Electro-osmotic water drag

Kreuer et al. [25] investigated the membrane properties, including water sorption, transport (proton conductivity, electro-osmotic water drag and water diffusion), microstructure and viscoelasticity of the short-side-chain (SSC) perfluorosulfonic acid ionomers (PFSA, Dow 840 and Dow 1150) with different lEC-values. The data were compared to those for Nafion 117, and the implications for using such ionomers as separator materials in direct methanol and hydrogen fuel cells discussed. Tire major advantages of PFSA membranes were seen to be (i) a high proton conductivity. 

The first part of this chapter returns to the topic of transport processes in a PEM. Proton transport and electro-osmotic water drag in PEMs have been discussed extensively in Chapter 2. To understand the operation of the PEM in a fuel cell, other transport phenomena, including liquid water diffusion, hydraulic permeation, and interfacial vaporization exchange of water, must be addressed as well. Eor this purpose, the corresponding transport parameters must be found and their impact on performance rationalized. 

Proton conduction in these fuel cells has an undesired side effect of osmotic water drag, also known as the electro-osmotic drag, from anode to cathode which causes the necessity for water recycling from anode back to the cathode at the system level. 

Zawodzinski, T. A., Davey, J., Valerio, J. and Gottesfeld, S. 1995. The water-con-tent dependence of electro-osmotic drag in proton-conducting polymer electrolytes. Electrochimica Acta 40 297-302. 

Ren, X., Henderson, W. and Gottesfeld, S. 1997. Electro-osmotic drag of water in ionomeric membranes—New measurements employing a direct methanol fuel cell. Journal of the Electrochemical Society 144 L267-L270. 

What are the mechanisms and the transport coefficients of water fluxes (diffusion, convection, hydraulic permeation, electro-osmotic drag) ... 

The study of the dynamical behavior of water molecules and protons as a function of the state of hydration is of great importance for understanding the mechanisms of proton and water transport and their coupling. Such studies can rationalize the influence of the random self-organized polymer morphology and water uptake on effective physicochemical properties (i.e., proton conductivity, water permeation rates, and electro-osmotic drag coefficients). 

Under fuel cell operation, a finite proton current density, 0, and the associated electro-osmotic drag effect will further affect the distribution and fluxes of water in the PEM. After relaxation to steady-state operation, mechanical equilibrium prevails locally to fix the water distribution, while chemical equilibrium is rescinded by the finite flux of water across the membrane surfaces. External conditions defined by temperature, vapor pressures, total gas pressures, and proton current density are sufficient to determine the stationary distribution and the flux of water. 

Electro-osmotic drag phenomena are closely related to the distribution and mobility of protons in pores. The molecular contribution can be obtained by direct molecular d5mamics simulations of protons and water in single iono-mer pores, as reviewed in Section 6.7.2. The hydrod5mamic contribution to n can be studied, at least qualitatively, using continuum approaches. Solution of the Poisson-Boltzmann (PB) equation. 

Below /ps, the membrane performs under uniform saturation conditions, like a linear ohmic resistance. According to Equation (6.53), two modes of water management can be applied to compensate for electro-osmotic drag and keep the membrane in a well-hydrated state. Sufficient replenishment of water in the membrane can be accomplished by (1) providing a steady external water supply j > at the anode side, or (2) applying an external gas... 

N effective number of polymer chains in resin N molar flux of liquid water in the membrane number of SO3 groups in the dry membrane ng. electro-osmotic drag coefficient in PEM... 

Figure 15. Electro-osmotic drag coefficients Adras of (a) Nafion 117 (EW — 1100 g/equiv) and (b) sulfonated poly-(arylene ether ketone)s, as a function of the solvent (water and/or methanol) volume fraction 174.212,219,274-281...

There are actually no experimental measurements of protonic streaming currents (Lu) and coupled water and methanol transport (L23 = L32) however, the first may be related to the hydrodynamic component of the electro-osmotic drag L /Ln, Lis/Lu) (see discussion in Section 3.2.1.1). The second is expected to be qualitatively related to the ratio of the electro-osmotic drag coefficient of water and methanol (L12/Z.13). In the following, the directly accessible transport coefficients o (Do), FH2O, MvieOH,... 

Figure 20. Electro-osmotic drag coefficients of diverse membranes based on perfluorinated polymers (Dow - and Nafion/silica composites ) and polyarylenes (S—PEK/ PSU blends, ionically cross-linked S—PEK/PBP ), as a function of the solvent (water/methanol) volume fraction Xy (see text for references). Lines represent data for Nafion and S—PEK (given for comparison) for data points, see Figure 15. Dashed lines correspond to the maximum possible electro-osmotic drag coefficients for water and methanol, as indicated (see text).

The transport properties that are most significantly affected by changes of the water volume fraction are the water/methanol electro-osmotic drag and permeation, both of which have significant contributions from viscous flow (see Section 3.2.1.1). For DMFC applications (where the membrane is in contact with a liquid water/methanol mixture), this type of transport determines the crossover, which is only acceptably low for solvent volume fractions smaller than 20 vol % (see Figures 14 and 15). Consequently, recent attempts have been focused on strengthening... 

Two other important electrolyte properties for the PEFC system are the water diffusion coefficient and electro-osmotic drag coefficient. These two param-... 

Figure 5. Electro-osmotic drag coefficient and water diffusivity as functions of water content in Nafion membranes.

In the above, D rn is the water diffusion coefficient through the membrane phase only. Note also that the water fluxes through the membrane phase, via electro-osmotic drag and molecular diffusion, represent a source/sink term for the gas mixture mass in the anode and cathode, respectively. 

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