Membrane/ionomer proton conductivity conduction mechanism

Perfluorosulfonic add (PFSA) membranes continue to be the industry standard for low-temperature PEMFCs due to their excellent proton conductivity, mechanical and chemical stability that is difficult to surpass. The Nafion membrane produced by DuPont has been the most studied (Mauritz and Moore, 2004 Grot, 2008). Nafion membranes are coded according to the polymer equivalent weight (EW) (first two digits), the membrane thickness (in mil, 1/1000 inch, corresponding to 25 pm) - third, or third and fourth digits) thus Nafion N117 is polymer EW 1100,7 mil thickness. In parallel with these developments, advances have been made on related perfluorinated ionomers that differ from the Nafion -type polymer... 

Of the most common air and fuel gaseous contaminants, ammonia is the most significant membrane contaminant, as the cationic species Nff + will interfere with the proton conduction mechanism and result in decreased membrane conductivity. Other gaseous contaminants may also affect the membrane and ionomer functionality through pH effects and decomposition products these are discussed in chapter 5. 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

The electrolyte membrane presents critical materials issues such as high protonic conductivity over a wide relative humidity (RH) range, low electrical conductivity, low gas permeability, particularly for H2 and O2, and good mechanical properties under wet-dry and temperature cycles has stable chemical properties under fuel cell oxidation conditions and quick start-up capability even at subfreezing temperatures and is low cost. Polyperfluorosulfonic acid (PFSA) and derivatives are the current first-choice materials. A key challenge is to produce this material in very thin form to reduce ohmic losses and material cost. PFSA ionomer has low dimensional stability and swells in the presence of water. These properties lead to poor mechanical properties and crack growth. 

For high temperature membranes, three avenues are being pursued a) synthesis of new ionomers, b) covalent modification of Nafion , and c) pol mier-inorganic composites. Some of the approaches aim to increase the attractive forces holding water within the membrane, enabling operation at low RH. Others seek to embody Bronstead bases as an alternative to water, which allow for proton mobility in the absence of water. This mechanism is exemplified by proton conductivity in e.g. H3PO4 or CSHSO4, in which the protons hop and the protonated base need not move macroscopic distances. 

To enhance membrane mechanical and hydro-thermal stability, Jiang et al. prepared a blend of side-chain sulfonated PFCB block copolymer and a PVDF fluoropolymer [129]. The chemical structure of the side-chain sulfonated PFCB ionomer is shown in Scheme 6.32. They evaluated the membrane s fiandamental properties, such as proton conductivity, gas permeability, water uptake, and... 

In order to increase the proton conductivity, a lower EW ionomer can be made. However, when the EW is too low (e.g., less than 700), the mechanical strength of the membrane becomes unacceptable, especially after the membrane is fully hydrated, which will in turn shorten the membrane lifetime. The proton conductivity and the mechanical strength of a membrane also depend on the side chain length and structure. The PFSAs made by Dow and 3M have shorter side chains than DuPont s Nafion, as shown in Pigure 1.3. 

In the case of sulfonated poly(phthalazinone ether ketone)s being used as a matrix, composite membranes showed less swelling in water and methanol, better mechanical and thermal stability, but lower proton conductivity [94, 95]. The results suggest that the amount of inorganic additives have to be optimized for each ionomer material and fuel cell operation conditions. Addition of 5 wt% of... 

Perfluorinated sulfonic acid containing polymers (PESAs) are the most commonly used membrane materials in fuel cells today. Membranes made from these ionomers provide the benefits of highly acidic pendant acid groups for high proton conductivity, good mechanical properties, excellent chemical stability, and fairly... 

One approach is to modify the PFSA side-chains such that they cany more than one acid site since the ciystallinify and morphological properties arise essentially from the ratio of non-substituted TFE to functionalised I FE of the backbone polymer repeat unit, multi-acid side-chain ionomer membranes have the potential to demonstrate the mechanical properties of a higher EW polymer (characteristic of a single acid site per side-chain), and the proton conduction properties of a lower EW material (conferred by the presence of multi-acid sites per side-chain). This direction is being followed at 3M, where introduction of... 

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