Direct methanol fuel cells (DMFC) are typically supplied under pressure or capillary action with a solution of methanol in water optimized for the best specific power and power density at an operating temperature of about 60 °C. Methanol and water consumption at the anode together with water and methanol losses through membrane due to crossover create an imbalance over time so the fuel concentration at the anode drifts from the optimal ratio. In the present study, we demonstrate a DMFC with a means for continuous adjustment of water and methanol content in the anode fuel mixture of an air-breathing DMFC to maintain the optimal concentration for maximum and continuous power. Two types of piezoelectric micropumps were programmed to deliver the two liquids at the designated rate to maintain optimal concentration at the anode during discharge. The micropumps operate over a wide range of temperature, can be easily reprogrammed and can operate in any orientation. A study of performance at different current densities showed that at 100 mA/cm2, the self-contained, free convection, air-breathing cell delivers 31.6 mW/cm2 of electrode surface with thermal equilibrium reached at 52 °C. The micropumps and controllers consume only 2.6% of this power during 43 h of continuous unattended operation. Methanol utilization is 1.83 Wh cm−3.
However, current technology for these cells relies on systems requiring mechanical pumps for moving liquid fuel and air through the cell, increasing the overall volume and weight. A direct methanol fuel cell (DMFC) using diffused air (air-breathing) and in which the methanol fuel/water fuel mixture can be accurately controlled and replenished with low power micropumps provides a cost-effective approach as fuel cell size is scaled down for small portable devices. The approach also allows the DMFC to operate unattended for longer periods of time.
Compact air-breathing direct methanol fuel cells (AB-DMFC) without blowers have been proposed for a variety of portable applications including cell phones [4,5], personal data assistants, cameras, and portable computers . Current approaches for making very small DMFCs rely on capillary action to supply active sites on the anode with solutions of methanol . Similar to a small cigarette lighter, methanol solution is supplied in the form of replaceable cartridges, each with limited operating time. Key factors limiting the power and discharge time for these cells include the following: (1) depletion of methanol from the anode by crossover to cathode side of the membrane, which results in oxidative losses on the air-side and additional polarization of the cathode and (2) a large enough reservoir of methanol solution to maintain the correct concentration results in excess volume and weight. The relationship between temperature and DMFC efficiency has been extensively studied with most practical demonstrations performed at temperatures above 60 °C in large stacks [8,9]. Performance at room temperature is at a considerable sacrifice in specific power and fuel efficiency. In addition, smaller portable fuel cells are less practical due to the greater relative parasitic losses associated with balance of plant and system control .
As DMFCs have matured for commercial applications, the design and development of small, low-power micropumps has also advanced . These pumps can be used to feed water and methanol at controlled rates to maintain fuel composition at the anode with a minimum penalty for weight, volume, and power. For example, a piezoelectric pump fabricated using microelectronics manufacturing methods by the Fraunhofer Institute for Reliability and Microintegration (IZM, Munich, Germany) for this research can deliver up to 1.2 cc/min liquid before cavitation or resonance interferes at about 200 Hz. The compact miniature pumps are silent and insensitive to changes in orientation or acceleration. They are also self-priming so they can cycle on and off without outside intervention or priming as long as the inlet port remains in contact with the liquid. When properly energized the micropumps do not create cavitation or large variations in hydrostatic pressure. Commercial piezoelectric micropumps are supplied with compact controllers, which accept input signals and adjust the delivery rate by altering the frequency. The controllers require about 20 mA. This power can be supplied by lightweight Li/MnO2 cells or by the fuel cell itself.
Since methanol crossover from the anode side of the membrane is driven by the electrochemical oxidation on the anode catalyst, the crossover rate typically increases with increasing electrode current density . This feature holds true for Nafion proton exchange membranes at methanol concentrations up to 4 M. At this level of methanol content, there is adequate rate of replacement of the alcohol on the anode to continuously feed crossover through the membrane. Proton drag also assists this crossover, so that in high methanol content solutions, the crossover will increase with increasing current density . Each proton generated at the anode transports 2–5 molecules of methanol to the cathode depending upon the temperature . Protons moving toward the cathode drag a mixture of both water and methanol.
