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。     

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.     

PEM燃料电池中质子交换膜内水和质子的迁移特性

质子交换膜的水含量及水和质子的迁移对PEM燃料电池的性能具有重要影响.提出了一个稳态两相流数学模型,用以研究质子交换膜中的水迁移和水含量及其与质子传递阻力的关系.模型耦合了连续方程、动量守恒方程、物料守恒方程和水在质子交换膜中的传递方程.通过与实验数据对比,验证了模型的有效性.分析模拟结果发现,当电流密度相同时,沿气体流动方向,质子交换膜中水的电渗拉力系数、反扩散系数和水力渗透系数逐步增大,而水的净迁移系数逐步减小;同时,质子交换膜的含水量增加,质子传递阻力逐步下降;增大电池的操作压力,电渗拉力系数、反扩散系数、水力渗透系数、水净迁移系数和质子膜的含水量增加,而质子传递阻力下降,使燃料电池的性能得到了提高.     

On the modeling of water transport in polymer electrolyte membrane fuel cells

Water management is a critical issue in polymer electrolyte membrane (PEM) fuel cells, and water transport through the membrane, catalyst layer and gas diffusion layer has significant impact on the cell performance and durability. In this study, the mechanism of water transport processes in PEM fuel cells has been analyzed through 3-D unsteady non-isothermal simulations, along with a comprehensive examination of various modeling approaches in literature. It is shown that the finite rates of sorption/desorption of water in membrane affect the cell current output and the cell response time. Water dissolved in the membrane should be taken as the proper mechanism of water formation in the cathode of practical PEM fuel cells. Capillary pressure and relative permeability have significant impact on the distribution of liquid water saturation and transport, implying a need for their determination under specific PEM fuel cell conditions.     

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。     

Water transport through a PEM fuel cell: A one-dimensional model with heat transfer effects

Models play an important role in fuel cell design/development. The most critical problems to overcome in the proton exchange membrane (PEM) fuel cell technology are the water and thermal management. In this work, a steady-state, one-dimensional model accounting for coupled heat and mass transfer in a single PEM fuel cell is presented. Special attention is devoted to the water transport through the membrane which is assumed to be a combined effect of diffusion and electro-osmotic drag. The transport of heat through the gas diffusion layers is assumed to be a conduction-predominated process and heat generation or consumption is considered in the catalyst layers. The analytical solutions for concentration and net water transport coefficient are compared with recent published experimental data. The operating conditions considered are various cathode and anode relative humidity (RH) values at and 2 atm. The studied conditions correspond to relatively low values of RH, conditions of special interest, namely, in the automotive applications. Model predictions were successfully compared to experimental and theoretical I-V polarization curves presented by Hung et al. [2007. Operation-relevant modelling of an experimental proton exchange membrane fuel cell. Journal of Power Sources 171, 728-737] and Ju et al. [2005a. A single-phase, non-isothermal model for PEM fuel cells. International Journal of Heat and Mass Transfer 48, 1303-1315]. The developed easy to implement model using low CPU consumption predicts reasonably well the influence of current density and RH on the net water transport coefficient as well as the oxygen, hydrogen and water vapour concentrations at the anode and cathode. The model can provide suitable operating ranges adequate to different applications (namely low humidity operation) for variable MEA structures.     

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。     

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.     

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.     

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.     

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.     

PEM fuel cells operated at 0% relative humidity in the temperature range of 23-120 °C

Operation of a proton exchange membrane (PEM) fuel cell without external humidification (or 0% relative humidity, abbreviated as 0% RH) of the reactant gases is highly desirable, because it can eliminate the gas humidification system and thus decrease the complexity of the PEM fuel cell system and increase the system volume power density (W/l) and weight power density (W/kg). In this investigation, a PEM fuel cell was operated in the temperature range of 23-120 °C, in particular in a high temperature PEM fuel cell operation range of 80-120 °C, with dry reactant gases, and the cell performance was examined according to varying operation parameters. An ac impedance method was used to compare the performance at 0% RH with that at 100% RH; the results suggested that the limited proton transfer process to the Pt catalysts, mainly in the inonomer within the membrane electrode assembly (MEA) could be responsible for the performance drop. It was demonstrated that operating a fuel cell using a commercially available membrane (Nafion^® 112) is feasible under certain conditions without external humidification. However, the cell performance at 0% RH decreased with increasing operation temperature and reactant gas flow rate and decreasing operation pressure.     

Numerical studies of cold-start phenomenon in PEM fuel cells

In this paper, a PEM fuel cell model for cold-start simulations has been employed for numerical investigations of the cell startup characteristics from subfreezing temperatures. The effects of many key parameters on fuel cell isothermal cold-start behaviors have been carefully examined. Numerical results indicate that a high gas flow rate in the cathode gas channel, a low initial membrane water content, a low current density under the constant current condition, and a high cell voltage under the constant cell voltage operation are beneficial for the PEM fuel cell isothermal cold-start processes. Increasing the startup cell temperature would significantly delay ice formation and consequently lead to longer cold-start time. Therefore, incorporating internal and external heating sources in the cell design scheme is very important for achieving fast and successful cold start of a PEM fuel cell from subfreezing temperatures.     

