Current Status of and Recent Developments in the Direct Methanol Fuel Cell

The direct methanol fuel cell (DMFC) has been discussed recently as an interesting option for a fuel‐cell‐based mobile power supply system in the power range from a few watts to several hundred kilowatts. In contrast to the favoured hydrogen‐fed fuel cell systems (e.g. the polymer electrolyte membrane fuel cell, PEMFC), the DMFC has some significant advantages. It uses a fuel which is, compared to hydrogen, easy to handle and to distribute. It also comprises a fairly simple system design compared to systems utilising liquid fuels (like methanol) to produce hydrogen from them by steam reforming or partial oxidation to finally feed a standard PEMFC. Nevertheless, many severe problems still exist for the DMFC, hindering its competitiveness as an option to hydrogen‐fed fuel cells. This work reviews the major research activities concerned with the DMFC by highlighting the problems (slow kinetics of the anodic methanol oxidation, methanol permeation through the membrane, carbon dioxide evolution at the anode) and their possible solutions. Special attention is devoted to the steady state and dynamic simulation of these fuel cell systems.     

直接甲醇燃料电池技术及应用

本文回顾了直接甲醇燃料电池（ＤＭＦＣ）的研究开发历史,系统阐述了ＤＭＦＣ系统中电催化剂选择与设计基本原则、电解质膜材料与甲醇渗透的关系。分析了电池工作温度、工作压力和甲醇进料方式对ＤＭＦＣ电化学性能的影响。     

Operation Characteristic Analysis of a Direct Methanol Fuel Cell System Using the Methanol Sensor‐less Control Method

The application of methanol sensor‐less control in a direct methanol fuel cell (DMFC) system eliminates most of the problems encountered when using a methanol sensor and is one of the major solutions currently used in commercial DMFCs. This study focuses on analyzing the effect of the operating characteristics of a DMFC system on its performance under the methanol sensor‐less control as developed by Institute of Nuclear Energy Research (INER). Notably, the influence of the dispersion of the methanol injected on the behavior of the system is investigated systematically. In addition, the mechanism of the methanol sensor‐less control is investigated by varying factors such as the timing of the injection of methanol, the cathode flow rate, and the anode inlet temperature. These results not only provide insight into the mechanism of methanol sensor‐less control but can also aid in the improvement and application of DMFC systems in portable and low‐power transportation.     

Influence of Methanol Crossover on the Fuel Utilization of Passive Direct Methanol Fuel Cell

The effect of methanol crossover on the fuel utilization of a passive direct methanol fuel cell (DMFC) was reported. The results revealed that the Faradaic efficiency decreased from 46.9 to 17.4% when methanol concentration increased from 1.0 to 8.0 mol L^–1 at the lower current density 11.1 mA cm^–2. However, the Faradaic efficiency increased from 14.7 to 31.3% when methanol concentration increased from 1.0 to 8.0 mol L^–1 at a higher current density of 44.4 mA cm^–2. On the other hand, although the amount of methanol was increased, the Faradaic efficiency did not change, obviously due to the uniform methanol crossover and methanol diffusion at the same methanol concentration and constant current.     

Model for the Degradation Performance of a Single‐Cell Direct Methanol Fuel Cell under Varying Operational Conditions

The effect of varying operating parameters on the degradation of a single‐cell direct methanol fuel cell (DMFC) with serpentine flow channels was investigated. Fuel cell internal temperature, methanol concentration, and air and methanol flow rates were varied in experimental tests and fuel cell performance was chronologically recorded. A DMFC semi‐empirical performance model was developed to predict the polarization curves of the DMFC and validated at different operating conditions. Performance degradation was observed and modeled over time by a linear regression model. Unlike previous studies, the cumulative exposure of the operating factors to the fuel cell was considered in the degradation analysis. The degradation model shows the cell voltage generation capacity does not significantly degrade. However, the Tafel slope of the cell changes with cumulative exposure to methanol concentration and air flow, and the ohmic resistance changes with cumulative exposure to temperature, methanol and air flow.     

