Constructing a pathway for mixed ion and electron transfer reactions for O2 incorporation in Pr0.1Ce0.9O2-x

In interfacial charge-transfer reactions, the complexity of the reaction pathway increases with the number of charges transferred, and becomes even greater when the reaction involves both electrons (charge) and ions (mass). These so-called mixed ion and electron transfer (MIET) reactions are crucial in intercalation/insertion electrochemistry, such as that occurring in oxygen reduction/evolution electrocatalysts and lithium-ion battery electrodes. Understanding MIET reaction pathways, particularly identifying the rate-determining step (RDS), is crucial for engineering interfaces at the molecular, electronic and point defect levels. Here we develop a generalizable experimental and analysis framework for constructing the reaction pathway for the incorporation of O-2(g) in Pr0.1Ce0.9O2-x. We converge on four candidates for the RDS (dissociation of neutral oxygen adsorbate) out of more than 100 possibilities by measuring the current density-overpotential curves while controlling both oxygen activity in the solid and oxygen gas partial pressure, and by quantifying the chemical and electrostatic driving forces using operando ambient pressure X-ray photoelectron spectroscopy. Identifying rate-determining steps (RDSs) is one of the most challenging aspects of catalysis. This work presents a general framework to identify the RDS of mixed ion and electron transfer reactions, and applies it to the four-electron/two-ion O-2 reduction in solid-oxide fuel cell cathodes, converging on four RDS out of more than 100 possible candidates.