Effect of the cathode open ratios on the water management of a passive vapor-feed direct methanol fuel cell fed with neat methanol

A novel approach has been proposed to improve the water management of a passive direct methanol fuel cell (DMFC) fed with neat methanol without increasing its volume or weight. By adopting perforated covers with different open ratios at the cathode, the water management has been significantly improved in a DMFC fed with neat methanol. An optimized cathode open ratio could ensure both the sufficient supply of oxygen and low water loss. While changing the open ratio of anode vaporizer can adjust the methanol crossover rate in a DMFC. Furthermore, the gas mixing layer, added between the anode vaporizer and the anode current collector to increase the mass transfer resistance, can improve the cell performance, decrease the methanol crossover, and increase the fuel efficiency. For the case of a DMFC fed with neat methanol, an anode vaporizer with the open ratio of 12% and a cathode open ratio of 20% produced the highest peak power density, 22.7 mW cm⁻², and high fuel efficiency, 70.1%, at room temperature of 25 ± 1 °C and ambient humidity of 25-50%.     

Effect of anode current collector on the performance of passive direct methanol fuel cells

The effect of anode current collector on the performance of passive direct methanol fuel cell (DMFC) was investigated in this paper. The results revealed that the anode of passive DMFC with perforated current collector was poor at removing the produced CO₂ bubbles that blocked the access of fuel to the active sites and thus degraded the cell performance. Moreover, the performances of the passive DMFCs with different parallel current collectors and different methanol concentrations at different temperatures were also tested and compared. The results indicated that the anode parallel current collector with a larger open ratio exhibited the best performance at higher temperatures and lower methanol solution concentrations due to enhanced mass transfer of methanol from the methanol solution reservoir to the gas diffusion layer. However, the passive DMFC with a smaller open ratio of the parallel current collector exhibited the best performance at lower temperatures and higher methanol solution concentrations due to the lower methanol crossover rate. Copyright © 2009 John Wiley & Sons, Ltd.     

Mass transport analysis of a passive vapor-feed direct methanol fuel cell

A two-dimensional, two-phase, non-isothermal model using the multi-fluid approach was developed for a passive vapor-feed direct methanol fuel cell (DMFC). The vapor generation through a membrane vaporizer and the vapor transport through a hydrophobic vapor transport layer were both considered in the model. The evaporation/condensation of methanol and water in the diffusion layers and catalyst layers was formulated considering non-equilibrium condition between phases. With this model, the mass transport in the passive vapor-feed DMFC, as well as the effects of various operating parameters and cell configurations on the mass transport and cell performance, were numerically investigated. The results showed that the passive vapor-feed DMFC supplied with concentrated methanol solutions or neat methanol can yield a similar performance with the liquid-feed DMFC fed with much diluted methanol solutions, while also showing a higher system energy density. It was also shown that the mass transport and cell performance of the passive vapor-feed DMFC depend highly on both the open area ratio of the vaporizer and the methanol concentration in the tank.     

Study on operational aspects of a passive direct methanol fuel cell incorporating an anodic methanol barrier

This paper presents an investigation concerning the effects of operating conditions on the performance of a passive direct methanol fuel cell (DMFC). A self-developed porous metal fiber sintered plate (PMFSP) is used as the methanol barrier between the fuel reservoir and current collector at the anode in order to alleviate the effect of methanol crossover. The effectiveness of using this method is validated. A series of operating conditions such as operating orientation, methanol concentration, ambient temperature, forced air convection and dynamic load are evaluated. Results show that the use of a PMFSP promotes a higher cell performance during vertical operation than horizontal orientation. The effect of methanol concentration depends on the PMFSP porosity. A relatively lower porosity is favorable for high-concentration operation. The cell performance gets improved when increasing the ambient temperature and adopting forced air supply at the cathode. Compared with the traditional structure, the use of a PMFSP makes the fuel cell insensitive to the change of blowing intensity. In addition, the dynamic characteristics of the PMFSP-based passive DMFC are also reported.     

