Species and Properties of Interest

Water content affects many processes within a fuel cell and must be properly managed. Proton conductivity within the polymer electrolyte typically decreases dramatically with decreasing water content (especially for perfhiorinated membranes such as Nation ), while excessive liquid water in the catalyst layers (CLs) and gas diffusion layers (GDLs) results in flooding, which inhibits reactant access to the catalyst sites. Water management is complicated by several types of water transport, such as production of water from the cathode reaction, evaporation, and condensation at each electrode, osmotic drag of water molecules from anode to cathode by 

Rodatz et al.10 studied a large PEM fuel cell stack used in automotive applications at different operating conditions. One of the main parameters that were studied was the pressure drop within the stack and its relationship with the flow field (bends in the channels) and single- and two-phase flows. It was observed that once the current in the fuel cell stack was reduced (i.e., applying dry conditions) the pressure drop decreased slowly until it reached a new value. This was attributed to the fact that the reduction of current reduces the flux of product water in the flow channels, and thus, it reduces the total mass flow in the flow channels. 

A transparent PEM fuel cell with a single straight channel was designed by Ma et al.11 to study liquid water transport in the cathode channel (this study is also mentioned in Section 2.5). The pressure drop between the inlet and outlet of the channel on the cathode side was used as a diagnostic signal to monitor liquid water accumulation and removal. The proper gas velocities for different currents were determined according to the pressure drop curves. 

Pei et al.12 studied the hydrogen pressure drop characteristics in a PEM fuel cell in order to use the pressure drop as a diagnostic tool for prediction of liquid water flooding in fuel cell stacks before flow channels have been blocked. 

The species distribution within a PEM fuel cell is critical to fully characterize the local performance and accurately quantify the various modes of water transport. The most commonly used analytic technique for measuring the gas composition within a fuel cell is gas chromatography (GC). Mench et al.13 demonstrated the measurement of water vapor, hydrogen, and oxygen concentration distributions at steady state. A micro gas chromatograph was utilized to measure the samples, which were extracted from eight 

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