燃料电池75度65％RH低湿度恒电流耐久性失效分析和使用不同阴极催化剂在DOE催化剂AST、膜AST下的衰减对比

Failure Mode Analysis of Membrane ElectrodeAssembly （MEA） for PEMFC under Low Humidity Operation

Cheng－hong Wang

Chien－ming Lai

Wei－han Huang

Jiunn－nan Lin

Li－duan Tsai

Abstract

The degradation mechanisms and phenomena ofmembrane electrode assembly （MEA） for proton exchange membrane fuel cell （PEMFCunder low humidity operation were investigated by means of several on－lineelectrochemical methods and transmission electron microscopy （TEM）． Electrochemicalactive surface area （ECSA） of catalyst and hydrogen crossover of MEA wereevaluated by cyclic voltagram （CV） and linear sweep voltagram （LSV），respectively． The decrease of catalytic active site and degradation of membranewere evident by ECSA loss and hydrogen crossover rate increase after long－termdischarging test． Membrane degradation played the major role for MEAsfailure under low humidity operation in this study． The Pt clusters werealso observed in the membrane， formed by Pt ions migration and reduction in themembrane， which can react with the crossover gases to produce free radicalsthat attack membrane and ionomers in the catalyst layer． Those phenomena willdrive membrane to rupture and cause a short circuit in MEA． By AST experimentsand fluoride release rate results， the MEA with Pt－alloy／C catalyst can showbetter performance and durability due to its high selectivity on ORR to depressthe free radicals formation．

读完全文才知道，标题中的低湿度是指75度65％RH

The MEA lifetime discharge experiment was  conducted under the conditions as the followings： the cell temperature was  at 75 °C． The hydrogen fuel and air oxidant were humidified to 65％ relative  humidity and were fed to each electrode at atmospheric pressure． The  current density was set at 480 mA／cm2 using the electronic loader （PLZ－664WA， KIKUSUI corp．， Japan）．

Table 1． Catalyst accelerated stress test  protocol

Table 2． Membrane accelerated stress test  protocol

Fig． 2． （a）Homemade MEA IV performance  results at 65％ relative humidity， the cell temperature was 80 °C， and fuel  stoichiometry was 1．5 and 2．5 at anode and cathode side， respectively． （b）Simulated  results of different potential losses for Pt－alloy／C and Pt／C at 480 mA／cm2  and low humidity operation．

Fig． 3． MEA durability test result at 65  ％RH， cell temperature 75 °C， and fuel stoic． is 1．5 and 2．5 at anode and  cathode side， respectively

Fig． 4． Electrochemical surface area of  （a） Pt／C and （b）Pt－alloy／C catalyst test before and after life test． Cell  temperature 75 °C， 100 ％RH， H2／N2， 100 sccm／100sccm at anode and cathode  side， respectively． Scan rate is 20 mV／s．

Table 3． Hydrogen crossover rate of  Pt－alloy／C MEA before and after durability test

Fig． 5． Hydrogen crossover test of  Pt－alloy／C MEA before and after life test． Cell temperature 75 °C， 100％RH，  H2／N2， 100sccm／100 sccm at anode and cathode side， respectively． Scan rate 2 mV／s．

Fig． 6． TEM images of MEA． Pt／C catalyst  （a） before， （b） after test ； Pt－alloy／C catalyst （c） before， （d） after test．

Fig． 7． Particle size distribution of Pt  catalyst in MEAs before and after test． （a）． Pt／C catalyst， （b）．Pt－alloy／C  catalyst．

Fig． 8． TEM cross－section images of  Pt－alloy／C MEA after life test under low humidity condition． （a）Pt particles  coarsen and （b）（d）migrate into membrane ， as well as （c） catalyst layer  deformation．

Fig． 9． Normalized ECSA of Pt／C and  Pt－alloy／C as function of cycle number． Cell temperature 65 °C， 100％ RH， 100 sccm／100 sccm at anode and cathode side，  respectively． Scan rate is 20 mV／s．

Fig． 10． OCV holding test of Pt／C and  Pt－alloy／CMEA as a function of operation time． Cell temperature 80 °C， 30  ％RH， 100 sccm （H2）／100 sccm （Air） at anode and cathode side， respectively．

CONCLUSIONS

In this study， MEAs with Pt／C and  Pt－alloy／C catalyst have been continually operated in constant current mode under  low humidity condition， and the degradation mechanisms were evaluated by  on－line electrochemical analysis method and TEM image analysis．

The Pt－alloy／C MEA has superior  performance and durability to Pt／C MEA． The Pt alloy／C MEA has operated over  3700 hr with a power loss less than 8％， and the Pt／C MEA showed only 1500 hr  of operation with a decay rate over 50 V／hr， about 7％ of power loss． TEM  images show the metallic particle growth over both Pt／C and Pt－alloy／C  catalyst and Pt－alloy particles are more resistant to particle growth， known  as Ostwald ripening phenomenon． For ORR in the cathode side， the Pt particle  size is less important than the ability to possess a higher coverage of  oxygen species over the catalyst．

Both MEA showed a failure of dramatic  voltage drop and incapability for discharge and an increase of hydrogen crossover  flux， meanwhile， can be observed， which indicates the membrane failure． This  suggests that the membrane and ionomer degradation played a fatal role on  MEA durability as operated at low humidity． From the fluoride release  rate monitoring results， Pt－alloy／C catalyst shows a better prohibition  against free radicals formation than Pt／C catalyst． That’s the reason why  the Pt－alloy／C MEA is more durable than the pure Pt／CMEA under low humidity  operation．