A Novel Composite Nafion/Anodized Aluminium Oxide Proton Exchange Membrane

Abstract

The high proton conductivity of proton exchange membranes (PEMs) made of Nafion® is attributed to a random 3D network of elongated water channels formed by bundles of rod‐like inverted micelles. Alignment of bundles, e.g., by stretching, is known to significantly enhance the in‐plane conductivity, yet achieving alignment normal to the membrane surface, desired for most applications, is challenging. A few attempts to obtain such alignment by confining Nafion in nanometric trans‐membrane pores of solid anodized aluminium oxide (AAO) membranes were reported, so far with limited success. Here we demonstrate that considerable pore filling of AAO with 200 nm pores by Nafion could be achieved through (i) use of Nafion concentrations below the threshold concentration C*, above which Nafion molecules form large aggregates; and (ii) control of filling rate through a combination of thermal and vacuum evaporation of Nafion solution through the membrane. The proton conductivity and NaCl diffusion permeability (sodium chloride diffusivity) tested by electrochemical impedance spectroscopy demonstrated superior intrinsic conductivity and selectivity of the composite membrane as compared to commercial Nafion membranes.

A Nafion‐filled Polycarbonate Track‐Etched Composite Membrane with Enhanced Selectivity for Direct Methanol Fuel Cells

“…σ values for both commercial and recast bulk Nafion are very similar and increase from about 7 S m −1 to about 12 S m −1 with the transition from the PH state to the FH state. The acid electrolyte used to eliminate the contact resistance increases the measured conductivity values roughly by 20–30% as compared with the “true” Nafion conductivity measured in water, in agreement with data previously reported , . This contribution is relatively small and applies to all the samples, so it should not affect significantly the comparison between the membranes.…”

Understanding methods of preparation and characterization of pore-filling polymer composites for proton exchange membranes: a beginner’s guide

“…The contact resistance can be eliminated by equilibrating the membrane with strong acids of concentration above 1 m rather than with dH 2 O (Gloukhovski et al 2016). Again, the proton conduction in pore-filling composite PEMs discussed later can result from Nafion presumably deposited in the pores (specific transport) as well as from free transport of acid ions through the unblocked pores (non-specific transport).…”

“…As a result, comparing conductivity and, correspondingly, selectivity values of different PEMs can be problematic due to limitations and uncertainties of the measurement techniques. Gloukhovski et al (2016) presented a detailed discussion of the subject, which may be summarized as follows.…”

“…Recently, Gloukhovski et al (2016) demonstrated successful filling of the AAO membrane (pore diameter, 200 nm) by through-the-membrane evaporation of Nafion solution (~1.2 g l −1 in IPA) under vacuum. Through-plane σ and η NaCl of the pore-filling Nafion were two and four times higher, respectively, than for bulk recast Nafion.…”

“…In a similar way, methanol permeability is commonly measured in diffusion cells, in which the membrane separates two stirred compartments. One compartment is filled with MeOH, and the other, with deionized water (dH 2 O) (Lin et al 2005, Yildirim et al 2008, Zhang et al 2009, Lin et al 2010, Gloukhovski et al 2016. The change in MeOH concentration in the second compartment is measured with gas chromatograph (GC) or density meter using a suitable calibration curve.…”