质子交换膜燃料电池冷启动的数值模拟

冷启动是质子交换膜燃料电池（PEMFC）商业化所面临的挑战之一，在PEMFC冷启动实验中，通过中子成像技术已经观测到电池内部存在过冷水，因此本文模型重点考虑过冷水对电池冷启动性能的影响。通过引入结冰概率函数对过冷水结冰过程的随机性进行描述，从而建立了PEMFC冷启动的三维、瞬态和多相流动数学模型。基于该模型，研究电池阴极催化层中离子聚合物的体积分数和质子交换膜的厚度对电池冷启动性能的影响。研究结果表明，增加阴极催化层中离子聚合物的体积分数，可有效促进阴极催化层中的反应生成水向质子交换膜中进行扩散，从而充分利用膜内的储水空间；减少质子交换膜的厚度，能促进质子交换膜中的离聚物水向阳极催化层扩散，在大电流密度工况下可有效缓解阳极的“膜干”现象。     

阳极进气湿度对质子交换膜水含量及电流密度分布影响

针对质子交换膜中水分布不均匀造成燃料电池性能降低的问题,将膜和催化层中水传递方程进行耦合实现水在膜和催化层之间连续传递,建立了质子交换膜燃料电池三维稳态模型。利用有限元分析软件COMSOL进行模拟计算,研究了阳极气体在不同湿度下膜电流密度分布并组装单电池进行了验证,分析实验模拟结果表明:模拟极化曲线与实验极化曲线吻合良好。湿度对电流密度分布影响很大,低湿度条件下,脊背下方电流密度大于气体流道下方;高湿度条件下,电流密度分布比较均匀;采用Nafion117较厚膜时,高电流密度下,即使阳极加湿,阳极侧也有脱水的可能。     

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。     

用非均匀团聚体模型模拟燃料电池老化过程

在燃料电池老化过程中,催化层中催化剂的迁移和粒径增加会造成燃料电池的性能下降。本文提出了一种新型的团聚体分层分布模型,且分层界面可沿膜厚方向发生线性变化。通过宏观电极模型和团聚体亚尺度模型的耦合,用有限元法分析了团聚体不均匀分布对质子交换膜燃料电池性能的影响。研究表明,靠近团聚体外侧的催化剂的流失对电池性能影响很大,当没有催化剂的外侧区域达到0.1倍半径时,输出电压为0.2V时的电流密度下降89.8%;当没有催化剂的内侧区域达到0.1倍半径时,输出电压为0.2V时的电流密度下降25%左右。相比于向质子交换膜方向的催化剂迁移,向气体扩散层方向的催化剂迁移对电池性能影响更大,其中分层递增分布的团聚体模型的电池电流密度是均匀分布团聚体模型的60%左右,分层递减分布模型的电流密度是均匀分布团聚体模型的10%左右。此外,铂负载量增加一倍对递增分布模型的电池性能有所提高。     

质子交换膜燃料电池两相流动和传递现象的三维数值模拟

为更好地模拟质子交换膜燃料电池内的复杂传递过程,发展了一个三维稳态的、非等温的气液两相流模型,模型综合考虑电池内的流动、传热、传质等过程,以及水的相变过程对电池内传质和温度场的影响.本模型的特性是可以详细地模拟和估计电极电化学动力学,考虑电子在扩散层和催化层,以及质子在膜相中的传递规律.通过数值计算得到了详细的组分浓度、电位和温度等在电池内的空间分布.比较了估算的极化性能与文献中的实验数据,结果表明两者较好地相符合.     

微型直接甲醇燃料电池阳极传质分析

针对微型直接甲醇燃料电池,将阳极流场板简化为规则结构的多孔介质,运用多孔介质理论建立了包括流场板在内的阳极传输模型。模型考虑了阳极流道内液体饱和度沿流动方向的变化、催化层的厚度以及甲醇渗透,计算并讨论了阳极流道内液体饱和度的分布和流量对电池电流密度的影响,分析了阳极过电位对甲醇浓度分布和电池性能的影响以及质子交换膜内的传质特性。     

PEMFC流道横截面二维两相流数学模型(Ⅰ)模型建立

建立了质子交换膜燃料电池（PEMFC）流道和脊横截面的二维两相流数学模型（across-the-channel model）。模型描述了PEMFC主要的传递和反应过程,包括阴、阳两极反应气的质量传递、动量传递、电子和质子的传递以及电化学反应等。模型细致地描述了水（液态和气态）在扩散层、催化层以及质子交换膜中的传递过程。模型可以用来研究流场、扩散层、催化层以及膜等对电池性能的影响,进而达到优化电池结构的目的。     

