High Temperature Polymer Electrolyte Membrane (HT-PEM) Fuel Cells provide an attractive alternative to Low Temperature (LT) PEM fuel cells with respect to fuel flexibility, simplified cooling system and balance of plant, higher value waste heat and no need for humidification of reactants. Polybenzimidazole (PBI) membranes doped with phosphoric acid have been demonstrated to be the most successful system to achieve high temperature operation of HT-PEM fuel cells. HT-PEM fuel cells face a number of challenges related to the performance that needs to be addressed.In this work we present the recent PBI membrane and single-cell fuel cell testing results with respect to performance and durability. In addition, we present results on the in-plane and through-plane ionic conductivity of phosphoric acid-doped PBI membrane. Lastly, we show preliminary results on post-mortem cell characterizations which are the key for understanding the breakdown mechanisms and maximizing cell performance and lifetime.
This communication will provide details on some of the high temperature polymer electrolyte membrane (HT-PEM) membrane-electrode-assembly (MEA) performance targets most recently achieved by Danish Power Systems. These include (i) MEA performances of >0.67 V at 0.2 A cm-2 using dry H2/Air, (ii) MEA lifetime of 12000 h at 0.23 A cm-2 using dry H2/Air with an average degradation rate of 9 µV h-1, and (iii) an integrated 5 kW stack/reformer system using methanol reformate as fuel. Studies using reformate have also led to promising results with an MEA performance of 0.58 V at 0.4 A cm-2 with a degradation rate of 24 µV h-1 using wet H2 (30 mol%)/Air for 500 hours. In addition to reaching these performance benchmarks, a reduction in the standard deviation for MEA performance to <5% has been achieved through efforts aimed at improving the uniformity of the membrane and catalyst layer thicknesses.
HT-PEMFC based on phosphoric acid-doped polybenzimidazole membranes are a technology characterized by simplified construction and operation along with methanol reformers. Durability issues including acid loss, platinum sintering and carbon corrosion are recognized for both steady state and start-stop cycling operations. This work reports experimental studies on the degradation of PBI-based fuel cells operating with synthetic reformate fuel and air. Degradation stressors include elevated temperatures, pressures, current densities, and start-stop cycles. An average degradation rate of 9.3 µV/h is observed for continuous operation at 0.4 A/cm 2 and 160 ˚C for 12,000 h. High pressure (1.5 bar abs ) operation at 170 o C and 0.8 A/cm 2 shows an average degradation rate of 12.6 µV/h during a period of 2,000 h. A startstop test from 50 o C consisting of 240 cycles between temperatures of 165 and 175 o C and current density of 0.31 and 0.55 A/cm 2 reveals a performance decay by 0.48-0.58 mV/cycle.
One of the major advantages of polybenzimidazole (PBI) based high temperature polymer electrolyte (HT-PEM) fuel cells compared to the low temperature representatives of this type of fuel cell technology is the higher tolerance against impurities like CO. Nevertheless lifetime and durability are still an issue. In this work, an improvement in degradation rates and therefore an extension of lifetime of HT-PEM fuel cells will be presented. Extended long term tests have been performed at different facilities under similar test conditions. After 3,000 h of operation, an average degradation rate of -1.7 µV/h has been achieved. One of the tests is still under operation; this MEA already reached a lifetime more than 9,000 hours with an actual degradation rate of -3 µV/h.
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