Bifunctional Electrocatalysts for Oxygen Reduction and Evolution Reactions (ORER) in Alkaline Electrolyte

Abstract

Oxygen reduction and evolution reactions (ORER) are vital for electrochemical conversion and storage in many electrochemical systems such as metal-air hybrid systems, conventional and regenerative fuel cells, photolysis and electrolysis (water and chlor-alkali). The breakage of oxygen bonds during reduction as well as formation of oxygen via oxygen evolution coupled in one and the same electrode, however shows sluggish kinetics for discharge and charge reactions with high overpotentials. The platinum group metals (PMG) such as Pt, RuO2 and IrO2 are considered to be the state-of-the-art electrocatalysts for both the ORR and OER (1-3). Although very low PMG loadings are currently used in the electrochemical reactions, the poor stability, high cost and rarity of these metals are of considerable challenge in the long run for high scale application and commercialization. Interest in rechargeable metal-air batteries in alkaline electrolyte is on the upsurge due to their high theoretical specific energy density and increased cycle life as well as abundance and low cost materials for both cathode and anode (4, 5). Among the class of promising electrocatalysts for the cathode, perovskites with ABO3 structure have shown bifunctionality, availability and stable performances in aqueous alkaline electrolytes. By fine-tuning the cations either by total or partial substitution of the site A having rare earth metals with alkali earth metal and B of the transition metals (Mn, Fe, Co, Ni, Al), the oxide ion mobility, electronic conductivity and oxygen vacancies can be modified in order to accommodate surface and catalytic properties on par or even better than the PMG (6,7). In this work, we report synthesis and various physicochemical characterizations of the perovskite type La0.1Ca0.9MnO3 both with and without co-mixing of nano Co3O4. Electrochemical activities for the ORR in a conventional three-electrode set-up were carried out by linear sweep voltammetry using an RDE at a sweep rate of 2 mV s-1, rotation rates between 500 and 2500 rpm and at the potential range of +0.1 to -0.9 V by oxygen saturation in 1M KOH. Furthermore, cyclic voltammetry measurements at a scan rate of 0.5 mV s-1 for composite electrodes in the potential region of +0.55 to -0.3 V and were obtained at 50 cycles in 6M KOH. The electrode kinetics, intrinsic activity and stability for the ORER were assessed for respective electrocatalysts and their mixtures.