Highly Active Transition Metal Dichalcogenides As Bifunctional Electrocatalysts for Li-Oxygen Batteries

Abstract

Lithium-air (Li-O2) batteries -comprising of an air (O2) cathode and a lithium anode- are able to deliver significantly higher amounts of theoretical energy densities compared to conventional Li-ion batteries. Therefore, these batteries have been extensively investigated lately to become the practical substitution for Li-ion batteries especially in transportation and electric vehicles. Performance of a Li-O2 battery is fundamentally governed by two main reactions occurring at the cathode: oxygen reduction reaction (ORR) during discharge and oxygen evolution reaction (OER) during charge. In order to improve the performance of Li-O2 batteries, seeking a bifunctional catalyst which operates favorably in both ORR and OER reactions, is a challenging issue. The role of various materials has been investigated in OER-ORR reduction in both aprotic and non-aprotic media, including: noble metals, metal oxides, transition metals, transition metal carbides, and etc. Among these catalysts, transition metal dichalcogenides (TMDCs) such as MoS2 indicated a superior performance towards a bifunctional catalytic activity for ORR and OER in aprotic electrolytes. Herein we report for the first time the catalytic activity of a number of novel two dimensional TMDCs with transition metals of groups V-IX and the three heavier chalcogens (S, Se, Te) in ORR-OER in an aprotic medium with Li salt. We have synthesized these materials via chemical vapor transport (CVT) and the nanoflakes accompanied by different characterization techniques such as: Raman, XPS, UPS, DLS, AFM and etc. We performed cyclic voltammetry experiements to comparatively study the TMDCs catalytic activity in ORR-OER reactions, their turnover frequency and so on. These catalysts outperformed all of the reported catalysts in aprotic media with Li salts during ORR and OER. A number of them exhibited an excellent bifunctional behavior such as: NbS2, VS2 and VSe2. Density functional theory (DFT) calculations were also performed on a number of these catalysts in order to understand their catalytic activity during ORR and OER in ionic liquid electrolyte. We believe these materials possess great potential to enhance the catalytic activity in core electrochemical reactions especially in Li-O2 battery applications.