Technology

Doing as plants do: stealing CO2

 By Eniday Staff

To reduce or just maintain the concentration of a substance in the environment there are probably three paths to take…

The first is not to add any others, which isn’t always easy and sometimes nearly impossible. Let’s take the most obvious example: none of us want to see climate change causing disastrous weather, but a lot of the energy we use comes from combustion processes that release carbon dioxide into the atmosphere. Of course, we’re all trying to reduce emissions, and to an extent, some of us are succeeding. Others, though, are failing to reverse the trend. The second path is CCS, which stands for carbon capture and storage. In practice, this means creating devices that can capture carbon dioxide generated by combustion processes. These often take the form of confined combustion using oxygen, which releases a mixture of water and CO2 that can be cooled down to separate the two. All these techniques are being tried, with fluctuating results. First of all, they are rather costly and can’t be used in large industrial facilities – like power plants, steelworks and chemical plants – which represent a very important source of CO2, albeit not the only one. Cars, stoves and boilers are by their nature unsuitable for this technique. The third path is carbon capture and utilisation, or CCU, which means capturing CO2 and using it to create products with high added value. There are already things that steal carbon dioxide straight from the atmosphere, in its diluted form, then transform it into useful organic substances, and they exist in nature: we are talking about plants!

imitate-plants-capture-CO2
A modular CO2 capture plant developed by the Swiss company Climeworks (climeworks.com)

Imitate the plants

Plants work on a simple principle. Sunlight provides a source of energy for converting carbon dioxide in the atmosphere into organic material, particularly sugars. The chain of chemical reactions at play here is fairly complex. Solar energy is converted into organic material at pretty inefficient rates, sometimes of even less than 1%. So, could an artificial device, a man-made machine, do any better?
That’s what they’ve started doing in the laboratories at the Collège de France in Paris. The idea is to use a photovoltaic system to convert solar radiation into electricity, which will then power an electrochemical cell that oxidises the water on one electrode and reduces the carbon dioxide onto another. This transforms the CO2 into other molecules like carbon monoxide, methane, methanol and others, depending on how the reaction is managed. This isn’t really a new idea, but until now these devices relied on rare, expensive materials, often pollutants like gallium, iridium and arsenic. And that’s where the challenge lies: to build a machine that imitates plants by stealing carbon dioxide from the air, without spending too much or producing pollution.
At the Collège de France they seem to have succeeded, at least on an experimental basis. The electrodes in the machine in Paris aren’t made of rare metals, just simple copper. This is covered with a porous structure with tiny cavities concealing small bumps, in order to increase the useful surface area of the electrode. The process for reducing carbon dioxide into hydrocarbons, mostly ethylene and ethane, has an efficiency rate of over 20%. The system efficiency, understood as the ratio between solar energy captured by the photovoltaic cell and solar energy contained in the hydrocarbons produced, is 2.3%, much higher than for most plants. Sure, it’s only an experiment, but an extremely successful one. And the road ahead is very promising, so much so that various French industrial groups have shown a lot of interest. French researchers say what’s needed is a device that can be replicated more easily with costs lowered to the point where it’s cheap compared to all other strategies for reducing CO2.

WATCH MORE: Energy Superfacts: CO2 buster by Eniday Staff

about the author
Eniday Staff