This paper deals with the design and testing of an AB-DMFC for unattended operation. Separate methanol and water micropump feeds and passive CO2 release minimize balance of plant power consumption, weight, and volume.
The membrane electrode assembly (MEA) structures evaluated in the 50-cm2 size single cell consisted of an anode catalyst of Pt/Ru on carbon bonded to Nafion® 117 membrane with a hot-bonded platinized-titanium sinter for the current collector. A platinized niobium foil tab was resistance-welded to the sinter and fed outside the cell through a gasket seal. The cathode used a platinum-on-carbon catalyst bonded to the membrane. A sintered/expanded metal composite diffusion media was hot-bonded to the cathode side of the membrane for current collection and control of evaporation/flooding. This custom design is self-supporting so that no electrode compression is required, thus saving substantial weight and volume from added fixturing. The cathode structure was optimized to lower oxygen reduction overpotential, provide the necessary electronic and ionic conductivity, and shed product water vapor. Ports for pumps, valves, and sensors were machined into the polysulfone body. Performance data was collected using neat methanol as the anode fuel with separate deionized water feed to maintain 3 M MeOH at the anode and passively diffused air on the cathode. No blowers or air compressors were necessary.
Methanol crossover was experimentally measured in a different fixture as follows: Vapor exiting the cathode was cooled by an ambient temperature condenser outside the fuel cell, passed through a separator and desiccant cartridge for water removal. The dry vapor was then analyzed for CO2 content by an infrared sensor (Vaisala, Inc., Woburn, MA). A flow meter (Agilent, Palo Alto, CA) downstream from the CO2 sensor provided accurate values for CO2 production rate at the cathode. The methanol crossover is calculated from the cathode CO2 production rate and is expressed as the equivalent current density that could be sustained anodically from the methanol that crossed over. Combining this direct rate of loss with the electrochemical consumption by the cell for useful power, the total rate at which methanol must be supplied is estimated as shown in Fig. 1.
With a set methanol concentration in the reservoir in commercial DMFCs supplied to the anode, alcohol concentration builds up at the anode as excess water crosses over the membrane. This requires the design to flow the methanol solution through the anode compartment, resulting in excess methanol which must be recycled and rebalanced together with water generated at the cathode. Although not discussed extensively in the literature, considerable volume and weight in the DMFC operating system must be dedicated to the liquid methanol and water pumps, reservoirs and recycling apparatus. The performance can vary widely over 0.5 M–4 M methanol concentrations  so the recover and remixing must be tightly controlled. These considerations severely limit the ability to design small, compact, lightweight DMFCs, which must operate for long periods of time unattended. The need for pumps, blowers, and gas/liquid separators further complicates designs for zero-gravity space station applications. However, the availability of methanol as a byproduct from space station bioprocesses may yet make DMFCs a desirable power supply to pursue for orbital and planetary missions.
Several approaches have been tried to fabricate micropumps, but the most suitable for small, portable AB-DMFCs uses the vibration of a piezoelectric ceramic plate made from lead zirconate titanate. The piezoelectric forms one wall of a small cavity (1–2 cm3) with oppositely hinged flaps on the opposing wall which permit exit and entry of the liquid as shown in Fig. 2 [16,17]. The finished IZM pump is about 15 × 5 mm in size with a thickness of 3 mm. Exit and entry tubes permit attachment to the fuel cell anode compartment and reservoir, respectively. A small alternating current (AC) voltage is applied to the plate to control flow rate through the pump. The micropumps are fabricated using industrial microelectronics manufacturing equipment.
Two piezoelectric commercial micropumps were selected based on their materials compatibility with methanol at elevated temperatures. The micropump selected to dispense water was built at the Fraunhofer IZM .
