|dc.description.abstract||Utilization of low-value, abundant and sustainable biomass materials for high-value energy storage application was the main goal of this thesis. Energy storage for vehicle electrification and intermittent renewable energy sources such as solar and wind energy made it an urgent necessity to look for next generation energy storage beyond lithium-ion batteries. Lithium-sulfur (Li-S) battery is one of the most promising candidates for the next generation energy storage solutions due to its high theoretical specific capacity (1675 mAh/g), high energy density (2567Wh/kg) and low-cost. However, the commercialization of this battery has been hindered due to several reasons such as the insulating nature of sulfur, intermediate polysulfide dissolution, low Coulombic efficiency and short cycle life. Confining sulfur and polysulfides using porous and conductive carbon materials have gained wide attention to improve Li-S battery performance. Porous carbon structures can physically adsorb sulfur molecule and prevent polysulfide dissolution, whereas nitrogen doping can adsorb polysulfides chemically to improve the Li-S battery performance. Biochar is a carbon-rich, inexpensive and porous material produced as a by-product during biomass pyrolysis for bio-oil production. Being highly porous, carbon-rich and conductive make activated biochar an excellent candidate to be used for sulfur-carbon (S/C) cathode composite.
This thesis work was performed focusing on two objectives. The first objective was to investigate the influence of biomass pyrolysis method on physical and chemical properties of chemically activated biochar/carbon. The second goal was to utilize fast pyrolysis biochar derived from inexpensive and abundant canola meal and Douglas-fir wood for the preparation of high-valued lithium-sulfur battery cathode composite. Fast pyrolysis biochar is obtained as a low-value byproduct of high valued liquid fuel through fast pyrolysis of biomass. Some studies with biomass biochar have been performed for lithium sulfur battery. However, fast pyrolysis derived biochar has rarely investigated for any battery application. Additionally, the protein content of canola meal biomass acts as a source of natural nitrogen-doping of activated biochar. Thus, our focus was on investigating the applicability of fast pyrolysis biochar for Li-S battery and to compare the performance with slow pyrolysis biochar and commercial conductive carbon black.
For achieving the goals, biochars were prepared using fast pyrolysis and slow pyrolysis method with canola meal and Douglas-fir wood at 500°C temperature. The biochars were further activated using potassium hydroxide (KOH) at 800°C temperature as KOH activation enhanced the carbon content, surface area and pore volume of the activated biochars. Total four types of activated biochars were prepared from two biomass such as CF-AB, CS-AB, DF-AB, and DS-AB. CF-AB and DF-AB were the activated biochars derived from fast pyrolysis biochar of canola meal and Douglas-fir wood, respectively. CS-AB and DS-AB were the activated biochars derived from slow pyrolysis of canola meal and Douglas-fir wood, respectively.
Physical and chemical characterizations were performed to evaluate physical and chemical characteristics of the samples (moisture content measurement, elemental analysis, thermogravimetric analysis for ash content and sulfur loading measurement, Raman and FTIR spectroscopy, scanning electron microscopy, and surface area and porosity analysis). CF-AB, CS-AB, and DF-AB activated biochars were selected based on the surface area and pore volume to synthesize S/C composite following melt-diffusion strategy at 155°C. For comparison, S/C composite was synthesized by following the same method using commercial conductive carbon black (CB). Samples were denoted as CF-AB-S, CS-AB-S, DF-AB-S, and CB-S. Additionally, CF-AB-S was washed with toluene to remove the sulfur that existed on the surface of CF-AB-S composite and developed another composite named as CF-AB-S-T. For electrochemical characterizations, such as galvanostatic charge-discharge measurement and cyclic voltammetry, lithium-sulfur cells were assembled in 2032 coin cells. Lithium chips and S/C composites were used as anode and cathode materials, respectively.
Activated biochars derived from fast pyrolysis biochars exhibited significantly higher surface area (3355-3277 m2/g) and pore volume (1.58-1.49 cm3/g) in comparison to activated biochars derived from slow pyrolysis biochars. CF-AB-S-T cathode composite exhibited superior initial discharge capacity of 1507 mAh/g at the 0.05 C rate (83.75 mA/g). All the prepared five cells had exhibited initial discharge capacity higher than 1000 mAh g-1 at the 0.05 C rate. Activated biochar derived S/C composite cathodes exhibited stable performance at 40th cycle, and even at the very high 2 C rate. These results indicated the applicability of canola meal and Douglas-fir derived activated biochar as cathode material for Li-S battery and excellent potential of fast pyrolysis derived biochars.||en_US