New insights for clean energy technology

Researchers at the University of Chicago Pritzker School of Molecular Engineering (UChicago PME) and at the Tandon School of Engineering of New York University have made a breakthrough in understanding how water transports charged ions across a critical component in clean energy technologies like fuel cells and redox flow batteries.

While scientists previously thought this component, called an anion exchange membrane (AEM), required high levels of free-flowing water, which can weaken the structure of the membranes over time. The new study, however, suggests that fast ion transport does not require high levels of free water.

Instead, AEMs can be optimized by using only enough water to create well-connected networks of water molecules within the membrane while also ensuring a dynamic shell of water around the ions.

The research is published in Nature Communications.

“Our study challenges the long-held idea that fast ion transport in energy membranes requires excess free water—in reality, it’s the structure of the water network that matters, not just the amount,” said UChicago PME Prof. Paul Nealey, a senior author of the new paper.

“This research provides us with a molecular-level blueprint for optimizing energy membranes, bringing us one step closer to more efficient fuel cells, better batteries, and more sustainable energy storage solutions,” said former UChicago PME Prof. Juan de Pablo, now at New York University and also a senior author.

Focused on the flow of ions

AEMs are thin, specially designed materials embedded with positively charged molecules. These positively charged molecules help attract and guide negatively charged ions—called anions—through the membrane while repelling positively charged ions—called cations. The membranes are used in various electrochemical devices, which use the charge differences created by AEMs to power other reactions (such as converting chemical energy to electricity in fuel cells or splitting water to produce clean fuel in water electrolyzers).

The efficiency of AEMs depends on how well ions move through them, and scientists knew that water helps ion flow. However, keeping high levels of free-flowing water in electrochemical devices limits their use in low-humidity settings and can make the structure of the AEMs swell, stretch and weaken.

In the new study, the researchers paired experimental data on AEM efficiency with computer simulations of how molecules within the systems behave to better understand the role of water. They used cutting-edge two-dimensional infrared spectroscopy (2D IR) to capture fast water dynamics.

“By integrating these approaches, we can precisely model what happens to the water molecules around AEMs on a timescale of picoseconds,” said Ge Sun, a UChicago PME graduate student and co-first author.

The team found that water molecules absorbed into an AEM create a network of hydrogen bonds within its structure. This network, as well as shells of water surrounding ions—rather than excess free water—helps the ions move efficiently. With the lowest levels of water, high amounts of energy are needed to move ions across an AEM because the hydrogen network is incomplete. As water levels increase and hydrogen networks become more structured, the energy required for ion movement decreases significantly.

“We observed that even without high levels of water, we see a boost to ionic conductivity and ion transport across the membrane. This happens because the water network is well-formed, and water molecules in the second layer can quickly adjust their orientation,” said Sun.

Optimizing future tech

When designing AEMs in the past, engineers erred on the side of using more water than necessary. The new results suggest that there is a better way to optimize water levels in the electrochemical devices that use these membranes.

“By uncovering how water molecules organize inside these membranes, we can design next-generation materials that work efficiently even in low-humidity environments, making clean energy technologies more practical and durable,” said UChicago PME. Assoc. Prof. Shrayesh Patel, a co-author of the study.

A key advance of this work was to rely on 2D IR, coupled with sophisticated molecular models, to elucidate the fine details of water dynamics in these systems. The new combination of experimental data and simulations used in this study provides a powerful framework that can be applied to many other scientific challenges that involve understanding molecular movements.