Internal membrane humidification

FIGURE 12.5 Progressive poisoning from 10,40, and 100 ppm CO on pure Pt and PIq jRuq j alloy anodes. Increased CO tolerance is shown by the PIq jRuq5 alloy anodes. The MEAs are based on catalyzed substrates bonded to Nafion 115. The single cell is operated at 80°C, 308/308 kPa, 1.3/2 stoichiometry with full internal membrane humidification. (From Ralph, T. R. and Hogarth, M. R, Platinum Metal Rev., 46,117, 2002. With permission.)... 

Internal humidification (actually a misnomer ) is another approach which has been successfully used. In this concept, a portion of the membrane is set aside to humidify the inlet gases and liquid water is injected directly into this inactive portion of the stack. In another method, Chow et al. (1995) developed an internal membrane humidification scheme for a PEMFC stack, where dry gas was run through a separate section of the stack to condition the gas before the electrochemically active portion of the cell. The advantage is that the gases are conditioned inside the stack, and the gas temperatures will be very close to the temperatures... 

Figure 6.13 Passive humidification via internal membrane water generation from embedded platinum particles.

Despite the fact that the PEFC is a water generation reactor, some humidification of the reactants is usually necessary to enable high performance and longevity. A dry inlet feed results in poor local performance, hot spots, and internal stresses that can lead to short lifetimes. There are various active and passive humidification methods used to accomplish humidification, including membrane humidification, direct injection, and internal or external recirculation. 

The experimental data (dots) are reproduced very well within the framework of the hydraulic permeation model (solid lines). For the basic case with zero gas pressure gradient between cathode and anode sides, APe = 0, the model (solid line) predicts uniform water distribution and constant membrane resistance at )p < 1 A cm and a steep increase in R/R beyond this point. These trends are in excellent agreement with experimental data (open circles) for Nafion 112 in Figure 6.15. A finife positive gas pressure gradient, APs = P/ - P/ > 0, improves the internal humidification of fhe membrane, leading to more uniform water distribution and significantly reduced dependence of membrane resistance on X. The latter trends are consistent with the predictions of fhe hydraulic permeation model. 

Recently, Dahr [1], Stonehart [2] and Watanabe [3] have made an attempt to reduce the humidification constraints in solid polymer electrolyte fuel cells (SPEFCs) by using modified perfluorosulfonic membranes. A recast Nafion film sandwiched between the two electrodes was first proposed by Dahr [1] for the realization of an internally humidified SPEFC. Stonehart [2] suggested the inclusion of small amounts of silica powder into the recast film in order to retain the electrochemically produeed water inside the membrane. Watanabe et al [3] have tried to exploit the H2/O2 crossover through the membrane to produce a chemical recombination to water on small Pt clusters inside the membrane. All of these membranes were operated with H2/O2 at 80°C and allowed the development of systems without assisted humidification or with near ambient humidification. 

Fig. 2.25 Steady-state polarization curves at different humidification is observed at high current densities and relative humidities for high temperature polymer electro- low RH. Reproduced from [32] with permission of the lyte fuel cell based on Aquivion (Solvay) membrane American Chemical Society and CNR-ITAE catalysts. The effect of internal...

Stack tests on Aquivion-containing MEAs have shown that only a moderate decrease in performance occurs at 110 °C, 1.5 bar abs. with 33 % RH. MEA hydration in the stack at intermediate temperature was mainly assured by the internal humidification and the back-diffusion of the water from the cathode to the anode through the thin (30 pm) low equivalent weight (790 g eq ) PFSA membrane. In the stack, the new Aquivion E79-03S membrane showed high conductivity, good water retention, and mechanical properties above 100 °C as compared to the conventional PFSA membranes and appropriate characteristics for a rapid startup in a cold environment as well as suitable operation in duty cycles. These promising characteristics were supported by improved MEA structures,... 

FIGURE 10.28 Effect of RH on fuel cell internal resistances (RC = electrode resistance, RM = membrane resistance). (Reprinted from /. Power Sources, 184, L. A. M. Riascos. Relative humidity control in polymer electrolyte membrane fuel cells without extra humidification, 204-211, Copyright (2008), with permission from Elsevier.)... 

Rpem/Rs beyond this point. These trends are in excellent agreement with experimental data for Nafion 112. A finite positive gas pressure gradient, = Pf — P > 0, improves the internal humidification of the membrane, leading to a more uniform water distribution and a significantly reduced dependence of membrane resistance on k. The latter trends are consistent with predictions of the hydraulic permeation model. 

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