采用不同流场的质子交换膜燃料电池内部传递现象模拟

A numerical model for proton exchange membrane (PEM) fuel cell is developed, which can simulate such basic transport phenomena as gas-liquid two-phase flow in a working fuel cell. Boundary conditions for both the conventional and the interdigitated modes of flow are presented on a three-dimensional basis. Numerical techniques for this model are discussed in detail. Validation shows good agreement between simulating results and experimental data. Furthermore, internal transport phenomena are discussed and compared for PEM fuel cells with conventional and interdigitated flows. It is found that the dead-ended structure of an interdigitated flow does increase the oxygen mass fraction and decrease the liquid water saturation in the gas diffusion layer as compared to the conventional mode of flow. However, the cathode humidification is important for an interdigitated flow to acquire better performance than a conventional flow fuel cell.     

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.     

质子交换膜燃料电池双极板导流场不同区域极化研究

质子交换膜燃料电池双极板导流场的不同区域极化研究可以诊断导流场工程设计的合理性。本文针对一种氢气、空气型燃料电池双极板导流场的设计,对导流场的四个相等部分进行了四个不同区域的燃料电池极化研究,进而可以判断整个导流场在各种燃料电池运行条件下对燃料电池性能的影响以及导流场设计的合理性。     

Influence of bio-inspired flow channel designs on the performance of a PEM fuel cell

Performance of the proton exchange membrane fuel cell(PEMFC) is appreciably affected by the channel geometry. The branching structure of a plant leaf and human lung is an efficient network to distribute the nutrients in the respective systems. The same nutrient transport system can be mimicked in the flow channel design of a PEMFC, to aid even reactant distribution and better water management. In this work, the effect of bio-inspired flow field designs such as lung and leaf channel design bipolar plates, on the performance of a PEMFC was examined experimentally at various operating conditions. A PEMFC of 49 cm~2 area, with a Nafion 212 membrane with a 40% catalyst loading of 0.4 mg·cm-2 on the anode side and also 0.6 mg·cm~(-2) on the cathode side is assembled by incorporating the bio-inspired channel bipolar plate, and was tested on a programmable fuel-cell test station.The impact of the working parameters like reactants' relative humidity(RH), back pressure and fuel cell temperature on the performance of the fuel cell was examined; the operating pressure remains constant at 0.1 MPa. It was observed that the best performance was attained at a back pressure of 0.3 MPa, 75 °C operating temperature and 100% RH. The three flow channels were also compared at different operating pressures ranging from 0.1 MPa to 0.3 MPa, and the other parameters such as operating temperature, RH and back pressure were set as 75 °C,100% and 0.3 MPa. The experimental outcomes of the PEMFC with bio-inspired channels were compared with the experimental results of a conventional triple serpentine flow field. It was observed that among the different flow channel designs considered, the leaf channel design gives the best output in terms of power density. Further,the experimental results of the leaf channel design were compared with those of the interdigitated leaf channel design. The PEMFC with the interdigitated leaf channel design was found to generate 6.72% more power density than the non-interdigitated leaf channel design. The fuel cell with interdigitated leaf channel design generated5.58% more net power density than the fuel cell with non-interdigitated leaf channel design after considering the parasitic losses.     

Optimization of the cathode geometry in polymer electrolyte membrane (PEM) fuel cells

A nonlinear constrained optimization procedure is used in the cathode design in order to maximize the average current density at a fixed voltage in a polymer electrolyte membrane (PEM) fuel cell with interdigitated fuel/air distributors. The operation of the PEM fuel cell is studied using a steady-state, two-phase, two-dimensional electro-chemical model. The following geometrical parameters of the cathode are considered: the thickness, and length per one shoulder of the interdigitated air distributor and the length of the shoulder. The optimization results obtained show that within manufacturability controlled lower and the space-limitation controlled upper bounds of these parameters, the optimal-cathode design corresponds to the lower bounds in the cathode length per one shoulder of the interdigitated air distributor and the fraction of the length associated with the shoulders and at a low (but larger than the lower bound) value of the cathode thickness. These findings are explained using an analogy with the effect of pipe dimensions on the fluid flow through a pipe and by considering the role of forced convection on the oxygen transport to the membrane/cathode interface.     

