Engineering Amorphous CoNiRuO_x Nanoparticles Grown on Nickel Foam for Boosted Electrocatalytic Hydrogen Evolution

Abstract

Designing efficient and cost-effective electrocatalysts is crucial for the large-scale development of sustainable hydrogen energy. Amorphous catalysts hold great promise for application due to their structural flexibility and high exposure of active sites. We report a novel method for the in situ growth of amorphous CoNiRuO_x nanoparticle structures (CoNiRuO_x/NF) on a nickel foam substrate. In 1 m KOH, CoNiRuO_x/NF achieves a current density of 10 mA/cm² with a hydrogen evolution reaction (HER) overpotential of only 43 mV and remains stable for over 100 h at a current density of 100 mA/cm². An alkaline electrolyzer assembled with CoNiRuO_x/NF as the cathode delivers a current density 2.97 times higher than that of an IrO₂||Pt/C electrode pair at the potential of 2 V and exhibits excellent long-term durability exceeding 100 h. Experimental results reveal that the combined replacement and corrosion reactions facilitate the formation of the amorphous CoNiRuO_x structure. This work provides valuable insights for developing efficient and scalable amorphous catalysts.

1. Introduction

The gradual depletion of fossil fuels and negative environmental impact have driven the search for cleaner and more sustainable energy solutions [1,2,3]. Among various potential alternatives, hydrogen (H₂) has garnered significant attention due to its high energy density and carbon-free emissions [4,5,6]. Water electrolysis is considered as one of the most promising hydrogen production pathways because of its environmental friendliness and compatibility with renewable energy [7,8,9,10]. However, the large-scale production and application of H₂ face numerous challenges, including improving the energy conversion efficiency of water electrolysis and reducing system costs [11,12]. The core issue lies in developing cost-effective and efficient electrocatalysts to simultaneously optimize the reaction kinetics of the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) during water splitting [13,14,15]. Currently, noble metals such as Pt and IrO₂/RuO₂ are benchmarks in the field of electrocatalysis due to their outstanding performance in HER and OER [16,17,18,19], respectively. However, the scarcity and high cost of noble metals pose significant barriers to the large-scale application of water electrolysis technology [20,21,22,23]. Therefore, developing high-performance catalytic materials with low cost is one of the critical steps for advancing water electrolysis technology.

Among various research directions, amorphous catalysts have attracted widespread attention due to their unique structural advantages [24,25]. Amorphous catalysts have atomic arrangements with short-range order, rich in dangling bonds and unique coordination structures, which often exhibit superior catalytic performance compared to their crystalline analogues [26,27,28,29]. Currently, many studies on amorphous catalytic materials have been reported [30,31,32]. For example, Dai et al. addressed the limitations of the smooth and densely stacked layered structure of crystalline Co−MOF by controlling the degree of crystallization in the synthesized electrode materials through ball milling, exfoliation, and adjusting the Fe doping levels. This approach transformed the crystalline Co−MOF into a defect-rich amorphous Fe@Co−MOF [33]. This catalyst demonstrated an HER overpotential of 150 mV at a current density of 10 mA/cm² in an alkaline electrolyte. In addition, Zhao et al. utilized a wet chemical method to etch Zn@Ni−P nanowires (NWs) with axial screw dislocations in an alkaline solution [34]. This process removed the Zn core of the Zn@Ni−P NWs and partially converted Ni into amorphous Ni(OH)₂, forming amorphous Ni−P/Ni(OH)₂ nanotubes (Ni−P/Ni(OH)₂ NTs). The Ni−P/Ni(OH)₂ NTs exhibited an HER overpotential of 54.7 mV at a current density of 10 mA/cm² in 1 m KOH. Considering the advantages of amorphous structures as electrocatalytic materials, we were motivated to explore new methods for synthesizing amorphous structures and further construct amorphous structures with novel compositions.

