Membrane Exchange Humidifier

A microporous pol mier membrane with a hydrophilic additive or filler has been used in an improved membrane exchange humidifier (156). Such membranes may be competitive with regard to water transmission rate and be dimensionally stable, and bondable by the use of adhesives or via melt bonding. The use of membranes with such properties allows a simpler configuration of a membrane exchange humidifier. 

An improved humidification is useful for a membrane exchange apparatus in situations where gaseous wet and dry streams are occurring. In such a membrane exchange apparatus, a suitable water permeable membrane is employed, and a first fluid stream is directed across one major surface of the water permeable membrane and the second fluid stream is directed across the opposing major surface. 

The water permeable membrane should contain sufficient amounts of a hydrophilic additive to render it wettable to water. Thus, when the second fluid stream contains liquid water, the membrane may become wetted and satiuated with liquid water. This may effectively seal the membrane sufficiently so as to hinder the unwanted transfer of other gases across it. The second fluid stream comprises liquid water when the dewpoint temperatiue of the second fluid stream is greater than its actual temperature. Suitable hydrophilic additives include sdica or alumina, in fiber or powder forms. 

Such water permeable membranes are characterized by pore structures in which the total porosity is greater than about 50%. Further, the average pore size may be from about 0.025 pm to about 0.1 pm. Such a humidifier is particularly suitable for use in humidifying a reactant gas supply stream for a solid pol5nner fuel cell. The fuel cell has a reactant gas inlet port and a reactant gas exhaust port (156). 

7 Poly(vinyl alcohoD/Titanium Dioxide Nanocomposites 

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A.D. Mossman, Membrane exchange humidifier for a fuel cell, US Patent 6 864 005, assigned to Ballard Power Systems Inc. (Burnaby,... 

A PEMFC uses a solid membrane that conducts protons as the electrolyte. Since it can start at ambient temperatures instantly, it is ideal for backup, portable, and motive power applications. The most important technologies concern the stack (like the heart of a human being) and the system controls (like the brain of a human being). The key components in a stack include the catalyst, PEM, GDM, plates, and gasket, while the controls include the operation algorithm, software, and electronic circuits. A fuel cell also needs various auxiliary components such as fans, blowers, compressors, pumps, heat exchangers, humidifiers, converters, valves, sensors, and batteries to work. A fuel cell system involves multidisciplinary skills and knowledge, and therefore it requires a team effort to develop. 

As most current automotive fuel cell efforts use proton exchange membrane (PEM) fuel cells, these will be described in a little more detail in this section and will be used as templates for the performance calculations presented in section 4.1.3. A typical passenger car PEM fuel cell system is depicted in Fig. 4.2. Included are heaters for bringing the equipment from ambient temperatures to the operating temperature of around 80°C and humidifiers for ensuring the level of water in the membrane and electrode areas required for... 

As to the membrane conductivity, only small losses of protonic conductivity, of the order of 5-10% after 4000 h, have been observed in well-humidified cells during PEFC life tests according to measurements of cell impedance at 5 kHz [42]. The deionized water employed in the humidification scheme [42] had very low levels of metal ions (e.g., Fe " / +, Ca + or Mg +). Such multivalent ions could exchange irreversibly with protons in the PFSA membrane, causing a drop in membrane conductivity. Deionizing the water used for PEFC humidification is therefore required, and appropriate plumbing should also be used in the humidification loop to avoid generation of ionic contaminants by corrosion processes. 

Membrane conductivity losses by ion exchange seem to be easier to prevent only small losses of protonic conductivity, of the order of 5-10% after 4000 hours, have been observed in well-humidified cells... 

Polymeric proton exchange membrane needs to be maintained properly humidified to guarantee a satisfactory ion conductivity during stack operation (see Sect. 3.2). In fact it exists a strong relationship between proton conductivity and water content of Nafion material used as membrane in PEMFC [24, 25]. Unfortunately the water produced at cathode side and the air moisture could be not sufficient to maintain properly wet the membranes in all working conditions, because of complex phenomena involving water within MEA [26] (Fig. 4.7). 

