PEMFC Nafion/silica

Wang et al. [19] developed a new method of electrode preparation with a novel silica gel additive (NSGA) instead of the traditional Nafion polymer additive (TNPA) for PEMFCs. The performance of the two types of electrodes was compared using electrochemical impedance spectroscopy coupled with a polarization curve and constant-current discharge. Figure 6.20 shows the AC... 

Figure 6.20. Nyquist plots for the electrodes fabricated according to the same preparation procedure [19]. Note NSGA stands for novel silica gel additive, and TNPA stands for traditional Nafion polymer additive. The values in parentheses are the ohmic drop corrected cell potential. (Reproduced from Wang C, Mao ZQ, Xu JM, Xie XF. Preparation of a novel silica gel for electrode additive of PEMFCs. Journal of New Materials for Electrochemical Systems 2003 6(2) 65-9, with permission from JNMES.)...

Addition of hydroscopic metal oxides such as silica, zirconia, or titania to a proton-conducting polymer is the most obvious way to improve water retention at elevated temperatures (Aparicio et al., 2003). Unfortunately, due to the negligible proton conductivity of these oxides, an increase in the overall resistance of the composite membrane is observed, especially at low temperatures. However, as the temperature is increased, the conductivity gain due to better hydration offsets the loss due to the excluded conducting volume, and the net fuel cell performance is improved, as compared to an unmodified membrane (Adjemian et al., 2002a,b). It should be stressed that there are limits to the water sorption capability of the oxides. While these membranes retain more water than traditional PEM materials such as Nafion at high temperature and low RH, water uptake is insufficient and ohmic losses are still unacceptably high for PEMFC applications. 

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