Dehydration membrane

Gas Dehydration Water is extremely permeable in polymer membranes. Dehydration of air and other gases is a growing membrane application. 

The experimental optimization of Nafion ionomer loading within a catalyst layer has attracted widespread attention in the fuel cell community, mainly due to its critical role in dictating the reaction sites and mass transport of reactants and products [15,128-134]. Nafion ionomer is a key component in the CL, helping to increase the three-phase reaction sites and platinum utilization to retain moisture, as well as to prevent membrane dehydration, especially at low current densities. Optimal Nafion content in the electrode is necessary to achieve high performance. 

Proton conductivities of 0.1 S cm at high excess water contents in current PEMs stem from the concerted effect of a high concentration of free protons, high liquid-like proton mobility, and a well-connected cluster network of hydrated pathways. i i i i Correspondingly, the detrimental effects of membrane dehydration are multifold. It triggers morphological transitions that have been studied recently in experiment and theory.2 .i29.i ,i62 water contents below the percolation threshold, the well-hydrated pathways cease to span the complete sample, and poorly hydrated channels control the overall transports ll Moreover, the structure of water and the molecular mechanisms of proton transport change at low water contents. 

Figure 15. Simulation results showing membrane dehydration (a) and cathode flooding (b). (a) 1 as a function of membrane position (cathode on the left) for different current densities. (Reproduced with permission from ref 14. Copyright 1991 The Electrochemical Society, Inc.) (b) Dimensionless oxygen mole fraction as a function of cathode-diffusion-medium position and cathode overpotential. (Reproduced with permission from ref 120. Copyright 2000 The Electrochemical Society, Inc.)...

The ionic resistance of a polymer electrolyte membrane is an important parameter in determining the mobility of protons through the membrane and the corresponding voltage loss across the membrane. Currently, the most commonly used membranes in PEM fuel cells are Nafion membranes produced by DuPont. However, these membranes are limited to low-temperature uses (usually below 80°C) because membrane dehydration at high temperatures can lead to reduced water content and then a lower proton transfer rate, resulting in a significant decrease in conductivity. The relationship between conductivity and the diffusion coefficient of protons can be expressed by the Nemst-Einstein equation ... 

Membrane dehydrated completely at this temperature incompletely dried membranes behave as in the... 

In practice, trade-offs between optimization of different membrane functions have to be accepted. For instance, the immobilization of the proton solvent will impede the leaking out of solvent and, thus, help to avoid membrane dehydration and cathode flooding. On the other hand this may only be achievable at the cost of lower proton conductivity. A good theoretical understanding of mechanisms of proton mobility in various aqueous and non-aqueous environments is thus of vital importance. 

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. 

Thus, local dehydration will most likely not be an issue for Nafion membranes, but that may not be the case for other membranes For instance, Eq. (28) emphasizes the importance of r. If the average pore size is too small, membrane dehydration becomes essential for the fuel cell operation. This is evident from Table 1 by the comparison of ypc values for different psds. 

The parameter yps is the current density at which membrane dehydration starts. Its value can be controlled via the external parameters ywandAf8. 

Understanding the laws of membrane dehydration and the possible measures of avoiding it should be taken into account in the regulation of the overall water balance in fuel cell stacks. 

Membrane performance characteristics in the hydraulic and diffusion limits are compared to each other in Fig. 9. Figure 9(a) illustrates that in the diffusion model considerable deviations from the purely ohmic performance of the saturated membrane arise already at small jv/Jj, well below the critical current density. This is in line with the comparison of the water-content profiles calculated in the diffusion model, Fig. 9(b), with those from the hydraulic permeation model, in Fig. 7. Indeed, membrane dehydration is much stronger in the diffusion model, affecting larger membrane domains at given values of jp/./j. Moreover, the profiles exhibit different curvature from those in Fig. 7. 

The membrane is assumed to be fully hydrated. The model of membrane water management, discussed in Sect. 8.2.2, suggests that for now, in the mostly used Nafion-type membranes, with thickness in the range of 50 pm, the critical current density of membrane dehydration exceeds by far the typical current densities of fuel cell operation (experimental studies, which corroborate that the membrane regulates water fluxes in the fuel cell but its own state of hydration is not critically affected by them [125,126]. 

S. Kato, K. Nagahama, H. Noritomi and H. Asai, Permeation rates of aqueous alcohol solutions in pervaporation through Nation membrane, J. Membr. Sci., 1992, 72, 31-41 V Freger, E. Korin, J. Wisniak and E. Komgold, Transport mechanism in ion-exchange pervaporation membranes Dehydration of water-ethanol mixture by sodium polyethylene sulphonate membranes, J. Membr. Sci., 1997, 133, 255-267. 

Permeation (4) Gas or liquid Forced flow through semipermeable membrane Membrane Dehydration of Isopropanol, Vol. 9, p. 284... 

Distillation Unit and Filtration Unit. In this system, purification and recycling of IPA is performed by the combination of the PV membrane dehydration unit, the distillation unit, and the microfiltration unit. In the distillation unit, impurities, which are difficult to separate in the PV membrane separation unit such as dissolved metal ions and high-boiling impurities, are completely eliminated. 

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