Electroosmotic drag

The importance of the ionomer in the electrode for the performance of the PEMFC has been well known since the pioneering work of Raistrick et al. [37]. In the PEMFC, the electroosmotic drag of water due to the proton transport from the anode to the cathode leads to the membrane drying out from the anode side (back diffusion of water from cathode to anode compensates partly for the water loss from the anode side of the membrane). Therefore, the loss of conductivity of the ionomer at the anode is also an additional important issue related to the membrane topic, since the ionomer in the electrode needs to connect ionically and chemically to the membrane. In an investigation of the transverse water profile in Nafion in PEMFCs with a... 

T.A. Zawodzinski, J. Davey, J. Valerio, and S. Gottesfeld. The water-content dependence of electroosmotic drag in proton-conducting polymer electrolytes. Electrochimica Acta 40, 291-302 1995. 

In order to evaluate the steady-state water profile in the membrane of a PEFC under given operating conditions, the necessary membrane transport properties required thus include water uptake by the membrane as function of water activity and membrane pretreatment conditions, A(aw) (covered in Section 5.3.1) the diffusion coefficient of water in the membrane as a function of membrane water content, D ) the electroosmotic drag coefficient as a function of membrane water content, (A) and the membrane hydraulic permeability, A hy(i(A). Section 5.3.2 includes a discussion of water transport modes in ionomeric membranes. 

The number of water molecules carried through the membrane per proton when a protonic current flows through the membrane is a central factor in the determination of the water profiles in the membrane of an operating PEFC. This number has been reported over the years with considerable variation. There is an important difference between the electroosmotic drag coefficient, (A), a characteristic of an ionomeric... 

Fuller and Newman [96] reported an elegant solution to the problem of obtaining electroosmotic drag coefficients ( transport numbers of water in their parlance) in ionomeric membranes under conditions of vapor phase equilibration. Their EMF method is based on the membrane potential which arises across a membrane sample exposed at each end to different water activities. The potential difference A is determined by (A) according to [96] ... 

The initial emphasis on evaluation and modeling of losses in the membrane electrolyte was required because this unique component of the PEFC is quite different from the electrolytes employed in other, low-temperature, fuel cell systems. One very important element which determines the performance of the PEFC is the water-content dependence of the protonic conductivity in the ionomeric membrane. The water profile established across and along [106]) the membrane at steady state is thus an important performance-determining element. The water profile in the membrane is determined, in turn, by the eombined effects of several flux elements presented schematically in Fig. 27. Under some conditions (typically, Pcath > Pan), an additional flux component due to hydraulic permeability has to be considered (see Eq. (16)). A mathematical description of water transport in the membrane requires knowledge of the detailed dependencies on water content of (1) the electroosmotic drag coefficient (water transport coupled to proton transport) and (2) the water diffusion coefficient. Experimental evaluation of these parameters is described in detail in Section 5.3.2. 

Springer and others were the first to use detailed, experimentally derived diffusion and electroosmotic drag coefficients of water in Nafion in a model for steady-state water profile and the resulting protonic conductivity in the membrane of an operating PEFC [87]. The distribution of water in a PEFC at steady, state (at constant current and reactant/water fluxes) was calculated in this model by considering water flow through five regions of unit cross-sectional area within the fuel cell two inlet... 

Having established the boundary conditions for the membrane by expressions for Xw2 and Xw3, the next step is to consider the components of water flow within the membrane. As stated above, they include electroosmotic drag, diffusion, and... 

However, in the cell the membrane hydration is affected by generic fuel cell processes, including the supply of humidified reactant gases to the electrodes, electroosmotic drag of water from anode to cathode, backtransport of water in the membrane, and production of water at the cathode. It is, therefore, generally important to consider the internal membrane water balance self-consistently and relate it to the membrane microstructure. 

Stationary fuel cell operation requires a steady flow of protons through all membrane cross sections, perpendicular to the transport direction. Proton flow induces water transport from anode to cathode by electroosmotic drag [78], Taken alone, this effect would lead immediately to membrane dehydration and to a drastic increase of its ohmic resistance. However, accumulation of water on one side of the membrane inevitably causes a backflow of water. The balance between this backflow and the electroosmotic flow leads to a stationary profile of water across the membrane. 

As a boundary condition to Eq. (7), / l can be fixed either on anode, P, or cathode side, P. In practice, that is, under operation conditions, it is justified to suppose full saturation of the membrane at the cathode side, due to the cathodic water production and the direction of electroosmotic drag to this side. Therefore,... 

This conclusion is straightforward as far as the electroosmotic drag coefficient is a weak function of r. If it decreases dramatically with the decrease of r, the conclusion will depend on the competition between the drag and permeation coefficients. If, however, the whole variation of the drag coefficient is between 1 and 1.4, the conclusions made will stay valid. 

Water is produced at the cathode by the cell process and can enter the cell as part of (humidified) feed streams. Water is transported in the PEFC through the membrane from anode to cathode by electroosmotic drag, that is, by a water flux associated with the protonic current. The flux of water due to electroosmotic drag (moles cm-2 sec-1) is given by ... 

The experimentally determined isotherm for water sorption by the membrane (see Fig. 9) was used to convert from water vapor activity in the gas phase at the interface to water content in the membrane at the surface and considering generation of water at the cathode, the water level in the membrane surface adjacent the cathode was assumed accordingly to be X = 14 (or somewhat above, see Ref. 36). The components of water flow considered within the membrane included electroosmotic drag and diffusion, whereas hydraulic flux was considered insignificant (i.e., very small hydraulic pressure drop across the membrane) [36]. Consequently, the basic equation describing the flux within the membrane becomes... 

Fig. 3 Electroosmotic drag co-efficient obtained by electro-phoretic-NMR as a function of the water content, n = [H2O]/ [-SO3H]. (From...

FCs. For the further development of FCs, membranes must be fabricated that allow FC operation at temperatures at or above 120°C. While direct methanol FCs are close to commercialization, membranes with lower methanol crossover and electroosmotic drag coefficients would greatly facilitate their wider use. 

Although it is beyond the scope of this chapter, a relatively straightforward analysis of the SECM data shown in Figure 19 allows one to compute the convective velocity of the solute, ueo, and the electroosmotic drag coef-... 

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