Performance of air-breathing direct methanol fuel cell with Au-coated aluminum current collectors

Current collectors of the direct methanol fuel cell (DMFC) are of significant importance for portable power sources, and greatly determine the weight energy density and cost of the cell. In this paper, the air-breathing aluminum (Al) current collectors have been developed for powering portable applications. The anode and cathode current collectors with the area of 4.5 cm² were fabricated on the Al substrates utilizing Computer Numerical Control (CNC) technology. To obtain strong anti-corrosion resistance, a 3-μm-Au layer was deposited on the current collectors using chemical plating. Compared with the graphite and stainless steel, the characterization of the Au-coated Al current collector was investigated to exhibit superior characteristics in electric conductivity, weight and electrochemical corrosion resistance. The current collector was applied to a DMFC and the cell performance was experimentally investigated under different operating conditions. The measured maximum power density of the DMFC could reach 19.8 mW cm⁻² at current density of 98 mA cm⁻² with 2 M methanol solutions. The results indicated that the Au-coated Al current collectors presented in this paper might be helpful for the development of portable power sources applied in future commercial applications.     

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.     

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.     

Operational characteristics of a passive air-breathing direct methanol fuel cell under various structural conditions

The operational characteristics of a small-scale passive air-breathing direct methanol fuel cell (PAB-DMFC) are comprehensively investigated under both steady-state and dynamic conditions. As the most important operating parameter, methanol concentration has significant effects on the cell performance. For different methanol concentrations (e.g., 0.5 and 8 M), the structural adaptations are particularly discussed. The results show that the structural factors are closely related to the influence degree of methanol concentration. In this study, the characteristics of the open circuit voltage (OCV) under various structural and methanol-concentration conditions are presented. Besides, the effects of other operating conditions such as running time, forced air convection and refueling action on the cell performance are also evaluated. In addition, a series of dynamic operations of the PAB-DMFC are conducted under different load cycles. Accordingly, the transient phenomena such as voltage undershoot and overshoot are explored. A fundamental principle for evaluating the operational characteristics of a PAB-DMFC is to simultaneously take into account the mass transfer requirements such as reactant delivery, product removal, methanol/water crossover control and so on.     

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.     

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.     

Performance of air-breathing direct methanol fuel cell with anion-exchange membrane

This report details development of an air-breathing direct methanol alkaline fuel cell with an anion-exchange membrane. The commercially available anion-exchange membrane used in the fuel cell was first electrochemically characterized by measuring its ionic conductivity, and showed a promising result of 1.0 × 10⁻¹ S cm⁻¹ in a 5 M KOH solution. A laboratory-scale direct methanol fuel cell using the alkaline membrane was then assembled to demonstrate the feasibility of the system. A high open-circuit voltage of 700 mV was obtained for the air-breathing alkaline membrane direct methanol fuel cell (AMDMFC), a result about 100 mV higher than that obtained for the air-breathing DMFC using a proton exchange membrane. Polarization measurement revealed that the power densities for the AMDMFC are strongly dependent on the methanol concentration and reach a maximum value of 12.8 mW cm⁻² at 0.3 V with a 7 M methanol concentration. A durability test for the air-breathing AMDMFC was performed in chronoamperometry mode (0.3 V), and the decay rate was approximately 0.056 mA cm⁻² h⁻¹ over 160 h of operation. The cell area resistance for the air-breathing AMDMFC was around 1.3 Ω cm² in the open-circuit voltage (OCV) mode and then is stably supported around 0.8 Ω cm² in constant voltage (0.3 V) mode.     

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.     

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.     

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 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.     

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.     

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.     

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.     

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.     

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.     

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.     

Air-breathing miniature planar stack using the flexible printed circuit board as a current collector

To maximize power density, the volume of a fuel cell stack should be reduced by miniaturizing the stack components. In this study, thin flexible printed circuit board was utilized as a current collector in order to reduce an air-breathing monopolar stack's volume. Also, the effects of varying the geometry and opening ratios of the ports to the cathode on stack performance were evaluated in order to determine the optimal cathode structure. Use of the thin current collector and cathode port optimization resulted in an output of 3.5 W from an 18 cm³ stack (power density of 350 mW/cm²). The effects of orientation under passive air-breathing operation were determined to be nearly negligible. All data was measured at ambient pressure and temperature, baseline conditions for mobile fuel cell intended for use in consumer electronics.     

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.     

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.