The more we use technology, the more we need energy storage devices. Supercapacitors are becoming a field of interest due to their high-power density and high cycling stability. Despite the advancement in the supercapacitor field, mysteries about the mechanism of charge storage are still hidden. Supercapacitors can store energy in the form of an electrical double layer (EDL) and/or using faradic process either redox reactions or diffusion process. Carbon materials usually prefer the EDL mechanism in which the charge accumulates on the surface of the carbon material, but it is limited by the surface area of the material. On the other hand, layered materials such as MoS2 has the ability to intercalate with the ions in the electrolyte forming a faradic process (diffusion) and acts as a pseudocapacitor. Unfortunately, pseudocapacitors have a limitation of losing stability over time and consequently losing one of the benefits of the supercapacitors. To overcome the drawbacks of the two types of materials, researchers are using a hybrid electrode of both carbon and MoS2 together. However, the contribution of the carbon material to the overall charge storage mechanism and the effect of the shape of the carbon material is not well understood. Herein, we have mixed MoS2 with carbon materials of different shapes (carbon nanotubes CNTs and graphene nanoflakes GNFs) in different ratios and we studied their effect on the charge storage mechanism in MoS2. Our results showed that mixing MoS2 with the small-diameter (8 nm) CNTs resulted in blocking the diffusion sites in MoS2 and increased the EDL capacitance percentage. While mixing the MoS2 with the large GNFs (1-2 µm) resulted in sandwiching the MoS2 between layers of GNFs and did not affect greatly the diffusion capacitance percentage. Our results showed that increasing the EDL capacitance contribution led to an enhancement in the total capacitance, where the higher the scan rate, the lower the diffusion of ions to the MoS2. A mixture of MoS2, CNT and GNFs with equal weight ratios showed the best capacitance of 123 F/g at a scan rate of 10 mV/s due to the enhancement of both EDL and diffusion capacitance. The composite showed a predominant pseudocapacitive behavior with the diffusion current representing 89% of the total current and the faradic capacitance representing 88.31% of the total capacitance. The retention after 1000 cycles of the mixture increased by 20.5% than that of the MoS2 alone.

Nature of carbon material determines the charge storage mechanism in 2H MoS2

2019 Spring National Meeting

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