Spontaneous H₂O₂ Accumulation under O₂ Driven by Sacrificial Oxidation of NiS₂ without External Electrical/Photonic Input

Although spontaneous H₂O₂ accumulation by sacrificial oxidation of metal disulfides (FeS₂, NiS₂, etc.) under oxic conditions without external electrical/photonic input has been observed, the mechanistic understanding of HCO₃^- on the production of H₂O₂ is still not clear. In this study, HCO₃^- apparently accelerates the production of H₂O₂ on NiS₂, and the produced H₂O₂ increases from 53.85 to 90.12 μM with increasing concentrations of HCO₃^- from 0 to 0.5 M at pH 9.0. Electrochemical analyses indicate that HCO₃^- effectively mediates the 2e reduction of O₂ to H₂O₂ on the surface of NiS₂. EPR and Raman analyses rule out the primary contributions of soluble ^•OH and O₂^•- to the production of H₂O₂ and reveal the presence of both ≡Ni-OO^• superoxo and ≡Ni-OOH peroxo on NiS₂, which are important precursors for H₂O₂. The presence of CO₃^•- also indicates that HCO₃^- is an important H-donor during the reduction of O₂ to H₂O₂. DFT calculations further reveal that it is thermodynamically more favorable for ≡Ni-OO^• and ≡Ni-OOH to abstract H from HCO₃^- than from H₂O to produce H₂O₂, confirming that HCO₃^- is a better H-donor than H₂O to produce H₂O₂ via the H-abstraction pathway. This study provides insight into the importance of HCO₃^- on spontaneous H₂O₂ accumulation by sacrificial oxidation of NiS₂ under oxic conditions without external electrical/photonic input.