| As the core component of the proton exchange membrane fuel cell?PEMFC?,the performance and durability of the membrane electrode assembly will affect the performance and durability of the PEMFC.There have been a lot of reports about the durability of MEAs,however,study on the structure of MEAs have rarely been reported.Especially for the effect of the structure of the MEA on the durability of the PEMFC and the structural change of the catalyst layer of the PEMFC stack during long-term dynamic loading conditions.In this paper,a metal BPs stack with 10-cell configuration has been assembled and tested its durability for 2,000 h by adapting a test protocol formulated according to the real road driving conditions for fuel cell vehicles.The performance degradation of the stack and individual cells have been analyzed after testing.In order to analyze the failure mechanism and the structural change of catalyst layers,individual cells were disassembled from the stack.Three representative cells,No.1,No.5 and No.10 were chosen for further comprehensive studies.1.After testing for 2,000 h,the voltage decay percentage of the stack was 14.34%at 800 mA cm-2 current density,and the voltage decay rate was 42.30?V h-1.The voltage non-uniformity has been found in the stack.The performance degradation of the No.1 cell which near to the inlet of the reactant gas was the most serious,while the No.10 cell was the weakest.Three representative cells?No.1,No.5,and No.10 cell?of the stack were characterized and analyzed after durability test.It was found that the carbon paper of the MEA changed from a superhydrophobic surface to a hydrophobic surface,and the static contact angles decreased by about 20°.The thicknesses of the cathode and anode CLs of three cells were thinner than the initial ones,but the PEMs have swelled.The catalyst loading of initial MEA is 0.3943 mg cm-2,while after 2,000h of durability testing,the catalyst loading of No.1 decreased to 0.2906 mg cm-2.2.The aged CLs have been analysized by TEM.After testing for 2000 h,the sizes of Pt particles increased from 3.4 nm to the range of 4.4-5.7 nm.Specimens embedded in epoxy resin and sliced to thin films using a ultramicrotome and analysized the morphology and elemental distribution,it was found that the carbon support,Pt and Nafion of cathode CL agglomerated after durability testing.The atomic ratios of the catalyst layers were detected by XPS,and it was found that the S/Pt of the catalyst layer of the No.1 was the highest after the durability test.3.To study the effect of the increase of S/Pt on the performance of MEA.The effect of sulfonate adsorption on the ORR of Pt have been investigated using CF3SO3H to exclude the effect of polytetrafluoroethylene backbone on Nafion.It was clearly shown that sulfonate groups would low the ORR kinetic when the content of sulfonate group was up to 5 mM,and futher increased the content of the sulfonate,the activity of Pt decreased.The kinetic loss of Pt in electrolyte with 10 mM CF3SO3H was 40%,and the 50 mM was 71.3%at 0.9 V.The sulfonate groups absorped on Pt surface will lower the ORR activity of Pt.4.The electrodes were imaged in three-dimension using Nano-CT to reconstruct the three-dimensional structures.The solid and pore phases of the catalyst layers have been separated after image segmentation.The result shows that the solid-phase of the cathode catalyst layer was smaller than the initial one.And after testing for 2,000 h,the pore volume fraction of cathode CL increased from 32.4%to 39.5%.Effective oxygen diffusivities and thermal conductivities were computed.It is showed that the effective gas diffusion coefficient of the cathode CL increased from 3.23×10-7 m2 s-1 for the initial MEA to 4.59×10-7 m2 s-1 after the durability test.The effective diffusion coefficient increased by 42.1%.In addition,the thermal conductivity of the cathode CL decreased from 0.142 W m-1 K for the initial MEA to 0.105 W m-1 K,and the thermal conductivity decreased by 26.1%. |
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