Doctor of Philosophy
Ram B. Gupta
Over the last century, almost all of the carbon materials developed for the energy industry are derived from fossil fuels. The growing global concerns about energy needs, fossil fuels consumption, and the related environmental issues have motived scientists to find new, green and sustainable energy resources such as the wind, solar and biomass energy. Essentially, biomass-derived materials can be utilized in energy storage and conversion devices such as Li-ion batteries, fuel cells, and supercapacitors. Among the biomass resources, lignin is a high volume byproduct from the pulp and paper industry and is currently burned to generate electricity and steam. The pulp and paper industry has been searching for high value-added uses of lignin to improve its overall process economics.
The importance of manufacturing valuable materials from lignin is, discussed in Chapter 2, demonstrating the need for a facile, green and scalable approach to synthesize bio-char and porous carbon for use in Li-ion batteries. From this context, lignin is first carbonized in water at 300 °C and 103 bar to produce bio-char, which is then graphitized using a metal nitrate catalyst at 900-1100 °C in an inert gas at 1 bar. Graphitization effectiveness of three different catalysts, iron, cobalt and manganese nitrates was examined. The obtained materials were analyzed for morphology, thermal stability, surface properties, and electrical conductivity. Both annealing temperature and the catalyst affects the degree of graphitization. High-quality graphitization is obtained by using Mn(NO3)2 at 900 °C or Co(NO3)2 catalysts at 1100 °C.
Research on various energy storage materials for supercapacitors has grown rapidly in the recent years. Various advanced materials have been shown as a promising candidate for future’s high-energy supercapacitor electrodes. For a material in a supercapacitor electrode to be considered, it must show promising results for its specific power and energy density, electrical conductivity, surface properties, durability, surface area and pore-size distribution in order to design and develop high-performance supercapacitor devices. The industrial applications of supercapacitors have not been satisfied due to the low energy density (the commercially available supercapacitors have between 5 to 10 times less energy density than that of batteries) and moderate charge-discharge rate of supercapacitor electrode. Thus, chapter 3 was aimed to design and synthesize nitrogen-doped carbon materials that show the characteristic of high-energy and high-power density supercapacitor electrodes with a long cycle life. With this aim, organosol lignin was successfully converted into N-doped carbon materials using a two-step conversion process. The nitrogen content in the carbon was up to 5.6 wt.%. The synthesize materials exhibit high surface area up to 2957 m2/g with micro/meso porosity and a sheet-like structure. The N-doped carbon produced at 850 oC exhibited a high capacitance value of 440 F g-1 at a 1 mV s-1 scan rate and demonstrated excellent cyclic stability over 30,000 cycles in 1 M KOH. In addition, the NC-850 delivers a high energy density of 15.3 W h kg-1 and power density of 55.1 W kg−1 at 1 mV s-1. Therefore, this study suggests that N-doped carbon materials synthesized from a pulp and paper byproduct, lignin, are promising environmentally-sustainable candidates for supercapacitor applications.
Challenges for commercialization of fuel cells include high operation cost, inadequate operational stability, and poisoning by H2O2. To address the challenge, costly Pt-based catalysts are needed in order to facilitate the oxygen reduction reaction (ORR) at the cathode and the hydrogen oxidation reaction (HOR) at the anode. In chapter 4, alternative metal-free ORR catalyst materials derived from lignin are studied in order to simultaneously enhance the catalytic activity, lessen the Pt dependency and reduce the excessive costs associated. Calcium sulfonate lignin was successfully converted into sulfur self-doped carbons via in-situ hydrothermal carbonization and followed by post-annealing treatment. The sulfur content in the as-prepared porous carbons is up to 3.2 wt.%. The resulting materials displayed high surface areas (up to 660 m2 g-1) with micro/meso porosity and graphitic/amorphous carbon structure. The as-prepared sulfur self-doped electrode materials (SC-850) were tested as a potential cathodic material for ORR. The number of electrons transferred per molecule was measured to be ~ 3.4 at 0.8 V, which approaches the optimum 4 electron pathway. Additionally, S-doped materials were also applied as a supercapacitor electrode material. The SC-850 electrode exhibited a high specific and volumetric capacitance values of 225 F g-1 and 300 F cm-3 at a scan rate of 0.5 A g-1. The SC-850 electrode also exhibited consistent response over 10,000 cycles at harsh conditions. It was shown that the metal-free SC-850 is a promising electrode material for supercapacitors and ORR applications.
All of the studies presented in this dissertation involve the development and application of carbon-based materials derived from lignin and its application towards the Li-ion batteries, supercapacitor, and fuel cell. Insight into the applicability of lignin-derived carbon materials towards electrochemical applications is made readily available, supplemented by detailed physical, chemical and electrochemical characterization, to examine the specific factors influencing the Li-ion batteries, supercapacitor, and electrocatalysis of fuel cell activity.
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Available for download on Monday, August 01, 2022