In Situ Measurements on Solid Oxide Fuel Cell Cathodes - Simultaneous X-Ray Absorption and AC Impedance Spectroscopy on Symmetrical Cells

A solid oxide fuel cell in operando is a complex multiphasic entity under electrical polarization and operating at high temperatures. In this work, we reproduce these conditions while studying transition metal redox chemistry in situ at the cathode. This was achieved by building a furnace that allowed for X-ray absorption near-edge structure and AC impedance spectroscopy data to be obtained simultaneously on symmetrical cells while at operating temperatures. The cell electrodes consisted of phases from the Ruddlesden-Popper family; La2NiO4+, La4Ni3O10-, and composites thereof. The redox chemistry of nickel in these cathodes was probed in situ through investigation of changes in the position of the X-ray absorption K-edge. An oxidation state reduction (Ni3+ to Ni2+) was observed on heating the cells; this was correlated to changing concentrations of ionic charge carriers in the electrode. Polarizing the cells resulted in dramatic changes to their electrical performance but not to the bulk redox chemistry of the electrode. The implications of this with respect to explaining the polarization behavior are discussed.