常规流场和交指型流场的质子交换膜燃料电池传热传质三维模拟

针对常规流场和交指型流场的质子交换膜燃料电池提出了三维非等温数学模型。模型详细考虑了电池内部的传热、传质和电化学反应，重点考察了多孔介质内的组分传递和膜内水的电渗和扩散作用，对氧气传递限制和膜内水迁移对电池性能的影响进行了分析和讨论。结果表明，流道的交指型设计加强了气体在多孔介质内的质量传递，提高了电池的输出性能，但相应地，阴极催化层界面水分的减少也使得膜的水合程度降低，这就需要更有效的水管理来防止膜脱水。     

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.     

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.     

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。     

Visualising Liquid Water in PEM Fuel Cells Using Neutron Imaging

In this article, we review the neutron imaging techniques that have been used to visualise liquid water in PEM fuel cells. A list of the various facilities engaged in this research is provided and the published literature in this field reviewed. Neutron imaging has been successfully used to visualise water dynamics in the flow channels of operating fuel cells. This technique has also been used to understand water removal mechanisms and the importance of membrane hydration and GDL flooding to optimal fuel cell performance. More recently this technique has been applied to imaging the water in fuel cell cross‐sections in order to quantify the water contents in the different components of an operating fuel cell. Finally, this technique has also been utilised to examine ice formation during sub‐zero operation of single fuel cells. With ongoing improvements in spatial and temporal resolution, neutron imaging can be expected to play a greater role in any fuel cell development related to water transport.     

Using poly(N,N-dimethylaminoethyl methacrylate)/polyacrylonitrile composite membranes for gas dehydration and humidification

The transport of water vapor through a composite membrane consisting of hydrophilic poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) as the active layer and polyacrylonitrile (PAN) as the substrate was investigated, and the performance of the membrane for gas dehydration and humidification applications was evaluated. For gas dehydration, methane/water vapor mixtures were used as feed and vacuum was applied on the downstream side. The feed composition and operating temperature were found to have a significant effect on the membrane performance. The PAN substrate had little effect on the permeation of methane, but the resistance of the substrate to water vapor permeation was significant because of the substantially higher permeability of water vapor in the membrane. For gas humidification, liquid water was brought to be in contact with the active layer of the membrane and nitrogen gas flowed on the other side. With an increase in the gas flow rate, the mass transfer rate of water through the membrane to reach the gas stream increased, and the humidity level of the gas stream decreased. The humidification can be enhanced significantly by operating at a higher temperature. A phenomenological mass transfer equation was derived for membrane humidifiers to correlate the overall mass transfer coefficient and membrane area, and this equation could be used in process design and scale up.     

质子交换膜燃料电池稳态自增湿性能分析

增湿及水管理系统使得燃料电池系统结构复杂,质子交换膜燃料电池（proton exchange membrane fuel cell,PEMFC）自增湿操作在实用化方面逐渐引起研究者的兴趣。提高PEMFC自增湿性能的关键在于对生成水的有效管理,保证质子交换膜的良好水合。实践证实采用自增湿膜电极组件是一个有效途径。本文建立催化层中增加保水层的水传递平衡模型预测膜中水的分布,考察自增湿操作的可行性和稳定性。数值分析表明：只有低于50 mm（如Nafion112）的薄膜能满足电池自增湿膜水合的要求。保证膜水合性能和电池操作稳定性的电池温度为60℃,操作压力为0.15 MPa,阴极气体过量系数可以增大到1.8。在上述操作条件下,电池自增湿性能与饱和增湿有可比性,与饱和增湿最佳条件有差距。因此PEMFC自增湿性能在综合考虑降低成本和费用,简化结构和操作时具有可行性,但不能替代增湿操作。     

Dynamic Response of a Subsaturated Polymer Electrolyte Fuel Cell in Counterflow Mode

This work demonstrates that the operation of a subsaturated polymer electrolyte fuel cell in counterflow mode results in a significantly elongated relaxation time after a load change, if compared to coflow mode. This effect is investigated here by using combined dynamic locally resolved measurements of the current density, the high frequency resistance, and the relative humidity. It is shown that the elongated relaxation time is a consequence of slow membrane hydration in the region of the cell, downstream the anode flow field, where the diffusive flux of water across the membrane occurs from the anode to the cathode. Here, the anode gas stream, which is humidified upstream the anode flow field via back diffusion of water from the cathode to the anode, is the only source of water for both membrane hydration and the internal humidification of the cathode gas stream, which passes the cell in opposite direction.     

