Nafion, the state-of-the-art-of proton exchange membrane in fuel cell, is a copolymer with a randomly tethered side-chain terminated by a pendant sulfonic acid groups adjacent to the polytetrafluoroethylene (PTFE) backbone. As a result of the intrinsic difference between the backbone and sulfonic acid groups, phase separation occurs during the formation of solid Nafion membranes from ionomer solution via a self-organizing process conducted at elevated temperature. The distribution of proton-conductive pathways not only inside but on the surface of the membrane plays a significant role in improving the cell performance. The latter determines the mass transport of H2O molecules and protons through the membrane. In addition, the three-phase boundary (TPB) plays a crucial role in the performance of fuel cells, which means that the electrocatalysts (such as Pt) in fuel cells must directly contact with the conductive hydrophilic regions. However, an excess of hydrophilic groups would cause flooding. Therefore, in order to further promote fuel cell performance, a good balance between the distributions of hydrophilic and hydrophobic groups is extremely crucial. In our study, we constructed a home-built AFM-EIS setup and measured the localized impedance by coupling AFM with an impedance tester in a Faraday cage. In contrast to traditional average and macroscopic bulk impedance measurements, we obtained a detailed and spatially diverse distribution of EIS spectra at selected points on Nafion with AFM-EIS. In addition, with a customized automatic procedure, we performed proton transfer resistance imaging on the Nafion surface in an area of 2 µm*2 µm at different humidities, the results of which confirmed the widely known fact that higher water activities significantly reduce the transport resistance. Furthermore, a numerical simulation was constructed to incorporate morphological and physical variations of the membrane, which revealed that at higher water activities, SO3H groups cluster to create a matrix of interconnected nano-water channels through which the ions are transported.

Localized electrochemical impedance measurements on nafion membranes: Observation and analysis of spatially diverse proton transport using atomic force microscopy

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.