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%).