质子交换膜燃料电池及其发电系统模拟

本文通过质子交换膜燃料电池单电池子模型、布气管子模型、流场子模型相互耦合,对电池堆传热传质过程加以数值模拟,得到单电池及电池堆的流场、温场、当地电流密度分布、过电位分布;给定平均电流密度下单电池及电池堆输出电压等参数;并比较了平均电流密度、布气管尺寸对电池输出电压、堆内反应物分配的影响;在此基础上,对整个ＰＥＭＦＣ发电系统进行流程模拟和参数分析,得到平均电流密度、重整器Ｓ／Ｃ等主要参数对系统性能影响．     

Fuel quality and operational issues for polymer electrolyte membrane (PEM) fuel cells

Polymer electrolyte membrane (PEM) fuel cells are susceptible to degradation due to the catalyst poisoning caused by CO present in the fuel above certain limits. Although the amount of CO in the fuel may be within the permissible limit, the fuel composition (% CO₂, CH₄, CO and H₂O) and the operating conditions of the cell (level of gas humidification, cell temperature and pressure) can be such that the equilibrium CO content inside the cell may exceed the permissible limit leading to a degradation of the fuel cell performance. In this study, 50 cm² active area PEM fuel cells were operated at 55–60 °C for periods up to 250 hours to study the effect of methane, carbon dioxide and water in the hydrogen fuel mix on the cell performance (stability of voltage and power output). Furthermore, the stability of fuel cells was also studied during operation of cells in a cyclic dead end / flow through configuration, both with and without the presence of carbon dioxide in the hydrogen stream. The presence of methane up to 10% in the hydrogen stream showed a negligible degradation in the cell performance. The presence of carbon dioxide in the hydrogen stream even at 1–2% level was found to degrade the cell performance. However, this degradation was found to disappear by bleeding only about 0.2% oxygen into the fuel stream.     

A novel design and micro-fabrication for copper (Cu) electroforming bipolar plates

A novel design and micro-fabrication were developed for micro-proton exchange membrane (PEM) fuel cell stack bipolar plates with cross section of 5 cm² and thickness of about 650 μm. Copper metals were used to make bipolar plates (BPs) by using a LIGA-like micro-fabrication process of deep UV lithography in order to obtain SU-8 resist patterns/and SU-8 mould. Through two-sided exposure and development, copper metal bipolar plates with serpentine (meandering) flow field configurations for a micro PEM fuel cell stack, were fabricated and performance tests through polarization characteristics were conducted; this made it possible for the first time to consider copper sheets as suitable for BPs in micro PEM fuel cell stacks.     

Polymer electrolyte membrane fuel cell as a hydrogen flow rate monitoring device

There is a substantial demand for hydrogen flow rate monitoring devices in applications where hydrogen is utilised or produced and which include oil refineries, ammonia production, food and chemical industries, coal gasification, methane steam reforming and water electrolysis. In this paper, a polymer electrolyte membrane (PEM) fuel cell has been demonstrated for measuring accurate flow rates of hydrogen or hydrogen-containing gases. The concept involves applying a constant voltage to a PEM fuel cell to oxidise the entire hydrogen supplied to the anode compartment of the fuel cell and observing limiting current values attained and relating these to the hydrogen flow rates. PEM fuel cells with an active area of 50 cm² were constructed and used to accurately measure the flow rates of hydrogen up to 170 mL/min. A device with a capability to monitor significantly higher hydrogen flow rates can be constructed by using several cells in a stacking arrangement or by using electrically isolated cells in a single device. The paper discusses advantages and limitations of the technique and the flow rate-measuring response for gases containing 5–100 % hydrogen. The response time for hydrogen gas was of the order of 1–2 min. However, the fuel cell flow field design can be optimised for faster response times.     

Studies on modified anode polymer hydrogen sensor

An improved polymer electrolyte membrane (PEM) fuel cell based amperometric hydrogen sensor that operates at room temperature has been developed. The electrolyte used in the sensor is PVA/H₃PO₄ blend, which is a proton conducting solid polymer electrolyte. A blend of palladium and platinum coated on the membrane is used as anode and platinum as cathode. The sensor functions as a fuel cell, H₂/Pd-Pt//PVA-H₃PO₄//Pt/O₂, and the short circuit current is found to be linearly related to the hydrogen concentration. The present study aims at investigating the dependence of sensor behaviour on the anode composition. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Challenges in hydrogen storage

Hydrogen is one possible medium for energy storage and transportation in an era beyond oil. Hydrogen appears to be especially promising in connection with electricity generation in polymer electrolyte membrane (PEM) fuel cells in cars. However, before such technologies can be implemented on a larger scale, satisfactory solutions for on-board storage of hydrogen are required. This is a difficult task due to the low volumetric and gravimetric storage density on a systems level which can be achieved so far. Possibilities include cryogenic storage as liquid hydrogen, high pressure storage at 70 MPa, (cryo)adsorptive storage, or various chemical methods of binding and releasing hydrogen. This survey discusses the different options and the associated advantages and disadvantages.     

