Effects of B-Site Nb Substitution on ORR Activity in Perovskite Cathode Materials for Solid Oxide Fuel Cells

Abstract

Solid oxide fuel cells (SOFCs) represent an advanced technology for achieving effective energy conversion, offering high efficiency and fuel flexibility. Perovskite-type oxide cathode materials are critical to their operation, due to their excellent electrochemical performance. Doping strategies are commonly employed to improve their physicochemical and electrochemical characteristics. However, the precise role of high-valence dopants in modulating oxygen reduction reaction (ORR) activity and oxygen ion transport remains inadequately understood. This study investigates the B-site engineering in the perovskite material Pr_0.4Sr_0.6Co_0.2Fe_0.8O_3-δ (PSCF) through niobium (Nb) doping, in which iron (Fe) is partially substituted to elucidate the influence of Nb on cathode performance. Density functional theory (DFT) calculations reveal that doping significantly reduces the oxygen vacancy formation energy (E_vac) at Co/Fe-related sites, thus promoting oxygen vacancy generation and enhancing oxygen mobility in the lattice. In contrast, the E_vac at Nb-related sites increases, indicating a site-dependent redistribution of oxygen defects and local charge compensation. This redistribution facilitates the ORR pathway associated with high valence Co⁴⁺/Fe⁴⁺ species at intermediate temperatures, even though the high temperature ORR involving Co³⁺/Fe³⁺ may be partially suppressed. As the Nb content increases, a decrease in polarization resistance is observed, with the optimal electrochemical performance achieved in PSCFN_0.05 and PSCFN_0.1, showing polarization resistances of 0.052 and 0.050 Ω cm², respectively. Notably, PSCFN_0.1 achieves more than 2.6 times the power density of the undoped PSCF at 500°C (77 vs. 29 mW·cm⁻²). These findings provide fundamental insights into rational B-site design, offering a clear strategy for enhancing the catalytic activity and ion transport properties of perovskite cathodes in SOFCs.

Keywords

cathode / oxygen reduction reaction / perovskite oxide / SOFC

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