Bi-functional oxygen electrocatalysts for reversible solid oxide cells: the influence of A-site non-stoichiometry on the system (La or Ba)_0.6–xSr_0.4Co_0.8Fe_0.2O_3−δ

Selective tailoring of stoichiometry in perovskite oxide generates excellent electrocatalytic activity toward redox reaction of oxygen [oxygen evolution reaction (OER) and oxygen reduction reaction (ORR)]. The redox reaction of oxygen is kinetically sluggish (spin relaxed reaction) and is the rate-limiting step for solid oxide cells (SOCs). We have reported that introducing non-stoichiometry at the A-site in A_0.6–xSr_0.4Co_0.8Fe_0.2O_3−δ imparts a bifunctional electrocatalyst for OER and ORR with excellent performance in fuel cell (FC) and electrolyzer cell (EC) mode. Two compositions, Ba_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (BSCF-6482) and La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF-6482), are designed by varying the non-stoichiometry from 0.6 to 0.52 for Ba and La, respectively. X-ray photoelectron spectroscopy (XP spectroscopy), iodometric estimation, and O₂-temperature-programmed desorption (O₂-TPD) reveal that Ba_0.54Sr_0.4Co_0.8Fe_0.2O_3−δ (BS-54) promotes oxygen surface exchange (higher β-oxygen population) and La_0.54Sr_0.4Co_0.8Fe_0.2O_3−δ (LS-54) accelerates α-oxygen desorption (charge-transfer reaction). A study of the distribution of relaxation time (DRT) using EIS (electrochemical impedance spectroscopy) on cell configuration LS-54 (or BS-54)/GDC-LS-54 (or BS-54)//GDC//Pt@800 °C reveals that LS-54 has minimum electrode polarization (comprising dual processes of charge transfer and oxygen surface exchange) at both +0.8 V (OER) and −0.8 V (ORR), which resonates at 103–104 Hz. It is corroborated that α-oxygen is associated with the charge-transfer process and controls the ORR, whereas the OER is assisted by oxygen surface exchange, primarily linked with β-oxygen. Inclusion of non-stoichiometry at the A-site promotes oxygen-vacancy formation and stabilizes a lower valence state for the B-site cations. The rate-controlling steps for the OER and ORR thereby alter in LS-54 and BS-54 compared to LSCF/BSCF-6482. Electrochemical measurements show superior reversible solid oxide cell (SOC) performance of LS-54 having a current density (CD) of 1.27 A cm^–2 @1.5 V and 0.66 A cm^–2 @0.5 V for a cell with dimensions as large as 5 cm × 5 cm. A similar cell operated with a CD of 1.0 A cm² under standalone mode of operation in FC. This work proposes a novel strategy to demonstrate A-site non-stoichiometric La_0.54Sr_0.4Co_0.8Fe_0.2O_3−δ to be a superior bifunctional electrocatalyst for both OER and ORR for SOCs.