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.     

Steady-state operation of a compressor for a proton exchange membrane fuel cell system

The performance of a system consisting of a proton exchange membrane (PEM) fuel cell coupled to a centrifugal air compressor is simulated. Two modes of operation of the system are investigated: one in which the speed of the compressor is constant, and the other in which the compressor speed is varied with the electric load on the fuel cell stack. The operating characteristics of the compressor and the PEM fuel cell stack and their influence on the system efficiency are analyzed for a step change in the stack current. The effects of the fuel cell stack back-pressure and the electric load on the compressor power consumption and the system efficiency are studied. It is found that the system efficiency is higher when the fuel cell stack is operated at a constant oxygen gas stoichiometry by varying the compressor speed instead of at a constant compressor speed. The system model can be used to determine the rotation speed of the compressor for various electric loads.     

空气流量对空冷燃料电池电堆性能的影响研究

空冷型氢燃料电池采用开放型阴极,具有自增湿、系统简单轻便等特点。为了揭示空气流量对输出性能的影响机制,对自组装的800 W空冷型燃料电池电堆进行了实验测试和数值分析,对比了不同空气风扇转速下电堆输出电压、净功率以及传质传热特性。结果表明：小电流条件下小空气流量可以保持电堆内较高的温度,减少活化损失,实现高净输出功率。然而,大电流条件下,小空气流量将导致电堆温度过高且分布不均匀。利用数值方法对组分和温度分布进行了可视化分析,结果表明低含水量引起的欧姆损失增加是限制输出功率的关键因素,通过提高风扇转速增加空气流量可以保证较好的冷却效果,从而提高含水量,减少欧姆损失。     

空气流量对空冷燃料电池电堆性能的影响研究

空冷型氢燃料电池采用开放型阴极,具有自增湿、系统简单轻便等特点。为了揭示空气流量对输出性能的影响机制,对自组装的800 W空冷型燃料电池电堆进行了实验测试和数值分析,对比了不同空气风扇转速下电堆输出电压、净功率以及传质传热特性。结果表明：小电流条件下小空气流量可以保持电堆内较高的温度,减少活化损失,实现高净输出功率。然而,大电流条件下,小空气流量将导致电堆温度过高且分布不均匀。利用数值方法对组分和温度分布进行了可视化分析,结果表明低含水量引起的欧姆损失增加是限制输出功率的关键因素,通过提高风扇转速增加空气流量可以保证较好的冷却效果,从而提高含水量,减少欧姆损失。     

A study of a microbial fuel cell battery using manure sludge waste

The cell voltage and power performance of a microbial fuel cell utilising waste carbohydrate as a fuel, that does not use a mediator, catalysts or a proton exchange membrane, is reported. Tests were conducted with the cell operated essentially as a battery using manure sludge as fuel and with oxygen reduction in an aqueous solution. Using carbon cloth as both anode and cathode, the cell produced peak power of the order of 5 mW m^?2. The cell performance was not greatly influenced by the quantity of fuel used and required a suitable separation between the cathode, anode and sludge/water interface. Agitation of the sludge did not adversely affect the cell performance, indicating that a continuous fuel cell system could be operated using the manure sludge. Using a platinised carbon cathode doubled the power density to over 10 mW m^?2. The use of nickel as an alternative cathode catalyst was not found to be effective under the conditions of operation of the cell. The cell power performance was found to be consistent and stable over the 3 month duration of the tests, after which point over 95% consumption of carbohydrate was achieved. Examination of the carbon anodes after the tests showed consistent formation of a biofilm on the surface of the fibres. A cell stack design based on multiple pocket anodes containing the fuel sludge has also been demonstrated. Copyright © 2007 Society of Chemical Industry     

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.     

