In situ imaging of electrode processes on solid electrolytes by photoelectron microscopy and microspectroscopy -: the role of the three-phase boundary

The electrochemical polarisation of metal catalyst films on solid electrolyte substrates can basically lead to three different effects: (a) the generation of mobile surface species (spillover) which spread over the catalyst surface and modify the catalytic activity, (b) potential-controlled segregation of impurities in the catalyst and (c) potential-dependent surface energy (electrocapillarity). The generation of spillover species occurs at the three-phase boundary between metal, solid electrolyte and gas phase and is highly localized. The spreading occurs via diffusion and leads to time-dependent and inhomogeneous surface concentrations. The kinetics of the spillover process can only be observed with in situ surface-analytical techniques in combination with electrochemical methods which offer sufficient resolution in space and time. Model experiments with UV and X-ray photoelectron emission microscopy (PEEM and SPEM) are summarized and discussed with respect to their relevance for the better understanding of electrochemical promotion in catalysis.