Synthesis of functionalized nanoporous biocarbons with high surface area for CO<inf>2</inf>capture and supercapacitor applications

Nanoporous biocarbons derived from biomass have attracted significant attention owing to their great potential for energy storage and conversion and water purification. However, the fabrication technology for these materials requires high cost and corrosive activating agents such as KOH or ZnCl2 which is a huge hurdle for their potential commercialization. In this study, a low-cost synthesis strategy is introduced for the preparation of O and N functionalized nanoporous biocarbons using non-corrosive and low-cost potassium citrate as an activating agent and casein as a biomass source via a single-step solid-state procedure. The presence of O functional groups in the activating agent and a large amount of N in the biomass precursor facilitated the successful incorporation of both O and N atoms into the final nanoporous biocarbons. A series of O and N functionalized nanoporous biocarbons with tunable porosity, specific surface area, and pore volume was prepared by varying the amount of the activating agent and casein. The optimized material CPC-3, which was synthesized using a 1 : 3 ratio of casein to potassium citrate, showed a high surface area (2212 m2 g-1), a large pore volume (1.11 cm3 g-1), and an appreciable amount of surface oxygen-containing functional groups. With the advantages of excellent surface parameters and the O and N functionalities on the porous surface, the functionalized nanoporous biocarbons achieved a high CO2 adsorption capacity of 25.4 mmol g-1 at 0 °C and 30 bar and showed an impressive specific capacitance of 177 F g-1 in a three-electrode and 95 A g-1 two-electrode system at 0.5 A g-1 using 3 M KOH as an electrolyte. Interestingly, the functionalized nanoporous biocarbon with a high amount of micropores displays a CO2 adsorption capacity of 5.3 mmol g-1 at 1 bar/0 °C, which is much higher than that of the reported activated porous carbon materials and ordered mesoporous carbons. It is surmised that the reported unique fabrication approach and the multifunctionality nature of these fascinating materials offer exciting opportunities for cost-effective CO2 adsorption systems and highly efficient energy storage devices