Microwave-assisted synthesis of graphene-like cobalt sulfide freestanding sheets as an efficient bifunctional electrocatalyst for overall water splitting

Total water splitting provides an appealing pathway for clean and sustainable energy conversion and storage. Therefore, development of a noble metal-free, efficient and robust electrocatalyst simultaneously towards the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is still a great challenge. Herein, an ultrafast and scalable method was introduced to synthesize CoS_x non-layered and freestanding nanosheets via microwave-assisted liquid-phase growth of a Co(OH)₂ precursor post-sulfurized at room temperature. The planar sheet area was up to several micrometres and exhibited an ultrathin thickness (<1 nm) with a mesoporous feature. Hence, the highly exposed surface atoms afforded high electrochemical activity, whereas the ultrathin and mesoporous features facilitated charge and mass transfer along the 2D nanostructure to efficiently perform the surface-dependent electrochemical reactions: HER, OER, and the overall water splitting. Notably, the amorphous CoS_x presented a low overpotential (η₁₀) of 127 mV to deliver a current density of 10 mA cm^-2 for HER. During the OER process, the surface of Co₉S₈ (annealed CoS_x) was mainly in situ oxidized to form CoOOH; this suggested CoOOH/Co₉S₈ as the real form of the catalyst that exhibited excellent OER activity with a low overpotential (η₁₀) of 288 mV to afford 10 mA cm^-2. Impressively, the assembled overall water electrolyzer required a cell voltage of 1.55 V to achieve a current density of 20 mA cm^-2 in 1 M KOH with excellent stability. Predominantly, our synthesis strategy is cost-effective and scalable to break the synthesis challenge of transition-metal-sulfide ultrathin nanosheets with high quality and suggests a possibility for commercial applications.