| Polymer electrolyte membrane(PEM)fuel cells are promising energy conversion devices.They have already been adopted for commercial applications.However,many problems remain in further reducing the cost and enhancing the performance,such as managing the water in the fuel cells,which is closely related to the cell performance.Sufficient water is useful for maintaining good hydration of the membrane and improving its proton conductivity.Nevertheless,excessive liquid water accumulation in the fuel cell will block the pores of the gas diffusion layers(GDLs)and hinder the transport of the reactant gases,decreasing the cell performance.This paper concerned on the water management in PEM Fuel Cells and takes GDL and flow channel as the object of the study.Firstly,the gas diffusion properties in GDL are experimentally studied and the effect of gas diffusion on the cell performance was numerical analyzed.And secondly,based on the above study,a new design of a cathode flow-field with a sub-channel was proposed to improve the water management and PEM fuel cell performance.Nowadays,because of the complex pore structure of the GDLs and the presence of water in them,the effective oxygen diffusivity(EOD)in porous medium of a PEM Fuel Cell was still not precisely understood.In this part,the EODs were measured in two typical porous materials(TGP-H-120 carbon paper and carbon cloth,commonly used as GDLs)using an oxygen sensor based on a galvanic cell under both dry and partially saturated conditions.The measurements were performed for various pore size distributions(PSDs)and water saturation distributions(WSDs).The EOD was found to strongly depend on the PSD and WSD in the GDL,not simply on the average porosity and average water saturation,as reported in previous studies.Finally,this paper presents the experimentally verified equations that account for PSD and WSD under both dry and partially saturated conditions for the prediction of the EODs in PEM Fuel Cells.Following the above study,the sensitivity of the cell performance to ? and ?(parameters in determining gas diffusivity)is quantitatively investigated at different porosities(0.75 and 0.8 for GDL,0.3 and 0.4 for CL)and CL thicknesses(5 ?m and 10 ?m)in both cathode and anode side by a three-dimensional two-phase fuel cell model.Numerical results demonstrate that the cell performance varies considerably with the variation of ? and ?.The sensitivity of cell performance to ? of the cathode GDL and CL side,especially in the cathode CL,is greatly stronger than in anode GDL and CL side.Hence,in the modeling of a PEM Fuel Cell,special care needs to be taken in the selection of the value of ? and ?,and at the meantime,to enhance cell performance,efforts should be focused on improving the gaseous diffusion and the water removal in cathode GDL and CL.Based on the above study,it was found that a suitable cathode flow-field design can result in perfect water management and thus high cell performance.In this part,a new design for a cathode flow-field with a sub-channel was proposed and has been numberically and experimentally analyzed in a parallel flow-field PEM Fuel Cell.Three sub-channel inlets were placed along the cathode channel.The main-channel inlet was fed with moist air to humidify the membrane and maintain high proton conductivity,whereas,the sub-channel inlet was fed with dry air to enhance water removal in the flow channel.The results indicated that the sub-channels inlet positions(SIP,where the sub-channel inlets were placed along the cathode channel)and flow rates(SFR,percentage of air from the sub-channel inlet in the total cathode flow rate)had a considerable impact on water removal and cell performance.A proposed design that combines the SIP and SFR can effectively eliminate water from the fuel cell,increasing the maximum power density by more than 13.2% compared to the conventional design. |
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