Date of Award

December 2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Ying Li

Committee Members

Junhong Chen, Chris Yuan, Benjamin Church, Christopher Fell

Keywords

Energy, Lithium Ion Battery, Lithium Sulfur Battery, Nanomaterials, Photocatalysis

Abstract

Non-renewable fossil fuels are the major sources to meet the energy, electricity and transportation demands of today's world. The over consumption of fossil fuels will lead to the increasing energy crisis and disastrous effects such as air pollution, global warming etc.

The primary greenhouse gas is CO2 mainly emits from the combustion of fossil fuels. Photocatalytic reduction of CO2 using sunlight as the energy input is a promising way to reduce CO2 level in the atmosphere and in the meantime produce alternative fuels such as CO, methane, methanol, etc. Among the various photocatalyst materials reported, nanomaterial TiO2 is the most widely studied due to its suitable band positions, high chemical stability, non-toxic nature, and low cost. However, the energy conversion efficiency using TiO2 for CO2 photoreduction is still low, mainly due to the reasons of (1) high probability of recombination of photo-induced electron-hole pairs, (2) fast backward reaction of hydrogen and oxygen to form water, and (3) limited ability of visible light utilization.

Another efficient way to decrease CO2 emissions is to reduce fossil fuels consumption. The invention of hybrid electric vehicles (HEVs) and electric vehicles (EVs) are great promise of replacing traditional gasoline driven automobiles. As one of the

new generation high energy density batteries, lithium-sulfur (Li-S) battery is very attractive because sulfur has a high theoretical capacity of 1,675 mA h g-1. However, the practical realization of Li-S batteries is limited by several problems: (1) poor electrical conductivity of sulfur (2) dissolution of the lithium polysulfide intermediates into the electrolyte, and (3) large volume expansion of sulfur during cycling.

One objective of this study is to demonstrate high-efficiency photocatalysts using innovative hybird nanostructures that consist of Ce doped TiO2 dispersed on mesoporous silica (SBA-15) or noble-metal nanoparticles Ag supported on TiO2 or MgAl-LDOs (layered double oxides) grafted on TiO2 (TiO2-MgAl LDOs). The use of Ce doping could result in smaller TiO2 nanocrystals and facilitate electron-hole separation, while SBA-15 provides good dispersion of TiO2 and a strong interaction between TiO2 and the substrate. And Ag species on TiO2 facilitate electron trapping and transport to the catalyst surface, and thus, can potentially enhance multi-electron transfer processes. TiO2-MgAl LDOs is favorable for CO2 species adsorption on the photocatalyst, therefore, compensating the weakened CO2 adsorption ability at higher temperature in the presence of H2O vapor. The other objective of this study is to find alternative materials as anode for Li-ion battries and demonstrate high-performance Li-S battery electrodes using hybrid nanomaterials consist of sulfur infiltrated porous micrsophere carbon (PMC). Carbon/TiO2 was found to be promising as anode alternative to replace graphite materials to avoid safety issues for Li-ion batteries. Cathodes made of sulfur infiltrated in such a multi-modal porous carbon framework provide advantageous properties that guaranttee the superior electrochemical performance.

Ce-doped TiO2 on SBA-15, Ag deposited TiO2 (Ag/TiO2) and MgAl-LDOs grafted on TiO2 (TiO2-MgAl LDOs) were synthesized and characterized for applications in CO2 photoreduction with H2O. Ce-doped TiO2 were synthesized using sol-gel method and SBA-15 was then added to the sol to prepare Ce-doped TiO2 on SBA-15 nanocomposites. Modification of TiO2 with Ce significantly stabilized the TiO2 anatase phase and increased the specific surface area, which contributed to an improvement of CO production from CO2 reduction. Dispersing Ce-TiO2 nanoparticles on the mesoporous SBA-15 support further enhanced both CO and CH4 production. The superior catalytic activity may be related to the partially embedded Ce-TiO2 nanoparticles in the ordered 1-D pores in SBA-15 that form synergies between the different components of the catalysts and enhance the diffusion and adsorption of CO2. Ag/TiO2 nanocomposites were synthesized by spray pyrolysis technique. This work has demonstrated the feasibility of syngas (H2 and CO) production from a gas mixture of CO2, H2O and CH3OH hrough a photocatalytic reduction process on Ag/TiO2 nanocomposite catalysts under solar irradiation.The material property analysis and photocatalytic activity results showed that the ultrasonic spray pyrolysis method is much superior to conventional wet impregnation process with the advantages of smaller Ag nanoparticles, a better Ag dispersion on TiO2, and a higher fraction of metallic Ag species, which facilitate charge transfer and improve photocatalytic activity. TiO2-MgAl LDOs were synthesized by hydrothermal and coprecipitation method. As the MgAl LDOs concentration increases, TiO2 crystal size was increased. MgAl LDOs grafting on TiO2 cuboids may help improve the adsorption ability of CO2 onto TiO2 which improves the photocatalytic activity of CO2 reduction.

Our work also entails the synthesis and characterization of carbon coated TiO2 for the application of Li-ion batteries and sulfur infiltrated porous microsphere carbon (PMC/S) for the application of Li-S batteries. Carbon decorated on commercial TiO2 nanoparticles (P25 and P90) composites with optimized carbon concentration and structure were fabricated by a facile process employing carbonization method. The electrochemical performance of C-P90 was superior to C-P25 because of its higher specific surface area and larger anatase fraction that can accommodate more lithium ions. 1.9% carbon was found to form an optimized carbon layer on TiO2 that can improve the electronic conductivity. The PMC was synthesized by spray pyrolysis method. Then PMC/S was fabricated via a liquid phase infiltration. The novel-structured porous carbon microspheres possess a controllable multi-modal pore size distribution, i.e., a combination of interconnected micropores, mesopores and macropores, which is beneficial for Li-S batteries electrochemical performance.

Future work includes further improvement of PMC/S composites to inhibit shuttle effect and improve the electrode performance including the cyclability and rate capability.

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