Nafion acid, isotherms

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... 

Nafion 120 in the acid and neutralized form has been studied (11). Isotherms have been obtained at 10°C and at lower temperatures. ... 

Figure 2. Room temperature sorption-desorption isotherms of acid Nafion. Key , room temperature dried absorption , 220° C dried primary absorption , 220° C dried desorption Q, 220° C dried secondary absorption.

As the membrane becomes more hydrated, the sulfonic acid sites become associated with more water allowing for a less bound and more bulk-like water to form. This is why there is a flattening out of the slope above X = 6 in the uptake isotherm, Figure 5.1. The extreme case is when the membrane is placed in a liquid-water reservoir, where the ionic domains swell and a bulklike liquid-water phase comes into existence throughout the membrane. The way in which water does this is unknown but is probably due to the interfadal properties of the membrane, such as the fluorocarbon-rich skin on the surface of Nafion [26,27] or the removal of a liquid-vapor meniscus at the membrane surface [28]. In essence, the reorganization results in a porous structure with an average pore size between 1 and 2 nm [29]. 

Fig. 11.10 Water uptake isotherms of various ionomers and sulfuric acid at 80 C. Dowex 50 is an ion-exchange resin made of 4% cross-linked polystyrene divinyl benzene BPSH 40 is a 2-mil 40% randomly sulfonated biphenol sulfone 700 EW PFSA is a 1-mil membrane with a structure similar to Nafion Nafion 112 is a 2-mil extruded membrane and, PAEK triblock is a 1-mil triblock polyaryl ether ketone with a sulfonated middle block (Reproduced from C.K. Mittelsteadt and H. Liu. [30] by permission from John Wiley Sons)...

Verbrugge and Hill (1990) presented the theoretical representation and experimental data to characterize the proton and water in perfluorosulfonic acid membrane. The analysis describes the transport of water molecules carried along with the transport of proton across the membrane. Springer et al. (1991) presented a one-dimensional, isothermal model of FEMFC in which detailed considerations of the Nafion-117 membrane characteristics in terms of water content and water transport properties including water drag coefficient and diffusion coefficient are given. Their results demonstrated increased membrane resistance with current density. 

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