Comparison of Isobaric and Non-isobaric Operations on a Two-Phase Dynamic Model for PEFC Cathode Gas Diffusion Layer

Key words,： Two 1-D dynamical and isothermal models of cathode gas diffusion layer（GDL） with isobaric and non-isobaric operations for polymer electrolyte fuel cells（PEFCs） were developed and implemented in COMSOL Multiphysics v3.5.The artificial diffusion coefficient was introduced as well to make the numerical computation be stable.In the non-isobaric model,the pressure of gas mixture was obtained by summing up the governing equations of gaseous components,instead of Navier-Stoks equation.Comparison of the two models were carried out with the steady-states and dynamical simulations under given conditions.The corresponding analysis based on the simulated results was also given simultaneously.This paper is contributed to finding the differences between the isobaric and non-isobaric operation in the two-phase model of cathode GDL.

Fractal Analysis of Gas Diffusion Layer in PEM Fuel Cells

The aim of this study is to show how fractal analysis can be effectively used to characterize the texture of porous solids. The materials under study were carbon papers, the backing material of the gas diffusion layer （GDL） in Proton Exchange Membrane Fuel Cell （PEMFC）. The fractal dimensions were calculated by analyzing data from mercury porosimetry. The polytotrafluoroethylene （PTFE） treated carbon paper shows a significantly high fractal dimension value than pare sample, and the high fractal dimension signifies that the physical complexity of the pore surface is enhanced. The fractal dimension can be used as a valid parameter to monitor the textural evolution of the samples as the treatment progresses, as this behaves in a similar way to other textural parameters. The use of fractal analysis in conjunction with the results of classical characterization methods leads to a better understanding of textural modificatious in the processing of materials.

Influence of gas-diffusion-layer current collector on electrochemical performance of Ni(OH)_(2) nanostructures

We report the electrochemical performance of Ni(OH)_(2) on a gas diffusion layer(GDL).The Ni(OH)_(2) working electrode was successfully prepared via a simple method,and its electrochemical performance in 1 M NaOH electrolyte was investigated.The electrochemical results showed that the Ni(OH)_(2)/GDL provided the maximum specific capacitance value(418.11 F·g^(−1))at 1 A·g^(−1).Furthermore,the Ni(OH)_(2) electrode delivered a high specific energy of 17.25 Wh·kg^(−1) at a specific power of 272.5 W·kg^(−1) and retained about 81%of the capacitance after 1000 cycles of galvanostatic charge–discharge(GCD)measurements.The results of scanning electron microscopy(SEM)coupled with energy-dispersive X-ray spectroscopy(EDS)revealed the occurrence of sodium deposition after long-time cycling,which caused the reduction in the specific capacitance.This study results suggest that the light-weight GDL,which can help overcome the problem of the oxide layer on metal–foam substrates,is a promising current collector to be used with Ni-based electroactive materials for energy storage applications.

Paired eye-control study of unilateral opaque bubble layer in femtosecond laser assisted laser in situ keratomileusis

AIM: To investigate the predictive factors of the opaque bubble layer(OBL) compared to the fellow eye of same patients in FS200 femtosecond laser assisted laser in situ keratomileusis(FS-LASIK).METHODS: This study consisted of 60 consecutive patients(120 eyes) with unilateral OBL during FS-LASIK. Eyes were divided into OBL(the OBL eyes) and OBL-free groups(the fellow eyes) based on the occurrence of OBL. The preoperative demographic data, refraction, keratometry, corneal astigmatism, pachymetry, intraocular pressure and intraoperative data including the outlet location of gas diffusing canal were collected. Conditional logistic regression analysis was performed to find the associated factors with OBL in the two groups by determining odds ratios(OR) and 95%CI.RESULTS: The preoperative demographic data, mean spherical errors, mean K value, suction time, intraocular pressure and central cornea thickness were not significantly different between the two groups. The outlet location of gas diffusing canal(P<0.01, OR 7.16, 95%CI 2.53-20.32) and the corneal astigmatism(P=0.013, OR 0.13, 95%CI 0.03-0.65) were significantly associated with the incidence of OBL by multivariate logistic regression analysis. Visual acuity, efficacy, and safety were comparable between the two groups two months after surgery except for a slightly lower predictability value for the hard OBL eyes.CONCLUSION: The reduction of the incidence of OBL is obvious when the outlet of gas diffusing canal located at the posterior border of the corneoscleral limbus. This is probably consequent to more effectiveness of gas diffusing canal. Corneal astigmatism is also an independent protective factor for OBL formation.

