Twin breakthroughs by researchers at King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia, overcome two issues holding back development of microbial electrosynthesis (MES) technology.
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MES involves using electrical currents to stimulate microorganisms into manufacturing useful chemical products.
The KAUST researchers are studying chemolithoautotrophs, microbes that typically reside in caves, hydrothermal vents and other locations where energy sources such as sunlight and organic carbon are in short supply, if they exist at all.
The microbes consume CO2 through their natural metabolism, creating small organic molecules as a byproduct. It’s this ability that Pascal Saikaly, an associate professor of environmental engineering at KAUST, and his team have harvested.
“The microbes obtain their energy from the oxidation of inorganic compounds, such as hydrogen, iron and sulfur,” explains Bin Bian, a PhD student from Saikaly’s team. The microbes strip the inorganic compounds of electrons while taking up CO2 and reducing it to organic products as part of the process.
The idea itself isn’t new; indeed, many research projects have focused on MES. However, the typical MES reactor uses chemolithoautotrophs grown on a submerged flat-sheet cathode, with CO2 then bubbled into the solution. According to team member Manal Alqahtani, this has two key problems: flat-sheet cathodes are difficult to scale up, and CO2 gas has poor solubility.
So, the team developed an alternative MES reactor using cathodes made from stackable, cylindrical porous nickel fibers that Saikaly’s group previously applied to recover water and energy from wastewater. Here, CO2 is pumped through each cylinder, and electrons flow along it.
“Using this architecture, we directly deliver CO2 gas to chemolithoautotrophs through the pores in the hollow fibers,” Alqahtani explains. “We provided electrons and CO2 simultaneously to chemolithoautotrophs on the cathode surface.”
In Alqahtani’s initial study, the porous nickel hollow fibers act as an inorganic electrocatalyst for hydrogen generation from proton reduction and as a gas‐transfer membrane for direct CO2 delivery to CO2‐fixing hydrogenotrophic methanogens (biological catalysts) on the cathode. These novel features create a suitable environment for the enrichment of methanogens, which utilize the hydrogen as a source of reducing equivalents for converting CO2 to methane.