Membrane fuel cells - concepts and system design

The low power range can be an interesting application market for the PEM fuel cell in the near future. With a possible function as battery in portable devices the banded structure PEM fuel cell will be an advantageous alternative to the conventional stack. The new system provides high output voltages, a flat cell geometry and can be optimized with respect to smaller cell volumes. The article describes the principle function of such a system and the necessary parameter for optimizing the cell geometry. A comparison with a conventional stack design is given and experimental results with an eight cell banded structure stack are presented.     

常规流场质子交换膜燃料电池阴极二维两相流模型

对采用常规条形流场的H₂-Air PEMFC阴极建立了二维两相流模型,控制方程耦合了电传导方程、O₂和气态H₂O的对流-扩散方程、多孔介质连续性方程以及液态H₂O输运方程.利用模型计算了阴极扩散层中O₂浓度、气态H₂O浓度、电流密度以及液态水饱和度的分布,分析了扩散层中H₂O的传递方式及各组分浓度分布的特点.     

Effects of flow channel geometry on cell performance for PEM fuel cells with parallel and interdigitated flow fields

A 3D numerical model was developed to explore the effects of the cathode flow channel configuration on the local transport phenomena and cell performance for parallel and interdigitated flow fields in proton exchange membrane (PEM) fuel cells. The effect of liquid water formation on the reactant transport is taken into account in the model. For operating voltages greater than 0.7 V, the electrochemical reaction rates are low with a small amount of oxygen consumption and liquid water production, and all cell designs provide sufficient oxygen for the electrochemical reactions. Thus, the flow channel aspect ratio and the flow channel cross-sectional area have little effect on the cell performance. For operating voltages lower than 0.7 V, as the operating voltage decreases the electrochemical reaction rates gradually increase with a large amount of oxygen consumption and liquid water production, so the cell performance is strongly dependent on the flow field design. For the parallel flow field design, lower flow channel aspect ratios and flow channel cross-sectional area areas improve liquid water removal, thus, decreasing both improves cell performance. However, the interdigitated design has an optimal aspect ratio of 1.00 and an optimal cross-sectional area of 1.000 mm × 1.000 mm.     

Non-isothermal effects of single or double serpentine proton exchange membrane fuel cells

Mathematical models on transport processes and reactions in proton exchange membrane (PEM) fuel cell generally assume an isothermal cell behavior for sake of simplicity. This work aims at exploring how a non-isothermal cell body affects the performance of PEM fuel cells with single and double serpentine cathode flow fields, considering the effects of flow channel cross-sectional areas. Low thermal conductivities of porous layers in the cell and low heat transfer coefficients at the surface of current collectors, as commonly adopted in cell design, increase the cell temperature. High cell temperature evaporates fast the liquid water, hence reducing the cathode flooding; however, the yielded low membrane water content reduces proton transport rate, thereby increasing ohmic resistance of membrane. An optimal cell temperature is presented to maximize the cell performance.     

Effect of the geometry and pattern of the flow channel on the performance of polymer electrolyte membrane fuel cell

This research focuses on the effect of the geometry and patterns of the gas flow channel on the PEM fuel cell performance. Simulation was conducted and the results were verified by experiments. Three-dimensional, single phase, compressible and isothermal models of 5 cm² electrodes, anode and cathode, were developed and studied by utilizing a commercial Computational Fluid Dynamics (CFD) software, FLUENT 4.5. Two types of gas flow channel were investigated: conventional and interdigitated. The results showed that the flow channel pattern does not have a significant effect on the anode cell performance, whereas it has a strong effect/influence on the cathode cell performance. The interdigitated design provides a higher limiting current density and cell performance than the conventional design on the cathode side. Moreover, the cell performance does not depend on the inlet and outlet channel widths. On the contrary, for the interdigitated design, it was influenced by the shoulder width. Finally, experiments were conducted to validate the simulation results.     

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.     

直通道流场PEMFC电流密度分布测定实验

大功率燃料电池研究中存在的主要问题是电池在放大过程中性能出现大幅度衰减．为提高电池和电池组性能,改善电池内部水管理,优化电极组成、电池操作参数和流场板结构,对电池内部电流密度分布进行测定很有必要．     

质子交换膜燃料电池流场板研究进展

流场板是质子交换膜燃料电池的核心部件之一,其结构直接影响着反应气体的利用效率以及燃料电池的排水及散热性能。综述了近十余年来质子交换膜燃料电池流场板的设计与研究进展。研究者们基于平行流场、蛇形流场、交指流场、点状流场,从流道尺寸、流道截面、进口分配段、流道布置等方面开展结构设计和优化,不同程度提高了燃料电池水热管理以及电性能。此外,各种形式的组合流场可综合不同流场优点,多级分形仿生流场优化了反应物、压力与电流密度分布,三维精细化流场通过改善供气方式降低了浓差极化。     

