The Research On Oxygen Transport In Membrane Electrode Assembly For PEM Fuel Cells

Oxygen transport resistance（OTR）is a critical factor to influence the performance of proton exchange membrane fuel cell（PEMFC）.The difficulty of oxygen transport within it directly determines whether the fuel cell can operate at high current densities.The oxygen transport in PEMFCs involves various transport mechanisms,including convection in the flow channel,and in the gas diffusion layer（GDL）,because most of the pore size of the material is above the micron level,the molecular diffusion is dominant,and gas transport mechanism in nanoscale the pore of the catalytic layer（CL）is Knudsen diffusion,and the presence of ionomer is also accompanied by the permeation of the ionic membrane.In general,ex-situ testing characterizes the OTR of fuel cell components,but OTR of separating the components allows to not be simply added when fuel cell operated.Therefore,ex-situ test can not truly reflect the OTR in fuel cell.The effective in-situ methode measured limiting current is used to evaluate OTR at the cathode.It is critical to understand the gas transport characteristics of individual components and optimizing the water and gas transmission of electrode materials（gas diffusion layer and catalyst layer）,reduce the concentration polarization of the cell,and improve fuel cell performance.The main results of this paper are as follows.（1）In-siut methode to measure OTR:This work measures a series of limiting current by using high excess H₂ and diluted O₂ in N₂ to calculate R_tot.From the R_tot-I curve,more than about 1500 mA/cm²,transport resistance is nearly constant,corresponding to relatively dry condition and less about than 1500 mA/cm²,transport resistance increases rapidly,corresponding to relatively wet condition.R_tot-I curve can be used to evaluate the oxygen transport condition under each current density,and can also directly reflect the total transport resistance.（2）Measure of the OTR of homemade GDL and modified CL.First of all,the OTR of a homemade GDL and a commercial GDL（JNTG-GDL）is measured under the same conditions.And by analyzing and researching experiment results,the ability of homemade GDL in oxygen transport is slightly worse than JNTG-GDL.In addition,an effective method to reduce OTR of CL was proposed by adding hydrophobic additive into CL.A kind of polytetrafluoroethylene（PTFE）with low molecule weight was selected for its feasibility to prepare emulsion.By mixing the emulsion with traditional catalyst ink and applying it onto membrane surface,we successfully add 10wt%hydrophobic phase into the CL.The microstructure of the surface for modified CL was observed by SEM already.The PTFE phase tends to distributes among the carbon particle uniformly.OTR for traditional CL and hydrophobic CL was measured by mean of limiting current under different gas pressure conditions（150,200,250,300KPa）at operating conditions of 65℃ and RH 70%.The results show that the hydrophobic phase obviously lower the OTR of CL in dry region as much as 24%compared to traditional CL.（3）An research is made of the influence of modifieding the surface of the micro porous layer（MPL）on the performance of a proton exchange membrane（PEM）fuel cell.The main requirements of a GDL are the provision of gas and the transport of water as well as significant electrical and thermal conductivity.In this paper,a modified MPL by flattening the screen printing is presented that enhances liquid water management and lowers contact resistance of MPL to ctatlyst layer（CL）.The surface of MPLs are characterized by SEM images.To study a correlation of the properties of the GDL,the electro-physical properties of modified GDL such as porosity,gas permeability,hydrophobicity,electrical resistance and thickness were measured.The polarization curve is studied with modified MPL and standard MPL at operating conditions of cell temperature 80 ℃,RH 50%and 100%and 100KPa back pressure.The experimental results show that generally the fuel cell performance with modified MPL precedes that with standard MPL,Especially,at the higher current density region.