Utilization of CO₂ as Energy Resources

– Electrochemical Bio-Conversion of Carbon Dioxide into Methane

Abstract

We are proposing a sustainable carbon cycle system which gives a solution not only to mitigate global warming but also to supply a carbon-neutral energy resource. Carbon Capture and Storage technology could become a practical countermeasure to reduce emission of the greenhouse gas. Depleted petroleum reservoirs and aquifer have been proposed as candidate sites of CCS. The long-term aim of this research is to establish a bio-technological system to convert geologically-stored CO₂ into methane, as energy resources.

To develop a means for the conversion, we focus on technological application of a bio-electrochemical system using microbial catalyzed electrode （bio-cathode）. On the surface of bio-cathode, methanogenic microorganisms utilize electrons to convert CO₂ to methane. Such system is an attractive option for energy conversion, as the bio-cathode yields methane from electrical current, which can be provided by renewable energy sources. In other words, intermittent electrical energy provided by, for instance, wind turbines and solar cells can be stored in a stable energy form, methane.

Toward technological application of the electro-methanogenic system, we examined electro-methanogenic activity of subsurface microorganisms. Indigenous microorganisms originated from a domestic oil reservoir were inoculated into bio-electrochemical reactor cells. Upon application of constant voltage of -0.70 V, the reactors produced methane at a rate of 1,100 mmol/day・m^－2（ cathode surface area）, which was the highest electro-methanogenic production rate. Moreover, current-to-methane conversion ef?ciency was almost 98%.

Thus, we concluded microorganisms indigenous to the subsurface reservoir are highly capable of electromethanogenic conversion of CO₂. Electrochemical and microbial analyses suggested a reaction mechanism, in which electron-releasing bacteria mediated electron transfer from the electrode to methanogenic archaea. These outcomes imply the possibility of electro-methanogenesis in subsurface CO₂ storage reservoirs. For further enhancement of the electromethanogenic activity, we are currently biotechnologically improving the biocatalysts as well as optimizing con?guration of the reactor system for subsurface reservoirs.