质子交换膜燃料电池仿真模型研究进展

质子交换膜燃料电池具有高效清洁等优势,是一种潜力巨大的绿色能源技术。数学模型作为一种合理可靠的工具,通过模拟电池内部的电化学传热传质过程,研究运行参数和结构参数对电池性能和寿命的影响,可以指导电池的优化设计。本文综述了近年来燃料电池催化层、气体扩散层和流道的研究模型,整理了各部件建模的影响因素和优化方法,以期对燃料电池建模以及电池各部件的优化设计起到参考作用。文中指出,考虑到现在仿真存在的局限性,未来主要研究方向为燃料电池系统研究与机理模型的结合、催化层微观结构的建模、非贵金属催化剂建模、气体扩散层衰减模型研究、大面积流道模型、三维模型温度分布研究以及全尺寸质子交换膜燃料电池模型的开发。     

球形团聚物模型在质子交换膜燃料电池过程模拟中的应用

在建立直通道质子交换膜燃料电池(PEMFC)的二维全电池数学模型中,将球形团聚物模型应用至两极的催化剂层。通过调节团聚物中质子传导介质的比例和催化层孔隙率,预测了基准供气状态下单电池的极化曲线,与文献报道的实验数据吻合良好。研究了电池运行过程中,膜电极内各化学组分和电流密度的分布情况及流向,比较了不同供气压力、催化剂铂颗粒尺寸等参数对电池性能的影响。计算结果表明,在阴极及时排出反应产生的水,并在阳极对燃料气进行加湿是保证单电池正常运行的前提,提高阴极的氧化剂气体压力,可显著改善PEMFC单电池性能,特别是在受浓差极化影响较大的大电流密度区;在催化剂铂载量相同的情况下,减小铂颗粒的尺寸可以提高电池的性能。     

流道布置对风冷燃料电池性能影响的研究

风冷燃料电池在无人机上应用具有较大的优势,但其仍然面临着性能低、水热管理复杂等问题。采用数值模拟研究了阴阳极流道顺流、逆流以及交叉流布置方式对风冷燃料电池性能的影响,讨论了各物理量之间的耦合机制。研究发现阴极流道布置对风冷燃料电池关键物理量分布影响显著,受水含量分布不均影响,交叉流布置催化层中电流密度呈现孤立点状分布,相较于顺流流道布置与逆流流道布置,交叉流流道布置具有较好的热管理特性,能够显著提高膜电极中水含量,继而提升风冷燃料电池性能,在0.6 V操作电压下,交叉流流道布置电流密度相比较顺流流道设计提升了约20%,此外研究还发现,环境相对湿度的降低会显著降低风冷燃料电池性能。     

质子交换膜燃料电池两维、两相流动模型

提出了考虑电池内部两相流动的质子交换膜燃料电池数学模型,模拟了阳极、阴极两侧的流道和扩散层中同时发生两相流动时电池内部的各种传递特性,并用实验数据验证了该模型的准确性。模拟结果显示,当电池阴极扩散层中有液态水存在时会大大降低膜中的局部电流密度;质子交换膜中水的净通量方向可正、可负,因此电池的增湿策略应根据不同的运行工况而不断变化。     

Mathematical Model of Proton Exchange Membrane Fuel Cell with Consideration of Water Management

One‐dimensional model on the membrane electrode assembly (MEA) of proton exchange membrane fuel cell is proposed, where the membrane hydration/dehydration and the possible water flooding of the respective cathode and anode gas diffusion layers are considered. A novel approach of phase‐equilibrium approximation is proposed to trace the water front and the detailed saturation profile once water emerges in either anode or cathode gas diffusion layer. The approach is validated by a semi‐analytical method published earlier. The novel approach is applicable to the polarization regime from open circuit voltage to the limiting current density under practical operation conditions. Oxygen diffusion is limited by water accumulation in the cathode gas diffusion layer as current increases, caused by excessive water generation at the cathode catalyst layer and the electro‐osmotic drag across the membrane. The existence of liquid water in the anode gas diffusion layer is predicted at low current densities if high degrees of humidification in both anode and cathode feeds are employed. The influences of inlet relative humidity, imposed pressure drop, and cell temperature are correlated well with the cell performance. In addition, the overpotentials attributed from individual components of the MEA are delineated against the cell current densities.     

Investigation of phosphoric acid and water transport in the high temperature proton exchange membrane fuel cells using a multiphase model

A three-dimensional, nonisothermal, and multiphase model of high temperature proton exchange membrane fuel cells is built to investigate water and phosphoric acid transportation, in which a spherical agglomerate model considering catalyst layer structure and liquid saturation is applied to determine the electrochemical kinetics in the cathode catalyst layer. Experimental polarization curve, water proportion in the anode outlet gas, and phosphoric acid distribution are selected for validation. It is found that the simulated results can represent the experimental data with reasonable accuracy. Based on the model, the effects of current density and stoichiometry on the variable distributions are analyzed. The results show that water in anode is mainly from cathode by concentration diffusion of liquid water, and the proportion of anode outlet water to the total produced water decreases slightly with the increase of current density. A higher current density leads to a greater electromigration of phosphoric acid from cathode to anode and a higher liquid phase fraction in anode, while a lower phosphoric acid concentration in the fuel cells.     

