Researchers convert CO2 into isobutanol using electricity

Currently, electrical energy generated by various methods is still difficult to store efficiently. Chemical batteries, hydraulic pumping and water splitting suffer from low-energy-density storage or incompatibility with current transportation infrastructure.

In a study published on 30 March in the journal Science, James Liao, UCLA’s Ralph M Parsons Foundation Chair in Chemical Engineering, and his team report a method for storing electrical energy as chemical energy in higher alcohols, which can 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,’ Liao said in a statement. ‘In addition, we have the potential to use electricity as transportation fuel without needing to change current infrastructure.’

Liao and his team are said to have genetically engineered a lithoautotrophic micro-organism (Ralstonia eutropha H16) to produce isobutanol and 3-methyl-1-butanol in an electro-bioreactor using CO₂ as the sole carbon source and electricity as the sole energy input.

Photosynthesis is the process of converting light energy into chemical energy and storing it in the bonds of sugar. It involves two steps: a light reaction and a dark reaction. The light reaction converts light energy into chemical energy and must take place in the light. The dark reaction converts CO₂ to sugar and doesn’t 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 CO₂ fixation to produce the fuel,’ Liao said. ‘This method could be more efficient than the biological system.’

Liao said that with biological systems the plants used require large areas of agricultural land. However, because Liao’s method does not require the light and dark reactions to take place together, solar panels, for example, can be built in the desert or on rooftops.

Theoretically, the hydrogen generated by solar electricity can drive CO₂ conversion in lithoautotrophic micro-organisms engineered to synthesise high-energy-density liquid fuels. But the low solubility, the low mass-transfer rate and the safety issues surrounding hydrogen limit the efficiency and scalability of such processes. Instead, Liao’s team reportedly found formic acid to be a favourable substitute and efficient energy carrier.

‘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 electrochemical formate production and the biological CO₂ fixation and higher alcohol synthesis now open up the possibility of the electricity-driven bioconversion of CO₂ to a variety of chemicals. In addition, the transformation of formate into liquid fuel will also play an important role in the biomass refinery process, according to Liao.

‘We’ve demonstrated the principle, and now we think we can scale up,’ he said.