PEM Fuel Cell Properties of Pre-Stretched Recast Nafion®

Abstract

Pre-stretched recast Nafion is a new membrane material with many desirable properties for PEM fuel cells. It was previously shown that the proton conductivity of this membrane in room temperature water was identical to that of commercial Nafion but its permeability to methanol was significantly reduced. Consequently, the power output from a direct methanol fuel cell with pre-stretched recast Nafion (draw ratio of 4) was 50% higher than that with Nafion 117. In an effort to better characterize pre-stretched recast Nafion, additional property measurements were made and compared to those for commercial Nafion. There was a substantial increase in the polymer crystallinity with increasing draw ratio which translated into improved membrane mechanical properties (a higher storage modulus and ultimate tensile strength). There was also a modest increase in the α-transition temperature when the membrane was elongated, as determined from dynamic mechanical analysis. Preliminary results indicated that the proton conductivity of pre-stretched membranes at a sub-zero temperature (-20oC) was about 35% greater than that for Nafion 212 or Nafion 117. On the other hand, the proton conductivity of a water vapor equilibrated pre-stretched Nafion film (draw ratio 4) at 95% relative humidity was identical to that of Nafion 212 for temperatures in the 15-60oC range.