(Invited) Controlling Defects and Oxidation at Molybdenum Disulfide Electrodes and the Impact on Capacitive Charging

Supercapacitors based on 1T-molybdenum disulfide (MoS2) have achieved record high volumetric capacitance due to a combination of high surface area, bulk density and the ability for ions to easily penetrate between restacked layers. While many promising MoS2-based devices have been demonstrated, few works have examined capacitive charging mechanisms at the MoS2/electrolyte interface and how defects and oxidation play a role. In fact, unlike similar materials like graphene and graphite, no fundamental studies on double-charging have ever been reported for this promising material. In this talk, I will first discuss recent work from my group which examines degradation mechanisms which act to rapidly and significantly oxidize MoS2 during aqueous processing. Light, oxygen and pH are found to play significant roles in the degradation mechanism and rate. In the second part of my talk, I will describe a model study of charging mechanisms at flat electrodes composed of various ratios of 2H and 1T MoS2 produced by chemical exfoliation and with various degrees of oxidation. Using this system we attempt to identify the intrinsic capacitance of the 1T and 2H phase, the mechanism of charging and the impact of defects and oxidation on double-layer charging and pseudocapacitance and relate these to the known theories of charging at the interface of semi-metals and semi-conductors. The intrinsic capacitance and theoretical surface area are used to discuss theoretical capacitance limits for MoS2 and MoS2-based heterostructures.