Revealing and quantifying the role of oxygen-ionic current in proton-conducting solid oxide fuel cells: A modeling study

Proton-conducting solid oxide fuel cells (H-SOFCs), as highly efficient energy devices for power generation, are allowed to be operated at intermediate temperatures (673 K-973 K), owing to the high ionic conductivities of "proton-conducting electrolytes". In fact, besides protons, oxygen vacancies also exist as ionic-charged carriers in proton-conducting electrolytes. However, the role of the oxygen-ionic current (jO) in the proton-conducting electrolyte has been overlooked to date. Since the type of conducting ions in the electrolytes has a significant impact on the electrode design and energy performance of SOFCs, it is significant to quantitatively evaluate the contribution of jO in proton-conducting electrolytes. In this study, a theoretical model, considering three types of mobile defects (proton, oxygen vacancy, electron-hole), is built. For the first time, a multifactor theoretical analysis is conducted for evaluating jO in H-SOFCs at various electrode humidity, operating temperature, and external current density (jext). The modeling results reveal that at relatively large jext (> 0.8 A/cm2), jO is mainly influenced by the humidity of fuel (PfuelH2O), rather than the humidity of air. Reducing Pfuel H2O leads to significant growth of jO. This effect becomes pronounced at higher temperatures. Suppressing jO by humidifying fuel is beneficial to improving the power density of H-SOFCs.