Green synthesis of high-performance supercapacitor electrode materials from agricultural corncob waste by mild potassium hydroxide soaking and a one-step carbonization

•A KOH soaking activation carbonization method is employed.•The process of KOH soaking activation is studied.•High-performance porous carbon is obtained by using much less (1/2−1/10) KOH.•Capacitive carbon exhibits high charge storage capacity and good stability.