Microbial electrosynthesis (MES) of organics from carbon dioxide has been recently put forward as an attractive technology for the renewable production of valuable multi-carbon reduced end-products and as a promising CO2 transformation strategy. MES is a biocathode-driven process that relies on the conversion of electrical energy into high energy-density chemicals. However, MES remains a nascent concept and there is still limited knowledge on many aspects.
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 and the cathode as sole carbon and electron source, respectively, with H2 as sole product. From a small inoculum of 15 mgCOD (in 250 mL), the bioelectrochemical system operating at -0.5 V vs. SHE enabled an estimated biofilm growth of 300 mg as COD over a period of 276 days.
A critical aspect is that reported performances of bioelectrosynthesis of organics are still insufficient for scaling MES to practical applications. Selective microbial consortia and biocathode material development are of paramount importance towards performance enhancement. A novel biocompatible, highly conductive three-dimensional cathode was manufactured by direct growth of flexible multiwalled carbon nanotubes on reticulated vitreous carbon (NanoWeb-RVC) by chemical vapour deposition (CVD). The results demonstrated that: (i) 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, and (ii) the nanostructure enhances the bacteria-electrode interaction, biofilm development, microbial extracellular electron transfer, and acetate bioproduction rate.
However, for scale-up beyond certain sizes, there are some limitations with the CVD technique. We harnessed the throwing power of electrophoretic deposition technique (EPD), suitable for industrial scale production, to form multi-walled CNT coatings onto RVC to generate a new hierarchical porous structure, hereafter called EPD-3D. A very effective mixed microbial consortium was successfully enriched and transferred to EPD-3D reactors and demonstrated drastic performance enhancement reaching biocathode current density of -102 ± 1 A m-2 and acetate production rate of 685 ± 30 g m-2 day-1.
An in-depth understanding on how electrons flow from the cathode to the terminal electron acceptor is still missing but crucial (e.g. for improved reactor design). High rates of acetate production by microbial electrosynthesis was shown to occur via biologically-induced hydrogen, likely through the biological synthesis of metal copper particles, with 99 ± 1% electron recovery into acetate. The acetate-producing bacteria showed the remarkable ability to consume a high H2 flux (ca. 1.15 m3H2 m-2 day-1), without H2 able to escape from the biofilm.
Finally, further investigation is needed towards reactor sizing and operating conditions optimization to further enhance the performance and applicability of this technology. The results demonstrated that (i) higher proton availability significantly increases the acetate production rate to 790 g m-2 day-1, at the optimal pH of 5.2 and -0.85 V vs. SHE, which will likely suppress methanogenic activity without inhibitor addition; and (ii) that potentials as low as -1.1 V vs. 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 rates of up to 1330 g m-2 day-1 (133 kg m-3 day-1) at pH 6.7. To achieve such high productivities in practice, the 3D macroporous electrode materials need to also be optimised to achieve a good balance between total surface area per volume available for biofilm formation and effective mass transfer between the bulk liquid and the electrode/biofilm surface. Using highly open, macroporous reticulated vitreous carbon electrodes with pore sizes of about 0.6 mm diameter was found to be optimal in this respect.
High product specificity and production rate are regarded as key parameters for large-scale applicability of a technology. The current density and acetate production rate reached in this study are at least an order of magnitude higher than obtained by any other group, to date. The high product specificity and product titer (11 g L-1) reported are remarkable for mixed microbial cultures, which would make the product’s downstream processing easier and the technology more attractive. Moreover, at zero wastage of H2 gas, it allows superior production rates and lesser technical bottlenecks over technologies that rely on mass transfer of H2 to microorganisms suspended in aqueous solution. Furthermore, we successfully demonstrated the use of a synthetic biogas gas mixture as cheap and readily available carbon dioxide source, yielding similarly high MES performance, which would allow this process to be used effectively for both biogas quality improvement and conversion of the available CO2 to acetate. These findings take microbial electrosynthesis a considerable step forward towards practical implementation.