LAUSR

We have synthesized a library of perovskite oxides with the composition La1- xSr xBO3-δ ( x = 0-1; B = Fe, Mn, Co) to systematically study anion-based pseudocapacitance. The electrochemical capacitance of these materials was evaluated by cyclic voltammetry and galvanostatic charging/discharging in 1 M KOH. We find that greater oxygen vacancy content (δ) upon systematic incorporation of Sr2+ linearly increases the surface-normalized capacity with a slope controlled by the B-site element. La0.2Sr0.8MnO2.7 exhibited the highest specific capacitance of 492 F g-1 at 5 mV s-1 relative to the Fe and Co oxides. In addition, the first all-perovskite asymmetric pseudocapacitor has been successfully constructed and characterized in neutral and alkaline aqueous electrolytes. We demonstrate that the asymmetric pseudocapacitor cell voltage can be increased by widening the difference between the B-site transition metal redox potentials in each electrode resulting in a maximum voltage window of 2.0 V in 1 M KOH. Among the three pairs of asymmetric pseudocapacitors constructed from SrCoO2.7, La0.2Sr0.8MnO2.7, and brownmillerite (BM)-Sr2Fe2O5, the BM-Sr2Fe2O5//SrCoO2.7 combination performed the best with a high energy density of 31 Wh kg-1 at 450 W kg-1 and power density of 10 000 W kg-1 at 28 Wh kg-1.