Improved Oxygen Reduction Reaction Activity of Nanostructured CoS₂ through Electrochemical Tuning

Searching for efficient Pt-free oxygen reduction reaction (ORR) electrocatalysts has been actively pursued among the current electrocatalyst research community. The family of transition-metal chalcogenides, especially cobalt disulfide (CoS₂), has been reported as competitive ORR catalysts. Here, we perform a detailed analysis of the intrinsic activity in terms of onset potentials and selectivity toward hydrogen peroxide of CoS₂ in both acid and alkaline medium. Our detailed characterizations of this system via X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and calculated bulk and surface thermodynamics and ORR mechanism reveal pH-dependent electrochemical evolution of the CoS₂ surfaces. Using XPS results before and after ORR in combination with density functional theory (DFT) calculations for individual surfaces reveals sulfur to oxygen substitution, and partial dissolution occurs in acidic media, while thin cobalt oxide films supported by CoS₂ are formed in alkaline media. The comprehensive DFT calculations of the ORR activities on these systems reveal that sulfur is an unlikely ORR active site, while undercoordinated Co metal site in the CoS₂ is less active than very active undercoordinated Co metal site in the Co oxide film. Using these guiding principles, we then demonstrate that electrochemical lithium (Li) tuning of CoS₂ in organic electrolyte increases its ORR performance in both acid and alkaline medium. Detailed characterizations demonstrate that the grain size of CoS₂ particle is considerably reduced and has a much richer surface oxygen content after electrochemical Li tuning (LiET-CoS₂) as the direct consequence of the Li galvanostatic cycling. The general efficacy of this method toward transition-metal chalcogenides (T-M-X) is further demonstrated by enhanced ORR activities of CoS and Ni₃S₂ in alkaline and neutral medium, respectively. This work opens up an opportunity for probing more advanced T-M-X-based catalysts.