Emission-free photodiodes

Materials World magazine
3 Jul 2011
stylised spark

A nanocrystalline photodiode, which splits hydrogen and oxygen, could result in emission-free energy harvesting devices.

The technology works by placing a photocatalyst, made via continuous hydrothermal flow synthesis, on the surface of a metal sheet that is then positioned in a water tank. The photocatalyst is able to absorb the high-energy portion of sunlight (UV and near UV) and produce a positive hole and an electron. The positive hole oxidises water on the sunlight side. The electron travels to the reverse side and reduces water-producing hydrogen.

The key feature of the system, being formed at University College London, UK, is the photo conversion of water by sunlight and the separation of oxygen and hydrogen.

Lead Researcher Ivan Parkin explains, ‘Oxygen and hydrogen are not produced in the same space and so do not recombine. We are therfore able to convert water into hydrogen and oxygen. These can be recombined in a fuel cell to produce electricity, or, more usefully, the hydrogen can be burned as a fuel source.’

He continues, ‘Our aim was to produce an alternative way of generating energy from sunlight with zero emissions. The beauty of the device is that once the water is split, the hydrogen can be used as a fuel, where on burning, it combines with oxygen to produce water. Hence no CO2 is evolved and there is a minimal environmental footprint’.

Another potential advantage is that the hydrogen can be piped with minimal energy – unlike electricity, which has large power losses.

However, ‘the challenge is to make the device competitive and more effective than photovoltaic solar panels (PV)’, adds Parkin. At present, the photodiode does not work at the efficiency levels of a PV device. Yet, by developing better photocatalysts, Parkin hopes the device could increase its efficiency. ‘New materials are required that are stable to the conditions used in the process (photostable), and that can harness more of the sun’s energy. Titanium dioxide can only use the fraction of sunlight that is below 385nm (the UV portion)’, explains Parkin.

The next phase will involve looking at visible light absorbing photocatalyst materials.

To test the performance of the cells, the team has ran a series of thermal thermal cycling tests. ‘We do not know the lifetimes of the devices yet, but it would need to run into years before commercial production,’ adds Parkin.

They are now looking to make a working demonstrator, with a more durable and lightweight design. Optimisation of the catalytic materials is also a priority, says Parkin. 


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