While electric vehicles have come a long way in the past decade, they still have many disadvantages when compared to internal combustion engine-driven vehicles. The lithium-ion batteries that power electric vehicles have a much lower energy storage density when compared to liquid fuel, they take longer to “refuel,” and they lack the supporting infrastructure that has built up around conventional vehicles over the past century. Now researchers at the UCLA Henry Samueli School of Engineering and Applied Science have developed a process that could allow liquid fuel to be produced using solar generated electricity.
Using only electricity for the energy input, the team was able to convert carbon dioxide into the liquid fuel isobutanol. They did this by genetically engineering a lithoautotrophic microorganism (one that utilizes inorganic compounds as energy sources) known as Ralstonia eutropha H16 to produce 3-methyl-1-butanol and isobutanol, a colorless, flammable liquid that holds potential as an alternative to gasoline in combustion engines. The microorganism was placed in an electro-bioreactor and used only carbon dioxide as the carbon source and electricity as the sole energy input.
The team says storing electrical energy as chemical energy in higher alcohols in this way would allow it to be used as liquid transportation fuels.
"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," said James Liao, UCLA's Ralph M. Parsons Foundation Chair in Chemical Engineering. "In addition, we have the potential to use electricity as transportation fuel without needing to change current infrastructure."
The process developed by the UCLA team carries on from previous success in producing isobutanol by genetically modifying a cyanobacterium to consume carbon dioxide. Like that previous work, this new method also mimics photosynthesis, but with an important difference. Photosynthesis has two parts – a light reaction, which converts light energy to chemical energy, and a dark reaction, which converts CO2 to sugar and doesn’t directly need light to occur.
"We've been able to separate the light reaction from the dark reaction and instead of using biological photosynthesis, we are using solar panels to convert the sunlight to electrical energy, then to a chemical intermediate, and using that to power carbon dioxide fixation to produce the fuel," Liao said. "This method could be more efficient than the biological system."
While the CO2 conversion process in the lithoautotrophic microorganisms could theoretically be driven by hydrogen generated by solar power, the team chose formic acid as a substitute energy carrier due to the low solubility, low mass-transfer rate and the safety issues surrounding hydrogen.
"Instead of using hydrogen, we use formic acid as the intermediary," Liao said. "We use electricity to generate formic acid and then use the formic acid to power the CO2 fixation in bacteria in the dark to produce isobutanol and higher alcohols."
The researchers say the method they have developed also has the potential to allow the bioconversion of CO2 to a variety of chemicals. Additionally, Liao says the transformation of formate into liquid fuel will also play an important role in the biomass refinery process. Now that they’ve proven the process works, they are looking to scale it up.
The team’s study is published in the journal Science.