Coupled pore network model for the cathode gas diffusion layer in PEM fuel cells

A pore network model(PNM)is developed for gas diffusion layer(GDL)in the cathode side of polymer electrolyte membrane fuel cells(PEMFCs).The model is coupled to network models of reactant oxygen and electron transport inside GDL and also to simple models of catalyst layer and membrane.The coupled model captures the simultaneous effect of reactant and charge access to reaction sites and the resulting water generation,allowing it a transient nature up to reaching the steady state,which is a notable modification to the available PNMs which assume uniform invasion of liquid water from catalyst layer.The results show strongly non-uniform water saturation distributions inside GDL with maxima under the current collector ribs.As an extra feature,the model can predict time evolution of oxygen concentration and water generation rate at catalyst layer as a result of liquid water build-up in GDL.Also included is a dry case coupled model in order to be compared with the main model.The local water blockages in GDL inflict an average of 38.8%loss on the produced limiting current of the fuel cell.Finally,the coupling allows prediction of concentration overvoltages which emerges to be most pronounced in the under-rib region.

Lattice Boltzmann Simulations of Water Transport from the Gas Diffusion Layer to the Gas Channel in PEFC

Water management is a key to ensuring high performance and durability of polymer electrolyte fuel cell(PEFC),and it is important to understand the behavior of liquid water in PEFC.In this study,the two-phase lattice Boltzmann method is applied to the simulations of water discharge from gas diffusion layers(GDL)to gas channels.The GDL is porous media composed of carbon fibers with hydrophobic treatment,and the gas channels are hydrophilic micro-scale ducts.In the simulations,arbitrarily generated porous materials are used as the structures of the GDL.We investigate the effects of solid surface wettabilities on water distribution in the gas channels and the GDL.Moreover,the results of X-ray computed tomography images in the operating PEFC are compared with the numerical simulations,and the mechanism of the water transport in PEFC is considered.

A Study on the Transport Process in Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells

Gas diffusion layer(GDL) plays a great important role in proton exchange membrane fuel cell(PEMFC).Water transport mechanism in GDL is still not clear.In the present study,an ex-situ transparent setup is built to visualize the transport phenomena and to measure the threshold pressure of water in GDL at different temperatures.It is found that the relationship between the breakthrough pressure and the temperature is nearly linear(i.e.the pressure decreases linearly with the increase of temperature).To avoid the problems faced by the continuum models,the pore network model is developed to simulate the liquid water transport through the carbon paper.A uniform pressure boundary condition is used in simulation and the results are similar to the ones obtained in the experiment.The reason is that the contact angle and surface tension coefficient of water in GDLs change accordingly with the change of temperature.

Evaluating Breakthrough Pressure in Gas Diffusion Layers of Proton Exchange Membrane Fuel Cells

This paper studied the breakthrough pressure for liquid water to penetrate the gas diffusion layer(GDL) of a proton exchange membrane fuel cell(PEMFC).An ex-situ testing was conducted on a transparent test cell to visualize the water droplet formation and detachment on the surface of different types of GDLs through a CCD camera.The breakthrough pressure,at which the liquid water penetrates the GDL and starts to form a droplet,was measured.The breakthrough pressure was found to be different for the GDLs with different porosities and thicknesses.The equilibrium pressure,which is defined as the minimum pressure required maintaining a constant flow through the GDL,was also recorded.The equilibrium pressure was found to be much lower than the breakthrough pressure for the same type of GDL.A pore network model was modified to further study the relationship between the breakthrough pressure and the GDL properties and thicknesses.The breakthrough pressure increases for the thick GDL with smaller micro-pore size.

Gas Transport Properties in Gas Diffusion Layers:A Lattice Boltzmann Study

The lattice Boltzmann method is applied to the investigations of the diffusivity and the permeability in the gas diffusion layer(GDL)of the polymer electrolyte fuel cell(PEFC).The effects of the configuration of water droplets,the porosity of the GDL,the viscosity ratio of water to air,and the surface wettability of the GDL are investigated.From the simulations under the PEFC operating conditions,it is found that the heterogeneous water network and the high porosity improve the diffusivity and the permeability,and the hydrophobic surface decreases the permeability.

