A novel carbon nanotube modified scaffold as an efficient biocathode material for improved microbial electrosynthesis

We report on a novel biocompatible, highly conductive three-dimensional cathode manufactured by direct growth of flexible multiwalled carbon nanotubes on reticulated vitreous carbon (NanoWeb-RVC) for the improvement of microbial bioelectrosynthesis (MES). NanoWeb-RVC allows for an enhanced bacterial attachment and biofilm development within its hierarchical porous structure. 1.7 and 2.6 fold higher current density and acetate bioproduction rate normalized to total surface area were reached on NanoWeb-RVC versus a carbon plate control for the microbial reduction of carbon dioxide by mixed cultures. This is the first study showing better intrinsic efficiency as biocathode material of a three-dimensional electrode versus a flat electrode: this comparison has been made considering the total surface area of the porous electrode, and not just the projected surface area. Therefore, the improved performance is attributed to the nanostructure of the electrode and not to an increase in surface area. Unmodified reticulated vitreous carbon electrodes lacking the nanostructure were found unsuitable for MES, with no biofilm development and no acetate production detected. The high surface area to volume ratio of the macroporous RVC maximizes the available biofilm area while ensuring effective mass transfer to and from the biofilm. The nanostructure enhances the bacteria-electrode interaction and microbial extracellular electron transfer. When normalized to projected surface area, current densities and acetate production rates of 3.7 mA cm-2 and 1.3 mM cm-2 d-1, respectively, were reached, making the NanoWeb-RVC an extremely efficient material from an engineering perspective as well. These values are the highest reported for any MES system to date

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

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.Fil: Flexer, Victoria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigación y Desarrollo en Materiales Avanzados y Almacenamiento de Energía de Jujuy - Universidad Nacional de Jujuy. Centro de Investigación y Desarrollo en Materiales Avanzados y Almacenamiento de Energía de Jujuy - Gobierno de la Provincia de Jujuy. Centro de Investigación y Desarrollo en Materiales Avanzados y Almacenamiento de Energía de Jujuy; ArgentinaFil: Jourdin, Ludovic. Delft University of Technology; Países Bajo

Critical biofilm growth throughout unmodified carbon felts allows continuous bioelectrochemical chain elongation from CO₂ up to caproate at high current density

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 melectrode -3 (-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

Bringing high-rate, CO2-based microbial electrosynthesis closer to practical implementation through improved electrode design and operating conditions

The enhancement of microbial electrosynthesis (MES) of acetate from CO2 to performance levels that could potentially support practical implementations of the technology must go through the optimization of key design and operating conditions. We report that higher proton availability drastically increases the acetate production rate, with pH 5.2 found to be optimal, which will likely suppress methanogenic activity without inhibitor addition. Applied cathode potential as low as 1.1 V versus SHE still achieved 99% of electron recovery in the form of acetate at a current density of around 200 A m(-2). These current densities are leading to an exceptional acetate production rate of up to 1330 g m(2) clay at pH 6.7. Using highly open macroporous reticulated vitreous carbon electrodes with macropore sizes of about 0.6 mm in diameter was found to be optimal for achieving a good balance between total surface area available for biofilm formation and effective mass transfer between the bulk liquid and the electrode and biofilm surface. Furthermore, we also successfully demonstrated the use of a synthetic biogas mixture as carbon dioxide source, yielding similarly high MES performance as pure CO2. This would allow this process to be used effectively for both biogas quality improvement and conversion of the available CO2 to acetate

Continuous long-term bioelectrochemical chain elongation to butyrate

We demonstrate here the long-term continuous bioelectrochemical chain elongation from CO2 and acetate by using a mixed microbial culture. The role of applied current (3.1 vs. 9.3 A m−2) on the performance was investigated. The main product was n-butyrate which was continuously produced over time. Trace amounts of propionate and n-caproate were also produced, but no alcohols were detected during the whole course of the experiment (163 days). Microbial electrosynthesis (MES) systems controlled with more current (9.3 Am−2) showed a butyrate concentration that was 4.5 times higher (maximum 0.59 g L−1) and increased volumetric production rates (0.54 g L−1 day−1) compared to the low-current reactors (0.12 g L−1 day−1), at 58.9 and 71.6 % electron recovery, respectively. Biocatalytic activity of the microbial consortia was demonstrated. This study revealed that the solid-state electrode does control the chain elongation reaction as an essential electron donor and determines the performance of MES systems. This study highlights MES as a promising alternative for acetate upgradin

Concentration-dependent effects of nickel doping on activated carbon biocathodes

This study aims to investigate the effect of nickel electrocatalyst addition on an activated carbon (AC) biocathode. Three types of electrodes were tested with abiotic and biological electrochemical experiments: Control AC electrodes without nickel; AC electrodes with 0.01% wt. metal loading; and AC electrodes with 5% wt. metal loading. High metal loading resulted in measurable improvement of electrocatalytic H2 production, compared to the other 2 electrodes, which achieved a similar electron recovery to H2. Higher acetate production was achieved with increasing metal loading. Microbial growth in the electrolyte and on the electrode was influenced by the addition of nickel at both loadings, but the effect was not concentration-dependent. Each electrode resulted in unique microbial community composition in the electrolyte. Evidently, nickel can affect the biocathode performance due to both catalytic and non-catalytic effects

Concentration-dependent effects of nickel doping on activated carbon biocathodes

This study aims to investigate the effect of nickel electrocatalyst addition on an activated carbon (AC) biocathode. Three types of electrodes were tested with abiotic and biological electrochemical experiments: Control AC electrodes without nickel; AC electrodes with 0.01% wt. metal loading; and AC electrodes with 5% wt. metal loading. High metal loading resulted in measurable improvement of electrocatalytic H2 production, compared to the other 2 electrodes, which achieved a similar electron recovery to H2. Higher acetate production was achieved with increasing metal loading. Microbial growth in the electrolyte and on the electrode was influenced by the addition of nickel at both loadings, but the effect was not concentration-dependent. Each electrode resulted in unique microbial community composition in the electrolyte. Evidently, nickel can affect the biocathode performance due to both catalytic and non-catalytic effects

Autotrophic hydrogen-producing biofilm growth sustained by a cathode as the sole electron and energy source

It is still unclear whether autotrophic microbial biocathode biofilms are able to self-regenerate under purely cathodic conditions without any external electron or organic carbon sources. Here we report on the successful development and long-term operation of an autotrophic biocathode whereby an electroactive biofilm was able to grow and sustain itself with CO2 as a sole carbon source and using the cathode as electron source, with H2 as sole product. From a small inoculum of 15mgCOD (in 250mL), containing 30.3% Archaea, the bioelectrochemical system operating at -0.5V vs. SHE enabled an estimated biofilm growth of 300mg as COD over a period of 276days. A dramatic change in the microbial population was observed during this period with Archaea disappearing completely