Lightweight current collector based on printed-circuit-board technology and its structural effects on the passive air-breathing direct methanol fuel cell

To realize lightweight design of the fuel cell system is a critical issue before it is put into practical use. The printed-circuit-board (PCB) technology can be potentially used for production of current collectors or flow distributors. This study develops prototypes of a single passive air-breathing direct methanol fuel cell (DMFC) and also an 8-cell mono-polar DMFC stack based on PCB current collectors. The effects of diverse structural and operational factors on the cell performance are explored. Results show that the methanol concentration of 6 M promotes a higher cell performance with a peak power density of 18.3 mW cm⁻². The combination of current collectors using a relatively higher anode open ratio and inversely a lower cathode open ratio helps enhance the cell performance. Dynamic tests are also conducted to reveal transient behaviors and its dependence on the operating conditions. To validate the real working status of the DMFC stack, it is coupled with an LED lightening system. The performance of this hybrid system is also reported in this study.     

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.     

The mass transport based on convection effects in a passive DMFC under open-circuit conditions

The study investigates the open-circuit characteristics of a passive direct methanol fuel cell (DMFC) based on temperature-induced convection effects, including the reactants distributions at anode, the non-uniform temperature distribution and the methanol crossover. A two-dimensional, well-thought-out numerical model coupling with mass transfer and momentum transfer is exploited for DMFC to investigate its inner component and temperature distributions under open-circuit condition. In addition, a 4.0 cm² passive DMFC has been designed and manufactured by the laser-cutting technology for experimental verification. The average methanol crossover flux, methanol diffusion coefficient and crossover current are obtained, which coincide with the simulation data well. The temperature-induced convection simulation results show that the distorted temperature distribution becomes more obvious with higher methanol concentration. Furthermore, the polarization curve, cell temperature and open-circuit voltage (OCV) are measured by varying the methanol concentration to conduct more in-depth research on DMFC performance at open circuit state. The results indicate that the temperature is increased, whereas the OCV is decreased with the increase of methanol concentration, accompanied by the phenomenon of methanol crossover is aggravated. The paper provides the theory basis and the optimal operating parameters for safe start-up of DMFC.     

Analysis of heat and mass transport in a miniature passive and semi passive liquid-feed direct methanol fuel cell

A two-dimensional, transient, multi-phase, multi-component, and non-isothermal model has been developed to solve the heat and mass transport in a passive and semi passive liquid-feed direct methanol fuel cell (DMFC). A semi passive DMFC uses channel at the cathode side to facilitate the oxidant transport. The transient characteristics of the temperature, methanol concentration, methanol crossover, useful current density and methanol evaporation are investigated. The results indicate that the temperature in the fuel cell increases during operation as much as 10 °C, due to the heat generation by internal phase change and the electrochemical reactions. However, it is revealed that the temperature distribution is nearly uniform at any time through all porous layers including the fuel cell and fuel delivery system. The effect of using an active feeding system in the cathode and passive methanol feeding in the anode (semi passive system) on the performance of a fuel cell is also studied. The active oxidant feeding to the cathode catalyst layer in the semi passive cell improved the fuel cell performance compared to that in a passive one. However, in general, the performance of passive cell is better than that in a semi passive one because of more temperature increase in the passive system.     

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.     

Double microporous layer cathode for membrane electrode assembly of passive direct methanol fuel cells

A novel membrane electrode assembly (MEA) is described that utilizes a double microporous layer (MPL) structure in the cathode of a passive direct methanol fuel cell (DMFC). The double MPL cathode uses Ketjen Black carbon as an inner-MPL and Vulcan XC-72R carbon as an outer-MPL. Experimental results indicate that this double MPL structure at the cathode provides not only a higher oxygen transfer rate, but enables more effective back diffusion of water; thus, leading to an improved power density and stability of the passive DMFC. The maximum power density of an MEA with a double MPL cathode was observed to be ca. 33.0 mW cm⁻², which is found to be a substantial improvement over that for a passive DMFC with a conventional MEA. A. C. impedance analysis suggests that the increased performance of a DMFC with the double MPL cathode might be attributable to a decreased charge transfer resistance for the cathode oxygen reduction reaction.     

