All-rice straw-derived self-supporting biochar to construct an ecological supercapacitor

The development of self-supporting electrodes is crucial for improving the energy density of energy storage devices. This study recovers black liquor during the alkaline pretreatment of straw raw materials and employs it in the preparation of self-supporting biochar, functioning as a bifunctional platform. During the hot pressing stage, a three-dimensional cross-linked structure is formed between the black liquor and fibers. In the subsequent carbonization process, in situ activation controls the pore structure and surface chemistry of the self-supporting biochar electrode. The resulting self-supporting electrodes with a mass loading of up to 69.0 mg cm −2 achieve an outstanding volumetric (areal) capacitance of 122.7 F cm −3 (28.2 F cm −2 ) at 10 mA cm −3 . Compared with the electrode prepared by replacing black liquor with lignin, the capacitance performance of this electrode was significantly improved, showing an increase of 99.97 %. The assembled symmetric supercapacitor (SSC) device demonstrated a high areal energy density of 0.82 mWh cm −2 at a power density of 0.60 mW cm −2 , outperforming most reported carbon-based energy storage devices. This strategy employs a "waste-to-value" approach to achieve the integrated utilization of rice straw. • The integrated rice straw is applied in the strategy for preparing green and low-cost electrodes. • Alkali pretreatment-hot pressing synergistic process to fabricate self-supporting electrode precursors. • The electrode achieved a volumetric capacitance of 122.7 F cm −3 . • The assembled symmetric supercapacitor (SSC) demonstrated an areal energy density of 0.82 mWh cm −2 . • The universality of the process was verified for sugarcane bagasse and wheat straw.