Increasing the Lifetime of PEM Fuel Cells:: A Characterization of some Degradation Mechanisms
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A segmented PEM fuel cell has been applied to measure spatial differences in key properties (polarization curves, electrochemical area and membrane resistance) of a fuel cell. The cell, consisting of 10 segments, was operated for 960 hours at a constant voltage of 0.6 V. During this period the loss of electrochemical active platinum surface area increased linearly along the flow field of the cell from segment 1 to 10. The Pt degradation was interpreted to be caused by excess amounts of water near the cell outlet stemming from the electrochemically generated water from the preceding segments on the cathode. XRD analysis of selected segments of the cathode and membrane was performed post mortem and confirms the electrochemical measurements of the reduction of active Pt area. On the other hand, measurements of fuel crossover in the membrane during fuel cell testing and post mortem XRF analysis on a selection of membrane pieces showed no apparent degradation of the membrane material over the 960 h. Carbon corrosion rates were measured for four types of polymer-electrolyte membrane (PEM) electrode assemblies (MEAs) during shut-down air purging and start-up. The corrosion rates were measured at different purge rates, temperatures, and relative humidities. The highest amount of carbon corrosion occurs when gases are humidified and when using low ow rates for the air purging during shut-down and low hydrogen ow during start-up. Dry conditions give the highest rates of corrosion during shut-down when comparing the same ow rates of hydrogen and air, while humid conditions gives the highest rate of corrosion during start-up when comparing the same ow rates of hydrogen and air. The stability of different types of platinum surfaces in the presence of chloride was evaluated by applying a potential of 1.2 V vs. RHE while the associated mass change of the Pt-electrode was monitored with an electrochemical quartz crystal microbalance (EQCM). The platinum metal surfaces based on particles and films show a large difference towards dissolution when exposed to small amounts of chloride. While a platinum metal film showed no degradation in a sulfuric acid solution containing 10 ppm of chloride, an electrode made from a fuel cell catalyst (50 wt.% Pt/C) lost 10 percent of its platinum content over a 24 hour period when exposed to the same amount of chloride. At a chloride concentration of 20 ppm the onset potential of the Pt oxide formation increased ~200 mV compared to an electrode in a chloride free solution. The degradation of nanoparticles thus appears to be much more significant than for electrodeposited platinum electrodes.