Unveiling Janus Chemical Processes in Contact-Electro-Chemistry through Oxygen Reduction Reactions.

Oxygen reduction reaction (ORR), operating via four-electron (H₂O) or two-electron (H₂O₂) pathways, underpins critical processes in energy conversion and biological metabolism. Solid-liquid contact electrification enables 2e^- ORR for both pollutant oxidation degradation and metal reduction without external metal catalysts. However, the criteria dictating oxidation versus reduction in such Janus contact-electro-chemistry (CE-Chemistry) systems remain unclear. This study systematically demonstrates that the redox selectivity in CE-Chemistry is controlled by the standard electrode potential (SEP) of the reactants, with a clear threshold distinguishing the oxidation and reduction pathways. Reduction of metal ions (e.g., [AuCl₄]^-, Pd²⁺, [PtCl₄]^2- Ag⁺, Rh³⁺, and Ir³⁺) was achieved when their SEPs lie between the 2e^- ORR (E⁰ = 0.695 V vs NHE) and the 4e^- ORR (E⁰ = 1.229 V vs NHE). Conversely, SEPs below the 2e^- ORR threshold favored oxidation (e.g., ferrocyanide). For the first time, methanol-to-formaldehyde oxidation was achieved in both aqueous and nonaqueous CE-Chemistry. Remarkably, the formaldehyde production rate in dimethyl sulfoxide was 25 times higher than in aqueous systems, which has already surpassed some photocatalytic processes. This study provides a comprehensive mechanistic framework for CE-Chemistry, highlighting the pivotal role of SEPs in regulating its Janus redox properties and the tunable radical reactivity in nonaqueous environments.