Purposely Designed Hierarchical Porous Electrodes for High Rate Microbial Electrosynthesis of Acetate from Carbon Dioxide

Abstract

Carbon-based products are crucial to our society, but their production from fossil-based carbon is unsustainable. Production pathways based on re-use of CO2 will achieve ultimate sustainability. Furthermore, the costs of renewable electricity production are decreasing at such a high rate, that electricity is expected to be the main energy carrier from 2040 onwards. Electricity-driven novel processes that convert CO2 into chemicals need to be further developed. Microbial electrosynthesis is a biocathode-driven process in which electroactive microorganisms derive electrons from solid-state electrodes to catalyse the reduction of CO2 or organics and generate valuable extracellular multicarbon reduced products. Microorganisms can be tuned to high-rate and selective product formation. Optimization and up-scaling of microbial electrosynthesis to practical, real life applications is dependent upon performance improvement while maintaining low cost. Extensive biofilm development, enhanced electron transfer rate from solid-state electrodes to microorganisms and increased chemical production rate require optimized microbial consortia, efficient reactor designs, and improved cathode materials. This Account is about the development of different electrode materials purposely designed for improved microbial electrosynthesis: NanoWeb-RVC and EPD-3D. Both type of electrodes are biocompatible, highly conductive three-dimensional hierarchical porous structures. Both chemical vapour deposition (CVD) and electrophoretic deposition were used to grow homogeneous and uniform carbon nanotubes layers on the honeycomb structure of reticulated vitreous carbon. The high surface area to volume ratio of these electrodes 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 high coulombic efficiencies. Fully autotrophic biofilms from mixed-cultures developed on both type of electrodes relying on CO2 as the sole carbon source and the solid-state-electrode as the unique energy supply.We present first the synthesis and characteristics of the bare electrodes. We then report the outstanding performance indicators of these novel biocathodes: current densities up to -200 A m 2, and acetate production rates up to 1330 g m-2 day-1, with electron and CO2 recoveries into acetate very close to 100 % for mature biofilms. The performance indicators are still amongst the highest reported by either purposely designed or commercially available biocathodes. Finally, we made use of the Titration and off-gas analysis sensor (TOGA) to elucidate the electron transfer mechanism in these efficient biocathodes. Planktonic cells in the catholyte were found irrelevant for acetate production. We identified the electron transfer to be mediated by biologically?induced H2. H2 is not detected in the head-space of the reactors , unless CO2 feeding is interrupted or the cathodes sterilized. Thus the biofilm is extremely efficient in consuming the generated H2. Finally, we successfully demonstrated the use of a synthetic biogas mixture as a CO2 source. We thus proved the potential of microbial electrosynthesis for the simultaneous upgrading of biogas, while fixating CO2 via the production of acetate.