Exsolution and electrochemistry in perovskite solid oxide fuel cell anodes: Role of stoichiometry in Sr(Ti,Fe,Ni)O_3

摘要

The exsolution of metal cations from oxides under reducing fuel conditions results in the formation of surface metallic nanoparticles, which can reduce Solid Oxide Fuel Cell anode polarization resistance. However, the loss of the B-site cations shifts the stoichiometry of the perovskite oxide. Depending on the amount exsolved and the initial stoichiometry, the exsolution can presumably shift the oxide away from its single-phase perovskite region. Herein, the direct comparison of initially stoichiometric composition Sr(Ti0.3Fe0.63Ni0.07)O3-delta (STFN0) with initially A-site deficient Sr-0.95(Ti0.3Fe0.63Ni0.07)O3-delta (STFN5) is conducted and reported. X-ray diffraction along with scanning and transmission electron microscopy analysis of the oxides, which are both reduced at 850?degrees C in H-2/H2O/Ar, shows a similar size and density of exsolved Fe-Ni alloy nanoparticles, albeit with slightly different alloy compositions. Whereas the oxide phase in reduced STFN5 shows a well-ordered perovskite structure, the greater B-site deficiency in reduced STFN0 results in a highly disordered and strained structure. The electrochemical performance of STFN0 anodes is inferior to that of STFN5 anodes, and even worse than SrTi0.3Fe0.7O3-delta (Ni-free) anodes. It appears that an initial Sr deficiency is important to avoid a too-high B-site deficiency after exsolution, which distorts the perovskite structure and impairs electrochemical processes.