CO2 Electrolysis and Fermentation

The electrochemical reduction of CO2 and H2O followed by fermentation could lead to an increase on sustainable production of useful chemicals. One of the fields that can benefit from it is the gas fermentation. Gas fermenting microorganisms that fix carbon dioxide (CO2) and carbon monoxide (CO), as anaerobic acetogens, can break the dependency on fossil sources to produce carbon-based materials as they can convert gaseous carbon to high value fuels and chemicals.

Different from other 2G technologies, gas fermentation not only allows for a number of different feedstocks to be utilized but also it uses it in its entirety. An overview of feedstocks and product options for gas fermentation are shown in the picture below1. Lignocellulosic biomass, for example, can be gasified to produce synthesis gas (syngas), a mixture of carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), and nitrogen (N2). This gas can be converted to useful products either by the Fischer-Tropsch process (FTP), which is very capital intensive among other drawbacks, or via gas fermentation, which takes place at low temperatures and atmospheric pressure, allowing for savings in energy and costs2.

Figure 1 - Overview of feedstock and product options for gas fermentation1.

To fix the oxidized carbon contained in syngas sources, gas-fermenting organisms as acetogens require reducing equivalents in the form of electrons to reduce the carbon to acetyl-CoA and further to reduce products such as alcohols. CO and H2 present in syngas can provide these reducing equivalents by oxidation to CO2 and water. Reducing equivalents can also be derived from sources. One approach is to use electricity for water electrolysis to produce H2 and/or CO2 electrolysis, which produces CO3,4.

CO2 represents a more diverse and plentiful waste stream compared to CO. However, metabolism of CO2 requires an energy source to be activated and then serve as both carbon and energy source in the gas fermentation process. In addition, if the electricity required for water or CO2 electrolysis comes from renewable sources, it will not contribute to additional carbon emissions.

Generating organic chemicals from CO2 and water powered by renewable electricity is being called “artificial photosynthesis”. In this process, the Faradaic efficiency – efficiency of electron transfer into products - must be high in terms of converting all the electricity into only the desired products. This favors the involvement of anaerobic microorganisms. A CO2 electrolyzer module operating at high current together with a conventional PV module providing electricity with an energy conversion efficiency of around 20% have already successfully demonstrated the anaerobic conversion of CO, H2, and CO2 from syngas to the desired alcohols butanol and hexanol5. The figure below shows a sketch of these modules.

Figure 2 - Modules used in technical photosynthesis of alcohols from CO2 and H2O5.

Furthermore, the decline in the cost of renewable electricity suggests that the production of reducing power at scale might become more economically interesting6. Such approaches allow for the expansion of the portfolio of products targets and waste streams that can be used, consequently expanding the options available to reduce carbon emissions.

This article is heavily based on the references below:

1. Liew, F. M. et al. Gas Fermentation-A flexible platform for commercial scale production of low-carbon-fuels and chemicals from waste and renewable feedstocks. Frontiers in Microbiology (2016). doi:10.3389/fmicb.2016.00694

2. Liew, F. M. et al. Gas Fermentation-A flexible platform for commercial scale production of low-carbon-fuels and chemicals from waste and renewable feedstocks. Frontiers in Microbiology (2016). doi:10.3389/fmicb.2016.00694

3. Heffernan, J. K. et al. Enhancing CO2-Valorization Using Clostridium autoethanogenum for Sustainable Fuel and Chemicals Production. Front. Bioeng. Biotechnol. (2020). doi:10.3389/fbioe.2020.00204

4. Köpke, M. & Simpson, S. D. Pollution to products: recycling of ‘above ground’ carbon by gas fermentation. Current Opinion in Biotechnology (2020). doi:10.1016/j.copbio.2020.02.017

5. Haas, T., Krause, R., Weber, R., Demler, M. & Schmid, G. Technical photosynthesis involving CO2 electrolysis and fermentation. Nat. Catal. (2018). doi:10.1038/s41929-017-0005-1

6. Glenk, G. & Reichelstein, S. Economics of converting renewable power to hydrogen. Nat. Energy (2019). doi:10.1038/s41560-019-0326-1