Bacteria coated electrodes lessen cost of CO2 conversion

Carbon is needed to make products such as clothing, perfume, jet fuel, concrete and plastic, but recycling CO₂ typically requires that it be separated from other gasses which is costly.

Now, new kinds of electrodes, enhanced with a coating of bacteria, can circumvent that step. While conventional metal electrodes react with sulphur, oxygen and other components of air and flue gasses, the bacteria appear less sensitive to them. The work is detailed in Environmental Science Nano.

"The microbes on these electrodes, or biocatalysts, can use smaller concentrations of CO₂ and seem more robust in terms of handling impurities when compared with electrodes that use metal catalysts," Joshua Jack, U-M assistant professor of civil and environmental engineering and first author of the paper said in a statement.

"Platforms that use metals seem to be much more sensitive to impurities and often need higher CO₂ concentrations to work. So if you wanted to take CO₂ directly out of power plants' emissions, the biotic catalyst may be able to do it with minimal clean-up of that gas."

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Because CO₂ is one of the most stable molecules, getting the carbon away from the oxygen takes a lot of energy, delivered in the form of electricity. Metal electrodes take off one of the oxygen atoms, resulting in carbon monoxide, which can be fed into further reactions to make useful chemicals, but other molecules can react with those electrons too.

According to U-M, the microbes can be more targeted: they not only work together to remove oxygen, but with help from electrons provided by the electrode, they also begin building the carbon into more complex molecules. 

To assess the potential cost savings from using biocatalysts to skip the gas separation step, Jack's team analysed data from previous studies, establishing efficiency rates for converting different waste gasses containing CO₂.

They then used that data to assess the carbon footprint and production costs for various CO₂-derived products.

The results showed that using renewable electricity with a concentrated CO₂ source, without gas separation, allows for the lowest carbon footprint and most cost-competitive products.

This scenario is possible only for especially clean and concentrated CO₂ sources, such as from fermentation at bioethanol plants. Separating CO₂ from flue gasses at fossil fuel burning operations can cost $40 to $100 per ton of CO₂. For exceptionally dilute sources such as regular air, the cost can reach $300 to $1,000 per ton.

The analysis showed that by using waste gasses or air directly, recycling CO₂ from dilute sources could become economically viable.

"Our hope is to accelerate the scalability of CO₂ conversion technologies to mitigate climate change and improve carbon circularity," said Jack. "We want to rapidly decarbonise energy and now even the chemical industry, in a much faster timeframe."