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.     

A three-dimensional numerical simulation of the transport phenomena in the cathodic side of a PEMFC

A three-dimensional numerical model is developed to simulate the transport phenomena on the cathodic side of a polymer electrolyte membrane fuel cell (PEMFC) that is in contact with parallel and interdigitated gas distributors. The computational domain consists of a flow channel together with a gas diffusion layer on the cathode of a PEMFC. The effective diffusivities according to the Bruggman correlation and Darcy's law for porous media are used for the gas diffusion layer. In addition, the Tafel equation is used to describe the oxygen reduction reaction (ORR) on the catalyst layer surface. Three-dimensional transport equations for the channel flow and the gas diffusion layer are solved numerically using a finite-volume-based numerical technique. The nature of the multi-dimensional transport in the cathode side of a PEMFC is illustrated by the fluid flow, mass fraction and current density distribution. The interdigitated gas distributor gives a higher average current density on the catalyst layer surface than that with the parallel gas distributor under the same mass flow rate and cathode overpotential. Moreover, the limiting current density increased by 40% by using the interdigitated flow field design instead of the parallel one.     

质子交换膜燃料电池可视化研究进展

质子交换膜燃料电池(包括氢氧质子交换膜燃料电池和直接甲醇燃料电池)内的两相流动以及相应的水管理、气管理对燃料电池的性能和寿命有很大的影响,而可视化方法是研究流场槽道内两相流动非常重要的方法之一。可视化实验可以真实地展示气泡或液滴在流场槽道内的生成以及发展过程,有利于了解其进化机制,从而进一步优化气管理、水管理并提高电池性能。本文主要综述了质子交换膜燃料电池两极流场内两相流动的可视化研究进展,讨论了扩散层的润湿性以及扩散层内水的传递机理,还介绍了实现可视化的方法,并提出了可视化研究的不足及发展方向。     

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。     

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.     

Analysis of coupled proton and water transport in a PEM fuel cell using the binary friction membrane model

Transport of liquid water within a polymer electrolyte membrane (PEM) is critical to the operation of a PEM fuel cell, due to the strong dependence of the membrane transport coefficients on water content. In addition, enhanced predictive abilities are particularly significant in the context of passive air breathing fuel cell designs where lower water contents will prevail in the membrane. We investigate and analyze the numerical predictions of a recently proposed rational model for transport of protons and water in a PEM, when compared to a widely used empirical model. While the performance is similar for a saturated membrane, for PEMs with low water content, the difference in computed current density and membrane water crossover can be substantial. The effects of coupling partially saturated gas diffusion electrodes (GDLs) with the membrane are studied in both a 1D and 2D context. In addition, a simplified 1D analytical membrane water transport model is validated against the complete 1D model predictions. Our numerical results predict a higher current density and more uniform membrane hydration using a dry cathode instead of a dry anode, and illustrate that the strongest 2D effects are for water vapor transport.     

分体式质子交换膜燃料电堆的膜加湿实验

在已开发的分体式质子交换膜燃料电池电堆基础上,对膜（Nafion115）加湿器子系统进行了详细的研究,揭示了质子交换膜燃料电池膜加湿方法的特性.初步分析了膜加湿的原理,并在不同操作条件下,定量地对膜透过水量与反应气体的润湿程度进行了测量,得到了Nafion115膜加湿器对电堆的润湿性能.发现操作温度在50～70℃之间,加湿速率最强;随着反应气体流量的增大,加湿速率呈非线性增加,但润湿程度反而降低;增加质子交换膜面积会增大反应气体的相对湿度,而加湿速率将下降;较高反应气体压力下不利于加湿.在适当的膜加湿器工况下,当电流密度为2.1A•cm^-2时,电堆最大功率密度可超过1.2W•cm^-2.     