Hydrogen and oxygen generation with polymer electrolyte membrane (PEM)-based electrolytic technology

With dwindling liquid fuel resources, hydrogen offers a credible alternative. The use of hydrogen in a fuel cell offers the highest fuel conversion efficiency compared with all other technologies and it also has the potential to substantially reduce greenhouse gas and particulate emissions at least at the end-user sites. One of the major barriers to the introduction of the hydrogen economy and its wider acceptance is the lack of the rather costly hydrogen generation, transportation and distribution infrastructure to meet the local transport fuel demands. On-site or distributed hydrogen generation would remove the need for this up-front infrastructure requirements and assist with the early large-scale trials of the fuel cell technology for both transport and stationary applications and also introduction of the hydrogen economy. In this paper, the development of polymer electrolyte membrane electrolysis technology for on-site, on-demand hydrogen generation has been discussed. The major emphasis is given on reducing catalyst cost; interface design and modifications; interconnect materials, design and fabrication; and investigation of the sources of degradation. Stacks to 2 kW_{H 2} capacity have been constructed and tested and show initial efficiencies of >87% at 1 A cm⁻².     

Influence of methanol impurity in hydrogen on PEMFC performance

Proton exchange membrane fuel cells [PEMFC] have become highly attractive for stationary as well as mobile energy applications due to their good efficiency compact cell design and zero emissions. PEM fuel cells mainly consist of anode and cathode containing platinum/platinum alloy electrocatalysts and Nafion membrane as the electrolyte. They operate on hydrogen fuel, which is generally produced by reforming of hydrocarbons, alcohols such as methanol and may contain large amounts of impurities such as methanol, carbon dioxide, trace amounts of carbon monoxide, etc. The studies on the effect of methanol impurity in hydrogen on fuel cell performance and methods of mitigation of poisoning are very important for the commercialization of fuel cells and are described in a limited number of papers only. In this paper, we present the studies on the influence of methanol impurity in hydrogen for the PEM fuel cells. The effect of various parameters such as methanol concentration, cell voltage, current density, exposure time, reversibility, operating temperature, etc. on the cell performances was investigated using pure hydrogen. Various methods of methanol poisoning mitigation were also investigated.     

Effect of nitrogen and carbon dioxide as fuel impurities on PEM fuel cell performances

Polymer electrolyte membrane (PEM) fuel cells are considered to have the highest power density of all the fuel cells. They operate on hydrogen fuel, which is generally produced by reforming of hydrocarbons, and may contain large amounts of impurities such as carbon dioxide, nitrogen, and trace amounts of carbon monoxide. We studied the effect of dilution of hydrogen gas with carbon dioxide on PEM fuel cells by polarization studies. The polarization curves were different when hydrogen gas was diluted with same quantities of carbon dioxide and with nitrogen. It may be due to carbon monoxide formation by reverse shift reaction and poisoning of anode platinum catalyst. Use of Pt–Ru alloy catalyst was found to suppress the poisoning. The effects of hydrogen gas composition, temperature, current density, and anode catalyst on fuel cell performances were examined in this study.     

Hydrogen storage using carbon adsorbents: past, present and future

Interest in hydrogen as a fuel has grown dramatically since 1990, and many advances in hydrogen production and utilization technologies have been made. However, hydrogen storage technologies must be significantly advanced if a hydrogen based energy system, particularly in the transportation sector, is to be established. Hydrogen can be made available on-board vehicles in containers of compressed or liquefied H₂, in metal hydrides, via chemical storage or by gas-on-solid adsorption. Although each method possesses desirable characteristics, no approach satisfies all of the efficiency, size, weight, cost and safety requirements for transportation or utility use. Gas-on-solid adsorption is an inherently safe and potentially high energy density hydrogen storage method that could be extremely energy efficient. Consequently, the hydrogen storage properties of high surface area “activated” carbons have been extensively studied. However, activated carbons are ineffective in storing hydrogen because only a small fraction of the pores in the typically wide pore-size distribution are small enough to interact strongly with hydrogen molecules at room temperatures and moderate pressures. Recently, many new carbon nanostructured absorbents have been produced including graphite nanofibers and carbon multi-wall and single-wall nanotubes. The following review provides a brief history of the hydrogen adsorption studies on activated carbons and comments on the recent experimental and theoretical investigations of the hydrogen adsorption properties of the new nanostructured carbon materials. Received: 16 October 2000 / Accepted: 15 November 2000 / Published online: 9 February 2001     

