Hydrodynamic modelling of direct methanol liquid feed fuel cell stacks

A model for the liquid feed, direct methanol fuel cell (DMFC), based on the homogeneous two-phase flow theory and mass conservation equation, which describes the hydraulic behaviour of internally manifolded cell stacks, is presented. The model predicts the pressure drop behaviour of the anode side of an individual DMFC cell and is used to determine the channel depth and width for fast and efficient carbon dioxide removal with minimum pressure drop. The model is used to calculate flow distribution through fuel cell stack internal manifolds. The effect of inlet and outlet manifold diameters on flow distribution is also determined. Two types of manifold design are compared, reverse flow and parallel flow. An iterative numerical scheme is used to solve the differential equations for longitudinal momentum and continuity.     

Gas evolution and power performance in direct methanol fuel cells

The use of acrylic cells and a CCTV camera for visually investigating the carbon dioxide gas evolution process inside an operating direct methanol fuel cell environment is demonstrated. Also, the effect of operating parameters on the system gas management, using a series of tests with different gas diffusion layer supporting materials, flow bed designs, cell sizes and exhaust manifold configurations, is studied. Carbon dioxide gas management is an important issue obstructing progress in viable direct methanol fuel cell systems development. Gas evolution mechanisms and gas management techniques are discussed and analysed with reference to several video picture and performance data. The data demonstrate that Toray carbon paper is not a suitable material for DMFCs due to its poor gas removal properties. A type carbon cloth shows relatively good gas removal behaviour. Increasing the liquid phase inlet flow rate is beneficial for gas removal. Increasing the current density results in higher gas production and in the formation of gas slugs, especially at low flow rates, which can lead to blocking of the channels and hence deterioration in the cell performance. A new flow bed design, based on a heat exchanger concept, is affective for gas management and gives a more uniform flow distribution in the flow bed channels. Using the results of this study, and the modelling techniques developed by our group, will are able to determine suitable operating conditions for our prototype 0.5kW cell DMFC stack.     

Portable Size DMFC‐Stack

A small, low temperature, direct methanol fuel cell stack for portable applications has been developed. Several flow field designs were investigated with respect to stable operation and high performance. Due to carbon dioxide and water production on the anode and cathode, respectively, methanol and oxygen access to the electrodes is hindered. During single cell operation the effect of both carbon dioxide evolution and water production on the current output was observed. The difference between parallel and serial feeding of both fuel and oxidant to the DMFC stack was also investigated. It was found that it is very important to remove reaction products from the active cell surface in order to ensure stable stack operation at low temperatures. The maximal power realised with the 12‐cell direct methanol fuel cell stack was 30 W.     

The Design and Usage of a Visual Direct Methanol Fuel Cell

In order to better understand the influence of gas evolution on the performance of the direct methanol fuel cell (DMFC) anode, a visual DMFC, comprising of a transparent anode and a cathode endplate with an integrated heat exchanger, and a picture analysis methodology were developed. The result was an inexpensive, but very powerful, tool for analyzing the role of two-phase flow. An important finding is that gas bubbles do not appear uniformly throughout the fluid flow matrix, but rather only at a few active sites. Another important finding is that the gas saturation (volume fraction of gas/volume fraction of liquid) increases along the streamwise direction.     

CO_2气泡脱离扩散层孔口过程的数值模拟

在直接甲醇燃料电池(DMFC)中,阳极催化层表面反应生成的CO2气体通过扩散层,及时排出阳极通道,对提高DMFC电流密度具有重要意义,因此研究气泡脱离孔口的过程很有益。今采用Fluent6.2.16对CO2气泡脱离扩散层孔口过程、两孔时气泡形成及聚并过程进行了数值模拟,考察了阳极通道内液体流速、扩散层孔道直径等因素对气泡脱离的影响。结果表明,阳极通道内液体流速越大,气泡脱离扩散层孔口所需的时间越短;扩散层孔道直径越大,气泡脱离扩散层孔口所需的时间越长,且生成的气泡越大;由于从相邻两扩散层孔道出来的气泡的阻挡和挤压作用,使得两气泡周围的压力分布与单气泡不同,气泡脱离过程与从单个扩散层孔口的脱离过程有所不同,脱离时间更早。     

