MicroEXAFS and microXANES for the study of Solid Oxide Fuel Cells

Chemical and structural compatibility between materials is a critical point in all-ceramic devices working at very high temperatures: in this respect, the degradation of the cell performance is often the critical factor limiting the working life of SOFC. A well-studied example is the formation of a resistive layer of La2Zr2O7 at the interface between lantanum-containing perovskite cathodes and zirconia electrolytes. LaNbO4 doped with 2% Ca2+ (LNC) represents the last real breakthrough in the field of proton-conducting oxides for use as electrolytes in SOFC, showing improved stability and high conductivity with respect to perovskites [1]. To develop efficient and robust devices based on LNC, the electrical performance and chemical compatibility of various electrode materials with LNC was extensively tested in recent literature. [2] We recently applied X-ray microspectroscopy to evaluate the chemical and local structural fate of cations interdiffusing across an LNC/cathode bilayer (where the cathode is either La0.6Sr0.4MnO3, La0.6Sr0.4Co0.8Fe0.2O3, etc.) after prolonged annealing at high temperatures (1150 °C), to simulate the operating conditions of a fuel cell. The interfaces of the devices were studied with space-resolved X-ray absorption spectroscopy (XAS) using the focused submicrometer-sized beam available at the SXM-II endstation on the ID21 beamline of ESRF. We collected microXANES and microEXAFS spectra at the Nb L3, La L3, Ca K, Fe K and Mn K edges. The microXRF composition maps spectra were also collected, giving information on the distribution of cations. In some cases, microXANES spectra show marked variations, so that chemical-contrast maps can be acquired by changing slightly the beam energy. A wealth of interesting results are obtained by combining chemically-resolved maps and microspectroscopy. In general, an unexpected and impressive exsolution of the Ca2+ dopant from the LNC electrolyte towards the cathode is observed in all samples. [3] The Ca and Nb change of coordination number and geometry between the perovskite cathode and the LNC electrolyte is well supported by microXANES simulations, pinpointing the greater structural flexibility of the perovskite structure as the driving force behind the incorporation of cations from the electrolyte. Fe and Mn edge microXANES spectra also show interesting variations as a function of the distance from the interface, hinting at some kind of oxygen vacancy accumulation at the interface. At the La, Fe and Mn edges, high quality microEXAFS spectra were collected and modeled up to about 8 Å-1. These results represent the first application of X-ray absorption microspectroscopy to the study of materials compatibility in SOFC. Such an approach can be extended to other kinds of SOFC materials, including ceramic anodes, cermets, and oxide-ion conductors, reproducing different working conditions of real devices: this is expected to give unprecedented insight on the mechanisms governing electrolyte-electrode compatibility and electrochemical performance in solid oxide fuel cells. [1] R. Haugsrud & T. Norby Nature Mater. 2006, 5, 193-196. [2] K.V. Kravchyk et al. Int. J. Hydrogen Energ. 2011, 36, 13059. [3] F. Giannici et al. Chem. Mater. 2015, 27, 2763.