From CO₂ to Bioplastic – Coupling the Electrochemical CO₂ Reduction with a Microbial Product Generation by Drop‐in Electrolysis

Abstract

CO2 has been electrochemically reduced to the intermediate formate, which was subsequently used as sole substrate for the production of the polymer polyhydroxybutyrate (PHB) by the microorganism Cupriavidus necator. Faradaic efficiencies (FE) up to 54 % have been reached with Sn‐based gas‐diffusion electrodes in physiological electrolyte. The formate containing electrolyte can be used directly as drop‐in solution in the following biological polymer production by resting cells. 56 mg PHB L−1 and a ratio of 34 % PHB per cell dry weight were achieved. The calculated overall FE for the process was as high as 4 %. The direct use of the electrolyte as drop‐in media in the bioconversion enables simplified processes with a minimum of intermediate purification effort. Thus, an optimal coupling between electrochemical and biotechnological processes can be realized.

A Comprehensive Modeling Analysis of Formate‐Mediated Microbial Electrosynthesis**

“…The trade‐off can be avoided by separating electrochemical formate production and biomass growth, as we discuss later, or by using gas diffusion electrodes (GDEs) to minimize the transport distance for CO 2 [45,46] . The latter strategy has been recently reported for bioplastic production using C. necator [14] . Second, volumetric and areal productivities cannot be simultaneously optimized.…”

“…Recently, Stöckl et al. developed a prototype system coupling a GDE‐based CO 2 electrolysis cell to formatotrophic growth and bioplastic production, and demonstrated production directly from the electrolyte used in the electrolysis cell [14] . In general, a two‐reactor system does have some challenges, however, including increased gas and liquid pumping requirements and the increased number of failure or contamination points.…”

“…Several groups have developed prototypical systems for mediated MES, relying on various redox mediators including H 2 , [6–9] inorganic ions (e. g., ferrous ions or ammonia), [10,11] simple organic molecules (e. g., carbon monoxide, formate, and methanol), [12–15] and complex organic molecules such as the dye neutral red [16,17] . Although these prototype systems have been able to achieve high efficiencies (∼10 % in the case of Wang et al [8] …”

Bioelectrochemical engineering analysis of formate-mediated microbial electrosynthesis

“…A simple equation for hydrogenotrophic growth of C. necator using the Calvin cycle is given by Ishizaki et al [37] : (14) We note that the cell stoichiometry measured by Ishizaki et al is slightly different from that used by Grunwald et al [23] In our model, cell mass is a single species that is not broken into its constitutive elements, so we don't account for this difference in our system.…”

“…ferrous ions or ammonia), [10,11] simple organic molecules (e.g. carbon monoxide, formate, and methanol), [12][13][14][15] and complex organic molecules such as the dye neutral red. [16,17] Although these prototype systems have been able to achieve high efficiencies (~10% in the case of Wang et al [8] ), the scalability and potential productivity of these systems remain unclear.…”