Electrocatalytic Water Oxidation at Neutral pH–Deciphering the Rate Constraints for an Amorphous Cobalt-Phosphate Catalyst System

摘要

The oxygen evolution reaction (OER) is pivotal in sustainable fuel production.Neutral-pH OER reduces operational risks and enables direct coupling toelectrochemical CO_2 reduction, but typically is hampered by low currentdensities. Here, the rate limitations in neutral-pH OER are clarified. Usingcobalt-based catalyst films and phosphate ions as essential electrolytebases, current–potential curves are recorded and simulated. OperandoX-ray spectroscopy shows the potential-dependent structural changesindependent of the electrolyte phosphate concentration. Operando Ramanspectroscopy uncovers electrolyte acidification at a micrometer distance fromthe catalyst surface, limiting the Tafel slope regime to low current densities.The electrolyte proton transport is facilitated by diffusion of either phosphateions (base pathway) or H_3O~+ ions (water pathway). The water pathway isnot associated with an absolute current limit but is energetically inefficientdue to the Tafel-slope increase by 60 mV dec~(?1), shown by an uncomplicatedmathematical model. The base pathway is a specific requirement in neutralpHOER and can support high current densities, but only with acceleratedbuffer-base diffusion. Catalyst internal phosphate diffusion or other internaltransport mechanisms do not limit the current densities. A proof-of-principleexperiment shows that current densities exceeding 1 A cm~(?2) can also beachieved in neutral-pH OER.