Valveless piezoelectric micropump for fuel delivery in direct methanol fuel cell (DMFC) devices

Fuel cells are being considered as an important technology that can be used for various power applications. For portable electronic devices such as laptops, digital cameras, cell phone, etc., the direct methanol fuel cell (DMFC) is a very promising candidate as a power source. Compared with conventional batteries, DMFC can provide a higher power density with a long-lasting life and recharging which is almost instant. However, many issues related to the design, fabrication and operation of miniaturized DMFC power systems still remain unsolved. Fuel delivery is one of the key issues that will determine the performance of the DMFC. To maintain a desired performance, an efficient fuel delivery system is required to provide an adequate amount of fuel for consumption and remove carbon dioxide generated from fuel cell devices at the same time. In this paper, a novel fuel delivery system combined with a miniaturized DMFC is presented. The core component of this system is a piezoelectric valveless micropump that can convert the reciprocating movement of a diaphragm activated by a piezoelectric actuator into a pumping effect. Nozzle/diffuser elements are used to direct the flow from inlet to outlet. As for DMFC devices, the micropump system needs to meet some specific requirements: low energy consumption but a sufficient fuel flow rate. Based on theoretical analysis, the effect of piezoelectric materials properties, driving voltage, driving frequency, nozzle/diffuser dimension, and other factors on the performance of the whole fuel cell system will be discussed. As a result, a viable design of a micropump system for fuel delivery can be achieved and some simulation results will be presented as well.     

Power management of a direct methanol fuel cell system

The effect of discharge rate of direct methanol fuel cell (DMFC) on fuel efficiency was comparatively investigated using a DMFC single cell and a DMFC system. The results obtained from the single cell were used to model the DMFC system. Several semi-empirical equations were derived that relate discharge current, voltage, power output, energy density and fuel consumption for a nominal 25 W DMFC system. The decrease in fuel efficiency with decreased power output that is observed for the DMFC system is attributable to the increase of methanol crossover that can be observed for an individual cell. A DMFC system can achieve maximum energy density and fuel efficiency at an appropriately high level of power output.     

Performance improvement in direct methanol fuel cells using a highly porous corrosion-resisting stainless steel flow field

Power generation with direct methanol fuel cell (DMFC) systems requires only simple equipment, and has the important advantage of using a liquid fuel with higher energy density and easier handling characteristics than hydrogen. However, the power output of DMFC is lower than hydrogen fuel cells. To improve the power output of DMFC it is very important to reduce diffusion polarization at higher current density conditions. This research used a corrosion-resisting type porous stainless steel developed based on the technology for metal–hydride battery electrodes in the separator flow fields for reactants and products in a single cell DMFC and analyzed its influence on performance characteristics.     

Li-ion batteries and portable power source prospects for the next 5–10 years

This paper describes the possible evolution of Li-ion technology, and evaluates the expected improvements, arising from new materials to cell technology. New active materials under investigation and electrode process improvements may allow an ultimate final energy density of more than 500 Wh/l and 200 Wh/kg, in the next 5–10 years, while maintaining sufficient power densities. A new rechargeable battery technology cannot be foreseen today that surpasses this. The possible use of small fuel cells is also discussed. The only solution would be direct methanol fuel cell (DMFC) technology, providing that the remaining important technological issues are solved. The association with a rechargeable battery could provide an optimised energy + power, hybrid power source. Several simulated comparisons for small- to medium-sized power sources are described, between fuel cells and batteries. Hybrid Li-ion/DMFC is a good option for systems larger than 1 kWh. The hybrid concept of high-energy–low-power primary lithium (as the fuel) with high-power Li-ion (as the cell stack) is an already available, cost effective solution where long run times are required.     

