Electrode in direct methanol fuel cells

Nanotechnology has recently generated a lot of attention and high expectations not only in the academic community but also among investors, scientists and researchers in both government and industry sectors. Its unique capability to fabricate new structures at the atomic scale has already produced novel materials and devices with great potential applications in a wide number of fields. Up to now, the electrodes in direct methanol fuel cells (DMFCs) have generally been based on the porous carbon gas diffusion electrodes that are employed in proton exchange membrane fuel cells. Typically, the structure of such electrodes is comprised of a catalyst layer and a diffusion layer, the latter being carbon cloth or carbon paper. It is a challenge to develop an electrode with high surface area, good electrical conductivity and suitable porosity to allow good reactant flux and high stability in the fuel cell environment. This paper presents an overview of electrode structure in general and recent material developments, with particular attention paid to the application of nanotechnology in DMFCs.     

Passive direct methanol fuel cells fed with methanol vapor

A vapor fed passive direct methanol fuel cell (DMFC) is proposed to achieve a high energy density by using pure methanol for mobile applications. Vapor is provided from a methanol reservoir to the membrane electrode assembly (MEA) through a vaporizer, barrier and buffer layer. With a composite membrane of lower methanol cross-over and diffusion layers of hydrophilic nanomaterials, the humidity of the MEA was enhanced by water back diffusion from the cathode to the anode through the membrane in these passive DMFCs. The humidity in the MEA due to water back diffusion results in the supply of water for an anodic electrochemical reaction with a low membrane resistance. The vapor fed passive DMFC with humidified MEA maintained 20–25 mW cm⁻² power density for 360 h and performed with a 70% higher fuel efficiency and 1.5 times higher energy density when compared with a liquid fed passive DMFC.     

Mixed-reactant,strip-cell direct methanol fuel cells

A feasibility analysis of a mixed-reactant, strip-cell direct methanol fuel cell concept is presented. In this type of cell, selective electrodes are mounted in an alternating fashion on the same side of a membrane electrolyte, and are exposed to a mixed-reactant feed. The presence of a single feed stream reduces flow system volume and sealing requirements, and potentially increases power density. Experimental polarization measurements of selective anodes and cathodes demonstrate the insensitivity of such electrodes to the presence of a mixed-reactant feed. The fuel efficiency of the direct methanol cell with selective electrodes is shown to be higher than that of a cell with more typical electrodes at low current density. The effect of geometric parameters on the performance of strip cells is discussed, and design recommendations are given for a simple geometry.     

Electron transport in direct methanol fuel cells

A three-dimensional (3D), two-phase, isothermal model of direct methanol fuel cells (DMFCs) was employed to investigate effects of electron transport through the backing layer and the land in bipolar plates. It was found that the electronic resistance of the backing layer, affected by backing layer electronic conductivity, backing layer thickness and flow channel width, played a relatively important role in determining the current density distribution and cell performance. In order to ignore the electron transport effect on the average current density, the minimum electronic conductivity of the backing layer has to be 1000 S m⁻¹, with the relative error in the average current density less than 5%, under the given conditions.     

New membranes for direct methanol fuel cells

The performance of direct methanol fuel cells (DMFC) is limited by the cross-over of methanol through the electrolyte. Electrolyte membranes prepared by blending of sulfonated arylene main-chain polymers like sulfonated PEEK Victrex (sPEEK) or sulfonated PSU Udel (sPSU) with basic polymers like poly(4-vinylpyridine) (P4VP) or polybenzimidazole (PBI) show excellent chemical and thermal stability, good proton-conductivity, and good performance in H₂ PEM fuel cells. Furthermore, these materials have potentially lower methanol cross-over when compared to standard Nafion-type membranes.In this work, membrane electrode assemblies (MEAs) have been prepared from such membranes according to the thin-film method. The catalyst layer was spray-coated directly on the heated membrane using an ink consisting of an aqueous suspension of catalyst powder and Nafion solution. Unsupported catalysts were used for anode and cathode. A rather high catalyst loading was chosen in order to minimize the effects of limited catalyst utilization due to flooding conditions at both electrodes.     

