Unveiling inactive sulfur residue and benzoquinone moiety formation in sulfur-doped carbon for water electrooxidation

Doping carbon materials with heteroatoms is an effective strategy to improve the catalytic performance of carbon materials through charge redistribution. Furthermore, heteroatom-doped carbon materials have been proven to be unstable and can be completely removed from the electrode via the electrochemical oxygen evolution reaction (OER). However, since S has a electronegativity similar to that of C, the behavior of S-doped carbon materials under OER conditions might differ and thus deserves special attention. In this study, we investigated the structural evolution of S-doped carbon materials during the alkaline OER. It was observed that the S-doped graphite flake (S-GP) underwent oxidization. Notably, only partial S dopants dissolved in the form of sulfates, resulting in the emergence of new forms of S- and O-containing groups on the electrode. The results from well-designed experiments demonstrated that despite remaining on the electrode, the S-containing groups had no effect on the OER activity, and the high OER activity was attributed to the derived benzoquinone moiety. The dissolved sulfates further promoted OER activity when S-doped carbon materials were used as substrates for the Ni(OH)2 anode. Our work reveals the real activity origin of S-doped materials towards OER, motivating researchers to reconsider the catalytic mechanism of the S-doped carbon materials and their supported composites for other reactions.