Performance characteristics of a novel tubular-shaped passive direct methanol fuel cell

The present work consists of a tubular-shaped direct methanol fuel cell (DMFC) that is operated completely passively with methanol solution stored in a central fuel reservoir. The benefit of a tubular-shaped DMFC over a planar-shaped DMFC is the higher instantaneous volumetric power energy density (power/volume) associated with the larger active area provided by the tubular geometry. Membrane electrode assemblies (MEAs) with identical compositions were installed in both tubular and planar-shaped, passive DMFCs and tested with 1, 2, and 3 M methanol solutions at room temperature. The peak power density for the tubular DMFC was 19.0 mW cm⁻² and 24.5 mW cm⁻² while the peak power density for the planar DMFC was 20.0 mW cm⁻² and 23.0 mW cm⁻² with Nafion^® 212 and 115 MEAs, respectively. Even though the performance of the fuel cell improved with each increase in methanol concentration, the fuel and energy efficiencies decreased for both the tubular and planar geometries due to increased methanol crossover. The tubular DMFC experienced higher methanol crossover potentially due to a higher static fluid pressure in the anode fuel reservoir (AFR) caused by the vertical orientation of the tubular fuel reservoir. The performance of the tubular DMFC in this work represents an 870% improvement in power density from the previous best, passive, tubular DMFC found in the literature.     

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.     

Compact direct methanol fuel cells for portable application

Consumers’ demand for portable audio/video/ICT products has driven the development of advanced power technologies in recent years. Fuel cells are a clean technology with low emissions levels, suitable for operation with renewable fuels and capable, in a next future, of replacing conventional power systems meeting the targets of the Kyoto Protocol for a society based on sustainable energy systems. Within such a perspective, the objective of the European project MOREPOWER (compact direct methanol fuel cells for portable applications) is the development of a low-cost, low temperature, portable direct methanol fuel cell (DMFC; nominal power 250 W) with compact construction and modular design for the potential market area of weather stations, medical devices, signal units, gas sensors and security cameras. This investigation is focused on a conceptual study of the DMFC system carried out in the Matlab/Simulink^® platform: the proposed scheme arrangements lead to a simple equipment architecture and a efficient process.     

Performance evaluation of direct methanol fuel cells for portable applications

This study examines the feasibility of powering a range of portable devices with a direct methanol fuel cell (DMFC). The analysis includes a comparison between a Li-ion battery and DMFC to supply the power for a laptop, camcorder and a cell phone. A parametric study of the systems for an operational period of 4 years is performed. Under the assumptions made for both the Li-ion battery and DMFC system, the battery cost is lower than the DMFC during the first year of operation. However, by the end of 4 years of operational time, the DMFC system would cost less. The weight and cost comparisons show that the fuel cell system occupies less space than the battery to store a higher amount of energy. The weight of both systems is almost identical. Finally, the CO₂ emissions can be decreased by a higher exergetic efficiency of the DMFC, which leads to improved sustainability.     

Polymer separator and low fuel concentration to minimize crossover in microfluidic direct methanol fuel cells

Microfluidic direct methanol fuel cell (DMFC) has some key issues, such as fuel crossover and water management, which typically hamper the development of conventional polymer electrolyte‐based fuel cells. Here a method of minimizing fuel crossover in microfluidic DMFC is reported. A polymer separator at the interface of the fuel and electrolyte streams in a single‐channel microfluidic DMFC is used to reduce the cross‐sectional area across where methanol can diffuse. Based on the optimized fuel rate, fuel concentration and pore radius, a maximum power density of 7.4 mW cm^?2 was obtained with the separator using 2 M methanol. This simple design improvement reduces the voltage loss at the cathode and leads to better performance. Copyright © 2014 John Wiley & Sons, Ltd.     

Fuel cell power trains for road traffic

Legal regulations, especially the low emission vehicle (LEV) laws in California, are the driving forces for more intensive technological developments with respect to a global automobile market. In the future, high efficient vehicles at very low emission levels will include low temperature fuel cell systems (e.g., polymer electrolyte fuel cell (PEFC)) as units of hydrogen-, methanol- or gasoline-based electric power trains. In the case of methanol or gasoline/diesel, hydrogen has to be produced on-board using heated steam or partial oxidation reformers as well as catalytic burners and gas cleaning units. Methanol could also be used for direct electricity generation inside the fuel cell (direct methanol fuel cell (DMFC)). The development potentials and the results achieved so far for these concepts differ extremely. Based on the experience gained so far, the goals for the next few years include cost and weight reductions as well as optimizations in terms of the energy management of power trains with PEFC systems. At the same time, questions of fuel specification, fuel cycle management, materials balances and environmental assessment will have to be discussed more intensively. On the basis of process engineering analyses for net electricity generation in PEFC-powered power trains as well as on assumptions for both electric power trains and vehicle configurations, overall balances have been carried out. They will lead not only to specific energy demand data and specific emission levels (CO₂, CO, VOC, NO_x) for the vehicle but will also present data of its full fuel cycle (FFC) in comparison to those of FFCs including internal combustion engines (ICE) after the year 2005. Depending on the development status (today or in 2010) and the FFC benchmark results, the advantages of balances results of FFC with PEFC vehicles are small in terms of specific energy demand and CO₂ emissions, but very high with respect to local emission levels.     

