PEMFC中粗糙GDL表面对液态水在GC中传输过程的影响

采用商业软件FLUENT中VOF模型模拟了质子交换膜燃料电池（PEMFC）中液态水在具有粗糙气体扩散层（GDL）表面的气体通道（GC）中的传递过程。考察了GDL表面润湿特性和粗糙度对液态水传输过程的影响。研究结果表明：和亲水GDL表面相比,疏水GDL表面有利于液态水的排出;和光滑疏水GDL表面相比,粗糙疏水GDL表面加快了液滴的排出,减小了液滴覆盖GDL表面的面积;同时,粗糙GDL表面增加了GC相似文献   

PEMFC气体通道表面润湿特性对气体扩散层中水分布的影响

采用格子Boltzmann方法（LBM）数值模拟研究了质子交换膜燃料电池（PEMFC）中气体通道（GC）表面润湿特性对气体扩散层（GDL）中液态水分布和含量的影响,从微观层次详细分析了液态水在GDL-肋板（land）交界面处的传输过程。研究结果表明,与亲水GC相比,疏水GC增加了GDL中的液态水含量。由于GDL相似文献   

PEMFC阴极扩散层结构特性对水淹影响的数值分析

建立质子交换膜燃料电池一维两相传递模型,通过达西定律和菲克定律的联立求解得到扩散层中的液体饱和度和氧气浓度分布。考察扩散层特性参数孔隙率、厚度、接触角、渗透率对阴极水淹的影响,结果表明扩散层表面憎水将有助于液态水移出,但当达到憎水条件后,增大接触角对液态水传输和氧气传质的影响逐渐变小。憎水条件下孔隙率和厚度对液态水传输的影响不是很明显,但孔隙率增大和扩散层厚度减小均有利于氧气传质,实际应用中孔隙率增大的同时,厚度也要适当增大,极限电流密度相差不大。模型计算结果与文献中不同PTFE含量条件下实验的Tafel斜率和极限电流密度比较,吻合较好。     

质子交换膜燃料电池双层扩散层特性三维分析

针对直流道质子交换膜燃料电池提出一种混合的两相三维非等温数学模型,考虑了液态水在多孔介质内的毛细流动和分布,分析了双层扩散层结构及碳纤维特性对电池性能的影响。结果表明,扩散层第一层（靠近催化层）厚度对质子膜电导率和气体传递特性有着相互制约的影响,需进行优化;在一定范围内,扩散层第一层碳纤维直径的减小可提高质子膜电导率,有利于电池性能的改善;在保持其他参数不变的前提下,应尽可能提高多孔介质的憎水性和孔隙率以提高电池输出性能。     

直接甲醇燃料电池阴极水的传输特性

利用COMSOL Multiphysics软件对直接甲醇燃料电池(DMFC)阴极模型进行计算,获得压力、速度、水、氧气和液态饱和度分布情况,研究扩散层在不同物理参数(如厚度、孔隙率、孔径大小和亲憎水性)下电池阴极水和氧气的传输情况,进一步建立扩散层孔隙率梯度的数学模型,研究扩散层孔隙率梯度以及支撑层参数对直接甲醇燃料电池性能和物质传输的影响。结果表明,扩散层具有大孔隙率、薄扩散层时均有利于氧气传质,可以使电池性能提高;扩散层孔隙率梯度的存在可以减轻氧气传输阻力,提高电池性能。     

直接甲醇燃料电池阴极水的传输特性

利用COMSOL Multiphysics软件对直接甲醇燃料电池（DMFC）阴极模型进行计算，获得压力、速度、水、氧气和液态饱和度分布情况，研究扩散层在不同物理参数（如厚度、孔隙率、孔径大小和亲憎水性）下电池阴极水和氧气的传输情况，进一步建立扩散层孔隙率梯度的数学模型，研究扩散层孔隙率梯度以及支撑层参数对直接甲醇燃料电池性能和物质传输的影响。结果表明，扩散层具有大孔隙率、薄扩散层时均有利于氧气传质，可以使电池性能提高；扩散层孔隙率梯度的存在可以减轻氧气传输阻力，提高电池性能。     

