A soft molecular single-source precursor approach to synthesize a nanostructured Co9S8 (pre)catalyst for efficient water oxidation and biomass valorization

Abstract

The molecular single-source precursor (SSP) route has emerged as a promising avenue for synthesizing highly efficient electro(pre)catalysts tailored for both oxygen evolution (OER) and organic electrooxidation reactions. This study introduces a novel [Co^II(PyHS)₄(OTf)₂] molecular complex, offering a facile route to access the nanocrystalline Co₉S₈ phase. Upon application in alkaline OER, Co₉S₈ displayed remarkably high electrocatalytic activity across various metrics, including overpotentials, Tafel slopes, faradaic efficiency, charge transfer resistance, turnover frequency, electrochemical surface area, Co-redox active sites, and long-term stability at industrially relevant current densities (for 80 h at 100 mA cm⁻²) outperforming its Co-based counterparts under identical conditions. In-depth analysis employing several ex situ techniques revealed the complete leaching of sulfur and irreversible reconstruction of Co₉S₈ into a cobalt oxyhydroxide active phase during the OER. Moreover, quasi in situ Raman spectroscopy provided insights into the presence of Co^IVO₂ active species under operational OER conditions, along with the formation of the Co superoxide intermediate. Beyond OER applications, the Co₉S₈-derived active phase demonstrated notable efficiency in catalyzing the selective oxidation of biomass-derived glycerol and furan-2-carboxaldehyde to formate and furan-2-carboxylic acid, respectively, achieving yields exceeding 80%, with excellent reusability. A full hybrid-water electrolysis cell has been developed, wherein biomass valorization with Co₉S₈, coupled with H₂ production, resulted in a significant improvement in energy efficiency compared to conventional water-splitting.