DataSheet1_Biomass-specific rates as key performance indicators: A nitrogen balancing method for biofilm-based electrochemical conversion.docx

Microbial electrochemical technologies (METs) employ microorganisms utilizing solid-state electrodes as either electron sink or electron source, such as in microbial electrosynthesis (MES). METs reaction rate is traditionally normalized to the electrode dimensions or to the electrolyte volume, but should also be normalized to biomass amount present in the system at any given time. In biofilm-based systems, a major challenge is to determine the biomass amount in a non-destructive manner, especially in systems operated in continuous mode and using 3D electrodes. We developed a simple method using a nitrogen balance and optical density to determine the amount of microorganisms in biofilm and in suspension at any given time. For four MES reactors converting CO2 to carboxylates, >99% of the biomass was present as biofilm after 69 days of reactor operation. After a lag phase, the biomass-specific growth rate had increased to 0.12–0.16 days−1. After 100 days of operation, growth became insignificant. Biomass-specific production rates of carboxylates varied between 0.08–0.37 molC molX−1d−1. Using biomass-specific rates, one can more effectively assess the performance of MES, identify its limitations, and compare it to other fermentation technologies.</p

data_sheet_1.PDF

<p>Current challenges for microbial electrosynthesis include the production of higher value chemicals than acetate, at high rates, using cheap electrode materials. We demonstrate here the continuous, biofilm-driven production of acetate (C2), n-butyrate (nC4), and n-caproate (nC6) from sole CO₂ on unmodified carbon felt electrodes. No other organics were detected. This is the first quantified continuous demonstration of n-caproate production from CO₂ using an electrode as sole electron donor. During continuous nutrients supply mode, a thick biofilm was developed covering the whole thickness of the felt (1.2-cm deep), which coincided with high current densities and organics production rates. Current density reached up to −14 kA m⁻³_electrode (−175 A m⁻²). Maximum sustained production rates of 9.8 ± 0.65 g L⁻¹ day⁻¹ C2, 3.2 ± 0.1 g L⁻¹ day⁻¹ nC4, and 0.95 ± 0.05 g L⁻¹ day⁻¹ nC6 were achieved (averaged between duplicates), at electron recoveries of 60–100%. Scanning electron micrographs revealed a morphologically highly diverse biofilm with long filamentous microorganism assemblies (~400 μm). n-Caproate is a valuable chemical for various industrial application, e.g., it can be used as feed additives or serve as precursor for liquid biofuels production.</p

High Acetic Acid Production Rate Obtained by Microbial Electrosynthesis from Carbon Dioxide

High product specificity and production rate are regarded as key success parameters for large-scale applicability of a (bio)­chemical reaction technology. Here, we report a significant performance enhancement in acetate formation from CO₂, reaching comparable productivity levels as in industrial fermentation processes (volumetric production rate and product yield). A biocathode current density of −102 ± 1 A m^–2 and an acetic acid production rate of 685 ± 30 (g m^–2 day^–1) have been achieved in this study. High recoveries of 94 ± 2% of the CO₂ supplied as the sole carbon source and 100 ± 4% of electrons into the final product (acetic acid) were achieved after development of a mature biofilm, reaching an elevated product titer of up to 11 g L^–1. This high product specificity is remarkable for mixed microbial cultures, which would make the product downstream processing easier and the technology more attractive. This performance enhancement was enabled through the combination of a well-acclimatized and enriched microbial culture (very fast start-up after culture transfer), coupled with the use of a newly synthesized electrode material, EPD-3D. The throwing power of the electrophoretic deposition technique, a method suitable for large-scale production, was harnessed to form multiwalled carbon nanotube coatings onto reticulated vitreous carbon to generate a hierarchical porous structure