Membrane humidification

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.). 

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.)... 

The water distribution within a polymer electrolyte fuel cell (PEFC) has been modeled at various levels of sophistication by several groups. Verbrugge and coworkers [83-85] have carried out extensive modeling of transport properties in immersed perfluorosulfonate ionomers based on dilute-solution theory. Fales et al. [109] reported an isothermal water map based on hydraulic permeability and electro-osmotic drag data. Though the model was relatively simple, some broad conclusions concerning membrane humidification conditions were reached. Fuller and Newman [104] applied concentrated-solution theory and employed limited earlier literature data on transport properties to produce a general description of water transport in fuel cell membranes. The last contribution emphasizes water distribution within the membrane. Boundary values were set rather arbitrarily. 

The sluggish oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) induces a major fraction of voltage losses in PEFCs. Due to the requirement of sufficient membrane humidification, discussed in Sect. 8.2.2, and the limitation it imposes on the working temperature range (< 100 °C) only platinum-based catalysts can provide acceptable reaction rates. Platinum, however, is costly, and its resources are limited. 

The operative parameters which can be regulated to optimize the stack performance are MEA humidification, reactant pressure, stack temperature, and stoichiometric ratio. While the role of membrane humidification, already partially discussed in Sect. 3.2, is closely considered in Sect. 4.5 and in case studies (Chaps. 6 and 7), the influence of the other parameters is examined here with reference to the stack of Fig. 3.5. These effects have already been described from a thermodynamic point of view (see Sect. 3.1), while kinetic implications are considered in this section for their importance in determining the stack efficiency. 

The analysis of the main aspects to be faced in design and realization of a fuel cell system as power source of an electric drive train is described in Chap. 4. Here the problems connected to the choice of auxiliary components, their energy consumption and integration in the overall system are discussed, paying particular attention to the management of membrane humidification, hydrogen purge and air supply as a way to optimize system efficiency and reliability. 

The polarization voltage of the cathode side is determined by reaction kinetics, transport of water and oxygen across the cell and by proton transport across the CCL. In this section, we will assume an ideal membrane humidification. We start with the model of the CCL. 

In this section, we re-formulate the model equations for the case of constant oxygen stoichiometry and obtain the criterion of ideal membrane humidification. 

The solutions (6.119-1.122) do not contain the water management parameter r and p. Furthermore, the exponential shapes of and j/J are governed by a single parameter, the oxygen stoichiometry A. Eqs. (6.119), (6.120) with p (6.122) and k (6.121) coincide with the solutions (6.43), (6.44) in Section 6.3.2, where ideal membrane humidification is assumed. This means that the current, which obeys (6.115) does not produce any significant non-uniformity of membrane resistance along z. Equation (6.115) is thus the condition of ideal membrane humidification. 

For PEFCs to function properly, a so-called conditioning step is required mainly to attain the desired level of membrane humidification and to obtain a quasi-steady performance that is reproducible under the applied test conditions. The start-up of the fuel cell and conditioning step can be performed following one of the following... 

The second solution corresponds to the water-limiting regime, when local current is hmited by membrane drying. This solution arises if /r >0 or, equivalently, r < 1. When r > 1 this solution disappears and for aU z we have an oxygen-limiting regime. Physically, at r > 1 insufficient membrane humidification only reduces the cell potential, but does not affect the limiting current density. 

Note that this is correct if the cell resistivity Rq is small, or the variation of the local cell current with z is not large. If, for example, membrane humidification is strongly nonuniform along z, the variation of the ohmic term is large, and this variation must be compensated by the respective variation in t]q. Writing Equation 5.60 in the form... 

In a membrane humidification system, dry air is forced through a moist membrane with small pores to absorb moisture. The membrane humidification system can have very high efficiency but also has a higher pressure drop compared to sparge systems. A third type of... 

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. 

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... 

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