Research On The Mass Transport And Durability Of Gas Diffusion Layer In Proton Exchange Membrane Fuel Cell

The poor durability of proton exchange membrane fuel cells still is their greatest barrier preventing commercialization.The gas diffusion layer(GDL)is an important component of the core component of PEMFCs,membrane electrode assembly(MEA).A GDL typically consists of a substrate layer and a micro porous layer(MPL),and the GDL is the water management center of a fuel cell.The most important role of a GDL is to allow gaseous reactants to move evenly towards the catalyst layer from gas channels,which is closely related to oxygen transport resistance.Another important function of a GDL is to drain the water generated in the cathode at a certain speed in order to avoid water flooding in the cathode CL,especially when the fuel cell is operated at a high current density.On the other hand,it is also necessary to prevent the membrane and catalytic layer from drying out due to excessive water loss,which will decrease the ionic conductivity and increase the internal resistance of the fuel cells.In long term durability tests,the degradation of GDL functions,especially the change of oxygen transport resistance,is a key factor for the performance degradation of single cell at high current density.The main results of this paper are as follows:(1)durability failure analysis of proton exchange membrane fuel cell and its effect on oxygen transport in gas diffusion layer:In this part,we perform a 1465 h in situ durability test with a single fuel cell operating at a constant-current of 800 mA/cm~2.Polarization curves,cyclic voltammetry,and electrochemical impedance spectroscopy are used for diagnostics and to understand changes in performance during the test.Limiting current method is used to analyze the oxygen transport resistance of the gas diffusion layer before and after durability test.The surface morphology and hydrophobicity of the gas diffusion layer are characterized using scanning electron microscopy and contact angle measurements,respectively.The results indicate that gas diffusion layer is the first component leading to fuel cell failure after 1465 h test.The oxygen transport resistances of the substrate layer and micro porous layer in the gas diffusion layer increase and the dry region narrows down after the 1465 h test,which implied the loss of water management capability of gas diffusion layer.Scanning electron microscope images and hydrophobicity analysis show that these may be caused by oxidation of carbon and loss of polytetrafluoroethylene in gas diffusion layer.(2)the effect of relative humidity on the durability of the gas diffusion layer is analyzed by means of limiting current.At constant current density of 800 mA/cm~2,and relative humidity(RH)of 70%and 100%,the situ durability tests are carried out for2040 and 1465 hours,respectively.The oxygen transport resistance of the gas diffusion layer(GDL)before and after durability test is analyzed by limiting current methods.It is found that the limiting current density of the single cells decrease after two durability tests,that is,the oxygen transport resistances increase.SEM and hydrophobicity analysis also show that although the aging time of single cell at 100%RH was nearly600 hours less than that of 70%RH,the GDL water management capability of the former was much lower than that of the latter.Severe flooding has occurred,and the water management capacity of the single cell has declined only slightly at 70%of RH,although the single cell has run for 2040 hours.