The process integrates electrochemical formate production and biological CO₂ fixation and higher alcohol synthesis, opening the possibility of electricity-driven bioconversion of CO₂ to commercial chemicals. The study was funded by a grant from the US Department of Energy’s Advanced Research Projects Agency–Energy (ARPA–E).

A man-made photovoltaic device is relatively efficient in converting sunlight to electricity, but the electrical energy generated is difficult to store. The biological photosystems, on the other hand, are limited by the intrinsic design and biomaterials available, for which no near-term improvements are in sight. One way to circumvent both problems is to link man-made solar cells to biological CO₂ fixation and fuel production. Theoretically, H₂ generated by solar electricity can drive CO₂ fixation in lithoautotrophic microorganisms engineered to synthesize high energy-density liquid fuels. However, the low solubility, low mass transfer rate, and safety issues of H₂ in microbial cultures limit the efficiency and scalability of such processes.

Compared with H₂, formic acid is a favorable energy carrier. Electrochemical production of formic acid from CO₂ and H₂O can achieve relatively high efficiency. Formate is highly soluble and is readily converted to CO₂ and NADH in the cells, providing a safe replacement for H₂. However, the high solubility of formate increases the cost of separation. Accumulated formate will decompose at the anode, decreasing the yield of the process. Therefore, simultaneous electrochemical formate production and biological formate conversion to higher alcohols is desirable. Unfortunately, introduction of electricity to microbial cultures may impede cell growth.

—Li et al.

According to the team, an integrated process for reduction of CO₂ to liquid fuel powered by electricity requires:

1. metabolic engineering of a lithoautotrophic organism to produce liquid fuels,

2. electrochemical generation of formate from CO₂ in fermentation medium, and

3. enabling microbes to withstand electricity.

Liao and his colleagues introduced a set of genes previously identified for the production of isobutanol and 3MB into R. eutropha, and altered the organism to use the heterologous isobutanol and 3MB production pathway as the new metabolic sink for carbon and reducing equivalents. In a pH-coupled formic acid feeding fermentor, the engineered strain produced fuels with the final titer of more than 1.4 g/l (~846 mg/l isobutanol and ~570 mg/l 3MB).

When they placed a cathode and anode in the culture medium to produce formate electrochemically, however, they found the growth the bacteria was inhibited upon introduction of the electric current. When the current stopped, cell growth resumed.

The researchers determined that O₂⁻ and NO in the medium triggered a stress response in Ralstonia, inhibiting growth. They then used a porous ceramic cup to shield the anode. The inexpensive shield provides “a tortuous diffusion path” for chemicals, enabling the reactive compounds produced by the anode to be quenched before reaching the cells growing outside the cup.

With this approach, the engineered Ralstonia produced more than 140 mg/l biofuels were achieved with the electricity and CO₂ as the sole source of energy and carbon, respectively.

The current way to store electricity is with lithium-ion batteries, in which the density is low, but when you store it in liquid fuel, the density could actually be very high. In addition, we have the potential to use electricity as transportation fuel without needing to change current infrastructure. We’ve demonstrated the principle, and now we think we can scale up.

—James Liao

Resources

• Han Li, Paul H. Opgenorth, David G. Wernick, Steve Rogers, Tung-Yun Wu, Wendy Higashide, Peter Malati, Yi-Xin Huo, Kwang Myung Cho, and James C. Liao (2012) Integrated Electromicrobial Conversion of CO₂ to Higher Alcohols. Science 335 (6076), 1596. doi: 10.1126/science.1217643