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

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 document
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commercial 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.

The beneficiaries range from (i) the general public and (ii) the Government and public sector, to (iii) a wide variety of industries and even (iv) charities and voluntary organisations. (i) General Public With depletion of the world's fossil fuels, industrialised countries must seek alternative energy sources before the costs of such resources become prohibitive. Burning fossil fuels to create electricity as at present is a major contributor to global warming, releasing huge quantities of carbon dioxide gas. The inexpensive and green production of fuel using free solar energy from the sun will be able to contribute towards the overall energy requirements of households and industry over the next decades as the world moves away from total reliance on polluting fossil or potentially hazardous nuclear fuels. The devices chosen are planned to inexpensive to make, durable and long lived. This approach has the ability to produce all the energy needed by a household in one day from a volume of water less than a single toilet flush. It is also non polluting as the only byproduct from the combustion of hydrogen is water. (ii) Commercial Sector Not only will our project partners such as Tata Steel (formerly Corus), TWI or Teer benefit from the commercial development of demonstrator photodiodes on a huge scale, but the supporting industries such as Cristal Global or Johnson Matthey will benefit from the large scale use or production of novel, UV and visble-light active semiconductor photocatalysts or precious metals as part of the water splitting devices. Such materials or coatings research will also benefit industries developing metal supported dye sensitised solar cells and also self cleaning products, such as architectural steels, glass, tiles and paints (e.g. Cristal global). Energy companies will also benefit from the development of such devices which can be used to support the national grid particularly at times of high demand. (iii) Government/Public Sector The proposed technology only uses and produces water and therefore does not pollute the environment via the production of CO2 such as occurs with fossil fuels. With ever more stringent legislation put in place to guarantee a cascade of international agreements to reduce CO2 and other greenhouse gases to acceptable levels, viable routes to a proven reduction of fossil fuels are clearly of prime importance to policy makers and legislators. (iv) Third Sector More speculative beneficiaries of this research are charities and voluntary organisations. With climate change leading to more frequent weather-induced disasters, such as hurricanes, floods and droughts, the call on voluntary aid organisations is increasing rapidly and climate stabilisation (via use of methods which negate the need for polluting fossil fuel technologies) would alleviate this burden to sustainable levels. The ability to remotely generate energy from the sun using water splitting devices could also help for localised or remote energy generation in areas such as those with no access to grid power or in case of emergencies.