SurfaceChemistry of Perovskite-Type Electrodes During High Temperature CO2 Electrolysis Investigated by Operando PhotoelectronSpectroscopy

摘要

Any substantial move of energy sources from fossil fuels to renewable resources requires large scale storage of excess energy, for example, via power to fuel processes. In this respect electrochemical reduction of CO2 may become very important, since it offers a method of sustainable CO production, which is a crucial prerequisite for synthesis of sustainable fuels. Carbon dioxide reduction in solid oxide electrolysis cells (SOECs) is particularly promising owing to the high operating temperature, which leads to both improved thermodynamics and fast kinetics. Additionally, compared to purely chemical CO formation on oxide catalysts, SOECs have the outstanding advantage that the catalytically active oxygen vacancies are continuously formed at the counter electrode, and move to the working electrode where they reactivate the oxide surface without the need of a preceding chemical (e.g., by H2) or thermal reduction step. In the present work, the surface chemistry of (La,Sr)FeO3−δ and (La,Sr)CrO3−δ based perovskite-type electrodeswas studied during electrochemical CO2 reduction by meansof near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS)at SOEC operating temperatures. These measurements revealed the formationof a carbonate intermediate, which develops on the oxide surface onlyupon cathodic polarization (i.e., under sufficiently reducing conditions).The amount of this adsorbate increases with increasing oxygen vacancyconcentration of the electrode material, thus suggesting vacant oxygenlattice sites as the predominant adsorption sites for carbon dioxide.The correlation of carbonate coverage and cathodic polarization indicatesthat an electron transfer is required to form the carbonate and thusto activate CO2 on the oxide surface. The results alsosuggest that acceptor doped oxides with high electron concentrationand high oxygen vacancy concentration may be particularly suited forCO2 reduction. In contrast to water splitting, the CO2 electrolysis reaction was not significantly affected by metallicparticles, which were exsolved from the perovskite electrodes uponcathodic polarization. Carbon formation on the electrode surface wasonly observed under very strong cathodic conditions, and the carboncould be easily removed by retracting the applied voltage withoutdamaging the electrode, which is particularly promising from an applicationpoint of view.