Membranes help multiply microbial CO2 munching

Microporous conductive membranes developed at KAUST are expected to help shape the future of microbial electrosynthesis for CO₂ conversion technologies. The membranes simultaneously stimulate the growth of CO₂-eating microbes and aid separation of the biochemical products.

Microbial electrosynthesis is a promising strategy to reduce the human carbon footprint. It uses specific living microbes to transform CO₂ into useful chemicals in an electrochemical cell via a reduction reaction under applied voltage. With the reduction of CO₂, the microbes multiply to form a biofilm on the cathode of the cell, but their growth involves a tedious multistep enrichment process that takes more than 30 days.

This enrichment process is a major bottleneck for achieving industrially attractive biochemical production and CO₂ bioconversion. A further drain is the complex and energy intensive techniques deployed to isolate the products, which mainly consist of acetate.

The lead author Bin Bian, a postdoc in Pascal Saikaly’s group, and coworkers had previously used electrochemical bioreactors equipped with conductive hollow fiber membranes to treat wastewater. When doing this, they discovered a thick biofilm formed on the hollow fiber membranes after microfiltration. “This suggested that a similar enrichment process for CO₂-eating biofilms could be achieved in microbial electrosynthesis systems,” Bian says.

Inspired by this discovery, the researchers designed metal-coated ceramic hollow fiber membranes to manufacture conducting cathodes that expedite microbial growth while making acetate easier to separate. The coating consisted of uniformly distributed nickel nanoparticles that catalyze the electrolysis of water into hydrogen, a key mediator in the electron transfer between membrane and microbes.

The researchers evaluated the performance of their membrane cathode in abiotic medium and in the presence of sludge. They found that, in both cases, the nickel-catalyzed production of hydrogen was essential in boosting the microbial growth and CO₂ conversion into acetate. “Moreover, the hollow fibers served as CO₂-delivery channels to the microbes adsorbed on their surface and consequently enhanced the efficiency of the CO₂ reduction,” Bian says.

Microbial electrosynthesis systems using the nickel-coated hollow fiber cathodes achieved a stable CO₂ bioconversion within one month. “This exceeded our expectations,” says Bian, noting that previous systems required at least three months to reach stable operation. “This is an important aspect for future scale-up,” he explains.

While working on performance improvements, the team is now expanding the reactor volume and treatment capacity of their microbial electrosynthesis system. They are also investigating ways to integrate their system with chain elongation technology to expand the bioconversion to value-added biochemicals other than acetate and methane.