Atomic layer deposition of ultrathin layered TiO_2 on Pt/C cathode catalyst for extended durability in polymer electrolyte fuel cells

This study shows the preparation of a TiO2 coated Pt/C（TiO2/Pt/C） by atomic layer deposition（ALD）,and the examination of the possibility for TiO2/Pt/C to be used as a durable cathode catalyst in polymer electrolyte fuel cells（PEFCs）. Cyclic voltammetry results revealed that TiO2/Pt/C catalyst which has 2 nm protective layer showed similar activity for the oxygen reduction reaction compared to Pt/C catalysts and they also had good durability. TiO2/Pt/C prepared by 10 ALD cycles degraded 70% after 2000 Accelerated degradation test, while Pt/C corroded 92% in the same conditions. TiO2 ultrathin layer by ALD is able to achieve a good balance between the durability and activity, leading to TiO2/Pt/C as a promising cathode catalyst for PEFCs. The mechanism of the TiO2 protective layer used to prevent the degradation of Pt/C is discussed.

气体扩散层亲疏水特性对流体分布的预测

采用伪势两相格子玻尔兹曼方法(LBM)研究了非增湿条件下碳纸和碳布气体扩散层(GDL)的流体流动状态。通过随机方法和X射线扫描法构建了气体扩散层样本。为确保模型中表面张力和接触角计算的准确性,采用玻璃微珠模型进行验证,随后通过调整气体扩散层的亲疏水特性,分析流体在气体扩散层中流动的实时状态,得到了亲疏水特性对孔隙结构内水饱和度的影响规律。结果表明:疏水性气体扩散层中的水分布与亲水性气体扩散层中的水分布明显不同,较大的疏水性更有利于气体扩散层内水的排出;疏水性更强的气体扩散层显著提高了液态水进入气体扩散层的入口压力,导致催化剂层(CL)更容易受到水渗透的影响。

气体扩散层变形模量与脊槽转角半径优化设计

提出了一种脊槽转角半径优化设计方法,以消除气体扩散层(GDL)应力集中现象,进而提高质子交换膜燃料电池(PEMFC)性能和可靠性。基于梁弯曲理论和几何概率理论分析,获得气体扩散层应力—应变非线性解析解。对气体扩散层基材商品TGP-H-060和MGL1902进行了压缩测试,以验证解析解。结果表明:基材TGP-H-060和MGL190的变形模量分别为240kPa和420kPa;说明本解析解可很好地拟合实验数据;当脊槽转角半径为340μm时,GDL上边缘最大接触压力最小,应力集中现象消失,应力分布的均匀性最好。变形模量影响了气体扩散层不均匀变形的程度,但不改变变形分布。因此,本优化设计改善了气体扩散层变形均匀性。

黏合物质与压缩变形对燃料电池气体扩散层孔隙结构和气体渗透特性的影响

在气体扩散层(GDL)生产过程中,疏水黏合处理和装配压缩变形导致GDL孔隙结构和渗透特性发生变化.首先基于随机重构算法,建立一种添加黏合物质和施加不均匀压缩的GDL建模方法;然后利用格子玻尔兹曼数值仿真气体单相流动,研究黏合物质与压缩形变对燃料电池GDL孔隙结构和气体渗透特性的影响规律.计算结果表明:黏合物质与压缩形变均会导致气体扩散小尺寸孔隙结构占比增大,整体孔隙率减小;GDL的渗透率变化趋势与孔隙率一致,均降低,变化规律基本符合理论预测关系;当孔隙率相近时,压缩变形是导致的渗透率降低的关键因素.

