Experimental And Simulation Studies On Mechanical Degradation Of Catalyst Layers In Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cell(PEMFC)is an energy conversion device which converts chemical energy directly into electric energy.It has advantages of high efficiency,low/zero emission and low operating temperature;and is a promising candidate for automobile applications.However,durability is an important technical challenge that limits the widespread applications of PEMFCs.In order to improve the durability,the degradation mechanism of fuel cell components should be fully understood.Catalyst layer(CL)is the key component of a PEMFC.Electrochemical reactions occur in CLs.Thus transport of protons,electrons,reactant gases,and products are all involved in CLs.The degradation of CLs leads to important effects on the fuel cell performance.In automobile fuel cells,the relative humidity(RH)and temperature vary significantly during the startup and shutdown processes,and these changes may cause the CLs to degrade.The objective of this study is to investigate the mechanical degradation of CLs under RH and temperature cycling conditions.Focusing on the structural changes of CLs,the corresponding effect on the fuel cell performance,and the related mechanism.The following work has been carried out:1.The influence of in-situ humidity cycling on the CL microstructure changes and fuel cell performance has been investigated.Membrane electrode assemblies(MEAs)are fabricated by gas diffusion electrode(GDE)method and catalyst coated membrane method(CCM)respectively.The PEMFC is connected with the fuel cell test station with hydrogen supply to the anode and nitrogen supply to the cathode.Humidity cycles are applied by changing the RH in the inlet gas streams.The fuel cell performance is tesed and the CL microstructure is observed.It is found that the fuel cell performance decreases significantly after the humidity cycling.Using CCM method to fabricate the MEA leads to more severe performance degradation.The ohmic resistance increases and electrochemical surface area decreases according to the result of result of impedance and cyclic voltammetry measurement.The microstructure of the CL is observed before and after the test,and it is found that agglomerate size grows significantly due to RH cycling,especially at locations under the rib.The growth of the agglomerate size may lead to the loss of electrochemical active surface area and reduction in the fuel cell performance.2.The influence of ex-situ RH cycles and temperature cycles on the CL structural changes has been investigated.The CL structure cannot be observed periodically for a fixed location in the in-situ experiments.Thus ex-situ experiments are designed to investigate the effect of RH and/or temperature cycles on the structural changes of the CL.The GDE samples and CCM samples are observed respectively.For the GDE samples,the experimental results indicate that cracks initiation and propagation,agglomerate detachment and growth,and surface bulges are the main structural changes of the CL.Applying RH and temperature cycles simultaneously causes the most significant crack propagation,while applying temperature cycles individually causes no significant change.It is indicated that the absolute humidity is the key parameter for the crack propagation.The performance of the samples after being subjected to the RH and temperature cycles decreases most significantly due to the increase in the ohmic resistance and charge transfer resistance and the reduction of electrochemical active surface area.The increase in the ohmic resistance may be caused by the crack propagation and surface height raise.The increase of charge transfer resistance and the reduction of electrochemical active surface area may be caused by the crack growth,agglomerate detachment,and agglomerate size growth.For the CCM samples,the microstructure of the CL show no significant changes;however,surface bulges occur which may caused by the plastic strain of the membrane.After RH and temperature cycles,CCM samples show more severe degradation than GDE samples which may caused by the chemical degradation of the membrane inside the CCM samples.3.A mathematical model has been developed based on the finite element method to investigate the mechanism of CL structural changes.The model is developed based on a simplified CL structure.Ionomer and Pt/C agglomerate are included in the model.The elastic-plastic model is adopted to describe the mechanical behavior of the ionomer.The cohesive zone model is adopted to describe the interfacial behavior between the ionomer and Pt/C agglomerate.The influence of RH,temperature,and clamping force cycles on the microstructure changes of the CL is investigated for different ionomer size.It is shown that the interface between the ionomer and Pt/C agglomerate start to delaminate near the end of the shutdown process,and the change of RH is the dominant factor that affects the delamination process.When more cycles are applied,the crack length and the maximum interface separation increase significantly.About 90% of the interface delaminates after 100 cycles.The initiation and propagation of the crack is mainly caused by the plastic strain inside the ionomer.The plastic strain is unrecoverable which makes the ionomer delaminates from the Pt/C agglomerate.In spite of the crack propagation,the plastic strain in the ionomer accumulates with the cycling.Long term operation may lead to the internal crack initiation inside the ionomer.In addition,it is found that small ionomer size may relief the crack propagation and plastic strain accumulation.Therefore,distributing the ionomer uniformly and control the total content of the ionomer may help to decrease the CL mechanical degradation.