Porous separators boost efficiency of electrolyzers for carbon monoxide reduction

Electrolyzers, devices that use electricity to drive desired chemical reactions, could enable the production of clean hydrogen (H₂) gas from water (H₂O) and the conversion of carbon dioxide (CO₂) into useful fuels or industrial chemicals. When it comes to the reduction of CO₂, the greenhouse gas can be first converted into carbon monoxide (CO) and then processed further to attain desired compounds.

Most existing electrolyzers consist of a cathode (i.e., the site where electrons enter and drive the reduction of compounds) and an anode (i.e., the site where electrons leave and oxidation occurs). These two layers are divided by a so-called separator, a material that allows ions to move between the two other layers.

In most conventional electrolyzers, this separator is a charge-selective membrane, a barrier that primarily allows one type of charged ion to pass through, while capturing the others. While these membranes have been widely used so far, they are known to slow down the transport of charged particles (i.e., ions) in electrolyzers, which can in turn limit their energy efficiencies.

Researchers at University of Toronto recently demonstrated the potential of substituting charge-selective membranes with porous separators, thin materials with tiny holes that allow positive and negative ions to move more freely. Their paper, published in Nature Energy, shows that porous separators can enhance the energy efficiency and conversion rates of electrolyzers for the reduction of CO into ethylene and other desired carbon-based products.

“For years, CO electrolyzers have been constrained below about 40%, which is a major barrier to making the technology viable at scale,” Rui Kai Miao, first author of the paper, told Tech Xplore. “These systems typically use anion exchange membranes (AEMs), materials designed to let negatively charged ions pass through while keeping gases separate. In theory, that is ideal, but in practice AEMs are still relatively immature and tend to cause high voltage losses. We started asking a simple question: Why do we need a charge-selective membrane at all?”

Substituting ion-selective materials with porous separators

Miao and his colleagues started exploring the idea that what they truly required was a physical barrier that prevents the transfer of gas between the two electrodes (i.e., cathode and anode) inside an electrolyte. While this is far from a new idea, it was previously applied primarily to alkaline water electrolyzers, as opposed to systems for the reduction of CO or CO₂.

“Once we revisited this basic assumption, a whole new design space opened up,” explained Miao. “In a CO electrolyzer, we take carbon monoxide (CO), which itself can be produced upstream from CO₂ using mature technology, and we convert it into more complex molecules such as ethylene using electricity and water. Essentially, this is a way of turning renewable electricity into chemical energy—capturing carbon in a useful form.”

The team’s design retains the original three-layer structure of electrolyzers. Their proposed system thus also consists of a cathode in which CO is reduced, an anode where water is oxidized, and a separator between them.

“The key difference is that our separator is uncharged and porous rather than ion-selective,” said Miao. “This allows ions to move more freely, lowers electrical resistance, and makes the whole cell operate more efficiently and stably.”

Initially, Miao and his colleagues tested various separator materials that were originally designed to be integrated in alkaline water electrolyzers. Unfortunately, they found that when they directly deployed these materials in their system for CO reduction, they did not improve its performance much.

“When we measured their properties—thickness and porosity—we found they were dense and relatively thick, which slows ion transport,” explained Miao. “In alkaline water electrolysis, these properties help limit hydrogen crossover, because H₂ diffuses very quickly in water. In CO electrolysis, the key species—CO (reactant) and ethylene (product)—diffuse much more slowly than H₂. That gave us room to rethink the separator, and we started to try materials that are porous and thin.”

A new strategy to improve energy systems

Ultimately, the researchers discovered that porous materials reduced the voltage of their electrolyzer cell and boosted its performance. In fact, they were able to attain an energy efficiency of 51% for the reduction of CO into multi-carbon chemicals, with the electrolyzer operating reliably for 250 hours.

“As the porous separators we use are also very chemically stable, we were able to operate the electrolyzers at higher temperatures, further reducing the voltage of the cell, and for a longer time,” said Miao. “We’re now adopting porous, uncharged separators across our projects. They’re inexpensive, robust, and easy to handle, and—crucially—we can tune porosity, thickness, and pore size to control ion transport and the local reaction environment for different target products.”

In the future, this recent study could inspire other energy engineers to develop electrolyzers with porous separators, as opposed to ion-selective transport membranes. Meanwhile, Miao and his colleagues plan to use a similar approach to enhance the performance of other electrolyzers or energy solutions that could benefit from improved ion transport.

“We’re already extending our approach to CO₂ reduction and related electrosynthetic pathways,” added Miao. “We are also working on scaling these separator-based electrolyzers. On scale-up, these separators offer very practical advantages. Unlike many charge-selective membranes that require careful hydration and can be mechanically fragile, the separators can be assembled dry or wet, are structurally strong, and simplify stack assembly.

“That’s helping us reduce assembly failures as we move to larger active areas and multi-cell stacks, while we push for longer-duration operation and integration with intermittent renewable power.”

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