Characterization of a direct methanol fuel cell using Hilbert curve fractal current collectors

The current collector or bi-polar plate is a key component in direct methanol fuel cells (DMFCs). Current collector geometric designs have significant influence on cell performance. This paper presents a continuous type fractal geometry using the Hilbert curve applied to current collector design in a direct methanol fuel cell. The Hilbert curve fractal geometry current collector is named HFCC (Hilbert curve fractal current collector). This research designs the current collector using a first, second and third order open carved HFCC shape. The cell performances of the different current collector geometries were measured and compared. Two important factors, the free open ratio and total perimeter length of the open carved design are discussed. The results show that both the larger free open ratio and longer carved open perimeter length present higher performance.     

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.     

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.     

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.     

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.     

Design and fabrication of light weight current collectors for direct methanol fuel cells using the micro-electro mechanical system technique

The direct methanol fuel cell (DMFC) is suitable for portable applications. Therefore, a light weight and small size is desirable. The main objective of this paper is to design and fabricate a light weight current collector for DMFC usage. The light weight current collector mainly consists of a substrate with two thin film metal layers. The substrate of the current collector is an FR4 epoxy plate. The thin film metal layers are accomplished by the thermo coater technique to coat metal powders onto the substrate surfaces. The developed light weight current collectors are further assembled to a single cell DMFC test fixture to measure the cell performance. The results show that the proposed current collectors could even be applied to DMFCs because they are light, thin and low cost and have potential for mass production.     

Performance of DMFC with SS 316 bipolar/end plates

This work mainly emphasizes the development of new materials and design for a bipolar/end plate in a direct methanol fuel cell (DMFC). According to the DOE requirements, preliminary studies show that SS 316 (Stainless Steel 316) is a suitable candidate. Several flow field designs were studied and a modified serpentine design was proposed. SS 316 end plates were fabricated with an intricate modified serpentine flow field design on it. The performance of a single stack DMFC with SS 316 end plates were studied with different operational parameters. A long-term test was carried out for 100 h with recycling the methanol and the contaminants in the MEA were characterized. The stack efficiency is found to be 51% and polarization losses are discussed. SS 316 with low permeability resulted in an increased pressure drop across the flow field, which increased the fuel cell performance. The use of SS 316 as bipolar plate material will reduce the machining cost as well as volume of the fuel cell stack.     

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%.     

Evaluation of a compression molded composite bipolar plate for direct methanol fuel cell

A polymer–graphite composite bipolar plate of direct methanol fuel cell (DMFC) was fabricated by a compression molding method. The electrical conductivity and electrochemical behavior of the composite material under DMFC operating conditions were evaluated. The results show that the composite bipolar plate has a good electrical conductivity. Moreover, the through-plane conductivity of the composite material is higher than the in-plane one, which is ascribed to the anisotropic property of the composite bipolar plate resulted from the compression molding process. Corrosion tests show that the stable current density is below 10 μA cm⁻² under both anode and cathode conditions of DMFC. The discharge test of the DMFC single cell also presents a satisfactory result.     

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.     

Integration of Heat Pipe into Fuel Cell Technology

This paper describes recent applications of heat pipe technology in fuel cell systems, which include new stack designs with heat pipes to improve heat transfer as well as work on fuel cell system level design and engineering with adopting the heat pipe concept. In one design, micro-heat pipes are inserted and bonded in bipolar plates for thermal control in the fuel cell stack. In another design, flat heat pipes are integrated with a carbon bipolar plate for improving thermal control in the fuel cell stack. Finally, based on the heat pipe concept, we specifically developed a series of direct methanol fuel cell (DMFC) systems characterized as passive technology for methanol fuel delivery, water recirculation, and air and thermal management. Long-term durability and stability of the passive DMFC systems have been proved experimentally.     

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.     

Influence of current collectors design on the performance of a silicon-based passive micro direct methanol fuel cell

In this paper, the influence of current collector open ratio on the performance of a passive micro direct methanol fuel cell is evaluated. The device is based on a hybrid approach consisting of two microfabricated silicon current collectors assembled together with a commercial membrane electrode assembly. The characterization was performed by measuring polarization curves of the fuel cell using current collectors with different open ratios on anode and cathode. Results show that the way in which the open ratio of current collectors is combined has an effect not only on the output power but also on the repeatability of polarization curves. This study allows the setting of some general design rules for current collectors of passive micro direct methanol fuel cells.     

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.     

Operational characteristics of the direct methanol fuel cell stack on fuel and energy efficiency with performance and stability

This paper is presented to investigate operational characteristics of a direct methanol fuel cell (DMFC) stack with regard to fuel and energy efficiency, including its performance and stability under various operating conditions. Fuel efficiency of the DMFC stack is strongly dependent on fuel concentration, working temperature, current density, and anode channel configuration in the bipolar plates and noticeably increases due to the reduced methanol crossover through the membrane, as the current density increases and the methanol concentration, anode channel depth, and temperature decreases. It is, however, revealed that the energy efficiency of the DMFC stack is not always improved with increased fuel efficiency, since the reduced methanol crossover does not always indicate an increase in the power of the DMFC stack. Further, a lower methanol concentration and temperature sacrifice the power and operational stability of the stack with the large difference of cell voltages, even though the stack shows more than 90% of fuel efficiency in this operating condition. The energy efficiency is therefore a more important characteristic to find optimal operating conditions in the DMFC stack than fuel efficiency based on the methanol utilization and crossover, since it considers both fuel efficiency and cell electrical power. These efforts may contribute to commercialization of the highly efficient DMFC system, through reduction of the loss of energy and fuel.     

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.     

Performance and long-term stability of Ti metal and stainless steels as a metal bipolar plate for a direct methanol fuel cell

In this study, STS 316L (Stainless Steel 316L), STS 430, and Ti metal are investigated as metal bipolar plates for a direct methanol fuel cell (DMFC). The corrosion resistance of these materials is investigated by potentiodynamic and chronoamperometry tests. Their cell performance and long-term stability are then studied under a real fuel cell test. The corrosion resistance of the metal bipolar plates is in the order of Ti > STS 316L > STS 430. However, the results of the real fuel cell test differ from the results of the corrosion resistance. Ti shows the lowest performance due to a sharp performance decrease in ohmic loss regions, while STS 430 shows a lower performance decrease in ohmic loss regions. Although STS 430 has less resistance to corrosion than STS 316L in the simulated environment, STS 430 performs better as a metal bipolar plate for a DMFC than STS 316L, particularly, with regard to cell performance, cell resistance, and durability.     

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.     

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.     

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.