Highly scalable and environment-friendly conversion of low-grade coal to activated carbon for use as electrode material in symmetric supercapacitor

The subbituminous type of coal is considered low-grade and unsuitable for power generation due to its low calorific value and high impurity content. As a result, this type of coal does not have any economic value and so is mostly lying unutilized. In this work, we have explored a simple and easily scalable approach to converting this low-grade subbituminous coal, abundantly found in the North-Eastern region (NER) of India, to valuable porous activated carbon material with the target of its potential use as electrode material in electrochemical energy storage. The synthesis process follows an environmentally friendly method, which includes ultrasonication-aided H2O2-oxidation of the raw coal and subsequent thermochemical activation with KOH. Through variation of KOH ratio and pyrolysis temperature, the optimized coal-derived activated carbon was found to exhibit the maximum specific surface area and pore volume of 1021 m(2)/g and 0.52 cm(3)/g respectively. When tested as electrode material for symmetric supercapacitor cells, this activated carbon delivers a high specific capacitance of 227 and 99 F/g at 0.5 A/g current density in aqueous and organic electrolytes respectively. Moreover, it shows excellent capacity retention of 82% at 5 A/g current density after 10,000 charge-discharge cycles.