UCLA

Developing novel functional materials for applications in energy storage and conservation is essential to solving today’s problems of meeting growing energy demands with minimal environmental impact. The first part of this dissertation describes pseudocapacitive Li-ion electrodes for fast charge storage. Pseudocapacitance, a charge-transfer process that is not diffusion-limited by definition, can be achieved by utilizing short diffusion lengths in nanostructures. In this section, three distinct transitional metal oxides with nanoscale architecture, Nb2O5, MoO2, and LiMn2O4, are examined. We focus on correlating charge storage mechanisms with structural properties, and exploring different aspects of pseudocapacitive charge storage with each system. First, we developed a core/shell Nb2O5/Nb4N5 composite via partial nitridation, achieving specific capacity over 500 C/g within 90 seconds. Our use of the nitride shell provides excellent electronic pathways that connect Nb2O5 nanoparticles and improve the interfacial electrical conductivity. The charge storage mechanism was found to depend on the thickness of the nitride shell. For MoO2, we developed an interconnected porous network that provides facile electrical and ionic pathways for fast charge storage. We also used it as a model system to understand the effects of structural variations on charge storage behavior. Phase transition suppression was found to be associated with highly capacitive systems, and a strong correlation between charge storage properties and crystallite size was established. Last, we developed needle-like nanostructures with selectively engineered surface facets to overcome its capacity loss due to surface Mn dissolution. This unique structure allowed for a higher capacity and less surface Mn dissolution due to a dominant presence of dissolution-resistant facets.

The second part of this dissertation studies thermal transport in mesoporous SiO2. SiO2 is widely used in many thermal insulating applications due to its facile synthesis and easy-to-control of structure. Porous amorphous SiO2 has strong boundary scattering effects that lead to significantly low thermal conductivity. In this section, we examined the effect of porosity, pore size, and framework texture (i.e., continuous or nanoparticulate) on thermal conductivity in mesoporous SiO2 thin films. Sol-gel and nanoparticle-based mesoporous SiO2 thin films were synthesized by evaporation-induced self-assembly using tetraethyl orthosilicate and pre-made SiO2 nanoparticles as the framework precursors, respectively, and a variety of template reagents. The thermal conductivity was measured by time-domain thermoreflectance at room temperature in vacuum. A porosity weighted simple effective medium approximation was employed to explain trends in thermal conductivity. The results give new insight into thermal transport in nanostructured amorphous materials, and suggest design rules of the nanoscale architecture to control the thermal conductivity of mesoporous materials for a wide range of applications.