Spatially Resolving Heterogeneous Chemistry on SOFC Electrodes with Operando Optical Methods

Abstract

Carbon fouling (or coking) is one of the primary degradation mechanisms that leads to performance loss and eventual failure in high temperature, solid oxide electrochemical cells (SOCs). Accumulated carbon blocks electrocatalytic sites at three phase boundaries, impedes transport through porous electrode structures and can react with common electrode materials such as Ni to form carbides that disintegrate in a process described as metal dusting. Operando Raman studies examining carbon remediation through gasification showed surprising results. (J. Phys. Chem. C, 119 (2015) 7637.) When comparing carbon removal from Ni-YSZ cermet anodes at 750˚C by gas phase H2O, O2, and CO2, experiments measuring intensity of the carbon ‘G’ band, Raman studies showed that H2O was most effective, removing all observable carbon in a matter of seconds. Similar amounts of O2 required approximately one minute to remove detectable carbon, and CO2 required even longer and never removed all of the observable carbon. These results were supported by changes in the SOC open circuit potential. These Raman measurements, however, sampled only a single location on the anode. Work described in this presentation shows the anode surface chemistry to be considerably more heterogeneous than might be inferred from the Raman data. A combination of near-IR thermal imaging (NIRTI), Fourier transform IR emission spectroscopy (FTIRES), voltammetry and downstream mass spectrometry (MS) exhaust analysis have begun to spatially resolve processes associated with carbon gasification from coked Ni-cermet anodes at 800˚C, and show that carbon gasification proceeds through a sequence of steps involving both surface and gas phase reactions.