PEM Electrocatalysis in a Stirred-Tank Bioreactor Enables Growth of Clostridium Ragsdalei with CO2 and Electrons
Authors: Schwarz Irina , Rieck Arielle , Mehmood Asad , Weuster-Botz Dirk , Fellinger Tim-Patrick 
Published: December 2024

Developing sustainable synthesis processes is a primary objective for the chemical industry since harsh process conditions and usage of fossil resources increase greenhouse gas emissions and the carbon footprint. Making use of CO2 as a feedstock to produce value-added organic chemicals is an attractive opportunity to realize a circular carbon economy. Moreover, electric energy generated by solar panels or wind power stations could be saved in stable chemical products. We combined both aspects in a novel bio-electrochemical system (BES) in which a PEM electrolysis cell is coupled to a standard stirred tank bioreactor.
Precious group metal-free (PGM-free) catalysts consisting of atomically dispersed active metal sites in nitrogen-doped porous carbon matrix (M-N-C where M= Co, Ni, Zn etc.) enabled electrocatalytical CO2 reduction to CO (CO2RR) in an aqueous environment. The acetogenic bacterium Clostridium ragsdalei directly converts the CO into organic acids and alcohols. Besides, the competing hydrogen evolution reaction (HER) delivers H2 as an additional electron carrier that can be consumed together with CO2. Bacteria are more flexible towards the stoichiometry of their substrates (CO, H2 and CO2), demonstrating an advantage over chemical synthesis processes that often require exact stoichiometries. Further, mild process conditions in bioprocesses allow for more sustainable production.
The BES consists of a stirred-tank bioreactor adapted to function together with an electrolysis cell. The cathode and anode are stacked in a membrane-electrode-assembly (MEA) mounted at the bottom with the cathode facing the bioreactor (Figure 1). The fully controlled BES is continuously gassed with CO2 and driven in a potentiostatic mode. A Zn/Co-N-C catalyst coated on carbon support was selective for the CO2 reduction to CO (Schwarz et al., 2024). CO and H2 were continuously produced at the cathode in an applied cell voltage range of -2.2 to -3.2 V. At lower applied voltages (≤ -2.5 V), the catalyst was more selective towards CO formation, whereas a higher cell voltage of -3.2 V caused the HER to take over the CO2RR. During electrofermentations, the cell voltage was increased stepwise to supply sufficient carbon and electron sources to C. ragsdalei.
The MEA construction was modified continuously to improve autotrophic batch electrofermentations with C. ragsdalei. Optimizing the compression between the cathode and anode improved the stability of current flows and prolonged the total process time from 3 to 6 days. Increasing the initially low area-to-volume ratio of the cathode with the electrocatalyst (0.0012 cm-1) allowed a higher CO formation rate. CO generation has been limiting the whole electrocatalytical CO2 fermentation process. Furthermore, developing more selective and stable electrocatalysts towards the CO2RR improved the faradaic efficiency. Switching from a two-electrode to a three-electrode setup enabled a more precisely controlled galvanostatic operation with defined CO formation rates sufficient for C. ragsdalei to grow and produce acetate and ethanol in considerable amounts.

Reference

Schwarz I, Rieck A, Mehmood A, Bublitz R, Bongers L, Weuster-Botz D, Fellinger T-P (2024): PEM electrolysis in a stirred-tank bioreactor enables autotrophic growth of Clostridium ragsdalei with CO2 and electrons. ChemElectroChem
11: e202300344.

Acknowledgements

Funding by the German Research Foundation (DFG) within the framework of the priority program SPP 2240 (research projects WE 2715/17-1 and FE 1590/1-1) is gratefully acknowledged.

Figure 1