External humidification

External humidification, 12 213 External interface management, in technology transfer, 24 366 External loop airlift bioreactors, 1 741, 742 Externally manifolded fuel cells, 12 200 External magnetic field, 23 835 External mass transfer, 15 728-729 External mass transfer resistance dimensionless parameter and,... 

M. V. Williams, H. R. Kunz, and J. M. Fenton. Operation of Nafion(R)-based PEM fuel cells with no external humidification Influence of operating conditions and gas diffusion layers. Journal of Power Sources 135 (2004) 122-134. 

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 PBI-based PEM fuel cell can operate from 120°C to 200°C without external humidification. AC impedance shows that kinetics resistance decreases at higher temperatures, as shown in Figure 6.53, which is different from the characteristics of Nafion -based PEM fuel cells at high temperatures, but is consistent with the performance trend of the PBI-based PEM fuel cells, as shown in Figure 6.54. Although there is no humidification of the reactant streams in the operation of PBI PEM fuel cells, mass transfer issues are still observed through AC impedance, as shown in Figure 6.55. 

The steady-state water profile across the ionomeric membrane for given cell current density, external humidification conditions, and differential pressurization, is the resultant of these electroosmotic, diffusive, and hydraulic fluxes. 

In designing a microscale fuel cell, there are several considerations that need to be accounted for. These include the following is the fuel cell to be completely active or passive will it operate at room temperature or elevated temperatures will the fuel be at atmospheric pressure or elevated pressure will external humidification be required and finally, will the fabrication techniques... 

The different humidification approaches could be classified as internal or external methods. Internal humidification means that humidification procedure concerns exclusively the inner spaces of fuel cell stack, while external humidification involves modifications in feeding stream humidity ratio outside of the stack [1. 29]. 

On the other hand, the use of external humidification is essential at high temperatures, because the concentration gradient of H2O in the membrane of individual cells would be more uniform if both air and hydrogen streams were humidified externally. The external water supply helps to balance the combined effects of electro-osmotic drag and back diffusion permitting to maintain the performance of the membrane. External humidification is practically useful also below 60°C, at least for medium large-size FCS. 

A possibility is to saturate at different temperatures the reactants before they enter into the stack [33]. This approach can be accomplished by several procedures based on external dewpoint, external evaporation, steam injection with downstream condensers, or flash evaporation. High temperature values allow to absorb significant water amount in gas streams and then transport it inside the stack compensating the water losses due to internal fast evaporation. However, the main problem with external humidification is that the gas cools after the humidifier device, the excess of water could condense and enter the fuel cell in droplet form, which floods the electrodes near the inlet, thereby preventing the flow of reactants. On the other hand, internal liquid injection method appears preferable for example with respect to the steam injection approach because of the need of large energy requirement to generate the steam. 

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

The control strategies are programmed in Matlab-Simulink, compiled in C-H-and then downloaded into the DSP processor of the d-Space board. The controlled management of the fuel cell system during the dynamic tests operates on hydrogen purge, air flow rate regulation (stoichiometric ratio), external humidification, and stack temperature. 

Other dynamic tests are affected varying the stack power and evaluating the stack response to hydrogen purge, external humidification, and air management... 

In order to clarify the effectiveness of the external humidification strategy in stack managing, a further experiment is shown in Figs. 6.18 and 6.19. This test is effected in a steady-state condition corresponding to the maximum efficiency of the FCS at 1.2 kW and R — 1.9. Stack current, voltage, and temperature are reported as function of time in Fig. 6.18, while Cy and hydrogen pressure are shown in Fig. 6.19. 

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.)... 

F. N. Buchi and S. Srinivasan, Operating Proton Exchange Membrane Fuel Cells without External Humidification of the Reactant Gases Fundamental Aspects, Journal of the Electrochemical Society, 144,2767 (1997). 

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. 

Another way to reduce cost and improve reliability is through advances in technology. As mentioned above, the pursuit of polymer electrolytes that are more tolerant to low RH conditions, that do not require external humidification, and that run at higher temperatures is a prime example of how technology can reduce cost and complexity. Elimination of sensors through intelligent and advanced controls is another example. 

External humidification uses additional equipment. Common methods include warm-up, dew point, permeable membrane, and liquid water spray. 

The different water movements are shown in Figure 4.10. Fortunately, all these water movements are predictable and controllable. Starting from the top of Figure 4.10, the water production and the water drag are both directly proportional to the current. The water evaporation can be predicted with care, using the theory outlined below in Section 4.4.2. The back diffusion of water from cathode to anode depends on the thickness of the electrolyte membrane and the relative humidity of each side. Finally, if external humidification of the reactant gases is used prior to entry into the fuel cell, this is a process that can be controlled. 

The key to running a fuel cell without external humidification is to set the air stoichiometry so that the relative humidity of the exit air is about 100% and to ensure that the cell design is such that the water is balanced within the cell. One way of doing this is described by BUchi and Srinivasan (1997) and is shown in Figure 4.13. The air and hydrogen flows are in opposite directions across the MEA. The water flow from anode to cathode is the same in all parts, as it is the electro-osmotic drag , and is directly proportional to the current. The back diffusion from cathode to anode varies, but is compensated for by the gas circulation. Other aids to an even spread of humidity are narrow electrodes and thicker gas diffusion layers, which hold more water. 

Although we have seen that small fuel cells can be operated without additional or external humidification, in larger cells this is rarely done. Operating temperatures of over 60°C are desirable to reduce losses, especially the cathode activation voltage drops described in... 

The need and the valne of external humidification is shown by revisiting equation 4.6, and taking an example... 

---