Temperature effects membrane conductivity

It has been widely reported [61-64] that temperature can significantly affect the proton conductivity (cr) of a membrane. For a PFSA membrane (e.g. Nafion membrane), the proton conductivity strongly depends on the water content of the membrane. Therefore, when studying the effect of temperature on membrane conductivity, the RH or water content of the membrane must be considered. At a low RH, an increase in the temperature will cause membrane dehydration, resulting in decreased proton conductivity, whereas with a well-hydrated membrane, the proton conductivity will increase with increasing temperatures. For example, the conductivity of Nafion 117 at 100% RH increases from 0.1 to 0.2 S cm when the temperature is raised from 30 to 85 °C [65]. Generally, under weU-hydrated conditions, the temperature dependence of conductivity can be expressed in an Arrhenius form [2] ... 

However, for a PFSA membrane operated at high temperatures, the situation can be more complex due to the tradeoff between increased conductivity and membrane dehydration when the temperature is increased. As shown in Fig. 4.5, the resistance of Nafion - 12 membrane decreases shghtly when the temperature is increased from 80 to 100 °C, which indicates an increase in proton conductivity, but the resistance increases when the temperature is raised to 120 °C, suggesting a decrease in proton conductivity. This is because the negative effect of dehydration is larger than the positive effect of temperature on membrane conductivity. 

Ammonia, produced due to the coexistence of H2 and N2 at high temperatures in the presence of catalyst, was estimated to be in the concentration range of 30 to 90 ppm [37, 38], Uribe et al. [39] examined the effects of ammonia trace on PEM fuel cell anode performance and reported that a trace in the order of tens of parts per million could lead to considerable performance loss. They also used EIS in their work. By measuring the high-frequency resistance (HFR, mainly contributed by membrane resistance) with an operation mode of H2 + NH3/air (feeding the anode with hydrogen and ammonia), they obtained some information related to membrane conductivity, and found that conductivity reduction due to ammonia contamination is the major cause of fuel cell degradation. 

Figure 10.5 Temperature and conversion profiles in an adiabaiic packed-bed membrane reactor with a dimensionless effective thermal conductivity of the membrane as a parameter [Itoh and Govind, 1989b]...

The effect of NaOH concentration and temperature on the swelling and conductivity of Nafion has been recently studied by Men shakova et The influence of NaOH concentration on the membrane conductivity is rather similar to the case of acid electrolytes in that a conductivity maximum occurs. The authors found that the conductivity maximum of the membrane is at —20% NaOH, i.e., close to the concentration where one observes the conductivity maximum of the free electrolyte. However, the conductivity decrease following the maximum is more drastic in the membranes... 

Caustic Soda Concentration. The maximum electric conductivity of caustic soda solution occurs at a concentration of about 20% at the ordinary electrolysis temperature, and the membrane conductivity tends to decline sharply with caustic soda concentration in the catholyte exceeding 20% (26). The boundary layer effect described in the previous section also makes relatively low concentrations preferable. With increasing concentration of caustic soda, moreover, the allowable concentration of multivalent cation in the brine must be decreased exponentially because the solubility products of multivalent cation hydroxides are constant, and operational difficulties... 

The thermal treatment of Nafion membrane has a significant effect on conductivity as shown by Sone et al. [318], who measured the conductivity of expanded Nafion 117 without heat-treatment and after heat-treatment at 80 °C (N-form), 105 °C (S-form), and 120 °C (FS-form). The thermal treatment reduces the conductivity of the expanded Nafion from 80 mS.cm down to 50 mS.cm for the N-form, and down to 30 mS.cm for the S- and FS-form. The reduction of conductivity correlates with the reduction of water uptake with increasing temperature of thermal treatment. 

Attempts to quantify the overall effect of CO2 on the effective anion conductivity in an anion exchange membrane have been done. For instance, Kim [20] and Lin et al. [21] reported effective anion conductivity reduction of 10-50 %, depending on temperature. Figure 5 shows a comparison of anion exchange membrane conductivity measured in full OH form... 

Maalouf M, Pyle B, Sun CN, Wu D, Paddison SJ, Schaberg M, Emery M, Lochhaas KH, Hamrock SJ, Ghassemi H, Zawodzinski TA (2009) Proton exchange membranes for high temperature fuel cells equivalent weight and end group effects on conductivity. ECS Trans 25 1473-1481... 

The effective PEM conductivity depends on the random heterogeneous morphology, namely, the size distribution and connectivity of the proton-containing aqueous pathways. Random network model of PEMs was developed in Eikerling et al. (1997). It included effects of the swelling of pores and the evolving connectivity of the pore network upon water uptake. The model was applied to study the dependence of membrane conductivity on water content and temperature. It could rationalize trends in... 

Temperature Effect on Membrane Conductivity and Hydrogen Crossover 132... 

Chapter 5 provides a more detailed discussion of the temperature effect on membrane proton conductivity. 

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