Facile room-temperature synthesis of cobalt sulphide for efficient oxygen evolution reaction

Water electrolysis is a promising strategy to generate high-purity hydrogen in large-scale industrial production, which is important for the green energy scenario and the safety of future energy [1–3]. Electrolysis of water consists of two half reactions, cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) [4]. In recent years, electrocatalysts for HER and OER have been intensively investigated and optimized in various electrolytes (acid, neutral and base) by different strategies, such as defect engineering, doping and confinement engineering [5–13]. Compared with HER with a two-electron transfer process, the pathway for the four-electron process, OER, is more sluggish with the breaking of O–H bonds and the formation of O–O bonds, which hinders the overall efficiency of the electrolysis of water [14–16]. Noble metal oxides, such as RuO₂, IrO₂, show excellent catalytic performance and are regarded as the benchmark for OER electrocatalysts [17, 18]. However, the high cost, scarcity and unstable features limit the application of noble metal-based catalysts in practical use. Thus, it is important to develop low-cost, high performance and stable electrocatalysts for OER.

Compared with noble metal-based OER electrocatalysts, transition metal-based materials have been explored largely due to their high activity and abundance in Earth's crust [19]. Much effort has been devoted to developing transition metals derivatives, such as Co [20], Ni [21], Mo [22], and Fe [23] oxides [24], hydroxides [25], nitrides [26], sulphides [27] and phosphides [28] for high-performance OER electrocatalysts. Currently, some transition metal-based electrocatalysts could even outperform the benchmark precious metal OER catalysts in regard to overpotential and stability [29, 30].

Transition metal sulphides (TMSs) were demonstrated as some of the most effective OER catalysts due to their unique physical and chemical properties. In addition, TMSs behave as small band gap semi-conductors and lead to good electrical conductivity [19, 31]. For example, Qu et al [32] developed a spinel MnCo₂S₄ nanowire array on Ti mesh by the sulphidation of MnCo₂O₄ precursor. The catalysts showed an excellent OER activity with a small overpotential of 325 mV at a current density of 50 mA cm⁻² in 1 M KOH, similar to that of commercial RuO₂. The Tafel slope of the material is 115 mV dec⁻¹, which implies fast OER kinetics. The catalysts also showed great stability at fixed current densities of 20 mA cm⁻² and 50 mA cm⁻² for 100 h. Lee et al [33] developed a series of compositional tunable Co_x Ni_y S_z by the template-assisted method. CoO@Co₉S₈ core–shell structure was achieved by the sulphidation of CoO and the addition of Ni precursor could promote the transition from Co₉S₈ to Ni₉S₈. Then the Co_x Ni_y S_z could be achieved by controlling the amount of Ni precursors and the reaction time. The optimized Co_9−x Ni_x S₈ catalysts showed an overpotential of 362 mV at a current density of 10 mA cm⁻² in 1 M NaOH and the corresponding Tafel slope is 74.7 mV dec⁻¹. However, the instability and surface reconstruction behaviour of metal sulphides during OER requires further study to uncover the specific active species.

In this work, we report the fabrication of Ni foam supported Co₄S₃ nanostructures (Co₄S₃/NF) by a single-step wet chemistry synthesis method at room temperature, which does not require external heating or annealing processes and is a low energy-process. Meanwhile, the wet chemistry synthesis provides a promising route to synthesize TSM materials at room temperature for large-scale production, which has great potential for industrial application. Owing to the unique electronic structures that endow rich active sites, the Co₄S₃/NF catalysts showed a small overpotential of 340 mV to reach a current density at 100 mA cm⁻² in 1 M KOH and with a small Tafel slope of 71.6 mV dec⁻¹. Furthermore, the catalysts showed excellent stability and possessed a stable output at a large current density of 100 mA cm⁻² for 100 h in 1 M KOH. The 1400 cycles of linear sweep voltammetry (LSV) tests showed an even better performance afterwards. The surface reconstruction behaviour behind the outstanding stability was investigated by ex-situ x-ray photoelectron spectroscopy (XPS), which showed Co³⁺/Ni³⁺-sulphate/sulphites species and the formed Co³⁺/Ni³⁺-(oxy)hydroxide contributed to the improved performance.

2.1. Material

2.2. Preparation of Co₄S₃/Ni(OH)₂ supported on Ni foam (Co₄S₃/Ni(OH)₂@NF)

2.3. Characterization

2.4. Electrochemical tests

A series of Co₄S₃/NF in this work were synthesized via a simple and scalable one-pot wet chemistry synthesis method. Generally, specific amounts of CoCl₂ · 6H₂O and Na₂S₂O₃ were dissolved in 10 ml of DI water in a 15 ml glass vial. Then a piece of Ni foam was immersed in the solution and kept at room temperature for 24 h. The colour of the electrode become darker after the reaction. Finally, the electrodes were collected and washed with DI water for several times. The as-prepared materials were used directly as self-standing OER electrodes. This method is energy-conserving, easy to operate and showed great potential for the large-scale fabrication of electrode materials.