Herein, we successfully synthesized an amorphous CoNiRuO_x nanoparticle structure (CoNiRuO_x/NF) via combining replacement reaction and corrosion engineering. The CoNiRuO_x/NF exhibits superb HER performance, requiring an overpotential of only 43 mV to achieve a current density of 10 mA/cm² and maintaining stability for over 100 h at 100 mA/cm². Moreover, an alkaline electrolyzer assembled with CoNiRuO_x/NF as the cathode achieves a current density of 100 mA/cm² at a voltage of only 1.73 V, and its current density at 2 V is 2.97 times higher than that of the IrO₂||Pt/C electrode pair. Based on experimental results, we reasonably propose a reaction mechanism in which the driving force of the replacement reaction (the transition of Ru³⁺ to its metallic state) and the driving force of the corrosion reaction (heterogeneous nucleation of Ni²⁺ and Co³⁺) work together to form the amorphous CoNiRuO_x structure. This study not only expands the preparation methods for amorphous materials but also designs a novel amorphous structure composition, providing a new pathway for developing efficient and stable electrocatalytic materials for hydrogen evolution.

2. Results and Discussion

The CoNiRuO_x/NF is synthesized through a one-step impregnation method (The detailed process is provided in the Section 3). Figures S1 and S2 shows the optimized Ru concentration and reaction time. The optical micrograph in Figure S3 reveals that CoNiRuO_x/NF retains the three-dimensional interconnected framework structure of the nickel foam (NF), which facilitates the rapid flow of electrolytes and efficient hydrogen release [35]. Scanning electron microscope (SEM) images (Figure 1a) demonstrate that CoNiRuO_x/NF exhibits a nanoparticle morphology, and the corresponding particle size distribution shows that the particle size of the nanoparticles is 15–20 nm (Figure 1a inset). Transmission electron microscopy (TEM) images further support this conclusion (Figure 1b). High–resolution transmission electron microscopy (HR−TEM) did not observe any lattice fringes (Figure 1c), indicating that the CoNiRuO_x grown on the NF is likely to be an amorphous structure [27]. This feature is further confirmed by selected area electron diffraction (SAED) (Figure 1c inset), where weak diffraction rings are observed near the central bright spot [36,37]. Meanwhile, the corresponding energy-dispersive X–ray spectroscopy (EDS) confirms that the Co, Ni, Ru, and O elements are uniformly distributed on the CoNiRuO_x/NF surface (Figure 1d), and the elemental contents of Co, Ni, Ru, and O are calculated to be 11.91%, 2.22%, 32.54%, and 53.33%, respectively. Inductively coupled plasma optical emission spectroscopy (ICP−OES) indicates that the metal Ru loading per unit area is approximately 2.64 mg/cm².

To clarify the structural information of the CoNiRuO_x/NF, two comparative samples were synthesized by directly immersing NF into solutions of CoCl₂·6H₂O and RuCl₃·xH₂O, respectively, with other conditions consistent with those for CoNiRuO_x/NF (synthesis details are provided in the Section 3). The SEM results (Figure 2a,b) show that Co/NF exhibits a nanosheet array structure with a lateral size of approximately 200 nm, whereas Ru/NF displays a nanoparticle morphology with particle sizes around 20 nm. Raman spectroscopy results (Figure 2c) indicate that Co/NF exhibits characteristic peaks around 465 and 525 cm⁻¹, which are attributed to the A_1g symmetric stretching and bending vibration modes of NiCo hydroxide [38]. Considering that Co²⁺ cannot undergo a replacement reaction with NF, combined with TEM results, lattice spacings of 0.193 nm, 0.203 nm, and 0.238 nm can be attributed to the (018), (107), and (104) facets of CoNi−LDH (Figure S4) [39], respectively. Hence, the main component of Co/NF can be identified as CoNi−LDH. Additionally, both Ru/NF and CoNiRuO_x/NF exhibit weak Raman peaks near 465 cm⁻¹. Considering that Ru³⁺ can carry out a replacement reaction with NF to generate Ru⁰ and Ni²⁺, the Raman peak near 465 cm⁻¹ in Ru/NF is attributed to Ni–O species, while the corresponding peak in CoNiRuO_x/NF may arise from the bonding of Ni−O/Co−O [40].