固体氧化物燃料电池堆多物理场耦合模拟的高效电化学-传质耦合算法

A multiphysics model for a production scale planar solid oxide fuel cell (SOFC) stack is important for the SOFC technology, but usually requires an unpractical amount of computing resource. The major cause for the huge computing resource requirement is identified as the need to solve the cathode O₂ transport and the associated electrochemistry. To overcome the technical obstacle, an analytical model for solving the O₂ transport and its coupling with the electrochemistry is derived. The analytical model is used to greatly reduce the numerical mesh complexity of a multiphysics model. Numerical test shows that the analytical approximation is highly accurate and stable. A multiphysics numerical modeling tool taking advantage of the analytical solution is then developed through Fluent®. The numerical efficiency and stability of this modeling tool are further demonstrated by simulating a 30-cell stack with a production scale cell size. Detailed information about the stack performance is revealed and brie y discussed. The multiphysics modeling tool can be used to guide the stack design and select the operating parameters.     

SOFC characteristics along the flow path

Solid oxide fuel cells (SOFCs) in metallic housings were integrally and locally characterised. The tests were performed in counter flow operation for hydrogen concentrations from 2% to 100%, to identify concentration limitations and to optimise fuel utilisation. Cell characterisations were performed by spatially resolved electrochemical impedance spectroscopy (EIS), current density/voltage (i–V) and temperature measurements as well as gas chromatography measurements at 16 distinct points across the cell. The results show a substantial variation of current density and voltage distribution along the flow path with varying hydrogen content and fuel utilisation. The fuel utilisation was calculated from the local current densities and compared to the values measured by gas chromatography. Both sets of results showed good agreement. At low hydrogen inlet concentrations the voltage at the fuel outlet drops to values that might be harmful for the stability of the anode since reoxidation of nickel can occur. The impedances obtained by local EIS did not show an overall coherent dependency on the hydrogen concentration. EIS under load revealed two distinct domains: in the range of hydrogen concentrations of 2–10% H₂ the impedance decreased significantly with increasing hydrogen content whereas at higher hydrogen contents the impedance was hardly affected. This indicates significant concentration and diffusion overpotential at low hydrogen concentrations. The local data showed differing behaviour in the middle of the cell compared to the fuel outlet. Leakage at the sealing could be identified as a possible reason. As an additional method of investigation, the voltage drop over the contact resistance of the cathode side was measured. Temperature measurements show that local temperatures differ significantly depending on the load applied to the cell. This observation emphasizes the importance of a thermal management adapted to the characteristics on operation conditions of the cells, particularly when the stack itself has only a low mass.     

操作参数对PEM燃料电池中水迁移的影响

质子膜内水分和阴极多孔电极中液态水含量是PEM燃料电池正常运行的控制因素。本文给出了一个用于研究PEM燃料电池内水迁移的稳态、等温、两相流模型。模型耦合了连续方程、动量守恒方程和物质守恒方程,以及水在质子膜中传递方程。运用试验结果验证了模型的有效性。分析模拟结果表明,增大系统操作压力、升高电池操作温度和降低加湿温度将会使质子膜中水的净迁移通量增大;增大操作压力、降低操作温度和升高加湿温度会增加阴极CTL与GDL界面上液态水含量。     

Effect of gas diffusion layer and membrane properties in an annular proton exchange membrane fuel cell

A complete three-dimensional and single phase computational dynamics model for annular proton exchange membrane (PEM) fuel cell is used to investigate the effect of changing gas diffusion layer and membrane properties on the performances, current density and gas concentration. The proposed model is a full cell model, which includes all the parts of the PEM fuel cell, flow channels, gas diffusion electrodes, catalyst layers and the membrane. Coupled transport and electrochemical kinetics equations are solved in a single domain; therefore no interfacial boundary condition is required at the internal boundaries between cell components. This computational fluid dynamics code is used as the direct problem solver, which is used to simulate the two-dimensional mass, momentum and species transport phenomena as well as the electron- and proton-transfer process taking place in a PEMFC that cannot be investigated experimentally. The results show that by increasing the thickness and decreasing the porosity of GDL the performance of the cell enhances that it is different with planner PEM fuel cell. Also the results show that by decreasing the thickness of the membrane the performance of the cell increases.     

