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.     

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.     

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.     

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.     

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.     

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.     

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

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

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.     

Development of an advanced MEA to use high-concentration methanol fuel in a direct methanol fuel cell system

Despite serious methanol crossover issues in Direct Methanol Fuel Cells (DMFCs), the use of high-concentration methanol fuel is highly demanded to improve the energy density of passive fuel DMFC systems for portable applications. In this paper, the effects of a hydrophobic anode micro-porous layer (MPL) and cathode air humidification are experimentally studied as a function of the methanol-feed concentration. It is found in polarization tests that the anode MPL dramatically influences cell performance, positively under high-concentration methanol-feed but negatively under low-concentration methanol-feed, which indicates that methanol transport in the anode is considerably altered by the presence of the anode MPL. In addition, the experimental data show that cathode air humidification has a beneficial effect on cell performance due to the enhanced backflow of water from the cathode to the anode and the subsequent dilution of the methanol concentration in the anode catalyst layer. Using an advanced membrane electrode assembly (MEA) with the anode MPL and cathode air humidification, we report that the maximum power density of 78 mW/cm² is achieved at a methanol-feed concentration of 8 M and cell operating temperature of 60 °C. This paper illustrates that the anode MPL and cathode air humidification are key factors to successfully operate a DMFC with high-concentration methanol fuel.     

Design and simulation of a liquid electrolyte passive direct methanol fuel cell with low methanol crossover

This study investigates an aqueous solution of sulfuric acid that serves as the liquid electrolyte (LE) in a passive direct methanol fuel cell (DMFC). The addition of an LE can reduce methanol crossover and increase the fuel utilization significantly. To improve the performance of an LE-DMFC, a mathematical model is developed to optimize the thicknesses of both the LE layer and the Nafion membrane. The maximum power density of the LE-DMFC is improved by approximately 30% compared with a conventional DMFC (C-DMFC) when each is fed by methanol solutions of the same concentration. Due to the low methanol crossover of the LE-DMFC, a highly concentrated methanol solution can be directly fed into the LE-DMFC. The discharge time and volume energy density of the LE-DMFC are two times longer and three times greater than those of the C-DMFC, respectively. In addition, fuel utilization increases by approximately 100%.     

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.     

Vapor feed direct methanol fuel cells with passive thermal-fluids management system

The present paper describes a novel technology that can be used to manage methanol and water in miniature direct methanol fuel cells (DMFCs) without the need for a complex micro-fluidics subsystem. At the core of this new technology is a unique passive fuel delivery system that allows for fuel delivery at an adjustable rate from a reservoir to the anode. Furthermore, the fuel cell is designed for both passive water management and effective carbon dioxide removal. The innovative thermal management mechanism is the key for effective operation of the fuel cell system. The vapor feed DMFC reached a power density of 16.5 mW cm⁻² at current density of 60 mA cm⁻². A series of fuel cell prototypes in the 0.5 W range have been successfully developed. The prototypes have demonstrated long-term stable operation, easy fuel delivery control and are scalable to larger power systems. A two-cell stack has successfully operated for 6 months with negligible degradation.     

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.     

Molecular sieve as an effective barrier for methanol crossover in direct methanol fuel cells

Methanol crossover is still a significant barrier to the commercialization of direct methanol fuel cells with wide-used Nafion® membrane. Herein, molecular sieve is introduced into the design of polymer electrolyte membrane to alleviate methanol crossover. The UZM-9 zeolite with an intermediate window size of 0.42 nm can effectively separate hydrated methanol (ca. 1.10 nm) and hydrated proton (ca. 0.23 nm). The methanol diffusion rate through the membrane is effectively suppressed after modified with UZM-9, which is about four times lower than the origin Nafion® membrane. The resulted peak power density reached 80 mW cm⁻² with 2 mol L⁻¹ methanol solution feed, which is 2.5-fold higher than that of direct methanol fuel cell with commercial Nafion® membrane. These results open a promising route to alleviate methanol crossover in direct methanol fuel cells.     

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.     

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

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

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.     

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.     

Modelling and experimental studies on a direct methanol fuel cell working under low methanol crossover and high methanol concentrations

A number of issues need to be resolved before DMFC can be commercially viable such as the methanol crossover and water crossover which must be minimised in portable DMFCs.