Cobalt precursors (1 g, 2 g, 3 g) were applied to obtain different electrocatalysts and denoted as Co₄S₃/NF-1, Co₄S₃/NF-2 and Co₄S₃/NF, respectively. A digital photo of electrodes prepared with different amount of Co precursors is shown in figure S1. The morphologies of electrodes and the distribution of elements were studied by SEM and EDX. The material barely grows on nickel foam for Co₄S₃/NF-1, which could also be demonstrated by SEM images in figures 1(a) and S2. Figures 1(b) and S3 showed the SEM images of Co₄S₃/NF-2. The colour of the electrode is darker than Co₄S₃/NF-1 and clusters of nanoparticles could be seen grown on the Ni foam skeleton. However, the materials grow unevenly on the surface of the Ni foam, which could not provide sufficient active sites for OER and thus weaken the performance. When using 3 g of Co precursor, the materials grow evenly on the surface of Ni foam and the colour is the darkest which indicates uniform and dense growth of active materials on the electrode. The SEM images of Co₄S₃/NF (figures 1(c) and S4) provided excellent growth of the active materials on Ni foam without blocking the 3D porous structures of Ni foams, which could provide sufficient channels for the diffusion of electrolytes and promote the OER performance. The EDX mapping results showed a uniform distribution of the Co, S and Ni elements (figure 1(d)). Therefore, the optimum amount of precursor is 3 g, was used for further analysis unless otherwise indicated.

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XRD patterns of the Co₄S₃/NF electrodes are shown in figure 2(a). As can be seen, the XRD peaks of the electrodes at 20.1°, 23.2°, 33.1° and 39.0° could be indexed to (111), (200), (220) and (311) planes of Ni (PDF No. 04-0850). The XRD peaks at 13.6°, 17.9° and 21.4° were assigned to (311), (331) and (511) planes of Co₄S₃ (PDF No. 02-1338). The XRD results indicated the main structure of Co₄S₃/NF was Co₄S₃ with strong Ni peaks originating from the Ni foam.

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The elemental valence states of the as-prepared catalysts were further investigated by XPS. The high resolution XPS spectra of Co 2p, Ni 2p and S 2p for Co₄S₃/NF electrodes are shown in figures 2(b)–(d). In the Co 2p region, two peaks at binding energies of 781.7 eV (Co 2p_3/2) and 797.7 eV (Co 2p_1/2) with satellite peaks of 786.8 eV and 803.5 eV were assigned to Co₄S₃ species [34]. The two peaks at binding energies of 856.0 eV (Ni 2p_3/2) and 873.7 eV (Ni 2p_1/2) with satellites peaks of 861.9 eV and 879.8 eV were ascribed to surface oxidized Ni²⁺ species [35]. In the S 2p region, the peak at 161.9 eV (S 2p_3/2) and 163.2 eV (S 2p_1/2) can be assigned to S²⁻ [36], which could be originated from Co₄S₃ species. The other two peaks at 168.1 eV (S 2p_3/2) and 169.4 eV (S 2p_1/2) could be assigned to a sulphate group [30], which may have come from the oxidation of residual Na₂S₂O₃ during the reaction or from the storage in air.

The OER electrocatalytic performance of Co₄S₃/NF, Co₄S₃/NF-1, Co₄S₃/NF-2 and the pure Ni foam were examined in 1 M KOH electrolyte. Figure 3(a) presents the relevant polarization curves. Electrodes prepared with a larger amount of Co precursor showed better OER performance, which could be due to more active materials grown on the surface and more even distribution of the active materials. At a geometric current density of 100/300 mA cm⁻², the overpotential of Co₄S₃/NF is 340/373 mV, which is better than that of Co₄S₃/NF-1 (385/430 mV) and Co₄S₃/NF-2 (370/420 mV). To obtain the kinetic information of the as-prepared electrodes, the corresponding Tafel plots are provided in figure 3(b). The Co₄S₃/NF electrode exhibits a Tafel slope of 71.6 mV dec⁻¹, which is among the best reported electrodes and suggests more rapid OER catalytic kinetics (table S2). The OER electrocatalytic performance of Co₄S₃/NF, Co₄S₃/NF-4, Co₄S₃/NF-5 and Co₄S₃/NF-6 showed a very close OER performance (figure S5), which could be due to the saturated active materials supported on the Ni foam when preparing with more than 3 g cobalt precursors. To better understand the remarkable catalytic property of Co₄S₃/NF, the electrochemically active surface area (ECSA) was investigated using a typical cyclic voltammetry (CV) method (figures S6–S8). As shown in figure 3(c), the double-layer capacitance (C_dl) of Co₄S₃/NF is 3.96 mF cm⁻², while the C_dl of Co₄S₃/NF-1 and Co₄S₃/NF-2 are 2.96 mF cm⁻² and 3.25 mF cm⁻², respectively. This result proved more active sites would be generated with increasing amounts of Co precursors, corresponding to the SEM results. Figure S9 showed the EIS results of different electrodes recorded in 1 M KOH under open circuit conditions. The inherent resistance values of Co₄S₃/NF, Co₄S₃/NF-1, Co₄S₃/NF-2 and pure Ni foam are 1.304 Ω, 1.351 Ω, 1.347 Ω and 1.275 Ω, respectively. The Co₄S₃/NF electrodes showed a small value of resistance which indicated a fast electron transfer ability. On the one hand, the surface reconstruction behaviour could increase the electron population on the metal centre and the conductivity could be enhanced [3]. On the other hand, the large surface area and large number of pores on Ni foam could promote the mass and electron transfer [5].

