Nafion saturated

This study, in conjunction with that discussed in 12.2.1.2, show that when using aqueous electrolytes or Nafion saturated with H20, the induction of NEMCA on finely dispersed noble metal catalysts is rather straightforward. The role of the electronically conducting porous C support is only to conduct electrons and to support the finely dispersed catalyst. The promoting species can reach the active catalyst via the electrolyte or via the aqueous film without having to migrate on the surface of the support, as is the case when using ceramic solid electrolytes. 

The usual working temperature of fuel cells with Nafion-type membranes is 80 to 90°C. Under these conditions, moisture must be supplied to keep the membranes wet, which usually is attained by passing the reactant gases through water that is somewhat warmer (by 5 to 10°C) than the cell s working temperature, thus saturating them with water vapor. 

A fiber optic sensor for the determination of sodium was reported by Burgess.<52) A bifurcated fiber with a reference fiber 5 mm apart from the tip was used to observe the changes of bromothymol blue (Amax = 620 nm) attached to Nafion in the presence of sodium ions. As the tip was saturated, the probe was renewed with fresh reagent. However, the epoxy holding the fibers was prone to damage from high sodium concentrations of around 2.5 M and the sensitivity of analysis was low. 

Ru" (0)(N40)]"+ oxidizes a variety of organic substrates such as alcohols, alkenes, THE, and saturated hydrocarbons. " In all cases [Ru (0)(N40)] " is reduced to [Ru (N40)(0H2)] ". The C— H deuterium isotope effects for the oxidation of cyclohexane, tetrahydrofuran, 2-propanol, and benzyl alcohol are 5.3, 6.0, 5.3, and 5.9 respectively, indicating the importance of C— H cleavage in the transitions state. For the oxidation of alcohols, a linear correlation is observed between log(rate constant) and the ionization potential of the alcohols. [Ru (0)(N40)] is also able to function as an electrocatalyst for the oxidation of alcohols. Using rotating disk voltammetry, the rate constant for the oxidation of benzyl alcohol by [Ru (0)(N40)] is found to be The Ru electrocatalyst remains active when immobilized inside Nafion films. 

Nation 7.4-nm-diameter Ag particles Silver ions are spontaneously reduced in basic air-saturated solutions of propanol in the presence of powdered Nafion 62... 

FIGURE 1.13 Polarization curves for the reduction of oxygen on a platinum disk covered with a film of a-Nafion in oxygen-saturated 0.5 M H2S04 at different rotation speeds. (From Ayad, A., et al., J. Power Sources, 149, 66, 2005.)... 

Figure 15.9 shows the esterification of different saturated carboxylic acids with dicyclopentadiene. In all depicted experiments the carboxylic acid/dicyclo— pentadiene molar ratio is about 4 and the amount of catalyst is 10% by weight of dicyclopentadiene at a reaction temperature of 80°C. In this reaction the Nafion/silica composite catalyst is more active compared to the Amberlyst resin, in particular with respect to the amount of the acid groups on the resin. 

In Weibel etal., (2005a, b) the 100 °C, fraction of 1 litre, experimental cell is described in detail, and its results are given. The cell shares features and terminology with Regenesys, in that it has an anolyte (5 M sulphuric acid, saturated with iron suphate and from 0.3 to 4g of SBC, separated by a Nafion 112 membrane, from the catholyte, VOj and VO (total concentration IM) in 5M sulphuric acid. The anode and cathode electrode structures are both of carbon felt. The cell has incompressible reactants, but its products, CO2 and H2O, at 100°C are both compressible. In an equilibrium diagram of the cell there would have to be, to avoid irreversible diffusion, an isothermal concentration cell... 

The most successful solid protonic membrane for fuel cell use is NAFION membrane (saturated with aqueous acid solution) used in kW-size fuel cells in the US Gemini space program up to 150 °C. NAFION is a perfluorinated polymer with sulfonic acid groups. 

Figure 3.46. Conductivity of Nafion-115 as a function of humidity (partial pressure of water relative to saturation pressure, the latter corresponding to 18% water by weight or 11 water molecules per sulphonate molecule). (From C. Yang, S. Sriniva-san, A. Bocarsly, S. Tulyani, J. Benziger (2004). A comparison of physical properties and fuel cell performance of Nafion and zirconium phosphate/Nafion composite membranes. /. Membrane Sci. 737,145-161. Used with permission from Elsevier.)...

Figure 3.46 shows that dry Nafion is a poor conductor and that operating the fuel cell membrane close to full water saturation is essential. Proton conductivity increases modestly with temperature in the range of 20-80°C, i.e., below the values shown in Fig. 3.46 (Gil et al., 2004). 

For Nafion 117, the aluminum recovery profile approached saturation after 12 h but the same was linear after 24 h for lonac 3470. 

Increasing the initial H2SO4 concentration, Cq, from 1 M to 2 M increased aluminum recovery uptake for both membranes but there was a drop in aluminum concentration at the end for Nafion 117. This observation of drop in aluminum concentration beyond saturation is counterintuitive and was reconfirmed through a replicate experiment. 

Zawodzinski et al. [58] have reported NMR relaxation measurements on water in Nafion membranes. In contrast with proton NMR relaxation studies, which are difficult to interpret because of various inseparable contributions to the observed relaxation rates, a direct relationship often exists between the observed relaxation rate and rotational dynamics of the deuteron-bearing species. The time scale probed by such measurements is in the pico- to nanosecond range, and thus very short-range motions are probed. In a membrane equilibrated with saturated water vapor, a Ti on the order of 0.2s was observed. This relaxation rate for D2O in the membrane is only higher by a factor of two than that in liquid D2O, indicating a bulk water-like mobility within the pore at high membrane hydration levels. The relaxation rate increases (i.e., local water motion in the membrane becomes slower) as the water to ion-exchange site ratio decreases. 

The conductivity of Nafion 117 (EW 1100), Membrane C (EW 900), and Dow membranes (EW 800) at 30 °C was measured by Zawodzinski et al. [58-60] for membranes immersed in liquid water and for partially hydrated membranes in contact with water vapor. The conductivity of Nafion 117 decreases roughly linearly with decreasing water content, as shown in Fig. 32. The value of the conductivity measured at A = 14 (saturated vapor), 0.06 0.01 S/cm, agrees with the value previously reported by Rieke and Vanderborgh [62]. The dependence of conductivity on water content for Membrane C and Dow membranes is somewhat more complicated. In both cases, the conductivity decreases roughly linearly with decreasing water content until an apparent threshold is reached, at which point the conductivity drops substantially (Fig. 32). Above the water content where the steep drop occurs, the specific conductivity of both membranes is substantially higher than... 

Fig. 39. Computed Rceii (upper panel) and H O/H net flux ratio (lower part) for Nafion 1100 membranes of different thicknesses. Cell and saturator temper-ture, 80 °C. Unity fuel utilization [87]. (Reprinted by permission of the Electrochemical Society).

Reference 28 reports pulsed field gradient NMR studies of water motion in Nafion 1100, yielding intradiffusion coefficients close to 1 x 10-5 cm2 sec-1 for samples in contact with saturated vapor. For liquid-immersed Nafion membranes, other NMR diffusion imaging studies [29]... 

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