Exploring the electrocatalytic activity of cobalt disulfide nanosheets towards the hydrogen evolution reaction with in situ ECAFM

Abstract

Exploring an efficient and durable electrocatalyst for the hydrogen evolution reaction (HER) is vitally necessary for sustainable energy conversion and storage systems. Herein, we fabricated CoS₂ nanosheets through a one-step hydrothermal reaction and investigated their electrocatalytic performance towards the HER in acidic and alkaline electrolytes. With sodium ethyl xanthate (SEXT) and cobalt acetate tetrahydrate as S and Co sources, the CoS₂ nanosheets were hydrothermally prepared under 200 °C for 10 hours. The CoS₂-2 nanosheets synthesized from a 7.5 atomic proportion of S to Co exhibited a remarkable electrocatalytic activity towards the HER with an overpotential of ∼145 mV in 0.5 M H₂SO₄ solution and 288.5 mV in 1 M KOH solution at 10 mA cm⁻², respectively. And after 2000 cycles, their current density remained almost constant. FESEM, TEM, AFM, and XRD showed that CoS₂-2 nanosheets possessed a good crystallinity and small size, facilitating sufficient exposure of electrocatalytically active sites. In situ electrochemical atomic force microscopy (ECAFM) demonstrated that after the HER, the morphology of individual CoS₂-2 nanosheets had no obvious change in acidic solutions and transformed significantly in alkaline media, while their height, width and area distinctly increased in both acidic and alkaline media. Nevertheless, the overall morphology of CoS₂-2 cuboids (aggregates) as-observed by FESEM hardly changed during the HER, confirming their excellent HER stability. The different change for atomic proportion and binding energy of Co³⁺ and Co²⁺ indicated that the electrocatalytic mechanism of CoS₂-2 towards the HER in acidic electrolytes was different from that in alkaline media.