Leiden chemists improve electrochemical production of sustainable chemical building blocks
If you could convert CO2 into building blocks for other molecules with the help of electricity, you could make the chemical industry considerably more sustainable. Leiden chemists have unravelled a fundamental part of this process and applied this knowledge in a real device, as they write in Nature Catalysis and Nature Communications.
For their research, the chemists from the group of professor Marc Koper used a process called electrochemical reduction. For this process, they dissolved CO2 in an electrolyte, a mixture of water and a salt. They then insert an electrode into the electrolyte, which in this case was made of gold.
‘As soon as we pass electricity through the electrode, the CO2 reacts on the surface of the electrode,’ explains PhD candidate Mariana Monteiro. ‘You then form CO, also known as carbon monoxide. This is a useful building block in chemistry, which we normally make from fossil resources. This reaction could therefore be a good way of using sustainable electricity to help the chemical industry to get rid of oil.’
Role of salt
Many aspects of the reaction have already been investigated, but there were still many open questions. Monteiro: ‘We knew that you could improve the reaction by changing the salt, but the mechanism behind this was still unknown. That is why we investigated what happens if you leave the positively charged ions, or metal cations, out of the electrolyte.
Surprising result
The outcome of the experiment was surprising, but very clear: nothing happened. The reaction does not work without metal cations. To be sure that this was not a specific mechanism for a gold electrode, Monteiro and her colleagues also tested it with electrodes made of copper and silver.
‘Here, too, we saw no reaction,’ says Monteiro. ‘But as soon as we added even a small amount of metal cation, we suddenly saw the reaction start up again.’ With the help of theoreticians from the Institute of Chemical Research of Catalonia, the researchers were also able to unravel the underlying mechanism. Monteiro: ‘The calculations show that the cations interact with the oxygen atom of the CO2, and this ensures that the reaction on the electrode can take place.’
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With this knowledge in mind, the chemists could work on a larger scale application of the process. They did this in collaboration with the chemical company Avantium, and with a slightly different set-up.
Instead of the normal gold electrode, they used a gas diffusion electrode, which forms a barrier with the water-like electrolyte on one side and CO2 gas on the other. ‘This is interesting for industry, because you can then let the CO2 gas flow past the electrode and thus produce CO continuously,’ says Monteiro.
Efficient, without by-products
The process proved to work well, and also reacted as expected to various cations. The researchers managed to produce CO with almost no by-products. Monteiro: ‘And we were also able to improve the energy efficiency by thirty per cent because we use an acidic electrolyte. This means that less sustainable electricity is needed to run the process.’
Monteiro is pleased with the findings and hopes that the knowledge gained can have a real impact in due course: ‘I am pleased that we can use this fundamental knowledge to improve the process on a larger scale and make it more sustainable. Hopefully, we can make a real difference.’
100 square centimetres
But the PhD candidate does not expect the system to be usable immediately: ‘We are now working with an electrode that is about 10 square centimetres in size. For the industry, an electrode of at least a 100 square centimetres is needed, so there is still some work to be done. We are bound to encounter new problems during the upscaling process, but if we work together with engineers, it should be possible.’
Papers
- Mariana C.O. Montero et al. Absence of CO2 electroreduction on copper, gold and silver electrodes without metal cations in solution, Nature Catalysis (26 July 2021)
- Mariana C.O. Montero et al. Efficiency and selectivity of CO2 reduction to CO on gold gas diffusion electrodes in acidic media, Nature Communications (16 August 2021)
Text: Renée Moezelaar