Performance and degradation of positrodes for proton ceramic electrolysers

Mengxin Wu, Jonathan Polfus, Truls Norby

Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo

mengxin.wu@smn.uio.no

Abstract

Proton ceramic electrolysers (PCEs) that utilise available steam and/or heat (renewable or industrial) as a supplementary energy source provide superior electrical efficiency compared to conventional water electrolysis. The positrode constitutes a major contribution to the overpotentials and hence losses of PCEs, resulting from its low solubility of protons, as well as limiting surface kinetics of the oxygen evolution reaction (OER) – which are challenging to characterize and improve.

The positrode needs to use its surface for the redox reaction between water vapour, electrons, and protons and to form oxygen. The protons are supplied from the electrolyte via diffusion in the positrode bulk and surfaces. The rate of the surface reaction and the proton charge transfer reaction at the 2pb as well as the ratio between the diffusivity of protons in the bulk and on the surface will determine how far the protons go on the surface before diving into the bulk. The sole transport on the surface to charge transfer only at the 3pb must be considered a limiting case. The microstructure of the positrode and its surface and interface to the electrolyte is key to efficient heterogeneous reaction kinetics and transport of electrons and protons as well as long-term stability.

In our ongoing research, we aim to establish theoretical frameworks and experimental methodology to measure and interpret the parameters that determine the kinetics and transport overpotentials and to evaluate which ones limit performance and cause degradation. The measured parameters are input to finite element modelling necessary to handle the complexity of processes and geometries. We report impedance spectroscopy and voltammetry for 3-electrode setups for selected positrode materials as a function of temperature and oxygen and steam partial pressures and extract parameters along geometrical and mechanistic models. The results are interpreted in terms of ohmic, charge transfer, and mass transfer (combined diffusional and surface kinetic Gerischer-type) polarisation. We also follow parameters over time to elucidate degradation mechanisms. The results will serve as input for finite element modelling, to evolve the mechanistic electrochemical model, improving the performance of operating cells, and counteracting degradation.