An investigation on antiflooding of PEMFC with inplate adverseflow flowfield【推荐论文】 .doc

精品论文An investigation on anti-flooding of PEMFC with in-plate adverse-flow flow-fieldLI Pengcheng1, PEI Pucheng2, HE Yongling1, ZHANG Hongfei25(1. School of Transportation Science and Engineering, Beihang University, Beijing 100191, China;2. State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China)Abstract: The stoichiometric ratios and related regimes, which can promote anti-flooding of polymer electrolyte membrane fuel cell (PEMFC) with in-plate adverse-flow flow-field (IPAF), were10investigated in this paper. Two flow combinations, which are the simple and complex adverse-flow between plates (ABP) that can be realized by IPAF, were employed to investigate. Constant stoichiometric ratios examination indicates that the complex ABP could improve anti-flooding of PEMFC better in the medium (greater than 200mA/cm2 and less than 1000mA/cm2) and high (greater than 1000mA/cm2) current densities than the simple ABP. More stoichiometric ratios were introduced15to find the cathode critical stoichiometry. Under the condition of cathode critical stoichiometry, the maximal local relative humidity of both electrodes of complex ABP is equal to 100% and below whilethe anti-flooding of the cathode of simple ABP is not satisfactory in the medium and high currentdensities. Further study shows that the mechanism of fuel cell, which is the interdependence between the electrodes effect, can make significant contribution to anti-flooding.20Key words: proton exchange membrane fuel cell; IPAF; stoichiometry; anti-flooding0IntroductionPEMFC is considered to be a clean and efficient power source which can be applied in many fields, such as batteries for portable devices, transportation power sources and residential stationary power. However, it may suffer many problems, of which fuel starvation 1, 2, oxidant25starvation 1, 3 and flooding 4, 5 are three typical issues. Compared with the other two issues, the flooding directly affects the operation and application of PEMFC. However, these issues are not independent. Therefore, it is necessary to understand their relationships and make use of them to improve the anti-flooding capability of the PEMFC.In the last two decades, a lot of efforts were made to improve the anti-flooding of PEMFC30and considerable progress was obtained. Su et al. 6 experimentally investigated the effect of different cathode flow-field on the flooding of PEMFC. They found that the parallel and interdigitated flow channels are easily flooded. ORourke et al. 7 proposed an early detection scheme of anode flooding in a PEMFC. They pointed out that anode flooding is suspected if the individual cells voltage of one or more cells significantly differs from the median cell voltage.35Kim et al.8 proposed a method to mitigate the cathode flooding in PEMFC. In their research they added hydrogen to the cathode reactant gas. Their experimental results showed that the liquid water in the cathode gas diffusion layer (GDL) can be removed by this method. However, the hydrogen addition method may cause voltage drop of PEMFC. Yousfi-Steiner 9 and Li 10 summarized the influencing factors of anti-flooding of PEMFC, including temperature, pressure,40inlet relative humidity (RH), flow-field configuration, gas feeding configuration, membraneproperty and operating current density. The mechanism of each factors contribution to theFoundations: Specialized Research Fund for the Doctoral Program of Higher Education (No. 20090002110074) Brief author introduction:LI Pengcheng, born in 1981, is currently a PhD candidate in School of Transportation Science and Engineering,Beihang University, China. He received his bachelor degree from BeihangUniversity, China, in 2004. His research interests include fuel cell design andcomputational fluid dynamicsCorrespondance author: PEI Pucheng,is currently a professor in Department of Automotive Engineering, TsinghuaUniversity, China. His research interests include fuel cell and engine design. E-mail: pchpei- 13 -anti-flooding of PEMFC may be different and they may affect each other. Therefore, it is also necessary to clarify each factors contribution and define their influence scope.A lot of researchers have noticed the significance of flow-field configuration to the45anti-flooding of PEMFC. But these studies focused on the traditional flow-field configuration. In our groups former work, Zhang 11 proposed a new flow-field that is IPAF, and preliminary proved that IPAF has enormous potential to improve anti-flooding. The first task of this paper is to find appropriate