(398p) Bioelectrosynthesis of Methane from CO2 for Energy Storage

The use of electricity to drive biologically-mediated reactions (bioelectrosynthesis) is a promising technique for CO2 utilization. Unlike most industrial electrochemical means to convert CO2, bioelectrosynthesis reactions like microbial methanogensis have been shown to have high electron capture efficiency (~90%), and overall energy efficiency (~80%) (Cheng et al., 2009). Bioelectrosynthesis also has the potential to produce a highly specific product stream with limited side reactions.

As we enter an era of abundant and intermittent renewable energy, the conversion of CO2 to fuels at the utility scale is especially desirable. Low-cost electricity during peak production hours can be used to generate fuels for distribution or for storage and later use for off-peak power generation. If the CO2 comes from biological sources or the atmosphere, the resulting fuel can be sold at a premium under Low Carbon Fuel Standards like California’s.

Bioelectrosynthesis is being pursued for conversion of CO2 to fuels or chemicals such as hydrogen peroxide or acetate, but the production of methane has several advantages. Methane (as natural gas) is already used as a transportation fuel and has an enormous transportation and distribution network in place. It can be inexpensively stored at large scale in underground reservoirs and can be converted back into electricity with relatively high efficiency at existing natural gas power plants.

An industrially viable scheme for bioelectrosynthesis requires a reactor with high throughput, low overpotential, and a stable biocatalyst. A recent report by Deutzmann et al. (2015) lends significant promise to bioelectrosynthetic methane production. They identified extracellular enzymes that mediate electron transfer from graphite electrodes to methanogenic microbes. The efficiency and stability (> 1 month) of these biocatalysts open a wide range of possibilities for innovative reactor design.

A challenge common among bioelectrosynthetic systems is the need to optimize the “triple junction,” where electrons, protons, and substrate are all supplied to the catalyst site while products are removed away before they can further react. The catalyst, in turn, should be supported on a high-surface-area (conductive) material. The biocatalysts reported by Deutzmann et al. have so far only been demonstrated on planar electrodes. However, with extracellular enzymes to mediate electron transfer, a porous electrode can provide radically higher surface area and volumetric throughput.

In this research, we test the activity of Methanococcus maropaludis cells and extracts in a variety of cutting-edge carbon aerogel electrodes. These custom-synthesized, electrically-conductive aerogels have controlled pore network sizes and structures yielding surface areas up to 500x106 m2/m3, 100 times more than what has been used previously in bioelectrosynthesis (Zhu et al., 2015). The aerogels can be 3D-printed into innovate reactor structures that create flow channels for mass transfer while limiting distances at the triple junction.

Here, we report the potential for improved reactor throughput using the combination of enzyme-mediated electron transfer and carbon aerogel electrodes for methane production. The results have implications for reactor design with bioelectrosynthesis and for the viability of CO2 to methane conversion for energy storage.

References

Cheng, S., Xing, D., Call, D. F. & Logan, B. E. Direct Biological Conversion of Electrical Current into Methane by Electromethanogenesis. Environ. Sci. Technol. 43, 3953–3958 (2009).

Deutzmann, J. S., Sahin, M. & Spormann, A. M. Extracellular Enzymes Facilitate Electron Uptake in Biocorrosion and Bioelectrosynthesis. mBio 6, e00496–15 (2015).

Zhu, C. et al. Highly compressible 3D periodic graphene aerogel microlattices. Nat. Commun. 6, 6962 (2015).