Radiation-grafted proton exchange membranes (PEMs) are promising alternatives to perfluorosulfonic acid polymer membranes for electrochemical applications like production electrodialysis and automobile fuel cells. Their attractive merits include potentially low fabrication cost, good polymer functionalization and synthesis adaptability and the tunability of the membrane properties by optimizing the grafting and material parameters. Importantly, a careful balancing of the membrane properties is necessary to yield the desired combination of mechanical integrity, chemical stability and proton conductivity under dynamic operating conditions.
Lately, partially fluorinated poly(ethylene-co-tetrafluoroethylene) (ETFE) has been identified as a promising base film for PEMs for the electrochemical applications due to its outstanding chemical, thermal, and mechanical stability, as well as its resistance to high-energy radiation. Radiation-grafting styrene or its substitutes onto ETFE base films followed by sulfonation is a feasible technique to fabricate ETFE-based PEMs, such as poly(styrenesulfonic acid)-grafted ETFE (ETFE-g-PSSA), with sufficient initial fuel cell performance even higher than Nafion®, though the long-term stability is still challenging.
To facilitate the fuel cell applications with ETFE-g-PSSA, it requires thorough understanding of the structure-property relationships. Scattering techniques are prominent among the available experimental tools with sufficient spatial resolution. However, the conventionally measured scattering intensity profile mixes structure information of hydrophobic polymer, hydrophilic polymer, ions, and water molecules, and fails to provide the detailed structure of individual components. This undesirable original data problem can be solved by partial scattering function (PSF) analysis, which is the quantitative decomposition of a series of intensity profiles obtained through contrast-variation small angle neutron scattering (CV-SANS) experiments.
To this note, Dr. Yue Zhao, Dr. Kimio Yoshimura, Dr. Shinichi Sawada, Dr. Toshinori Motegi, Dr. Akihiro Hiroki and Dr. Yasunari Maekawa from National Institutes for Quantum Science and Technology (QST), together with Dr. Aurel Radulescu from Jülich Centre for Neutron Science at MLZ utilized CV-SANS technique with PSF analysis to visualize, qualify and characterize the exact structure of individual components of hydrated ETFE-g-PSSA PEMs. In addition, the authors performed cross-term analysis to explore the correlation between two components to establish their locations. The characterization was performed in multiple length scales, and the structural insights of the PEMs were provided. Their work is currently published in the journal, Macromolecules.
The results suggested three-component domains consisting of ETFE base-polymer (BP), PSSA graft-polymer (GP) and water. The formation of polymer grains with a radius of gyration of ~150 nm through the aggregation of phase-separated GP domains in the BP matrix in the mass fractal structure with a dimension of 2.4 was observed on a large length scale. Each GP domain had an average radius of gyration of 9.5 nm and was made up of homogenously distributed water and GP, characterized by a bicontinuous-like local structure with a mean separation distance of 2 nm. This indicated the formation of a well-connected ion channel network – a crucial factor in improving membrane conductivity. Furthermore, the PSF analysis revealed the repulsion between the water and GP at molecular lengths less than 3 nm, leading to a lower hydration number compared to Nafion® membranes.
In summary, this is the first study to apply PSF analysis to establish the quantitative role of each component in the hydrated radiation-grafted ETFE-g-PSSA PEMs. The results showed that PSF analysis could provide mechanistic insights into membrane conductivity and structural correlations over a wide range of length scales. In a statement to Advances in Engineering, Dr. Yue Zhao said that the structural insights at the molecular level provide a path forward toward establishing superior design guidelines for fuel-cell membranes.
Zhao, Y., Yoshimura, K., Sawada, S., Motegi, T., Hiroki, A., Radulescu, A., & Maekawa, Y. (2022). Unique structural characteristics of graft-type proton-exchange membranes using SANS partial scattering function analysis. Macromolecules, 55(16), 7100-7109.