High temperature mapping of surface electrolyte oxide concentration in solid oxide fuel cells with vibrational Raman spectroscopy

Abstract

Experiments described in this work employ Raman spectroscopy and electrochemical impedance spectroscopy (EIS) to characterize the properties of a YSZ electrolyte in situ at 715 °C and under electrochemical potential control. The YSZ itself is the electrolyte in a functioning, electrolyte supported SOFC membrane electrode assembly (MEA). Experiments measure the changes in surface chemistry of the YSZ as a function of through-cell overpotential, and experiments are designed to identify what role—if any—surface reduced YSZ plays in electrochemical oxidation and charge transfer. Raman spectra show that the change in YSZ from fully oxidized to surface reduced is reversible and happens on a timescale of minutes at these temperatures. Spatially resolved experiments carried out with polarized cells show that the oxide ion concentration near the three phase boundary (TPB) is depleted even further when an overpotential is applied. The change in relative oxide concentration with respect to overpotential is also reversible. EIS measurements probing lateral impedance across the electrolyte surface show that the surface reduced phase has a lower polarization resistance (Rpol) as through-cell overpotential increases. Results are discussed in terms of reduced resistance to processes involving charge and mass transfer reactions specific to the reduced YSZ surface state.