High capacitive performance of exfoliated biochar nanosheets from biomass waste corn cob

A high performance exfoliated biochar carbon with a layered nanosheet structure was prepared from a low cost agricultural residue (corn cob) via a novel synthesis strategy involving biomass pre-treatment, nitrogen pyrolysis, and a high temperature thermal-chemical flash exfoliation. The exfoliation strategy resulted in porous carbon nanosheets with BET specific surface area of 543.7 m² g^-1, far higher than the 7.9 m² g^-1 of the natural biochar produced without any pre- or post-treatment modifications. The exfoliated material also showed increased oxygen functionality in the form of electrochemically active quinone and pyrone surface groups. This combination of high specific surface area and highly active surface functional groups resulted in very promising capacitive performance, demonstrating a high capacitance of 221 F g^-1, over 100 times greater than the natural biochar. The exfoliated biochar electrodes fabricated without any conductive or organic additives showed outstanding high rate capability retaining 78% of their low rate capacitance at a fast 40 A g^-1 discharge. This combination of high capacitance and fast charge-discharge capability distinguishes this material from most other high surface area activated carbons; in fact, the electrochemical behaviour more closely resembles that of designer nanomaterials such as graphene and carbon nanotubes. The biochar electrodes were also extremely durable showing only a 3% reduction in capacitance after 5000 successive potential cycles. The exfoliation strategy developed here could provide a novel route for the low cost production of high performance energy storage materials from a variety of waste biomass feedstocks.