Theoretical understanding of stability of the oxygen electrode in a proton-conductor based solid oxide electrolysis cell

Abstract

The oxygen electrode in a proton-conductor based solid oxide cells is often a triple-conducting material that enables the transport and exchange of electrons (e−), oxygen ions (O2−), and protons (H+), thus expanding active areas to enhance the oxygen electrode activity. In this work, a theoretical model was developed to understand stability of tri-conducting oxygen electrode by studying chemical potentials of neutral species (i.e., μO2, μH2, and μH2O) as functions of transport properties, operating parameters, and cell geometry. Our theoretical understanding shows that (1): In a conventional oxygen-ion based solid oxide cell, a high μO2 (thus high oxygen partial pressure) exists in the oxygen electrode during the electrolysis mode, which may lead to the formation of cracks at the electrode/electrolyte interface. While in a proton-conductor based solid oxide cell, the μO2 is reduced significantly, suppressing the crack formation, and resulting in improved performance stability (2). In a typical proton-conductor based solid oxide electrolyzer, the dependence of μO2 on the Faradaic efficiency is negligible. Hence, approaches to block the electronic current can improve the electrolysis efficiency while achieving stability (3). The difference of the μO2 (thus pO2) between the oxygen electrode and gas phase can be reduced by using higher ionic conducting components and improving electrode kinetics, which lead to further improvement of electrode stability.