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.     

Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.     

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.     

Effects of anode microporous layers made of carbon powder and nanotubes on water transport in direct methanol fuel cells

The effects of the design parameters of the anode diffusion layer (DL), including the PTFE loading in the backing layer (BL), and the carbon and PTFE loading in the microporous layer (MPL), on water transport through the membrane and the performance of a liquid-feed direct methanol fuel cell (DMFC) are experimentally investigated. The results indicate that increasing the PTFE loading in the BL and introducing a MPL could decrease water crossover through the membrane without sacrificing cell performance when the feed methanol concentration is increased. It is also found that changing the PTFE loading in the MPL has little effect on water crossover, whereas increasing the carbon loading in the MPL could noticeably decrease the water-crossover flux. Nevertheless, the ability of the MPL to reduce water crossover is limited by the presence of a number of mud cracks. To reduce further the water-crossover flux, a crack-free MPL made of multi-walled carbon nanotubes (MWCNTs) and PTFE is proposed. Tests indicate that the DMFC with the nanotube MPL results in a much lower water-crossover flux than a conventional carbon-powder MPL. More importantly, the use of the nanotube MPL allows the DMFC to be operated with a higher methanol concentration, and thereby increases the fuel cell system energy density.     

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.     

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.     

Systematic approach for modeling methanol mass transport on the anode side of direct methanol fuel cells

A systematic method for modeling direct methanol fuel cells, with a focus on the anode side of the system, is advanced for the purpose of quantifying the methanol crossover phenomenon and predicting the concentration of methanol in the anode catalyst layer of a direct methanol fuel cell. The model accounts for fundamental mass transfer phenomena at steady state, including convective transport in the anode flow channel, as well as diffusion and electro-osmotic drag transport across the polymer electrolyte membrane. Experimental measurements of methanol crossover current density are used to identify five modeling parameters according to a systematic parameter estimation methodology. A validation study shows that the model matches the experimental data well, and the usefulness of the model is illustrated through the analysis of effects such as the choice fuel flow rate in the anode flow channel and the presence of carbon-dioxide bubbles.     

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.     

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.     

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.     

Enhancement of water retention in the membrane electrode assembly for direct methanol fuel cells operating with neat methanol

It is desirable to operate a direct methanol fuel cell (DMFC) with neat methanol to maximize the specific energy of the DMFC system, and hence increasing its runtime. A way to achieve the neat-methanol operation is to passively transport the water produced at the cathode through the membrane to the anode to facilitate the methanol oxidation reaction (MOR). To achieve a performance of the MOR similar to that under the conventional diluted methanol operation, both the water transport rate and the local water concentration in the anode catalyst layer (CL) are required to be sufficiently high. In this work, a thin layer consisting of nanosized SiO₂ particles and Nafion ionomer (referred to as a water retention layer hereafter) is coated onto each side of the membrane. Taking advantage of the hygroscopic nature of SiO₂, the cathode water retention layer can help maintain the water produced from the cathode at a higher concentration level to enhance the water transport to the anode, while the anode retention layer can retain the water that is transported from the cathode. As a result, a higher water transport rate and a higher water concentration at the anode CL can be achieved. The formed membrane electrode assembly (MEA) with the added water retention layers is tested in a passive DMFC and the results show that this MEA design yields a much higher power density than the MEA without water retention layers does.     

Modeling of water transport through the membrane electrode assembly for direct methanol fuel cells

In this work, a one-dimensional, isothermal two-phase mass transport model is developed to investigate the water transport through the membrane electrode assembly (MEA) for liquid-feed direct methanol fuel cells (DMFCs). The liquid (methanol–water solution) and gas (carbon dioxide gas, methanol vapor and water vapor) two-phase mass transport in the porous anode and cathode is formulated based on classical multiphase flow theory in porous media. In the anode and cathode catalyst layers, the simultaneous three-phase (liquid and vapor in pores as well as dissolved phase in the electrolyte) water transport is considered and the phase exchange of water is modeled with finite-rate interfacial exchanges between different phases. This model enables quantification of the water flux corresponding to each of the three water transport mechanisms through the membrane for DMFCs, such as diffusion, electro-osmotic drag, and convection. Hence, with this model, the effects of MEA design parameters on water crossover and cell performance under various operating conditions can be numerically investigated.     

