Nanocharacterization and nanofabrication of a Nafion thin film in liquids by atomic force microscopy

New Insights into Perfluorinated Sulfonic-Acid Ionomers

In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers' complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, especially when considered with their applications, are at the forefront of bridging electrochemistry and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various physical (e.g., mechanical) and transport properties with morphology and structure across time and length scales. In addition, topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degradation phenomena, and PFSA thin films are presented. Throughout, the impact of PFSA chemistry and side-chain is also discussed to present a broader perspective.

Wettability of Nafion and Nafion/Vulcan carbon composite films

The wettability of the Pt/carbon/Nafion catalyst layer in proton exchange membrane fuel cells is critical to their performance and durability, especially the cathode, as water is needed for the transport of protons to the active sites and is also involved in deleterious Pt nanoparticle dissolution and carbon corrosion. Therefore, the focus of this work has been on the first-time use of the water droplet impacting method to determine the wettability of 100% Nafion films, as a benchmark, and then of Vulcan carbon (VC)/Nafion composite films, both deposited by spin-coating in the Pt-free state. Pure Nafion films, shown by SEM analysis to have a nanochanneled structure, are initially hydrophobic but become hydrophilic as the water droplet spreads, likely due to reorientation of the sulfonic acid groups toward water. The wettability of VC/Nafion composite films depends significantly on the VC/Nafion mass ratios, even though Nafion is believed to be preferentially oriented (sulfonate groups toward VC) in all cases. At low VC contents, a significant water droplet contact angle hysteresis is seen, similar to pure Nafion films, while at higher VC contents (>30%), the films become hydrophobic, also exhibiting superhydrophobicity, with surface roughness playing a significant role. At >80% VC, the surfaces become wettable again as there is insufficient Nafion loading present to fully cover the carbon surface, allowing the calculation of the Nafion:carbon ratio required for a full coverage of carbon by Nafion.

Study of the nanoscopic deformation of an annealed nafion film by using atomic force microscopy and a patterned substrate

We demonstrated the repetitive imaging of the same area of a nafion film before and after annealing by using atomic force microscopy (AFM). In order to find the exact same area of the same sample after changing the cantilever and reattaching the sample, a micropatterned substrate was developed. A micropattern with a 250-500 microm pitch was prepared on the backside of a transparent glass substrate. This pattern includes various signs such as colored letters and numbers at the center of each lattice of the pattern. The nanostructures fabricated by AFM nanolithography on a nafion film using this new method were successfully characterized before and after annealing (over 100 degrees C). The AFM images clearly showed that the nanostructures on a nafion film were dramatically changed by annealing. The data indicated an evidence to understand why the nafion fuel cell does not work well at high temperatures. Our method is probably effective for the study of nanoscopic dynamics in various surface structures.

Electrochemical atomic force microscopy study of proton conductivity in a Nafion membrane

High membrane conductivity is one of the key parameters in polymer electrolyte fuel cell applications. We introduce an electrochemical atomic force microscopy method that provides simultaneously the surface topography of a Nafion 112 membrane and the conductivity of ion channels with an unprecedented resolution of ca. 10 nm. For given conditions, a large fraction of the channel ports is found to conduct exactly the same number of protons per unit time. This is taken as evidence for an optimum pore size and structure for proton conduction, or alternatively, for an efficient connectivity of the ion channel network, so that the same conductivity is measured at all exit pores. The time response following a potential step and the influence of the relative humidity on the transport properties is investigated. The method will be of relevance for tailoring the production technology to yield an optimised micromorphology, and it permits detailed tests of membrane models and provides data for theoretical modelling of proton conductivity.