A small mono-polar direct methanol fuel cell stack with passive operation

A passive direct methanol fuel cell (DMFC) stack that consists of six unit cells was designed, fabricated, and tested. The stack was tested with different methanol concentrations under ambient conditions. It was found that the stack performance increased when the methanol concentration inside the fuel tank was increased from 2.0 to 6.0 M. The improved performance is primarily due to the increased cell temperature as a result of the exothermic reaction between the permeated methanol and oxygen on the cathode. Moreover, the increased cell temperature enhanced the water evaporation rate on the air-breathing cathode, which significantly reduced water flooding on the cathode and further improved the stack performance. This passive DMFC stack, providing 350 mW at 1.8 V, was successfully applied to power a seagull display kit. The seagull display kit can continuously run for about 4 h on a single charge of 25 cm³ 4.0-M methanol solution.     

Local entropy generation analysis on passive high-concentration DMFCs (direct methanol fuel cell) with different cell structures

In this paper, the local entropy generation analysis has been conducted based on a two-dimensional, two-phase, non-isothermal DMFC (direct methanol fuel cell) model, the entropy generation contributed by the chemical reactions, heat transfer, mass diffusion, and viscous dissipation is investigated. Then, the performance of fuel cells with different methanol barrier layers and electrolyte membranes have been studied based on the local entropy generation analysis. Results indicate that the entropy generation during cell operation is mainly caused by the irreversible electrochemical reactions, and that the entropy generated by mass diffusion and viscous dissipation can be considered negligible. The entropy generated by heat transfer is about two magnitudes less than the entropy generated by the electrochemical reactions in the passive DMFCs operating near room temperature. The overall entropy generation rate in a DMFC can be decreased by increasing the thickness of the methanol barrier layer and decreasing the thickness of the electrolyte membrane.     

One-dimensional and non-isothermal model for a passive DMFC

Passive direct methanol fuel cells (DMFCs) are promising energy sources for portable electronic devices. Different from DMFCs with active fuel feeding systems, passive DMFCs with nearly stagnant fuel and air tend to bear comparatively less power densities. A steady state, one-dimensional, multi-component and thermal model is described and applied to simulate the operation of a passive direct methanol fuel cell. The model takes into consideration the thermal and mass transfer effects, along with the electrochemical reactions occurring in the passive DMFC. The model can be used to predict the methanol, oxygen and water concentration profiles in the anode, cathode and membrane as well as to estimate the methanol and water crossover and the temperature profile across the cell. Polarization curves are numerically simulated and successfully compared with experiments for different methanol feed concentrations. The model predicts with accuracy the influence of the methanol feed concentration on the cell performance and the correct trends of the current density and methanol feed concentration, on methanol and water crossover. The model is rapidly implemented and is therefore suitable for inclusion in real-time system level DMFC calculations. Due to its simplicity the model can be used to help seek for possibilities of optimizing the cell performance of a passive DMFC by studying impacts from variations of the design parameters such as membrane thickness, catalyst loading, diffusion layers type and thicknesses.     

Effect of cell orientation on the performance of passive direct methanol fuel cells

A passive liquid feed direct methanol fuel cell (DMFC) with neither liquid pump nor a gas compressor was tested at different orientations. The experimental results showed that the vertical operation always yielded better performance than did the horizontal operation. It was further demonstrated that the improved performance in the vertical orientation was caused by the increased operating temperature as a result of a higher rate of methanol crossover, which resulted from the stronger natural convection in the vertical orientation. The constant current discharging tests showed that, although the vertical operation of the passive DMFC can yield better performance, the fuel utilization at this orientation is lower as a result of the increased rate of methanol crossover. It was also shown that the horizontal orientation with the anode facing upward rendered an effective removal of both CO₂ bubbles on the anode and liquid water on the cathode and thereby a relative stable operation. Finally, it was revealed that the horizontal orientation with the anode facing downward exhibited rather unstable and short discharging duration because of the difficulties in removing CO₂ bubbles from the anode and the liquid water from the cathode at this particular orientation.     

