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The sun delivers more energy into the Earth’s atmosphere in just over 1 hour than we use in a year – 3,850,000 EJ (exajoules) is delivered in a year, whilst in 2005 human activity used an estimated 550 EJ. Although humans are getting better at harnessing renewable energy sources, we are still unable to compete with plants and directly convert the carbon dioxide and water in our atmosphere into a fuel source in an economically viable manner. Therefore, there still remains an opportunity to develop systems that harness the power of the sun, while simultaneously tackling concerns over rising levels of atmospheric carbon dioxide. In an uncertain world there is a further benefit of bringing energy security and off grid access to power generation.

In the School of Engineering and Materials Science at Queen Mary University of London there is a growing interest in harnessing sunlight for useful work. We are approaching the problem of getting energy from light in a variety of interesting ways that include mimicking plants in a process called artificial photosynthesis and developing new types of photovoltaic (solar cell) devices.

There are three main problems associated with the use of semiconductor systems when splitting water to provide a fuel source.

The first is that, to be economically viable, the semiconductor must absorb and become active under visible light illumination. It must also produce excited species that are capable of driving a chemical reaction in the absence of external power supplies. These factors and characteristics are relatively well understood for most semiconductor systems.

The second problem is that the photoexcited carriers can simply recombine within the semiconductor, producing no useful work. This is termed internal recombination and must be prevented as far as possible in the semiconductor system.

Finally, reactants and products are held very closely at the surface of the catalyst as they are produced very close to each other. Therefore, the equilibrium of the reaction is not pushed completely to products due to the possibility of back reactions. All three of these factors combine to reduce the overall efficiency of the system.

The work being undertaken around the globe on blended, core shell, nanostructured and other forms of semiconductor structure will continue to move forward. Perhaps an answer might lie by thinking outside the box and investigating new types of materials with inherent properties that can directly address some of the inherent problems associated with the systems being developed now. 

http://www.sems.qmul.ac.uk/staff/research.php?s.c.dunn@qmul.ac.uk