Miniature DMFCs with passive thermal-fluids management system

A new miniature DMFC system that includes a fuel cell stack, a fuel tank and a passive ancillary system (termed “thermal-fluids management system” in this paper) is presented. The thermal-fluids management system utilizes passive approaches for fuel storage and delivery, air breathing, water management, CO₂ release and thermal management. With 5.1 g of neat methanol in the fuel cartridge, a prototype has successfully demonstrated 18 h of continuous operation with total power output of 1.56 Wh.     

A direct methanol fuel cell system with passive fuel delivery based on liquid surface tension

The existing direct methanol fuel cell (DMFC) systems are fed with a fixed concentration of fuel, which are either a diluted methanol solution or an active fuel delivery driven by an attached active pump. Both approaches limit the power conversion density or degrade the overall efficiency of the DMFC system significantly. Such disadvantages become more severe in small-scale DMFCs, which require a high conversion efficiency and a small physical space suitable for portable electronics. In this paper, passive fuel delivery based on a surface tension driving mechanism was designed and integrated in a laboratory-made prototype to achieve consumption depending on fuel concentration and power-free fuel delivery. Unidirectional methanol-to-water smooth flow is achieved through the capillaries of a Teflon PTFE (polytetrafluoroethylene) membrane based on the difference in liquid surface tension. The prototype was demonstrated to exhibit a better polarization performance and to last for an extended operating time compared to conventional DMFCs. Its high efficiency and load regulation performance were also demonstrated in contrast to an active DMFC supplied with a constant concentration fuel. The fuel delivery driven by the liquid surface tension effect demonstrated here is believed to be more applicable for future small-scale DMFCs for portable electronics.     

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.     

A self-regulated passive fuel-feed system for passive direct methanol fuel cells

Operating a passive direct methanol fuel cell (DMFC) with high methanol concentration is desired because this increases the energy density of the fuel cell system and hence results in a longer runtime. However, the increase in methanol concentration is limited by the adverse effect of methanol crossover in the conventional design. To overcome this problem, we propose a new self-regulated passive fuel-feed system that not only enables the passive DMFC to operate with high-concentration methanol solution without serious methanol crossover, but also allows a self-regulation of the feed rate of methanol solution in response to discharging current. The experimental results showed that with this fuel-feed system, the fuel cell fed with high methanol concentration of 12.0 M yielded the same performance as that of the conventional DMFC running with 4.0 M methanol solution. Moreover, as a result of the increased energy density, the runtime of the cell with this new system was as long as 10.1 h, doubling that of the conventional design (4.4 h) at a given fuel tank volume. It was also demonstrated that this passive fuel-feed system could successfully self-regulate the fuel-feed rate in response to the change in discharging currents.     

Water and air management systems for a passive direct methanol fuel cell

In this paper water and air management systems were developed for a miniature, passive direct methanol fuel cell (DMFC). The membrane thickness, water management system, air management system and gas diffusion electrodes (GDE) were examined to find their effects on the water balance coefficient, fuel utilization efficiency, energy efficiency and power density. Two membranes were used, Nafion^® 112 and Nafion^® 117. Nafion^® 117 cells had greater water balance coefficients, higher fuel utilization efficiency and greater energy efficiency. A passive water management system which utilizes additional cathode gas diffusion layers (GDL) and a passive air management system which makes use of air filters was developed and tested. Water management was improved with the addition of two additional cathode GDLs. The water balance coefficients were increased from −1.930 to 1.021 for a cell using a 3.0 mol kg⁻¹ solution at a current density of 33 mA cm⁻². The addition of an air filter further increased the water balance coefficient to 1.131. Maximum power density was improved from 20 mW cm⁻² to 25 mW cm⁻² for 3.0 mol kg⁻¹ solutions by upgrading from second to third generation GDEs, obtained from E-TEK. There was no significant difference in water management found between second and third generation GDEs. A fuel utilization efficiency of 63% and energy efficiency of 16% was achieved for a 3.0 mol kg⁻¹ solution with a current density of 66 mA cm⁻² for third generation GDEs.     

Exergy analysis of a passive direct methanol fuel cell

An analytical, one-dimensional, steady state model is employed to solve for overpotentials at the catalyst layers along with the liquid water and methanol distributions at the anode, and oxygen transport at the cathode. An iterative method is utilized to calculate the cell temperature at each cell current density. A comprehensive exergy analysis considering all possible species inside the cell during normal operation is presented. The contributions of different types of irreversibilities including overpotentials at the anode and cathode, methanol crossover, contact resistance, and proton conductivity of the membrane are investigated. Of all losses, overpotentials in conjunction with the methanol crossover are considered as the major exergy destruction sources inside the cell during the normal operation. While the exergy losses due to electrochemical reactions are more significant at higher current densities, exergy destruction by methanol crossover at the cathode plays more important role at lower currents. It is also found that the first-law efficiency of a passive direct methanol fuel cell increases as the methanol solution in the tank increases in concentration from 1 M to 3 M. However, this is not the case with the second-law efficiency which is always decreasing as the concentration of the methanol solution in the tank increases.     

