Trees to supercapacitors: green energy storage for sustainable future

Sammanfattning: Currently the world is transitioning to a fossil free and circular economy-based development to achieve the environmental and climate objectives along with the sustainable development goals. To achieve this target, efficient use of resource materials is important, which can increase the lifetime and economic value of the resources while decreasing both the extraction of new raw materials and landfill waste. Hence replacing conventional fossil based raw materials with bio-based raw materials play an important role in the circular economy. In addition to that, lack of green and environment friendly energy storage systems to meet the increasing global energy demand is another major challenge faced by countries across the world. Within this context, the overall objective of this study is to use biomass based raw materials for the development of biocarbon that can make energy storage “greener”, thereby addressing the sustainability objectives and energy storage demands. In this work, the second most abundant biopolymer on earth, lignin has been utilized as the raw material for making carbon aerogels (CAs) which are extremely light, hierarchically porous, having high specific surface area with interconnected microstructure and mechanically stable. CAs were prepared by carbonizing the aerogels prepared using lignin and cellulose nanofibers (CNFs). Lignin and CNFs were mixed to make aqueous lignin/CNF suspensions which were subjected to unidirectional ice-templating process to obtain directionally arranged ice crystals in the frozen suspension. Freeze drying was performed for sublimating the ice crystals resulting in lignin/CNFs aerogels with directional porosity. These lignin/CNFs aerogels were further carbonized to obtain CAs which were evaluated for their suitability as supercapacitor electrodes. Results from this work demonstrated the potential of lignin-based carbon aerogels as suitable materials for future energy storage applications. Kraft lignin (KL) based CAs showed superior properties compared to soda lignin (SL) based CAs. The type of CNF used, whose function is to modify the rheology of lignin/CNF suspensions and act as sacrificial template during carbonization, was also shown to have an impact on the microstructure of CAs. Mechanically fibrillated CNFs resulted in CAs with more mesoporous (pore size between 2-50 nm) microstructure which is advantageous of energy storage applications. Graphene quantum dots (GQDs) prepared from biochar was used as capacitance enhancer for KL based CAs. Introduction of GQDs on the surface of CAs resulted in the improvement of specific capacitance. Further improvement in the microstructure and specific capacitance was achieved by controlling the process parameters during ice-templating process. Ice templating cooling rate and solid content of lignin/CNFs suspensions were varied systematically and observed that ice templating cooling rate had predominant influence on the microstructure of CAs. Effect of carbonization temperature on the final carbon structure was investigated by preparing carbon particles from different technical lignins and the results showed that carbon particles produced from SL is more graphitic and electrically conductive. The dependance of microstructure on the ice templating cooling rate was utilized for the preparation of CAs reinforced carbon epoxy composites with superior mechanical properties. In summary, this thesis demonstrates that unidirectional ice-templating, freeze drying followed by direct carbonization route of preparation of lignin based CAs is a green and effective strategy for making supercapacitor electrodes for energy storage application. Results indicated that type of lignin, ice-templating cooling rate, and carbonization temperature affect the microstructure and electrochemical performance of the carbon electrodes. This study opens new opportunities to investigate the usage of lignin-based carbon materials for meeting the demands of future energy storage devices.