采用常规条形流场的质子交换膜燃料电池阴极数值模拟(Ⅰ)模型建立

对采用常规条形流场的H₂-Air PEMFCs阴极建立了二维数学模型,模型的控制方程耦合了连续性方程、Darcy方程、电传导方程以及O₂和H₂O的对流-扩散方程,对氧的电化学还原反应过程采用Butler-Volmer方程描述.利用模型计算了阴极扩散层中电流密度、O₂和H₂O浓度、催化层界面上局部电流密度的分布,分析了采用常规条形流场时气体在阴极扩散层中的传递机制及各组分浓度分布的特点.     

Benefits of Membrane Electrode Assemblies with Asymmetrical GDL Configurations for PEM Fuel Cells

Membrane electrode assemblies (MEAs), based on commercial catalyst‐coated membranes combined with various gas diffusion layers (GDLs) on anode and cathode, were studied in terms of their specific advantages for different operations regimes of proton exchange membrane fuel cells (PEMFCs.) It is verified that MEAs with optimized gas diffusion layer designs (backing and micro‐porous layers) on anode and cathode are able to provide improved cell performance combined with a largely reduced sensitivity towards changes in the relative humidity as compared to MEAs with symmetrical gas diffusion layer configuration.     

Three-dimensional numerical simulation of straight channel PEM fuel cells

The need to model three-dimensional flow in polymer electrolyte membrane (PEM) fuel cells is discussed by developing an integrated flow and current density model to predict current density distributions in two dimensions on the membrane in a straight channel PEM fuel cell. The geometrical model includes diffusion layers on both the anode and cathode sides and the numerical model solves the same primary flow related variables in the main flow channel and the diffusion layer. A control volume approach is used and source terms for transport equations are presented to facilitate their incorporation in commercial flow solvers. Predictions reveal that the inclusion of a diffusion layer creates a lower and more uniform current density compared to cases without diffusion layers. The results also show that the membrane thickness and cell voltage have a significant effect on the axial distribution of the current density and net rate of water transport. The predictions of the water transport between cathode and anode across the width of the flow channel show the delicate balance of diffusion and electroosmosis and their effect on the current distribution along channel.     

A thin-film/agglomerate model of a proton-exchange-membrane fuel cell cathode catalyst layer with consideration of solid-polymer-electrolyte distribution

A thin-film/agglomerate model for the cathode part of a proton-exchange-membrane fuel cell is developed. Parameter estimation is employed to determine the exchange current density in the catalyst layer, proton conductivity of the recast ionomer, and oxygen diffusivity in the solid polymer electrolyte. The effects of catalyst and polymer electrolyte loadings in the catalyst layer on the cell performance are demonstrated using this model. The influence of polymer electrolyte distribution in the catalyst layer is correlated with the oxygen diffusion and proton migration rates within the electrolyte. It is found that proton migration in the polymer electrolyte is the dominant factor for cell current density under normal operating conditions. A better cell performance is achieved by a concentrated polymer electrolyte near the catalyst layer/membrane interface.     

Experimental study on the effect of reactant flow arrangements on the current distribution in proton exchange membrane fuel cells

Current distribution in a proton exchange membrane fuel cell (PEMFC) is significantly influenced by reactant flow configurations. In this study, the current distribution has been measured experimentally using a segmented flow-field plate and printed circuit board (PCB). Local current distributions for a PEMFC with serpentine flow field and three different flow arrangements including co-flow, cross-flow, and counter-flow arrangements for the anode and cathode streams are investigated along with the effect of flow channel orientation. It is shown that the counter-flow arrangement yields most uniform distribution for the current density, whereas the co-flow arrangement results in a considerable variation in the current density from the reactant gas stream inlet to exit. Flow channel orientation can also impact the cell performance and the current distribution appreciably. The limiting hydrogen concentration at the anode side due to the low stoichiometry condition can have a predominant effect on the current distribution and cell performance.     

Improvement of the Ohmic Loss Process Model of the Proton Exchange Membrane Fuel Cell

The impedance characteristics of the ohmic overpotential of the proton exchange membrane (PEM) fuel cells are studied analytically using the process modeling approach. The water transport in the membrane, the cathode catalyst layer, and gas diffusion layer are analyzed. The analytical relation of the impedance of the ohmic loss is determined and is converted to an equivalent circuit. Then, the impedance of a PEM fuel cell is measured experimentally in different current densities, operating temperatures and the anode and cathode relative humidities. The measured impedances are compared with the predicted ones from the analytical model. It is shown that the predicted impedance characteristics are in great agreement with the measured ones in all different operating conditions.