Proton-Conductivity Enhancement in Polymer Thin Films

Abstract

Highly proton conductive polymers have long attracted the attention of researchers for use in energy conversion, sensors, catalysts, and other applications. From the viewpoint of the scientific history of the creation of highly proton conductive polymers, one fundamental approach is based on the strategy of phase-segregated structures with strong acid groups. This Feature Article presents a new approach to enhancing the proton conductivity of the polymer thin films using an interface that can modify the degrees of freedom for a polymer structure through interaction between the substrate surface and polymers. I introduce suppressed proton conductivity into Nafion thin films and then specifically examine the enhancement in proton conductivity by the molecular orientation of the polymers. As the last topic, a highly proton conductive organized polyimide thin film is demonstrated using the lyotropic liquid-crystal property. Both molecular ordering and the in-plane oriented structure can enhance proton conductivity. Moreover, the optical domain and degree of molecular ordering derived from the molecular weight can contribute strongly to the proton transport property.

Molecular Dynamics Simulations of Substrate Hydrophilicity and Confinement Effects in Capped Nafion Films

“…Amide thin films on different substrates, such as silica and MgO(110), have shown differences in proton conductivity up to an order of magnitude because of differences in an interfacial structure. 13 …”

Ionomer Thin Films in PEM Fuel Cells

“…In a DPD model of a 10-nm-thick Nafion on carbon, diffusion was found to be anisotropic, higher along the film and lower in thickness direction [93]. Similarly, the conductivity of ionomer films along the ionomersubstrate interface is strongly influenced by the molecular orientation and ordering of the ionomer moieties therein [91]. Thus, the impact of morphological changes on transport is largely dependent on the direction on which the mechanism is probed.…”

“…It is clear that a thin film's internal and surface morphology poses significant limitations to transport and overall device performance [7,27,33,56,57,62,65,91]. Due to similar transport limitations between the Nafion electrolyte films formed in PEFC electrodes [6,7,9,11,14,18,92] and Nafion thin films [20,27,32,33,36,57,91], thin films are used as model systems to investigate and model electrode ionomers [1,2,7]. Key for electrode performance is good oxygen permeability with proton conductivity across/through an ionomer thin film on and between carbon/Pt agglomerates.…”

“…When confined to thin films, PFSA exhibits not only lower diffusion [27,54,65,68,71,72] but also a few orders of magnitude slower relaxation, indicating more restricted chain dynamics [68]. An important implication of confinement effect for fuel cell ionomers is the reduced conductivity with decrease in film thickness [5,34,36,37,53,64,70,91] and exhibits a substrate dependence [34] ( Fig. 4), which can be associated with decreased water uptake [34,36,64,70,91] and the number of conducting sites (from conductive AFM) [56], as the conductivity-l relationship was shown to extend to the thin-film regime, albeit with differences from that in the bulk regime [69].…”

Synthesis of self-assembled nucleobases and their anhydrous proton conductivity

“…7,10,11 Namely, the existence of a proton conducting pathway in a material is one of the most important factors for the high proton conductor. [11][12][13] However, the simple mixing of acidic and basic molecules generally forms a random proton conducting pathway and, as a result, does not provide the maximum proton conductivity. Therefore, various anhydrous proton conductors with the proton conducting pathway, such as periodic-mesoporous-organosilica encapsulated imidazole molecule, 14 self-assembled organic phosphonates consist of long alkyl chains, 15 and Kevlar® nanobers with cadmium telluride nanocrystals and phosphoric acid, 16 have been reported and these materials showed the anhydrous proton conductivity of 7.11 Â 10 À3 S cm À1 at 180 C, 10 À2 S cm À1 at 140 C, and 2.35 Â 10 À1 S cm À1 at 160 C, respectively.…”