Water management in direct methanol fuel cells

Water management is an important challenge in portable direct methanol fuel cells. Reducing the water and methanol loss from the anode to the cathode enables the use of highly concentrated methanol solutions to achieve enhanced performances. In this work, the results of a simulation study using a previous developed model for DMFCs are presented. Particular attention is devoted to the water distribution across the cell. The influence of different parameters (such as the cathode relative humidity (RH), the methanol concentration and the membrane, catalyst layer and diffusion media thicknesses) over the water transport and on the cell performance is studied. The analytical solutions of the net water transport coefficient, for different values of the cathode relative humidity are successfully compared with recent published experimental data putting in evidence that humidified cathodes contribute to a decrease on the water crossover. As a result of the modelling results, a tailored MEA build-up with the common available commercial materials is proposed to achieve low methanol and water crossover and high power density, operating at relatively high methanol concentrations. A thick anode catalyst layer to promote methanol oxidation, a thin anode gas diffusion layer as methanol carrier to the catalyst layer and a thin polymer membrane to lower the water crossover coefficient between the anode and cathode are suggested.     

Towards operating direct methanol fuel cells with highly concentrated fuel

A significant advantage of direct methanol fuel cells (DMFCs) is the high specific energy of the liquid fuel, making it particularly suitable for portable and mobile applications. Nevertheless, conventional DMFCs have to be operated with excessively diluted methanol solutions to limit methanol crossover and the detrimental consequences. Operation with diluted methanol solutions significantly reduces the specific energy of the power pack and thereby prevents it from competing with advanced batteries. In view of this fact, there exists a need to improve conventional DMFC system designs, including membrane electrode assemblies and the subsystems for supplying/removing reactants/products, so that both the cell performance and the specific energy can be simultaneously maximized. This article provides a comprehensive review of past efforts on the optimization of DMFC systems that operate with concentrated methanol. Based on the discussion of the key issues associated with transport of the reactants/products, the strategies to manage the supply/removal of the reactants/products in DMFC operating with highly concentrated methanol are identified. With these strategies, the possible approaches to achieving the goal of concentrated fuel operation are then proposed. Past efforts in the management of the reactants/products for implementing each of the approaches are also summarized and reviewed.     

Compact direct methanol fuel cells for portable application

Consumers’ demand for portable audio/video/ICT products has driven the development of advanced power technologies in recent years. Fuel cells are a clean technology with low emissions levels, suitable for operation with renewable fuels and capable, in a next future, of replacing conventional power systems meeting the targets of the Kyoto Protocol for a society based on sustainable energy systems. Within such a perspective, the objective of the European project MOREPOWER (compact direct methanol fuel cells for portable applications) is the development of a low-cost, low temperature, portable direct methanol fuel cell (DMFC; nominal power 250 W) with compact construction and modular design for the potential market area of weather stations, medical devices, signal units, gas sensors and security cameras. This investigation is focused on a conceptual study of the DMFC system carried out in the Matlab/Simulink^® platform: the proposed scheme arrangements lead to a simple equipment architecture and a efficient process.     

Carbon dioxide vent for direct methanol fuel cells

Passive, stand-alone, direct methanol fuel cells require a pressure management system that releases CO₂ produced in the anode chamber. However, this must be done without allowing the methanol fuel to escape. In this paper, two siloxane membranes are investigated and shown to selectively vent CO₂ from the anode chamber. The addition of hydrophobic additives, 1,6-divinylperfluorohexane and 1,9-decadiene, improved the selectivity of the siloxane membranes. The best performing CO₂ vent was obtained with 50:50 wt% poly(1-trimethyl silyl propyne) and 1,6-divinylperfluorohexane.     

Current density measurements in direct methanol fuel cells

The current density in the fuel cell is the direct consequence of reactions taking place over the active surface area. Thus, measurement of its distribution will lead to identification of the location and nature of reactions and will give opportunity to improve the overall efficiency of fuel cells. Within this study, the current density distribution in a direct methanol fuel cell was analyzed by segmenting the current collector into nine sections. Besides, the effect of the different operating parameters such as molarity, flow rate and reactant gas on the current density distribution was analyzed.     

Mass transport phenomena in direct methanol fuel cells

Clean and highly efficient energy production has long been sought to solve energy and environmental problems. Fuel cells, which convert the chemical energies stored in fuel directly into electrical energy, are expected to be a key enabling technology for this century. This article is concerned with one of the most advanced fuel cells – direct methanol fuel cells (DMFCs). We present a comprehensive review of the state-of-the-art studies of mass transport of different species, including the reactants (methanol, oxygen and water) and the products (water and carbon dioxide) in DMFCs. Rather than elaborating on the details of the previous numerical modeling and simulation, the article emphasizes: i) the critical mass-transport issues that need to be addressed so that the performance and operating stability of DMFCs can be upgraded, ii) the basic mechanisms that control the mass-transport behaviors of reactants and products in this type of fuel cell, and iii) the previous experimental and numerical findings regarding the correlation between the mass transport of each species and cell performance.     

