Atmospheric greenhouse gas content, particularly content of carbon dioxide (CO2), is one of the most discussed topics nowadays in science and throughout society. Rising demand on fuels and increasing atmospheric CO2 content initiate the idea of reducing the greenhouse gas on the one hand and regarding CO2 as chemical feedstock for fuels and chemicals on the other hand.
In this work the biocatalytic and bio electrocatalytic reduction of CO2 is presented. Microorganisms and dehydrogenase enzymes are used as catalysts. One approach shows the application of methanogenic microorganisms, grown on a carbon felt electrode, for microbial electrolysis cells (MEC), converting CO2 to methane by electron uptake. Such microorganisms are known for their ability to convert CO2 together with H2 selectively to methane. However up to now, H2 had to be added artificially. This is not practical for sustainable processes in industrial scale due to the necessity of production, storage and transport of the required H2 to the microbial reactors for CO2 conversion. Here the direct electron uptake of the microorganisms from an electrode without adding H2 artificially is shown for the reduction of CO2 to methane.
In a second approach dehydrogenase enzymes are investigated for being addressed electrochemically. Dehydrogenase enzymes are known from natural processes, were co enzymes like Nicotinamide adenine dinucleotide (NADH) deliver electrons and protons for the reduction of CO2 to hydrocarbons. Nevertheless, such co enzymes are oxidized during such reactions and need to be steadily supplied or regenerated. This limits the technical use of enzymes for CO2 reduction to small scale experiments, due to high costs for the supply of co enzymes and the regeneration of such. In this work the substitution of the co enzyme NADH for the reduction of CO2 using dehydrogenase enzymes is presented. Modification of a carbon felt electrode with an alginate-silicate hybrid gel containing dehydrogenase enzymes enables direct electron injection from the electrode into the enzymes and makes the necessity of NADH redundant. This approach is shown for addressing single dehydrogenase enzymes for the conversion of CO2 to formate and for the conversion of butyraldehyde to butanol as well as for addressing three dehydrogenase enzymes at the same time for the reduction of CO2 to methanol.
Both approaches depict direct electron injection into biocatalytic systems. This substitutes the addition of hydrogen and electron equivalents. Further, such electrochemical methods can possibly be driven by renewable energy sources like solar or wind, which are lacking in transport and storage options. Combining biocatalytic reduction of the greenhouse gas CO2 with e.g. solar or wind driven electrochemical systems, offers the possibility of renewable energy storage in fuels and valuable chemicals.