The chemical industry is transitioning to more sustainable and biobased processes. One key element of this transition is coupling energy fluxes and feedstock utilization for optimizing processes, routes and efficiencies. Here, we show for the first time the coupling of the Kolbe electrolysis at the anode with a subsequent microbial conversion of the cathodically produced coproduct hydrogen. Kolbe electrolysis of valeric acid yields the liquid drop-in fuel additive n-octane. Subsequently, the solvent isopropanol is produced by resting Cupriavidus necator cells using gaseous electrolysis products (esp. CO 2 and H 2 ). The resting microbial cells show carbon efficiencies of up to 41 % and Coulombic/ Faradaic efficiencies of 60 % and 80 % for anodic and cathodic reactions, respectively. The implementation of a paired electrolyser resulted in superior process performances with overall efficiencies of up to 64.4 %.

The transition of today's fossil fuel based chemical industry toward sustainable production requires improvement of established production processes as well as development of new sustainable and bio-based synthesis routes within a circular economy. Thereby, the combination of electrochemical and biotechnological advantages in such routes represents one important keystone. For the electrochemical generation of reactants from gaseous substrates such as O 2 or CO 2 , gas diffusion electrodes (GDE) represent the electrodes of choice since they overcome solubility-based mass transport limitations. Within this article, we illustrate the architecture, function principle and fabrication of GDE. We highlight the application of GDE for conversion of CO 2 using abiotic catalysts for subsequent biosynthesis as well as the application of microbial catalysts at GDE for CO 2 conversion. The reduction of oxygen at GDE is summarized for the application of oxygen depolarized cathodes in microbial fuel cells and generation of H 2 O 2 to drive enzymatic reactions. Finally, engineering aspects such as scale-up and the modeling of GDE-based processes are described. This review presents an update on the application of GDE in bio-based production systems and emphasizes their large potential for sustainable development of new pathways in bioeconomy.bioeconomy, C1-biotechnology, CO 2 conversion, electrobiotechnology, gas diffusion electrode, hydrogen peroxide dependent enzymes | INTRODUCTIONThe combination of electrochemical and microbial as well as enzymatic reactions is well-established in the field of biosensors (Bedendi et al., 2022). In bioeconomy in general, this combination is believed to be highly effective to optimize established processes or to setup new production routes (Harnisch & Urban, 2018). Often, the high selectivity of the biocatalysts is combined with a high energy efficiency of the electrochemical reaction step. Common examples are the electrochemical substitution or regeneration of cofactors

The chemical industry is transitioning to more sustainable and biobased processes. One key element of this transition is coupling energy fluxes and feedstock utilization for optimizing processes, routes and efficiencies. Here, we show for the first time the coupling of the Kolbe electrolysis at the anode with a subsequent microbial conversion of the cathodically produced coproduct hydrogen. Kolbe electrolysis of valeric acid yields the liquid drop-in fuel additive n-octane. Subsequently, the solvent isopropanol is produced by resting Cupriavidus necator cells using gaseous electrolysis products (esp. CO 2 and H 2 ). The resting microbial cells show carbon efficiencies of up to 41 % and Coulombic/ Faradaic efficiencies of 60 % and 80 % for anodic and cathodic reactions, respectively. The implementation of a paired electrolyser resulted in superior process performances with overall efficiencies of up to 64.4 %.