Intrinsic Capacitance of Molybdenum Disulfide.

The metallic, 1T polymorph of molybdenum disulfide (MoS ₂ ) is promising for next-generation supercapacitors due to its high theoretical surface area and density which lead to high volumetric capacitance. Despite this, there are few fundamental works examining the double-layer charging mechanisms at the MoS ₂ /electrolyte interface. This study examines the potential-dependent and frequency-dependent area-specific double-layer capacitance ( C _a ) of the 1T and 2H polymorphs of MoS ₂ in aqueous and organic electrolytes. Furthermore, we investigate restacking effects and possible intercalation-like mechanisms in multilayer films. To minimize the uncertainties associated with porous electrodes, we carry out measurements using effectively nonporous monolayers of MoS ₂ and contrast their behavior with reduced graphene oxide deposited layer-by-layer on atomically flat graphite single crystals using a modified, barrier-free Langmuir-Blodgett method. The metallic 1T polymorph of MoS ₂ ( C _a,1T = 14.9 μF/cm ² ) is shown to have over 10-fold the capacitance of the semiconducting 2H polymorph ( C _a,2H = 1.35 μF/cm ² ) near the open circuit potential and under negative polarization in aqueous electrolyte. However, under positive polarization the capacitance is significantly reduced and behaves similarly to the 2H polymorph. The capacitance of 1T MoS ₂ scales with layer number, even at high frequency, suggesting easy and rapid ion penetration between the restacked sheets. This model system allows us to determine capacitance limits for MoS ₂ and suggest strategies to increase the energy density of devices made from this promising material.