质子交换膜燃料电池气体扩散层结构与设计研究进展

气体扩散层（GDL）在质子交换膜燃料电池（PEMFC）中起到支撑催化层、传输反应气体和排出反应过程中产生的水的作用，设计和优化GDL的结构对提升燃料电池的性能有重要作用。本文首先介绍了氢燃料电池应用前景，简述了PEMFC的结构和工作原理，指出了目前GDL的气液传输能力不足的问题，分析了孔结构、碳材料、微孔层微观结构、润湿性和耐久性五个因素对GDL性能的影响，并归纳了当前的研究进展，同时还涵盖了与GDL内传质过程相关的建模方法。最后总结了影响GDL性能的各种因素，并对质子交换膜燃料电池内的GDL发展进行了展望，指出用新型金属泡沫材料代替传统碳材料构建气体扩散层-双极板集成结构从而缩短传质路径并降低传质阻力，提出利用新兴的3D打印技术去构建高精度具有复杂结构的气体扩散层。本综述对未来优化GDL结构、提高燃料电池性能具有一定的指导意义。     

PEM燃料电池阴极中的液态水及其影响因素

阴极多孔介质中液态水的含量对PEM燃料电池阴极中的传质及其性能具有极其重要的影响。提出了一个二维、两相、稳态数学模型,研究PEM燃料电池阴极中两相水的传递及其对电池性能的影响。模型耦合了连续方程、动量方程和组分守恒方程,并将质子膜中的净水迁移通量作为边界条件之一来处理。通过实验的方法和数值模拟的方法,研究了电池操作压力和温度对电池性能的影响,同时验证了模型的有效性。模拟发现:提高操作压力和升高阴极加湿温度使电池阴极催化剂层(CTL)和扩散层(GDL)界面上的液态水含量大幅提高;升高阳极加湿温度,电池阴极CTL和GDL界面上的液态水含量变化不明显;而升高燃料电池的操作温度,阴极CTL和GDL界面上液态水的含量降低。     

分形多孔材料的一种改进化气体扩散分形模型

基于分形理论与技术,提出了气体在由一簇弯弯曲曲、横截面积大小不等的椭圆形毛细管组成的多孔材料中的气体扩散率分形模型。研究结果表明:归一化气体扩散率是最大孔隙面积、最小与最大孔隙面积之比、多孔材料总横截面积、形状因子及分形维数等多孔材料微结构参数的函数;模型能清楚地揭示影响气体扩散率的物理机制。文中气体扩散率分形模型与已有的实验数据进行对比,结果显示它们之间吻合较好;提出的改进化气体扩散分形模型更具有一般性。     

质子交换膜燃料电池二维全电池两相流综合数值模型

针对直通道质子交换膜燃料电池（PEMFC）建立了一个二维全电池综合数值模型,模型综合考虑参与电化学反应的三个要素反应物质、电子和质子的传输过程以及液态水的淹没和膜内水传输现象。研究了供气压力、液态水淹没对电池性能的影响;比较了不同输出电压、供气湿度等条件对阴极液态水饱和度分布以及电解质膜含水率的影响;预测了基准供气状态下电池的极化曲线和文献报道的实验结果吻合很好。计算结果显示：输出电压越小液态水淹没电极现象越严重;阴极液态水的生成有利于膜的浸润保持较高电导率,但是会淹没电极使有效电极面积减小,导致电池性能下降。     

Pore-network modeling of liquid water transport in gas diffusion layer of a polymer electrolyte fuel cell

A pore-network model is developed to study the liquid water movement and flooding in a gas diffusion layer (GDL), with the GDL morphology taken into account. The dynamics of liquid water transport at the pore-scale and evolution of saturation profile in a GDL under realistic fuel cell operating conditions is examined for the first time. It is found that capillary forces control liquid water transport in the GDL and that liquid water moves in connected clusters with finger-like liquid waterfronts, rendering concave-shaped saturation profiles characteristic of fractal capillary fingering. The effect of liquid coverage at the GDL–channel interface on the liquid water transport inside GDL is also studied, and it is found that liquid coverage at the GDL–channel interface results in pressure buildup inside the GDL causing the liquid water to break out from preferential locations.     

