Simple one-pot strategy for converting biowaste into valuable graphitized hierarchically porous biochar for high-efficiency capacitive storage

•A one-pot strategy to convert Idesia polycarpa fruit oil residue into porous biochar.•K2FeO4 simultaneously works as porogen and graphitization catalyst.•K2FeO4 fulfils the synchronous pore-forming and graphitization of carbon skeleton.•The resultant biochar owns good hierarchical porosity and high graphitization degree.•The porous biochar exhibits a good capacitive performance in various electrolytes.