Oxygen Electrocatalysis on Mixed-Anion Perovskite Compounds in Alkaline Media

Abstract

Enhancement of oxygen reduction and evolution reaction (ORR and OER) performance has been considered to play a key role for the development of next-generation energy storage devices such as metal-air batteries and water electrolyzers. Recently, mixed-anion compounds have been found to be highly efficient bifunctional electrocatalysts for ORR and OER[1,2]. Our group has been also clarified that layered perovskite oxychloride Sr2Co1 − x Fe x O3Cl serves as a good bifunctional catalyst, whose OER activity outperformed that of Ba0.5Sr0.5Co0.8Fe0.2O3 −δ [3]. While detailed mechanism of its enhanced OER activity has been discussed previously[4], that of ORR has not been evaluated in detail, even though ORR pathway is generally considered to be much more complicated. Moreover, further exploration of highly active bifunctional mixed-anion perovskites has to be continued for better understanding of oxygen electrocatalysis on mixed-anion perovskite compounds. In the present research, therefore, we focused on the evaluation of ORR pathway for Sr2Co1 − x Fe x O3Cl, and oxygen electrocatalysis on other mixed-anion perovskite compounds in alkaline media.Solid-state method was applied to the synthesis of Sr2Co1 − x Fe x O3Cl and other mixed-anion perovskites. Electrochemical measurements were conducted by a three-electrode cell with a rotating disk electrode (RDE). Glassy carbon RDEs coated by a catalyst layer consisting of catalysts, Vulcan XC-72 (Cabot), and AS-4 (Tokuyama) were used for the working electrode. Pt wire and reversible hydrogen electrode were used for the counter and reference electrodes, respectively. Oxygen-saturated 1 mol dm− 3 KOH solution was used for ORR activity tests, and Argon-saturated 1 mol dm− 3 KOH solution containing 5 mmol dm− 3 H2O2 was used for the activity tests of peroxide reduction reaction (PRR). Peroxide decomposition rates of Sr2Co1 − x Fe x O3Cl were performed by measuring evolved oxygen gas in the same manner as previously described[5].While the highest OER activity was observed at x = 0.2, ORR activity of Sr2Co1 − x Fe x O3Cl was monotonously decreased with increase of Fe doping. Figure 1 shows decomposition rates of Sr2Co1 − x Fe x O3Cl in 1 mol dm− 3 KOH. Surprisingly, their HO2 − decomposition rate was higher than those of cobalt-based perovskite oxides such as LaCoO3, whose ORR activity was comparable to those of oxychlorides. Further discussion about ORR pathway of Sr2Co1 − x Fe x O3Cl, and oxygen electrocatalysis of other mixed-anion perovskites will be provided at the conference.Figure 1. H2O2 decomposition rates of Sr2Co1 − x Fe x O3Cl powders.