Direct methanol fuel cell relative permeability

The effect of annealing temperatures (65 - 250 °C) and blend composition of Nafion 117, solution-cast Nafion , poly(vinyl alcohol) (PVA) and Nafion /PVAblend membranes for application to the direct methanol fuel cell is reported in [148], These authors have found that a Nafion /PVAblend membrane at 5 wt% PVA (annealed at 230 °C) show a similar proton conductivity of that found to Nafion 117, but with a three times lower methanol permeability compared to Nafion 117. They also found that for Nafion /PVA (50 wt% PVA) blend membranes, the methanol permeability decreases by approximately one order of magnitude, whilst the proton conductivity remained relatively constant, with increasing annealing temperature. The Nafion /PVA blend membrane at 5 wt% PVA and 230 °C annealing temperature had a similar proton conductivity, but three times lower methanol permeability compared to unannealed Nafion 117 (benchmark in PEM fuel cells). 

Perfluorinated copolymers, Nafion (DuPont), are the current fuel cell PEM materials of choice. They are chemically very stable and have good proton conductivity. Conversely, Nafion and similar perfluorinated vinylidene copolymers have several limitations, some of which include (1) low modulus as well as modest glass-transition temperatures (2) reduced conductivity at temperatures above 80°C and (3) relatively high methanol permeability, which limits efficient applications for direct methanol fuel cells. They are also recognized to be currently quite expensive. 

Nafion , which is one of the best PEMs available today, satisfies requirements b (for gases only) d and e, and perhaps even requirement c. However, the conductivity of Nafion drops rapidly with decreasing relative humidity (RH), methanol permeability through Nafion is very high (a disadvantage for direct methanol fuel cells), and the material is presently extremely expensive ( 700 m ), although cost reductions with increased production rates have been projected by the manufacturer. These drawbacks have prompted an extensive effort to improve the properties of Nafion and identify alternate materials to replace Nafion . 

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