Proton ceramic electrochemical cells (PCECs) comprising fuel cells (PCFCs), electrolysers (PCEs), and reactors (PCERs) require well-conducting electrolytes of mechanical and thermochemical stability, but even more so electrodes with sufficient electrocatalytic activity at moderate temperatures. Oxide positrodes for PCFCs and PCEs are particularly challenging due to limited surface catalytic activity for the oxygen redox reaction and solubility of protons. Current research aims to improve these by, e.g., exsolution of catalytic nanoparticles and optimisation of hydration thermodynamics, in addition to microstructure. Interpretation of experimental polarisation results and degradation phenomena requires physicochemical understanding of the reaction mechanisms and microstructural paths and application of appropriate mathematical models, a field underdeveloped for solid oxide electrochemical cells (SOECs) in general and for PCECs in particular.

The polarization processes at solid-state electrodes comprise space charge layer depletion resistance, charge transfer between the electrolyte and electrode phases, and mass transfer in and on the electrode material. The latter comprises diffusion in and on the solid electrode material and in the gas phase as well as surface reaction kinetics. For PCECs, the processes on Ni metal negatrodes involves diffusion of dissociated atomic hydrogen,[1] while the processes on mixed proton-electron conducting oxide positrodes involves proton-proton charge transfer, proton diffusion, and the surface oxygen-steam electron transfer redox-reaction.

Electrochemical impedance spectroscopy (EIS) enables separation of the different polarisation processes by their capacitances. The proton-proton charge transfer may be expected to follow Butler-Volmer (BV) type kinetics. The mass transfer polarisation of porous mixed conducting electrodes involves coupled diffusion, reaction kinetics, and chemical capacitance that in the simplest case can be modelled as a Gerischer impedance when it polarises the BV charge transfer. This approach for mixed proton-electron conducting (MPEC) electrodes for PCECs follows essentially all the principles for corresponding mixed ion-electron conducting (MIEC) electrodes for SOECs laid down in what we may call the Adler-Lane-Steel (ALS) model. [2,3] Individual polarisation resistances measured by EIS under DC bias can be integrated over current to obtain individual overpotential-current curves and help to identify processes and predicting behaviours under operation in fuel cell or electrolyser mode, also involving complications due to competing oxide ion and p- and n-type electronic conduction.[4]