Effect of the porous carbon layer in the cathode gas diffusion media on direct methanol fuel cell performances

The effect of cathode gas diffusion media with microporous layers (MPLs) on direct methanol fuel cell (DMFC) performances is studied by combining electrochemical analysis and physicochemical investigation. The membrane electrode assemblies (MEAs) using MPL-modified cathode gas diffusion layers (GDLs, GDL-1) showed slightly better performances (117 mW cm⁻²) at 0.4 V and 70 °C than commercial GDL (SIGRACET^® product version: GDL-35BC, SGL Co.) DMFC MEAs (110 mW cm⁻²). This might be due to high gas permeability, uniform pore distributions, and low water transport coefficient including methanol crossover. For GDL-1, the air permeability was 31.0 cm³ cm⁻² s⁻¹, while the one for SGL 35BC GDLs was 21.7 cm³ cm⁻² s⁻¹. Also, the GDL-1 in the pore-size distribution diagrams had distinct peaks due to more uniform distributions of macropores and micropores with smaller holes between aggregates of carbon particles compared to GDL-35 BC as confirmed by SEM images. Furthermore, the MEA using GDL-1 for the cathode had a lower water transfer coefficient compared to an MEA with a commercial 35 BC GDL.     

Performance and design analysis of tubular-shaped passive direct methanol fuel cells

To achieve the maximum performance from a Direct Methanol Fuel Cell (DMFC), one must not only investigate the materials and configuration of the MEA layers, but also consider alternative cell geometries that produce a higher instantaneous power while occupying the same cell volume. In this work, a two-dimensional, two-phase, non-isothermal model was developed to investigate the steady-state performance and design characteristics of a tubular-shaped, passive DMFC. Under certain geometric conditions, it was found that a tubular DMFC can produce a higher instantaneous Volumetric Power Density than a planar DMFC. Increasing the ambient temperature from 20 to 40 °C increases the peak power density produced by the fuel cell by 11.3 mW cm⁻² with 1 M, 16.3 mW cm⁻² with 2 M, but by only 8.4 mW cm⁻² with 3 M methanol. The poor performance with 3 M methanol at a higher ambient temperature is caused by increased methanol crossover and significant oxygen depletion along the Cathode Transport Layer (CTL). For a 5 cm long tubular DMFC to maintain sufficient Oxygen transport, the thickness of the CTL must be greater than 1 mm for 1 M operation, greater than 5 mm for 2 M operation, and greater than 10 mm for 3 M or higher operation.     

Effect of cathode gas diffusion layer on water transport and cell performance in direct methanol fuel cells

The optimal design of the cathode gas diffusion layer (GDL) for direct methanol fuel cells (DMFCs) is not only to attain better cell performance, but also to achieve better water management for the DMFC system. In this work, the effects of both the PTFE loading in the cathode backing layer (BL) as well as in the micro-porous layer (MPL) and the carbon loading in the MPL on both water transport and cell performance were investigated experimentally. The experimental data showed that with the presence of a hydrophobic MPL in the GDL, the water-crossover flux through the membrane decreased slightly with increasing the PTFE loading in the BL. However, a higher PTFE loading in the BL not only lowered cell performance, but also resulted in an unstable discharging process. It was also found that the PTFE loading in the MPL had little effect on the water-crossover flux, but its effect on cell performance was substantial: the 40-wt% PTFE loading in the MPL was found to be the optimal value to achieve the best performance. The experimental results further showed that increasing the carbon loading in the MPL significantly lowered the water-crossover flux, but a too high carbon loading would decrease the cell performance as the result of the increased oxygen transport resistance; the 2.0-mg C cm⁻² carbon loading was found to exhibit the best performance.     

Stable operation of air-blowing direct methanol fuel cells with high performance

A membrane electrode assembly (MEA) that is a combination of a catalyst-coated membrane (CCM) for the anode and a catalyst-coated substrate (CCS) for the cathode is studied under air-blower conditions for direct methanol fuel cells (DMFCs). Compared with MEAs prepared by only the CCS method, the performance of DMFC MEAs employing the combination method is significantly improved by 30% with less methanol crossover. This feature can be attributed to an enhanced electrode|membrane interface in the anode side and significantly higher catalyst efficiency. Furthermore, DMFC MEAs designed by the combination method retain high power density without any degradation, while the CCM-type cell shows a downward tendency in electrochemical performance under air-blower conditions. This may be due to MEAs with CCM have a much more difficult structure of catalytic active sites in the cathode to eliminate the water produced by electrochemical reaction. In addition, DMFCs produced via combination methods exhibit a lower water crossover flux than CCS alternatives, due to the comparatively dense structure of the CCM anode. Hence, DMFCs with a combination MEA structure demonstrate the feasibility of a small fuel cell system employing the low noise of a fan, instead of a noisy and large capacity air pump, for portable electronic devices.     

