High Performance PEM Fuel Cells - From Electrochemistry and Material Science toEngineering Development of a Multicell Stack

摘要

Under Task 1, it was shown that apparently identical MEAs of 50 Cm2 active areawith 1.4 mg/cm2 Pt./C cathodes (20 wt % Pt on C) and 0.3 mg/cm2 Pt/C anodes with 40 microns thickness Gore-Select(TM) PEM material did not give identical performance, except in the Tafel region. This indicates that their overall active surface areas at low current density were identical, and that performance suffered at high current density in the range of interest. In all cases, this is shown as a change in polarization slope in the linear region. The slope of the best of these cells was 0.25 ohms cm2, and that of the worst was ca. 0.36 ohms cm2. In consequence, the performance of the best cell at 0.7 V with humidified gases was 0.44 A/cm2, and that of the worst was 0.3 A/cm2. These are substantially less than 0.7 A/cm2 at 0.7 V, which has been achieved in 5 cm2 cells. This is the fuel cell performance level required to achieve the overall system' performance goals (i.e., 0.7 A/cm2 and 0.7 V on hydrogen and air at atmospheric pressure). The variable polarization slope gives the impression of an internal resistance component, but the internal resistance measured at high frequency is rather low, about 0.12 ohms cm2. Thus, the differences in performance observed are either due to problems with the flow-field, or to dispersion in performance between individual MEAs, which otherwise contain identical components made by identical methods.