Regarding the analysis of the valence states, X-ray photoelectron spectroscopy (XPS) results (Figure 2d,e) reveal that the Co 2p spectrum of Co/NF shows the fitting peaks near 780.8 and 796.4 eV corresponding to the 2p_1/2 and 2p_3/2 of Co³⁺, and the fitting peaks located at 782.9 and 797.9 eV match the 2p_1/2 and 2p_3/2 of Co²⁺ [20], respectively. Additionally, the fitting peaks for the 2p_1/2 and 2p_3/2 of Ni²⁺ are identified at 855.6 and 873.3 eV in the Ni 2p spectrum [41], respectively. The above results indicate the coexistence of Co³⁺, Co²⁺, and Ni²⁺, which further supports the presence of the CoNi−LDH structure. For CoNiRuO_x/NF, the oxidation states of Co are similar to those in Co/NF. However, CoNiRuO_x/NF shows weaker fitting peak intensities for Co²⁺, Co³⁺, and Ni²⁺ compared to Co/NF, indicating that the introduction of Ru reduces the relative content of Co²⁺ and Ni²⁺. This is consistent with the trend of decreased intensity of the M−O peaks in the Raman spectrum, suggesting that the introduction of Ru alters the interaction between the metal and oxygen. From the Ru 3p spectrum (Figure 2f), the fitting peaks at 462.4 and 484.6 eV for Ru/NF correspond to the 3p_1/2 and 3p_3/2 of Ru⁰, while those at 466.3 and 488.6 eV are ascribed to the 3p_1/2 and 3p_3/2 of Ru^x⁺ [42]. Compared to Ru/NF, the fitting peaks in CoNiRuO_x/NF (463.5 eV and 485.7 eV) shift toward higher binding energies, indicating a higher oxidation state of Ru in CoNiRuO_x/NF. Additionally, the elemental ratio of Co to Ru is determined to be 1:2.84 based on XPS results, further clarifying the elemental composition of the amorphous CoNiRuO_x.

Based on the above experimental results, the synthesis mechanism of CoNiRuO_x/NF is further speculated. First, when the NF is immersed in a solution containing dissolved oxygen (O₂), an oxygen corrosion reaction would occur on the NF, resulting in the generation of Ni²⁺ and OH⁻ [43]. This reaction process can be described by Equation (1). Additionally, the Co²⁺ in the solution can also be oxidized to Co³⁺ by dissolved oxygen, as described by Equation (2) [44]. For Co/NF, the generated Co³⁺ will combine with the Ni²⁺ released from the corrosion reaction, as well as with OH⁻ and CO₃²⁻ in the solution to form [Ni²⁺_1-xCo³⁺_x(OH)₂]^x+[(CO₃²⁻)_x/2·yH₂O] (Equation (3)), which corresponds to the CoNi−LDH structure. For Ru/NF, due to the difference in standard electrode potentials between Ru³⁺ and Ni²⁺ (E⁰(Ru³⁺/Ru⁰) > E⁰(Ni²⁺/Ni⁰)), the replacement reaction can occur spontaneously [45]. The NF will reduce Ru³⁺ to metallic Ru while itself being oxidized to Ni²⁺, as shown in Equation (4). As for the CoNiRuO_x/NF, considering that both the oxygen corrosion reaction and the replacement reaction may occur in the solution containing Ru³⁺ and Co²⁺, this suggests that Ru³⁺ near the NF tends to be reduced to its metallic state (driving force of the replacement reaction), and Ni²⁺, Co³⁺, and OH⁻ should simultaneously exist in the surrounding environment (driving force of the corrosion reaction). The source of Ni²⁺ is from both the substitution reaction and the corrosion reaction, while the presence of Cl⁻ in the system can accelerate the corrosion process [46]. Under the combination of these two reactions, we can infer that during the transformation of Ru³⁺ to its metallic state, a heterogeneous nucleation process of Ni²⁺ and Co³⁺ occurs simultaneously, ultimately leading to the formation of the thermodynamically stable CoNiRuO_x catalytic structure on the NF.

