Determination of Fe oxidation states in the B-site ordered perovskite-type Ba2Ca0.67Fe0.33NbO6−δ at the surface (nano-scale) and bulk by variable temperature XPS and TGA and their impact on electrochemical catalysis

Abstract

In this study, we analyzed the surface and bulk valences of Fe in the double perovskite-type Ba₂Ca_0.67Fe_0.33NbO_6−δ by both variable temperature X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). The concentration of Fe²⁺ near-surface was found to be about 3 times higher than that in the bulk sample. The electrochemical performance of Ba₂Ca_0.67M_0.33NbO_6−δ (M = Mn, Fe and Co) was assessed by ac impedance spectroscopy using a Zr_0.84Y_0.16O_1.92 (YSZ) supported half-cell. The area specific polarization resistances (ASR) of all samples were found to decrease with increasing temperature. We speculate that ASR for H₂ oxidation are correlated with the higher concentrations of low valence Fe²⁺ species near-surface (nano-scale), as demonstrated for Ba₂Ca_0.67Fe_0.33NbO_6−δ.