Proton exchange membrane chemical stability

The current state-of-the-art proton exchange membrane is Nafion, a DuPont product that was developed in the late 1960s primarily as a permselective separator in chlor-alkali electrolyzers. Nation s poly(perfluorosulfonic acid) structure imparts exceptional oxidative and chemical stability, which is also important in fuel cell applications. 

Polyphosphazene-based PEMs are potentially attractive materials for both hydrogen/air and direct methanol fuel cells because of their reported chemical and thermal stability and due to the ease of chemically attaching various side chains for ion exchange sites and polymer cross-linking onto the — P=N— polymer backbone. Polyphosphazenes were explored originally for use as elastomers and later as solvent-free solid polymer electrolytes in lithium batteries, and subsequently for proton exchange membranes. 

In the PEFC, the membrane, together with the electrodes, forms the basic electrochemical unit, the membrane electrode assembly (MEA). The first and foremost function of the electrolyte membrane is the transport of protons from anode to cathode. On one hand, the electrodes host the electrochemical reactions within the catalyst layer and provide electronic conductivity, and, on the other hand, they provide pathways for reactant supply to the catalyst and removal of products from the catalyst. The components of the MEA need to be chemically stable for several thousands of hours in the fuel ceU under the prevailing operating and transient conditions. PEFC electrodes are wet-proofed fibrous carbon sheet materials of a few 100 ttm thickness. The functionality of the proton exchange membrane (PEM) extends to requirements of mechanical stability to also ensure effective separation of anode and... 

Therefore, an ideal proton exchange membrane must possess high proton conductivity but low electronic conductivity, high mechanical strength and size stability, and good chemical, electrochemical, and thermal stabihty. Among aU these characteristics, the bottleneck property is high proton conductivity. 

As described before in this chapter, conventional DEFCs can be divided into two types as a function of fhe employed membrane, namely proton exchange membrane DEFCs (PEM-DEFCs) and anion exchange membrane DEFCs (AEM-DEFCs), used in acidic and alkaline medium, respectively. As previously reported, Pt-based catalysts undergo rapid poisoning of the catalytic sites, which compromises cell performance. On the other hand, the kinetics of both ethanol oxidation (OER) and the oxygen reduction reaction (ORR) in alkaline medium are much faster than the corresponding kinetics in acidic medium, which substantially improves cell performance. The main limitation to the cell performance in AEM-DEFCs is the physical and chemical stability of the AEM [71]. Another problem encountered with the AEM is that its ionic conductivity is about one order of magnitude lower than that of Nafion membranes. 

Sol gel synthesis, electrochemical characterization, and stability testing of Tiu7Wa302 nanoparticles for catalyst support applications in proton-exchange membrane fuel cells. Journal of the American Chemical Society, 132 (49), 17531-17536. 

The most widely studied fuel-ceU membrane is DuPont s Nafion , a copolymer of tetrafluoroethylene and perfluoro(4-methyl-3,6-dioxa-7-octene-l-sulfonic acid). Nafion is the membrane material of choice for most proton-exchange membrane fuel cells that operate at a temperature <80 °C. While Nafion offers high conductivity combined with exceptional chemical and mechanical stability [3], it suffers from several critical drawbacks. When used in a direct methanol fuel cell, Nafion shows significant methanol leakage (crossover from the anode to the cathode) with the resultant reduction in fuel-ceU performance. To overcome this shortcoming the methanol concentration in the anode feed is usuaUy reduced to 0.5-2.0 M, which necessitates... 

---