2Ni + O₂ + 2H₂O → 2Ni²⁺ + 4OH⁻

(1)

4Co²⁺ +O₂ + 2H₂O → 4Co³⁺ + 4OH⁻

(2)

Ni²⁺ + Co³⁺ + OH⁻ + CO₃²⁻ → [Ni²⁺_1-xCo³⁺_x (OH)₂]^x+[(CO₃²⁻)_x/2·yH₂O]

(3)

3Ni + 2Ru³⁺ → 3Ni²⁺ + 2Ru

(4)

The HER performance of CoNiRuO_x/NF was evaluated via a standard three-electrode system in 1 m KOH. The linear sweep voltammetry (LSV) curves in Figure 3a show that CoNiRuO_x/NF achieves a current density of 10 mA/cm² at a low overpotential of only 43 mV, which outperforms Ru/NF (82 mV), Co/NF (235 mV), and NF (274 mV) and is second only to Pt/C (27 mV). The Tafel slope was used to further investigate the reaction kinetics of the catalysts. As shown in Figure 3b, the Tafel slope of CoNiRuO_x/NF is 49.2 mV dec⁻¹, which is significantly lower than that of Ru/NF (72.6 mV dec⁻¹), Co/NF (105.8 mV dec⁻¹), and NF (131.5 mV dec⁻¹) and is close to that of Pt/C (43.0 mV dec⁻¹). This indicates that CoNiRuO_x/NF possesses faster reaction kinetics and superior electron transfer efficiency. Additionally, the electrochemical impedance spectroscopy (EIS) results in Figure S5 reveal that CoNiRuO_x/NF has a smaller charge transfer resistance compared to the Ru/NF, Co/NF, and NF, further confirming its ultrafast electron transfer rate.

We further compared the electrochemical surface area (ECSA) and intrinsic activities of these materials. The ECSA of the catalyst was first evaluated through the double-layer capacitance (C_dl). Accordingly, the ECSA of CoNiRuO_x/NF is approximately 3.1, 6.7, and 9.2 times that of Ru/NF, Co/NF, and NF, respectively (Figure S6). This reveals that the abundant atomic disorder of CoNiRuO_x/NF facilitates the exposure of catalytic active sites, further confirming the superiority of the amorphous structure in enhancing catalytic performance. The comparison of intrinsic activities in Figure 3c shows that the intrinsic activity of CoNiRuO_x/NF at an overpotential of 100 mV (η = 100 mV) is 0.25 mA/cm², which is higher than that of Ru/NF (0.19 mA/cm²), Co/NF (0.04 mA/cm²), and NF (0.03 mA/cm²), demonstrating the excellent inherent catalytic activity of CoNiRuO_x/NF. At a potential of 0.043 V (vs. RHE), the turnover frequency (TOF) of CoNiRuO_x/NF was further determined to be 0.001 s⁻¹. In addition, we compared CoNiRuO_x/NF with many previously reported Ru-based catalysts (Figure 3d and Table S1), and CoNiRuO_x/NF exhibits the best HER catalytic activity among them.

The stability of CoNiRuO_x/NF was evaluated using a chronopotentiometry curve (V–t). As shown in Figure 3e, CoNiRuO_x/NF operated continuously for over 100 h at a current density of 100 mA/cm² with negligible current decay, demonstrating its outstanding resistance to deactivation. Characterization of CoNiRuO_x/NF after the stability test via SEM and XPS revealed that it retained its original nanoparticle morphology (Figure S7), and the valence states of the constituent elements Ru, Co, and Ni remained unchanged (Figure S8). However, based on the elemental ratio analysis from XPS, the Co:Ru ratio in the CoNiRuO_x/NF after the stability test was found to be 1:1.03, indicating partial Ru leaching during the catalytic process. This may be due to the relatively weaker bonding in the amorphous structure compared to the crystalline phase. To further explore the industrial application potential of CoNiRuO_x/NF, an alkaline flow electrolyzer was assembled using CoNiRuO_x/NF as the cathode and an OER electrode prepared by our research group (NiFe−LDH) as the anode, and then the couple was designated as NiFe−LDH||CoNiRuO_x/NF. For comparison, another electrode pair was assembled using Pt/C as the cathode and IrO₂ as the anode (IrO₂||Pt/C), and the activity tests were conducted under the same alkaline flow electrolyzer conditions. The LSV results (Figure S9) show that NiFe−LDH||CoNiRuO_x/NF required a voltage of only 1.73 V to deliver a current density of 100 mA/cm², which is lower than the voltage needed by IrO₂||Pt/C to achieve the same current density (1.88 V). The V–t curve results (Figure S10) indicate that NiFe−LDH||CoNiRuO_x/NF also demonstrated stable operation for over 100 h at 100 mA/cm², further confirming its superb stability and broad application prospects for large–scale hydrogen production.