High concentration methanol fuel cells: Design and theory

Use of highly concentrated methanol fuel is required for direct methanol fuel cells (DMFCs) to compete with the energy density of Li-ion batteries. Because one mole of H₂O is needed to oxidize one mole of methanol (CH₃OH) in the anode, low water crossover to the cathode or even water back flow from the cathode into the anode is a prerequisite for using highly concentrated methanol. It has previously been demonstrated that low or negative water crossover can be realized by the incorporation of a low-α membrane electrode assembly (MEA), which is essentially an MEA designed for optimal water management, using, e.g. hydrophobic anode and cathode microporous layers (aMPL and cMPL). In this paper we extend the low-α MEA concept to include an anode transport barrier (aTB) between the backing layer and hydrophobic aMPL. The main role of the aTB is to act as a barrier to CH₃OH and H₂O diffusion between a water-rich anode catalyst layer (aCL) and a methanol-rich fuel feed. The primary role of the hydrophobic aMPL in this MEA is to facilitate a low (or negative) water crossover to the cathode. Using a previously developed 1D, two-phase DMFC model, we show that this novel design yields a cell with low methanol crossover (i.e. high fuel efficiency, ∼80%, at a typical operating current density of ∼80-90% of the cell limiting current density), while directly feeding high concentration methanol fuel into the anode. The physics of how the aTB and aMPL work together to accomplish this is fully elucidated. We further show that a thicker, more hydrophilic, more permeable aTB, and thicker, more hydrophobic, and less permeable aMPL are most effective in accomplishing low CH₃OH and H₂O crossover.     

Basic model for membrane electrode assembly design for direct methanol fuel cells

This research proposes a model that predicts the effect of the anode diffusion layer and membrane properties on the electrochemical performance and methanol crossover of a direct methanol fuel cell (DMFC) membrane electrode assembly (MEA). It is an easily extensible, lumped DMFC model. Parameters used in this design model are experimentally obtainable, and some of the parameters are indicative of material characteristics. The quantification of these material parameters builds up a material database. Model parameters for various membranes and diffusion layers are determined by using various techniques such as polarization, mass balance, electrochemical impedance spectroscopy (EIS), and interpretation of the response of the cell to step changes in current. Since the investigation techniques cover different response times of the DMFC, processes in the cell such as transport, reaction and charge processes can be investigated separately. Properties of single layers of the MEA are systematically varied, and subsequent analysis enables identification of the influence of the layer's properties on the electrochemical performance and methanol crossover. Finally, a case study indicates that the use of a membrane with lower methanol diffusivity and a thicker anode micro-porous layer (MPL) yields MEAs with lower methanol crossover but similar power density.     

Water-transport in outer micro-porous layers for direct methanol fuel cells

The outer micro-porous layer (MPL) between the gas diffusion layer and channel of the bipolar plate was studied for both sides of the electrodes in DMFC, with particular attention to the effects of the hydrophobicity of the MPL on mass transport as well as cell performance. Water-transport behavior from the electrodes to the channel was observed through the transparent window of the single cell with membrane-electrode assemblies (MEAs) including three combinations of outer MPLs. The crossover amount of methanol as well as water through the membrane was measured, and the mass balance, based on the measured flux, was established to understand the mass transport in MEAs. The design of outer MPLs is discussed for the best cell performance.     

Study on water transport in PEM of a direct methanol fuel cell

Water transport phenomenon in PEM and the mechanism of occurrence and development of a two-phase countercurrent flow with corresponding transport phenomenon in the PEM are analyzed. A one-dimensional steady state model of heat and mass transfer in porous media system with internal volumetric ohmic heating is developed and simulated numerically. The results show that two dimensionless parameters D and N, which reflect the liquid water flow rate and inner heat source in the PEM, respectively, are the most important factors for the water fraction and thermal balance in the PEM. The saturation profiles within the two-phase region at various operating modes are obtained. Smaller mass flow rate of liquid water and high current density are the major contributions to the membrane dehydration.