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Advancing Microelectronics • Volume 29, No. 5 • September/October, 2002
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LTCC Fuel Cell System for Portable Wireless Electronics
Jeanne Pavio, Joseph Bostaph, Allison Fisher, Jerry Hallmark, Billy-Joe Mylan, Chenggang Xie, Motorola Labs, Energy Technologies Laboratory, 7700 S. River Parkway, Tempe, AZ 85284, 480-755-5313 phone, 480-755-5350 fax, jeanne.pavio@motorola.com
Abstract
A unique electronic power technology is described for wireless communications and portable computing systems. The direct methanol fuel cell prototype system, packaged using Low Temperature Cofired Ceramic Technology, has been shown to power or charge a range of portable electronic wireless systems successfully, encompassing both Personal Digital Assistants (PDAs) and cellular phones. Excellent initial performance has been demonstrated in multilayer ceramics technology which allows for prototype design changes that can be realized within a few days. System considerations and electronic controls will be reviewed along with basic technology and performance characteristics. The system consists of two LTCC ceramic multilayers with embedded microchannels for the fuel, electronic controls, embedded piezo micropumps, sensors and CO2 separator on one ceramic, and an air-breathing or forced air cathode ceramic on the opposite side. Compressed between the two ceramics is a polymer membrane with electrodes on either side. The electrodes consist of Pt and PtRu catalyst layers. Elements to be discussed will encompass targeted market applications, system considerations, materials and performance.
Key Words: Direct Methanol Fuel Cell (DMFC), Low Temp Cofired Ceramics (LTCC), Catalyst, Micropump, Anode, Cathode, Membrane Electrode Assembly (MEA)
Introduction
Low Temp Cofired Ceramics (LTCC) have proven to be an excellent material of choice for the development of Direct Methanol Portable Energy Systems currently underway at Motorola Labs. These MEMS systems are focused on providing power to a wide range of communications and computing devices including cell phones, pagers, two-way radios, personal digital assistants (PDAs), and laptop computers. The unique properties of LTCC allow for operation under temperatures ranging from below freezing to 250oC. These ceramics produce no warpage during compression. And, most importantly, they provide for rapid feedback on design changes, which can then be assessed in days or sometimes in a matter of hours.
The following discussion will review some of the market applications for Portable Fuel Cells as well as the system considerations of these applications. We will then focus on the materials themselves used in the design and the final performance achieved.
Applications and System Considerations
Although it was initially thought that the transportation market would be the first and foremost venue for widespread proliferation of alternative energy sources such as fuel cells, it is now widely believed that broad-based markets such as communications and computing are the areas where truly portable micro-energy systems will be championed and can be implemented relatively quickly. The need for such energy sources is quite clear in developing nations where infrastructure either does not exist or is compromised for one reason or another. A requirement for alternative power, such as fuel cells, was underlined recently during catastrophe and emergency conditions where off-the-grid availability was viewed as a necessity rather than an extravagance. On a daily basis, for business travelers, off-the-grid availability is a relief when trying to cope with cell phones, which invariably run out of charge at airports, and laptops which, all too often, cannot be used during off-times due to lack of a plug-in.
The first market of choice for any potential portable energy product is that of the cell phone. With over four hundred million cell phones sold in 2000, capturing only a small percentage of the battery total available market (TAM) would be profitable and lucrative. If we examine battery TAM for three critical communications and computing products, as seen in Table 1, it is evident that a substantial impact can be made by replacement of even a small percentage of batteries with portable energy sources, such as fuel cells. Each of these opportunities, however, present different constraints as well as different advantages in the development of alternative power sources. For instance, miniaturization is key if the goal is replacement of the internal battery of a cell phone with a fuel cell. This becomes less of an issue with two-way radios or desktop chargers. Both of these radio applications, however, must be able to achieve fast start-up and function after storage in temperatures that may be well below freezing. The requirement for fast start-up and for miniaturization dictate that the best alternative may be a hybrid solution where a battery is integrated within the device or within the fuel cell and provides not only the start-up capability, but also handling of peak power demands. Thus, the fuel cell can be sized for average load requirements of the phone.
