Design of graphene-chitosan-based aerogels for CO2 capture
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2023-08
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Abstract
The significant effects of global CO2 emissions on our environment and climate highlight the importance of finding feasible solutions for capturing CO2, to tackle the consequences of anthropogenic greenhouse gases. This study introduces two novel approaches for producing highly effective CO2 adsorbents using graphene and chitosan precursors. The first approach involves combining glycated chitosan (GC) with electrochemically exfoliated graphene (EEG) nanoplatelets, through a physical crosslinking mechanism. Glycation of chitosan improves its dispersion in water by introducing more hydroxyl functional groups and reduces its amorphous parts, resulting in a bimodal particle size distribution. The unique and specific interactions between amine and phosphate groups proposed here trap hydrogen ion-water crystals within its network, which on freeze drying generates mesoporous microstructures. By inducing mesoporosity through the physical crosslinks of amine groups on GC and phosphate groups on EEG, the CO2 adsorption capacity of the EEG-GC aerogels is significantly enhanced. The extent of glycation and the weight ratio of EEG to GC is demonstrated to impact the microstructure and mesoporosity of the precursor gel and derived aerogels. The optimized EEG-GC18 aerogels exhibit a high BET surface area of 136.29 m2 g-1 and exceptional CO2 adsorption capacity (2.88 mmol g-1) and selectivity (43.8) at 298 K and 1 bar. In the second approach, the hierarchically structured aerogels are synthesized by generating covalent amide crosslinks between chitosan (CS) and manganese dioxide (MnO2) functionalized electrochemically exfoliated graphene (MEEG). An innovative approach was undertaken to achieve a two-fold enhancement in surface area where MnO2 functionalization prevents graphene aggregation by acting as a steric barrier between consecutive graphene sheets, while the strong amide bond formation between MEEG and CS inhibits preferred ice-crystal growth during aerogel synthesis. The resulting MEEG-CS aerogels exhibit a high surface area of 374.2 m2 g-1 and exceptional CO2 adsorption capacity (3.94 mmol g-1) and selectivity (65.2) at 298 K and 1 bar, surpassing previously reported hierarchically structured CO2 adsorbents. Although, in order to incorporate hierarchical structuring, for both EEG-GC and MEEG-CS, primary amine adsorption sites have been sacrificed, however, the enhanced surface area improved the accessibility to the available adsorption sites, thus increasing overall CO2 uptake. The electrical conductivity of the aerogels from both routes enables direct electrical heating, leading to quicker regeneration times and lower energy costs. The utilization of EEG-GC and MEEG-CS aerogels offers new avenues for designing innovative porous materials that act as hybrid adsorbents combining both physisorption and chemisorption mechanisms to address climate change concerns.
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Pal, S. (2023). Design of graphene-chitosan-based aerogels for CO2 capture (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.