Synergistic Effect of Nitrogen Doping and Ultra-Microporosity on the Performance of Biomass and Microalgae-Derived Activated Carbons for CO2 Capture

We report a unique naturally derived activated carbon with optimally incorporated nitrogen functional groups and ultra-microporous structure to enable high CO2 adsorption capacity. The coprocessing of biomass (Citrus aurantium waste leaves) and microalgae (Spirulina) as the N-doping agent was investigated by probing the parameter space (biomass/microalgae weight ratio, reaction temperature, and reaction time) of hydrothermal carbonization and activation process (via the ZnCl2/CO2 activation) to generate hydrochars and activated carbons, respectively, with tunable nitrogen content and pore sizes. The central composite-based design of the experiment was applied to optimize the parameters of the prehydrothermal carbonization procedure resulting in the fabrication of N-enriched carbonaceous products with the highest possible mass yield and nitrogen content. The resulting hydrochars and activated carbon samples were characterized using elemental analysis, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and Brunauer–Emmett–Teller surface area analysis. We observe that while N-doping and the activation process can individually enhance the CO2 adsorption capacity to some extent, it is the combined effect of the two processes that synergistically work to greatly increase the adsorption capacity of the N-doped activated carbon by an amount which is more than the sum of individual contributions. We analyze the origins of this synergy with both physical and chemical characterization techniques. The resulting naturally derived activated carbon demonstrates one of the highest CO2 adsorption capacities (8.43 mmol/g) with rapid adsorption kinetics and good selectivity and reusability.