Effect of interface on surface morphology and proton conduction of polymer electrolyte thin films

Abstract

To understand the relationship between surface morphology and proton conduction of polymer electrolyte thin films, perfluorinated ionomer Nafion® thin films were prepared on different substrates such as glassy carbon (GC), hydrophilic-GC (H-GC), and platinum (Pt) as models for the ionomer film within a catalyst layer. Atomic force microscopy coupled with an electrochemical (e-AFM) technique revealed that proton conduction decreased with film thickness; an abrupt decrease in proton conductance was observed when the film thickness was less than ca. 10 nm on GC substrates in addition to a significant change in surface morphology. Furthermore, thin films prepared on H-GC substrates with UV-ozone treatment exhibited higher proton conduction than those on untreated GC substrates. However, Pt substrates exhibited proton conduction comparable to that of GCs for films thicker than 20 nm; a decrease in proton conduction was observed at ∼5 nm thick film but was still much higher than for carbon substrates. These results indicate that the number of active proton-conductive pathways and/or the connectivity of the proton path network changed with film thickness. The surface morphology of thinner films was significantly affected by the film/substrate interface and was fundamentally different from that of the bulk thick membrane.

Proton Transport Property in Supported Nafion Nanothin Films by Electrochemical Impedance Spectroscopy

“…The structural rearrangement and corresponding low proton conductivity for both Nafion thin film and membrane subjected to higher temperature treatment was also reported in many literature. 17,[43][44][45][46][47] It can be summarized that the proton conductivity of ultra-thin film is lower than that of the freestanding membrane regardless of the temperature and relative humidity. The finding of lower proton conductivity in Nafion thin films is consistent with the depressed surface current density and proton conductivity of thin film measured by current sensing AFM 46 and impedance technique 21,26 respectively.…”

“…Low proton conductivity of thin films is consistent with several other literature evidences. 21,26,46 of Research. The authors acknowledge Bryan Szeto at the National Institute for Nanotechnology for fabrication of the IDE arrays.…”

Current Understanding of Proton Conduction in Confined Ionomeric Systems

“…While proton conductivity of a 4 nm thick self-assembled Nafion film varies just by the variation of oxide layer thickness 5,7 on SiO 2 substrate, it does not show any clear trend with change in substrates in others' work. 13 The author have found quite similar deprotonation ratio (I d /I p ) for spin coated nafion thin films ( 70 nm) at similar thickness on Au and native SiO 2 substrates. 4 Ohira et al 13 have obtained a proton conduction trend: Pt> hydrophilic glassy carbon (GC) > unmodified GC for nafion film thickness lower than 10 nm.…”

“…13 The author have found quite similar deprotonation ratio (I d /I p ) for spin coated nafion thin films ( 70 nm) at similar thickness on Au and native SiO 2 substrates. 4 Ohira et al 13 have obtained a proton conduction trend: Pt> hydrophilic glassy carbon (GC) > unmodified GC for nafion film thickness lower than 10 nm. The substrate effect on surface current density disappears in films thicker than 10 nm.…”

“…The substrate effect on surface current density disappears in films thicker than 10 nm. 13 Nafion films prepared by different techniques (self-assembly, drop casting, spin coating) have not been studied at similar thickness so far (Table 1). While the effect of film preparation conditions on proton conductivity needs further investigation, film treatment conditions have been shown to distinctly affect morphology of the film as well as proton conductivity.…”