High-performance supercapacitors fabricated with activated carbon derived from lotus calyx biowaste

The clean, green, and renewable energy source and its storage have attracted considerable interest from both industry and academia to address the ongoing global climate change. The waste biomass-derived porous carbon is an important research topic complementing its low-cost, eco-friendly, and renewable nature. Three-dimensional (3D) porous carbon was prepared by the one-step simultaneous carbonization and activation of natural lotus (Nelumbo nucifera) calyx. The as-prepared 3D-lotus calyx-derived activated carbon (3D-LCAC) electrode, with a specific surface area of 798 m2 g−1 delivered a remarkable specific capacitance of 223 F g−1 at 1 A g−1, with exceptionally high cycling stability, showing 97% retention of its initial capacitance, even after 50,000 charge-discharge cycles in a KOH electrolyte. It also demonstrated superior rate capability approximately 3-times higher than the commercial AC. The electrochemical performance of the 3D-LCAC electrode in a symmetric supercapacitor device was measured in aqueous (6 M KOH, 1 M Na2SO4) and ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) electrolytes. The ionic liquid electrolyte facilitated 3D-LCAC symmetric supercapacitor device delivered approximately ten-times higher energy density than that of aqueous electrolytes under similar electrochemical conditions. Overall, the 3D-LCAC from renewable and sustainable biowaste is a good candidate for high-performance electrode materials in practical supercapacitor applications.