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...

The ionic conductivity of Nafion® 1100 extruded membranes re‐cast from solutions of butan‐1‐ol and propan‐2‐ol is measured in 0.5 mol dm–3 H2SO4 at 295 K, using an immersed, four‐electrode d.c. technique. The general trend is an increasing conductivity for the thicker membranes. Materials which were solution‐cast from butan‐1‐ol yielded the highest conductivity while a series of membranes with lower conductivities (similar to those of an extruded Nafion® 1100 series of membranes) was found using propan‐2‐ol. The conductivity results indicate that membranes manufactured by extrusion and casting from various solvents might have different structures. Differences in the water content and conductivity of the membranes are considered to arise from the impact of processing conditions on the surface and bulk structure of the membranes.

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