带增湿区的质子交换膜燃料电池

Water management is of great importance to maintain performance and durability of proton exchange membrane fuel cells. This paper presents a novel proton exchange membrane (PEM) fuel cell with a humidification zone in the membrane electrode assembly (MEA) of each cell, in which the moisture of the cathode exhaust gas could transfer through the membrane to humidify anode or cathode dry gas. With a simple model, the relative humidity (RH) of the dry air exhaust from a membrane humidifier with 100% RH stream as a counter flow is calculated to be 60.0%, which is very close to the experimental result (62.2%). Fuel cell performances with hydrogen humidifying, air humidifying and no humidifying are compared at 50, 60 and 70˚C and the results indicate that humidifying is necessary and the novel design with humidifying zone in MEA is effective to humidify dry reactants. The hydrogen humidifying shows better performance in short term, while water recovered is limited and the stability is not as good as air hu-midifying. It is recommended that both air and hydrogen should be humidified with proper design of the humidifying zones in MEA and plates.     

Effect of water transport properties on a PEM fuel cell operating with dry hydrogen

In this work, membrane resistance measurement and water balance experiment were implemented to investigate the feasibility for a PEM fuel cell operating with dry hydrogen. The results showed that when a thin membrane was used in a cell the performance and the membrane resistance changed a little while the anode humidity changed from saturated to dry. Comparing with the anode humidity, the influence of the cathode humidity was serious on the cell performance. The water balance experiments showed that the net water transport coefficient was negative even the anode was humidified and liquid water existed not only in the cathode but also in the anode. High cathode humidity was disadvantage for the removal of water both in the anode and the cathode.     

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.     

Characterising PEM Fuel Cell Performance Using a Current Distribution Measurement in Comparison with a CFD Model

The characterisation of a proton exchange membrane (PEM) fuel cell with a straight channel flow field design is performed. Spatially resolved current distribution measurements, at different air flow rates, are compared to numerical simulation results. The numerical model is validated by agreement of the measured and simulated current distribution. The test cell is segmented. It is operated at steady state conditions and the gas flow rates and cell temperature are controlled. The numerical simulation is realised with a PEM fuel cell model based on FLUENT™ computational fluid dynamics (CFD) software. It accounts for mass transport in the gaseous phase, heat transfer, electrical potential field and the electrochemical reaction. It provides three‐dimensional distributions of, e.g., current densities, reactant concentrations and temperature.     

Transport Properties,Local Coordination,and Thermal Stability of the Water/Diethylmethylammonium Methanesulfonate Binary System

Ammonium based protic ionic liquids are highlighted for their great potential to sustain proton transport in proton exchange membrane (PEM) fuel cells. Yet, there remain questions concerning the effect of water produced by the fuel cell at the cathode side on the performance of the ionic liquid. In this contribution we report the effect of water on the transport properties and the local coordination in the binary system of the protic ionic liquid diethylmethylammonium methanesulfonate ([DEMA][OMs]) and water, employing ¹H NMR, Raman, and infrared spectroscopy. We observe that the self‐diffusion of cations and anions increases with the water content and that cations and anions diffuse at the same rate at all concentrations investigated. ¹H NMR and vibrational spectroscopy, on the other hand, indicate that added water interacts primarily with the anion and slightly affects the ionicity of the ionic liquid. In addition, by investigating the thermal stability of the binary system we find that although [DEMA][OMs] displays a continuous loss of water upon increasing temperature a fraction of water molecules can be retained even above 120 °C, and that the complete loss of water is immediately followed by decomposition, which is observed to occur at about 185 °C.     

Effects of Potential on Corrosion Behavior of Uncoated SS316L Bipolar Plate in Simulated PEM Fuel Cell Cathode Environment

The effect of potential on the corrosion behavior of uncoated stainless steel SS316L as bipolar plate material in proton exchange membrane (PEM) fuel cell cathode environment is studied. Electrochemical methods, X‐ray photoelectron spectroscopy, scanning electron microscope are employed to characterize the corrosion behavior of SS316L at different polarization potentials in PEM fuel cell cathode environment. The results show that the corrosion current density of SS316L increases with the increase of polarization potential significantly. When the potential is higher than 0.7 V versus SCE, severe corrosion occurs on SS316L. The work also shed light on the corrosion mechanisms of SS316L at different potential in the PEM fuel cell cathode environment.     

质子交换膜燃料电池膜中气态水管理模型

分析质子交换膜燃料电池的膜水含量与运行参数的关系,从工程方法的角度建立水传输模型.模型分析得到,要提高膜的水合程度,需要通过增湿反应气体.过高的增湿反应气体又会引起阴极扩散层水的泛滥,需通过调节反应气体流量来缓解水的泛滥.为保证膜的高水合程度和低的阴极扩散层水的泛滥,建立了膜水含量的神经网络控制模型,为电池水管理奠定了基础.