Page 51 - Introduction to Transfer Phenomena in PEM Fuel Cells

40 Introduction to Transfer Phenomena in PEM Fuel Cells

Figure 1.16. Chemical structure of Nafion

Other suppliers also provide polymer membranes: Dow Chemical (Dow
Membrane with a shorter chain), Asahi Glass (Flemion), Asahi Chemical
(Aciplex), Gore, Pemeas, etc. These polymers are physically in the form of
sheets, rolls or in solution, which is cast to be shaped [BOU 07]. Nafion
membranes are available in different sizes and thicknesses; they are usually
marked with the letter “N” followed by three or four digits, the first two of
–1
which represent the equivalent weight (EW) expressed in g.eq , divided by
100, whereas the remaining numbers represent the thickness of the
membrane expressed in mills (1 mill = 1/1,000 inch = 25.4 µm). The typical
TM
thicknesses of Nafion membranes are 2; 3.5; 5; 7 and 10 mills, that is, 50;
TM
89; 127; 178 and 254 µm respectively. For example, Nafion N117 has an
–1
equivalent weight of 1 100 g eq and a thickness of 7 mills (178 µm) [BAR
05]. The smaller the thickness, the greater the membrane is permeable to gas,
but the fewer the proton conductionlosses.

1.3.1.1.2. The concept of equivalent weight
The equivalent weight (EW) is the weight of polymer per mole of active
–
sites (SO 3 ). It corresponds to the inverse of the ion exchange capacity (IEC)
which represents the amount of sulfonate groups introduced per gram of
polymer in a fuel cell [BAR 05]. It is expressed as follows:

+
=
EW 100n 446 [1.19]