Water content distribution in a polymer electrolyte membrane for advanced fuel cell system with liquid water supply

To better understand the operation of a new fuel cell design, we used magnetic resonance imaging (MRI) to measure the water content distribution in a polymer electrolyte membrane under fuel cell operation with and without a supply of liquid water. The supply of liquid water to the membrane improved the cell performance by increasing the water content in the membrane and thus reducing the electrical resistance of the membrane. The study also showed that MRI is a promising method to investigate the distribution of water in the membrane of a fuel cell under operating conditions.     

Composites of Proton-Conducting Polymer Electrolyte Membrane in Direct Methanol Fuel Cells

Fuel cells are a replacement for the conventional energy resources. As early as 1839, William Grove discovered the basic operating principle of fuel cells by reversing water electrolysis to generate electricity from hydrogen and oxygen. A Direct Methanol Fuel Cell (DMFC) operates on liquid fuel, which is one of the exciting varieties of fuel cells. There are many problems with DMFCs such as the high cost of electrolyte membranes, high platinum loading, CO poisoning, fuel cross-over, and so on. In this review, research regarding the solution of these problems will be cited and discussed. The electrical performance (in respect to power density) of the composites for the Nafion® and other perfluorinated membranes in DMFC are evaluated. The effect of these modifications on various aspects, such as mobility of protons through the membrane, permeation of hydronium ions, and cross-over of methanol through the membrane leading to the negative potential, have previously been discussed. Therefore, the main focus of this review is on the effect of composites of Nafion® and non-fluorinated membranes on the DMFC performance.     

质子交换膜中的传质分析

质子交换膜燃料电池是最有应用前景的汽车动力替代系统。质子交换膜中的传质是质子交换膜燃料电池性能的控制因素之一。论文从宏观和微观角度分析了质子交换膜中的质子和水分的传递机理,分析了操作参数对质子在质子交换膜中传递的影响。研究发现:外载荷对质子和水分在质子交换膜中的传递有很大影响;（H₅O₂）⁺是水合质子的主要结构形式;通过（H₅O₂）⁺中氢氧键不断形成与断裂,电荷在质子交换膜中得以传递。研究结果对理解质子交换膜中的传质机理及其推广应用具有积极意义。     

具有可渗透阳极的自呼吸微流体燃料电池性能特性

微流体燃料电池去除了质子交换膜,避免了膜退化、水管理等问题,是微型燃料电池领域新的研究热点。本文构建了具有可渗透阳极和空气自呼吸阴极的微流体燃料电池,采用甲酸溶液作为燃料对其性能特性进行了实验研究。结果表明:具有可渗透阳极的自呼吸微流体燃料电池性能随燃料浓度或流量的增加先升高后下降,随电解液浓度的增加而升高;阳极侧反应产生的CO₂气泡对自呼吸微流体燃料电池的性能和燃料利用率的影响较大,适当提高燃料流量有利于气泡的排除。     

Membrane gas diffusion measurements with MRI

Gas transport across polymeric membranes is fundamental to many filtering and separation technologies. To elucidate transport mechanisms, and understand the behaviors of membrane materials, accurate measurement of transport properties is required. We report a new magnetic resonance imaging (MRI) methodology to measure membrane gas phase diffusion coefficients. The MRI challenges of low spin density and short gas phase relaxation times, especially for hydrogen gas, have been successfully overcome with a modified one-dimensional, single-point ramped imaging with T(1) enhancement, measurement. We have measured the diffusion coefficients of both hydrogen gas and sulfur-hexafluoride in a model polymeric membrane of potential interest as a gas separator in metal hydride batteries. The experimental apparatus is a modified one-dimensional diaphragm cell which permits measurement of the diffusion coefficient in experimental times of less than 1 min. The H(2) gas diffusion coefficient in the membrane was 0.54 +/- 0.01 mm(2)/s, while that of sulfur-hexafluoride was 0.14 +/- 0.01 mm(2)/s, at ambient conditions.     

Improved dimensional stability of Nafion membrane modified using a layer by layer self-assembly of biophilic polymers

The modification of perfluorinated proton exchange membranes (e.g. Nafion) utilizing a layer by layer (LbL) electrostatic assembly of oppositely charged biophilic polymers such as poly-l-lysine as positive layer and dsDNA as a negative layer is developed to protect the interface between the catalyst layer and the membrane in a low temperature fuel cell. Various physicochemical measurements including water uptake, proton conductivity and surface tension were investigated for both the as-received Nafion and the biopolymeric LbL modified Nafion. The smaller water contact angle and the less swelling characteristics of the biopolymer modified Nafion membrane was founded compared to that of as-received Nafion. This clearly indicates that the more hydrophilic surface of biopolymeric layers on Nafion plays an important role in the enhanced dimensional stability. In addition, a potential hypothesis explaining the higher proton conductivity from the LbL biopolymeric film coated Nafion is suggested.     

