Intrinsic\nCapacitance of Molybdenum Disulfide

Abstract

The\nmetallic, 1T polymorph of molybdenum disulfide (MoS₂) is\npromising for next-generation supercapacitors due to its high\ntheoretical surface area and density which lead to high volumetric\ncapacitance. Despite this, there are few fundamental works examining\nthe double-layer charging mechanisms at the MoS₂/electrolyte\ninterface. This study examines the potential-dependent and frequency-dependent\narea-specific double-layer capacitance (<i>C</i>_a) of the 1T and 2H polymorphs of MoS₂ in aqueous and organic\nelectrolytes. Furthermore, we investigate restacking effects and possible\nintercalation-like mechanisms in multilayer films. To minimize the\nuncertainties associated with porous electrodes, we carry out measurements\nusing effectively nonporous monolayers of MoS₂ and contrast\ntheir behavior with reduced graphene oxide deposited layer-by-layer\non atomically flat graphite single crystals using a modified, barrier-free\nLangmuir–Blodgett method. The metallic 1T polymorph of MoS₂ (<i>C</i>_a,1T = 14.9 μF/cm²) is shown to have over 10-fold the capacitance of the semiconducting\n2H polymorph (<i>C</i>_a,2H = 1.35 μF/cm²) near the open circuit potential and under negative polarization\nin aqueous electrolyte. However, under positive polarization the capacitance\nis significantly reduced and behaves similarly to the 2H polymorph.\nThe capacitance of 1T MoS₂ scales with layer number, even\nat high frequency, suggesting easy and rapid ion penetration between\nthe restacked sheets. This model system allows us to determine capacitance\nlimits for MoS₂ and suggest strategies to increase the\nenergy density of devices made from this promising material.