Fuel cell membranes water management

Water is generated at the cathode reaction and must be removed from the fuel cell to prevent blockage of reaction sites. In the case of PEM fuel cell, proper humidification of the membrane is necessary to improve proton transfer (and efficiency), so commonly exhaust gas water is recycled to inlet air and hydrogen streams to carry water to the membrane. Thus, water management is very important in PEM fuel cells. The water management system in PEM fuel cells will contain a humidification system for both air and hydrogen streams. 

Polymer Electrolyte Fuel Cell (PEFC) The electrolyte in this fuel cell is an ion exchange membrane (fluorinated sulfonic acid polymer or other similar polymer) that is an excellent proton conductor. The only liquid in this fuel cell is water thus, corrosion problems are minimal. Water management in the membrane is critical for efficient performance the fuel cell must operate under conditions where the byproduct water does not evaporate faster than it is produced because the membrane must be hydrated. Because of the limitation on the operating... 

The membrane is assumed to be fully hydrated. The model of membrane water management, discussed in Sect. 8.2.2, suggests that for now, in the mostly used Nafion-type membranes, with thickness in the range of 50 pm, the critical current density of membrane dehydration exceeds by far the typical current densities of fuel cell operation (experimental studies, which corroborate that the membrane regulates water fluxes in the fuel cell but its own state of hydration is not critically affected by them [125,126]. 

Water management can be most simply achieved by providing the gas feed humidification level required to maintain the conductivity of the fuel-cell membrane and of the ionomers in the catalyst layers. Gas feed humidification has been achieved by a variety of methods including, for example, enthalpy exchangers [6] and porous bipolar plates [62]. The two latter approaches rely on utilization of stack-produced water, thereby eliminating the need of frequent water refill . The system in Fig. 29a, can use a condenser to... 

Many different types of fuel-cell membranes are currently in use in, e.g., solid-oxide fuel cells (SOFCs), molten-carbonate fuel cells (MCFCs), alkaline fuel eells (AFCs), phosphoric-acid fuel cells (PAFCs), and polymer-electrolyte membrane fuel cells (PEMFCs). One of the most widely used polymers in PEMFCs is Nalion, which is basically a fluorinated teflon-like hydrophobic polymer backbone with sulfonated hydrophilic side chains." Nafion and related sulfonic-add based polymers have the disadvantage that the polymer-conductivity is based on the presence of water and, thus, the operating temperature is limited to a temperature range of 80-100 °C. This constraint makes the water (and temperature) management of the fuel cell critical for its performance. Many computational studies and reviews have recently been pubhshed," and new types of polymers are proposed at any time, e.g. sulfonated aromatic polyarylenes," to meet these drawbacks. 

Okada, T., Xie, G., Tanabe, Y. 1996. Theory of water management at the anode side of polymer electrolyte fuel cell membranes. Journal of Electroanalytical Chemistry, 413,49-65. 

In addition to the preceding properties, hydration of the membrane (water management) and thickness also play important roles in affecting the overall performance of fuel cells. 

Okada, T., Xie, G. and Meeg, M. 1998. Simulation for water management in membranes for polymer electrolyte fuel cells. Electrochimica Acta 43 2141-2155. 

Because of its lower temperature and special polymer electrolyte membrane, the proton exchange membrane fuel cell (PEMFC) is well-suited for transportation, portable, and micro fuel cell applications. But the performance of these fuel cells critically depends on the materials used for the various cell components. Durability, water management, and reducing catalyst poisoning are important factors when selecting PEMFC materials. 

Figure 8. Two-dimensional sketch of water management in a PEM fuel cell whereby the membrane—electrode assembly separates the anode feed channel from the cathode, and a diagram of water uptake profiles in anode and cathode channels.

Distributions of water and reactants are of high interest for PEFCs as the membrane conductivity is strongly dependent on water content. The information of water distribution is instrumental for designing innovative water management schemes in a PEFC. A few authors have studied overall water balance by collection of the fuel cell effluent and condensation of the gas-phase water vapor. However, determination of the in situ distribution of water vapor is desirable at various locations within the anode and cathode gas channel flow paths. Mench et al. pioneered the use of a gas chromatograph for water distribution measurements. The technique can be used to directly map water distribution in the anode and cathode of an operating fuel cell with a time resolution of approximately 2 min and a spatial resolution limited only by the proximity of sample extraction ports located in gas channels. 

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.). 

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