Synthesis of Multiporous Carbons from the Water Caltrop Shell for High-Performance Supercapacitors

In this study, an economic, sustainable, and green synthesis method of multiporous carbons from agricultural waste, water caltrop shell (denoted as WCS), was presented. To prepare the WCS biochar, the dried WCS was first carbonized to a microporous carbon with a surface area of around 230 m² g^-1 by using a top-lit-updraft method. Then, the microporous WCS biochar was directly mixed with an appropriate amount of ZnO nanoparticles and KOH as activating agents via a solvent-free physical blending route. After further activation at 900 °C, the resulted carbons possess both micropores and mesopores that were named as WCS multiporous carbons. The carbon yield of the prepared WCS multiporous carbons with high surface area in the range of 1175-1537 m² g^-1 is up to 50%. Furthermore, the micropore/mesopore surface area ratio can be simply tuned by controlling the ZnO content. For supercapacitor applications, the as-prepared WCS multiporous carbon electrodes showed high specific capacitance (128 F g^-1 at 5 mV s^-1) with a good retention rate at 500 mV s^-1 scan rate (>60% compared to the capacitance at 5 mV s^-1) and low Ohmic resistance in a 1.0 M LiClO₄/PC electrolyte. In addition to the ZnO nanoparticles, CaCO₃ nanoparticles with low environmental impact were also used to prepare the WCS multiporous carbons. The assembled supercapacitors also demonstrate high specific capacitance (102 F g^-1 at 5 mV s^-1) and good retention rate (∼70%).