2D Layered Manganese Oxide Nanosheets as a Bifunctional Electrocatalyst for Zn-Air Batteries

Abstract

The sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is a crucial issue that limits a practical utilization of metal-air batteries. Manganese oxides with various structures, δ-, β-, α-, or γ-MnO2, have been studied and reported as electrocatalysts for the ORR and/or OER. Here, we have suggested birnessite-typed layered manganese oxides (δ-MnO2) with their sheet-like structure and interlayer spacing of ca. 7 Å containing different alkaline intercalated cations within the layers as bifunctional electrocatalysts. Surprisingly, the experimental results together with a fundamental explanation by density functional theory (DFT) calculations show that the intercalated cations between layers of the birnessite (Bir) play an important role on the ORR and OER catalytic activities which have the activity trend as follow: Li-Bir > Na-Bir > K-Bir > Rb-Bir > Cs-Bir.[1] Apart from the fundamental study, primary Zn-air batteries with these materials are also demonstrated. Zn-air battery with the Li-Bir catalyst shows the highest performance among these catalysts. Moreover, we also integrated the Li-Bir with reduced graphene oxide (rGO) as a composite to improve the electronic conductivity of the MnO2. The mechanical rechargeable Zn-air battery with Li-Bir/rGO composite as an air electrode demonstrates better performances than the Zn-air battery with a pristine Li-Bir catalyst. Interestingly, even the Zn-air battery with the Li-Bir/rGO composite (0.687 V) shows higher potential gap between discharge and charge potentials than state-of-art Pt/C+RuO2 (0.667 V) in the first cycle (2 hours per cycle), however, the Zn-air with our composite catalyst exhibits smaller potential gaps of 0.697 V and 0.719 V (0.720 and 0.761 V for Pt/C+RuO2) at the second and third cycles, respectively. In addition, our mechanical rechargeable Zn-air battery displays an impressive cyclability of discharge-charge processes (10 minutes per cycle) of 500 cycles or over 83 hours at a current density of 2 mA cm-2. Therefore, the Li-bir/rGO composite could be used as a bifunctional electrocatalyst in a practical application for Zn-air batteries. Reference [1] S. Kosasang, N. Ma, P. Wuamprakhon, N. Phattharasupakun, T. Maihom, J. Limtrakul, M. Sawangphruk, Chem. Commun. 2018, 54, 8575.