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Water and Heat Management in PEM Fuel Cells

Water has an important role in fuel cell operation as protonic conductivity of the polymer electrolyte membrane strongly depends on the membrane water content. Maintaining the required water level in a fuel cell is tightly related to the temperature as the water vapor content in the gaseous reactants streams (both air and hydrogen) is an exponential function of their temperature. On top of that, water is produced in the electrochemical reaction on the fuel cell cathode side and must be taken away from the cell. Although the fuel cell produces water, most of fuel cells being tested in the laboratories, as well as those in demonstrations or in pre-commercial applications, need external humidification of the reactant gases, which requires additional devices (tank, pump(s), heat exchanger(s), etc.). Operation with dry gases would result in drying out of the polymer membrane, as shown in Figure 1, which would adversely affect the fuel cell performance and life time. However, with adequate temperature profile along the cathode channel, as shown in Figure 2, it is possible to operate a fuel cell with ambient air without external humidification, using water and heat produced inside the fuel cell. The goal is to keep the temperature in the cathode channel at a level that corresponds to saturation conditions. As more water is generated the temperature is allowed to increase just enough to evaporate that water. This requires a specific design of the fuel cell in terms of spatially variable heat transfer rates, and corresponding control of the cooling subsystem. This novel concept results in simplification of the fuel cell support system by eliminating the devices needed for external humidification, without affecting fuel cell performance and durability in various applications.

PEM Fuel Cell; temperature control; humidity control

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