Carbon materials from high ash biochar for supercapacitor and improvement of capacitance with HNO3 surface oxidation

The hierarchical carbon, with high specific surface area (2959 m2 g−1) and high pore volume (1.65 cm3 g−1) was prepared from sustainable feedstock – biochar, which was a waste from a thermochemical process optimized for bio-oil production, using KOH catalytic activation. The hierarchical carbon after HNO3 treatment, as electrode materials, showed improved specific capacitance 260 F g−1 in 6 mol L−1 KOH at a current density of 0.6 A g−1, while the specific capacitance of carbon without oxidation exhibits relative high (200 F g−1) at a higher current density (0.5 A g−1) after 2000 cycles. More importantly, the capacitive performances of the hierarchical carbons are much better than general bio-inspired activated carbons, ordered mesoporous carbons and commercial graphene, thus highlighting the success of preparing high performance supercapacitor electrode material from biochar and potential for improving economic viability of thermochemical biofuel processes by converting biochar to a high value added carbon materials.

► Nanostructured carbon with high capacitance >200 F g−1 from biochar. ► Nanostructured carbon containing graphene nanosheets. ► HNO3 hydrothermal oxidation greatly increase surface oxygen groups. ► HNO3 hydrothermal oxidation greatly improved internal resistance. ► HNO3 hydrothermal oxidation greatly improved the capacitance at higher current density.