A two-dimensional, two-phase mass transport model for liquid-feed DMFCs

A two-dimensional, isothermal two-phase mass transport model for a liquid-feed direct methanol fuel cell (DMFC) is presented in this paper. The two-phase mass transport in the anode and cathode porous regions is formulated based on the classical multiphase flow in porous media without invoking the assumption of constant gas pressure in the unsaturated porous medium flow theory. The two-phase flow behavior in the anode flow channel is modeled by utilizing the drift-flux model, while in the cathode flow channel the homogeneous mist-flow model is used. In addition, a micro-agglomerate model is developed for the cathode catalyst layer. The model also accounts for the effects of both methanol and water crossover through the membrane. The comprehensive model formed by integrating those in the different regions is solved numerically using a home-written computer code and validated against the experimental data in the literature. The model is then used to investigate the effects of various operating and structural parameters, such as methanol concentration, anode flow rate, porosities of both anode and cathode electrodes, the rate of methanol crossover, and the agglomerate size, on cell performance.     

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。     

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.     

Material aspects of the liquid feed direct methanol fuel cell

A study of a small scale Liquid Feed Direct Methanol Fuel Cell (LFDMFC), based on solid polymer electrolyte membrane, is reported. Two flow cell designs, one with a parallel flow channel arrangement and the other with a spot design of flow bed, are used. The structure of the DMFC comprises a composite of two porous electrocatalytic electrodes; Pt–Ru–carbon catalyst anode and Pt–carbon catalyst cathode, on either side of a solid polymer electrolyte (SPE) membrane. The performance of three Pt–Ru catalysts is compared. The influence of the degree of Teflon loading on the electrode structure is also reported. The effect of the following parameters: cell temperature, oxygen gas or air pressure, methanol liquid flow rate and methanol concentration on the power performance is described.     

A Pumpless Methanol Feeding Method for Application in Direct Methanol Fuel Cell Systems

This paper presents a simple and reliable pumpless methanol feeding (PLMF) method for application in direct methanol fuel cell (DMFC) systems. The primary feature and advantage of the PLMF is as follows: it employs an approach that allows the cathode gas pressure to be connected with a fuel container for supplying the methanol fuel into the anode fuel loop, instead of using any feeding pump or other specially designed apparatuses. The PLMF has been used in a portable 25 W DMFC system and realised feeding methanol in real time for meeting the requirements of the system. The PLMF method not only is suitable for the DMFC system, but also can be used in other liquid‐feeding fuel cell systems.     

Three-dimensional two-phase mass transport model for direct methanol fuel cells

A three-dimensional (3D) steady-state model for liquid feed direct methanol fuel cells (DMFC) is presented in this paper. This 3D mass transport model is formed by integrating five sub-models, including a modified drift-flux model for the anode flow field, a two-phase mass transport model for the porous anode, a single-phase model for the polymer electrolyte membrane, a two-phase mass transport model for the porous cathode, and a homogeneous mist-flow model for the cathode flow field. The two-phase mass transport models take account the effect of non-equilibrium evaporation/ condensation at the gas-liquid interface. A 3D computer code is then developed based on the integrated model. After being validated against the experimental data reported in the literature, the code was used to investigate numerically transport behaviors at the DMFC anode and their effects on cell performance.     

Performance Tests and Pressure Drop Measurements in the Anode Flowfield of a μDMFC

Cell performance tests and measurements of the pressure drops in the anode flow channels of a custom‐made microdirect methanol fuel cell (μDMFC) are conducted and studied for different methanol concentrations (0.5–2 M), flow rates (10–20 sccm) and operating temperatures (40–80 °C). The anode flowfields consist of three channel/four pass flow channels with widths of 500–2000 μm and a total length of 300–400 mm. Moreover, flow characteristics of the CO₂ gas bubbles and methanol solution in the anode flow channels are identified and analysed for CO₂ fraction through visualisation. Finally, an optimal channel size for the present μDMFC is obtained.     

