The Importance Of Water Control To Pem Fuel Cell Performance

Alan Cisar, Oliver J. Murphy, and Stanley F. Simpson Lynntech, Inc. 7610 Eastmark Dr., Suite 105 College Station, TX 77840

All membranes currently in use in polymer electrolyte membrane (PEM) fuel cells have sulfonate (-S03") groups as the anionic functionalities attached to the backbone of the polymer electrolyte. As a consequence of this fact, all PEM membranes depend on the presence of water in the electrolyte to facilitate proton transport. This includes perfluorinated membranes, such as Nafion® (DuPont) (1 ), and Gore Select™ (W. L. Gore) (2 ), partially fluorinated membranes, such as the Ballard membrane, which is a derivatized trifluorostyrene (3 ), non-fluorinated membranes, including both sulfonated polyparaphenylene (Maxdem's Poly-X™) (4 ) and sulfonated styrene-butadiene (DAIS) (5 ), and the various grafted materials that have been described in the literature (6,7 ). In every case, without water, the proton conductivity of the membrane is insufficient to support fuel cell operation.

Since in every case currently reported increasing the water content of the membrane reduces the resistance to proton conduction (8,9 ), there is a clear advantage to maintaining the membrane in a state of complete saturation. Maintaining the membrane in a state of saturation requires that either the gas in contact with the membrane is saturated with the water vapor, or that the membrane is in contact with liquid water. The presence of liquid water within the cell is not desirable, since the water can cover the surface of the electrocatalyst, blocking access for gases and water droplets in small passages can cause pressure fluctuations.

As a result of this factor, water management has been a major concern to PEM fuel cell developers from the beginning of the technology. Over the last three decades a variety of methods have been developed for controlling the amount of water in a fuel cell stack, each with its own advantages and disadvantages. As the dual drivers of cost and performance continue to push improvements, this problem has become more important. This paper reviews the full range of recorded techniques, from the simplest sparging bottle systems to the most sophisticated internal transfer systems that use water permeable separators to permit the transfer of water from cathode to anode or the back diffusion of waterfrom the cathode to the anode through exceedingly thin membranes.

Humidification techniques can be broadly categorized into two categories, internal to the stack and external from the stack. Internal, or in-the-stack methods, typically limit the gas dew point to the operating temperature of the stack. External methods permit greater flexibility in gas dew point, but require the system to have additional controls as well and generally a significantly to the size of the stack as well.

The most commonly used in-stack humidification method is the "Dummy cell". A dummy cell humidifier, as shown in Figure 1, has separate chambers for gas and water with a water permeable membrane between them. The water is heated to the stack temperature, typically by circulating through the stack as cooling water. The gas is humidified by water evaporating from the membrane. (10) In one variation of this method the gas stream inside the stack passes repeatedly over a cell, and then over a short section of humidifier, with each cell in the stack having its own humidification section, instead of using a common humidifier section at one end of the stack, as is the more general practice. (11)

Figure 1. Cross section view illustrating the function of a "Dummy Cell" type humidifier.

Another method of internal humidification involves the use of membranes with internal passages for water flow. This method permits the direct humidification of the electrolyte membrane, and the use of completely dry gases. This approach is especially useful for regenerative fuel cells, which function both as fuel cells and electrolyzers. (12) Direct injection of water as a fine mist into the gas manifolds of a fuel cell stack is another means for humidifying gas inside a fuel cell stack. For multikilowatt systems this can be a compact, volume efficient method. This method, like all in-stack methods, utilizes the phase change enthalpy of evaporation to cool the stack while humidifying. One recent patent reveals a method for fabricating hydrophilic micro-porous bipolar plates with internal cooling channels. By proper control of pressures within the stack, liquid water formed in the cathode compartment can be forced into the cooling water channels for removal and water from the cooling channels can be forced into the anode chamber to humidify the fuel. (13 )

Still another approach to humidification is the use of thin membranes. These membranes are sufficiently thin that the water made by the fuel cell can diffuse from the cathode to the anode along the concentration gradient at a rate comparable to the electro-osmotic flow and keep the membrane sufficiently saturated to support good conductivity. (14 ) While this approach is a simple and elegant solution to the humidification problem, this is also a solution that requires careful control of the stack produce stable operation.

The simplest external humidification system is the sparger. In a sparger the gases entering the fuel cell are dispersed as fine bubbles and the bubbles permitted to rise through a volume of heated water. As the bubbles rise, the gas quickly becomes saturated with water vapor, reaching equilibrium with the liquid phase. While spargers offer good control of the water content of the gas stream, the required contact time for humidification produces humidifiers that are generally larger than the stack for which they humidify gas.

More recent developments include the use of water permeable tubes surrounded with water to supply a large evaporation surface for the gas passing through the tubes. This approach, which is illustrated in Figure 2, has proven quite flexible and efficient. One variation on this method uses this tube structure in manner more like the one for which it was designed; gas drying. (15 ) In this approach the wet air exiting the fuel cell passes through the tubes where the gas is indicated in Figure 2, while the fuel gas passes through the section of the device labeled hot water reservoir in the figure. (There is no heating element used in this configuration.) This leads to the transfer of part of the fuel cell's product water from the exiting air stream to the incoming fuel stream.

Humidified Gas -«-Out

H,0 Out

Water Permeable Tubes

H,0 Out

Water Permeable Tubes

Humidified Gas -«-Out

Common Endcap

Heated Shell, Heater (not shown) Is wrapped around the shell.

Figure 2. Cut-away view of a parallel tube type humidifier, illustrating the function of this compact device.

Common Endcap

Dry Gas In

Heated Shell, Heater (not shown) Is wrapped around the shell.

Figure 2. Cut-away view of a parallel tube type humidifier, illustrating the function of this compact device.

Many other methods have been proposed and tested for humidifying feed gases for fuel cells, some of which will also be discussed. (16 ) Fuel cells are not the only devices requiring gas humidification. There are many varieties of medical equipment that require precise control of humidity, and an even wider variety of methods have been developed to humidify gases. This area can offer many potential approaches to fuel cell humidity control, some of which will be considered here.(17,18 )

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