Preparation and electrochemical performance of coal-based composite carbon fibers using water-soluble fraction from lignite by thermal dissolution

• A composite carbon fibers was prepared by water-soluble fraction from lignite . • The energy density of AC-PVA-SPs-1 is 8.5 Wh/kg at 250 W/kg. • The surface area of AC-PVA-SPs-1 is 1477 m 2 /g and capacitance is 229.9 F/g. Lignite serves as prefer carbon sources for preparation of composite carbon fibers, as the good reactivity of carbon units and good solubility of reaction products. However, low yield of soluble products from lignite at mild condition and high volatility of toxic organic solvent during spinning are not beneficial for the preparation of coal-based composite carbon fiber. In this work, the sodium hydroxide was introduced to enhance the thermal dissolution of lignite with methanol at mild condition. The water-soluble fraction, separated from the thermal dissolution products, was then mixed with certain amount of polyvinyl alcohol to afford a solution. The solution was treated by electrostatic spinning, pre-oxidation, carbonization, acid washing, and activation, ultimately producing the coal-based composite carbon fibers. In these treatment process, acid washing is beneficial for the removal of surface impurities, resulting an increase in the specific surface area and the exposure of active sites on the surface of the composite carbon fibers. The specific capacitance is enhanced from 73.5 F/g to 197.1 F/g at a current density of 1 A/g. The activated coal-based composite carbon fibers were assembled into a symmetric snap capacitor with an energy density of 8.5 Wh/kg at a power density of 250 W/kg. This work explored a new approach for preparation of coal-based composite carbon fibers by electrostatic spinning in aqueous solution system. It is also a value-added and environmentally friendly pathway of carbon fraction from lignite for preparation of supercapacitor electrode materials.