3. Materials and Methods

3.1. Chemicals and Reagents

Ni foam (NF, thickness: 1 mm) was purchased from Kunshan Guangjiayuan new material Co., Ltd. (Kunshan, China). Ruthenium (III) chloride hydrate (RuCl₃·xH₂O, 99.99%), potassium hydroxide (KOH, 95%), and iridium dioxide (IrO₂, 99.9%) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Cobalt chloride hexahydrate (CoCl₂·6H₂O, AR) and Nafion 117 solution (5%) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). Isopropyl alcohol (C₃H₈O, AR) and hydrochloric acid (HCl, AR) were purchased from Xilong Chemical Factory (Shantou, China). Deionized water (>18 MΩ cm resistivity) was produced by a Water Purification System of Heal Force (Shanghai, China).

3.2. Synthesis of CoNiRuO_x/NF

CoNiRuO_x/NF was synthesized via a simple impregnation process. First, a nickel foam (NF) substrate (3 cm × 3 cm) was ultrasonically cleaned for 5 min in 6 m HCl, ethanol, and deionized water to remove potential oxide layers on the surface. Subsequently, the cleaned NF was immersed in a solution containing 0.3 mg/mL RuCl₃·xH₂O and 3 mg/mL CoCl₂·6H₂O at room temperature (~25 °C) for 3 h to obtain the amorphous CoNiRuO_x/NF structure. Additionally, we adjusted the concentration of RuCl₃·xH₂O (0.1 mg/mL, 0.2 mg/mL, 0.4 mg/mL) and the reaction time (1 h, 5 h) to obtain electrode materials, and the corresponding catalytic activities are provided in Figures S1 and S2.

3.3. Synthesis of Ru/NF

The preparation process of Ru/NF is similar to that of CoNiRuO_x/NF, with the only difference being the absence of CoCl₂·6H₂O in the reaction solution.

3.4. Synthesis of Co/NF

The preparation process of Co/NF is similar to that of CoNiRuO_x/NF, with the only difference being the absence of RuCl₃·xH₂O in the reaction solution.

3.5. Preparation of NiFe-LDH, Pt/C, and IrO₂ Electrodes

NiFe–LDH was prepared based on a modified corrosion method [13]. First, a clean NF (3 cm × 4 cm) was immersed in a 0.1 m FeCl₂·4H₂O solution, acting as the cathode, while a stainless-steel mesh served as the anode. Under stirring conditions at 500 r/min, a constant current of 0.6 A was applied for 2 min to perform electrochemical deposition. The material was then removed and immersed in a 0.1 m NiSO₄·6H₂O solution, followed by heating in a water bath at 80 °C for 3 h to obtain the NiFe–LDH electrode.

Next, 4 mg of Pt/C catalyst was dispersed in a solution containing 760 μL of isopropanol and 40 μL of Nafion, then it was stirred for 30 min to obtain a homogeneous solution. Then, 200 μL of the solution was dropped onto a 1 cm × 1 cm NF substrate and air-dried at room temperature for 12 h to obtain a Pt/C catalyst with a loading of approximately 1.0 mg/cm². The preparation of IrO₂ follows a similar procedure, except that Pt/C was replaced by IrO₂.

