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

Abstract Oil cake, as the main by-product in the he production process of idesia polycarpa oil, is usually discarded, bringing seriously environmental pollution. It is necessary to improve the utilization of oil cake biowaste and even to convert oil cake into high-value-added materials. In this work, we proposed a simple and less time-demanding approach to convert oil cake into graphitized hierarchically porous biochar (GHPB) by a one-pot heat-treatment. Potassium ferrate works as both as pore-forming agent and graphitization catalyst to fulfil the synchronous hierarchical pore generation and graphitization of oil cake. The resultant GHPB materials exhibited a superior hierarchical porosity with a high proportion of microporosity and a suitable mesoporosity, a highly graphitized carbon skeleton and a moderate nitrogen doping. As the electrodes for supercapacitor, GHPB samples presented a satisfactory electrochemical capacitive performance in various aqueous electrolytes (KOH, H2SO4 and Na2SO4). Meanwhile, the assembled GHPB-based symmetric capacitors also achieved the superb capacitive property in KOH and Na2SO4 electrolytes, including satisfactory capacitance, superior rate capability, high energy density and prominent cycling stability. Our proposed strategy not only relieved the environment pollution caused by oil cake biowaste, but also obtained the high-added-value biowaste-derived carbon-based electrodes for supercapacitors in energy storage. Therefore, it demonstrated an example of trash-to-treasure transformation, boosting the utilization of abundant biowaste resource.