Modelling of performance of PEM fuel cells with conventional and interdigitated flow fields

A simple mathematical model is developed to investigate the superiority of the interdigitated flow field design over the conventional one, especially in terms of maximum power density. Darcy's equation for porous media and the standard diffusion equation with effective diffusivity are used in the gas diffuser, and a coupled boundary condition given by the Butler–Volmer equation is used at the catalyst layer interface. The performance of PEM fuel cells with a conventional flow field and an interdigitated flow field is studied with other appropriate boundary conditions. The theoretical results show that the limiting current density of a fuel cell with an interdigitated flow field is about three times the current density of a fuel cell with a conventional flow field. The results also demonstrate that the interdigitated flow field design can double the maximum power density of a PEM fuel cell. The modelling results compared well with experimental data in the literature.     

5kW氢-空质子交换膜燃料电池堆的模型研究

采用机理模型和经验模型相结合的建模方法建立了一个5kw质子交换膜燃料电池堆实际装置的电化学模型。利用电池堆与单电池之间的内在关系,首先给出了单电池的数学描述,进而建立燃料电池堆的数学模型,其中包括热力学平衡电势、活化极化电势和欧姆极化电势等表达式,以及单电池内阻的经验公式。由于难以得到机理方程中的某些关键参数,因此采用实验设计,获得燃料电池堆的实验数据,运用线性回归的数学方法获得机理模型中活化极化电势方程中的相关参数,通过模拟研究和模型验证,所建立的模型可以较准确的描述燃料电池的极化曲线,预估出燃料电池的输出电压。     

Mitigating distortions during debinding of a monolithic solid oxide fuel cell stack using a multiscale,multiphysics model

Improving the power density of solid oxide fuel cell stacks would significantly enhance this technology for transportation. Using a monolithic structure to downsize the stack dimension offers a key to elevate the power density of solid oxide fuel cell stacks. This innovative design is promising but manufacturing is a challenge. The monolith is co-sintered in one firing step, and the gas channels are formed by burning off sacrificial organic materials. Structure distortion or fracture was observed in post-mortem investigations. In this work a multiscale, multiphysics modelling approach is proposed to describe and resolve this challenge in the debinding process occurring in a monolithic stack, i.e. the burning of organics and transportation of gases through the gradually opening microstructure, as well as the pressure build-up in the microstructure due to gas development. Simulation results show that a prominent pressure peak is experienced in the stack when a plasticiser (polyethylene glycol) and a pore-former (polymethyl methacrylate) are decomposed simultaneously. To reduce the high pressures, we investigate two possible strategies: (i) changing the mixture of organic additives; (ii) modifying the debinding temperature profile. Three tapes with different pore-formers are prepared, and the generated pressures during debinding of the three stacks are compared. The corresponding stack shapes after debinding are recorded. Numerical investigations show a good agreement with the post-mortem observations. By changing the composition of organics the distortion or fracturing of the stack can be avoided. Furthermore, to facilitate stack manufacturing, the high pressures can also be reduced by adjusting the heating rates and dwell temperatures of debinding. By using the new temperature profile suggested by the simulation study, the duration of debinding can also be reduced.     

应用于大尺寸平板型固体燃料电池电堆的Al2O3-SiO2-CaO基密封材料软化特性及其验证

设计研制了Al₂O₃-SiO₂-CaO基密封材料,对其高温晶化与软化、热性能、界面黏结特性开展了原位观察,并进行了电堆实际应用验证。结果表明:在不高于1 100℃时该密封材料均为非晶态,850℃开始软化,900~1 000℃出现球化。热重分析表明密封材料在0~960℃的质量损失较小,约为0.06%;密封材料与连接板、电池界面黏结紧密,利于固体氧化物燃料电池(SOFC)电堆密封应用。采用研制的密封材料组装了2个5单元SOFC短堆,分别进行了热循环与稳定性研究。结果表明:2个5单元电堆的开路电压达到6.0 V,平均开路电压1.2 V,电堆1热循环前后在35 A(0.56 A/cm²)条件下输出功率为运行无衰减,电堆2在27 A(电流密度0.43 A/cm²)进行恒流放电,运行300 h较为稳定。     

An investigation into water and thermal balance for a liquid fueled fuel processor

