Study On Low-Platinum Load PEMFC Membrane Electrode Assembly And Its Durability Failure Process

Performance and durability have always been the most urgent problems that hinder the commercialization of proton exchange membrane fuel cell（PEMFC）in fixed and mobile power applications.Membrane electrode assembly is the core component of fuel cell.The performance of membrane electrode assembly determines the performance of the PEMFC.The decline of membrane electrode assembly is one of the decisive factors for the durability of the cell.The performance of low Pt-loading membrane electrode assembly decreases seriously in the high current density area.The decline of low Pt load membrane electrode assembly is more serious than that of high Pt-loading membrane electrode assembly,which is very unfavorable to the pursuit of economical and practical fuel cell.Therefore,it is very important to study the specific failure mechanism of the performance and durability of membrane electrode assembly in low Pt-loading fuel cell stack.This could not only provide ideas for the design of low Pt-loading fuel cell and further reduce the cost of fuel cell,but also provide practical and theoretical basis for further improving the reliability of membrane electrode assembly in the specified time.Firstly,in order to improve the performance of low Pt-loading membrane electrode assembly,SiO₂ particles（pore forming agent）of 15nm,20nm,30nm and50nm were used to make pores in Pt/C catalyst layer by hard template method.It is found that when the mass ratio of pore forming agent to catalyst is 0.5%,1%and 3%,the performance of membrane electrode assembly hardly changes,while when using pore forming agents with the same content and different sizes,the performance of membrane electrode assembly changes slightly.Generally,when using 20 nm pore forming agent,the performance is the best,but this modification method does not reduce its oxygen transport resistance.Using hard template method to make pores can not increase the performance of low Pt-loading membrane electrode assembly in high current density region.Then,in order to better explain the problems related to the durability of low Pt-loading fuel cell stack,our research group cooperated with ZBT GmbH laboratory in Germany and used the self-designed low humidification cycle test condition to make the short stack composed of five cells with a load of 0.28mg/cm² run for 3910 hours.The test data are analyzed,and the cell no.5 with the worst performance is tested and characterized offline in different regions,and the specific cause of its failure is obtained.The details of the study are as follows:（1）The short stack composed of five membrane electrode assembly with a load of 0.28mg/cm² was tested in a low humidification variable load cycle for 3910 hours.By analyzing the durability data,it is found that the voltage consistency of the five batteries changes after 3910 hours of operation.At the current density of 800m A/cm²,the standard deviation of the stack increases from 7.58mv before durability to 15.33mv after durability.There is an active region in the operation of the stack,and the voltage in this region increases with the increase of test time.By analyzing the open circuit voltage data before and after start up and shutdown,it is found that the open circuit voltage of cell no.5 decreases linearly with the number of start up and shutdown.By analyzing the I-V test data of the stack,it is found that the decrease of the cell’s voltage is mainly due to the increase of the activation overpotential at low current density.After data analysis,it is concluded that there may be a protection mechanism in the system,which makes the hydrogen supplied by membrane electrode assembly and anode become dry after the accident.（2）Make a single cell fixture with the same flow field structure as the fuel cell stack,and explore the loading and testing conditions of the new fixture.Then,after 3910 hours of variable load cycle test,the cell no.5 with the most serious performance degradation is assembled into a single cell,I-V test,CV test,LSV test and EIS test are carried out,and the test data are analyzed.The results show that the degradation of cell no.5 performance may be due to the reduction of the number of active sites in the catalyst layer.The tested crossover increases slightly,which may be one of the reasons for the performance degradation at low current density.It is worth noting that at high current density,R_mt decreases,which may also lead to poor water retention performance of membrane electrode assembly at low current density and large proton transmission resistance,resulting in performance degradation at low current density.（3）The fresh membrane electrode and cell no.5 were analyzed off-line by region-based segmentation method.The contact angle of each region was measured by the contact angle measuring instrument.It was found that the contact angle of the microporous layer（the side close to the catalytic layer）decreased steadily.The cross-sectional morphology of the cathode inlet and outlet and the middle region were characterized by scanning electron microscope（SEM）.It was found that the thickness of the proton membrane in the middle region changed from 14.1μm to10.1μm.Combined with EDS to characterize the distribution of Pt elements,it is found that a small amount of Pt migrates to the proton exchange membrane and a large amount of Pt migrates to MPL.The Pt particle size in different areas of the cathode catalyst layer is observed by TEM.It is found that the particle size at the inlet and outlet of the cathode increases,but the particle size in the middle part hardly changes.The details of the interface morphology between the catalytic layer and the proton exchange membrane were characterized by means of cold inlaid resin embedding and ultra-thin sections.It was found that the loss of Pt particles was the most serious in the cathode catalytic layer at the inlet of the cathode.The cathode/anode catalytic layer in the cathode outlet area also has a certain degree of Pt loss.There is a large accumulation of platinum particles at the boundary between the catalytic layer and the membrane where platinum loss occurs.