Performance of miniaturized direct methanol fuel cell (DMFC) devices using micropump for fuel delivery

A fuel cell is a device that can convert chemical energy into electricity directly. Among various types of fuel cells, both polymer electrolyte membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) can work at low temperature (<80 °C). Therefore, they can be used to supply power for commercial portable electronics such as laptop computers, digital cameras, PDAs and cell phones. The focus of this paper is to investigate the performance of a miniaturized DMFC device using a micropump to deliver fuel. The core of this micropump is a piezoelectric ring-type bending actuator and the associated nozzle/diffuser for directing fuel flow. Based on the experimental measurements, it is found that the performance of the fuel cell can be significantly improved if enough fuel flow is induced by the micropump at anode. Three factors may contribute to the performance enhancement including replenishment of methanol, decrease of diffusion resistance and removal of carbon dioxide. In comparison with conventional mini pumps, the size of the piezoelectric micropump is much smaller and the energy consumption is much lower. Thus, it is very viable and effective to use a piezoelectric valveless micropump for fuel delivery in miniaturized DMFC power systems.     

Active direct methanol fuel cell: An overview

Direct methanol fuel cells (DMFC) are attractive for portable applications such as phone chargers, LEDs, sensors, etc. This study will present an overview of an active system of DMFC. An active system of DMFC is suitable for portable applications due to its high power output. A complete active system of DMFC consists of internal and external components. However, most of the research work only focused on the fundamental issues of internal components such as membranes and catalysts. Very few works have presented on a complete active system of DMFC. This study presents a review of both external and internal components used in active systems, including the fuel and product management utilized in active DMFC. The performance of a DMFC tested in an active system is also included. Finally, this paper summarizes the challenges and future advances in the design of active system DMFC.     

Passive direct methanol fuel cells for portable electronic devices

Due to the increasing demand for electricity, clean, renewable energy resources must be developed. Thus, the objective of the present study was to develop a passive direct methanol fuel cell (DMFC) for portable electronic devices. The power output of six dual DMFCs connected in series with an active area of 4 cm² was approximately 600 mW, and the power density of the DMFCs was 25 mW cm⁻². The DMFCs were evaluated as a power source for mobile phone chargers and media players. The results indicated that the open circuit voltage of the DMFC was between 6.0 V and 6.5 V, and the voltage under operating conditions was 4.0 V. The fuel cell was tested on a variety of cell phone chargers, media players and PDAs. The cost of energy consumption by the proposed DMFC was estimated to be USD 20 W⁻¹, and the cost of methanol is USD 4 kW h. Alternatively, the local conventional electricity tariff is USD 2 kW h. However, for the large-scale production of electronic devices, the cost of methanol will be significantly lower. Moreover, the electricity tariff is expected to increase due to the constraints of fossil fuel resources and pollution. As a result, DMFCs will become competitive with conventional power sources.     

Microfluidic direct methanol fuel cell by electrophoretic deposition of platinum/carbon nanotubes on electrode surface

Carbon nanotubes (CNTs) supported platinum (Pt) nanoparticles prepared via electrophoretic deposition are used as catalyst layer of a microfluidic direct methanol fuel cell (DMFC), to study the influence of catalyst layer materials and deposition methods on the cell performance. A Y‐shaped channel is designed and microfabricated. It is verified by cyclic voltammetric measurements that shows ca. 317.7% increase in the electrochemical active surface area for the electrode with CNTs over that without CNT. Scanning electron microscopy observations indicate the network formation within the electrode because of a 3‐D structure of CNTs, which could be beneficial to the increasing electrode kinetics and to the improvement in fuel utilization. Comparison between the DMFCs with and without CNTs as support shows that the proof‐of‐concept microfluidic DMFC with Pt/CNTs electrode is able to reach a maximum power density of 5.70 mW cm^?2 at 25 °C, while the DMFC with plain Pt electrode only has a maximum power density of 2.75 mW cm^?2. Copyright © 2015 John Wiley & Sons, Ltd.     

