Charge and Mass Transfer in Positrodes for Proton Ceramic Electrochemical Cells

Proton ceramic electrochemical cells (PCECs) comprising fuel cells (PCFCs), electrolysers (PCEs), and reactors (PCERs) require well-conducting electrolytes of mechanical and thermochemical stability as well as electrodes with sufficient electrocatalytic activity at moderate temperatures. Oxide positrodes for PCFCs and PCEs are particularly challenging due to poor surface catalytic activity for the oxygen redox reaction and limited transport of protons due to low concentrations. Current research aims to improve these by, e.g., exsolution of catalytic nanoparticles and optimisation of hydration thermodynamics and microstructure. Studies of polarisation and degradation phenomena require physicochemical understanding of the mechanisms of charge and mass transfer and application of appropriate mathematical models.<br/>The polarization processes at solid-state mixed proton-electron conducting oxide positrodes involves proton-proton charge transfer, proton diffusion in the bulk and on the surface of the positrode, and the surface oxygen-steam electron transfer redox-reaction.<br/>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 is called the Adler-Lane-Steel (ALS) model. [1,2] Measurements under oxidising conditions and moderate humidities at temperatures (typically &gt; 400°C) where p-type electronic and oxide ionic conductivities in the electrolyte are significant must take the transport numbers of these into account when fitting impedance spectra to avoid major overestimates of the performance and activation enthalpies of proton ceramic positrodes. [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.<br/>We illustrate the models and parameterisation by results for Ba-Ln-Co-based perovskite positrodes on doped BaZrO₃ electrolytes using EIS on 3-electrode button cells as a function of temperature, <i>p</i>O₂, <i>p</i>H₂O, and DC bias.<br/> <br/>Acknowledgements: The work is in part supported by the Research Council of Norway (RCN) through projects “Electrolyser 2030 (MODELYS)” #326809 and FME HYDROGENi #333118, the latter financed by its industry partners and the Norwegian government through the RCN’s Centres for Environment-friendly Energy Research programme (FMETEKN).<br/> <br/>¹ Adler, S.B.; Lane, J.A.; Steele, B.C.H. Electrode Kinetics of Porous Mixed-Conducting Oxygen Electrodes. J. Electrochem. Soc., 1996, 143, 3554.<br/>² Adler, S.B. Mechanism and kinetics of oxygen reduction on porous La_1-<i>x</i>Sr<i>_x</i>CoO_3-<i>_δ</i> electrodes. Solid State Ionics, 1998, 111, 125.<br/>³ Poetzsch, D.; Merkle, R.; Maier, J., Investigation of oxygen exchange kinetics in proton-conducting ceramic fuel cells: Effect of electronic leakage current using symmetric cells, J. Power Sources, 2013, 242, 784.