Nitrogen-doping activated biomass carbon from tea seed shell for CO₂ capture and supercapacitor

Activated carbon is a promising material that has a broad application prospect. In this work, biomass (tea seed shell) was used to prepare activated carbon with KOH activation (referred to as AC), and nitrogen was doped in activated carbon using melamine as the nitrogen source (referred to as NAC-x, where x is the mass ratio of melamine and activated carbon). The obtained activated biomass carbon (activated bio-carbon) samples were characterized by Brunauer–Emmett–Teller (BET)-specific surface area analysis, ultimate analysis, X-ray photoelectron spectroscopy (XPS) analysis, Raman spectrum analysis, and X-ray diffraction (XRD) patterns. The specific surface areas of activated bio-carbons were 1503.20 m²/g (AC), 1064.54 m²/g (NAC-1), 1187.93 m²/g (NAC-2), 1055.32 m²/g (NAC-3), and 706.22 m²/g (NAC-4), revealing that nitrogen-doping process leads to decrease in specific surface area. XPS analysis revealed that the main nitrogen-containing functional groups were pyrrolic-N and pyridinic-N. The capacity of CO₂ capture and electrochemical performance of activated bio-carbon samples were investigated. The CO₂ capturing capacity followed this order: AC (3.15 mmol/g) > NAC-2 (2.75 mmol/g) > NAC-1 (2.69 mmol/g) > NAC-3 (2.44 mmol/g) > NAC-4 (1.95 mmol/g) at 298 K at 1 bar, which is consistent with the order of specific surface area. The specific surface area played a dominant role in CO₂ capturing capacity. As for supercapacitor, AC-4 showed the highest specific capacitance (168 F/g) at the current density of 0.5 A/g, but NAC-2 showed the best electrochemical performance (89 F/g) at 2 A/g. Nitrogen-containing functional groups and specific surface area both had an important impact on electrochemical performance. In general, NAC-3 and NAC-2 produced excellent electrochemical performance. Compared with NAC-3, less melamine was used to prepare NAC-2; therefore, NAC-2 was considered as the best activated bio-carbon for supercapacitor for 141 F/g (at 0.5 A/g), 108 F/g (at 1 A/g), and 89 F/g (at 2 A/g) in this work.