Understanding and Tuning Nanostructured Materials for Chemical Energy Conversion

Abstract

The conversion of energy that employs chemical reaction is termed chemical energy conversion. In my dissertation, I have focused on chemical energy conversion systems involving energetic materials and lithium ion batteries, where performance is strongly dependent on the properties of materials and their architecture. The objective of this study is to enhance our understanding and tuning of nanostructured materials that might find application toward energetic materials and electrode materials in lithium ion batteries.

Rapid heating diagnostics tools, i.e. temperature-jump techniques, have been used to study the ignition of aluminum nanoparticles, nanothermite reaction mechanism and metal oxides nanoparticles decomposition under rapid heating conditions (~10<super>5</super>-10<super>6</super> K/s). Time-resolved mass spectra results support the hypothesis that Al containing species diffuse outwards through the oxide shell. Low effective activation energies were found for metal oxides nanoparticles decomposition at high heating rates, implying the mass transfer control at high heating rates. The role of oxygen release from oxidizer in nanothermite reactions have been examined for several different systems, including some using microsized oxidizer (i.e., nano-Al/micro-I₂O₅). In particular, for periodate based nanothermites, direct evidence from high heating rate SEM and mass spectrometry results support that direct gas phase oxygen release from oxidizer decomposition is critical in its ignition and combustion.

Efforts have also been made to synthesize nanostructured materials for nanoenergetic materials and lithium ion batteries applications. Hollow CuO spheres were synthesized by aerosol spray pyrolysis, employing a gas blowing mechanism for the formation of hollow structure during aerosol synthesis. The materials synthesized as oxidizers in nanothermite demonstrated superior performance, and of particular note, periodate salts based nanothermite demonstrated the best gas generating performance for nanothermite materials. Energetic composite nanofibrous mats (NC/Al-CuO, NC/Al-Fe₂O₃, and NC/Al-Bi₂O₃) were also prepared by an electrospinning method and evaluated for their combustion performance.

Aerosol spray pyrolysis was employed to produce carbon coated CuO hollow spheres, Mn₃O₄ hollow spheres, and Fe₂O₃ mesoporous spheres. These hollow/mesoporous spheres demonstrated superior electrochemical performance when used as anode materials in lithium ion batteries. The effects of the amorphous and crystal structures on the electrochemical performance and the structure evolution during electrochemical tests were also investigated.