Evaluation of SrCo0.8Nb0.2O3-δ, SrCo0.8Ta0.2O3-δ and SrCo0.8Nb0.1Ta0.1O3-δ as air electrode materials for solid oxide electrolysis and reversible solid oxide cells

Abstract

Strontium cobalt-based perovskites display high electrochemical activity but poor stability for solid oxide electrolysis cells (SOECs). Here, we evaluate the stability of three strontium cobalt-based materials, i.e. SrCo0.8Nb0.2O3-δ (SCN), SrCo0.8Ta0.2O3-δ (SCT) and SrCo0.8Nb0.1Ta0.1O3-δ (SCNT) as air electrodes for SOECs. The electrochemical results show that the area-specific resistance (ASR) for SCN and SCNT cells decreases and then becomes almost constant whereas the ASR of the SCT cell continuously increases, during the 240 h SOEC test. The improved performance of SCN and SCNT cells is ascribed to the lower energy barrier required for the oxide ion movement in the presence of niobium and the less concentration of undesired phases formed during SOEC operation. SCN is further tested under reversible solid oxide cell (RSOC) conditions with 16 h solid oxide fuel cell (SOFC) and 7 h SOEC cycles. The cell presents excellent stability as its impedance increases only slightly from 0.188 to 0.193 Ω cm2 after 114 h RSOC test. A single cell with NiO-SSZ/SSZ/SDC/SCN configuration shows high electrolysis current densities of 0.51, 0.42 and 0.34 A cm−2 at 800, 750 and 700 °C, respectively, in 10% H2O/90% H2 and 0.79 A cm−2 at 800 °C in 50% H2O/50% H2 at 1.3 V.