Performance evaluation of passive direct methanol fuel cell with methanol vapour supplied through a flow channel

This work examines the effect of fuel delivery configuration on the performance of a passive air-breathing direct methanol fuel cell (DMFC). The performance of a single cell is evaluated while the methanol vapour is supplied through a flow channel from a methanol reservoir connected to the anode. The oxygen is supplied from the ambient air to the cathode via natural convection. The fuel cell employs parallel channel configurations or open chamber configurations for methanol vapour feeding. The opening ratio of the flow channel and the flow channel configuration is changed. The opening ratio is defined as that between the area of the inlet port and the area of the outlet port. The chamber configuration is preferred for optimum fuel feeding. The best performance of the fuel cell is obtained when the opening ratio is 0.8 in the chamber configuration. Under these conditions, the peak power is 10.2 mW cm⁻² at room temperature and ambient pressure. Consequently, passive DMFCs using methanol vapour require sufficient methanol vapour feeding through the flow channel at the anode for best performance. The mediocre performance of a passive DMFC with a channel configuration is attributed to the low differential pressure and insufficient supply of methanol vapour.     

Single passive direct methanol fuel cell supplied with pure methanol

A new single passive direct methanol fuel cell (DMFC) supplied with pure methanol is designed, assembled and tested using a pervaporation membrane (PM) to control the methanol transport. The effect of the PM size on the fuel cell performances and the constant current discharge of the fuel cell with one-fueling are studied. The results show that the fuel cell with PM 9 cm² can yield a maximum power density of about 21 mW cm⁻², and a stable performances at a discharge current of 100 mA can last about 45 h. Compared with DMFC supplied with 3 M methanol solution, the energy density provided by this new DMFC has increased about 6 times.     

Small direct methanol fuel cells with passive supply of reactants

Small, stand-alone, direct methanol fuel cells (DMFCs) that have no auxiliary liquid pumps and gas blowers/compressors are known as passive DMFCs. The devices are ideal for powering portable electronic devices, as this type of fuel cell uniquely has a simple and compact system and no parasitic power losses. This article provides a comprehensive review of experimental and numerical studies of heat and mass transport in passive DMFCs. Emphasis is placed on the mechanisms and key issues of the mass transport of each species through the fuel cell structure under the influence of passive forces. It is shown that the key issue regarding the methanol supply is how to feed high-concentration methanol solution but with minimum methanol crossover through the membrane so that both the system specific energy and cell performance can be maximized. The key issue regarding the oxygen supply is how to enhance the removal of liquid water from the cathode under the air-breathing condition. For water transport, the aim is to transport the water produced on the cathode through the membrane to the anode by optimizing the design of the membrane electrode assembly so that the fuel cell can be operated with pure methanol and with minimum flooding at the cathode. The heat loss from a passive DMFC is usually large and it is therefore critically important to reduce this feature so that the fuel cell can be operated at a sufficiently high temperature, which critically affects the 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.     

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.     

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.     

Transient behavior analysis of a new designed passive direct methanol fuel cell fed with highly concentrated methanol

A new structure of passive direct methanol fuel cell (DMFC) with two methanol reservoirs separated by a porous medium layer is designed and a corresponding mathematical model is presented. The new designed passive DMFC can be directly fed with highly concentrated methanol solution or neat methanol. The porosity (?_pr) of the porous medium layer is optimized using the proposed model. Some operation parameters are also optimized by both the numerical calculation and experimental measurement. The new designed DMFC can be continuously operated for about 4.5 times longer than a conventional passive DMFC with the optimum parameters. The methanol crossover during the same discharging is only about 50% higher.     

Development of a passive direct methanol fuel cell stack for high methanol concentration

In order to develop a vertically arranged passive DMFC with a porous carbon plate, PCP, the effect of the head height of the methanol solution in contact with the porous carbon plate on the power generation was investigated for a 55 mm height using a single cell. The single cell was operated at several methanol concentrations greater than 70 wt%. By filling the reservoir with 90 and 100 wt% methanol solutions, power densities greater than 30 mW cm⁻² for over 10 h were demonstrated. Based on the result of the single cell study, a passive DMFC stack consisting of 8 unit cells with the PCP was designed and fabricated. The power generation characteristics were then experimentally measured. The maximum power output of 1.8 W, which was almost 10% lower than that expected from the single cell performance, was obtained with 100% methanol. At the same time, a nonuniform cell voltage among the 8 unit cells was found as a reason for the decreasing power output with the increasing current.     