装配工况下PEMFC不同双极板截面接触行为研究

为进一步探究双极板几何形状对层间接触行为的影响,以金属双极板为对象,搭建了双极板压缩试验台,同时建立了双极板压缩试验有限元模型,根据压敏纸采集到的试验结果验证了模型的有效性.然后,分别搭建了梯形、矩形和波浪形三种常见截面形状的双极板有限元模型,并搭建了相应的双极板压缩试验有限元模型.通过仿真分析研究在不同装配载荷作用下双极板截面形状对层间接触行为的影响.结果表明:装配载荷大小和双极板截面形状对双极板与气体扩散层(Gas Diffusion Layer, GDL)之间的接触行为有显著影响;在不同装配载荷作用下,三种双极板肋下的GDL表面接触应力幅值均在0.5~2 MPa之间,均符合装配要求;在同等条件下,双极板肋宽度是决定双极板与GDL之间整体平均接触应力幅值和GDL平均侵入面积的主要因素;在三种双极板中,矩形双极板下的接触行为对装配载荷的变化更为敏感,并且矩形双极板下平均接触应力分布均匀性最差.

A comprehensive assessment on the durability of gas diffusion electrode materials in PEM fuel cell stack

Polymer electrolyte membrane (PEM) fuel cell is the most promising among the various types of fuel cells. Though it has found its applications in numerous fields, the cost and durability are key barriers impeding the commercialization of PEM fuel cell stack. The crucial and expensive component involved in it is the gas diffusion electrode (GDE) and its degradation, which limits the performance and life of the fuel cell stack. A critical analysis and comprehensive understanding of the struc-tural and functional properties of various materials involved in the GDE can help us to address the related durability and cost issues. This paper reviews the key GDE components, and in specific, the root causes influencing the durability. It also envisages the role of novel materials and provides a critical recommendation to improve the GDE durability.

车用质子交换膜燃料电池膜电极关键材料与结构设计进展

各国氢燃料电池汽车已逐渐开始商用,但是氢燃料电池成本高、寿命差等问题仍未解决。从材料与结构出发,重点阐述了膜电极关键材料性能要求、研究进展、模拟仿真和重点企业方面内容。研究结果表明超薄高温低湿质子交换膜、高稳定性低铂催化剂以及高透氧离聚物能有效改善燃料电池膜电极的性能,并在当前实践中得到了应用。同时,在总结现阶段氢燃料电池膜电极研究进展的基础上,指出当前膜电极研究中的发展趋势和不足,并为膜电极产品开发和产业化提出了新的发展思路。

质子交换膜燃料电池扩散层水传递可视化研究

质子交换膜燃料电池(polymer electrolyte fuel cell,PEFC)扩散层(gas diffusion layer,GDL)水管理对其性能十分关键,扩散层可视化研究对指导微观模型的建立,进而指导电池的设计意义重大。搭建了一个离线扩散层可视化研究实验台,通过对不同碳纸进行液态水穿透实验,观测了液态水突破现象,并得到碳纸突破压力随温度近似呈线性变化且成反比。同时通过分析碳纸中传递过程,建立了一种微观模型,通过MATLAB编程计算突破压力大小,并与实验值进行了对比,得到比较一致的结果。

PEMFC用气体扩散层中微孔层的干法制备

比较了用干法和湿法制备的微孔层(MPL)对质子交换膜燃料电池(PEMFC)输出性能的影响。结果表明干法MPL具有较湿法MPL更优的“水管理”功能。对基于干法MPL和湿法MPL的气体扩散层(GDL)分别进行了表征,包括扫描电子显微镜法(SEM)、透气率以及表面接触角。表征结果支持上述实验结论。干法MPL中PTFE质量百分含量的最优值为30%。

PEMFC气体扩散层的双层结构和两相流

气体扩散层(GDL)是质子交换膜燃料电池(PEMFC)的关键部件之一,担负集流和传质的重要功能。概述了气体扩散层中水和反应气的传质、GDL的双层结构、微孔层的“水管理”机制以及GDL的研究进展。

固体聚合物电解质水电解池用阳极气体扩散层

采用刷涂法在钛网基体上制备扩散层,结合固体聚合物电解质(SPE)水电解池中的交流阻抗及极化曲线测试,探讨浆料中PVDF、PTFE和锑掺杂二氧化锡(ATO)配比对阳极扩散层厚度、阻值、孔隙率、疏水性和单体电池性能的影响。PVDF与PTFE添加量的增加,使阳极扩散层的厚度增加、阻值增大、孔隙率下降及疏水性增强;PVDF对阻值、孔隙率的影响更大。合理的m(ATO)∶m(PVDF)∶m(PTFE)为15∶1∶2,在常压、80℃下,电流密度为1 A/cm2时,槽压仅为1.59 V。