Thermodynamic Assessment of Sacrificial Oxidant Potential, H2O/O2 Potential, and Rate-Overpotential Relationship to Examine Catalytic Water Oxidation in Nonaqueous Solvents.

Abstract

The water oxidation reaction (WOR), which is pivotal to storing energy in chemical bonds, requires a catalyst to overcome its inherent kinetic barrier. In bulk solutions, sacrificial oxidants (SOs) can regenerate the catalysts to ensure that the homogeneous WOR can be operated with long-term consistent performance. To implement this strategy for organic WOR systems, we modified four common SOs with tetra-n-butylammonium ([NBu4]+)─[NBu4]2[Ce(NO3)6], [NBu4][IO4], [NBu4][HSO5], and [NBu4]2[S2O8]─and examined their chemical stability and electrochemical behaviors in various organic solvents. We also derived the organic-solvent-associated redox potential of H2O/O2 in organic media (EH2O/O2(org)) using open-circuit potential measurements of the H+/H2 redox couple and the related thermochemical cycle. Our findings indicate that the EH2O/O2(org) varies with solvent identity and can be adjusted by changing the [H2O], [acid], and [base] levels; thus, the SO should be carefully selected for WOR, because the innate redox potentials of SOs are not always higher than EH2O/O2(org) under the studied conditions. Lastly, we obtained catalyst-performance-related insights via a rate-overpotential free-energy relationship by calculating the overpotentials of previously studied WOR systems in organic media.