质子交换膜燃料电池阴极气场设计优化

通过建立质子交换膜燃料电池阴极气场的三维简化模型,研究了不同进气方式、不同进出气总管尺寸对电池堆流量分布的影响,并提出从单电池流道深度的角度优化气场分布的改进方案。结果表明,与单入口进气方式相比,双入口进气方式效果更优;采用2-2型进气方案时流量分布最为均匀,进气功耗最小;增大进气总管的宽度可以使电堆流量分布更加均匀;各单电池合理采用非均匀深度流道可以有效提高电池堆流量分布均匀性。     

直流道PEMFC两相流数学模型

建立直流道质子交换膜燃料电池(PEMFC)三维非等温两相流数学模型,基于质子交换膜与气体之间的水分传递特征,综合考虑电渗、浓度扩散及电化学反应作用的影响,发展了膜电极水分传递的非平衡扩散模型.并自主开发程序代码对电池内复杂的多物理场耦合传递过程进行数值模拟,研究PEMFC电极内气态水、液态水分布、质子膜含水量分布和水迁移特性等,分析单电池内部的温度分布特征,并获得电池极化性能曲线.     

Effect of Nafion dispersion solvent on the interfacial properties between the membrane and the electrode of a polymer electrolyte membrane-based fuel cell

A new Nafion binder solution was prepared using a different organic solvent, dimethylacetamide (DMAc), and applied to a polymer electrolyte membrane-based fuel cell. Wide angle X-ray diffraction (WAXD), electrochemical impedance spectroscopy (EIS), and polarization of the fuel cell were carried out to determine the crystallinity of the Nafion binder film, the cell resistance, and the fuel cell performance. This new Nafion binder film, which was created using a homemade Nafion solution containing DMAc, dissolved slower than a recast Nafion film that was made using a commercial Nafion solution in methanol (2 M). It was found that the slow dissolution of the homemade Nafion binder film was due to a more highly developed crystalline morphology, which can lead to good structural integrity in the catalyst layer for long-term operation of the fuel cell. The micellar structure of Nafion in the commercial Nafion binder solution is broken by new organic solvent, which leads to higher physical chain entanglement between the Nafion membrane and the Nafion binder during preparation of the membrane/electrode assembly (MEA), thereby improving the interfacial stability between the membrane and the electrode and providing long-term stability of the fuel cell.     

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.     

Direct methanol fuel cells : Methanol crossover and its influence on single DMFC performance

In the present investigation, the methanol crossover rate through Nafion®-115 membrane at different temperatures and different concentrations had been investigated in a fuel cell test apparatus by using gas chromatography analysis. The singledirect methanol fuel cell (DMFC) tests were carried out to investigate the effect of the concentration of methanol aqueous solutions and cell temperature on methanol crossover and consequently, on the open circuit voltage and the cell performance of DMFC. It can be found that the methanol crossover rate through Nafion® membrane increases as methanol concentration and temperature increase. It can also be found that methanol crossover presented a negative effect on the open circuit voltage and the single DMFC performance. Single DMFC test results showed that an improved cell performance was obtained as temperature increased although the methanol crossover rate increased with temperature increment.     

Effects of flow field design on water management and reactant distribution in PEMFC: a review

The performance of a proton exchange membrane fuel cell (PEMFC) stack is affected by many factors, including the operating conditions, flow field and manifold design, and membrane performance. To achieve the desired PEMFC performance, the reactant must be uniformly distributed and effectively diffused into the catalyst layer for the electrochemical reaction. Water management and reactant distribution in fuel cells are crucial because they affect the distribution and diffusion rate of the reactant. This paper reviews the important research results reported in recent years related to the effects of water and reactant distribution on the performance and life span of PEMFC stacks.     

Solid hydrocarbon conversion in a fuel cell with molten carbonate electrolyte

A new architecture of a molten carbonate electrolyte fuel cell is considered. The architecture ensures efficient implementing the processes of direct electrochemical solid hydrocarbon oxidation. Experimental results demonstrate the possibility of achieving high specific characteristics of the fuel cell (current density and specific power values) by using graphite, various types of coal, and plastic as the dispersive fuel. The effect of the organic fuel composition on the energy parameters of the electrochemical cell is illustrated. It is shown that the fuel oxidation rate and the achievable maximal values of the specific power increase with the relative amount of hydrogen. It is concluded that application of the proposed ideology is promising for creating real energy devices with direct electrochemical oxidation of solid hydrocarbons.     

