Mns-MWCNT Composite, as a Superior Electrode Material for Asymmetric Flexible Quasi Solid-State Supercapacitors

Abstract

Nowadays, renewable and sustainable energy sources are being explored and promoted to curb global pollution and to solve the issue of depleting fossil fuel resources. However, these renewable energy sources are intermittent in supply, therefore, it becomes necessary to store them for future usage. The traditional storage devices for electrical energy storage are different kinds of batteries but the sluggish performance of batteries, use of toxic heavy metals, issues pertaining to safety and low cycling efficiency are some of its major drawbacks. Recent proliferation of portable, wearable and flexible electronics and adoption of electric vehicles have forced the scientists to look for fast charging, light weight, leak proof, safe, toxic material free, durable energy storage devices like supercapacitors. Supercapacitors possess fast charge-discharge characteristics, high cycling stability and eco-friendliness in comparison to batteries. But they suffer from extremely low energy density (<10 Wh kg-1), high cost, bulk size, high self-discharge and significant drop in power density while increasing its energy density. The rational synthesis of electrode-materials has a significant impact on the development of high-performance electrodes for energy storage devices. Traditionally semiconducting materials like transition metal oxides are used as pseudocapacitive materials but transition metal oxides have very low electronic and ionic conductivity. Recently, transition metal sulfides, another class of semiconductors, are being investigated as they exhibit better conductivity and reaction kinetics compared to oxides. In this work, α-MnS is selected as an alternative to transition metal oxides. α-MnS is the most stable polymorph among other possible crystal structures of MnS, and has a hexagonal sheet like structure along with high operational potential window than many other metal sulfides. Although it is more conducting than transition metal oxides, it still needs conductivity enhancers like activated carbon, graphene, multiwall carbon nanotubes (MWCNT) etc. External mixing of conductivity enhancers is not effective enough to enhance the conductivity. Hence, MWCNT is mixed during the synthesis step to have a better bonding with α-MnS and create high conductivity channels. α-MnS + MWCNT composite shows a specific capacitance of 115 F g-1 at 1 A g-1 in 3 M KOH electrolyte under 3 electrode configuration. α-MnS + MWCNT composite is deposited on flexible conducting carbon cloth as cathode material and Vulcan carbon is deposited on carbon cloth as anode material. A mixture of poly vinyl alcohol PVA+ 3 M KOH is used as the gel type electrolyte. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), galvanostatic charge discharge (GCD), electrochemical impedance spectroscopy (EIS), cycling stability are performed to characterize the active material and the device performance. The assembled device shows a potential window of 1.4 V, alongside a specific capacitance of 27.7 F g-1 at 1 A g-1 and with 80% of capacity retention after 10,000 cycles. These results prove that transition metal sulfides can be viable alternative materials for supercapacitor application. This quasi-solid state or gel type electrolyte significantly reduces the conductivity and specific capacitance. Enhancing the ionic conductivity of the gel and finding out the optimum thickness of gel electrolyte layer between the electrodes are expected to open new pathways in enhancing the performance of the device. Figure 1