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Maintaining American energy independence requires minimizing reliance on foreign countries to produce commodity chemicals and fuels. Using carbon dioxide electrolyzers to produce valuable chemical precursors such as ethylene provides one way to diversify domestic feedstocks. But, so far, these devices have been limited by their low efficiency, which makes them energy-intensive and costly.

In a new study, published in , researchers at Lawrence Livermore National Laboratory (LLNL) designed a new polymer ink, called an ionomer, that controls how gas and water move in electrochemical devices. By carefully balancing and directing the device chemistry, the ionomer improves energy efficiency of the conversion process.

"Adding the right ionomer lowered the overall voltage needed to run the device," said LLNL scientist and author Aditya Prajapati. "That means the device requires less electricity usage to make the same amount of product."

The ionomer is one small but crucial part of the electrochemical device.

"Our device is about the size of a sandwich, with several thin layers stacked together. Carbon dioxide gas flows in on one side and electricity drives the reaction inside," said LLNL postdoctoral researcher and author Nicholas Cross.

Inside the device, carbon dioxide encounters a copper catalyst layer that triggers a reaction to turn it into ethylene, a building block for plastics. The ionomer was sprayed as a coating onto this copper layer.

"You can think of the ionomer as a traffic controller for molecules: it controls the chemistry of the catalyst surface and makes sure the right amount of water and carbon dioxide reach the catalyst," said LLNL scientist and author Maxwell Goldman. "Without it, too much water can flood the device, or too little can starve the reaction."

The ionomer keeps the reaction balanced, which cuts down the energy needed to make valuable products like ethylene.

To create this important traffic controller, the team carefully attached chemicals to a stable polymer backbone. They tested the device with and without the ionomer and explored a number of ionomers with different water uptakes.

"We discovered that the amount of water the ionomer holds—the 'water content'—is a powerful lever that controls how much ethylene is produced," said LLNL scientist and author Chris Hahn. "Too little ionomer and the device floods; too much and it wastes energy. Finding the right balance allowed us to run the device at high performance and low voltage."

The researchers emphasized that the improved device performance and the deep understanding of the ionomer were made possible by combining polymer chemistry, experiments and multiphysics modeling. They hope this work will guide the design of the next generation of polymers in electrochemical devices.