Humidification PEMFC

Furthermore, Watanabe and co-workers [51] modified Nafion PEMs by the incorporation of nano-sized particles of SiO, TiO, Pt, Pt-SiO, and Pt-TiO to decrease the humidification requirements of PEMs. When operated at 80°C under low humidification PEMFC, the modified PEMs showed lower resistance than Nafion . This improvement was attributed to the suppression of crossover by in situ Pt along with the subsequent sorption of the water produced on the incorporated oxides. 

In PEMFCs working at low temperatures (20-90 °C), several problems need to be solved before the technological development of fuel cell stacks for different applications. This concerns the properties of the components of the elementary cell, that is, the proton exchange membrane, the electrode (anode and cathode) catalysts, the membrane-electrode assemblies and the bipolar plates [19, 20]. This also concerns the overall system vdth its control and management equipment (circulation of reactants and water, heat exhaust, membrane humidification, etc.). 

Until recently (i.e., till early 1990s), most of the efforts to develop DMFCs has been with sulfuric acid as the electrolyte. The recent success with a proton conducting membrane (perfluorosulfonic acid membrane) in PEMFCs has steered DMFC research toward the use of this electrolyte. The positive feature of a liquid feed to a DMFC is that it eliminates the humidification subsystem, as required for a PEMFC with gaseous reactants. Another positive point is that the DMFC does not require the heavy and bulky fuel processor. Two problems continue to be nerve-wracking in the projects to develop DMFCs (1) the exchange current density for methanol oxidation, even on the... 

Figure 6.11. Nyquist plots for MEAs containing different proton-conducting ionomers at 0.85 V without external humidification catalyst loading = 0.4, 0.7 mg Pt/cm2 for anode and cathode, respectively TceU = 25°C Pressure = 1 atm and H2/02 flow = 400 cmVmin [8]. (Reprinted from Electrochimica Acta, 50(2-3), Ahn SY, Lee YC, Ha HY, Hong SA, Oh IH. Effect of the ionomers in the electrode on the performance of PEMFC under non-humidifying conditions, 673-6, 2004, with permission from Elsevier.)...

The membrane and ionomer humidification requirements are of paramount importance for PEMFC operation since the proton conductivity is a fundamental necessity in the membrane as well as in the electrode for the fuel cell to function. The operating conditions of current PEMFCs are dictated by the properties of the membranes/ionomers. Now, the most important membrane type (e.g., Nafion membranes from DuPont) is based on PFSA ionomers that are used in the membrane... 

Direct methanol fuel cell (DMFC) was developed in 1950s-1960s, based on the liquid alkaline or aqueous acid solution as the electrolyte. It converts the methanol directly into electricity, instead of using indirectly produced hydrogen from methanol through the reforming process. Today, DMFC commonly refers to as the one that employs PEM as the electrolyte. Fuel for DMFC is a dilute solution of methanol, usually 3-5 wt% in water. The size of DMFC can be considerably smaller than PEMFC because of the elimination of fuel processor, and complex humidification and heat management systems. The performance of DMFC is relatively low compared to that of PEMFC. 

Thus, the optimization of PEMFC performance in terms of efficiency and reliability requires a proper design and management of reactant feeding sections, as well as of cooling and humidification sub-systems [4]. 

Biichi FN, Srinivasan S (1997) Operating PEMFC fuel cells without external humidification... 

DMFCs have potential near-term applications mainly in the portable power source market, as they are smaller, lighter, simpler, and cleaner than conventional batteries. Liquid methanol is consumed directly in a DMFC, which implies a higher energy density of the fuel cell system. But the power densities achievable with state-of-the-art DMFCs are still very small in comparison to hydrogen-fuelled PEMFCs. One of the major problems lies in the use of liquid methanol solution on the anode of the DMFC, which, on the one hand, keeps the ionomeric membrane water saturated (and thus no humidification is needed) but, on the other hand, does not keep fuel (methanol or any other organic fuel, e.g., formic acid, ethanol) and water from permeating to the cathode side, since the basic PFSA membranes are permeable to both methanol and water. - The fuel and water crossover from anode to cathode hampers the performance of the air cathode. 

