Internal humidification

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. 

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]. 

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... 

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,... 

A highly water-permeable PEM would facilitate water removal via liquid transport toward the anode, alleviating the problem of cathode flooding and anode dehydration. Erom a system perspective, it is deemed beneficial to make use of internal humidification of CLs and PEM by water that is produced at the cathode. This mode of internal water management obviates the need for external humidifiers. It demands, however, precise control of water permeation rates through the PEM and of vaporization rates in partially saturated porous electrodes. Therefore, it is cmcial to know how relevant parameters of water transport (diffusion, hydraulic permeation, electro-osmotic drag, vaporization, and condensation) depend on PEM morphology and thermodynamic conditions. 

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. 

FIGURE 6-8. Stack configuration with internal humidification. 

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... 

The shape of particles is normally that of more or less regular spheres, dense or hollow, with smooth surfaces and sometimes cracks. This is related to the composition and the rate of solvent evaporation, with possible existence of internal pressure inside the drops when a rigid surface layer is being formed (Walton and Mumford 1999). All these characteristics will have some effect on handling properties of powders such as bulk and tapped densities, particle density, (mixing with other powders, storage) wettability and solubility, porosity, specific area (rehydration, instantisation) flowability (size, surface asperities), friability and creation/existence of dust, stability in specific atmosphere and medium (oxidation, humidification, active component release) (Huntington 2004). 

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.)... 

FIGURE 27.7 Performance of a GORE 5510 MEA in a single cell at a temperature of 60°C as well as of 90°C under reduced humidification conditions (ZSW measurements Internal publication). 

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. 

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. 

Interference due to uneven mass transportation may be avoided by dilution of the solution for analysis, by adjustment of the matrix of the sample solution and reference solution or by humidification of the carrier gas. In addition, the influence of variations in mass transportation can be corrected with the aid of an internal standard. 

The difference in deformations of neighbouring elements or layers due to different shrinkage induces tensile stress, which also may exceed local strength. This occurs, for example, between the external layer and internal core of every cast concrete element, because conditions of water loss are different if careful humidification is not ensured during curing. The phenomenon called differential shrinkage also occurs when a layer of fresh cement-based mix is put on an old concrete element during repair. 

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...

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.)... 

PEMFC humidification technology primarily involves external, internal, and self-humidification. 

A fuel cell by itself can also carry out humidification by changing its internal structure. The water generated by the fuel cell is used to achieve humidification. 

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