Proton conducting BaCe0.6Zr0.3Y0.1O3-delta (BCZY63) pellets were fabricated by the solid-state reactive sintering method whereby a small extra amount of a metal oxide additive (5 mol%) was included in the precursor mixture before sintering. The effect of the addition of six different metal oxide additives (CuO, ZnO, Fe2O3, MnO2, PdO, and Cr2O3) on the transport properties of BCZY63 was investigated. Although most additives (e.g. ZnO, Fe2O3, MnO2, Cr2O3) led to a decrease (sometimes minor) in conductivity, CuO and PdO dopants yielded an increase in total conductivity compared to the BCZY63 control. The enhancement in the total conductivity is found to be related to two factors: 1) the additive can act as a sintering aid to produce larger grain size; 2) the presence of the metal ions from additives in the structure can lead to increased bulk proton concentration. Due to its excellent sinterability and moderately improved proton conductivity, CuO-doped BCZY63 was subsequently applied as the dense electrolyte layer in protonic ceramic fuel cells utilizing a recently developed single firing step fabrication technique to produce high quality single cells that showed exceptional performance and long-term stability. (C) 2016 Elsevier B.V. All rights reserved.