Determination of oxygen effective diffusivity in porous gas diffusion layer using a three-dimensional pore network model

In proton exchange membrane fuel cell (PEMFC) models, oxygen effective diffusivity is the most important parameter to characterize the oxygen transport in the gas diffusion layer (GDL). However, its determination is a challenge due to its complex dependency on GDL structure. In the present study, a three-dimensional network consisting of spherical pores and cylindrical throats is developed and used to investigate the effects of GDL structural parameters on oxygen effective diffusivity under the condition with/without water invasion process. Oxygen transport in the throat is described by Fick's law and water invasion process in the network is simulated using the invasion percolation with trapping algorithm. The simulation results reveal that oxygen effective diffusivity is slightly affected by network size but increases with decreasing the network heterogeneity and with increasing the pore connectivity. Impacts of network anisotropy on oxygen transport are also investigated in this paper. The anisotropic network is constructed by constricting the throats in the through-plane direction with a constriction factor. It is found that water invasion has a more severe negative influence on oxygen transport in an anisotropic network. Finally, two new correlations are introduced to determine the oxygen effective diffusivity for the Toray carbon paper GDLs.     

Effect of hydrophobicity and pore geometry in cathode GDL on PEM fuel cell performance

The effect of hydrophobic and structural properties of a single/dual-layer cathode gas diffusion layer on mass transport in PEM fuel cells was studied using an analytical expression. The simulations indicated that liquid water transport at the cathode is controlled by the fraction of hydrophilic surface and the average pore diameter in the cathode gas diffusion layer. Deposition of a hydrophobic microporous layer reduces the average pore diameter in the macroporous substrate. It also increases the hydrophobic surface, which improves the mass transport of the reactant. The optimized hydrophobicity and pore geometry in a dual-layer cathode GDL leads to an effective water management, and enhances the oxygen diffusion kinetics.     

A Two‐dimensional Network Study on Oxygen Transport in Porous Gas Diffusion Layer

Oxygen transport in the porous gas diffusion layer (GDL), which is generally characterised by the oxygen effective diffusivity, is of great importance for the performance of proton exchange membrane fuel cells (PEMFCs). The determination of the oxygen effective diffusivity is challenging due to the complex structure of the porous GDL samples. In the present study, a two‐dimensional network consisting of arms and nodes is adopted to illustrate how oxygen effective diffusivity is affected by the GDL structure under the condition with/without water invasion. Water permeation in the network is simulated using the invasion percolation algorithm and oxygen transport in the arms is described by Fick's law. The simulation results reveal that oxygen effective diffusivity under dry condition decreases with increase in the network heterogeneity. With water permeation, the oxygen effective diffusivity goes to zero even though water saturation is rather less than unity. The critical water saturation, above which the oxygen effective diffusivity becomes zero, is found to decrease with increasing heterogeneity. To enhance oxygen transport, four different modified networks are introduced in the present study. It is found that the network with large arms in oxygen transport direction has the best oxygen and water transport properties.     

Mesoscopic modeling of two-phase behavior and flooding phenomena in polymer electrolyte fuel cells

A key performance limitation in polymer electrolyte fuel cells (PEFC), manifested in terms of mass transport loss, originates from liquid water transport and resulting flooding phenomena in the constituent components. Liquid water covers the electrochemically active sites in the catalyst layer (CL) rendering reduced catalytic activity and blocks the available pore space in the porous CL and fibrous gas diffusion layer (GDL) resulting in hindered oxygen transport to the active reaction sites. The cathode CL and the GDL play a major role in the mass transport loss and hence in the water management of a PEFC. In this work the development of a mesoscopic modeling formalism coupled with realistic microstructural delineation is presented to study the influence of the pore structure and surface wettability on liquid water transport and interfacial dynamics in the PEFC catalyst layer and gas diffusion layer. The two-phase regime transition phenomenon in the capillary dominated transport in the CL and the influence of the mixed wetting characteristics on the flooding dynamics in the GDL are highlighted.     

