Effect of non-uniform parallel channel on performance of passive direct methanol fuel cell

In this paper, the effects of current collector on passive direct methanol fuel cell's (passive DMFC) performance and removing CO₂ gas is studied. For this purpose, a single cell with two arrangements of current collector in anode and cathode side is considered. In first arrangement, non-uniform parallel channels with 53.76% open ratio is used in the anode side and a perforated flow field with 34.5% open ratio is applied in the cathode side. In second arrangement, uniform parallel channels with 42.28% open ratio has been used in both anode and cathode sides. At the first arrangement, a maximum power of 20 mW cm⁻² in 4 M methanol concentrations and in the second arrangement a maximum power of 17.7 mW cm⁻² in 5 M methanol concentrations has been obtained. Furthermore, it is shown that using the current collector with non-uniform parallel channels is more effective in removing CO₂ gas than other parallel channels.     

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.     

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%.     

Influence of cathode oxygen transport on the discharging time of passive DMFC

In this paper, the long-term discharge performances of passive DMFC at different currents with different cell orientations were investigated. Water produced in the cathode was observed from the photographs taken by a digital camera. The results revealed that the passive DMFC with anode facing upward showed the best long-term discharge performance at high current. A few independent water droplets accumulated in cathode when the anode faced upward. Instead, in the passive DMFC with vertical orientation, a large amount of produced water flowed down along the surface of current collector. The passive DMFC with vertical orientation showed relatively good performance at low current. It was concluded that the cathode produced less water in a certain period of time at smaller current. In addition, the rate of methanol crossover in the passive DMFC with anode facing upward was relatively high, which leaded to a more rapid decrease of the methanol concentration in anode. The passive DMFC with anode facing downward resulted in the worst performance because it was very difficult to remove CO₂ bubbles produced in the anode.     

Evaluation of structural aspects and operation environments on the performance of passive micro direct methanol fuel cell

Passive micro direct methanol fuel cell (μDMFC) which operates based on fuel diffusion is preferred for portable applications for its structural simplicity. In this work, we have systematically investigated multiple variables including the hot-press conditions, current collector channel patterns, current collector open ratios, and their effects on the performance for passive μDMFC by experiments and simulations. Results indicate that vertical stripe pattern (VSP) is preferred for both anodes and cathodes due to the upward reaction products drift by natural convection. Open ratio of 45.6% and 35.8% are found to yield the best performance for anode and cathode, respectively. In addition, the external environmental conditions of vibration frequency, cell orientation, environmental temperature and atmospheric pressure are all discussed in detail in this work. The optimized fabrication, assembly and operation parameters shed light on the design considerations necessary for the wide adaptation of high-performance and durable passive μDMFC for portable applications.     

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.     

Experimental investigation of the effect of free openings of current collectors on a direct methanol fuel cell

A current collector is one of the key components of a direct methanol fuel cell (DMFC). For a planar-type DMFC, the current collector is usually fabricated from a thin metal that has open holes. The geometry of the current collector may have a significant effect on DMFC performance. Therefore, the design of the current collector is important for DMFC design. The objective of this study is to make a systematic experimental investigation of DMFC performance in the presence of current collectors with different free open ratios and total perimeter lengths of the free openings. Current collectors with 5 × 5, 7 × 7, and 10 × 10 hole arrangements under different total free open ratios of 30%, 40%, 50%, and 60% are investigated. The results show that the total free open ratio can significantly affect cell performance; they also show that decreasing the total free open ratio decreases cell performance, and increasing the total free open ratio increases cell performance. A high total free open ratio affects the total contact area between the membrane electrolyte assembly (MEA) and current collectors. Proper consideration of both the total free open ratio and the total contact area between the MEA and current collectors is necessary for the design of DMFC current collectors. In addition, a longer total perimeter of the free openings yields higher cell performance with the same free open ratio of the current collectors.     

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.     

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.     

