At a set potential of less than -0.7 V (vs Ag/AgCl), carbon dioxide was reduced to methane using a two-chamber electrochemical reactor containing an abiotic anode, a biocathode, and no precious metal catalysts. At -1.0 V, the current capture efficiency was 96%. Electrochemical measurements made using linear sweep voltammetry showed that the biocathode substantially increased current densities compared to a plain carbon cathode where only small amounts of hydrogen gas could be produced. Both increased current densities and very small hydrogen production rates by a plain cathode therefore support a mechanism of methane production directly from current and not from hydrogen gas. The biocathode was dominated by a single Archaeon, Methanobacterium palustre. When a current was generated by an exoelectrogenic biofilm on the anode growing on acetate in a single-chamber MEC, methane was produced at an overall energy efficiency of 80% (electrical energy and substrate heat of combustion).

—Cheng et al. (2009)

Methanogenic microorganisms do produce methane in marshes and dumps, but scientists thought that the organisms turned hydrogen or organic materials, such as acetate, into methane. However, while trying to produce hydrogen in microbial electrolysis cells, the researchers found that the cells produced much more methane than expected.

We were studying making hydrogen in microbial electrolysis cells and we kept getting all this methane. We may now understand why...All the methane generation going on in nature that we have assumed is going through hydrogen may not be. We actually find very little hydrogen in the gas phase in nature. Perhaps where we assumed hydrogen is being made, it is not.

—Bruce Logan, Kappe Professor of Environmental Engineering

Microbial electrolysis cells do require an electrical voltage to be added to the voltage that is produced by bacteria using organic materials to produce current that evolves into hydrogen. The researchers found that Archaea, using about the same electrical input, could use the current to convert carbon dioxide and water to methane without any organic material, bacteria or hydrogen usually found in microbial electrolysis cells.

Logan, working with Shaoan Cheng, senior research associate; Defeng Xing, post doctoral researcher, and Douglas F. Call, graduate student, environmental engineering, confirmed that the microscopic organisms produced the methane. The researchers created a two-chambered cell with an anode immersed in water on one side of the chamber and a cathode in water, inorganic nutrients and carbon dioxide on the other side of the chamber. They applied a voltage, but recorded only a minute current. The researchers then coated the cathode with the biofilm of Archaea and not only did current flow in the circuit, but the cell produced methane.

The process does not sequester carbon, but it does turn carbon dioxide into fuel, said Logan. “If the methane is burned and carbon dioxide captured, then the process can be carbon neutral.”

Logan suggests the method for off peak capture of renewable energy in a portable fuel. Methane is preferred over hydrogen because a large portion of the US infrastructure is already set up to easily transport and deliver methane.

The National Science Foundation and Air Products and Chemicals, Inc. supported this project.

Resources

• Shaoan Cheng, Defeng Xing, Douglas F. Call and Bruce E. Logan (2009) Direct Biological Conversion of Electrical Current into Methane by Electromethanogenesis. Environ. Sci. Technol., Article ASAP