The proton conductivity of a series of extruded Nafion membranes ͓of equivalent weight ͑EW͒ of 1100 and nominal dry thickness of 51, 89, 127, and 178 m͔ has been studied. Measurements were made in 1 M H 2 SO 4 at 298 K using a four-electrode, dc technique. The membrane area resistance increases with thickness, as expected, from 0.07 to 0.16 ⍀ cm 2 for Nafion 112 and Nafion 117, respectively. However, in contrast to the published literature, after correcting for the membrane thickness, the conductivity of the membranes decreases with decreasing membrane thickness. For example, values of 0.083 and 0.16 S cm Ϫ1were obtained for Nafion 112 and 117 membranes, respectively. In situ current-interrupt measurements in a proton exchange membrane fuel cell confirmed the relatively poor conductivity of the membrane electrode assemblies ͑MEAs͒ based on the thinner membranes. While a high contact resistance to the electrodes may have contributed to the in situ MEA resistance, water balance measurements over the MEA showed that the high resistance was not due to a low water content or to an uneven water distribution in the MEAs. The implications of the findings for the understanding of the membrane properties are discussed. Nafion membranes.-Nafion membranes have a wide range of applications due to their high chemical and electrochemical stability, reasonable mechanical strength ͑particularly when reinforced͒, extremely low permeability to reactant species, selective and high ionic conductivity, and their ability to provide electronic insulation. Industrial applications of these materials involve industrial sectors such as gas separation, gas sensors, electrodialysis, chlor-alkali cells, salt splitting, and as a solid polymer electrolyte in fuel cells and batteries. [2][3][4] This study has focused on the application of the Nafion range of cation-exchange membranes in proton exchange membrane fuel cells ͑PEMFCs͒. In the PEMFC the proton conductivity of the membrane is particularly important since it plays a significant role in controlling the performance of the fuel cell. 5,6 Higher levels of proton conductivity allow much higher power densities to be achieved. This is particularly important for automotive applications of PEMFCs. The two common strategies to improve the conductivity of the membrane are to raise the specific conductivity and to reduce the thickness. There is, however, a practical limit on the thickness since, much below 25 m, mixing of the hydrogen and air ͑or oxygen͒ reactant gasses due to crossover through the ion-exchange material is too high for pure Nafion membranes and there is a loss of efficiency. Reducing the membrane thickness also increases the risks with respect to mechanical properties such as strength, raising concerns regarding the durability and ease of handling of the membranes.The structure of Nafion membranes.-The proton conductivity of Nafion membrane materials is complex, being favored by a high level of hydration and being strongly dependent on the pretreatment ͑especially the thermal͒ his...

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For widespread exploitation of proton exchange membrane fuel cells (PEMFCs) the cost of the stack must be reduced, and the performance per unit volume increased. Significant cost reduction has been achieved by the development of a high-volume, low cost, electrode manufacturing process and from reductions in the electrode precious metal catalyst loadings. The performance of membrane electrode assemblies (MEAs) employing printed cathodes (0.6 mg Pt/cm 2 ) and anodes (0.25 mg Pt/cm 2 , 0.12 mg Ru/cm 2 ) in Ballard Mark V single-cell and advanced-stack hardware are at least comparable to current stack MEAs comprising high loading unsupported platinum black electrodes containing 4.0 mg Pt/cm 2 . Optimum cell performance has provided high power densities of 0.42 W/cm 2 at 0.7 V Furthermore, under motive and utility test conditions, the low-cost electrodes show minimal loss in performance after over 3000 h of stack operation and, in short and full sized stacks, the cell-to-cell reproducibility is excellent, highlighting the high consistency of product available from the electrode manufacturing process. Incorporation of the low cost electrodes in commercial PEMFC stacks is anticipated in the near future. J.St.-P. wishes to thank the Natural Sciences and Engineering Research Council of Canada for an Industrial Research Fellowship. The authors wish to acknowledge the contributions made by Shanna Knights, Ross Bailey, and other employees of Ballard Power Systems and Malcolm Gascoyne, Jan Denton, and other employees of Johnson Matthey, who have contributed to the development of low cost electrode technology.