A cathode with mixed hole, oxygen vacancy, and proton conductivity extends the reactive zone for the oxygen reduction to water beyond the triple phase boundary, making the whole cathode surface an active electrocatalyst. The defect chemistry (i.e., concentrations and mobilities of point defects) of such materials with three charge carriers is complex, and some of the desired properties for a PCFC cathode material are in mutual conflict (i.e., proton uptake, electronic conductivity, catalytic activity, and long-term chemical stability). And the promising protonic cathode material needs a high catalytic activity for the oxygen reduction reaction to water to improve its performance. Nevertheless, the reactions both require the dissociation of the strong oxygen-oxygen bond.

The mechanism for this reaction is not exactly identical to that in oxide-ion-conducting cells (where the resulting oxide ions are incorporated into the cathode material, while on PCFC cathode, they are desorbed in the form of steam). The dependence of proton uptake on cation composition in cathode perovskites in order to extract the parameters that are most important for a high proton concentration. Regarding the optimization of PCFC cathode materials, refraining from striving for very high electronic conductivities is anticorrelated with proton uptake. It is prospected that the role plays due to oversized dopants in barium ferrate for enhancing the hydration properties. The beneficial effect on protonation is attributed to a higher degree of disorder in the local structure of doped samples, which translates in B-O-B bonds buckling. The B-O-B buckling reduces the Fe-O bond covalency (i.e., less Fe 3d-O 2p orbital overlap), thus decreasing the hole transfer from iron to oxygen. This leaves more negative charge density on the oxide ions, which increases their basicity and propensity for protonation.

In addition, although the existing thermogravimetric and electrical conductivity relaxation methods still have certain limitations in measuring proton concentration in mixed ionic conductors. The in-situ characterization techniques (such as in-situ transmission electron microscopy, in-situ spectroscopy, neutron diffraction, etc.) can be used to analyze the process of proton absorption and transport in mixed ionic conductors. Also, combining multi-scale simulations with experiments can further analyze numerical values such as the binding energy of proton absorption and the activation energy of diffusion. For instance, in-situ transmission electron microscopy is used to directly observe water entering Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ via introducing a small amount of water vapor into the TEM chamber. Electron energy loss spectroscopy is also used to determine the formation of oxygen bubbles on the material surface, while the protons ultimately remain within the material. This result provides the most direct evidence for studying the reactions of water and protons in mixed ionic conductors.