Nanostructured Materials for Energy Generation and Storage

Development of new energy generation and storage technologies is important for increasing the share of renewable energy sources and wider use of the plug-in electric vehicles. Thermoelectric energy conversion is suitable for the waste-heat recovery allowing for energy reuse. The limited use of thermoelectric generators is explained by the low efficiency energy conversion defined by the thermoelectric figure of merit ZT. The factor that limits the scalability of energy storage in reversible electrochemical cells is high Ohmic losses, which degrade the battery performance. In this dissertation, I investigate innovative approaches for increasing the efficiency of the thermoelectric energy generation and battery storage via the use of nanostructured materials. It was theoretically predicted that strong quantum confinement of charge carriers and acoustic phonons can lead to drastic enhancement of ZT. I have used "graphene-like" mechanical exfoliation of Bi$_2$Te$_3$ and TiTe$_2$ to produce thin films with variable thickness. The exfoliated films were stacked together to form quasi-superlattices. Unlike the epitaxially grown superlattices, the stacked structures are characterized by nearly infinite potential barriers for electrons and phonons. Using the thermal and electrical measurements I have shown that ZT of such structures can be increased over a large temperature range via reduction of the phonon thermal conductivity. It has been previously shown that the core-shell nanostructure-based electrodes enable the combined strength of two or more materials to obtain enhanced energy-power densities in the batteries. The improvement is due to reduction of the Ohmic effects. I investigated the thermal and electrical conductivities of the ultra-long vertically-aligned core-shell carbon nanotubes utilized in the Li-ion battery electrodes. The thermal conductivity, measured with the "laser-flash" technique, of the carbon nanotubes coated with amorphous silicon and carbon was in the range from 400 to 600 W/mK near room temperature. This is substantially higher than that of the reference battery electrodes. The electrical resistivity was shown to be two orders of magnitude lower than that of the conventional electrode. The results obtained for the Li-ion battery electrodes suggest that the use of nanostructured materials can substantially improve the thermal management of the batteries and their energy storage efficiency