Effect of Porosity in Catalyst Layers on Direct Methanol Fuel Cell Performances

Effects of porosity of catalyst layers (CLs) on direct methanol fuel cell (DMFC) performances are investigated using silicon dioxide (SiO₂) particles as a pore former. The pore size and volume of CLs are controlled by changing the size and content of SiO₂. As the size of pore formed by removal of SiO₂ increases, DMFC performances are enhanced. The augmentation in performances can be explained by facilitation of fuel transport to catalyst particles, increase of utilization efficiency of catalysts, diminishment in methanol crossover, reduction in activation loss and facilitation of water discharging out of CLs of cathode due to the controlled porosity in CLs. The enhanced fuel transport, accessibility of fuels to Pt catalyst surface, is proved by the active areas of Pt catalyst. In addition to the active area of Pt catalyst, porous CLs exhibit a decline in methanol crossover, leading to increase of open circuit voltage (OCV). The porous CLs also show improvements in activation loss due to high porosity, causing enhancement in DMFC performances. In aspect of pore volume contribution to cathode performance, the SiO₂ content is optimized. Based on the DMFC performances, it can be suggested that the optimum conditions of SiO₂ are 500 nm in size and 20 wt.% in content. The porosity effect on both electrodes appears as follows: the pores in cathode are more effective on DMFC performances (55.5%) than those of anodes (44.5%) based on the maximum power of DMFC, indicating that the pores in CLs facilitate removal of water from electrodes.     

直接甲醇燃料电池质子交换膜研究进展

直接甲醇燃料电池(DMFC)以其燃料来源丰富、储存方便、结构简单、安全等优点而日益受到广泛的关注。预计将在很多领域中能得到广泛的应用。过去,人们对DMFC做了很多研究,针对直接甲醇燃料电池中的质子交换膜(PEM)的阻甲醇性能方面的研究进展作如下评述。     

Operating Temperature Dependency on Performance Degradation of Direct Methanol Fuel Cells

The effect of operating temperature on performance degradation of direct methanol fuel cell (DMFCs) is examined to disclose the main parameter of the degradation mechanism and the degradation pattern in the membrane electrode assemblies (MEAs). The DMFC MEA degradation phenomenon is explained through the use of various electrochemical/physicochemical tools, such as electrochemical impedance spectroscopy, electrode polarization, methanol stripping voltametry, field emission‐scanning electron microscopy, X‐ray diffraction, inductively coupled plasma‐atomic emission spectroscopy, and X‐ray photoelectron spectroscopy analysis. The operation of DMFC under high temperature accelerates the degradation process of the DMFC. The higher degradation rate under high temperature DMFC operation is mainly attributed to the formation of membrane pinhole with interfacial delamination and cathode degradation. A high operating temperature may result in more considerable thermal and mechanical stress of the polymeric membrane continuously due to frequent dry–wet cycling mode and substantial uneven distribution of water between the anode and the cathode during a long period of DMFC operation. On the other hand, the electrochemical surface area deterioration by Pt coarsening and ionomers loss is not directly related to the larger DMFC performance decay at high temperature.     

直接甲醇燃料电池溶胶-凝胶流动相的制备与表征

对直接甲醇燃料电池溶胶-凝胶流动相的制备工艺进行了分析,采用溶胶-凝胶法以正硅酸甲酯为前驱体制备出了溶胶-凝胶流动相.分别以溶胶-凝胶流动相和液体流动相为燃料对比研究了直接甲醇燃料电池的放电性能,测定了溶胶-凝胶流动相在Nation117膜中的甲醇渗透率,研究了溶胶-凝胶流动相的质子电导率.实验结果表明,使用溶胶-凝胶...     

直接甲醇燃料电池(DMFC)改性阳极催化剂的研究进展

介绍了国内外直接甲醇燃料电池（DMFC）的研究状况及其工作原理,阐述了DMFC阳极改性催化剂的作用机理,重点对目前国内外研究的各种改性催化剂体系进行了比较和评价,探讨了电催化剂的发展方向。     

A Sulfophenylated Polysulfone as the DMFC Electrolyte Membrane – an Evaluation of Methanol Permeability and Cell Performance

