(353e) Activity of CoS2 for Electrochemical Oxygen Reduction Reaction Under Different Reaction Conditions

The family of transition-metal chalcogenides (T-M-X) has been reported as promising nonprecious oxygen reduction reaction (ORR) electrocatalysts. It has been since hypothesized that intrinsic ORR activity of T-M-X is closely related to the structural evolution of their surfaces under electrochemical conditions. However, a very little is understood about the evolution of the T-M-X surfaces under oxidizing potentials due to limited theoretical investigations of the active sites coupled with experimental validation. With cobalt disulfide (CoS2) as a representing material, here we construct a comprehensive bulk and surface Pourbaix diagrams of the system and use density functional theory (DFT) to determine the preferential ORR active sites. We show that sulfur to oxygen substitution and partial dissolution occurs in acidic media, while thin cobalt-oxide films supported by CoS2 are formed in alkaline media. Furthermore, our calculations reveal that sulfur is unlikely ORR active site, while an undercoordinated Co-metal site in the CoS2 is less active than the very active undercoordinated Co-metal site in the Co-oxide film. To demonstrate the validity of our theoretical results, we perform the controllable decrease of grain size and the increase of surface oxygen content of CoS2 particle through the electrochemical lithium (Li) ion cycling method. Compared with pristine CoS2 (C-CoS2), detailed characterizations via transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and rotating ring disk electrode (RRDE) experiments reveal the smaller grain size, richer surface oxygen content and better ORR performance of the processed CoS2 (LiET-CoS2) in both acid and alkaline medium, and thus strongly confirm our calculated thermodynamics as well as the ORR mechanism regarding the pH-dependent electrochemical evolution of the CoS2 surfaces. This work not only helps to elucidate the mechanism of ORR activity of T-M-X in broad pH values but also opens a new perspective for probing more advanced T-M-X-based catalysts in the future.