Trade-off between oxygen reduction reaction activity and CO2 stability in a cation doped Ba0.9Co0.7Fe0.3O3−δ perovskite cathode for solid oxide fuel cells

Abstract

The excellent oxygen permeability of the perovskite BaCoO₃ and its applicability as a cathode in solid oxide fuel cells are hindered by stability issues of carbonate formation in the presence of CO₂, which degrades its excellent oxygen surface-exchange kinetics during the oxygen reduction reaction (ORR) process. In this work, we present a new type of Ba_0.9Co_0.7Fe_0.2M_0.1O_3−δ (M = Zr, Nb, Y) perovskite oxide with high CO₂ tolerance with respect to their original compound Ba_0.9Co_0.7Fe_0.3O_3−δ (B90CF) by doping cations at the B-site. These cathodes show low polarization resistances (R_p) of 0.0687, 0.0719, and 0.0853 Ω cm² for Ba_0.9Co_0.7Fe_0.2Y_0.1O_3−δ (B90CFY), Ba_0.9Co_0.7Fe_0.2Zr_0.1O_3−δ (B90CFZr), and Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3−δ (B90CFNb) at 700 °C, respectively, which are comparable to B90CF (0.0655 Ω cm²). Moreover, after exposure to air containing 1% CO₂ for 1500 min, the electrochemical impedance spectroscopy (EIS) test demonstrated the improved CO₂ tolerance for the B90CFM cathode with Zr (Nb, Y) doping, where the R_p of B90CFZr only increased by 31%, ∼1/10 of that for B90CF. Barium carbonate on the B90CFZr cathode was obviously suppressed compared to B90CF according to SEM combined with EDS, XRD and FT-TR analysis. Doping high-valence metals at the B site of the B90CFM perovskite may increase the acidity on the surface and metal-oxygen bond free energy (〈ABE〉), leading to enhanced resistance to CO₂. The trade-off between electrocatalytic activity and stability enable B90CFZr to be a promising cathode material for low and medium temperature SOFCs.