3.6. Characterization

The optical microscopy imaging was performed on an optical microscope (MA2001, Chongqing, China). The microscopic topographies of the samples were characterized via field emission scanning electron microscope (FESEM, HITACHI S–4800, Tokyo, Japan) and transmission electron microscope (TEM, FEI Tecnai G2 F20, Hillsboro, OR, USA). The elemental valence states of the samples were obtained from X–ray photoelectron spectroscopy (XPS, Thermo SCIENTIFIC Nexsa, Hillsboro, OR, USA). All the XPS spectra have been calibrated by the binding energy of C1s at 284.8 eV. The Raman spectra were collected via a laser Raman spectrometer (Renishaw Invia, London, UK) with an excitation source of a 514 nm laser. The element content of the samples was analyzed by inductively coupled plasma optical emission spectroscopy (ICP–OES, Agilent 5110, Santa Clara, CA, USA).

3.7. Electrochemical Measurements

All electrochemical measurements were carried out at a standard three-electrode system on the German Zahner electrochemical workstations. The as–prepared materials (1 cm × 1 cm), carbon rod, and Hg/HgO were used as the working electrodes, counter electrode, and reference electrode, respectively. All measured potentials versus Hg/HgO were calibrated to the reversible hydrogen electrode (RHE) according to the Nernst equation (E_RHE = E_Hg/HgO + 0.098 + 0.059 × pH). All polarization curves were conducted over linear sweep voltammetry (LSV) with a scan rate of 2 mV/s and dealt with 95% iR drop unless otherwise mentioned. Electrochemical impedance spectroscopy (EIS) was collected at a frequency range of 0.1 Hz to 100 kHz with an amplitude of 10 mV. The electrochemical double-layer capacitance (C_dl) was calculated via cyclic voltammetry (CV) within a potential range of −0.1 V to 0 V at a scan rate range of 20 to 100 mV/s, and the electrochemical surface area (ECSA) value was calculated according to the formula ECSA = C_dl/C_s, where C_s is 40 μF/cm².

Turnover frequency (TOF) was calculated using the equation TOF = (J × A)/(2 × F × n), where the J, A, F, and n refer to the current density (10 mA/cm²) at a given overpotential (0.043 V), geometric surface area of the work electrode (1 cm²), Faradaic constant (96,485 C mol⁻¹), and mole numbers of metal in the electrode (3.7 × 10⁻⁵ mol), respectively.

The alkaline flow electrolyzer uses NiFe-LDH as the anode and CoNiRuO_x/NF as the cathode, and the membrane is ZF-500. The loading amount of IrO₂ and Pt/C on nickel foam was set at 1 mg/cm², and the electrode working area was 1 cm². The electrolyzer was carried on 1 m KOH at a flow rate of 17 mL/min at a temperature of 25 °C. LSV measurements were conducted on a potential range of 1–2 V with a scan rate of 5 mV/s without iR compensation. The stability of the alkaline flow electrolyzer was assessed by applying a constant current density of 50 mA/cm² for 100 h without iR compensation.

4. Conclusions

In summary, a simple and easily scalable method is proposed to construct CoNiRuO_x nanostructures in situ grown on NF with high disorder (CoNiRuO_x/NF). The optimized CoNiRuO_x/NF demonstrates excellent catalytic activity for HER in 1 m KOH, with an overpotential mere 43 mV at a current density of 10 mA/cm² and maintains stability more than 100 h at 100 mA/cm². Furthermore, the assembled NiFe–LDH||CoNiRuO_x/NF alkaline electrolyzer can deliver a current density of 100 mA/cm² at a voltage of only 1.73 V, and its current density is 2.97 times higher than that of the IrO₂||Pt/C electrode pair at 2 V. Systematic experimental results indicate that the formation of the amorphous CoNiRuO_x structure involves the combined effects of replacement and corrosion reaction. Specifically, the transition of Ru³⁺ to its metallic state (driving force of replacement reaction) and the heterogeneous nucleation of Ni²⁺ and Co³⁺ (driving force of corrosion reaction) occur simultaneously on the NF surface, resulting in the formation of the amorphous CoNiRuO_x structure. This work not only expands the synthesis methods for amorphous materials but also constructs a new composition of amorphous structures, offering a new direction for the development of efficient and scalable electrocatalytic materials.