Overview on the application of direct methanol fuel cell (DMFC) for portable electronic devices

Technologically advanced human societies require specialized tools and equipment to enable their diverse and mobile activities. Portable electronic devices like laptop, PDA, handphone, etc. are now an essential tool for many people in their daily lives. The rechargeable batteries used to power the portable electronic devices could be improved upon with regards to power density, and there is a crucial need for efficient, renewable and more environmentally friendly power sources. Many researchers have shown that the direct methanol fuel cell (DMFC) is an appropriate alternative to rechargeable battery technology, although many factors must be resolved before it can be commercialized. This paper gives an overview on the possibilities for using the DMFC as portable electronic devices power source along with some views on current and future trends in DMFC development, economic analysis and presents the current problems and solutions by DMFC researchers.     

Towards operating direct methanol fuel cells with highly concentrated fuel

A significant advantage of direct methanol fuel cells (DMFCs) is the high specific energy of the liquid fuel, making it particularly suitable for portable and mobile applications. Nevertheless, conventional DMFCs have to be operated with excessively diluted methanol solutions to limit methanol crossover and the detrimental consequences. Operation with diluted methanol solutions significantly reduces the specific energy of the power pack and thereby prevents it from competing with advanced batteries. In view of this fact, there exists a need to improve conventional DMFC system designs, including membrane electrode assemblies and the subsystems for supplying/removing reactants/products, so that both the cell performance and the specific energy can be simultaneously maximized. This article provides a comprehensive review of past efforts on the optimization of DMFC systems that operate with concentrated methanol. Based on the discussion of the key issues associated with transport of the reactants/products, the strategies to manage the supply/removal of the reactants/products in DMFC operating with highly concentrated methanol are identified. With these strategies, the possible approaches to achieving the goal of concentrated fuel operation are then proposed. Past efforts in the management of the reactants/products for implementing each of the approaches are also summarized and reviewed.     

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

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

A silicon‐based micro direct methanol fuel cell stack with a serial flow path design

A 6‐cell silicon‐based micro direct methanol fuel cell (μDMFC) stack utilized the serial flow path design was developed. The effect of the structure of flow path on the performance of the stack was investigated using polarization characterization and electrochemical impedance analysis. Further, the voltage distribution for individual cells under different current density was discussed. The results indicated that the μDMFC stack with the serial flow path design exhibited better performance than that utilized the parallel flow path due to uniform mass transfer of methanol as a result of the use of the serial flow path. Such a μDMFC stack generates a peak output power of ca. 187 mW, corresponding to an average power density of ca. 21.7 mWcm^‐2, and exhibits a steady‐state power output for more than 100 h. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Portable DMFC system with methanol sensor-less control

This work develops a prototype 20 W portable DMFC by system integration of stack, condenser, methanol sensor-less control and start-up characteristics. The effects of these key components and control schemes on the performance are also discussed. To expedite the use of portable DMFC in electronic applications, the system utilizes a novel methanol sensor-less control method, providing improved fuel efficiency, durability, miniaturization and cost reduction. The operating characteristics of the DMFC stack are applied to control the fuel ejection time and period, enabling the system to continue operating even when the MEAs of the stack are deteriorated. The portable system is also designed with several features including water balance and quick start-up (in 5 min). Notably, the proposed system using methanol sensor-less control with injection of pure methanol can power the DVD player and notebook PC. The system specific energy and energy density following three days of operation are 362 Wh kg⁻¹ and 335 Wh L⁻¹, respectively, which are better than those of lithium batteries (∼150 Wh kg⁻¹ and ∼250 Wh L⁻). This good energy storage feature demonstrates that the portable DMFC is likely to be valuable in computer, communication and consumer electronic (3C) markets.