Heteroatoms co-doped carbon from biowaste for capacitive energy storage: Dependence of physicochemical properties and electrochemical performances on precursor grain sizes

Biomass wastes are widely used as precursors for synthesizing heteroatoms doped carbons for capacitive energy storage. Based on one step of carbonization and chemical etching strategy, the effect of precursor grain sizes on physicochemical properties and electrochemical performances of resulting carbons is not paid enough attention. A biowaste of macadamia nutshell with different grain sizes is employed for the synthesis of heteroatom-doped porous carbons by chemical etching in the present study. Benefiting from more pore-forming agents loaded on the precursor with smaller grain sizes, the specific surface area and specific capacitance of carbons are significantly controlled by the grain size. As the nutshell decreased from 200 to 75 μm and was treated at 700 °C, the specific surface area enlarged from 717 to 1989 m2 g−1, while the total content of heteroatoms including oxygen, nitrogen, sulfur, and phosphorus decreased from 12.4 to 9.8 at.%. In a three-electrode system, the specific capacitance of the corresponding carbon increased from 231 to 291 F g−1 at 1 A g−1. The assembled analog supercapacitor achieves a maximum specific energy of 7.8 Wh kg−1 at the specific power of 239 W kg−1 in an aqueous electrolyte. Results demonstrate smaller precursor grain sizes lead to a higher specific surface area of carbons via one step of carbonization and chemical etching, thus achieving a stronger capacitance storage capacity of supercapacitors.