Unveiling Partial Transformation and Activity Origin of Sulfur Vacancies for Hydrogen Evolution

Introducing various vacancies into electrocatalysts has been recognized as an effective strategy for enhancing electrocatalytic performance. However, the structural evolution of vacancies during electrocatalysis is rarely explored. Herein, we fabricate Co9S8 nanosheets having abundant sulfur vacancies (SV-Co9S8) for an enhanced alkaline hydrogen evolution reaction (HER). Compared with perfect Co9S8, SV-Co9S8 has a much higher HER activity and can be further activated after a simple pretreatment. Further characterizations demonstrate that sulfur vacancies (SVs) can induce the formation of Co(OH)2, which produces a synergetic effect with residual SVs. Combined experimental and theoretical results indicate that SVs and Co(OH)2 can cooperatively optimize the combination of adsorbed hydrogen (H*). Additionally, the water dissociation promoted by Co(OH)2 and the favorable conductivity from the remaining SVs also contribute to the high HER activity. This work not only unveils the electrochemical structural evolution induced by SVs but also stimulates researchers to rethink the structure–activity relationship of electrodes with anion vacancies.