Impact of first-row transition metals in (Ba,La)(Fe,TM)O3-δ on proton uptake and electronic conductivity

Abstract

Oxides with mixed proton, oxygen vacancy, and hole conductivity are key functional materials for fuel and electrolyzer cells based on protonic ceramic electrolytes. Here, we explore the effect of partial B-site substitution of Ba0.95La0.05FeO3-δ with first-row transition metals, clarifying the impact on hydration and electronic conductivity. An anti-correlation between proton uptake and electronic conductivity is observed: Zn- doped compositions favor protonation and decrease electronic conductivity; the opposite holds for Co-containing samples. Cu- and Ni- doped materials deviate positively from this general trend. We ascribe this behavior to an interplay of Cu (Ni)-O bond covalency, charge transfer, and d orbital occupancy. Codoping with 10 mol% of late transition metals and 10 mol% of zinc strongly increases the proton uptake while yielding a reasonable electronic conductivity. In particular, Ba0.95La0.05Fe0.8(Co,Ni,Cu)0.1Zn0.1O3-δ samples show both high proton uptake and electronic conductivity, suggesting it as promising cathode material.