Performance characterization of passive direct methanol fuel cells

The performance of a fuel cell is usually characterized by a polarization curve (cell voltage versus current density) under stabilized operating conditions. However, for passive direct methanol fuel cells (DMFC) that have neither fuel pumps nor gas compressors, the voltage at a given current density varies with time because methanol concentration in the fuel reservoir keeps decreasing during the discharging process. The important question brought up by this transient discharging behavior is: under what conditions should the polarization data be collected such that the performance of the passive DMFC can be objectively characterized? In this work, we found that the performance of the passive DMFC became relatively stable as the cell operating temperature rose to a relatively stable value. This finding indicates that the performance of the passive DMFC can be characterized by collecting polarization data at the instance when the cell operating temperature under the open-circuit condition rises to a relatively stable value.     

An algorithm for sensor-less fuel control of direct methanol fuel cells

We report an algorithm for real-time control of the fuel of a DMFC. The MEA voltage decay coefficients [e₁, e₂], and I-V-T, M′-I-T, and W′-I-T curves (where I is the current, V the voltage, T the temperature, and M′ and W′ the methanol and water consumption rates, respectively) of n fuels with specified methanol concentrations C_M,k (k = 1, 2,…, n) are pre-established and form (I,V,T), (M′,I,T), and (W′,I,T) surfaces for each C_M,k. The in situ measured (I,V,T)_u after voltage decay correction is applied to the n preset (I,V,T) surfaces to estimate C_M,u (the C_M corresponding to (I,V,T)_u) using an interpolation procedure. The C_M,u is then applied to the n preset (M′,I,T) and (W′,I,T) surfaces to estimate cumulated “methanol” and “water” consumed quantities . Thus in a real-time system, the C_M and total quantity of fuel can be controlled using the estimated C_M,u and cumulated “methanol” and “water” consumed quantities.     

Advanced air-breathing direct methanol fuel cells for portable applications

A novel MEA is fabricated to improve the performance of air-breathing direct methanol fuel cells. A diffusion barrier on the anode side is designed to control methanol transport to the anode catalyst layer and thus suppressing the methanol crossover. A catalyst coated membrane with a hydrophobic gas diffusion layer on the cathode side is employed to improve the oxygen mass transport. It is observed that the maximum power density of the advanced DMFC with 2 M methanol solution achieves 65 mW cm⁻² at 60 °C. The value is nearly two times more than that of a commercial MEA. At 40 °C, the power densities operating with 1 and 2 M methanol solutions are over 20 mW cm⁻² with a cell potential at 0.3 V.     

Optimum operating strategies for liquid-fed direct methanol fuel cells

This study focuses on optimum operating strategies for liquid-fed direct methanol fuel cells (DMFCs) to minimize methanol consumption. A mathematical model is developed and verified with experimental data from the literature using the parameter estimation method. The model consists of a set of differential and algebraic equations and makes it possible to describe zero initial hold-up conditions. Based on the model, steady-state simulation results are obtained and explain the dependence on the feed concentration of key variables such as cell voltage, cell power density, overpotentials of both electrodes, and methanol crossover ratio. Dynamic simulation results are also presented to check the transient behaviour of a DMFC operated from start-up to shut-down. Dynamic optimization allows determination of the optimum transient strategies of feed concentration required to maximize the fuel efficiency. With six scenarios of power density load, it is demonstrated that the optimum transient strategies depend heavily on both the load of power density and the number of control actions. The main advantage of these approaches is to reduce fuel consumption and, ultimately, to enable DMFCs to be operated more efficiently.     

Mass and heat transport in direct methanol fuel cells

The direct methanol fuel cell (DMFC) is a better alternative to the conventional battery. The DMFC offers several advantages, namely, faster building of potential and longer-lasting fuel, however, there are still several issues that need to be addressed to design a better DMFC system. This article is a wide-ranging review of the most up-to-date studies on mass and heat transfer in the DMFC. The discussion will be focused on the critical problems limiting the performance of DMFCs. In addition, a technique for upgrading the DMFC with an integrated system will be presented, along with existing numerical models for modeling mass and heat transfer as well as cell performance.     

