Nanostructured oxides for advanced energy conversion. Synthesis and properties

Providing a sustainable energy supply to the world population is one of the most important scientific and technological challenges of the 21st century. Enhancement of the existing energy supply can come from renewable sources, including solar, wind, biomass, etc. as well as from more efficient energy conversion systems, such as fuel cells, thermoelectric generators, piezoelectric energy harvesting and others. The performance of the energy conversion systems mainly depends on the availability of innovative materials with optimized performances. Nanostructured materials have attracted great attention because of the peculiar electrical, magnetic and optical properties arising from the confinement of matter in a small volume or in a reduced dimensionality. New properties are also related to the high surface to volume ratio and the significant contribution of surface properties to the overall behaviour. Although the large scale application of materials with reduced dimension is currently limited to microelectronics and a few other fields, nanostructured materials are becoming increasingly important in energy conversion and storage. Typical examples are Li-ion batteries, supercapacitors, photovoltaic devices, photocatalysis, fuel cells, etc. The efficiency of the device is largely determined by the ability to tailor the chemical composition and the structure of the material at the nanoscale. The synthesis of some nanostructured oxides with application in advanced energy conversion systems will be discussed in this presentation by means of examples taken from the current research activities performed at IENI-CNR as well as from literature. The oxygen and proton conduction in ceramic electrolytes used in solid-oxide fuel cells is strongly dependant on grain size. Oxygen transport is strongly enhanced in doped zirconia and ceria nanoceramics when the grain size decreases below a critical value. Photocatalytic and photovoltaic properties of TiO2 dramatically depends on phase composition (anatase, rutile, brookite), size and shape of the nanocrystals. These parameters can be controlled by systematically changing temperature, pH and duration of the hydrothermal synthesis process. Nickel hydroxide, Ni(OH)2, is an important electrode material in alkaline Ni batteries and supercapacitors. The morphology of the Ni(OH)2 particles obtained by hydrothermal synthesis can be tailored by means of surfactants and hydrophilic polymers, producing platelets, tabular crystals, flower-like aggregates, whiskers and spherical superstructures. The final morphology is determined by self-assembly and solvent mediated recrystallization processes.