PFSA dispersions

Fig. 24 Estimated models of PFSA dispersed in diluted aqueous solutions...

Fig. 26 Basic model assumed in the analysis of PFSA dispersion by UAS...

It should be noted that as both PFSAs and PFCAs are surfactants, water solubility increases abruptly when the critical micelle concentration (CMC) is reached. CMC is the concentration at which equilibrium of singly dispersed molecules and molecules in aggregated or the micelle form is reached. CMC values of PFOS and PFOA are approximately 4 and 3.6 to 4g/L respectively [4]. 

One activity at DuPont s Fuel Cells Business Center is the development of a thinner membrane with sufficient mechanical stabdity. Thinner membranes translate into higher current density, which in turn means a higher electrical efficiency. The tradeoff is a less mechanically robust membrane. Nafion membranes are nonreinforced films based on Nafion resin, a PFSA/PTFE copolymer in the acid (H+) form. DuPont is especially marketing Nafion PFSA NR-111 and NR-112 membranes as nonreinforced dispersion-cast films for that purpose. These membranes are dehvered as a composite with the membrane positioned between a backing film and a coversheet. This composite is wound on a 6 in. i.d plastic core, with the backing film facing out, as shown in Figure 27.21. 

The application to fuel cells was reopened by Ballard stacks using a new Dow membrane that is characterized by short side chains. The extremely high power density of the polymer electrolyte fuel cell (PEFC) stacks was actiieved not only by the higher proton conductance of the membrane, but also by the usage of PFSA polymer dispersed solution, serpentine flow separators, the structure of the thin catalyst layer, and the gas diffusion layer. Although PFSA membranes remain the most commonly employed electrolyte up to now, their drawbacks, such as decrease in mechanical strength at elevated temperature and necessity for humidification to keep the proton conductance, caused the development of various types of new electrolytes and technologies [7], as shown in Fig. 2. 

The structures of PFSA membranes have been analyzed and discussed by many researchers, and the cluster-network model for hydrated membranes proposed by Gierke [22] has been a basic model symbolic of the PFSA characteristics up to now. As for the structure of the diluted aqueous solution of PFSA, it is important to understand the structure of ionomer dispersion and catalyst ink, comprising catalyst particles, ionomer, and solvent, for the preparation of cast membrane and catalyst layer, respectively. Aldebert et al. 

Gebel proposed a schematic model of the structural evolution from dry membrane to colloidal dispersion of rodlike particles based on the results of the scattering analysis of PFSA over a wide range of water contents, combined with energetic considerations [94]. 

Typically, the membranes for low-temperature PEM fuel cells are made of perfluoiocaibon-sulfonic acid ionomers (PFSA). The best material known is Nafion produced by DuPont, though similar materials have been developed for commercial or development purposes by other manufacturers such as Asahi Glass (Flemion) or As Chemical (Aciplex). In the electrode part, the catalyst is generally dispersed as small particles on a conductive support (carbon powders). Despite many attempts to develop a non-noble-metal catalyst, platinum remains the best known electrocatalyst for ORR in PEM fuel cells. The most extensive limitations to large-scale commercial use of these materials arise from the fact that... 

Cui and co-workers performed classical molecular-dynamics simulations of two dilferent perfluorosulfonic-acid (PFSA) membranes to investigate the hydrated morphology and the hydronium-ion dilfusion. They put special emphasis on the water content of the membrane (5% to 20%) and compared the properties for two dilferent lengths of the side chains carrying the sulfonic-acid groups. The short side chains lead to a more disperse distribution of water clusters inside the membrane. At low water content this results in a more connected water-channel network, which enhances the proton transport. 

The catalyst ink usually includes catalyst, carbon powder, binder, and solvent. Sometimes, other additives are added to improve the dispersion of the components and stabilize the catalyst ink. The catalyst either covers the surface of the GDL or directly coats the surface of the membrane (catalyst coated membrane, CCM). The CL usually consists of (1) an ionic conductor such as perfluorosulfonate acid (PFSA) ionomer to provide a passage for protons to be transported in or out, (2) metal catalysts supported on a conducting matrix like carbon, to provide a means for electron conduction, and (3) a water-repelling agent such as polytetrafluoroethylene (PTFE) to provide sufficient porosity for the gaseous reactants to be transferred to catalyzed sites [5, 6]. Every individual factor must be optimized to provide the best overall performance of a CL. 

In sulfonated hydrocarbon polymers, the hydrocarbon backbones are less hydrophobic and the sulfonic acid functional groups are less acidic and polar. As a result, the water molecules of hydration may be completely dispersed in the nanostructure of the sulfonated hydrocarbon polymers. Both PFSA and sulfonated hydrocarbon membranes have similar water uptakes at low water activities, whereas at high relative humidity (100%) PFSA membranes have a much higher water uptake due to the more polar character of the sulfonic acid functional groups. The sulfonated aromatic polymers have different microstructures from those of PFSA membranes (Fig. 8) (Li et al., 2003). 

Casting Method In a novel method suggested by Matsubayashi et al. (1994) PFSA solution is mixed with the catalyst and dried in a vacuum. Then, the PFSA coated catalyst is mixed with a PTFE dispersion, calcium carbonate used to form pores, and water. The mixture is passed through a filter and the filtrate is formed into a sheet. The sheet is then dipped in nitric acid to remove any calcium carbonate. The sheet is then dried and PFSA solution is applied to one side of the electrode catalyst layer. Finally catalyst layer is applied to the membrane. 

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