Determination of Fe oxidation states in the B-site ordered perovskite-type Ba2Ca_(0.67)Fe_(0.33)NbO_(6-δ) at the surface (nano-scale) and bulk by variable temperature XPS and TGA and their impact on electrochemical catalysis

摘要

In this study, we analyzed the surface and bulk valences of Fe in the double perovskite-type Ba2Ca_90.67)Fe_(0.33)NbO_(6-δ) by both variable temperature X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA), The concentration of Fe~(2+) near-surface was found to be about 3 times higher than that in the bulk sample. The electrochemical performance of Ba2Ca_(0.67)M_(0.33)NbO_(6-δ) (M = Mn, Fe and Co) was assessed by ac impedance spectroscopy using a Zr_(0.84)Y_(0.16)O_(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 Fe~(2+) species near-surface (nano-scale), as demonstrated for Ba2Ca_(0.67)Fe_(0.33)NbO_(6-δ).