Three-dimensional two-phase mass transport model for direct methanol fuel cells

A three-dimensional (3D) steady-state model for liquid feed direct methanol fuel cells (DMFC) is presented in this paper. This 3D mass transport model is formed by integrating five sub-models, including a modified drift-flux model for the anode flow field, a two-phase mass transport model for the porous anode, a single-phase model for the polymer electrolyte membrane, a two-phase mass transport model for the porous cathode, and a homogeneous mist-flow model for the cathode flow field. The two-phase mass transport models take account the effect of non-equilibrium evaporation/ condensation at the gas-liquid interface. A 3D computer code is then developed based on the integrated model. After being validated against the experimental data reported in the literature, the code was used to investigate numerically transport behaviors at the DMFC anode and their effects on cell performance.     

Hydration of α-pinene over molybdophosphoric acid immobilized in hydrophobically modified PVA membranes

A polyvinylalcohol/molybdophosphoric acid (PVA/HPMo) membrane crosslinked with succinic acid was modified by treatment with acetic anhydride in order to improve its hydrophobic properties, and was used as catalyst in the hydration reaction of α-pinene. The increase of membrane hydrophobicity with acetylation is documented not only by the water droplet contact angle but also by the sorption coefficients of α-pinene and water. The introduction of acetyl groups improves the membrane transport properties, as reflected by pinene diffusivity calculated from permeation data.A kinetic-diffusion model was developed assuming that the reaction product α-terpineol affects the transport of water and α-pinene across the acetylated PVA membranes. When membrane acetylation increases, the model predicted kinetic constants for hydration and isomerization reactions decrease, although the initial water diffusivity increases. These results suggest that the increase of catalytic activity may be due to an improvement of water transport across the membrane.     

增湿温度与气体流速对PEMFC阴极流道内液态水分布及排水影响

通过直条单流道可视化PEM单池研究了阴极流道内液态水的传递行为,给出了液态排水与流道进、出口压降间的直接关系。结合可视化与压降测量,开发一种评价流道内液态水积累及排出的新方法,考察了增湿温度与气体流速对流道内液态水分布及排水的影响。随着增湿温度提高,存水区域由流道下部向进口扩展。对于给定流道,存在一特定流速。在该流速下液滴临界直径与流道尺寸相当,流道内液态水积累最严重。在本实验条件下,该特定流速为2 m·s^-1。为了及时排出液态水,气体流速不能低于3 m·s^-1。     

The effect of cathodic water on performance of a polymer electrolyte fuel cell

A simple analytical model of water transport in the polymer electrolyte fuel cell is developed. Nonlinear membrane resistance and voltage loss due to incomplete membrane humidification are calculated. Both values depend on parameter r, the ratio of mass transport coefficients of water in the membrane and in the backing layer. Simple equation for cell performance curve, which incorporates the effect of cathodic water is constructed. Depending of the value of r, the cell may operate in one of the two regimes. When r ≥ 1, incomplete membrane humidification simply reduces cell voltage; the limiting current density is determined by oxygen transport in the backing layer (oxygen-limiting regime). If r < 1, limiting current density is determined by membrane drying (water-limiting regime). In that case there exists optimal current density, which provides minimal membrane resistance. It is shown that membrane drying may lead to parasitic “in-plane” proton current.     

Effects of heat and water transport on the performance of polymer electrolyte membrane fuel cell under high current density operation

Key challenges to the acceptance of polymer electrolyte membrane fuel cells (PEMFCs) for automobiles are the cost reduction and improvement in its power density for compactness. In order to get the solution, the further improvement in a fuel cell performance is required. In particular, under higher current density operation, water and heat transport in PEMFCs has considerable effects on the cell performance. In this study, the impact of heat and water transport on the cell performance under high current density was investigated by experimental evaluation of liquid water distribution and numerical validation. Liquid water distribution in MEA between rib and channel area is evaluated by neutron radiography. In order to neglect the effect of liquid water in gas channels and reactant species concentration distribution in the flow direction, the differential cell was used in this study. Experimental results suggested that liquid water under the channel was dramatically changed with rib/channel width. From the numerical study, it is found that the change of liquid water distribution was significantly affected by temperature distribution in MEA between rib and channel area. In addition, not only heat transport but also water transport through the membrane also significantly affected the cell performance under high current density operation.