Oxygen Reduction at Dense Thin-Film Microelectrodes on a Proton-Conducting Electrolyte

Abstract

The presence of protons in mixed conducting cathode materials contacted with proton conducting electrolytes introduces additional possible mechanistic pathways for the surface oxygen reduction reaction compared to cathodes on oxide ion conducting electrolytes. A reaction network that includes these additional paths with protonated intermediate species is developed. The corresponding rate equations are derived, reflecting the point defects' contributions to the overall oxygen and water partial pressure dependencies. An equivalent circuit is derived for the general case of electrodes that exhibit proton, oxygen ion and electronic conductivity on proton conducting electrolytes. A characteristic circuit element is a chemical capacitance comprising proton and oxygen ion concentration changes. Complications due to nonnegligible electronic transference numbers arising in proton conducting electrolytes at high pO 2 are discussed. In solid oxide fuel cells oxygen is adsorbed at the cathode side while taking up electrons. This applies for both oxygen-ion and pro-ton conducting electrolytes. Differences arise at later stages of the reduction reaction: cathodes on oxide ion conducting electrolytes finally transfer the reduced oxygen in form of oxide ions into the electrolyte, whereas cathodes on proton conducting electrolytes desorb the reduced oxygen in form of water after it has received also protons. While oxygen-ion conductivity is of explicit advantage in the former case, this is not so in the latter. In both cases the reaction comprises several elementary reaction steps. As to the former case, it could be clarified for a wide range of materials whether the reaction is limited to the three-phase-boundary (TPB) where oxygen, electrolyte and electrode meet (Fig. 1a, relevant for cathode materials with very low oxide ion conductivity), or whether the ionic conductivity of the primarily electronically conducting cathode enables a " bulk path " 1,2