This concept enables the operation of PEMFCs without any humidification, which should also be interesting for higher temperatures. The corresponding U-I characteristics are displayed in Figure 21.42. The disadvantage to this concept, however, is that the cost of the membrane is significantly... 

FIGURE 21.42 (a) U-I curve and (b) ohmic resistance of a PEMFC using Pt-Ti02-PEM operated at 80°C and ambient pressure with no external humidification at the reactant utilization of H2 56% and O2 54%. An OCV was measured at a flow rate of 7 mL min- for both dry H2 and dry O2. The amount of Pt dispersed in the PEM = 0.1 mg cm-, the amount of Ti02 = 0.42 mg cm- (4 wt%). Full symbols measured on increasing current density, and open symbols measured on decreasing current density. (Reproduced from Uchida, H. et al., J. Electrochem. Soc., 150, A57, 2003. With permission of the Electrochemical Society, Inc.)... 

Stevens, D. A. Hicks, M. T. Haugen, G. M. Dahn, J. R. (2005). Ex situ and in situ stability studies of PEMFC catalysts Effect of carbon type and humidification on degradation of the carboa J. Electrochem. Soc., 152, A2309-A2315. 

Balancing water is the most difficult task for a PEMFC due to the high water production rate and the various water movement possibilities. Although a previous estimate is very helpful in determining the values of certain key parameters, such as the reactant humidification temperature, the stack inlet and outlet temperatures, the coolant flow rate, and the reactant stoichiometric ratio (mainly air), automatic adjustment by the fuel cell system itself is important in achieving the optimal operation conditions, especially if the fuel cell is in the load-following mode. 

Because of their operating temperature below 100 °C, PEMFCs are best suitable for mobile applications which need a quick starting time. On the other hand, the liquid product water has to be removed without interfering with the humidification of the electrolyte. The functionaHty of a PEMFC is shown in Eigure 35.19 (11). 

The proton conduction based on the phosphoric acid is the basis of HT-PEMFC Celanese technology [37], mostly referred to as phosphoric acid-doped PBI (polybenzimidazole) This membrane enables operation at temperatures as high as 180°C, without the need for external humidification. Heat dissipation at this temperature is much easier than at the 70-80°C operating temperature of fuel cell systems using standard PFSA membranes. The CO tolerance at 180°C is such that even 1 % CO leads to a minor loss of power density compared to that using the same membrane on pure hydrogen. The downside of this membrane is its low conductivity below 1(X)°C, making a cold start impossible, as well as the lower power density at its optimal temperature. 

For the rate of commercialization of PEMFCs to continue to increase, system costs must continue to decrease. One way to do this is to eliminate the fuel ceU temperature and humidification requirements described above, allowing operation over a wide range of temperatures without the need for humidification of the incoming gases. To do this, new materials are needed. These include new ionconducting materials for membranes and electrodes and new catalysts that can function with less water. This entry wiU review how these materials function in a PEMFC and some of the approaches to new materials that may overcome the humidification and temperature barriers. 

Yu J, Matsuura T, Yoshikawa Y, Islam MN, Hori M. In situ analysis of performance degradation of a PEMFC under nonsaturated humidification. Electrochem Solid-State Lett 2005 8 A156-8. 

It can be observed fliat PEMFC performance employing carbon cloth GDLs is different from that when using carbon paper GDLs. Ralph et al. [39] showed that at high current density wifli internal humidification in Ballard Mark V cells the carbon cloth offered a distinct advantage compare to carbon paper. They deemed the surface porosity and hydrophobieity of the carbon cloth substrate to be more favorable for the movement of liquid water, and thus both water management and... 

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