a Pt sheet counter electrode, and an Ag/AgCl (saturated KCl) reference electrode. Potentials measured versus Ag/AgCl were referenced to the reversible hydrogen electrode (RHE) by adding a value of (0.197 + 0.059 pH) V. A mixed solution of 1 M KOH and 0.33 M urea was used as the electrolyte.

Catalyst samples were electrochemically activated via potential cycling at a sweep rate of 20 mV/s for 20 cycles. All polarization curves were recorded at a sweep rate of 20 mV/s. Electrochemical impedance spectroscopy (EIS) measurements were conducted in the same three-electrode system at 1.29 V (vs. RHE). Impedance spectra were collected over a frequency range of 0.01 Hz to 100 kHz with a 10 mV amplitude.

The electrochemical active surface area (ECSA) was estimated by measuring the double-layer capacitance of the system via cyclic voltammetry (CV). A series of CV tests were conducted within the potential window of 1.064 to 1.164 V (vs. RHE) at different scan rates (20, 40, 60, 80, 100 mV s⁻¹, etc.). Subsequently, a linear plot was obtained by establishing the relationship between the difference of anodic and cathodic currents (i_a–i_c) at 1.114 V (vs. RHE) and the scan rate. The double-layer capacitance (C_dl) was half of the slope value of the fitted line. The ECSA is calculated based on the equation ECSA = C_dl/Cs, where Cs represents the specific capacitance of a smooth planar electrode (typically reported as 0.04 mF cm⁻² in previous studies) [22].

All tests were conducted without IR correction. The working electrode was prepared by cutting the as-synthesized catalyst into a 1 × 1.5 cm² piece, which was fixed with a platinum plate electrode clip to ensure an immersed area of 1 cm² in the electrolyte.

4. Conclusions

A simple staged SCDM was used to efficiently deposit the amorphous Co-NiB@NF ternary boride onto a nickel foam substrate. The catalyst achieved a current density of 10 mA cm⁻² at only 1.29 V, and a current density of 100 mA cm⁻² at 1.41 V. In a 10 h constant potential test, the current density retention reached 83%. In summary, the improved catalytic performance of the NiCoB-based composite is attributed to the synergistic optimization of its electronic structure and microstructure achieved during synthesis. The electron-donor effect of boron atoms toward cobalt establishes directed charge transfer pathways, which effectively lower the d-band center of the Ni active sites and promote the formation of Ni³⁺ active species. Meanwhile, the metallic NiCoB core and its in situ reconstructed amorphous hydroxide shell form a unique core–shell architecture: the high electrical conductivity of the core ensures rapid charge transport, while the shell’s wrinkled, sheet-like morphology not only exposes abundant active sites but also significantly enhances reactant mass transport and bubble detachment through its interwoven porous framework. By integrating compositional design with structural control, this study demonstrates the practical potential of transition metal borides in the UOR system and provides a valuable strategy for developing non-precious metal electrocatalysts that exhibit both high activity and high stability.