Abstract
There are three questions, which must be answered in order to better utilize the potentials of biopolymers in the areas of energy storage. Questions - Can non-conductive biopolymers be employed in applications where highly conductive materials are needed? Is it possible to harness the flexibility, tailorability, compatibility and sustainability of biopolymers into energy chemistry research? Will research on biopolymer applications into energy storage be economical and sustainable? The answers to these three questions form the bridge between the under-exploitation of biopolymers and their optimal applications in the field of energy, most specifically sustainable energy.
Biopolymers are naturally abundant and green polymers that are gaining the interest of researchers for sustainable energy applications. They can be well-tailored to design fundamental energy storage devices that can store, convert and even generate renewable and non-renewable energies. Examples of biopolymers are starch, cellulose, alginates, chitosans, gelatins, poly lactic acid, poly caprolactones, etc. Examples of energy storage devices are batteries, supercapacitors, fuel cells, their hybrid forms, and their flow forms.
The work we have achieved in this research is to design biopolymer nano-architectures. They are miniaturized forms of biopolymers, hybridized with specific metal oxide nanoparticles, and possessing special material, electronic and covalent interactions. We worked on starch, gelatin, cellulose, and poly butylene succinate biopolymers. In addition, we worked on polyaniline and poly(3,4-ethylene dioxythiophene) conductive polymers with Fe3O4, TiO2, SnO2, and gold nanoparticles. Each biopolymer was activated with a novel procedure that involves protonation in the presence of NH4Cl and propylene carbonate. The activation helps to improve the conductivity of the biopolymers by supplying polarons and bipolarons for effective charge transfer within the polymer backbones. The major breakthrough in this work is that we achieved highly conductive biopolymers in their uncarbonized forms, with conductivity values as high as 85 𝜇Scm-1. This is even higher than the conductivity obtained in various carbon forms of biopolymers. The use of conductive uncarbonized form satisfies green chemistry and makes the research a sustainable energy storage research. The work was designed to answer four major questions in the design of biopolymer nanoarchitectures towards supercapacitor applications. These questions form the knowledge gap of this research, and they include (a) what is the effect of nano and micro designs of biopolymers? (b) what is the effect of molecular assembly of biopolymers? (c) what is the effect of surface modification of biopolymers? (d) what is the effect of phase changes in the conductive polymers? To address the effect of nano and micro design of biopolymers on supercapacitor performance, we designed starch and gelatin-based supercapacitor electrodes of various sizes in the nano and micro ranges. The starch-gelatin-TiO2 nanohybrids with average sizes of 87 nm showed the highest specific capacitances of 808 Fg-1 and an energy density of 208 Whkg-1 at a scan rate of 5 mVs-1. The maximum retention capacity was shown to be 95.5%, the other nanohybrids with relatively smaller sizes showed lower supercapacitive performances. The effect of the molecular assembly was fully investigated by designing various reverse micelles from starch and poly (1,4-butylene succinate). The uncarbonized starch forms were activated through protonation and incorporated with magnetite nanoparticles. The reverse micelles had better electrochemical and transport properties than the ordinary composites. This proves that biopolymers can take up molecular assemblies that show better electronic interactions with electrolytes more than ordinary composites. St-PBS micelle II, one of the reverse micelles, which was designed through the single emulsion technique from a 1:1 DCM: acetonitrile solvent and introduced to 5% Tween-80 solution, had the highest supercapacitive performance. St-PBS micelle II showed 143 Whkg-1 as compared to the 79 Whkg-1 and 77 Whkg-1 shown by St-PBS micelle I and St-PBS micelle III. The work further shows how magnetite incorporation improved the specific capacitances, energy densities, and power densities by providing more charge storage sites. After Fe3O4 incorporation, the St-PBS micelle showed an energy density of 154 Whkg-1. The St-PBS-Fe3O4-micelle showed up to 250% increase in energy storage performance compared to ordinary St-PBS that is not molecularly assembled. The effect of phase changes in the conductive polymers was studied; thus: calcination was used to induce phase changes in starch-TiO2 hybrids. It was confirmed that TiO2 grown on activated starch templates, heated at 500 °C, consisted of 28% rutile and 72% anatase. This phase change was responsible for a better supercapacitive performance of 388 Fg-1, 194 Whkg-1, and 4473 WKg-1; as compared to 74 Fg-1, 37 Whkg-1 and 2991 WKg-1 in 100% anatase TiO2 that have not been subjected to phase change. The asymmetric supercapacitor designed from this electrode and polyaniline spheres showed a wide operational voltage 1.6 V. In addition, pH gradient was used to create a pH overpotential on the surface of gold metallized
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regenerative cellulose amide, leading to high energy density and specific capacitance as high as 96 Whkg-1 and 603 Fg-1. The retention capacity achieved was 94%. This was because the pH gradient created in the modified cellulose based electrode led to additional diffusive current and potential at the double layer region of the electrode. Finally, a supercapacitor electrode obtained from biochar nanosheet was also designed. In the three-cell configuration, a specific capacitance (Csp) of 283 Fg-1 was increased to 517 Fg-1 after exfoliation, with an energy density (Ed) of 258.5 Whkg-1 and retaining 95% of its capacity after 5000 cycles with an average time of charge of 120 s. The CPE fitting of the EIS showed ‘n’ value of 0.773 to confirm diffusion based charge storage and a very low ESR. In the two-cell real life test using the PAT El-cell, a retention capacity of 96.5% was obtained, with a time of charge of 210 s. In addition, a Csp and Ed of 493 Fg-1 and 246 Whkg-1 was obtained. The self-discharge profile showed a two-stage self discharge before exfoliation (0.75-0.2-0.08 A), and a one stage stable self-discharge profile from 0.9 to 0.18 A, confirming a high stability of the biochar nanoshhets after exfoliation This doctoral research has demonstrated that uncarbonized biopolymer nano-architectures can be employed in energy storage purposes, provided they are well activated and modified.