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Electrolysing carbon-dioxide

Carbon capture is often suggested as a way to reduce carbon emissions, so far it has normally just meant collecting carbon-dioxide and hiding it somewhere – normally underground. But the ideal solution would be some way of converting the carbon-dioxide back to carbon, the natural way of doing this is of course to harvest sunlight with plants and then some of this biomass is sequestered. But the highest efficiency plants only convert a few percent of the sun's energy into biomass, and if you want to use it as fuel, the best conversion efficiency is less than 0.5%.

A more radical rethinking of this problem has been to use solar electricity to electrolyse – or split – the carbon-dioxide into carbon and oxygen. Whilst in comparison to photosynthesis this is good, it still wastes large amounts of solar energy, as most solar cells can't use the huge amount of infra-red thermal energy in sunlight.

Stuart Licht and collegues from washington DC have come up with a way of increasing this efficency. They are electrolysing carbon-dioxide dissolved in lithium carbonate, so still using electricity, but they have discovered that if you heat the system to 950C you need 40% less electricity to split the carbon-dioxide.

The really neat thing about this is that you can use the infra-red part of the solar spectrum to heat up the electrolysis cell while still using a solar cell at maximum efficiency – in fact slightly better than normal, as there are less overheating problems. Using the most efficient solar cells available, they think they can use between 30 and 55% of the sun's energy to either produce carbon or at a slightly higher temperature carbon-monoxide.

This could either be used to sequester carbon, or the carbon-monoxide could be used as a feedstock to produce hydrocarbon-fuels or hydrogen.

25th Jul 2010

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