XPS study of SOFC anode surface chemistry at high temperature and controlled oxygen activity

Mixed conducting oxides gain increasing interest as anodes in solid oxide fuel cells, due to their large electrochemically active surface area and excellent redox stability, compared to state-of-the-art Ni-YSZ cermets.... [ view full abstract ]

Mixed conducting oxides gain increasing interest as anodes in solid oxide fuel cells, due to their large electrochemically active surface area and excellent redox stability, compared to state-of-the-art Ni-YSZ cermets. However, to further understand and optimize these materials, information on the surface chemistry and reaction mechanisms is important. We present a new method to investigate mixed conducting anodes at in a standard UHV-based XPS analyzer. By heating and application of a voltage versus an oxygen buffering reference electrode, the effective oxygen partial pressure in the investigated working electrode can be precisely controlled within the range that is typical for reducing gas mixtures, down to to the reducing decomposition of the working electrode. The virtual absence of molecules in the gas phase of the UHV chamber largely prohibits presence of atmospheric adsorbates, while the effect of effective oxygen pressure on the defect concentrations at the surface and the electrochemical stability window are experimentally accessible. By combination with additional synchrotron-based ambient pressure XPS measurements, the UHV characterization can also help differentiating between surface species from atmospheric adsorbates and species arising from different defect energetics of electrode bulk and surface. This technique was applied to investigate the near surface oxidation states of transition metals found on La_0.6Sr_0.4FeO_3-δ, SrTi_0.3Fe_0.7O_3-δ and La_0.7Sr_0.2Cr_0.9Ni_0.1O_3-δ at different oxygen partial pressures. When the oxygen partial pressure in the working electrode goes 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, La_0.7Sr_0.2Cr_0.9Ni_0.1O_3-δ turns out to be more oxidized than the bulk, and can include a Cr⁶⁺ species.