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Water and Thermal Management Due to operation at less than 100 °C and atmospheric pressure, water is produced as a liquid. A critical requirement is to maintain high water content in the electrolyte to ensure high ionic conductivity. Maintaining high water content is particularly critical when operating at high current densities (approximately 1 A/cm2) because mass transport issues associated with water formation and distribution limit cell output. The ionic conductivity of the electrolyte is higher when the membrane is fully saturated: this impacts the overall efficiency of the fuel cell. Without adequate water management, an imbalance will occur between water production and water removal from the cell. Water content is determined by balance of water12 during operation. Contributing factors to water transport are the water drag through the cell, back-diffusion from the cathode, and the diffusion of water in the fuel stream through the anode. Water transport is not only a function of the operating conditions12 but also the characteristics of the membrane and the electrodes. Water drag refers to the amount of water that is pulled by osmotic action along with the proton (28). One estimate is that between 1 to 2.5 molecules are dragged with each proton (29). As a result, transported water can be envisioned as a hydrated proton, H(H2O)n. During operation, a concentration gradient may form whereby the anode is drier than the cathode. Under these conditions, there is back-diffusion of water from the cathode to the anode. Membrane thickness is also a factor in that the thinner the membrane, the greater the transport of water back to the anode. The objective of the stack engineer is to ensure that all parts of the cell are sufficiently hydrated, and that no excessive flooding occurs (29, 30, 31, 32). Adherence of the membrane to the electrode will be adversely affected if dehydration occurs. Intimate contact between the electrodes and the electrolyte membrane is important because there is no free liquid electrolyte to form a conducting bridge. Because this type of degradation is largely irreversible, operation under dry conditions will severely impact membrane lifetime (33). Reliable forms of water management have been developed based on continuous flow field design and appropriate operating adjustments. For this reason, flow field designs often feature serpentine channels or unstructured flow passages. The flow-plates (which also serve as bipolar plates) are typically made of graphite, an injection-molded and cured carbon material, or a metal. If more water is exhausted than produced, then humidification of the incoming anode gas becomes important (31). If there is too much humidification, however, the electrode floods, which causes problems with gas diffusion to the electrode. A temperature rise between the inlet and outlet of the flow field increases evaporation to maintain water content in the cell. There also have been attempts to control the water in the cell using external wicking connected to the membrane to either drain or supply water by capillary action. Much progress has been made towards PEFC commercialization. Figure 3-4, from Gore Fuel Cell Technologies, demonstrates the company’s newest commercial offering, PRIMEA® Series 56 MEA that has demonstrated over 15,000 hours of cell operation (34). 12. A smaller current, larger reactant flow, lower humidity, higher temperature, or lower pressure will result in a water deficit. A higher current, smaller reactant flow, higher humidity, lower temperature, or higher pressure will lead to a water surplus. 3-9PDF Image | Fuel Cell Handbook (Seventh Edition)
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