A group of microbes called chemolithoautotrophs consumes CO 2  through their natural metabolism, spitting out small organic molecules as a byproduct. A  new concept  developed by Pascal Saikaly and his team at KAUST shows that these microbes could be enlisted to convert industrial CO 2  emissions into valuable chemicals.

Chemolithoautotrophs are one of those microbes which could become key allies in global efforts to curb carbon emissions and avoid dangerous climate change . These are commonly found in the deep sea, in caves and hydrothermal vents, where conventional energy sources, such as sunlight and organic carbon, are lacking.

"The microbes obtain their energy from the oxidation of inorganic compounds , such as hydrogen, iron, and sulfur," explains Bin Bian. The microbes strip the inorganic compounds of electrons while taking up CO 2  and reducing it to organic products as part of the process.

To harness chemolithoautotroph capabilities for recycling CO 2  emissions into useful chemicals, supply electrons to the microbes in a process called microbial electrosynthesis (MES). Typically, MES reactors have grown chemolithoautotrophs on a submerged flat-sheet cathode and bubbled CO 2  gas into the solution, but this setup has two key limitations. Flat-sheet cathodes are difficult to scale up and CO 2  gas has poor solubility.

A porous cylindrical electrode to inhale CO 2

The team developed an alternative MES reactor using cathodes made from stackable, cylindrical porous nickel fibers that Saikaly's group had previously applied to recover water and energy from wastewater. CO 2  is pumped through each cylinder, and electrons flow along it.

"Using this architecture, we directly deliver CO 2  gas to chemolithoautotrophs through the pores in the hollow fibers," Alqahtani says. "We provided electrons and CO 2  simultaneously to chemolithoautotrophs on the cathode surface."

In an initial study, methane-producing microbes were able to convert CO 2  to methane with 77 percent efficiency. A follow-up study improved performance further by coating the electrodes with carbon nanotubes. These offered a more biocompatible surface for microbial growth and improved the hollow fibers' CO 2  adsorption capability 11-fold. Additionally, the nanotubes enhanced the electron transfer from the electrode to chemolithoautotrophs.

In tests using acetate-producing microbes, production of the chemical almost doubled when the nanotube coating was applied. Alqahtani's ongoing work includes researching easier approaches in developing porous cylindrical cathodes, with the optimization of CO 2  flow rates and investing renewable MES energy sources, such as solar.