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An experimental technique has been developed to conduct simultaneous measurements of species concentration and current density distributions using a segmented cell of 50 cm 2 . Twelve segmented subcells in conjunction with a multichannel potentiostat were employed to measure the current distribution, and two sets of multiposition microactuators and micro gas chromatographs were used to measure gas concentration distributions on the anode and cathode sides simultaneously. Distribution data are shown systematically for a polymer electrolyte fuel cell ͑PEFC͒ operated under low-humidity cathode conditions and at 80°C and 2 atm. The O 2 concentration profile for the PEFC is generally linear and can be well predicted by a simple mass balance analysis, the anode water concentration profile is closely tied to the net water transport rate through the membrane, and the net water transport coefficient ranges between 0.05 and 0.30 even under very dry cathode conditions, suggesting the presence of strong back-diffusion of water from the cathode to the anode despite rather dry cathode gas.Low-humidity operation of polymer electrolyte fuel cells ͑PEFCs͒ is a current focus for automotive applications. In such an operation, membrane dehydration and electrode flooding by liquid water can coexist in a PEFC, thereby significantly compounding the fundamental phenomena governing the cell performance and operational stability. Detailed quantification of localized behaviors in this type of PEFC is highly desirable for a better understanding of cell performance and durability.Various experimental techniques to measure the current distribution have been developed, including the printed circuit board approach by Brown et al. 1 and Cleghorn et al., 2 the partial membrane electrode assembly ͑MEA͒/subcell approach and passive current mapping technique by Stumper et al., 3 the magnetic loop current sensor approach by Wiser et al., 4 the segmented cell/resistor network approach by Noponen et al. 5 and Brett et al., 6 and the segmented cell/multipotentiostat approach of Wang and co-workers. [7][8][9] In addition, localized ac impedance and current distribution have been measured on a segmented PEFC with a single linear channel at 30°C with 1 atm fully humidified H 2 and dry air inlets. 6 Mench et al. 9 demonstrated the first measurement of water, hydrogen, and oxygen concentration distributions using a micro gas chromatograph. A review of these prior efforts in detail was provided most recently by Wang. 10 A common problem prevailing in all prior work to date is the inability to produce a similar level of current density expected from a nonsegmented single cell under operating conditions characteristic of automotive applications. The problem largely originates from excessive contact resistances existing between many segment pieces and the gas diffusion layer ͑GDL͒.The objective of the present work is twofold. One is to present an improved segmented cell technique that enables measurement of the current distribution that closely matches the perfo...

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