CONSPECTUS:Carbon-based products are crucial to our society,but their production from fossil-based carbon is unsustainable.Production pathways based on the reuse of CO2will achieve ultimatesustainability. Furthermore, the costs of renewable electricityproduction are decreasing at such a high rate, that electricity isexpected to be the main energy carrier from 2040 onward. Electricity-driven novel processes that convert CO2into chemicals need to befurther developed. Microbial electrosynthesis is a biocathode-drivenprocess in which electroactive microorganisms derive electrons fromsolid-state electrodes to catalyze the reduction of CO2or organics andgenerate valuable extracellular multicarbon reduced products. Micro-organisms can be tuned to high-rate and selective product formation.Optimization and upscaling of microbial electrosynthesis to practical,real life applications is dependent upon performance improvementwhile maintaining low cost. Extensive biofilm development, enhanced electron transfer rate from solid-state electrodes tomicroorganisms and increased chemical production rate require optimized microbial consortia, efficient reactor designs, andimproved cathode materials.This Account is about the development of different electrode materials purposely designed for improved microbial electrosynthesis:NanoWeb-RVC and EPD-3D. Both types of electrodes are biocompatible, highly conductive three-dimensional hierarchical porousstructures. Both chemical vapor deposition (CVD) and electrophoretic deposition were used to grow homogeneous and uniformcarbon nanotube layers on the honeycomb structure of reticulated vitreous carbon. The high surface area to volume ratio of theseelectrodes maximizes the available surface area for biofilm development, i.e., enabling an increased catalyst loading. Simultaneously,the nanostructure makes it possible for a continuous electroactive biofilm to be formed, with increased electron transfer rate and highCoulombic efficiencies. Fully autotrophic biofilms from mixed cultures developed on both types of electrodes rely on CO2as the solecarbon source and the solid-state electrode as the unique energy supply.We presentfirst the synthesis and characteristics of the bare electrodes. We then report the outstanding performance indicators ofthese novel biocathodes: current densities up to−200 A m−2and acetate production rates up to 1330 g m−2day−1, with electron andCO2recoveries into acetate being very close to 100% for mature biofilms. The performance indicators are still among the highestreported by either purposely designed or commercially available biocathodes. Finally, we made use of the titration and off-gasanalysis sensor (TOGA) to elucidate the electron transfer mechanism in these efficient biocathodes. Planktonic cells in the catholytewere found irrelevant for acetate production. We identified the electron transfer to be mediated by biologically induced H2.H2is notdetected in the headspace of the reactors, unless CO2feeding is interrupted or the cathodes sterilized. Thus, the biofilm is extremelyefficient in consuming the generated H2. Finally, we successfully demonstrated the use of a synthetic biogas mixture as a CO2source.We thus proved the potential of microbial electrosynthesis for the simultaneous upgrading of biogas, whilefixating CO2via theproduction of acetate.