If we examine power sources for laptop and notebook computers, even the average load demand is high at about 20 watts. This dictates an entirely different fuel cell device when compared to that of a cell phone, or even a small PDA. Size constraints are less demanding for portable computers as are requirements for operation under varying temperature extremes, since laptops are not usually left unprotected in an Alaskan winter, or left to melt in the desert sun. However, weight per unit of power may be an important consideration for the business traveler and must be seriously engineered for the final product. Figure 1 describes some of these system considerations for three representative product families in the communications and computing arena.
Design and Materials for a Direct Methanol Fuel Cell (DMFC)
At Motorola Labs the focus has initially been on development of direct methanol fuel cells for power levels of one watt and under. These DMFCs could be used for a battery desktop trickle charger. In a hybrid design system with a small battery providing peak power and instant start up, they could also be used to power a cell phone or PDA. This paper will discuss one design providing 100mW of net power to the device. It was demonstrated both as a desktop trickle charger as well as a hybrid power source for PDAs.
The system was designed using LTCC technology, in part, because of the capability to fabricate diverse and extensive channelization within the ceramic, and also because of the ability to provide rapid prototyping design changes at low relative cost. The rapid prototyping advantage contracts the development cycle, allowing for many successive iterations in the time it would take to machine or mold one iteration in alternate materials such as plastic or graphite.
After ample testing using a single cell test device, performance characteristics could be mapped and modeled to create the four cell design which became the standard for the 100mW fuel cell. Elements of the fuel cell are shown in Figure 2.
In order to produce energy, a direct methanol fuel cell oxidizes methanol at a PtRu anode electrode, forming CO2 and hydrogen ions, or protons, and negatively charged electrons. The electrons then go through a current collector to the outside circuit, providing power to an outside device. Meanwhile, protons pass through a polymeric Proton Exchange Membrane (PEM) to the cathode side of the fuel cell. At the cathode side, oxygen from air is reduced at the Pt catalyst site, which combines with these protons to form water, thereby completing the oxidation-reduction process [1]. This sequence is depicted in Figure 3.
The combination of polymeric membrane, electrocatalysts, and electrically conductive gas diffusion layers (GDLs), are known as the fuel cell “membrane electrode assembly,” or MEA. When multiple MEAs are sandwiched between current collectors, this fuel cell “stack” forms the heart of the fuel cell. However, in order to create a fuel cell device which is capable of powering real-world devices, a true system must be developed. In the Motorola Labs 100mW fuel cell system, this system consists of components for fuel delivery, the microfluidics, the fuel cell stack, and, finally, the electronics that it takes for control of the cell [2]. In our particular system, fuel delivery is accomplished through the use of a piezoelectric pump which pumps methanol directly into a mixing chamber which can be within the ceramic. Within the mixing chamber, which is part of the microfluidics section of the device, a methanol concentration sensor detects the level of methanol, sending a signal such that fuel concentration can be adjusted accordingly for optimum performance in the cell. The fuel is then transported via channels within the ceramic into the fuel cell stack area where the actual chemical reaction takes place. In order to manage the CO2 byproduct of the methanol oxidation, a mechanism for CO2 separation and venting is also included in the system. On the cathode side, air enters passively or actively (forced air). Both passive and active air systems have been demonstrated in our design. Oxygen is reduced at active Pt catalyst sites and combines with the protons, coming through the membrane, to form water. The water is then collected and recirculated through the system, through a pumping mechanism, and back into the water storage tank. The electronics controlling the system include a DC to DC converter, which, in the 100mW system, up-converts the voltage to appropriately match the communication’s device. Since this is a hybrid system, a small battery (5V) is included, which provides peak power capability and is continuously charged by the fuel cell. Materials used in the 100mW fuel cell include: the two LTCC ceramics for the microfluidics anode and cathode sides with current collectors on both sides; piezoelectric pumps for fuel transport (plastic encased and mounted in cavities in the ceramic); plastic (polypropylene) tanks for storage of water and methanol; gas diffusion layers on either side of the membrane; and Pt and PtRu catalysts deposited directly on a NafionTM 117 PEM membrane. Discrete electronic components forming the control circuitry are mounted directly on the ceramic. Extensive empirical testing of performance was completed, both short and long term, utilizing a single cell test device. From these results, predictive models were developed which simulated the 100mW net performance. A series of iterative prototypes, based on combinations of the single cell design, demonstrated 100mW performance while progressively adding functionality and reducing size. The final prototype can be seen in Figure 4.
