Understanding how protons move at the interface between polymers and electrode materials is essential for improving fuel cells and related energy devices. However, conventional impedance measurements under inert conditions have long masked these interfacial contributions, showing only a single, merged signal.
Now, researchers at the Japan Advanced Institute of Science and Technology, in collaboration with Tokyo University of Science and University of Calgary, have developed a method to experimentally separate and quantify proton transport at individual interfaces in ultrathin ionomer films. The study was published in the journal ACS Applied Materials & Interfaces on May 1, 2026.
By extending impedance measurements to lower frequencies and systematically varying the electrode pad length, the team shifted the characteristic response of each interface, making it possible to separate contributions that had previously overlapped. (Figure 1) This allowed them to directly evaluate proton conductivity at polymer–substrate interfaces such as SiO2, platinum, and carbon, independent of measurement geometry.
"What we have done is not to show that interfaces are very different, but that we can finally separate and evaluate them individually," explains Professor Yuki Nagao, who led the study. "Until now, these contributions were effectively hidden in a single semicircle."
While the present study focuses on Nafion, a widely used benchmark ionomer, the method is not limited to this material. It can be extended to a broad range of ion-conducting polymers, enabling systematic studies of interfacial transport properties.
"This approach allows us to evaluate how suitable a material is for an interface, not just in the bulk," Nagao notes. "Even for newly developed materials in industry, which have typically been assessed only as bulk membranes, we can now examine their interfacial performance directly."
The study revealed that proton transport at different interfaces is of a similar order of magnitude, with only modest differences. More importantly, the method provides a reliable framework for isolating and comparing interfacial transport properties.
This advance opens a new way to study ion transport in thin-film materials, particularly in realistic operating environments where measurements are often conducted under inert atmospheres.
"We did not expect that impedance measurements alone could lead to this level of insight," Nagao adds. "But it turns out that, with the right approach, they can reveal much more than we assumed."
The findings are expected to contribute to the rational design of ionomer interfaces in electrochemical devices, including fuel cells, electrolyzers, and batteries.