Effects of structural aspects on the performance of a passive air-breathing direct methanol fuel cell

This study systematically investigates the effects of structural aspects on the performance of a passive air-breathing direct methanol fuel cell (DMFC). Three factors are selected in this study: (1) two different open ratios of the current collector; (2) two different assembly methods of the diffusion layer; and (3) three membrane types with different thicknesses. The interrelations and interactions among these factors have been taken into account. The results demonstrate that these structural factors combine to significantly affect the cell performance of DMFCs. The higher open ratio not only provides a larger area for mass transfer passage and facilitates removal of the products, but also promotes higher methanol crossover. The hot-pressed diffusion layer (DL) can mitigate methanol permeation while the non-bonded variant is able to enhance product removal. The increase of membrane thickness helps obtain a lower methanol crossover rate and higher methanol utilisation efficiency, but also depresses cell performance under certain conditions. In this research, the maximum power density of 10.7 mW cm⁻² is obtained by selecting the current collector with a lower open ratio, the non-bonded DL, and the Nafion 117 membrane. The effect of methanol concentration on the performance of DMFCs is also explored.     

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.     

Toward using porous metal-fiber sintered plate as anodic methanol barrier in a passive direct methanol fuel cell

A porous metal-fiber sintered plate (PMFSP) based on multi-tooth cutting and high-temperature solid-phase sintering is used as the methanol barrier at the anode of a passive DMFC in order to reduce the effect of methanol crossover. Its roles in controlling the mass transfer mechanisms related to reactant supply and product removal are also considered in this study. Results show that the cell performance can be significantly improved by using such a macroporous material, especially at a higher methanol concentration. The porosity of the PMFSP has great effects on the cell performance in the form of interacting with the current collector setup. When the combination of anodic circular-hole-array with an open ratio of 28.3% and cathodic parallel fence with 58% is used, it is favorable to use a lower porosity of 70%. When the above current collectors are reversed, a higher porosity of 80% is recommended. Results also demonstrate that the PMFSP with a medium thickness of 2 mm achieves a higher cell performance. Moreover, the PMFSP assembled in an outside manner is proved to be more able to enhance the cell performance than that based on inside-type. The mechanisms related to the roles of the PMFSP in mass transfer process are provided in detail.     

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.     

Utilization and positive effects of produced CO2 on the performance of a passive direct methanol fuel cell with a composite anode structure

This study investigates the CO₂ behavior and its effect on the performance of a passive direct methanol fuel cell (DMFC) by use of visualization method. Different flow field designs are introduced to the anode of the DMFC. The parallel-fence structure and the new composite structure with a sintered porous metal fiber felt (SPMFF) are experimentally compared in terms of the cell performance and mechanisms of reactant and product managements. Results show that the cell based on the composite structure yields the best performance during high-concentration operation. The vertical parallel-fence structure is more suitable at a lower methanol concentration. The visualization tests indicate that the use of a composite structure can help enhance convection due to CO₂ self-promoting behavior and also enhance gas storage in the channel to control the methanol concentration. The presence of CO₂ helps to restrain methanol crossover, which should be actively controlled to enhance internal convection and thereby improve the cell performance. The use of a composite structure enables the cell to perform stably for continuous operation at a higher methanol concentration. The open-circuit and dynamic characteristics of this composite-structure-based DMFC are also evaluated in this study.     

Structural diversity and orientation dependence of a liquid-fed passive air-breathing direct methanol fuel cell

This paper investigates the interesting effects of structural diversity and operating orientation on the performance of a liquid-fed passive air-breathing direct methanol fuel cell (PAB-DMFC). The results indicate that a higher thickness of the GEFC^®-10N membrane helps enhance the cell performance due to its ability in reducing methanol crossover (MCO). When the cell uses carbon cloth at the anode but carbon paper at the cathode as the diffusion media, it produces higher performances than other combinations. The work also confirms the merit of using a cathode diffusion layer since it improves water, methanol and heat management. As for the structural optimization of current collector, it is recommended to use the circular-hole-array pattern with a lower open ratio at the anode but the parallel-fence pattern with a higher open ratio at the cathode. It is further demonstrated that the vertical operation yields a higher cell performance at a lower methanol concentration while the horizontal operation performs better at a higher methanol concentration. Besides, the effects of opening pattern and working orientation on the CO₂ evolvement behaviors are analyzed by using visualized methods. Detailed mechanisms related to the resultant phenomena are comprehensively provided in this work.     