stoichiometric ratios to improve the anti-flooding capability of IPAF. At the same time, the new flow-field provides an opportunity to understand the inter-relationship of various50species transports and define the influence scope of each factor. This is the second task of this paper.1Three-dimensional numerical simulation1.1 Flow combinations of oxidant and fuelAs described in Ref 11, the IPAF flow-field has two independent groups of flow path and55the flow direction of either group could be changed to the same direction as to that of the other group. Therefore, the IPAF flow-field has several oxidant and fuel flow combinations, of which three typical ones are listed in Table.1. Fig.1 shows the sketch of three flow combinations. In this work, the simple and complex ABP, i.e. CAC and AAA, are taken for comparison.60Fig. 1 Scheme representation of three flow combinationsTab. 1 Three typical combinations of oxidant and fuel flows that IPAF flow-field plates can support.Combination Oxidant in-plate flow Between-plate relative flow of fuel and oxidantFuel in-plate flowCCC C C C CACC A CAAA A A AA: adverse-flow; C: co-flow.1.2 Brief introduction of simulation method3D simulations were performed using the commercial code ANSYS Fluent-Fuel Cell model.65In the software there are 8 sub-modules for PEMFC simulation, but only three sub-modules: Electrochemistry Sources, Butler-Volmer Rate and Membrane Water Transport, are chosen in this work. Model equations and detailed information of this software for fuel cell simulation can be found in Fluent user manual provided by ANSYS, Inc.In our former work 11, we systematically described the numerical simulation method,70simulation region, sample fuel cell parameters and main assumptions. In addition to the adjustment of stoichiometry and fuel cell dimension, this content is also applied to this paper and will not be repeated here. The adjustment of fuel cell geometry parameters is based on simulation optimization results and the MEA we purchased. The geometry and operating parameters arelisted in Table.2.75Tab. 2 Parameters of PEMFC modelItem Value Item ValueChannel length 230 mm Cathode diffusion/catalyst layer permeability3.0×10-13/3.0×10-13m212Cathode channel width/depth0.6/0.8 mm Anode diffusion/catalyst layer permeability3.0×10-13/3.0×10-13m212Anode channel width/depth 0.6/0.4 mm PEM permeability 1.8×10-18m2Cathode/anode rib width 1.0/1.0 mm Diffusion/catalyst layer electric conductivity570 Sm-1Cathode/anode platethickness2.0/1.2 mm Bipolar plate electricconductivity3703 Sm-1Cathode/anode diffusion layer thickness200/200m PEM ionic conductivity Springer model13Cathode/anode catalystlayer thickness15/10m Cathode/anode referencecurrent density1.05×106/9.23×108Am-3PEM thickness 50m Transfer coefficient at cathodeActive area 3.68cm2 Transfer coefficient atanode1.50.5Diffusion/catalyst layer tortuosity1.5/1.5 O2/H2 concentration dependence1.0/0.5PEM tortuosity 10 Open circuit voltage 0.994VCathode diffusion/catalyst layer porosityAnode diffusion/catalystlayer porosity0.496/0.20312 Temperature 343 K0.203/0.203a12 H2/air inlet pressure (bar) 1.1/1.1PEM porosity 0.28 H2/air inlet RH 100/50%a: The anode diffusion layer of low porosity is adopted to suppress the across-rib transport of hydrogen to adapt to that of oxygen, which may be in favor of the function of IPAF flow fields.1.3 Stoichiometric ratiosFirstly, 9 stoichiometric ratios have been employed to investigate. These 9 stoichiometric80ratios combinations are 1.2, 1.8, 2.4 for anode side and 2.0, 3.0, 4.0 for cathode side. Then in order to obtain the cathode critical stoichiometries, more levels of cathode stoichiometry are introduced to investigate at certain values of operating current density.1.4 Characterization of anti-floodingIt is quite imprecise to estimate the degree of flooding of PEMFC by experimental methods.85In experiments, the flooding of PEMFC is detected by the pressure drop along channels measurement 5, differential pressure through GDL measurement 14 and water balance calculation 15. It is difficult to accurately estimate the degree of flooding of fuel cell through experiment, except for no flooding. So it is unfavorable for quantitative evaluation of flooding. However, the numerical method shows clear advantage over the experimental method in studying90the flooding of PEMFC. First the parameters at any site of fuel cell can be probed by numerical simulation. On the other hand, the single phase flow can be allowed by numerical simulation,95100105110115which makes quantitative analysis of flooding of fuel cell possible.The study of flooding should include the anode and