Characterization and evaluation of BaCo0.7Fe0.2Nb0.1O3-δ as a cathode for proton-conducting solid oxide fuel cells

This study characterizes BaCo0.7Fe0.2Nb0.1O3-delta (BCFN) perovskite oxide and evaluates it as a potential cathode material for proton-conducting SOFCs with a BaZr0.2Ce0.7Y0.2O3-delta (BZCY) electrolyte. A four-probe DC conductivity measurement demonstrated that BCFN has a modest electrical conductivity of 2-15 5 cm(-1) in air with p-type semiconducting behavior. An electrical conductivity relaxation test showed that BCFN has higher D-chem and K-chem than the well-known Ba0.5Sr0.5CO0.8Fe0.2O3-delta oxide. In addition, it has relatively low thermal expansion coefficients (TECs) with values of 18.2 x 10(-6) K-1 and 14.4 x 10(-6) K-1 at temperature ranges of 30-900 degrees C and 30-500 degrees C, respectively. The phase reaction between BCFN and BZCY was investigated using powder and pellet reactions. EDX and XRD characterizations demonstrated that BCFN had lower reactivity with the BZCY electrolyte than strontium-containing perovskite oxides such as SrCo0.9Nb0.1O3-delta and Ba0.6Sr0.4Co0.9Nb0.2O3-delta. The impedance of BCFN was oxygen partial pressure dependent. Introducing water into the cathode atmosphere reduced the size of both the high-frequency and low-frequency arcs of the impedance spectra due to facilitated proton hopping. The cathode polarization resistance and overpotential at a current density of 100 mA cm(-2) were 0.850 Omega cm(-2) and 110 mV in dry air, which decreased to 0.43 Omega cm(-2) and 52 mV, respectively, in wet air (similar to 3% H2O) at 650 degrees C. A decrease in impedance was also observed with polarization time; this was possibly caused by polarization-induced microstructure optimization. A promising peak power density of similar to 585 mW cm(-2) was demonstrated by an anode-supported cell with a BCFN cathode at 700 degrees C. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.