Performance Improvements in High-Temperature PEM Fuel Cells

Abstract

The work presented here focuses on recent results obtained on degradation of PBI membranes used in membrane electrode assemblies (MEAs) for high temperature polymer electrolyte fuel cells (HTPEM) that operates in the temperature range 140-180 °C. The testing is done at 160 °C using either pure hydrogen or reformate of different compositions. Pure platinum versus PtCo alloys have been studied. The latest developments show that it is possible to reduce the Pt loading, increase the platinum utilization and achieve a long lifetime. We have shown that cells based on a thermally cured membrane proved a degradation rate of as little as 0.5 μV/h over an extended period of time. This is, to the authors’ knowledge, lower than what is ever reported for HTPEM. The degradation mechanisms over time have been studied using SEM and TEM characterization tools. It has been shown that the Pt nanocatalyst particles grow from approx. 3 to 9 nm over 17,000 hours on the cathode side, while the anode is far less affected. The durability of HTPEM can now be considered similar to low temperature PEM as shown in Fig. 1. Here, more than 15,000 hours is demonstrated in single cells at 300 mA/cm2. The degradation rate is around 4 μV/h for approx. 13,000 hours. The high operating temperature makes it possible to make commercial fuel cell systems using methanol (methanol-water mixtures) as fuel. The HTPEM cells can tolerate CO impurities up to more than 3 vol-% without significant losses. Several demonstration projects have been made, including a Fiat 500 equipped with a 5 kW methanol based fuel cell system. This car is in every day operation at a catering company in Denmark. Another project is a street sweeper with the same type and size of system as shown in Fig. 2. Figure 1