Progress in understanding triple ionic–electronic conduction in perovskite oxides for protonic ceramic fuel cell applications

Protonic ceramic fuel cells offer a promising route to effectively generate electricity from various fuels at reduced temperatures. However, the viability of this technology is impeded by the sluggish kinetics of the oxygen reduction reaction at the cathode. Recently, triple ionic–electronic conductors have shown their promise as cathode materials with improved catalytic activity because of their enhanced mixed electron and ionic conductivities that can maximise the active sites for the reaction. This review examines the transport mechanism of holes, oxygen ions, and protons within triple ionic–electronic conductors. This review highlights the equilibrium among these charge carriers and their requirement for specific cationic environments to facilitate rapid transport. As a result, triple ionic–electronic conductors need to balance the transport of these charges to realise optimum oxygen reduction reaction activity. The review further identifies the transport of oxygen ions or protons as the current limiting factor in triple ionic–electronic conductors. This review concludes by emphasizing the importance of understanding the role of ionic transport in the oxygen reduction reaction to enhance the performance of triple ionic–electronic conductors.