Expensive gas separation may not be necessary to recycle CO₂ from the air and industrial installations

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Overview of emerging CO2 capture and conversion paths. Proposed process flow for electrified CO2 conversion with selection of the waste gas raw material, omission or inclusion of a gas separation device and choice of CO2 conversion nanotechnology. Credit: Environmental Sciences: Nano (2024). DOI: 10.1039/D3EN00912B

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Overview of emerging CO2 capture and conversion paths. Proposed process flow for electrified CO2 conversion with selection of the waste gas raw material, omission or inclusion of a gas separation device and choice of CO2 conversion nanotechnology. Credit: Environmental Sciences: Nano (2024). DOI: 10.1039/D3EN00912B

A costly step in the process of converting carbon emissions into useful products such as biofuels and pharmaceuticals may not be necessary, according to researchers at the University of Michigan.

The article is published in the magazine Environmental Sciences: Nano.

Carbon dioxide in Earth’s atmosphere is a major driver of climate change, with fossil fuel combustion responsible for 90% of all CO2 emissions. New EPA regulations introduced in April call on fossil fuel plants to reduce their greenhouse gas emissions by 90% by 2039.

Many researchers claim that storing that CO2 would be a shame if carbon is needed for the production of many products we depend on every day, such as clothing, perfume, jet fuel, concrete and plastic. But recycling CO2 typically requires it to be separated from other gases – a process that comes with a price tag that can be prohibitive.

Now new types of electrodes, provided with a layer of bacteria, can skip that step. While conventional metal electrodes react with sulphur, oxygen and other components of air and flue gases, the bacteria appear to be less sensitive to this.

“The microbes on these electrodes, or biocatalysts, can use smaller concentrations of CO2 and appear to be more robust in terms of handling impurities compared to electrodes using metal catalysts,” said Joshua Jack, assistant professor of civil and environmental engineering at UM, and first author of the Environmental Science Nano cover article.

“Platforms that use metals appear to be much more sensitive to impurities and often require higher CO2 concentrations. So if you wanted to take CO2 directly from power plant emissions, the biotic catalyst can potentially do this with minimal cleanup of that gas.”

Because CO2 is one of the most stable molecules. Removing the carbon from the oxygen requires a lot of energy, supplied in the form of electricity. For example, metal electrodes take away one of the oxygen atoms, resulting in carbon monoxide, which can be used in further reactions to make useful chemicals. But other molecules can also react with those electrons.

The microbes, on the other hand, can be much more targeted. Not only do they work together to remove oxygen, but using electrons provided by the electrode, they also begin to build the carbon into more complex molecules.

To assess the potential cost savings from using biocatalysts to skip the gas separation step, Jack’s team analyzed data from previous studies, establishing efficiency rates for converting various waste gases containing CO.2. They then used that data to assess the carbon footprint and production costs for different CO2 emissions2-derivatives.

The results showed that using renewable electricity, such as solar cells, produces concentrated CO2 emissions2 source, without gas separation, ensures the lowest ecological footprint and the most cost-competitive products.

But this ideal scenario is only possible for particularly clean and concentrated CO2 sources, such as from fermentation in bioethanol plants. Separate CO2 from flue gases from burning fossil fuels can cost $40 to $100 per ton of CO2. And for exceptionally dilute sources such as regular air, costs can be as high as $300 to $1,000 per ton.

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

“Our hope is to accelerate the scalability of CO22 conversion technologies to mitigate climate change and improve carbon circularity,” said Jack. “We want to quickly decarbonize energy and now even the chemical industry, in a much faster time frame.”

More information:
Joshua Jack et al., Electrified CO2 valorization in emerging nanotechnologies: an engineering analysis of the purity of gas feedstocks and nanomaterials in electrocatalytic and bioelectrocatalytic CO2 conversion, Environmental Sciences: Nano (2024). DOI: 10.1039/D3EN00912B

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