Membrane wetting

The usual working temperature of fuel cells with Nafion-type membranes is 80 to 90°C. Under these conditions, moisture must be supplied to keep the membranes wet, which usually is attained by passing the reactant gases through water that is somewhat warmer (by 5 to 10°C) than the cell s working temperature, thus saturating them with water vapor. 

Do not allow the membrane to dry during this process. Keep the membrane wet (e.g., submerged in water) at all times (unless using the partial drying procedure). 

Membrane-wetting properties may be carefully considered in the membrane selection. In general, the membrane surface where the droplet is formed should not be wetted by the disperse phase. Therefore, a w/o emulsion is prepared using a hydrophobic membrane and an o/w emulsion is prepared using a hydrophilic membrane. On the other hand, w/o and o/w emulsions were successfully prepared using pretreated hydrophilic and hydrophobic membranes, respectively. The pretreatment basically consisted in absorbing the continuous phase on the membrane surface so that to render the membrane nonwetted by the disperse phase [14, 23, 25]. The presence of emulsifier in the disperse phase represents another strategy that permits the preparation of emulsions with a membrane wetted by the disperse phase. 

The permeate flux increases with increasing feed velocity due to the reduction of the boundary layer thickness. Therefore, the temperamre and concentration at the liquid-vapor interface are approaching values close to those in the bulk solution. Garcia et al. [71] have observed that the feed pressure measured at the inlet of the feed cell frame increased with an increase in the feed flow rate consequently increasing the risk of membrane wetting. 

One of the benefits of VMD relative to the other MD configurations is that conductive heat loss through the membrane is negligible. This enables estimation of the boundary layer heat transfer coefficient. Care must be taken in VMD to prevent membrane wetting, because APjnterface is typically higher in VMD than in other MD configurations. 

Water concentration, g/formulation Swelling-agent concentration, g ZnCl / formulation Description of membrane Wet thickness of unheated membrane, mm X 10 Gravimetric swelling ratio of unheated membrane, wet wt/dry wt Rate of water transport, ml/(cm )(day)+ Salt retention, %... 

Figure 7. Comparison of experimentally measured and theoretically predicted membrane wet density vs. Na+ molarity for standard Nafion in the sodium sulfonate form...

Water management is important in the optimization of the cell operation, affecting both internal resistance and cathode polarization losses of the cell.273 Because protons (H30+) permeate through the membrane together with hydrated water and the fuel cell is generally operated at 80 °C, water should be supplied to the anode side. Fuel (hydrogen) and oxygen (air) also should be humidified to keep the membrane wet. 

Another major issue is curing. This can be done with curing membranes, wet hessian, plastic sheeting. Well cured repairs will perform far better than those left to air cure . 

Zhang HY, Wang R, Liang DT, Tay JH. Theoretical and experimental studies of membrane wetting in the membrane gas-liquid contacting process for CO absorption. J Membr Sci 2008 308 162-170. 

Rongwong, W., Jiraratananon, R., Atchariyawut, S. 2009. Experimental study on membrane wetting in gas-liquid membrane contacting process for CO2 absorption by single and mixed absorbents. Sep. Purif. Technol. 69 118-125. 

It is important to be extremely careful when manipulating the membrane. Avoid touching the membrane even with gloves and lift it by the corners with forceps. It is also important that the membrane wetting and transfer solutions (steps 6 and 7) are sterile. 

Dry papers] Wet paper— DB membrane— Blot membrane—> Wet paper— ... 

Similarly, gas leaves the compartment along its entire length. This minimizes pressure drop and, with an elevated discharge header, keeps the entire surface of the membrane wet. Tests with a demonstration electrolyzer showed a cell voltage of 3.02-3.05 V (depending on the membrane) at a current efficiency of 96-97% at 6kAm . ... 

Manufacturers most commonly supply membranes wet, pre-treated, expanded, cut to size, and ready to use. An exception here is an alternative form of Nafion supplied dry... 

AGMD is more versatile than DCMD because permeate is condensed on a chilled surface rather than directly in the chilled permeate. This is why AGMD can be used for both DCMD applications and in the removal of trace volatile components from aqueous alcohol solutions. In DCMD this operation is not possible because alcohol, due to lower surface tension and/or smaller contact angle with the membrane, is likely to wet the membrane at the permeate side. In contrast, in AGMD the concentration of the condensed permeate is not in direct contact with the membrane surface, meaning there is no danger of membrane wetting at the permeate side. 

Using a clean disposable scalpel, cut out the desired protein band, taking care to minimize the area of the PVDF slice. Wash the slice six times in a clean Eppendorf tube with deionized water. Vortex the tube for the final wash. Leave a little water in the tube after the final wash to keep the membrane wet. Parafilm-seal the tube and store on dry ice... 

Dehydration can be caused by insufficient humidification, high temperature, and electroosmosis. At high current density, the electroosmotic force is strong, and the water present at the anode side by back diffusion is not sufficient to keep the membrane wet. It has been shown that during a step increase in the current density, the electroosmotic force will immediately pull water molecules from the anode to the cathode (Wang, 2006). 

The practical problem encountered in PEMFC is keeping the membrane wet under operating conditions. The balance of water with its transport determines the water content in the cell during the reactive mode of operation. Water-transport processes are a function of the current and the characteristics of both the membrane and the electrodes (Mosdale, 1996 Motupally, 2000). The temperature of operation ofthe cell has a significant influence on its performance. The performance of the PEMFC increases with temperature until 90°C, which reflects a lowering of the internal resistance of the cell. This decrease is due largely to the decrease in ohmic resistance of the electrolyte. Mass transport, which is limited (i) by diffusion of reactants through the PEMFC assembly to the active Pt sites. 

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