Structure-property relationships at Nafion thin-film interfaces: Thickness effects on hydration and anisotropic ion transport

Significance 

Nafion is a synthetic polymer with ionomeric properties and is entirely made up of sulfonated tetrafluoroethylene-based fluoropolymer-copolymer. Most baffling is its ionic conductivity property that is coupled by its chemo-thermo-mechanical stability and has enabled Nafions’ wide applicability in batteries, polymer electrolyte membrane fuel cells (PEMFCs), water electrolyzers and in chemical sensors. Studies have revealed that its properties are dependent on the phase segregation between its two primary moieties. Therefore, for fabrication of durable and efficient electrochemical devices, an exquisite understanding and superb control of Nafion’s structural and functional properties is necessary, particularly for PEMFCs.

In line with this, much research has of late been focused on Nafion properties in the catalyst layer, i.e. the heterogeneous region where limiting charge transfer processes occur in PEMFCs. Consequently, researchers have been utilizing planar thin-films as model systems to obtain fundamental insight into the catalyst layer Nafion properties. Unfortunately, most of these studies report on average sample properties which do not offer adequate insight into the thin-film Nafion.

Recently, Colorado School of Mines researchers Dr. Steven DeCaluwe and Mr. John Fischer from the Department of Mechanical Engineering in collaboration with scientists at NIST Center for Neutron Research: Dr. Andrew M. Baker, Dr. Pavan Bhargava and Dr. Joseph A. Dura assessed the thickness dependence of water uptake, interfacial ionic domain structure, and anisotropic ionic conductivity in ultrathin Nafion films (averaging between 5–153 nm) on silica substrates. For this purpose, they used the in-situ neutron reflectometry technique. Their work is currently published in the research journal, Nano Energy.

In brief, the research method they used commenced with the deposition of Nafion thin films onto polished silicon wafers followed by neutron reflectometry measurements. Next, the researchers estimated the transport properties in the thin-film Nafion and lamellae. They then used the observed interfacial lamellae to understand anomalous transport limitations in the films. Lastly, the depth profiles from neutron reflectometry fitting were used to model the thin-film ionic conductivity parallel to and normal to the substrate plane.

The authors observed that the transition between regimes roughly coincided with the point where the bulk-like layer thickness exceeded the radius of gyration for thin-film Nafion. Additionally, they noted that the bulk-like layer of the thick-film regime absorbed the same amount of water as in bulk membranes, whereas in the thin-film regime the water uptake of the bulk-like layer decreased with decreasing thickness.

In summary, Dr. Steven DeCaluwe and colleagues study demonstrated the importance of spatial morphology variations in thin-films and coatings when determining average properties. In general, the results obtained in their study showed that water uptake in the lamellae was influenced by its interactions with both the silica substrate and either the vapor interface or the bulk-like layer. Altogether, they presented a means to deconvolute the effects of thickness, substrate, and chemical environment on thin-film copolymer properties.

Reference

Steven C. DeCaluwe, Andrew M. Baker, Pavan Bhargava, John E. Fischer, Joseph A. Dura. Structure-property relationships at Nafion thin-film interfaces: Thickness effects on hydration and anisotropic ion transport. Nano Energy, volume 46 (2018) page 91–100

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