Polyelectrolyte Nanocomposite Membranes Using Surface Modified Nanosilica for Fuel Cell Applications

The preparation and characterization of a new type of nanocomposite polyelectrolyte membrane (PEM), based on Nafion® (E. I. du Pont de Nemours and Co., Ltd., for its copolymer of tetrafluoroethylene and perfluorinated vinyl ether) and sulfonic acid (-SO₃H) or phosphotungstic acid (PWA) modified nanosilica (Si-SO₃H or Si-PWA, respectively), for direct methanol fuel cell (DMFC) applications are described. Physical characteristics of these manufactured nanocomposite membranes were investigated by scanning electron microscopy (SEM), water uptake, methanol permeability and ion exchange capacity, as well as proton conductivity. The Nafion®/Si-PWA and Nafion®/Si-SO₃H membranes showed higher proton conductivity, lower methanol permeability and, as a consequence, a higher selectivity parameter, in comparison to the neat Nafion® or Nafion®/pristine nanosilica membranes. The obtained results indicated that both the Nafion®/Si-PWA and Nafion®/Si-SO₃H membranes could be utilized as promising polyelectrolyte membranes for direct methanol fuel cell applications.     

仪器分析技术在直接甲醇燃料电池研究中的应用

介绍了仪器分析技术在直接甲醇燃料电池电催化剂及其载体结构,电池反应动力学、产物分析、固体聚合物电解质膜以及电池性能评价等各方面的应用及研究进展。     

不同流场下进气对质子交换膜燃料电池性能的影响

流场的结构对于质子交换膜燃料电池(PEMFC)的水管理和气体的传递具有十分重要的影响,相关研究一直是燃料电池的研究热点与重点。本文以纯氧气和空气作为阴极氧化剂,通过电池的性能测试、极化曲线和电化学阻抗分析等原位实验,分析了气体的流动与传输、不同流场下的电流密度、入口反应气体浓度等条件对电池性能的影响。实验结果表明,提高氧气浓度可以获得更好的质子交换膜燃料电池性能和最小化活化损失,纯氧气、波状流场的使用效果随进气量的变化而有明显的变化。     

Perfluorinated surfactants as model charged systems for understanding the effect of confinement on proton transport and water mobility in fuel cell membranes. A study by QENS

We have investigated the dynamical properties of water confined in mesomorphous phases of perfluorinated sulfonic surfactants. These systems mimic the physico-chemical properties of the perfluorinated Nafion membranes which are used as electrolyte in fuel cells. As the surfactants offer the advantage to self-assemble in well defined organized phases (such as hexagonal and lamellar phases), they could be used as model charged systems to understand the structure-transport relationship in complex real materials. Indeed, the geometry as well as the typical confinement size can be easily controlled and tuned through water concentration and temperature. A QENS study of hexagonal and lamellar phases has been performed on both time-of-flight and backscattering spectrometers to cover a dynamic range from picoseconds to nanoseconds. Analysis of the data with localized translational diffusion models shows the existence of a strong confinement effect that depends on the geometry. Typical confinement sizes and diffusion coefficients can be extracted from the QENS analysis and compared to the Nafion membrane.     

Solid-oxide fuel cells with hydrocarbon fuels

Solid-oxide fuel cells can directly use hydrocarbon or hydrocarbon-derived fuels. Conversion efficiencies can be considerably greater than those of heat engines, with hybrid cycles in combination with heat engines and co-generation promising conversion efficiencies as high as 70%. This paper discusses the fundamental concepts of fuel cells, concentrating on the underlying chemical and electrochemical processes. Fully understanding fuel cell function requires attention to physical and chemical processes that span length scales ranging from atomistic to meter-scale systems. Beyond the electrochemistry that is responsible for electrical energy production, fuel cell function relies on chemically reacting flow, porous-media transport, and heterogeneous thermal chemistry. Especially with hydrocarbon and hydrocarbon-derived fuels, there are interesting scientific and engineering connections, and analogies with combustion science and technology.     

EXPERIMENTAL INVESTIGATION OF THE PERFORMANCE OF A SINGLE PROTON EXCHANGE MEMBRANE FUEL CELL USING DRY FUEL

Recently, the requirements for cellular phones, portable computers, and digital cameras have increased dramatically. A portable electric power supply with long duration and high performance is needed for these products. A proton exchange membrane fuel cell (PEMFC) can meet these requirements and becomes one of the best candidates for a portable power source. It is impossible to install an extra humidifier into small-scale portable electric products for PEMFC water management. This article presents a series of experiments to investigate the performance of a single PEMFC. The effects of different operating conditions on cell performance, including the temperature, pressure, and inlet fuel/oxidant flow rate, are discussed. The test results confirm the positive effect of these parameters on cell performance and power output. The interaction effect of temperature--flow rate is related to the cell humidity, and is important for cell performance. The dry-out problem for a PEMFC is also significantly revealed in the experiments for higher cell temperature and flow rate. Current experimental results can provide useful information for investigating the cell performance and its operating effects under dry fuel/oxidant flow conditions and as a benchmark for simulation work in future studies.     

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.