Nafion membranes temperature effect

It is favorable for fuel cell operation when reduced methanol transport across the membrane is accompanied by proper water management. In particular, a low water crossover from the anode to the cathode is necessary to avoid flooding of the cathode. The dependence of water permeation on the membrane thickness is weak. Only a small decrease in water permeation is observed for the commercial Nafion membranes, whereas the thickness of the recast membranes has no significant influence on the water transport rate. In contrast, the effect of temperature on water permeation is strong. At 65°C, the rates are higher by a factor of 5 compared to those at 25°C. 

Water, sodium ion, and hydroxide ion concentrations have been measured within the membrane phase as a function of bulk caustic solution concentration and temperature. These internal membrane concentrations are important because of their influence on the membrane polymer morphology, structural memory, plasticity and the resultant effects on its internal resistance, viscoelasticity and material transport. In addition, the self-diffusion coefficient of the sodium ions in various Nafion membranes has been measured as a function of temperature and external caustic concentration... 

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. 

In Chapter 1 of this book, the necessary parameters for both RDE/RRDE analysis in ORR study, such as O2 solubility, O2 diffusion coefficient, and the viscosity of the aqueous electrolyte solutions, are discussed in depth in terms of their definitions, theoretical backgroimd, and experimental measurements. The effects of type/concentration of electrolyte, temperature, and pressure on values of these parameters are also discussed. To provide the readers with useful information, the values of these parameters are collected from the literature, and summarized in several tables. In addition, the values of both the O2 solubility and diffusion coefficient in Nafion membranes or ionomers are also listed in the tables. Hopefully, this chapter would be able to serve as a data source for the later chapters of this book, and also the readers could find it useful in their experimental data analysis. 

C.F. Martins, L. Neves, I.M. Coelhoso, F. Vaca Chavez, J.G. Crespo, P.J. Sebastiao, Temperature effects on the molecular dynamics of modified NAFION membranes incorporating ionic liquids cations a NMRD study, Fuel Cells 13 (2013) 1166-1176. 

Ho KC, Hung WT (2001) An amperometric N02 gas sensor based on Pt/Nafion electrode. Sens Actuators B 79 11-18 Ho KC, Liao JY, Yang CC (2005) A kinetic study for electrooxidation of NO gas at a Pt/membrane electrode-apphcation to amperometric NO sensor. Sens Actuators B 108 820-827 Imaya H, Ishiji T, Takahashi K (2005) Detection properties of electrochemical acidic gas sensors using halide-halate electrolytic solutions. Sens Actuators B 108 803-807 Ives DJG, Janz GJ (eds) (1961) Reference electrodes theory and practice. Academic, New York, NY Jordan LR, Hauser PC, Dawson GA (1997) Humidity and temperature effects on the response to ethylene of an amperometric sensor utilizing a gold-Nafion electrode. Electroanalysis 9 1159-1162 Katayama-Aramata A, Nakajima H, Fujikawa K, Kita H (1983) Metal electrodes bonded on sohd polymer electrolyte membranes (SPE)—the behaviour of platinum bonded on SPE for hydrogen and oxygen electrode processes. Electrochim Acta 28 777-780... 

A rational analysis of filler effects on structural, proton transport properties and electrochemical characteristics of composite perfluorosulfonic membranes for Direct Methanol Fuel Cells (DMFCs) was reported [7]. It has been observed that a proper tailoring of the surface acid-base properties of the inorganic filler for application in composite Nafion membranes allows appropriate DMFC operation at high temperatures and with reduced pressures [7]. An increase in both strength and amount of acidic surface functional groups in the fillers would enhance the water retention inside the composite membranes through an electrostatic interaction, in the presence of humidification constraints, in the same way as for the adsorption of hydroxyl ions in solution [7]. 

Proton conductivity in PFSA membranes depends upon the polymer EW (nrrmber of charge carriers), and the hydration number (A, number of water molecrrles/srrlforric acid group), polymer stracture, membrane morphology and temperatirre, all of which affect proton mobility. The proton conductivity of some recent PFSA membrane materials is shown in Fig. 2.3-2 5. Figure 2.3 illustrates for 3MTM PFSA membranes the effect of polymer EW and hydration niunber on proton conductivity at 80 °C, Fig. 2.4 displays the variation of proton conductivity of Aquivion membranes with temperature and relative hirmidity, and Fig. 2.5 shows the conductivity of Nafion NR-211 at 30 °C, 50 °C and 80 °C over a range of relative humidity values. 

The work of Adachi et al. (2009) represented a first attempt to correlate and validate ex situ and in situ water permeation phenomena in PEMs. Water permeabilities of Nafion PEMs and water transport in operating PEFCs were investigated under comparable ex situ and in situ values of temperature and RH. The examined parameters included the type of driving forces (RH, pressure), the phases of water at PEM interfaces, PEM thickness, and the effect of catalyst layers at the membrane interfaces. Several experimental setups and schemes were designed and explored. Water permeability at 70°C was determined for Nafion membranes exposed to either liquid or vapor phases of water. Chemical potential gradients of water across the membrane are controlled through the use of differences in RH (38-100%), in the case of contact with water vapor, and hydraulic pressure (0-1.2 atm), in the case of contact with liquid water. Three types of water permeation experiments were performed, labeled as vapor-vapor permeation (VVP), liquid-vapor permeation (LVP), and liquid-liquid permeation (LLP). Ex situ measurements revealed that the flux of water is largest... 

Water Content and Hydration Temperature Effects. As mentioned earlier, the Dow membrane is more amorphous than Nafion 117. This allows the Dow membrane polymer matrix to adsoib more water than Nafion 117. The temperature at which the membrane is hydrated also influences the swelling of the ionomer matrix. The highest temperature used during the membrane preparation procedure controls the water content of the membrane. The water content of the membrane was calculated by dividing the weight of water absorbed by the total weight of the hydrated membrane. The water content of a membrane is primarily controlled by the inherent structure of the ionic polymer (Table I). 

Temperature Effects. The temperature for each transport experiments was controlled with a water bath. The membranes were tested at 5,25 and 35 The results obtained from both Nafion 117 (Figure 4) and Dow membrane (Figure 5) show that the permeability increased as the temperature was raised. 

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