To better understand the sizing and performance issues associated with water and energy balances of a fuel processor for a PEM fuel cell, a process flow system has been programmed into the ASPEN^® simulator for a reference system supplying 600 W electrical power. The fuel processor consists of an autothermal reformer (ATR), single-stage water gas shift (WGS), dual-stage PROX reactors and an anode gas burner (AGB) operating on sulfur-free JP8. The reactor components were modeled using RGIBBS for the ATR and actual experimental kinetics for the WGS and PROX reactors. Simulations investigating thermal management, ATR and WGS performance and system efficiencies were done. Using realistic temperature approaches for heat exchangers it is likely that integrated systems will have to discard heat to the surroundings. Also, depending on the operating conditions, water will either be in surplus or deficit. At 1 atm pressure there is no condition that will enable complete water capture. The minimum pressure where water independence can be achieved is at 1.23 atm with a fuel cell utilization of 60% and a S/C = 1.5 and O/C = 1.0. The complete fuel processor is expected to be 0.46 L and 2.02 kg. This paper will expand on these findings with suggestions for optimal operating efficiencies and sizing for JP8 fuel processors.     

液相进样直接甲醇燃料电池性能研究

报道了用研制的Pt-Ru/C催化剂, 采用特殊工艺制备了膜电极, 并组装了直接甲醇质子交换膜单电池系统。考察了电极扩散层制备方法、催化剂层中催化剂、Teflon-C以及Nafion液的用量等电极制备工艺条件以及空气作为氧化剂对单电池性能的影响。结果表明：采用刷涂法制备电极扩散层比喷涂法好,催化剂层中催化剂的优化含量为0.6mg·cm-2,Teflon-C、Nafion液的最佳用量分别为0.3 mg·cm-2、0.5 mg·cm-2。当工作温度为80℃时,输出电压为0.3V,氧气作为阴极气体的输出电流密度为36mA·cm-2;而空气作为阴极气体的输出电流密度为22.5mA·cm-2。膜电极有效面积为9cm2的的液相进样直接甲醇/氧气燃料电池三电池电堆的最大功率为0.285W,此时输出电压为0.7V,输出电流为0.407A;而液相进样直接甲醇/空气三电池电堆的输出电压为0.635V,输出电流为0.252A时,最大功率为0.160W。     

Analysis of PEM fuel cell stacks using an empirical current–voltage equation

An empirical equation was developed to describe the electrode processes (activation, ohmic and mass-transfer) of PEMFC stacks over the entire current range. The potential–current and power–current curves of a strip PEMFC stack were fitted with the empirical equation under a variety of experimental humidity, temperature and stack length conditions. The concept of mass transfer impedance was defined mathematically in the present research. For the strip PEMFC stack, mass transfer impedance was only important at high currents. With decreasing humidity the mass transfer impedance increased considerably. With increasing temperature or stack cell number the mass transfer impedance increased only slightly.     

Experimental and numerical studies of portable PEMFC stack

The objective of this work is to establish the design principles of a proton exchange membrane (PEM) fuel cell (FC) stack for portable applications. A combination of experiments and numerical simulations were carried out and the results analyzed to enhance understanding of the behavior of this portable PEMFC stack. A three-dimensional (3D) computational fluid dynamics (CFD)-based methodology was used to predict such as the current and temperature distributions of this portable PEMFC stack. The results show how the baseline operation and original design of this stack impact the local temperature, water content, water transport, and kinetic variables inside the individual cells. The outcome of this work will pursue the development of universal heuristics and dimensionless numbers correlated to portable PEMFC stack design.     