Evaluation of composite membranes for direct methanol fuel cells

The performance of direct methanol fuel cells (DMFCs) can be significantly affected by the transport of methanol through the membrane, depolarising the cathode. In this paper, the literature on composite membranes that have been developed for reduction of methanol crossover in DMFCs is reviewed. While such membranes can be effective in reducing methanol permeability, this is usually combined with a reduction in proton conductivity. Measurements of methanol permeability and proton conductivity are relatively straightforward, and these parameters (or a membrane ‘selectivity’ based on the ratio between them) are often used to characterize DMFC membranes. However, we have carried out one-dimensional simulations of DMFC performance for a wide range of membrane properties, and the results indicate that DMFC performance is normally either limited by methanol permeability or proton conductivity. Thus use of a ‘selectivity’ is not appropriate for comparison of membrane materials, and results from the model can be used to compare different membranes. The results also show that Nafion^® 117 has an optimum thickness, where DMFC performance is equally limited by both methanol permeability and proton conductivity. The model also indicates that new composite membranes based on Nafion^® can only offer significant improvement in DMFC performance by enabling operation with increased methanol concentration in the fuel. A number of composite membrane materials that have been reported in the literature are shown to deliver significant reduction in DMFC performance due to reduced proton conductivity, although improved performance at high methanol concentration may be possible.     

Passive direct methanol fuel cells for portable electronic devices

Due to the increasing demand for electricity, clean, renewable energy resources must be developed. Thus, the objective of the present study was to develop a passive direct methanol fuel cell (DMFC) for portable electronic devices. The power output of six dual DMFCs connected in series with an active area of 4 cm² was approximately 600 mW, and the power density of the DMFCs was 25 mW cm⁻². The DMFCs were evaluated as a power source for mobile phone chargers and media players. The results indicated that the open circuit voltage of the DMFC was between 6.0 V and 6.5 V, and the voltage under operating conditions was 4.0 V. The fuel cell was tested on a variety of cell phone chargers, media players and PDAs. The cost of energy consumption by the proposed DMFC was estimated to be USD 20 W⁻¹, and the cost of methanol is USD 4 kW h. Alternatively, the local conventional electricity tariff is USD 2 kW h. However, for the large-scale production of electronic devices, the cost of methanol will be significantly lower. Moreover, the electricity tariff is expected to increase due to the constraints of fossil fuel resources and pollution. As a result, DMFCs will become competitive with conventional power sources.     

Investigation on the durability of direct methanol fuel cells

This study addresses the durability of direct methanol fuel cells (DMFCs). Three performance indices including permanent degradation, temporary degradation and voltage fluctuation are proposed to qualify the durability of DMFC. The decay rate, associated with permanent degradation, follows from such failure mechanisms as dissolution, growth and poisoning of the catalyst, while temporary degradation reflects the elimination of the hydrophobic property of the gas diffusion layer (GDL). However, voltage fluctuations reveal different results which cannot stand for degradation phenomenon exactly. In this investigation, such methods of examination as scanning electron microscope (SEM), and X-ray diffraction (XRD) are employed to check the increase in the mean particle size in the anode and cathode catalysts, and the degree is higher in the cathode. The Ru content in the anode catalyst and the specific surface area (SSA) of the anode and cathode catalysts decrease after long-term operation. Moreover, the crossover of Ru from the anode side to the cathode side is revealed by energy dispersive X-ray (EDX) analysis. Electro-catalytic activity towards the methanol oxidation reaction (MOR) at the anode is verified to be weaker after durability test by cyclic voltammetry (CV). Also, the electrochemical areas (ECAs) of the anode and cathode catalysts are evaluated by hydrogen-desorption. SSA loss simply because of agglomeration and growth of the catalyst particles, of course, is lower than ECA loss. The observations will help to elucidate the failure mechanism of membrane electrode assembly (MEA) in durability tests, and thus help to prolong the lifetime of DMFC.     

Fabrication of electrocatalyst layers for direct methanol fuel cells

To optimize the performance of the membrane electrode assembly (MEA), a manufacturing process for electrocatalyst layers is systematically studied by controlling physical parameters such as electrocatalyst loadings at each electrode, electrocatalyst compositions, and layer thickness. The MEA is evaluated in an air-breathing direct methanol fuel cell (DMFC) with various methanol concentrations. The investigation focuses on finding the best compromise between electrocatalyst loadings and utilization of methanol concentration. Surprisingly, the power density is influenced more by the Pt loading than by the Pt–Ru loading, and can be increased further by using a methanol concentration above 3 wt.% for a certain level of electrocatalyst loading. Current–voltage characteristics indicate that increasing Pt and Pt–Ru loadings at each electrode can reduce the activation overpotentials, but the respective variation of current density with cell voltage differs in the voltage range (0.3–0.8 V). Although MEA performance can be improved by increasing the Pt (and Pt–Ru) concentration, a penalty is paid due to the tendency towards increased nanoparticle aggregation. The MEAs are also applied to a small pack of air-breathing DMFCs to assess their operability in mobile phones.