Investigation On The Behavior And Mechanism Of Water And Gas Transport In The Cathode Of Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cell(PEMFC)is one of the main conversion devices for hydrogen energy,and its maximum output power is closely associated to the water and gas transport in the cathode gas diffusion layer(GDL)and catalyst layer(CL).At present,there are theoretical limitations in studying the capillary force drainage mechanism of GDL water transport.First,it only considers the discharge process of water from the hydrophobic pores of the GDL,and ignores the capillary force that liquid water needs to overcome when entering the pores.Secondly,the definition of water saturation in two-phase flow is not completed,resulting in the theoretical value of the gas transport coefficient probably far from the actual value.In addition,the influence of catalyst layer structure and surface morphology on the water and gas transport inside the catalyst layer and the interface between the catalyst layer and the proton exchange membrane(PEM)is less studied.However,these two factors seriously affect the activity expression of Pt.Therefore,in this dissertation,a water-gas transport model in a microporous layer(MPL)based on breakthrough pressure and clogging degree is established to improve the traditional capillary force drainage mechanism.And,it is verified by different GDL,and the factors affecting the water-gas transport of MPL are analyzed.By optimizing the pore size distribution of the catalyst layer and reducing the surface roughness,the gas transport within the catalyst layer and the CL|PEM interface is improved.The following innovative researches of this dissertation is as follows:(1)By studying the polarization curves of GDL with type of SGL28BC under different back pressures,it is found that the higher the back pressure,the more serious the concentration polarization of the cell is at high current density.However,theoretical analysis based on the traditional capillary force drainage mechanism shows that the back pressure has little effect on the water saturation and concentration polarization overpotential.(2)Considering the capillary force that liquid water needs to overcome when entering the pores and the deficiencies in the definition of water saturation in two-phase flow,the breakthrough pressure and clogging degree of hydrophobic pores are newly defined.Based on the seepage equation in porous media and the pore size distribution test of MPL,the relationship between the water clogging degree of small pores,current density and external pressure is established.On this basis,an MPL water-gas transport model based on breakthrough pressure and plugging degree is constructed.(3)In order to verify the correctness and feasibility of the model,using four types of GDLs,including SGL28BC,the new MPL water vapor transport model is verified,and the calculated results show a trend consistent with the measured polarization curves.In addition,based on the new model,the effect of back pressure on the concentration polarization overpotential of SGL28BC is analyzed,showing that the higher the back pressure,the more serious the polarization.Referring to the theoretical guidance,by increasing the proportion of small pores in the self-made MPL and the hydrophobicity of the carbon surface,leading to the reduction of the water blockage of the small pores and the increase of the breakthrough pressure,the optimization of the water-gas transport performance in the MPL is realized.(4)In order to quantitatively study the effect of catalyst layer pore size optimization on cell performance and oxygen transport resistance.The pore size distribution of the catalyst layer is adjusted by polystyrene microsphere(PS).After adding the pore-forming agent,the molecular diffusion resistance and local oxygen transport resistance of catalyst layer with Pt loading of 0.3 mg/cm~2 decrease by 0.079s/cm(0 k Pa)and 0.068 s/cm,respectively,while it decreases by 0.149 s/cm(0 k Pa)and0.219 s/cm at the Pt loading of 0.1 mg/cm~2,respectively.It can be seen that under the condition of low platinum,the optimization of the pore size improves the oxygen transport performance more obviously.In addition,the Pt loading and the addition of pore-forming agents have little effect on the Knudsen diffusion resistance.(5)Effects of optimization on the PEM|CL interface on the water and gas transport inside the single cell.The surface roughness of the catalyst layer is reduced by a rolling process,and an optimized CL|PEM interface is formed by a decal transfer process.The roughness is reduced from 0.347μm to 0.266μm.After rolling the catalyst layer,the internal resistance of the cell is significantly reduced and the oxygen transport resistance is reduced from 0.21 s/cm to 0.15 s/cm.The power density is increased by28.5%.The optimized interface alleviates the accumulation of interfacial liquid water and improves the oxygen transport property.