Self-Supported Polyhedral-like Co3S4 Nanostructures Enabling Efficient High Current Hydrogen Evolution Reaction

Abstract

The advancement of overall water-splitting technologies relies on the development of earth-abundant electrocatalysts that efficiently produce H₂ as a chemical fuel while offering high catalytic efficiency, structural robustness, and low-cost synthesis. Therefore, we aim to develop a cost-effective and durable non-noble electrocatalyst for overall water splitting. A straightforward hydrothermal approach was employed to fabricate freestanding polyhedral Co₃O₄ on a microporous Ni foam scaffold, followed by anion-exchange transformation in the presence of Na₂S solution to yield its conductive sulfide analog. The engineered Co₃S₄ electrode delivers remarkable HER activity in 1.0 M KOH, requiring a low overpotential (<100 mV) to drive 10 mA cm^-2, far outperforming its pristine oxide counterpart and even closely benchmarking with a commercial Pt/C catalyst. This exceptional performance is governed by the synergistic effects of enhanced electrical conductivity, abundant catalytic sites, and accelerated charge-transfer kinetics introduced through sulfur substitution. Furthermore, the optimized Co₃S₄ electrodes enable a bifunctional overall water-splitting device that achieves a cell voltage of >1.76 V at 100 mA cm^-2 and maintains prolonged operational stability for over 100 hrs. of continuous operation. Post-stability analyses confirm insignificant phase preservation during testing, ensuring sustained activity throughout the electrolysis process. This study highlights the potential of anion-exchanged Co₃S₄ as a cost-effective and durable catalyst for high-performance HER and full-cell water-splitting applications.

Keywords: Co3S4; anion-exchange; hydrogen evolution reaction; hydrothermal synthesis; overall-water electrolysis; polyhedral structure.

Figure 1

Schematic illustration of the synthesis process for three-dimensional polyhedral Co₃O₄ and its subsequent transformation into Co₃S₄ through anion-exchange. In the first step (I), Co₃O₄ polyhedral structures are obtained by hydrothermal growth of cobalt precursor, followed by controlled air annealing to induce crystallization and phase stabilization (II). In the following step (III), the Co₃O₄ template undergoes a sulfurization process in Na₂S solution, where oxygen anions are gradually replaced by sulfide ions, leading to the formation of porous Co₃S₄ microstructures. Figure S1a shows the photograph image of Co₃O₄ and Co₃S₄ electrode films.

Figure 2

(a) XRD spectra; (b) Raman spectra of Co₃O₄ and Co₃S₄ electrode films.

Figure 3

XPS spectra of Co₃O₄ and Co₃S₄ electrode films: (a) Survey spectra; (b) Narrow ranged Co 2p; (c) Narrow ranged O 1s; and (d) Narrow ranged S 2p spectra. All the narrow-range spectra were fitted using a Gaussian curve fitting model.

Figure 4

FESEM images recorded at low and high magnifications for (a) Co₃O₄ electrode film; (b) Co₃S₄ electrode film. FESEM-EDS elemental mapping images for the constituent (c) Co; (d) O; and (e) S elements.

Figure 5

Electrocatalytic HER performance of Co₃O₄ and Co₃S₄ catalysts was examined in an alkaline 1.0 M KOH condition. (a) LSV curves; (b) Tafel slopes; and (c) Comparative HER performance of Co₃S₄ catalyst and reported metal sulfide-based catalysts at 10 mA cm⁻² in 1.0 M KOH, with the additional details are summarized in Table S1.

Figure 6

(a) LSV curves for the bifunctional Co₃S₄ (BF-Co₃S₄) electrolyzer cell recorded before and after the prolonged chronopotentiometric stability test; (b) voltage step profile for BF-Co₃S₄ electrolyzer at various current densities; and (c) chronopotentiometric stability profiles recorded at 10 mA cm⁻² for 100 hrs. for the Co₃S₄ electrolyzers.