Elucidating the proton-coupled oxygen reduction pathway in protonic ceramic fuel cells

Abstract

The proton-coupled oxygen reduction reaction (PC-ORR) at protonic ceramic fuel cell (PCFC) cathodes involves multiple charges, which are proton, oxygen ion, and electron/electron hole, and its complexity has long impeded unambiguous identification of the reaction pathway and the rate-determining step (RDS). The difficulty is amplified by the prevailing practice of subjectively positing an a priori “most probable” pathway to infer the RDS—a procedure that heightens the risk of decisive bias and error. Consequently, mutually inconsistent pathways have been proposed for ostensibly the same reaction. Here, we present a generalized microkinetic framework that infers the RDS without prior pathway construction. Applying this approach, we resolve the RDS for two widely studied PCFC cathodes, PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF) and BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ (BCFZY). PBSCF is limited by vacancy-assisted O₂ dissociation, whereas BCFZY is limited by a proton-coupled OH adsorbates formation step involving adsorbed atomic oxygen and bulk protons. While both exhibit sufficient proton transport and fast bulk diffusion such that surface reactions dominate porous cathode performance, the origin of the contrasting RDSs is traced to their different proton uptake/release mechanisms.