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Electrochemical stability is an important factor when evaluating the performance of the electrocatalysts. A chronovoltammetry test was performed on Co₄S₃/NF at a fixed current density of 100 mA cm⁻² for 100 h. The overpotential of Co₄S₃/NF increased by 3% from 406 mV to 419 mV after 100 h (figure 3(d)), which proved the excellent stability of the catalysts. It is to be noted that the electrolyte was not replaced or refilled during the stability test. Thus, the negligible change in overpotential could be due to the decreased area of the electrodes immersed in the electrolyte, which was caused by the consumption of the electrolyte under a large current density for long working times. Furthermore, figure 3(e) shows the LSV test of Co₄S₃/NF for 1400 cycles in 1 M KOH. Surprisingly, an enhanced OER performance was achieved after the 1400th cycle. The overpotential was even lowered by 20 mV at 100 mA cm⁻² and a lower onset potential could also be noted after stability test.

To further analyse the remarkable stability of Co₄S₃/NF, an ex-situ XPS test was performed to study the change of valence states of the elements before and after OER tests at a constant current density of 10 mA cm⁻² for one day. To avoid further possible oxidation of the materials, the electrode after the OER stability test was stored in a vacuum box before XPS analysis and transferred to XPS test chamber within 5 min. According to figure 4(a), by comparing the change of XPS binding energy before and after the reaction, the peaks of Co shifted to a lower binding energy, which could be due to the transformation from Co₄S₃ to Co³⁺ [37]. It is reasonable that the Co²⁺ species in Co₄S₃ are oxidized to Co³⁺ during the OER reaction. It was also observed that there was a change of binding energy of Ni before and after the reaction. The peaks of Ni shifted to a higher binding energy, which could be due to the transformation from Ni²⁺ to Ni³⁺ [38, 39]. The transformation of Co and Ni could also be proved by the polarization curves of the 1st cycle of Co₄S₃/NF as shown in figure S10. Two oxidation peaks could be seen around 1.25 V vs RHE and 1.38 V vs RHE, which correspond to the oxidation peaks of Co and Ni [33]. These oxidation peaks disappeared after 1400 cycles of the LSV, which proved the total transformation of Co²⁺ and Ni²⁺ species. In the S 2p spectra, the original S²⁻ peak of the Co₄S₃ disappeared after the stability test. The changes in S XPS spectra indicates the catalyst surface undergoes a reconstruction process as the OER proceeds. The surface of the catalysts seems to change from Co₄S₃ to Co³⁺/Ni³⁺-sulphate/sulphites and Co³⁺/Ni³⁺-(oxy)hydroxide species during the OER process in the alkaline medium [30]. The XRD result of sample proved the formation of Co-sulphites species after OER stability test while the Co₄S₃ species remained, which could indicate the surface of Co₄S₃ undergoes reconstruction process (figure S11). The morphologies of materials did not show significant changes after OER stability test (figure S12).

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In summary, a facile and energy-conserving method to obtain Ni foam supported Co₄S₃ at room temperature was developed. Superior electrocatalyst performance was achieved with an overpotential of 340 mV at 100 mA cm⁻² and a Tafel slope of only 71.6 mV dec⁻¹ in alkaline media. Excellent stability was realized with a negligible change in overpotential at a constant current density of 100 mA cm⁻² for 100 h. The LSV curves showed that the catalytic performance become even better after 1400 cycles, demonstrating excellent stability. Ex-situ XPS tests proved the surface reconstruction process of the catalyst and showed the Co³⁺/Ni³⁺-sulphate/sulphites and Co³⁺/Ni³⁺-(oxy)hydroxide species could be the more active species as the OER electrocatalyst. This work provides cost-effective OER electrocatalysts with high performance and stability and provides a useful strategy to produce transition metal sulphides at room temperature.

The work was supported by Engineering and Physical Sciences Research Council (EPSRC, EP/V027433/1, EP/L015862/1, EP/R023581/1) and the Royal Society (RGS\R1\211080; IEC\NSFC\201261). S Z thanks the funding support from China Scholarship Council/University College London for the joint PhD scholarship.

The data that support the findings of this study are available upon reasonable request from the authors.