Leonardite humic acid activated carbon/MnO\(_{2}\) composite nanostructures for supercapacitors

Abstract This work reports the preparation and electrochemical studies of activated carbon derived from leonardite humic acid composited with MnO2 for supercapacitors. Activated carbon contains high conductivity, high specific surface area, and accommodates large volume expansion/contraction during charging/discharging process. Meanwhile, MnO2 has very high theoretical specific capacity (1370 F∙g‒1). Their composite could significantly improve both the storage performance and cycle stability of supercapacitors. Moreover, humic acid from leonardite was selected to add value to this waste and reduce environmental pollution.  By varying the carbonization temperature (500℃ to 800℃), the prepared samples carbonized at 800℃ exhibited fascinating properties. The oxidation state of Mn ions was in the mixed state of Mn+2 (41.2%) and Mn+2, +3 (52.8%). A gravimetric capacitance of 329 F∙g‒1 and 294 F∙g‒1 were observed at 2 mVs-1 and 0.5 Ag-1, respectively. The remaining gravimetric capacitance of 193 F∙g‒1 was evaluated at 1000 cycles, indicating its high cycle performance.  Moreover, the gravimetric energy of 37.51 Wh∙kg‒1 and gravimetric power of 272.96 W∙kg‒1 were observed. When combined, the interesting electrochemical properties of leonardite humic acid-activated carbon/MnO2 composite nanostructures make them important options for supercapacitor application.