A sulfophenylated polysulfone (PSU‐sph), carrying 0.8 sulfonic acid units per repeating unit of the polymer, is evaluated as a membrane electrolyte for DMFC applications. The liquid uptake, methanol transport characteristics, electrolyte conductivity, and fuel cell performance are investigated. The methanol transport and DMFC performance results are compared to those of Nafion® 117. The PSU‐sph membrane investigated shows superior qualities with regard to methanol crossover, with a methanol permeability of approximately 25% compared to that of Nafion®. The conductivity was measured to be 15% compared to that of Nafion®. However, this could not fully account for the internal resistance of the cell, implying that the contact resistance between the electrodes and electrolyte is higher when PSU‐sph is used, probably because the electrodes are developed for use with Nafion® membranes. The stability of the PSU‐sph membrane seems promising, with very low degradation observed over a period of 72 hours. It was concluded that although the mass transport properties of the PSU‐sph membrane sample investigated were superior, it could not match the performance of Nafion® 117 in a DMFC application. However, a higher degree of sulfonation may have a significant positive effect on cell performance. The results also showed that a fully intergrated MEA is needed to fully assess new menbrane materials.     

Influence of Contamination with Inorganic Impurities on the Durability of a 1 kW DMFC System

IEK‐3 at Forschungszentrum Jülich successfully replaced the battery tray of a horizontal order picker (ECE 220, Jungheinrich) with a direct methanol fuel cell (DMFC) hybrid system in the kilowatt class. The DMFC combined with a lithium‐ion battery forms the hybrid drive for electrically driven horizontal order pickers. Such vehicles are used to transport and collect goods for orders in warehouses. The order picker is fueled by pure methanol to achieve a maximum range with the smallest possible space for the battery tray. An advantage of such energy systems is that there is no need for the relatively complicated and time‐consuming recharging of the conventional lead‐acid batteries, nor are spare batteries required for multishift operation. This study focuses on the influence of contamination with inorganic impurities on the durability of the 1 kW DMFC system V3.3‐1. The data from a long‐term test will be presented, in which the DMFC system was subjected to a realistic dynamic load profile for 3,000 h. The impurities identified in the post‐mortem analysis of the membrane electrode assembly will be outlined as well the influence of selected impurities, the sources of these impurities and how the DMFC V3.3‐2 system was modified on the basis of these findings.     

Ni-Cr-Co氧化物纳米催化剂对甲醇阳极氧化的电催化性能研究

采用了热分解法合成Ni-Cr-Co氧化物纳米粒子,并用作直接甲醇燃料电池(DMFC)的阳极电催化剂。通过X射线衍射(XRD)、透射电子显微镜(TEM)和扫描电子显微镜(SEM)对催化剂进行表征,制得的纳米催化剂均匀分散,且粒径为25~50 nm。利用循环伏安法(CV)对不同金属摩尔比和焙烧温度下的催化剂在甲醇氧化反应中的活性进行了研究。结果表明,Ni-Cr-Co(摩尔比为1∶1∶1.5)的纳米氧化物对甲醇氧化反应的起始电位、峰值氧化电流密度和If/Ib分别为0.38 V,19.3 mA/cm2和1.72,表现了很好的电催化性能。     

Development of a Self‐Adaptive Direct Methanol Fuel Cell Fed with 20 M Methanol

A passive and self‐adaptive direct methanol fuel cell (DMFC) directly fed with 20 M of methanol is developed for a high energy density of the cell. By using a polypropylene based pervaporation film, methanol is supplied into the DMFC's anode in vapor form. The mass transport of methanol from the cartridge to the anodic catalyst layer can be controlled by varying the open ratio of the anodic bipolar plate and by tuning the hydrophobicity of anodic diffusion layer. An effective back diffusion of water from the cathode to the anode through Nafion film is carried out by using an additive microporous layer in the cathode that consists of 50 wt.% Teflon and KB‐600 carbon. Accordingly, the water back diffusion not only ensures the water requirement for the methanol oxidation reaction but also reduces water accumulation in the cathode and then avoids serious water flooding, thus improving the adaptability of the passive DMFC. Based on the optimized DMFC structure, a passive DMFC fed with 20 M methanol exhibits a peak power density of 42 mW cm^–2 at 25 °C, and no obvious performance degradation after over 90 h continuous operation at a constant current density of 40 mA cm^–2.     

