Membranes, hydration

F. Volke, A. Pampel 1995, (Membrane hydration and structure on a subnanometer scale as seen by high resolution solid state nuclear magnetic resonance POPC and P0PC/C12E04 model membranes), Biophys.J. 68, 1960-1965. 

This intercept is referred to as the lag time [4], and it provides a means for estimating the diffusion coefficient provided the membrane thickness is known and with the assumption that other time-dependent processes, such as membrane hydration, do not occur during the lag phase. 

In a series of papers, Dupuis and co-workers simulated the effect of temperature and membrane hydration on membrane nanostructure and... 

To expand on the last remark, the simulation results from Fuller and Newman are shown in Figure 17. The curves clearly show a nonuniform current distribution that is mainly due to the change in the gas concentrations and the membrane hydration. In the simulation, the initial decrease in the current density is due to the change in the oxygen concentration. However, once enough water is generated to hydrate the membrane, the increased conductivity yields higher local current densities. What... 

Water management is one of the most critical and widely studied issues in PEFC. Water management is referred to as balancing membrane hydration with flooding avoidance. These are two conflicting needs to hydrate the polymer electrolyte and to avoid flooding in porous electrodes and GDL for reactant/ product transport. 

This gives a time constant between 0.1 and 1 s. The slowest process, however, is membrane hydration, the time constant of which can be estimated by... 

Dynamic characteristics of a fuel cell engine are of paramount importance for automotive application. Three primary processes govern the time response of a PEFC. They are (1) electrochemical double-layer discharging, (2) gas transport through channel and GDL, and (3) membrane hydration or dehydration (i.e., between a dry and a hydrated state). The time constant of double-layer discharging is between micro- and milliseconds, sufficiently short to be safely ignored for automotive fuel cells. The time constant for a reactant gas to transport through GDL can be estimated simply by its diffusion time, i.e.,... 

It is typically on the order of several hundred nanometers. In practice the minimum thickness for polymeric membranes is 50gm or greater, which is far more than one would expect from (6.53). This is apparendy due to the fact that these membranes hydrate in the bulk, thus increasing the dielectric constant. They also form a hydrated layer at the solution/membrane interface (Li et al 1996) which affects their overall electrochemical properties and selectivities. Macroscopic ISEs use relatively thick membranes ( 500jU.m). In contrast, it is desirable to use thin membranes in the construction of asymmetric solid-state potentiometric ion sensors, in order to make their preparation compatible with the thin-layer preparation techniques. 

Figure 12 under the counter-flow mode at 0.5 A/cm2. The index with the MPL is larger than that without the MPL for an index of 0-1 as depicted in Fig. 12. This result also shows that the MPL enhances water back-transport from the cathode side to the anode side. However, when the index is negative, meaning that the internal water circulation from the anode channel to the cathode channel, the index with the MPL is slightly higher than the index without the MPL. Therefore, the MPL at the cathode suppressed water vapor absorption at the anode, which is explainable by membrane hydration attributable to the MPL at the cathode. Consequently, the MPL promotes membrane hydration, leading less internal water circulation from the anode to the cathode side.

The effect of halo thane (CF3CHBrCl) on the lateral surface conductance and membrane hydration has been studied by Yoshida and coworkers [41]. Below the pretransition temperature, the activation energy of the ion movement (H30++0FT) was 18.1 kj/mol, which corresponds to that of the spin-lattice relaxation time of water (18kJ/mol) above pretransition the activation energy increased to 51.3 kj/mol. Halothane did not show any effect on the ion movement when the temperature was below the pretransition temperature. When the temperature exceeded the pretransition temperature, the authors observed at 0.35 mM halothane (equilibrium concen-... 

Figure 14.2 Typical permeate flow decrease upon start up of new membranes due to compaction and possibly degree of membrane hydration.

As shown in Figure 6.37(M6), the impedance spectra eventually return to the shape observed at the beginning of the measurement. The charge transfer arcs are slightly smaller and the spectrum of cell 2 has shifted towards the lower real parts, which is a result of better membrane hydration after the flooding event. 

Fig. 15. Critical exponent for protonic percolation on purple membrane. Hydration dependence of the conductivity for HjO (O) and ( ) hydration of lyophilized...

Commonly used GDL materials are made of porous carbon fibers, including carbon cloth and carbon paper. Carbon cloth is more porous and less tortuous than carbon paper and has a rougher surface. Experimental results showed that carbon cloth GDL has better performance under high-humidity conditions because its low tortuosity (of the pore structure) and rough textural surface facilitate droplet detachment. " However, under dry conditions, carbon paper GDL has shown better performance than carbon cloth GDL because it is capable of retaining the membrane hydration level with reduced ohmic loss. 

In most fuel cell operations, humidified gases are used to ensure proper membrane hydration. Hence, the ability to remove liquid water becomes the primary concern of GDL selection. PTEE is often used to increase the GDL hydrophobic-ity. Contact angle is commonly used to measure the hydrophobicity (typically in the range between 120 and 140°). However, Gurau et al. suggested that external contact angle measurements were more indicative of the GDL surface roughness than the capillary forces in the GDL pores (which reflects the real measurement of water removal capacity). They presented a new method for... 

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 state-of-the-art proton-conducting polymer membranes use water networks to conduct protons. Thus, they require sufficient membrane hydration for functioning, which limits operation of the present-day cells to temperatures lower... 

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. 

The necessity of adequate membrane hydration, with the associated risk of drying out, limits the operative temperature of PEM fuel cells under 100°C. 

As water in a PEM fuel cell is initially produced in the vapor phase, the membrane hydration from this phase is more relevant with respect to water uptake from liquid phase. In particular, when water is collected by vapor phase, two distinct mechanisms can be individuated at lower vapor concentration ion solvation occurs inside the membrane, while in conditions of pre-saturated vapor phase polymer swelling is observed with accommodation of a larger amount of water. At this regard an experimental polynomial equation has been proposed to correlate water content of membrane with water partial pressure [10] ... 

FCS water management is the key factor for an efficient and reliable operation of a PEMFC stack. Membrane hydration control and water balance for a durable operation of FCS are the main objectives of this sub-system, whose design and control issues, strictly connected to thermal management but also to reactant subsystem components, are discussed in Sect. 4.5. The possibility of interactions between the wet and warm cathode outlet stream and the components of thermal and water management sub-systems is also discussed. 

---