Raising the penetration of renewable —an intermittent—sources of energy into the grid will require large scale electrical energy storage and retrieval. However, at present, no existing technology provides such storage and retrieval at a low financial and environmental cost.

A team at Cornell University is now suggesting that engineered electroactive microbes could address many of the limitations of current energy storage technologies by enabling rewired carbon fixation—a process that spatially separates reactions that are normally carried out together in a photosynthetic cell and replaces the least efficient with non-biological equivalents.

If successful, this could allow storage of renewable electricity through electrochemical or enzymatic fixation of carbon dioxide and subsequent storage as carbon-based energy storage molecules including hydrocarbons and non-volatile polymers at high efficiency.

In an open-access paper published in the Journal of Biological Engineering, the team compiles performance data on biological and non-biological component choices for rewired carbon fixation systems and identifies research and engineering challenges.

We think biology plays a significant role in creating a sustainable energy infrastructure. Some roles will be supporting roles and some will be major roles, and we’re trying to find all of those places where biology can work.

—Buz Barstow, senior author and assistant professor of biological and environmental engineering

Adding electrically engineered (synthetic or non-biological) elements could make this approach even more productive and efficient than microbes alone. At the same time, having many options also creates too many engineering choices. The study supplies information to determine the best design based on needs.

We are suggesting a new approach where we stitch together biological and non-biological electrochemical engineering to create a new method to store energy.

—Farshid Salimijazi, first author

Natural photosynthesis already offers an example for storing solar energy at a huge scale, and turning it into biofuels in a closed carbon loop. It captures about six times as much solar energy in a year as all civilization uses over the same time. But, photosynthesis is really inefficient at harvesting sunlight, absorbing less than one percent of the energy that hits photosynthesizing cells.

Electroactive microbes enable replacing biological light harvesting with photovoltaics. These microbes can absorb electricity into their metabolism and use this energy to convert CO₂ to biofuels. The approach shows a lot of promise for making biofuels at higher efficiencies.

Electroactive microbes also allow for the use of other types of renewable electricity to power these conversions. Also, some species of engineered microbes may create bioplastics that could be buried, thereby removing carbon dioxide (a greenhouse gas) from the air and sequestering it in the ground. Bacteria could be engineered to reverse the process, by converting a bioplastic or biofuel back to electricity. These interactions can all occur at room temperature and pressure, which is important for efficiency.

The authors point out that non-biological methods for using electricity for carbon fixation (assimilating carbon from CO₂ into organic compounds, such as biofuels) are starting to match and even exceed microbes’ abilities. However, electrochemical technologies are not good at creating the kinds of complex molecules necessary for biofuels and polymers. Engineered electroactive microbes could be designed to convert these simple molecules into much more complicated ones.

Combinations of engineered microbes and electrochemical systems could greatly exceed the efficiency of photosynthesis. For these reasons, a design that marries the two systems offers the most promising solution for energy storage, according to the authors.

The paper includes performance data on biological and electrochemical designs for carbon fixation. The current study is the first that gathers in one place all of the data needed to make an apples-to-apples comparison of the efficiency of all these different modes of carbon fixation, Barstow said.

In the future, the researchers plan to use the data they have assembled to test out all possible combinations of electrochemical and biological components, and find the best combinations out of so many choices.

The study was supported by Cornell and the Burroughs-Wellcome Fund.

Resources

• Farshid Salimijazi, Erika Parra and Buz Barstow (2019) “Electrical energy storage with engineered biological systems” Journal of Biological Engineering 13:38 doi: 10.1186/s13036-019-0162-7