Unraveling the Transformation of Cobalt Sulfide in the Hydrogen Evolution Reaction at a High Overpotential: A Step Toward Deciphering the Mechanism

The hydrogen evolution reaction (HER) plays a crucial role in various renewable energy technologies, and its efficiency is contingent upon the utilization of effective catalysts. Cobalt disulfide (CoS 2 ), like platinum, is utilized as an HER electrocatalyst. Nevertheless, CoS 2 has the advantage of being sourced from elements that are abundantly available on Earth, compared to platinum, for the HER. In this study, the stability of a commercially accessible nanostructured CoS 2 powder is investigated. To achieve this aim, analytical techniques, including scanning electron microscopy, transmission electron microscopy, energy-dispersive spectrometry, X-ray diffraction, X-ray absorption spectroscopy, electron paramagnetic resonance, and in situ Raman spectroscopy, are employed to characterize CoS 2 both prior to and subsequent to HER treatment. Our results indicate that when subjected to a high overpotential of 400 mV for 10 days, CoS 2 transforms into an alternative Co species, likely metallic Co, and subsequently into Co (hydr)oxide (CoH y O x ) under air exposure. These discoveries provide valuable insights into the development of state-of-the-art catalysts for the HER and elucidate the underlying mechanisms that contribute to efficient water splitting for energy storage applications.