Abstract
Alkaline-based metal-air batteries require a low-resistance, bifunctional oxygen electrode to perform fast oxygen electrocatalysis during charging and discharging cycles. However, achieving good oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) bifunctionality has proven challenging. In this work, a perovskite-structured and oxygen-deficient oxide, La0.6Sr0.4CoO3- δ (LSCO), has been investigated as a model bifunctional thin-film oxygen electrode for alkaline metal-air cells. The rotating disk electrode (RDE) configuration in combination with common electrochemical techniques such as linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were applied to characterize the behavior of the ORR and OER taking place on LSCO in 0.1 M KOH solution. The results show that the oxygen electrocatalysis process on LSCO in the alkaline media follows a multistep charge-transfer pathway. A physics-based, generalized electrochemical model, encompassing two sequential 2e- steps with HO2 - as an intermediate species and one parallel 4e- step, along with peroxide dissolution, has been established to account for the multi-step charge transfer behavior with very satisfactory results, yielding a series of important electrode kinetic parameters for the elementary reactions considered. After forming the base concept of oxygen electrocatalysis on the LSCO, the effect of the Vulcan carbon (XC-72R) mass ratios on the ORR/OER were also investigated and a synergistic effect between the perovskite-oxide and the XC-72 was observed to affect the reaction mechanism. Finally, the bifunctionality of the LSCO towards ORR was clearly elevated through the addition of a commercial, noble-metal 20% wt Pt/C, where even the slightest addition of the Pt/C dominated the ORR trending and the smallest amounts of LSCO trumped the OER characteristic profiles. Keywords: oxygen electrocatalysis; perovskite-oxide; rotating disk electrode, linear sweep voltammetry.
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