PFSIs ionomers

Equivalent weights for PFSI ionomers immediately following precipitation from the reaction medium and for membranes formed as described in section 22.5 were measured by acid-base titration. In brief, the procedure is as follows. A known amount of dried polymer (in a typical measurement, approximately 0.0670 g of polymer) and 0.10 g of NaCl were dissolved in 30 mL of deionized water, and the resulting solution was refluxed in a 150-mL conical-shaped flask at 80°C overnight with constant stirring. The flask was covered with a latex balloon and then filled with nitrogen gas through a septum. The polymer swelled up in water. A few drops of phenolphthalein were added to the solution, which was then titrated with a 0.0100 M... 

TABLE 22.3 Ionic Conductivity of PFSI Ionomer Membranes... 

An adequate structure of polymer molecules promotes the advantageous phase separation into hydrophobic and hydrophilic domains upon water uptake. The most notable class of membranes based on this principle are the perfluorosulfonic acid ionomers (PFSI), Nafion [26] and similar membranes [27]. In these membranes, perfluorosulfonate side chains, terminated with hydrophilic —SO3H groups, are attached to a hydrophobic fluorocarbon backbone. The tendency of ionic groups to aggregate into ion clusters due to the amphiphilic nature of the ionomer leads to the formation of basic aqueous units. At sufficient humidity these units first get connected by narrow channels and then may even fuse to provide continuous aqueous pathways [28]. 

Perfluorosulfonate ionomer (PFSI) - Dow Cemical Company Fig. 1.5. Chemical structures of perfluorosulfonate ionomer materials. 

Figure 13.1 Generic chemical formula of perfluorosulphonate ionomer (PFSI), with variable indices m and n. For Flemion m = 0-1, n = 1-5 Nafion m =, n = 2 Dow m=, n= Aciplex m = 0-2, n = 1-4. Typical molecular weights are estimated to be in the range 10 -10 g moF (Mauritz and Moore [2]), which would correspond a random copolymer with x = 595-5945, y = 91-909 for 1100 EW molecules of Nafion.

Subsequently, we will discuss effective rates of current conversion at the agglomerate level. Most commonly it is assumed that agglomerates in CCLs either comprise a homogeneous mixture of carbon/catalyst particles and perfluorosulfonated ionomer (PFSI) or that they consist of aggregated carbon/catalyst particles and water-filled pores which are coated on the external surface by a fihn of ionomer. These types of agglomerates represent the two limiting structures in terms of ionomer distribution at the mesoscopic scale, and both of them may exist simultaneously. 

Colloid Method In order to improve gas transport through the ionomer-bonded hydrophilic catalyst layer, some modified hydrophilic electrodes have also been developed. One technique is called the colloid method, which changes Nafion into a colloid state. The colloid ionomer (such as Nafion) can adsorb catalyst particles to form larger catalyst/Nafron agglomerates. It is believed that the colloid method benefits the construction of the CL microstructure and enhances gas transportation [41-45]. Uchida et al. [41, 42] first proved that butyl acetate with a e of 5.01 was the best solvent to form PFSI colloids for the preparation of a catalyst layer. The detailed steps are as follows ... 

Add butyl acetate into the PFSI solution to form an ionomer colloid. 

In PEMFCs, the CLs are typically comprised of carbon-supported platinum catalyst and PFSls. These CLs usually contain 20-40 wt% PFSI. The ionomer in the CLs has three roles (1) to act as a binder between the platinum/carbon particles, (2) to provide a proton-conductive link between the bulk membrane and platinum catalyst sites for protonic current flow, and (3) to make the platinum catalyst electro chemically active by transferring protons to and from it (Young et al., 2010). 

Unlike hydrophobic CLs, hydrophilic CLs use a hydrophilic perfluorosulfonate ionomer (PFSI) such as Nafion as a binder instead of PIPE. Hence, this kind of CL can be called an ionomer-bonded hydrophilic CL. During preparation, the catalyst powder (e.g. Pt/C), PFSI (e.g. Nafion ), and solvent (e.g. ethanol or isopropanol) are mixed thoroughly to form a uniform hydrophilic catalyst ink/paste that is then transferred to a GDL or a membrane. Hydrophilic CLs can be classified into two groups, according to the transfer method GDL-based hydrophilic CL and catalyst coated membrane (CCM). 

While lots of PEMs have been developed, the sulfonated hydrocarbon polymer electrolyte (SHCPE) and perfluorocarbon sulfonated ionomer (PFSI) membranes are most widely investigated for LT-PEMFCs and DMFCs, and the PBI and its modified polymer membranes doped with H3PO4 are most widely applied to HT-PEMFCs. In the following sections, we discuss the morphologies and conducting mechanisms of the SHCPEs, PFSIs, and PBI doped with H3PO4 (PBI/H3PO4) membranes. 

In spite of many PEMs developed and reported in literature, Nafion (a trade name by DuPont Co., which is a perfluorosulfonic acid ionomer [PFSI]) is considered the benchmark of PEMs [23,32], and it has the largest number of researchers devoted for its study. In the following sections, we discuss the morphology of Nation PEM (Sections 1.3.2.2.2 and 1.3.2.2.3) and sulfonated hydrocarbon PEM (Section 1.3.2.2.4). 

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