Effects of sintering additives on defect chemistry and hydration of BaZr0.4Ce0.4(Y,Yb)0.2O3_ δ proton conducting electrolytes

The effects of NiO, ZnO, and CuO sintering additives (0.5, 1.0, or 2.0 wt%) on the sintering behaviour, effective acceptor concentration, and hydration thermodynamics are examined for BaZr0.4Ce0.4Y0.1Yb0.1O3_delta and BaZr0.4Ce0.4Y0.2O3_delta proton conducting electrolytes. Thermogravimetry of hydration shows that the sintering additives - except for 0.5 wt% CuO - lead to a decrease in the effective acceptor concentration, and the decrease per mole sintering additive is the largest for NiO. The absence of typical secondary phases such as BaY2NiO5 and the homogeneous distribution of Ni determined from elemental mapping imply that sintering additives dissolve into the perovskite lattice. Exsolution of metallic Ni and Cu upon reduction leads to a substantial recovery of the effective acceptor concentration. Subsequent oxidation is accompanied by a repeated decrease in the effective acceptor concentration as the sintering additives appear to re-dissolve. Defect chemical reactions are proposed to explain the observed results and these are supported by energetics from first principles calculations. Overall, NiO has the highest positive impact on densification and grain growth, and a relatively small amount of 0.5 wt% NiO or CuO is preferable to optimise both sintering and hydration.