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90度30%RH下氧浓度、膜厚度对燃料电池电解质膜降解速率的影响
Degradation of Polymer-ElectrolyteMembranes in Fuel Cells I. Experimental
T. Madden
D. Weiss
N. Cipollini
D. Condit
M. Gummalla
S. Burlatsky
V. Atrazhev
Abstract
In this work, chemical degradation isstudied using highly controlled measurements of the fluoride ion release fromsubscale cells in degrading environments using perfluorosulfonic-acid-basedmembrane electrode assemblies, primarily with cast, 25um/1mil thick membranes.Effects of key variables, such as oxygenconcentration, relative humidity (RH), temperature, and membrane thickness onthe fluoride ion emission rate (FER) are described under open-circuit decayconditions. Some of the observed trends are expected or consistent withprevious observations, such as decreasing FER with decreasing temperature andincreasing RH. Other trends observed are not expected, such as a logarithmic decrease of FER with oxygenconcentration and increasing FER with increasing membrane thickness.Cross-sectional transmission electron microscopy analysis of decayed membranesindicates a surprisingly homogeneous distribution of small Pt particles (3 to 20nm in diameter), presumably from dissolution and migration from the cathode.The experimental results are consistent with radical generation at these Ptparticles from crossover hydrogen and oxygen, subsequent radical migration, andpolymer attack. The response of the FER to new experimental conditions in thisstudy suggests that the attack can existat any plane within the membrane, not just the “Xo” plane of maximum Ptprecipitation.
Figure 1. BOL performance curve for the cast 1 mil MEA with type 1 electrodes showing cell voltage data (uncorrected) and corrected for ohmic resistance data (current interrupt method). Performance curves were measured with hydrogen and air at saturator dew points of 65 and 62°C, respectively, and the cell temperature controlled to 65°C.
Two types of electrodes were used in this study: type 1 electrodes were applied by a vendor, had higher performance, and exhibited higher FER under equivalent conditions vs 2 electrodes. Type 2 electrodes were laboratory-made samples that included high-surface-area 50 wt % Pt on Ketjen carbon support (supplied by Tanaka Kikinzoku Group) mixed with Nafion 1100 equivalent weight solution (supplied by Solution Technologies).
Figure 2. Hydrogen crossover currents measured at 0.5 V with pure hydrogen/nitrogen (100% RH) applied at the anode/cathode, respectively, for the cast 1 mil MEA with type 1 electrodes at BOL. Balanced (0 psig) and unbalanced (5 psi) anode overpressure measurements are used to characterize cell integrity at BOL.
Figure 3. Plot of cumulative fluoride emitted (umol/cm2) vs time for various O 2 concentrations during a single-cell test using the cast 1 mil MEA with type 1 electrodes at 90°C, 30% RH, and open-circuit conditions. The FER is obtained by least-squares fit of the data during the O 2 introduction.
Table I. Replicate values for the fluoride emission rate measured for six unique cell builds at the following conditions: 90°C, 30% RH, 100% O 2 (dry basis), and open-circuit potential. All samples employed type 1 electrodes.
Figure 4. Plot of FER (umol/cm 2 h) and cathode-to-anode FER ratio vs O 2 concentration during a single-cell test using the cast 1 mil MEA with type 1 electrodes at 90°C, 30% RH, and open-circuit conditions. The line is a least-squares logarithmic fit.
Figure 5. Plot of FER (umol/cm 2 h) vs RH using the cast 1 mil MEA with type 1 electrodes at 90°C, 100% cathode O 2 , and open-circuit conditions. The dotted line is a guide for the eyes.
Figure 6. Plot of ln FER (umol/cm 2 h) vs temperature (at 55, 70, 80, and 90°C) using the cast 1 mil MEA with type 1 electrodes at 30% RH, 100% cathode O 2 , and open-circuit conditions. The line is a least-squares linear fit corresponding to ~70 kJ/mol.
Figure 7. Plot of FER (umol/cm 2 h) vs membrane thickness using type 2 electrodes at 90°C, 30% RH, 100% cathode O 2 , and open-circuit conditions.
Figure 8. Plot of Pt particle locations using two TEM imaging techniques. DF-STEM was better for determining particle location but was not accurate for particle size. BF-TEM was more difficult but yielded more accurate particle sizes.
Two different techniques were used to determine the Pt particles: bright-field (BF) TEM and dark-field (DF) scanning TEM (STEM)
Conclusions
Experimental methods have been refined to provide accurate determinations of FER over a wide range of conditions. Some of the observed trends are expected, such as decreasing FER with decreasing temperature and increasing RH.
Other trends observed were not expected, such as a logarithmic decrease of FER with oxygen concentration and increasing FER with increasing membrane thickness.
Cross-sectional TEM analysis of decayed membranes indicates a surprisingly homogeneous distribution of small Pt particles, presumably from dissolution and migration from the cathode. The experimental results are consistent with radical generation and migration at these Pt particles from crossover hydrogen and oxygen. The response of the FER to new experimental conditions in this study suggests that the attack throughout the volume of the membrane populated with Pt particles and not just the “Xo” plane of maximum Pt precipitation.
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