A study on the overall efficiency of direct methanol fuel cell by methanol crossover current

This paper was presented to determine the methanol crossover and efficiency of a direct methanol fuel cell (DMFC) under various operating conditions such as cell temperature, methanol concentration, methanol flow rate, cathode flow rate, and cathode backpressure. The methanol crossover measurements were performed by measuring crossover current density at an open circuit using humidified nitrogen instead of air at the cathode and applied voltage with a power supply. The membrane electrode assembly (MEA) with an active area of 5 cm² was composed of a Nafion 117 membrane, a Pt–Ru (4 mg/cm²) anode catalyst, and a Pt (4 mg/cm²) cathode catalyst. It was shown that methanol crossover increased by increasing cell temperature, methanol concentration, methanol flow rate, cathode flow rate and decreasing cathode backpressure. Also, it was revealed that the efficiency of the DMFC was closely related with methanol crossover, and significantly improved as the cell temperature and cathode backpressure increased and methanol concentration decreased.     

Characterization of direct formic acid fuel cells by Impedance Studies: In comparison of direct methanol fuel cells

We characterized direct liquid fuel cells by electrochemical impedance spectroscopy (EIS) combined with reversible hydrogen electrode (RHE) under fuel cell operating conditions. EIS has been successfully implemented as an in-situ diagnostic tool using an impedance setup with RHE, capable of singling out individual contributions to the overall polarization of fuel cells and separating the anode and cathode contributions. While a direct methanol fuel cell (DMFC) anode was subject to substantial poisoning by reaction intermediates due to better accessibility of methanol to catalyst surface regardless of anode diffusion media, a direct formic acid fuel cell (DFAFC) anode suffered from significant mass transfer limitation depending on the anode diffusion media property and formic acid concentration. The high frequency resistance of a DFAFC cathode increased linearly with an increase of formic acid concentration by membrane dehydration effect. Interestingly, on both the DMFC and DFAFC cathodes, decrease in the mixed charge transfer resistance with an increase of fuel crossover was observed together with a drop in the cathode potential.     

Methanol and water crossover in a passive liquid-feed direct methanol fuel cell

Methanol crossover, water crossover, and fuel efficiency for a passive liquid-feed direct methanol fuel cell (DMFC) were all experimentally determined based on the mass balance of the cell discharged under different current loads. The effects of different operating conditions such as current density and methanol concentration, as well as the addition of a hydrophobic water management layer, on the methanol and water crossover were investigated. Different from the active DMFC, the cell temperature of the passive DMFC increased with the current density, and the changes of methanol and water crossover with current density were inherently coupled with the temperature rise. When feeding with 2–4 M methanol solution, with an increase in current density, both the methanol crossover and the water crossover increased, while the fuel efficiency first increased but then decreased slightly. The results also showed that a reduction of water crossover from the anode to the cathode was always accompanied with a reduction of methanol crossover. Not only did the water management layer result in lower water crossover or achieve neutral or reverse water transport, but it also lowered the methanol crossover and increased the fuel efficiency.     

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.     

Characterization of a liquid feed direct methanol fuel cell with Sierpinski carpets fractal current collectors

The bipolar plate/current collector plays an important role in direct methanol fuel cells (DMFC). A current collector with different geometries could have a significant influence on cell performance. This paper presents fractal geometry application to current collector design in a direct methanol fuel cell (DMFC). This new current collector design is named CCFG (Current Collectors with Fractal Geometry). This research determined how to make a better free open design for the current collector on a printed circuit board based DMFC. The results show that both the free area ratio and total holes perimeter length on the bipolar plate affect the cell performance. The total number of holes on the perimeter presents greater effects than the free open ratio. The cell performance is more sensitive using a cathode current collector than the anode current collector.     

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.