A second micropump provided by PAR Technologies (Hampton, VA) was chosen to feed methanol because the seals used are compatible with methanol at the temperatures used in this study. Compact battery-powered AC signal generators were obtained from two vendors. The pumps accept signals from the microprocessor programmed for the desired delivery rate. The pumps can deliver liquid with power as low as 75 and 22 mW. The release of product CO2 from the anode compartment was provided by a poppet valve which opens intermittently at a gauge pressure of 3 psi to minimize solvent loss.
Piezoelectric micropumps offer several advantages including low volume, quiet running, and low power demand. The pumps can be distributed in parallel across a large MEA or bipolar stack of MEAs or placed in series to provide added back pressure in larger designs. Use of a small lithium battery button cell makes the pump independent of the DMFC operating state.
A programmable microcontroller (Xeltec PIC) unit reported temperature with an embedded thermocouple. The microprocessor uses this data to determine the volume delivery rate necessary for the water and methanol based on a pre-programmed algorithm. Commercial microprocessors like the PIC variety convert sensor data to instructions for the inverter. The IZM Microcontroller accepts a signal to determine the pump rate by adjusting the square wave frequency delivered to the piezoelectric wall of the pump . An algorithm was developed to program the microcontroller to control liquid flow rates under load. Table 1 compares characteristics of the two types of micropumps.
A breadboard test fixture was machined from polystyrene to accommodate the 50-cm2 MEA, current collectors, pumps, vents, and thermocouple. Voltage and cell temperature were continuously monitored.
The two micropumps were tested with their respective liquids over a range of amplitudes and frequencies to examine the response behavior with 3 M methanol formed at the anode. Since each pump tends to have slightly different characteristics, it is important to establish the best operating parameters for each. Figure 3 shows the frequency-dependent pump rates for the PAR pump at various amplitudes.
The PAR pump rate can be controlled linearly between 40 and 100 Hz. Choosing an optimal value for the pump amplitude, stable and consistent behavior was observed over 4 h (Fig. 4).
The Fraunhofer pump was evaluated at 40, 80, and 160 Hz and with an amplitude of 25 volts direct current (VDC) (Fig. 5). Continuous, stable operation at three different frequencies is shown in Fig. 6.
Figure 7 shows the voltage of the 50-cm2 cell operating continuously with one-time 20-mL charges of neat methanol and water over 43 h. The cell was discharged at 2 amps during which time the measured methanol concentration in the anode compartment was maintained at 3 M. With the electrodes, water and fuel reservoirs contained in the same polysulfone housing, the cell temperature rose to 55 °C from room temperature over the first 15 h due to Ohmic heating and methanol crossover. Cell power leveled off at 0.64 W. The methanol volume dropped from 20 mL to 8 mL over this time, limiting the discharge time to 43 h.
Additional adjustments in the methanol rate and duty cycle were made. Methanol was fed to the top and water to the bottom of the anode compartment in the vertically oriented cell. Six aliquots of methanol/water were removed from the anode side over the two days of operation (50 mL per sample) and analyzed by gas chromatography. The results showed a limited fluctuation in methanol concentration between 2.6 M and 3.1 M. Some of this variation was due to the sampling times relative to the methanol and water injection times. This implies that some mechanical mixing with carefully placed micromixers will further extend operation time .
The cell operating time was extended to 43 h to yield 56 Wh. Fuel consumption was 37 mL of methanol solution for a fuel efficiency of 1.96 Wh/ccmethanol. Figure 8 shows the variation in cell voltage and power over a range of current densities with optimal performance close to 100 mA cm−2.
At approximately 30 h into operation, the fuel cell during polarization gave 1.24 W at 5 amps as shown in Fig. 8. During the automated, dual-pump operation, the power consumption of each of the components was determined as shown in Table 2. The measured power consumptions for the Fraunhofer and PAR micropumps are low with respect to the manufacturers' specifications. This is due in part to the very low backpressures of this design. Small, occasional excesses in liquid volume can be vented from a small “beak” valve. The Fraunhofer pump, used to feed water to the reservoir, is set to a pump rate of 1 mL/min for one minute each hour and the PAR, used to feed the neat methanol, is set to 80 mL/min for 0.124 min each hour. The pumps are operated intermittently to work at efficient points in their performance curves and also to create some turbulence in the fuel mix. The maximum power output of this type of single cell is 1320 mW. If the micro-ancillary components were powered by the fuel cell, the net power output of the single-cell device would be in the 1299 mW range (96% power utilization). This does not take into account any improvements in MEA structure that could be used to increase performance.