直接甲醇燃料电池

介绍了直接甲醇燃料电池的原理、结构,并与发展较早的氢气燃料电池进行优劣比较。针对近期商业化便携式燃料电池的技术指标,主要讨论了直接甲醇燃料电池在性能和成本上的现状和问题,并着重阐述了阳极催化剂和电解质膜(决定其性能的两个关键因素)的研发进展。     

直接甲醇燃料电池

介绍了直接甲醇燃料电池的原理、结构,并与发展较早的氢气燃料电池进行优劣比较。针对近期商业化便携式燃料电池的技术指标,主要讨论了直接甲醇燃料电池在性能和成本上的现状和问题,并着重阐述了阳极催化剂和电解质膜（决定其性能的两个关键因素）的研发进展。     

直接甲醇燃料电池阳极催化剂的制备

直接甲醇燃料电池（DMFC）阳极催化剂是DMFC的关键材料之一,其电化学活性的大小对燃料电池的输出性能及成本起着关键作用。不同催化剂的制备技术决定了催化剂电化学活性的高低。介绍了DMFC阳极催化剂的几种制备方法,并对这些方法进行了评述,对制备DMFC阳极催化剂具有很好的参考价值。     

自呼吸式直接甲醇燃料电池性能及其传质特性

针对有效面积为1 cm2的自呼吸式直接甲醇燃料电池（direct methanol fuel cell,DMFC）单电池,阳极采用燃料罐供液,将阴极侧集流体和夹具设计为一体式结构,并用自制的七合一膜电极组件对其进行测试,讨论了催化剂类型、扩散层材料、集流体结构等因素对其性能的影响,分析了电池内部的传质特性,优化了电池特别是其在中高电流密度条件下的性能。实验结果表明：采用Pt黑、Pt-Ru黑催化剂制作的自呼吸式DMFC能强化反应物的传质;采用碳布制作的膜电极更倾向于获得更高的极限电流密度;低电流密度时,因甲醇渗透电池电压随着甲醇浓度的增加而降低,但在中高电流密度下,电池性能随甲醇浓度的增大先升高后降低;平行集流体有利于阴阳极生成物的排出和反应物的传质,因此易获得较高的电池性能。     

Two‐Phase 1D+1D Model of a DMFC: Development and Validation on Extensive Operating Conditions Range

A two‐phase 1D+1D model of a direct methanol fuel cell (DMFC) is developed, considering overall mass balance, methanol transport in gas phase through anode diffusion layer, methanol and water crossover. The model is quantitatively validated on an extensive range of operating conditions, 24 polarisation curves. The model accurately reproduces DMFC performance in the validation range and, outside this, it is able to predict values under feasible operating conditions. Finally, the estimations of methanol crossover flux are qualitatively and quantitatively similar to experimental measures and the main local quantities' trends are coherent with results obtained with more complex models.     

Performance evaluation of the incorporation of different wire meshes in between perforated current collectors and membrane electrodeassembly on the Passive Direct methanol fuel cell

Passive Direct methanol fuel cells (DMFC) are more suitable for charging small capacity electronic devices. In passive DMFC, the fuel and oxidant are supplied by diffusion and natural convection process on the anode and cathode sides respectively. Current collectors (CC) play a vital importance in fuel cell performance. This paper presents the combined impact of perforated and wire mesh current collectors (WMCC) on passive DMFC performance. Three types of open ratios of perforated current collectors (PCC), such as 45.40％, 55.40％and 63.40％and two types of wire mesh current collectors with open ratios of 38.70％and 45.40％were chosen for the experimental work. A combination of Taguchi-L9 rule is con-sidered. A combination of three PCC and two WMCC on both anode and cathode was used. Methanol con-centration was varied from 1 mol·L-1–5 mol·L-1 for nine combinations of PCC and WMCC. From the experimental results, it is noticed that the combination of PCC and WMCC with an open ratio of 55.40％ and 38.70％ incorporated passive DMFC produced peak power density at 5 mol·L-1 of methanol concentration. The passive DMFC performance was evaluated in terms of maximum power density and maximum current density. The combined current collectors of PCC and WMCC open ratios of 55.40％+38.70％ have more stable voltage than single PCC of open ratio 63.40％ at 4 mol·L-1 of methanol concentration.     

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.     

直接甲醇燃料电池阳极非铂催化剂的研究进展

介绍了直接甲醇燃料电池(DMFC)的特点及其阳极催化剂存在的主要问题。综述了DMFC阳极非铂催化剂的研究进展及非铂催化剂对甲醇氧化的催化活性和抗CO等中间物毒化能力等性能,提出了DMFC阳极非铂催化剂存在的问题以及发展趋势。