Valorization of carbon dioxide by electrocatalytic reduction coupled to acetogens via multiple electron carriers (Ecat-acetogens II)

• Applicants: Dr. Tim-Patrick Fellinger; Professor Dr.-Ing. Dirk Weuster-Botz
• Subject Area: Biological Process Engineering ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 445928567

Acetogenic bacteria produce CO2-based chemicals in aqueous media by hydrogenotrophic conversion of CO2, but CO is the preferred carbon and electron source. Consequently, coupling CO2 electrolysis with bacterial fermentation within an integrated bio-electrocatalytical system (BES) is promising, if CO2 reduction catalysts are available for the generation of CO in the complex biotic electrolyte. In the first funding period, a standard stirred-tank bioreactor was coupled to a zero-gap PEM electrolysis cell for CO2 conversion, allowing potential control and separation of the anode in one single cell. Novel M-N-C electrocatalysts for the cathodic CO2 reduction and the competing hydrogen evolution enabled in-situ feeding of acetogenic bacteria with CO and H2. The proof-of-concept for the designed BES was successfully shown in the first funding period with C. ragsdalei as an example. As compared to other BES, a very low Ohmic cell resistance in the range of Rcell≈1 Ω proves the potential of the coupling of a standard stirred-tank bioreactor to the zero-gap PEM electrolysis cell for CO2 reduction to reach high production rates and energy efficiencies. Besides a required mitigation of electrochemical side reactions, a key challenge is to reduce unassigned voltage losses. The results so far indicate catalyst degradation throughout autoclaving of the BES and an unclear, but harmful impact of the bacterial medium. In addition, the H2- and CO-conversion kinetics of C. ragsdalei could be responsible for the fact that complete conversion of the gases produced at the cathode was not observed even at low H2 production rates in the BES. The objective of the second funding period is: (1) To increase the Faradaic efficiency of the CO2 reduction by modifications of the cathode catalyst to avoid degradation throughout autoclaving and reduce side-reactions. (2) To identify and replace the bacterial medium components showing a harmful impact on the catalyst (or adaption of the catalyst itself). (3) To reduce blocking of the catalyst surface by integration of CO2-sparging directly onto or into the cathode (‘self-cleaning cathode’) and thus further improving the mass transport of CO2 to the gas diffusion electrode. (4) To identify the H2- and CO-conversion kinetics of C. ragsdalei to enable the selection of optimum operating states of the BES with high CO- and H2-conversion rates. (5) To systematically optimize the BES by assessing individual voltage losses (voltage loss analysis). At the end of this project optimal process conditions for the energy efficient electrofermentation of ethanol, acetate, and 2,3 butandiol with C. ragsdalei using CO2 and electrons will be determined. Additionally, the Ecat-acetogens team will contribute to cooperation and knowledge-transfer between the projects of eBiotech, including the exchange of catalysts and the benchmarking of different BES.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2240: Bioelectrochemical and engineering fundamentals to establish electro-biotechnolgy for biosynthesis – Power to value-added products (eBiotech)