Using the calculated weights given in Table 3, the cell with battery-powered pumps provides 0.89 mW g−1 while a self-powered unit will provide 1.24 mW g−1.
With a successful 43-h operation, consideration was given to determining the specific power and energy for the bench-top unit. Each component was weighed and summarized in Table 3.
The bench top unit uses a thick polysulfone case (1.25″) which can be reduced in volume and weight by at least a factor of two using three-dimensional printing methods. The electronics packages of the two pumps can also be consolidated into one unit.
Previous investigators have shown that AB-DMFC performance is enhanced at elevated temperatures. In the present design, the cell self-heats without external temperature control. Heating occurs for the anode at 100 mA cm−2 from air oxidation of methanol which crosses over to the cathode. The exothermic mixing of water and methanol at the anode from the two micropumps helps to stabilize the operating temperature of 45 °C over 25 h before rising to 75 °C and falling as fuel is exhausted. Cells operating with 2.0 M and 4.0 M methanol feed with the same membrane typically operate at 23 °C and 28 °C, respectively .
This work provides an initial assessment of a functioning AB-DMFC with the use of methanol and water micropumps to control fuel mixture and reduce weight for improved efficiency. The design is scalable and takes advantage of new manufacturing methods developed for microelectronics. Additional features that can be used to increase running time and energy efficiency for larger sizes include: (1) additional liquid feed ports to supply larger electrode sizes; (2) addition of temperature and methanol sensors in the liquid anode compartment to adjust liquid feed rates; and (3) addition of one or more piezoelectric micromixers to improve solution homogeneity. Developments in low-methanol-crossover membranes [22–24] and microfluidic recycling of product water  will provide further enhancements for this approach. Compact ultrasonic sensors with 0.1% sensitivity for methanol content at the anode will provide improved micropump regulation during changes in temperature and duty cycle . Electrochemical methanol sensors have also been demonstrated , although both approaches may require too much operating power for small power supplies. Specific applications will benefit from injection-molded plastic designs that integrate membrane, anode chamber, current collectors, reservoirs, cell terminals and micropumps to achieve further reductions in weight and volume. Microfabrication methods developed in the semiconductor industry will enable the incorporation of more complex microfluidic capillary transport of separate methanol and water feeds . Developments in bonding of Nafion to polymer structures using interpenetration polymer networks will make it possible to create edge seals for the MEA [29,30] to improve durability.
Testing with this prototype fuel cell resulted in an understanding of the interactions between pump rate, frequency, temperature, back-pressure, and solution viscosity. At the pump rates used, the small differences in methanol and water viscosity and density did not affect pump behavior. The pumps worked continuously, holding the programmed pump rate constant. The test was performed over the temperature range of 22–40 °C. The fluid delivery rates were unchanged over this range and the periods of testing. The testing of the cell design ended after 43 h of continuous operation with a 9 V battery providing 26 mW of power necessary to control both micropumps.
Low-power, self-priming piezoelectric micropumps were successfully used to extend the operating time and specific power of a compact AB-DMFC. The results show that by controlling the methanol/water ratio continuously, methanol crossover can be minimized while maintaining the optimal methanol concentration at the anode. The use of micropumps to precisely control water and methanol feed rates for AB-DMFCs enables the design of new small fuel cells having long, unattended operating time required for lightweight power source needs. The pulsing behavior of these pumps assists in solution mixing and thermal stability at the anode. The volume and weight of methanol and water required for a given operating time are minimized.
The authors wish to thank Mr. Robert Stone for his technical assistance in preparing the MEAs, Mr. Simon Stone for the CO2 detector.
National Aeronautics and Space Agency Glen Research Center (NNC04CA47C).