Characterization and evaluation of BaCo _0.7Fe _0.2Nb _0.1O _3-δ as a cathode for proton-conducting solid oxide fuel cells

This study characterizes BaCo _0.7Fe _0.2Nb _0.1O _3-δ (BCFN) perovskite oxide and evaluates it as a potential cathode material for proton-conducting SOFCs with a BaZr _0.1Ce _0.7Y _0.2O _3-δ (BZCY) electrolyte. A four-probe DC conductivity measurement demonstrated that BCFN has a modest electrical conductivity of 2-15 S 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 Ba _0.5Sr _0.5Co _0.8Fe _0.2O _3-δ oxide. In addition, it has relatively low thermal expansion coefficients (TECs) with values of 18.2 × 10 ^-6 K ^-1 and 14.4 × 10 ^-6 K ^-1 at temperature ranges of 30-900 °C and 30-500 °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 SrCo _0.9Nb _0.1O _3-δ and Ba _0.6Sr _0.4Co _0.9Nb _0.1O _3-δ. 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 0.85 Ω cm ^-2 and 110 mV in dry air, which decreased to 0.43 Ω cm ^-2 and 52 mV, respectively, in wet air (∼3% H ₂O) at 650 °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 ∼585 mW cm ^-2 was demonstrated by an anode-supported cell with a BCFN cathode at 700 °C.