3D structuring of sustainable carbon electrodes for energy storage

As the world is moving towards decarbonization, electric vehicles and other clean energy technologies offer a path towards a sustainable future. Sustainable electrochemical energy storage (EES) devices such as supercapacitors (SC) and batteries will be essential to the transition to net-zero emissions. However, these devices today use electrode materials comprising mined minerals and other materials manufactured from petroleum or coal products, both of which are non-sustainable approaches. One way of improving the sustainability of an EES device is to develop sustainable carbon electrodes. Carbon, the fourth most abundant element in nature, plays a unique and dominant role in ecosystems and the economy and is already an integral part of most EES devices. State-of-the-art preparation of carbon-based electrodes for SC relies on conventional slurry-based electrode fabrication that utilizes inactive materials such as current collectors, insulating binders, and conductive agents. Furthermore, this type of fabrication often requires multiple processing steps such as coating, drying, calendaring, and punching and also possesses limitations in the electrode geometries or dimensions, which restrict the overall performance of the devices. To lower the production costs, environmental impact, and use of inert materials, while improving the energy storage efficiency, this dissertation focused on fabricating 3D structured, free-standing, sustainable electrodes from renewable resources such as biomass precursors. Green and naturally abundant polysaccharide-based biopolymers were used as the substrate or skeleton to develop 3D structured free-standing biomass-derived carbon aerogel (BCA) energy storage materials. 
The main goal of the dissertation was to fabricate advanced, free-standing, 3D structured BCA electrode architectures with low tortuosity and tailored macro-, meso- and microstructure using single or multiple materials by utilizing the versatility and ability of 3D printing to control the geometry in all three dimensions. For this, three different fabrication strategies involving 3D printing for patterning the BCA were developed. Furthermore, a novel strategy for mineralization of hydrogels was proposed and optimized, allowing to significantly enhance the micro-, meso- and nanoporosity of the BCA electrodes. Efforts were also directed towards understanding the structure-property relationship, including the interfacial molecular structure, morphology, chemical composition, surface area, porosity, and pore-size distribution of the as-developed, multi-scale architectured, free-standing BCAs in comparison with state-of-the-art slurry-based electrodes. Electrochemical experiments revealed that the free-standing, BCA-based electrodes reported in this dissertation delivered an outstanding specific capacitance (322 F g^–1 (Paper II), 547.7 F g_{_carbonizedbiomass}^-1 (Paper III), 2819.7 F g_{_carbonizedbiomass} ^–1 (Paper IV) and 331 F g^–1 (Paper V)) in comparison with those of other free-standing BCA (285 F g^–1) and most powder-based activated carbons (usually < 250 F g–1). Furthermore, all the electrodes exhibited an outstanding rate capability and extremely high cyclic stability with high capacitance retention (>100%), confirming their superior energy storage performance compared with commercially available activated carbons and biomass-derived analogues fabricated using slurry-based techniques.
Architecting the geometry of porous BCA electrodes at multiple length scales using 3D printing not only substantially reduced the number of steps associated with conventional electrode fabrication but also improved the electrochemical performance owing to the enhanced surface area within the same footprint. Furthermore, it can eliminate the requirement of additional high-weight inactive materials (binders and conductive agents) and high-cost current collectors, thus making the fabrication cost-effective and less energy-consuming. The multi-scale approach of designing biomass-derived, 3D structured, free-standing carbon electrodes reported in this dissertation may open up for substantial performance-enhancing capabilities for various EES devices such as SC, batteries, flow batteries, and fuel cells, paving the way for truly sustainable energy storage.