Nabin Aryal

Performance of different Sporomusa species for the microbial electrosynthesis of acetate from carbon dioxide

Sporomusa ovata DSM-2662 produces high rate of acetate during microbial electrosynthesis (MES) by reducing CO2 with electrons coming from a cathode. Here, we investigated other Sporomusa for MES with cathode potential set at -690mVvsSHE to establish if this capacity is conserved among this genus and to identify more performant strains. S. ovata DSM-2663 produced acetate 1.8-fold faster than S. ovata DSM-2662. On the contrary, S. ovata DSM-3300 was 2.7-fold slower whereas Sporomusa aerivorans had no MES activity. These results indicate that MES performance varies among Sporomusa. During MES, electron transfer from cathode to microbes often occurs via H2. To establish if efficient coupling between H2 oxidation and CO2 reduction may explain why specific acetogens are more productive MES catalysts, the metabolisms of the investigated Sporomusa were characterized under H2:CO2. Results suggest that other phenotypic traits besides the capacity to oxidize H2 efficiently are involved.

An overview of cathode materials for microbial electrosynthesis of chemicals from carbon dioxide

The applicability of microbial electrosynthesis (MES) for chemical synthesis from carbon dioxide (CO2) requires improved production and energetic efficiencies. Microbial catalysts, electrode materials, and reactor design are the key components which influence the functioning of such processes. In particular, cathode materials critically impact the electricity-driven CO2 reduction process by microorganisms. Interest in cathode surface modifications for improving MES processes is thus consistently increasing. In this paper, the recent developments and spatial modification of cathode materials for microbial CO2 reduction are systematically reviewed. The characteristics of commercially available materials, their modifications, and developments in new materials that have been used as cathodes for MES are summarized. Key cathode–microorganism interactions that led to improved CO2 conversion are then discussed. The cathode surface modification approaches have focused mainly on improving the surface area and surface chemistry of the materials. Although the modified cathode surfaces improved biofilm growth in direct electron uptake based bioconversions, they have achieved lower acetate production rates than that of hydrogen-based MES processes thus far. Research efforts on different materials suggest that the three-dimensional cathodes that can retain more biomass, in particular in hydrogen-based bioconversions, are promising for further improvements in production efficiencies. Further efforts toward reducing the energy inputs for achieving energetically efficient MES processes by using electrocatalytically efficient cathodes are needed.

Multifactorial evaluation of the electrochemical response of a microbial fuel cell

A lab-scale microbial fuel cell (MFC) with a reticulated vitreous carbon (RVC) anode and a non-catalyzed multi-layered carbon air-cathode was electrochemically characterized under various physicochemical factors: temperature (15–25 °C), phosphate buffer concentration (4–8 mM), acetate concentration (7.1–14.3 mM), and equivalent solution conductivity (2.5–5 mS cm−1). A fundamental step was undertaken to identify and characterize the electrochemical mechanisms through multifactorial evaluation of the simultaneous effect of such factors on the functioning of the MFC. This type of analysis of cyclic voltammetry and impedance spectroscopy parameters revealed complementary features to model the electrochemical response. This multifactorial approach finds broad application in a wide variety of MFC and environmental technology studies.

Freestanding and flexible graphene papers as bioelectrochemical cathode for selective and efficient CO2 conversion

During microbial electrosynthesis (MES) driven CO2 reduction, cathode plays a vital role by donating electrons to microbe. Here, we exploited the advantage of reduced graphene oxide (RGO) paper as novel cathode material to enhance electron transfer between the cathode and microbe, which in turn facilitated CO2 reduction. The acetate production rate of Sporomusa ovata-driven MES reactors was 168.5 ± 22.4 mmol m−2 d−1 with RGO paper cathodes poised at −690 mV versus standard hydrogen electrode. This rate was approximately 8 fold faster than for carbon paper electrodes of the same dimension. The current density with RGO paper cathodes of 2580 ± 540 mA m−2 was increased 7 fold compared to carbon paper cathodes. This also corresponded to a better cathodic current response on their cyclic voltammetric curves. The coulombic efficiency for the electrons conversion into acetate was 90.7 ± 9.3% with RGO paper cathodes and 83.8 ± 4.2% with carbon paper cathodes, respectively. Furthermore, more intensive cell attachment was observed on RGO paper electrodes than on carbon paper electrodes with confocal laser scanning microscopy and scanning electron microscopy. These results highlight the potential of RGO paper as a promising cathode for MES from CO2.

An overview of microbial biogas enrichment

Biogas upgrading technologies have received widespread attention recently and are researched extensively. Microbial biogas upgrading (biomethanation) relies on the microbial performance in enriched H2 and CO2 environments. In this review, recent developments and applications of CH4 enrichment in microbial methanation processes are systematically reviewed. During biological methanation, either H2 can be injected directly inside the anaerobic digester to enrich CH4 by a consortium of mixed microbial species or H2 can be injected into a separate bioreactor, where CO2 contained in biogas is coupled with H2 and converted to CH4, or a combination hereof. The available microbial technologies based on hydrogen-mediated CH4 enrichment, in particular ex-situ, in-situ and bioelectrochemical, are compared and discussed. Moreover, gas-liquid mass transfer limitations, and dynamics of bacteria-archaea interactions shift after H2 injection are thoroughly discussed. Finally, the summary of existing demonstration, pilot plants and commercial CH4 enrichment plants based on microbial biomethanation are critically reviewed.

Highly Conductive Poly(3,4-ethylenedioxythiophene) Polystyrene Sulfonate Polymer Coated Cathode for the Microbial Electrosynthesis of Acetate From Carbon Dioxide

Microbial electrosynthesis (MES) is a bioelectrochemical technology developed for the conversion of carbon dioxide and electric energy into multicarbon chemicals of interest. As with other biotechnologies, achieving high production rate is a prerequisite for scaling up. In this study, we report the development of a novel cathode for MES, which was fabricated by coating carbon cloth with conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) polymer. Sporomusa ovata-driven MES reactors equipped with PEDOT:PSS-carbon cloth cathodes produced 252.5 ± 23.6 mmol d-1 acetate per m2 of electrode over a period of 14 days, which was 9.3 fold higher than the production rate observed with uncoated carbon cloth cathodes. Concomitantly, current density was increased to -3.2 ± 0.8 A m-2, which was 10.7-fold higher than the untreated cathode. The coulombic efficiency with the PEDOT: PSS-carbon cloth cathodes was 78.6 ± 5.6%. Confocal laser scanning microscopy and scanning electron microscopy showed denser bacterial population on the PEDOT:PSS-carbon cloth cathodes. This suggested that PEDOT:PSS is more suitable for colonization by S. ovata during the bioelectrochemical process. The results demonstrated that PEDOT: PSS is a promising cathode material for MES.