空气自呼吸式直接甲醇燃料电池传输特性数值研究

针对空气自呼吸式直接甲醇燃料电池（DMFC）建立了二维两相非等温的传质模型,通过自编程序模拟了电池内的传热、传质和电化学反应过程。并基于此模型研究了主要的操作参数,包括甲醇进口温度、环境中空气的温度、甲醇进口浓度,及对电池性能的影响。研究表明甲醇溶液的进口温度和环境温度上升都会使电池的性能得到提高,其中甲醇溶液的进口温度对电池性能的影响更大;当甲醇进口浓度提高后,在大电流密度下电池的性能得到了显著的提高。     

Toward Fully‐ and Semi‐passive Operation of a Liquid‐Fed Direct Methanol Fuel Cell

This work reports the performance characteristics of a liquid‐fed direct methanol fuel cell (DMFC) operated in both fully‐ and semi‐passive conditions. For the latter case, a blower is used to provide forced air convection at the cathode so as to reveal how and how much a passive DMFC suffers from its structural constraint and also the mass and heat transfer limitations. The results based on the fully passive operation suggest that the cell performance is greatly affected by the level of methanol concentration. In this study, 2 M performs the best when the cell uses different structural setups. Besides, the effects of ambient temperature and the cathode self‐heating mechanisms are also explored under a fully passive condition. For the semi‐passive operation, forced air convection is proved to be helpful in enhancing oxygen delivery but may lead to faster heat and water dissipation and thus significantly reduces the cell performance. An optimal blowing intensity is obtained when the blower operates at a half speed. When the cathode diffusion layer is removed, the effects of active air supply become weakened. Considering the limited performance improvement and parasitic losses caused by a blower, we believe the self‐breathing mode is still an attractive choice.     

Prediction of methanol and water fluxes through a direct methanol fuel cell polymer electrolyte membrane

Development of a direct methanol fuel cell (DMFC) mass flux model, using conventional transport theory, is presented and used to predict the fluid phase superficial velocity, methanol and water molar fluxes, and the chemical species (methanol and water) dimensionless concentration profiles in the polymer electrolyte membrane, Nafion^® 117, of a DMFC. Implementation of these equations is illustrated to generate the numerical data as functions of the variables such as the pressure difference across the membrane, methanol concentration at the cell anode, temperature, and position in the membrane.     

Effect of operating conditions on energy efficiency for a small passive direct methanol fuel cell

Energy conversion efficiency was studied in a direct methanol fuel cell (DMFC) with an air-breathing cathode using Nafion 117 as electrolyte membrane. The effect of operating conditions, such as methanol concentration, discharge voltage and temperature, on Faradic and energy conversion efficiencies was analyzed under constant voltage discharge with quantitative amount of fuel. Both of Faradic and energy conversion efficiencies decrease significantly with increasing methanol concentration and environmental temperature. The Faradic conversion efficiency can be as high as 94.8%, and the energy conversion efficiency can be as high as 23.9% if the environmental temperature is low enough (10 °C) under constant voltage discharge at 0.6 V with 3 M methanol for a DMFC bi-cell. Although higher temperature and higher methanol concentration can achieve higher discharge power, it will result in considerable losses of Faradic and energy conversion efficiencies for using Nafion electrolyte membrane. Development of alternative highly conductive membranes with significantly lower methanol crossover is necessary to avoid loss of Faradic conversion efficiency with temperature and with fuel concentration.     

Model-based analysis of the feasibility envelope for autonomous operation of a portable direct methanol fuel-cell system

Portable power systems based on direct methanol fuel cells (DMFC) have to provide power in various environmental conditions: it is advantageous for such a power-supply system to be autonomous, i.e. able to operate without water refills for the methanol solution. It is shown that system autonomy depends mainly on environmental humidity, condenser temperature and air excess ratio: this result is valid in general for any DMFC, as cell parameters have only a marginal role. The environmental conditions in which a portable DMFC system may be autonomous are considered, delineating a feasibility envelope. Two methods are proposed to extend this envelope: operating with a diluted methanol reservoir, which improves the autonomy of the system only marginally and at the cost of a large loss in energy density, and system pressurisation, which delivers a more significant improvement in autonomy properties, but at the cost of system efficiency and simplicity.     

Empirical Model Equations for the Direct Methanol Fuel Cell DMFCs

Empirical model equations, proposed for polymer electrolyte fuel cells, are used to predict the cell voltage vs. current density response of a liquid feed direct methanol fuel cell. The model equations are validated against experimental data for a small-scale fuel cell over a wide range of methanol concentration and temperatures. A new empirical equation is presented which is able to predict the voltage response of liquid feed direct methanol fuel cells over a wide range of operating conditions and even in the case of very low current densities caused by, for example, the use of dilute methanol solutions or low cell temperatures.