cathode flooding because flooding may occur in the anode side, or the cathode side, or both sides. Under the single phase hypothesis, the anti-flooding of each electrode can be characterized in the maximal local relative humidity (MLRH) of the entire electrode. If the MLRH is greater than 100%, flooding will be considered to occur in the fuel cell. Obviously, the MLRH indicates the degree of flooding of fuel cell in this theory.2Results and discussion2.1 Location of MLRHIt is found that the location of the MLRH is catalyst layers of the electrode. This is easy to understand. RH is a special expression of the water vapor concentration. Due to the electrochemical reaction in the electrode, the minimum local concentration of hydrogen and oxygen are always located in the anode and cathode catalyst layers respectively. According to the species replacement principle, the location of the MLRH should be the location of the minimum reactant concentration. This species replacement principle also explains the relationship between fuel starvation, oxidant starvation and flooding.Fig.2 displays the contours of RH and mole fraction of oxygen in the middle plane of the cathode flow channel, diffusion layer and catalyst layer at stoichiometric ratio 1.2/3.0,800mA/cm2 under the AAA flow combination. The simulation results validate the species replacement effect. Although the location of MLRH may be in other plane of the catalyst layers, there is no big difference between the local RHs. Therefore, the MLRH of the middle plane of the anode and cathode catalyst layers will be considered to be the maximum RH of the entire cell in the following sections.Fig. 2 Contours of a) RH and b) mole fraction of O2 in middle plane of cathode flow channels, diffusion layer and catalyst layer at stoichiometric ratio 1.5/3.0, 800mA/cm2 under AAA flow combination.1202.2 Constant stoichiometric ratios1251302.2.1Performance of fuel cellsFig.3 shows the polarization curves of the CAC and AAA flow combinations at 9 stoichiometric ratios. From Fig.3, it can be concluded that the stoichiometries of reactants has a little impact on the performance of the fuel cell, which coincides with other researchers results16,17. However, it is clearly seen that the performance of AAA flow combination in the low current density range (less than 100mA/cm2) is abnormal when the cathode stoichiometry is2.0. This abnormality occurs only when the operating current density is very low, so it will not have a significant impact on the output power.This abnormal phenomenon is caused by the fuel starvation in the middle of the active area at a low operating current density, which mainly attributes to the negative effect of the across-ribtransport. This will be explained in our next paper, so it is not discussed hereFig. 3 Polarization curves of a) CAC and b) AAA flow combination at 9 stoichiometric ratios.1352.2.2Anti-flooding under constant stoichiometric ratiosFig.4 shows the variation of the anti-flooding of the CAC and AAA flow combinations under9 constant stoichiometric ratios. From Fig.4a and 4b, it can be concluded that the variation features of the anti-flooding of CAC flow combination are:140145150155160165170175180ØWith current density increasing, the MLRHs of the anode decrease gradually and most of them are below 100% except when the current density is below 100mA/cm2 and the cathode stoichiometry is 2.0;ØMost of the MLRHs of the cathode are larger than 100% especially when the cathode stoichiometry is 2.0 and 3.0;ØWith the current density increasing, the MLRHs of the cathode go up and reach a plateau when the operating current density exceeds a certain value which depends on the cathode stoichiometry. This means the stoichiometry regulation cannot affect the MLRH of the cathode significantly in the high current densities.ØWith the cathode stoichiometry increasing, the anti-flooding of each electrode is improved while it is opposite when the anode stoichiometry is elevated.In our simulation, the RH of incoming hydrogen (100%) is higher than that of air (50%). As is known to all, one proton takes several water molecules with itself, travelling through the membrane from anode to cathode in a PEMFC and the product of water is mostly formed in the cathode. Therefore, most of the MLRHs of the anode at different stoichiometric ratios combinations are below 100% while those of cathode are larger than 100%. With current density increasing, there will be more water transporting from the anode to the cathode and formed in the cathode, which makes the MLRH of the anode decrease and that of the cathode increase. When the anode stoichiom