Conductivities of Nafion membranes

Figure 5.16. Schematic illustration of a conductivity cell for Nafion membranes (1) graphite current collectors (2) carbon paper electrodes (3) membrane (4) electrical connections (5) beaker filled with deionized water and (6) thermocouple [15]. (Reprinted from Journal of Power Sources, 134, Silva RF, De Francesco M, Pozio A. Tangential and normal conductivities of Nafion membranes used in polymer electrolyte fuel cells, 18-26, 2004, with permission from Elsevier.)...

Silva RF, De Francesco M, Pozio A (2004) Tangential and normal conductivities of Nafion membranes used in polymer electrolyte fuel cells. JPower Sources 134 18-26... 

The high proton conductivity of hydrated Nafion is the main reason of its wide use in PEM fuel cells. Many studies were devoted to determine this property under different conditions of temperature and water uptake (from vapor and liquid phase) on membranes subjected to different pretreatments, using AC impedance and DC techniques. Slade et al. [305] reviewed the conductivity of Nafion membranes measured tUl 2002, and recently Yadav and Fedkiw [306] have updated the information collected during the last decade. 

Huang QM, Zhang QL, Huang HL, Li WS, Huang YJ, Luo JL (2008) Methanol permeability and proton conductivity of Nafion membranes modified electrochemically with polyaniline. J Power Sources 184 338-343... 

Table 19.1. Conductivity of Nafion membranes in the protonic form in various electrolytes at 25 °C...

The conductivity of Nafion membranes depends on the equivalent weight of the ion exchange polymer and its water content, these two parameters being closely related. On the other hand, the conductivity has been found to be affected strongly by the pretreatment of the membrane At room temperature, the maximum in conductivity reaches 0.078 Q" cm for a Nafion 1100 EW without sorbed acid and 0.098 2 cm for a Nafion 1200 EW when HBr is present in the membrane phase . Some experimental values of conductivities are summarized in Table 19.1. 

TABLE 4.8.3 Effect of Pretreatment on Conductivity of Nafion Membrane in 40% KOH... 

TABLE 4.8.4 Conductivity of Nafion Membrane of 1200 EW in Various Electrolytes ... 

Costamagna, R, S. Grosso, and R. D. Felice. 2008. Rercolative model of proton conductivity of Nafion membranes. /. Power Sources 178 537-546. 

As already mentioned in previous sections, the sulfonic acid groups in PFSA membranes exhibit remarkably high acidity. In the presence of water, the membrane hydrates to form a continuous hydrated domain in which protons are solvated by water with the inunobDized anionic group (R-SOs ) as counter-ions [8], High italic conductivity of Nafion membranes can also be achieved in solvent environments other than pure water. In 1994, the first attempt was made to... 

In the FTIR study on chemical aging of Nafion conducted by Alentiev et al. (2006), (-S(0)j-0-) groups not observed in native Nafion were found after thermal treatment of Nafion films at 95°C. The same result was obtained by the authors for a Nafion film that had been stored for 14 years. Their findings indicate that sulfo-ethers and crosslinking can appear during storage, exploitation, and thermal treatment of perfluorosulfonic membranes, which should reduce the concentration of sulfo-groups and decrease the proton conductivity of Nafion membranes in the process of exploitation, such as in fuel cells. 

More recently, EC-AFM [11,66,67] has been applied to study the morphology and proton conductivity of Nafion membranes. Using EC-AFM, the spatially resolved proton current driven by the electrochemical reactions occurring on the two sides of the Nafion membrane can be measured. The experimental setup is shown in Fig. 5.15. A commercial AFM is equipped with an electrochemical cell and a biopotentiostat. The AFM tip is modified with a platinum electrode to act as the cathode catalyst for the fuel cell reaction, whereas the Nafion membrane coated with Pt catalyst on the side opposite the AFM tip serves as the anode of the electrochemical cell. The applied voltage leads to water oxidation and generates protons on the electrode with the Pt catalyst. The current can then be detected when the AFM tip is in contact with an ion channel that is connected with the ionic network in the membrane. Because water is required for the electrochemical reactions, the experiment must be performed... 

Fig. 1 Conductivity of Nafion membranes exposed to various saturation levels versus temperature near the freezing point. (Reproduced with permission from Mukundan et al. (2005). Copyright...

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