Nanocrystalline Water Splitting Photodiodes II: Device Engineering, Integration and Scale-up

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Summary Fossil fuels are the primary source of energy for most industrialised countries and global stocks are being rapidly depleted, prompting a growing interest in alternative energy sources. In recent years this keen interest has sharpened considerably with the increasingly politically and socially accepted observation that burning fossil fuels to create electricity is a, if not the, major contributor to global warming, releasing into the atmosphere every year ca. 8.0 Gig tonnes of carbon dioxide, CO2, i.e. ca. 10% of present atmospheric levels. Of the renewable energy resources that might substitute for the fossil fuels, only sunlight, or solar energy, has the capability to satisfy current global energy demands. Indeed, the amount of solar energy falling on the Earth is exceeding humankind's present energy requirements by > 5000 times. Unfortunately it is not in a form that can be always readily utilised but, instead, needs to be converted into electricity or stored as a chemical fuel. The conversion of solar to electrical energy using photovoltaic devices, such as the silicon solar cell or using dye-sensitised solar cells, is well-established. However, electrical energy is not easily stored in large amounts and solar energy is diurnal and intermittent and there is least of it when we most need it, i.e. at night in winter. As a consequence, there is a real need for an efficient (> 10%), inexpensive (< £5 per m2) solar energy conversion device that generates a readily utilised chemical fuel. The advantage of a direct solar-driven, water-splitting system is that it converts the sun's energy into a chemical form, i.e. hydrogen, that can be readily stored or transported at minimal energy cost and used when needed and is non-polluting when used as a fuel, since the product is water. In stage 1 of this project, the researchers were able to investigate the fundamental properties and develop small laboratory prototype water splitting diodes. In particular, it was shown that careful engineering of the semiconductor-metal support interface was critical to high activity, as was the need to obtain a high as possible surface area, porosity and composition of the photocatalyst layer. Further improvements with regard to photocatalyst film adhesion, viable scale-up production methods are required to create practical prototypes which would run efficiently and effectively under real life conditions. This device will also require integration with a fuel cell of some kind in order to be of broad use as an energy solution. Therefore, the target at the end-point of this proposed project (Stage 2) is the fabrication of an efficient, scaled up, Proposal original proforma documentPage 3 of 10 Date printed: 12/10/2010 10:48:54TS/I002855/1 Date saved: 12/10/2010 10:39:12commercial demonstrator capable of harvesting solar energy to (i) split water into hydrogen and oxygen process streams on or near to domestic scale, (ii) have a final, inexpensive optimised design, which is sustainable in terms of life cycle analysis and comprehensive materials selection and (iii) gain significant and sufficient know how in terms of device integration into domestic utilisation model; most notably addressing the issues of hydrogen storage and subsequent usage via a commercial fuel cell or burner. The end point of Stage 2 will deliver a viable device, using optimised catalyst(s) and photocatalysts and the most suitable coating method in terms of (i) high activity and robustness and scalability (ii) economic impact; and (iii) environmental, ethical and societal considerations.