As the need for energy is increasing fast these days, it is imperative to develop new and reliable energy conversion and storage technology strategies that are alternative to the fossil fuels. In the first part of my thesis, we investigate the possibility of utilizing silicon as potential photocatalyst in the organic waste degradation. We first demonstrate that the platinum nanoparticle loaded porous silicon nanowires can be used as effective photocatalysts for photocatalytic degradation of organic dyes and toxic pollutants under visible irradiation. Secondly, we demonstrate a facile and simple solution phase method to enhance and stabilize the photoactivity of silicon nanowires, by functionalizing the silicon nanowires with reduced graphene oxide sheets. Photocatalytic IC dye degradation testing shows that the photoactivity of silicon nanowires can be enhanced and maintained without decay, featured with an initial activation process. In the second part of my thesis, we investigate various high energy density air battery systems. First of all, a silicon-air battery using an alkaline solution as electrolyte is investigated, achieving a high anode specific capacity. Second we demonstrate a new family of silicide based anode materials for high energy density primary air batteries. We show various silicide anodes (Mg2Si, TiSi2, CoSi2 and VSi2) can exhibit excellent electrochemical performance with unparalleled capacity in both thin film and bulk power pellet form. Lastly, we invent a hydrophobic three-dimensional (3D) graphene membrane as a moisture-resistive cathode for high performance Li-air batteries, enabling a robust Li-air battery with exceptional performance. All of these new technology strategies will open up exciting opportunities for many applications in the energy field.