Electrochemical XPS investigation of metal exsolution on SOFC electrodes: Controlling the electrode oxygen partial pressure in ultra-high-vacuum

Abstract

Mixed conducting oxides gain increasing interest as anode materials in solid oxide fuel cells (SOFCs), due to their large electrochemically active surface area and excellent redox stability, compared to state-of-the-art Ni-Yttria-Stablilzed Zirconia cermets. However, further optimization of these materials requires more information on the surface chemistry and the reaction mechanisms. Here we present a new concept for electrochemical XPS measurements close to SOFC anode operation conditions even in a UHV chamber. Application of a voltage versus an oxygen buffering reference electrode enables control of the effective oxygen partial pressure in the investigated mixed conducting thin film working electrode within the range that is typical for SOFC anodes or SOEC cathodes. A lower limit is given by the reductive decomposition of the working electrode. The virtual absence of molecules in the gas phase of the UHV chamber largely prohibits presence of atmospheric adsorbates. However, surface oxidation states of metal ions can be tuned by the over potential, and exsolution of metallic species can be monitored in situ. With this technique, we investigated the oxygen partial pressure dependent oxidation states of transition metals in La0.6Sr0.4FeO3-δ, SrTi0.3Fe0.7O3-δ and La0.7Sr0.2Cr0.9Ni0.1O3-δ thin film electrodes. When the oxygen partial pressure in the working electrode goes sufficiently below the NiO/Ni and FeO/Fe redox pairs, the exsolution of metallic Ni or Fe nanoparticles could be monitored in-situ. While most oxides are easier to reduce at the surface, Cr on the La0.7Sr0.2Cr0.9Ni0.1O3-δ surface turns out to be more oxidized than the bulk and can include a Cr6+ species.