This prototype, basically a small desktop charger, was shown to operate for approximately one week without refueling, and delivered a minimum of 100 mW of power. It also was capable of directly powering a Personal Digital Assistant device. The desktop charger with fuel measured 55mm X 55 mm X 55 mm. In another version a taller, more slim profile was achieved. The taller aspect ratio design, measuring 55 cm in width and 110cm in length is shown in Figure 5. The single cell test device from which this prototype was initially conceived can be seen in Figure 6. Note that an air screen is visible in the cathode side ceramic through which oxygen passes to the cathode electrode on the membrane.
In that way, the cathode screen is utilized for passive air conduction. Flow fields for methanol fuel distribution can be seen on the ceramic anode side. Prior to testing, these ceramics are bolted together, sandwiching the MEA.
Performance
Performance was measured for one week’s time without refueling, achieving 100mW net power under a resistive load of 100 ohms. It quickly became apparent during initial testing that a number of key issues must be resolved before such a device could be commercially viable as a core Motorola energy product. For instance, in a fuel cell system, pumps for fuel delivery and for water recovery are needed as well as a DC to DC converter, battery management electronics and methanol control management. All of these components require power to operate, power which is delivered by the fuel cell device itself. In the first prototypes of the 100 mW fuel cell, ancillary system components consumed 50% of the power output from the DMFC device. Subsequent redesign of the pump drivers and redesign and optimization of the control accessories are expected to provide a net power output of at least 75% of the gross output power of the DMFC, consuming only 25% for accessories and control.
Power management and efficiency optimization, as well as miniaturization, are critical issues to solve in order to develop the portable fuel cell into a product which presents a viable alternative to the best battery technology available. Currently, today’s Li-ion battery technology has been able to achieve 180-280 W. Hrs/Liter for small cells. The performance of the initial prototype developed was less than half that, with recent DMFC designs expected to meet or exceed the battery mark.
Demonstration testing of the 100mW fuel cell under constant power is shown in Figure 7.
Conclusions
A portable direct methanol fuel cell design has been presented, which is capable of powering communications devices such as PDAs, or performing as a desktop charger for a cell phone. The fuel cell is packaged using low temperature cofired ceramic (LTCC) technology. This ceramic media is suitable for producing specialized design features such as the channelization requirements for fuel delivery and distribution as well as those required for oxygen intake and for water recovery. Rapid prototyping of the ceramics is a real advantage, allowing assessment of major design changes in only a few days with minor adjustments, in hours.
The TAM for communications and computing system energy sources confirm that the focus on fuel cells for these devices could be profitable as well as beneficial to the marketplace, offering functions not currently available with batteries or plug-in power.
The 100mW fuel cell showed sustained and stable performance for approximately a week, without refueling. It is expected that, with additional fuel, prolonged operation beyond one week’s time would have been achievable.
New and future work is targeted at higher power levels of one to five watts of power. Analysis has shown that these power levels offer more efficient power management relevant to balance of plant accessories.
References
[1] Leo J.M.J. Blomen and Michael N. Mugerwa, Fuel Cell Systems, Plenum Press Publishers, New York, 1993.
[2] J. Bostaph, et al. “Microfluidic fuel delivery system for 100mw DMFC,” 199th meeting of the Electrochemical Society, Washington D.C., March 2001.
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