Establishing a sustainable methanogenic carbon dioxide reduction in bioelectrochemical systems and identification of kinetic and thermodynamic constraints.

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Bioelectrochemical systems allow for the storage of electric energy in form of methane (CH4), which is a transportable gas that can be benefit from pre-existent natural gas infrastructure. In these biological reactors, a cathode provides electrons for the carbon dioxide (CO2) reduction into methane (CH4) through an anaerobic microbiome. The objectives of this proposal can be divided in two lines: i) development and investigation of Fe4.5Ni4.5S8 electrodes, which mimic the catalytic properties of important enzymes of the methanogenic processes; and ii) use of carbon isotope analysis to support a comprehensive process analysis and simulation of the CH4 formation from CO2 in bioreactors. The hypothesis for the studies on the kinetic isotope effects is that in the CH4 formation using direct electron transfer pathways the 13C fractionation is dependent on the available free energy for the methanogenic metabolism, analogously to the hydrogenotrophic methanogenesis. For instance, a variable 13C fractionation is observed in autotrophic CO2-fixation processes through bacteria, archaea and algae. Hence, if we confirm this hypothesis we can develop a model framework using 13C analysis data for detailed description of product formation yields with thermodynamic dependent growth kinetics and detailed calculation of the stable carbon isotope fractionation. This model should be valid for the methanogenic CO2 reduction route using different electron donors. Therefore, gas diffusion cathodes will be applied to allow for an immediate change in the electron donor source, i.e. gassing of hydrogen (H2) or providing an electric current. Ultimately, through the 13C-based thermodynamic diagnosis, ideal conditions for comparing the novel Fe4.5Ni4.5S8 against benchmark electrodes will be provided. We expect that the hydrogenase and CO-dehydrogenase functional mimics from the Fe4.5Ni4.5S8 electrodes can enhance the methanogenic CO2-reduction, which might be directly inferred by lowered thermodynamic constraints. The investigations will include parallel biological experiments with open microbiomes and pure cultures. Assays with methanogens from the Methanothrix genus holds the promise to allow for a direct determination of 13C fractionation for H2-free methanogenesis from CO2 as they can perform the CO2 reduction only through direct electron transfer mechanisms.