Ultrahigh-surface-area activated biocarbon based on biomass residue as a supercapacitor electrode material: Tuning pore structure using alkalis with different atom sizes

This work aims to develop an electrode material based on activated biocarbon made from rubber wood sawdust waste using alkali activating agents – KOH, NaOH, and LiOH – via a solvent-free mechanical mixing. The role of different alkali metal atom sizes plays in resulting pore structure has been studied for the first time. The influence of standard reduction potentials on the pore structure and electrochemical performance of the activated biocarbon are also investigated. The biocarbons activated with KOH or NaOH exhibit a microporous characteristic and a wide range of specific surface area of 1323–3040 m2/g, while the one activated with LiOH exhibits a mesoporous nature. The comparatively small atom size of Li and its low standard reduction potential may have caused the slow reactivity of LiOH with carbon matrix. This leads to the low specific surface area of 1134 m2/g and the formation of tiny lithium particles that could intercalate into biocarbon structure more readily than potassium or sodium particles. Mesoporosity tends to be generated by using alkali activating agent with small metal atom size and low standard reduction potential, whereas microporosity has an opposite trend. Cyclic voltammetry curves and charge-discharge profiles confirm the dominant EDLC behavior along with partial pseudocapacitance for activated biocarbons. The maximum specific capacitance is 195.3 F/g at 0.25 A/g for biocarbon activated with KOH at 4:1. This value is reduced to 69.4% of its original values at 4 A/g. The biocarbons activated with LiOH or NaOH retain the rate capability by 82.9% and 80.4%, respectively.