Development of an air-breathing direct methanol fuel cell with the cathode shutter current collectors

An air-breathing direct methanol fuel cell with a novel cathode shutter current collector is fabricated to develop the power sources for consumer electronic devices. Compared with the conventional circular cathode current collector, the shutter one improves the oxygen consumption and mass transport. The anode and cathode current collectors are made of stainless steel using thermal stamping die process. Moreover, an encapsulation method using the tailor-made clamps is designed to assemble the current collectors and MEA for distributing the stress of the edges and inside uniformly. It is observed that the maximum power density of the air-breathing DMFC operating with 1 M methanol solution achieves 19.7 mW/cm² at room temperature. Based on the individual DMFCs, the air-breathing stack consisting of 36 DMFC units is achieved and applied to power a notebook computer.     

A flow field enabling operating direct methanol fuel cells with highly concentrated methanol

In this work, an anode flow field that allows a direct methanol fuel cell (DMFC) to operate with highly concentrated methanol is developed and tested. The basic idea of this flow field design is to vaporize methanol solution in the flow field by utilizing the heat generated from the fuel cell so that the methanol concentration in the anode catalyst layer can be controlled to an appropriate level. The flow field is composed of two parallel flow channel plates, separated with a gap. The upper plate with a grooved serpentine flow channel is to vaporize a highly concentrated methanol solution to ensure the fuel to be completely vaporized before it enters the gap, while the lower plate, perforated to form a serpentine flow channel and located between the gap and the membrane electrode assembly (MEA), is to uniformly distribute the fuel onto the anode surface of the MEA. The test results show that this unique flow field design enables the DMFC operating with 16.0-M methanol to yield a power output similar to that with the conventional flow field design with 2.0-M methanol, significantly increasing the specific energy of the DMFC system. Finally, the effects of methanol solution flow rates and operating temperature on cell performance are investigated.     

Dominance evaluation of structural factors in a passive air-breathing direct methanol fuel cell based on orthogonal array analysis

This study comprehensively investigates the dominance of various structural factors in a passive air-breathing DMFC by means of orthogonal array analysis (OAA). Two membrane types, two assembly patterns of the diffusion layer and two open ratios of the current collector are prepared. Three target variables are selected as the performance indexes including the maximum power density (MPD), limiting current density (LCD) and open circuit voltage (OCV). The range analysis (RA) method and effect curves (ECs) are used to characterize the OAA data. The RA results demonstrate that the current collector and diffusion layer combine to dominate the values of MPD and LCD in a wide range of methanol concentrations from 0.5 to 8 M. The dominant structural factors related to the value of OCV at different methanol concentrations are also explored. In addition, the effect curves show that a medium methanol concentration like 2 M generally promotes higher values of MPD and LCD, while a relatively lower methanol concentration like 0.5 M benefits a higher value of OCV than others in a general statistical sense.     

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.     

Two-dimensional two-phase thermal model for passive direct methanol fuel cells

A two-dimensional two-phase thermal model is presented for direct methanol fuel cells (DMFC), in which the fuel and oxidant are fed in a passive manner. The inherently coupled heat and mass transport, along with the electrochemical reactions occurring in the passive DMFC is modeled based on the unsaturated flow theory in porous media. The model is solved numerically using a home-written computer code to investigate the effects of various operating and geometric design parameters, including methanol concentration as well as the open ratio and channel and rib width of the current collectors, on cell performance. The numerical results show that the cell performance increases with increasing methanol concentration from 1.0 to 4.0 M, due primarily to the increased operating temperature resulting from the exothermic reaction between the permeated methanol and oxygen on the cathode and the increased mass transfer rate of methanol. It is also shown that the cell performance upgrades with increasing the open ratio and with decreasing the rib width as the result of the increased mass transfer rate on both the anode and cathode.