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.