humidity. Interestingly, the slope of the PFIA and PFICE-4 lines (on a log scale) were approximately parallel to each other and to the PFSA controls.
Why All the Trouble to Develop New Ionomers?The objective of this work was to increase the proton conductivity of the ionomer while maintaining a water-insoluble membrane with good physical integrity. By starting with a perfluoro sulfonyl fluoride polymer backbone of 700 or 800 g/mol, it was our expectation that the backbone crystallinity would remain, and, therefore, the water insolubility, while the ion exchange capacity would increase. In other words, each additional bis(sulfonyl)imide functionality increased the overall acid content but left the backbone unchanged.It is clear from Fig. 3 that the ionic conductivity increased with these new ionomers over all humidity ranges, but it is worth considering the implications of these improvements in actual fuel cell operating conditions. Starting with the higher humidity conditions at 90% RH, the conductivity of the PFIA ionomer was over two times higher than Nafion (335 vs. 147 mS/cm) and nearly four times higher for the PFICE-4 ionomer (567 vs. 147 mS/cm). Making a few simple assumptions, such as a 20 μm thick membrane and operating at 1.5 A/ cm 2 , voltage loss due only to membrane resistance at these conditions can be calculated using Ohm's law (V=I*R). For the Nafion membrane the voltage loss was calculated to be 20 mV, the PFIA about 9 mV and the PFICE-4 about 5 mV. While these differences can be significant at times, they represent only a modest improvement at best and would not likely justify the additional effort and cost associated with these new, ultra-low equivalent weight ionomers. However, looking at the other end of the humidity range, where the conditions are significantly drier, the same comparisons were made with dramatically different conclusions. At 30% RH the ionic conductivities of Nafion, PFIA and PFICE-4 membranes were 11, 44, and 70 mS/cm, respectively. Using these values in the same analysis as above, the voltage loss due to membrane resistance at a current density of 1.5 A/cm 2 was calculated to be 275, 68, and 43 mV respectively. This several hundred millivolt reduction in voltage loss due to lower membrane resistance can be put into context by converting these values into loss of efficiency. For instance, one simple measure is the ratio of the calculated voltage loss to the equilibrium potential of the hydrogen-oxygen reaction (1.23 V). Under fully hydrated conditions, the efficiency loss due to membrane resistance only was 1.7, 0.7 and 0.4% for Nafion, PFIA, and PFICE-4 respectively. Again, these differences may be significant in some situations but are generally very small values. The more interesting case, however, is where the membrane humidification is low. For the Nafion membrane a 22% efficiency loss was calculated. This value is unacceptable and would generally prevent a system design that would be expected to operate at these dry conditions. PFIA membrane provides a much larger co...