Using bifunctional ionomers as electrolytes to synthesize ethylene from carbon dioxide

Over the past century or so, human activities have led to the rapid deterioration of the environment on Earth, with detrimental effects such as climate change and a rise in atmospheric CO₂. Many scientists worldwide have thus been trying to devise new technologies and solutions that could help to tackle these existing environmental issues.

A possible way of reducing the presence of CO₂ in the atmosphere is to devise effective strategies to recycle CO₂ and convert it into liquid fuels or other industrial materials, ideally using sustainably produced electricity. To do this, however, researchers should be able to produce highly valuable products from CO₂ at industrial-scale current densities using a low amount of electrical energy.

Researchers at Wuhan University have recently introduced a new strategy that could be used to synthesize ethylene, a flammable hydrocarbon gas, from CO₂ and pure water. This strategy, introduced in a paper published in Nature Energy, entails the use of bifunctional ionomers, a reactive polymer, as polymer electrolytes to activate CO₂ and enable its efficient co-electrolysis with water to produce ethylene.

"Many CO₂ electrolyzers under development use liquid electrolytes (KOH solutions, for example), yet using solid-state polymer electrolytes can in principle improve efficiency and realize co-electrolysis of CO₂ and pure water, avoiding corrosion and electrolyte consumption issues," Wenzheng Li and his colleagues wrote in their paper. "However, a key challenge in these systems is how to favor production of multicarbon molecules, such as ethylene, which typically necessitates a strong alkaline environment."

To synthesize ethylene from CO_2, Li and his colleagues used an alkaline polymer electrolyte (APE), which can bring the gap between cathodes and anodes down to less than tens of micrometers in membrane electrode assembly architectures (MEAs). This can in turn reduce the so-called internal ohmic loss (i.e., drop in voltage caused by the transfer of electrons in circuits or movement of ions through electrolytes and membranes), improving the technology's energy conversion efficiency at high current densities.

"We use bifunctional ionomers as polymer electrolytes that are not only ionically conductive but can also activate CO₂ at the catalyst–electrolyte interface and favor ethylene synthesis, while running on pure water," Li and his colleagues wrote in their paper. "Specifically, we use quaternary ammonia poly(ether ether ketone) (QAPEEK), which contains carbonyl groups in the polymer chain, as the bifunctional electrolyte."

The polymer electrolyte proposed by Li and his colleagues could significantly outperform liquid electrolytes, which are integrated in most existing CO₂ electrolyzers, as it could improve the devices' energy efficiency. Most notably, the team was able to render the ionomer they used bifunctional, so that it would be ionically conductive and activate CO_2, interfering with the reaction necessary to reduce it down to ethylene.

The researchers evaluated their electrolyte by integrating it in an electrolyzer running on CO₂ and pure water. In these tests, they found that their polymer electrolyte increased the selectivity of ethylene to 50%, even in the absence of a strong alkaline environment.

"The electrolyzer running on CO₂ and pure water exhibits a total current density of 1,000 mA cm⁻² at cell voltages as low as 3.73 V. At 3.54 V," Li and his colleagues wrote in their paper. "Ethylene is produced with the industrial-scale partial current density of 420 mA cm⁻² without any electrolyte consumption."

The recent work by this team of researchers opens new possibilities for the conversion of CO₂ into ethylene on an industrial scale. In the future, it could inspire similar approaches for synthesizing hydrocarbons or other industrial gases from CO₂ using alkaline polymer electrolytes.