Unveiling the mechanistic implications of water oxidation reactions boosted by guanidine proton relays: a chemical-electrochemical-chemical pathway and a non-concerted proton-electron transfer

Abstract

Concerted or non-concerted are two competitive pathways involved in proton-coupled electron transfer–driven artificial water oxidation reactions (WORs). It is known that designed metal-based electrocatalysts applied in WORs afford high oxidation states to accelerate the electron transfer (ET), so that the rate-determining step (RDS) of the reaction is controlled by the proton transfer (PT) step of a commonly accepted concerted proton-electron transfer pathway. An alternative idea is installing an organic super-base in the closest position to the catalyst nerve center to capture protons at the end of each catalytic loop. In this work, the mentioned repeatable cycle is operated through a guanidine (present in arginine) proton relay (GPR) with a low-energy-barrier hydrogen bonding characteristic, as was confirmed by proton inventory isotopic studies and Gerischer impedance from a semi-infinite transmission line. Direct experimental evidence was provided by kinetic isotope effects (KIEs), proton inventory, pH-dependency on the RHE scale, atom proton transfer, Tafel slope, and Gerischer impedance behavior. The obtained Gerischer impedance indicates a sinking process and efficient proton shuttling, in which a chemical-electrochemical-chemical (CEC)-type mechanism controls the overall WOR. This evidence specifies that the rate of the proton exchange reaction on guanidine sites is extremely larger than the natural transport rate in the polymer/electrolyte interface. The results of experimental studies unveil a PT-accelerated non-concerted mechanism with the first ET-step as the RDS of WORs.