Pore-network analysis of two-phase water transport in gas diffusion layers of polymer electrolyte membrane fuel cells

A pore-network model was developed to study the water transport in hydrophobic gas diffusion layers (GDLs) of polymer electrolyte membrane fuel cells (PEMFCs). The pore structure of GDL materials was modeled as a regular cubic network of pores connected by throats. The governing equations for the two-phase flow in the pore-network were obtained by considering the capillary pressure in the pores, and the entry pressure and viscous pressure drop through the throats. Numerical results showed that the saturation distribution in GDLs maintained a concave shape, indicating the water transport in GDLs was strongly influenced by capillary processes. Parametric studies were also conducted to examine the effects of several geometrical and capillary properties of GDLs on the water transport behavior and the saturation distribution. The proper inlet boundary condition for the liquid water entering GDLs was discussed along with its effects on the saturation distribution.     

Two-phase transport and the role of micro-porous layer in polymer electrolyte fuel cells

Two-phase transport of reactants and products constitutes an important limit in performance of polymer electrolyte fuel cells (PEFC). Particularly, at high current densities and/or low gas flow rates, product water condenses in open pores of the cathode gas diffusion layer (GDL) and limits the effective oxygen transport to the active catalyst sites. Furthermore, liquid water covers some of the active catalytic surface, rendering them inactive for electrochemical reaction. Traditionally, these two-phase transport processes in the GDL are modeled using so-called unsaturated flow theory (UFT), in which a uniform gas-phase pressure is assumed across the entire porous layer, thereby ignoring the gas-phase flow counter to capillarity-induced liquid motion. In this work, using multi-phase mixture (M²) formalism, the constant gas pressure assumption is relaxed and the effects of counter gas-flow are studied and found to be a new oxygen transport mechanism. Further, we analyze the multi-layer diffusion media, composed of two or more layers of porous materials having different pore sizes and/or wetting characteristics. Particularly, the effects of porosity, thickness and wettability of a micro-porous layer (MPL) on the two-phase transport in PEFC are elucidated.     

Effect of cathode GDL characteristics on mass transport in PEM fuel cells

The effect of the content of the hydrophobic agent in the cathode gas diffusion layer (GDL) on the mass transport in the proton exchange membrane fuel cells (PEMFCs) was studied using mercury porosimetry, scanning electron microscopy, and electrochemical polarization techniques. The mercury intrusion data and SEM micrograph indicated that the hydrophobic agent alters the surface and bulk structure of the GDL, thereby controlling gas-phase void volume and liquid water transport. The electrochemical polarization curves were measured and quantitatively analyzed to determine the oxygen transport limitation both in the catalyst layer and the GDL. Evaluation of the parameter ζ, which represents the cathode GDL characteristics for liquid water transport, indicated that the optimized content of the hydrophobic agent and effective water management results from a trade-off between the hydrophobicity and the absolute permeability for faster water drainage.     

Three-dimensional CFD modelling of PEM fuel cells: An investigation into the effects of water flooding

In this work, a three-dimensional PEM fuel cell model has been developed and is used to investigate the effects of water flooding on cell performance parameters. The presence of liquid water in the cathode gas diffusion layer (GDL) limits the flow of reactants to the cathode catalyst layer, thereby reducing the overall reaction rate and curtailing the maximum power that can be derived from the cell. To characterize the effects of water flooding on gas diffusion, effective diffusivity models that account for the tortuosity and relative water saturation of the porous fuel cell electrodes have been derived from percolation theory and coupled with the CFD model within a single phase flow skeleton. The governing equations of the overall three-dimensional PEM fuel cell model, which are a representative of the coupled CFD and percolation theory based effective diffusivity models, are then solved using the finite volume method. Parametric studies have been conducted to characterize the effects of GDL permeability, inlet humidity and diffusivity of the reactants on the various cell performance parameters such as concentration of reactants/products and cell current densities. It is determined that the GDL permeability has little or no effect on the current densities due to the diffusion dominated nature of the gas flow. However, through the incorporation of percolation theory based effective diffusivity model; a marked reduction in the cell performance is observed which closely resembles published experimental observations. This is a reasonable approximation for effects of water flooding which has been inherently used for further parametric studies.