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。     

质子交换膜燃料电池动态过程的数值模拟

为描述质子交换膜燃料电池的动态过程,发展了一个基于计算流体动力学的非稳态、非等温的三维两相流数学模型.应用模型对一蛇形流道结构的质子交换膜燃料电池单体进行了数值计算,得到了电池启动过程中电池阴极侧膜表面温度和电流密度等特征参数的动态过程变化曲线.最后,分析了阴极入口速度、湿度和电池电压阶跃变化后电池特性的动态响应特性.结果表明：电池的启动时间和阶跃响应时间均为秒的数量级,与大多数模型模拟的结果相一致.     

Novel approach to membraneless direct methanol fuel cells using advanced 3D anodes

A conventional membrane electrode assembly (MEA) for a direct methanol fuel cell (DMFC) consists of a polymer electrolyte membrane (PEM) compressed between an anode and cathode electrode. Limitations with this conventional design include: cost, fuel crossover, membrane degradation or contamination, ohmic losses and reduced active triple phase boundary (TPB) sites for catalyst located away from the electrode/membrane interface. In this work, ex situ and in situ characterization of a novel electrode assembly based on a membraneless architecture and advanced 3D anodes was investigated. The approach was shown to be fuel independent and scaleable to a conventional bi-polar fuel cell arrangement. The membraneless configuration exhibits comparable performance to a conventional ambient (25 °C, 1 atm) air-breathing DMFC. However, it has the additional advantages of a simplified design, the elimination of the membrane (a significant component expense) and enhanced fuel and catalyst utilization through the extension of the active catalyst zone.     

自呼吸式微型燃料电池阳极流场的优化设计与研究

设计了点状、蛇形和点蛇混合三种微型质子交换膜燃料电池(μPEMFC)阳极流场,模拟计算表明三种结构中点蛇混合结构更能有效协调燃料利用率与接触电阻之间的矛盾。利用微电子机械技术(MEMS)制备了两种点蛇混合阳极结构,组成自呼吸式电池。实验研究发现,当点蛇混合结构的集流条由2增加到4时,流道总长度增大约一倍,氢气在流道内的流速明显增加,电池的极限电流密度和峰值功率密度分别提高18.56%和15.26%,在100~500 mA恒电流放电下,可节省燃料平均达6.18%。     

带有蛇形流场的微生物燃料电池串联堆性能特性

以四个成功启动的带有蛇形流场的单电池构造了微生物燃料电池串联堆(MFCS-S),测试了MFCS-S性能,探讨了其性能提升的限制因素,研究了增加反极电池阴、阳极电解液流量,采取混联的方式运行,移除反极电池和反接反极电池对电堆性能的影响。实验结果表明:MFCS-S在输出电压为2.11 V时获得最大功率密度(2226 mW·m^-2);在一定电流条件下,性能较差的单电池发生电压反极,这是限制MFCS-S性能提高的主要原因;增加反极电池阴、阳极流量虽然不能较大幅度地改善单电池反极,但是却能大幅度提高电堆功率密度;采用混联方式运行不但可以有效避免电池反极,而且可以大幅度提高电堆功率密度;移除反极电池并不能有效地避免电池的反极,反接反极电池反而进一步加剧反极。     

三维石墨毡阳极自呼吸微流体燃料电池性能数值模拟

针对采用三维石墨毡可渗透阳极的空气自呼吸无膜微流体燃料电池,建立了三维等温稳态数学模型,对电池中燃料及电解液的流动和传输、电极过程动力学及电荷传递过程进行了模拟,计算获得了具有石墨毡阳极的微流体燃料电池内的传质和燃料渗透特性,研究了石墨毡厚度及反应物（燃料和电解液）流量对电池性能的影响。结果表明：当入口流量为333 μl·min^-1时,采用石墨毡可渗透阳极相比碳纸和碳布可渗透阳极,极限电流密度和极限功率密度分别提升12%和50%;电池电压为0.8 V时燃料渗透引起的寄生电流密度仅占电流密度的0.86%。电池性能随着石墨毡电极厚度增加而升高,但增幅逐渐减小;反应物流量增大,电池的性能先增加后逐渐趋于稳定。     

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。