Changing the oxygen reaction mechanism in composite electrodes by the addition of ionic- or ambipolar-conducting phases: Series or parallel pathways

In this work, we conduct a systematic investigation of composite oxygen electrodes for Solid Oxide Cells (SOCs) based on the recently emerging misfit calcium cobaltite material, Ca3Co4O9+delta - C349. Its electrochemical performance is shown to be limited by its low intrinsic level of ionic conductivity, thus, requiring the addition of an ionic-conducting composite phase. In this respect, Ce0.8Gd0.2O2-delta (CGO) is the state-of-the-art choice due to its excellent ionic conductivity. Nonetheless, little attention has been given to the use of alternative ionic phases, such as Ce0.8Pr0.2O2-delta, which also offer minor levels of p-type conductivity. Therefore, we try to elucidate the influence of this minor electronic component in this phase, on the resultant oxygen reaction mechanism of the composite electrode. We demonstrate that a preferential series pathway for the oxygen reaction occurs in the CGO-based electrode, performing better at high temperatures and low oxygen partial pressures. Conversely, the CPO-based electrode presents superior performance under highly oxidising conditions and at low temperatures, related to an increasing importance of possible parallel reaction pathways. The results are supported by a detailed mechanistic study, based on a Distribution Function of Relaxation Times (DFRT) analysis, allowing clarification of potential mechanistic models for both electrodes.

Automated summary

This study investigates the use of composite oxygen electrodes in Solid Oxide Cells (SOCs) using the material Ca3Co4O9+delta - C349. The low intrinsic level of ionic conductivity in this material restricts its electrochemical performance, necessitating the addition of an ionic-conducting composite phase such as Ce0.8Gd0.2O2-delta (CGO). However, little attention has been given to alternative ionic phases like Ce0.8Pr0.2O2-delta, which also offer p-type conductivity. The study demonstrates that the oxygen reaction pathway in the CGO-based electrode is preferential and performs better at high temperatures and low oxygen partial pressures, while the CPO-based electrode performs better under highly oxidising conditions and at low temperatures.