可平行或独立分开运行的120kW双燃料电池动力系统研制

描述了一种可以平行或独立分开运行的120kW双燃料电池动力系统。这种大功率的双燃料电池动力系统可以用作城市大巴动力发动机或地面发电站。该系统包括两套集成式的燃料电池堆,为每套集成式燃料电池堆支持运行的空气输送子系统,氢气输送子系统,冷却流体循环子系统都实行了统一的一体化设计,以达到减少体积、减少质量,符合车载或固定式发电需要。     

Development and operation of a 150 W air-feed direct methanol fuel cell stack

A five-cell 150 W air-feed direct methanol fuel cell (DMFC) stack was demonstrated. The DMFC cells employed Nafion 117^® as a solid polymer electrolyte membrane and high surface area carbon supported Pt-Ru and Pt catalysts for methanol electrooxidation and oxygen reduction, respectively. Stainless steel-based stack housing and bipolar plates were utilized. Electrodes with a 225 cm² geometrical area were manufactured by a doctor-blade technique. An average power density of about 140 mW cm^–2 was obtained at 110 °C in the presence of 1 M methanol and 3 atm air feed. A small area graphite single cell (5 cm²) based on the same membrane electrode assembly (MEA) gave a power density of 180 mW cm^–2 under similar operating conditions. This difference is ascribed to the larger internal resistance of the stack and to non-homogeneous reactant distribution. A small loss of performance was observed at high current densities after one month of discontinuous stack operation.     

On efficiency of both single fuel cells and stacks I

A method to estimate the efficiency of a stack of several identical cells is described on the basis of the electrochemical behavior of a single cell. Efficiency of fuel cell stacks is defined by means of a combination of semi-empirical models of polarization curves and dimensionless variables such as reaction extent and utilization. The connection of flows among the cells is basically divided in two extreme cases and one intermediate case. Higher efficiencies are obtained when the same main flow (both anodic and cathodic) passes consecutively through the stack cells (Chain Flow), because it is favored thermodynamically. It is less favored when the main flow is strictly divided among all the cells (Separate Flow). In the intermediate case, the main flow is divided among all the stack cells and all the outlets are collected in one flow. The latter can spontaneously evolve to the more thermodynamically stable behavior of Chain Flow.     

Theoretical Model for the Optimal Design of Air Cooling Systems of Polymer Electrolyte Fuel Cells. Application to a High‐Temperature PEMFC

The cooling system of a high‐temperature PEM fuel cell with a nominal electric power of 1.5 kW for a combined heat and power unit (CHP) has been designed using a thermochemical model. The 1D model has been developed as a simple, predictive, and useful tool to evaluate, design, and optimize cooling systems of PEM fuel cells. As proved, it can also be used to analyze the influence of different operational and design parameters, such as the number and geometry of the channels, or the air flow rate, on the overall performance of the stack. To validate the model, predicted results have been compared with experimental measurements performed in a commercial 2 kW air‐forced open‐cathode stack. The model has then been applied to calculate the air flow required by the designed prototype stack as a function of the power output, as well as to analyze the influence of the cooling channels configuration (cross‐section geometry and number) on the heat management. Results have been used to select the optimum air‐fan cooling system, which is based on compact axial fans.     

PEM fuel cell current regulation by fuel feed control

We demonstrate that the power output from a PEM fuel cell can be directly regulated by limiting the hydrogen feed to the fuel cell. Regulation is accomplished by varying the internal resistance of the membrane-electrode assembly in a self-draining fuel cell with the effluents connected to water reservoirs. The fuel cell functionally operates as a dead-end design where no gas flows out of the cell and water is permitted to flow in and out of the gas flow channel. The variable water level in the flow channel regulates the internal resistance of the fuel cell. The hydrogen and oxygen (or air) feeds are set directly to stoichiometrically match the current, which then control the water level internal to the fuel cell. Standard PID feedback control of the reactant feeds has been incorporated to speed up the system response to changes in load. With dry feeds of hydrogen and oxygen, 100% hydrogen utilization is achieved with 130% stoichiometric feed on the oxygen. When air was substituted for oxygen, 100% hydrogen utilization was achieved with stoichiometric air feed. Current regulation is limited by the size of the fuel cell (which sets a minimum internal impedance), and the dynamic range of the mass flow controllers. This type of regulation could be beneficial for small fuel cell systems where recycling unreacted hydrogen may be impractical.     

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.