The humidification of the reactant gases could be successfully realized by using membrane humidifiers [34, 35]. The water content of a wet stream can be transferred across a semi-permeable membrane to a dry stream (Fig. 4.8). The membrane separates the dry stream compartment from the other compartment crossed by with liquid water or wet stream. Theoretically the dry stream could increase its water vapor content along the entire interface area of the membranes from the inlet up to close the saturation value at the exit of the device. The design includes a tubular form for the humidifier and a counterflow to optimize the exchange. This device could be more suitable for FCS management with respect not only to bubbler humidifier but also to water vapor injection method, as the last systems need additional equipments, make the whole system complex and increase... 

The electrons are drawn to an external circuit through conductive backing layers to generate electricity, and the protons migrate through a humidified solid electrolyte (proton exchange membrane, PEM) and react with oxygen at the cathode ... 

Fig. 8.23 Performance of an H2-O2 AEMFC with EDA-CoFe-C and Pt/C cathodes at 50 °C. Cathode catalyst loadings 4 mg cm for EDA-CoFe-C and 0.4 mgpt cm for Pl/C. Anode and cathode gases humidified at 50 °C. Gas flow rates 200 mL min (H2) and 400 mL min (O2). The A201 membrane thickness 28 )rm ion-exchange capacity 1.8 mmol g conductivity 42 mS cm (reprinted from ref. [75] with permission from Elsevier)...

K.W. Feindel, S.H. Bergens, R.E. Wasybshen, Insights into the distribution of water in a self-humidifying H2/O2 proton-exchange membrane fuel cell using H NMR microscopy, J. Am. Chem. Soc. 128 (2006) 14192—14199. 

A proton exchange membrane (PEM) fuel cell plant of 100 kWe capacity is to be designed in conjunction with the experimental polarization curve of Fig. 3b. The stack voltage requirement is 200 V, and cell area is limited to 30 x 30 cm set by the method of assembly. The stack runs on humidified H2 and air at 1 atm and 80 °C. The cost ofH2is 5/kg. The stack operates 8,150 h/year and has a lifetime of 5 years with no salvage value. (In reality, the Pt in the stack would be of value). The objective of this example is to determine the number of cells, cell area, and power density of the stack and costs of the stack, fuel cell plant, fuel, and electricity (per kWhe). 

Bellows RJ, Marucchi-Soos E, Reynolds RP. The mechanism of CO mitigation in proton exchange membrane fuel cells using dilute H2O2 in the anode humidifier. Electrochem Solid State I,ett 1998 1 69-70. 

Figure 23.23. Platinum surface area of the cathode with TKK 46 wt% Pt/Vulcan catalyst over 10,000 potential cycles at 20 mV/s in the voltage range 0.6-1.0 V at 80 °C under humidified H2-N2 (anode-cathode) [33]. (Reprinted by permission of ECS— The Electrochemical Society, from Ferreira PJ, la O GJ, Shao-Hom Y, Morgan D, Makharia R, Kocha S, Gasteiger HA. Instability of PEC electrocatalysts in proton exchange membrane fuel cells.)...

Vengatesan, S., Kim, H.J., Lee, S.Y., Gho, E., Ha, H.Y., Oh, I.H., Lim, T.H. 2007 Operation of a proton exchange membrane fuel cell under non-humidified conditions using a membrane-electrode assemblies with composite membrane and electrode. Journal of Power Sources, 167,325-329. 

Sim V, Han W, Poon HY, Lai YT, Yeung KL (2014) Confinement as a new architecture for self-humidifying proton-exchange membrane for PEMFC Abstracts of Papers. In 248th ACS National Meeting Exposition, San Ftandsco, CA, United States, 10-14 Aug 2014... 

Yang B, Fu Y Z, and Manthiram A (2005), Operation of thin Nafion-based self-humidifying membranes in proton exchange membrane fuel cells with dry and Oj , J. Power Sources., 139,170-175. 

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