质子交换膜燃料电池电流分布的动态响应

动态特性是理解质子交换膜燃料电池性能的重要参数之一.运用燃料电池测试系统、恒电流/恒电压多通道测试仪和燃料电池电流密度分布测试装置,试验测量了质子交换膜燃料电池在不同加湿温度、电池温度和压力下的电流分布动态响应和动态特性.研究发现:不同区域的局部电流达到新的平衡所需的时间不同;加湿温度变化时,不同区域的局部电流的变化趋...     

流道结构和几何尺寸对燃料电池性能影响的实验研究

流场板是质子交换膜燃料电池重要部件之一。本文对以氢气和氧气作为反应气体的质子交换膜电池的极化曲线进行了实验测定,研究了不同流场板结构、流场板深度和宽度对电池性能的影响．研究发现采用组合流道的电池性能最佳．     

Characterisation of wettability in gas diffusion layer in proton exchange membrane fuel cells

This paper presents a detailed study of the wetting properties of three fuel cell gas diffusion layers (GDLs) having different morphologies and different contents of hydrophobic agent. An internal contact angle to water at temperature representative of PEM fuel cell operating conditions was directly obtained using the Washburn method with 66 ° C water as test liquid. These results show that the surface of the carbon fibres is hydrophilic under fuel cell operating conditions. Surface energy values of the GDL fibres, determined by a combination of the Washburn method and the Owens-Wendt two parameters theory, were found to be low, indicating that GDLs are wet very poorly by most liquids. About 40% of the total surface free energy values was related to a polar component allowing dipole-dipole and hydrogen bonding interactions with water. The origin of this polar character is discussed.     

质子交换膜燃料电池内传递现象的数值模拟

本文发展了一个质子交换膜燃料电池的三维数学模型,用于研究整个电池内的传递现象,模型全面考虑了流体 流动、热量传递、电化学动力学和多组分传递等物理化学过程。数值求解数学模型获得了电池内详细的温度、反应物浓度 和电流等的空间分布情况。为验证模型的正确性,将估算的电池性能与文献中的实验数据进行了比较。     

基于可逆固体氧化物电池堆的电能高效存储

研究了固体氧化物电池堆中电能的可逆存储与产生,通过相变金属存储燃料电池模式下的热能并在电解池模式下加以利用. 系统的荷电状态(即氢燃料百分比)可显著增加开路电压,系统压力的增加有效提高了开路电压. 较高的系统压力可促进电极表面的物质扩散和输运,相应地改善电极的极化电阻. 通过有效的热能管理,系统的电能可逆存储的循环效率可高达92%.     

Development of catalytically enhanced sodium aluminum hydride as a hydrogen-storage material

The dehydriding of sodium aluminum hydride, NaAlH₄, is kinetically enhanced and rendered reversible in the solid state upon doping with selected titanium compounds. Following the initial reports of this catalytic effect, further kinetic improvement and stabilization of the cyclable hydrogen capacity have been achieved upon variation in the method of the introduction of titanium and particle-size reduction. Rapid evolution of 4.0-wt % hydrogen at 100 °C has been consistently achieved for several dehydriding/rehydriding cycles. An improved, 4.8-wt % cyclable capacity has been observed in the material doped with a combination of Ti and Zr alkoxide complexes. Doping the hydride with Ti(OBuⁿ)₄ and Fe(OEt)₂ also produces a synergistic effect, resulting in materials that can be rehydrided to 4 wt % at 104 °C and 87 atm of hydrogen within 17 h. The improved kinetics allowed us to carry out constant-temperature, equilibrium-pressure studies of NaAlH₄ that extended to temperatures well below the melting point of the hydride. The 37-kJ/mol value determined for enthalpy of the dehydriding of NaAlH₄(s) to Na₃AlH₆ and Al and the hydrogen plateau pressure of 7 atm at 80 °C are in line with the predictions of earlier studies. The nature of the active catalyst and the mechanism of catalytic action are unknown. The catalytically enhanced hydrides appear to be strong candidates for development as hydrogen carriers for onboard proton exchange membran (PEM) fuel cells. However, further research and development in the areas of rehydriding catalysts, large-scale, long-term cycling, safety and adjustment of the plateau hydrogen pressure associated with dehydriding of AlH₆ ^- are required before these materials can be utilized in commercial onboard hydrogen-storage systems. Received: 7 September 2000 / Accepted: 14 November 2000 / Published online: 9 February 2001