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

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 [CoII(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 CoIVO2 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.