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.     

Regulating methanol feed concentration in direct methanol fuel cells using feedback from voltage measurements

Regulating methanol feed concentration in direct methanol fuel cells (DMFCs) is important for improving electrical performance and fuel utilization. Low methanol concentration reduces the reaction rate at the anode due to Nernstian effects resulting in a lower operating voltage. However, simply increasing the methanol concentration does not always lead to improved performance due to increased methanol crossover from the anode to the cathode resulting in mixed-potential losses and the associated fuel loss. Hence, there exists an optimal intermediate value of methanol concentration for each current density that will yield the highest electrical performance (V). In this paper, we describe the development of an in situ methodology which uses the measured cell voltage as the feedback to regulate the methanol feed concentration for maximum power density. This methodology is demonstrated at the current densities of 50, 100, and 250 mA cm⁻²and the results for optimal concentration are presented. Fuel loss as a function of methanol concentration is evaluated by oxidizing the crossover methanol at the cathode exhaust and measuring the CO₂ mass flux.     

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.     

Non-isothermal modeling of direct methanol fuel cell

In this work, a two-dimensional, two-phase non-isothermal model is developed for DMFC. The natural convection heat transfer at the out surface of the current collector is considered as the thermal boundary conditions to obtain a more realistic simulation of the DMFC working conditions. The heat and mass transfer, along with the electrochemical reactions occurring in the DMFC are modeled and numerically solved by a self-developed simulation code. The numerical results show that cell performance is enhanced with the increase in the inlet temperature. The distribution of temperature in the DMFC mainly depends on the inlet temperature of the dilute methanol aqueous in the anode side. The mean temperature of MEA and temperature difference in MEA increase with the increase in current density and the profiles show the same trend. With the decrease in MEA thermal conductivity and the increase in the inlet temperature the temperature difference in MEA becomes larger.     

A transient two-phase mass transport model for liquid feed direct methanol fuel cells

A transient two-phase mass transport model for liquid feed direct methanol fuel cells (DMFCs) is developed. With this model, various processes that affect the DMFC transient behaviors are numerically studied. The results show that the cell voltage exhibits an overshoot behavior in response to a sudden change in the current density. The magnitude of the overshoot depends on the magnitudes of the change in the cell current density and the initial current density. It is found that the dynamic change in the methanol permeation through the membrane to the cathode results in a strong cathode overpotential overshoot, which is believed to be the predominant factor that leads to the cell voltage overshoot. In contrast, the anode overpotential is relatively insensitive to the changes in the methanol concentration as well as CO surface coverage in the anode catalyst layer. Moreover, the effect of the double layer capacitance (DLC) on the cell dynamic behavior is studied and the results show that the DLC can smoothen the change in the cell voltage in response to a change in the cell current density. Furthermore, the dynamic response of mass transport to a change in the cell current density is found to be rather slow. In particular, it is shown that the slow response in the mass transport of methanol is one of the key factors that influence the cell dynamic operation.     

Water management in a passive direct methanol fuel cell

Passive direct methanol fuel cells (DMFCs) are under development for use in portable applications because of their enhanced energy density in comparison with other fuel cell types. The most significant obstacles for DMFC development are methanol and water crossover because methanol diffuses through the membrane generating heat but no power. The presence of a large amount of water floods the cathode and reduces cell performance. The present study was carried out to understand the performance of passive DMFCs, focused on the water crossover through the membrane from the anode to the cathode side. The water crossover behaviour in passive DMFCs was studied analytically with the results of a developed model for passive DMFCs. The model was validated with an in‐house designed passive DMFC. The effect of methanol concentration, membrane thickness, gas diffusion layer material and thickness and catalyst loading on fuel cell performance and water crossover is presented. Water crossover was lowered with reduction on methanol concentration, reduction of membrane thickness and increase on anode diffusion layer thickness and anode and cathode catalyst layer thickness. It was found that these conditions also reduced methanol crossover rate. A membrane electrode assembly was proposed to achieve low methanol and water crossover and high power density, operating at high methanol concentrations. The results presented provide very useful and actual information for future passive DMFC systems using high concentration or pure methanol. Copyright © 2012 John Wiley & Sons, Ltd.