Methanol Fuel Cell
Modeling Environmental and Performance Differences in Miniaturized Micro Direct Methanol Fuel Cells
Interest in the use of micro direct methanol fuel cells in applications with low power needs has been growing since the technology was first developed, and the many different devices and scenarios in which such fuel cells would be of enormous benefit continues to drive research in the area. Direct experimental observation has been conducted on many fronts in an effort to ascertain optimum operating environments and levels of internal elements within the micro direct methanol fuel cell system — such as fuel concentration and rate of delivery — yielding higher outputs and/or longer operational periods for individual and stacked micro direct methanol fuel cells. In this review of current literature on the subject, several such experimental studies are utilized to develop a more comprehensive and multi-faceted view of the design and operational characteristics of the micro direct methanol fuel cell. This review essentially finds that operational temperature and fuel rates are the most influential aspects of the micro direct methanol fuel cell in terms of performance in power output and duration as well as in operational efficiency.
Introduction
As the number of electricity-dependent technological devices increases and size generally decreases (at least to a point), the need to develop innovative new power sources to be used in or in conjunction with these devices becomes more pressing. Micro direct methanol fuel cells have been shown to be one effective means of addressing this issue; though dependent on a fuel source in a way that a standard battery is not, in proper conditions and with proper design characteristics these fuel cells can provide power for much longer and are completely self-starting, requiring no additional energy input to begin generation (Mench et al. 2001; Lu et al. 2003). These fuel cells are also exponentially smaller than current battery technologies, and though this does limit their power generation capacity it increases their portability and the multitude of applications to which they are suited (Lu et al. 2003). Design techniques utilizing and even creating micro direct methanol fuel cells continue to evolve and develop, creating better understandings of the interacting forces that influence the performance of these micro direct methanol fuel cells. As the technology continues to grow more powerful through innovative cell design and stacking, smaller fuel cells and cell arrangements are able to deliver more power, creating even greater portability and expanding applicability (Lu et al. 2003; Lim et al. 2006; Kamitani et al. 2008).
Nomenclature
A
Area term, cm2
T
temperature, C
H
channel height, micrometers
W
channel width, micrometers
L
low flow channel length, cm
V
potential
M
molecular weight
P
pressure, Pa
Review of Current Design and Environmental Research
Though temperature has a major effect on the performance of micro direct methanol fuel cells, it is not generally something that can be easily controlled in practical applications, and thus much research has focused on improving the efficiency of micro direct methanol fuel cells at room temperature. Silicon micro structures for these fuels cells have drawn increasing attention, with experimental research showing that the reduction of the flow rate of methanol in room temperature micro direct methanol fuel cells can greatly improve fuel efficiency, though at the cost of actual power density (Kamitani et al. 2008). Reducing the fuel cell area also played a role in this experiment; reducing the size of the fuel cell surface area to 0.3cm2, the researchers were able to achieve a maximum power output o 12.5 mW cm-2 at a fuel rate of 5.52 microliters per minute-1, at an efficiency of 14.1% (Kamitani et al. 2008). Efficiency becomes 20.1% when the fuel rate is slowed to less than 2 microliters over the same time period, with a less extreme but still observable drop in power density (Kamitani et al. 2008).
Silicon was also a material of interest in an experiment that set out to design a micro direct methanol fuel cell that was specifically meant to be used in portable devices. Using a microelectromechnical technique, fuel delivery channels were etched into the surface of silicon wafers to create individual fuel cells capable of producing up to 50mW/cm2 at T. 60, though this power output was reduced to 16mW/cm2 at room temperature (Lu et al. 2003). This was achieved with a channel only 750W and 400H, and power output remained consistent despite tests using 2M, 4M, and 8M methanol solutions as fuels (Lu et al. 2003). This shows that temperature has a much higher influence on performance than fuel solution or rate.
A more recent and in many ways more innovative use of silicon materials, in combination with others, shows potential to further increase the efficacy and efficiency of micro direct methanol fuel cells. By utilizing high-aspect-ratio carbon nanotubes as fuel delivery and reaction area structures for either the cathode or anode end of a micro direct methanol fuel cell, the reaction area and thus the efficiency of the fuel cell can be greatly increased (Wu et al. 2008). Though this conclusion has yet to be borne out by direct observational evidence, initial experimentation has shown that these nanotubes can be controlled in their growth to produce consistent and effective fuel channels ranging from 100H 10 150H, 80W to 100W, and only 3L-5L. (Wu et al. 2008). This network of micro fuel channels would allow for far greater control of fuel flow and will also increase the exposure of the fuel to the fuel cell (and vice versa), increasing fuel efficiency and ultimately energy production, as the researchers predict with careful addendums (Wu et al. 2008).
None of the current research into these increasingly smaller and more productive micro direct methanol fuel cells would be possible, of course, were it not for foundational work that first developed and described this emerging technology. The essential quality of the micro direct methanol fuel cell design is that it is a pumpless and totally energy-independent system; as long as a fuel supply is maintained the cell will operate through gravitational forces, capillary action, and natural buoyancy and air movement (Mench et al. 2001). The fact that it requires no purposeful energy to operate is key to the continued interest in these fuel cells.
It is this feature that makes micro direct methanol fuel cells so attractive in very small applications, and that allows for the ever-decreasing size of the fuel cells themselves. The creation of micro direct methanol fuel cells on printed circuitry boards has even been achieved, with fairly encouraging results; though temperatures were at 80C, improving performance vastly over room temperature performance according to previous research, the fact that stacked eight cell system was able to produce 180mW/cm2 with channels only 200W and a fuel of 2M methanol shows a high efficiency and energy output per the amount of fuel consumed (Lim et al. 2006). Utilizing microtechnologies to further control the flow rate and exposure of fuel in micro direct methanol fuel cells has increasing benefits in fuel efficiency and in ultimate energy output, and it is expected that a similar system operating at room temperature, though undoubtedly reduced in its efficiency, would show greater energy output than many other devices with similar fuel consumption rates (Lim et al. 2006).
Conclusion
As the capabilities to produce smaller channels for fuel delivery increase, fuel efficiency in micro direct methanol fuel cells is also expected to rise. The ability to control fuel flow in these microstructures is essential to increasing efficiency, and it cannot be easily achieved through mechanical movement. The alteration of the fuel delivery system and the environment in which the micro direct methanol fuel cells operate through design will lead to greater levels of efficiency.
References
Kamitani, a. Morishita, S.; Kotaki, H.; Arscott, S. (2008). “Miniaturized microDMFC using silicon microsystems techniques: performances at low fuel flow rates.” Journal of micromechanics and microengineering 18.
Lim, S.; Kim, S.; Kim, H.; Ahn, J; Han, H.; Shul, Y. (2006). “Effect of operation parameters on performance of micro direct methanol fuel cell fabricated on printed circuit board.” Journal of power sources 161, pp. 27-33.
Lu, G.; Wang, C. Yen, T.: Zhnag, X. (2003). “Development and characterization of a silicon-based micro direct methanol fuel cell.” Electrochimica Acta 49, pp. 821 — 828.
Mench, M.; Wang, Z.; Bhatia, K.; Wnag, C. (2001). “Design of a micro direct methanol fuel cell.” Proceedings of the IMECE’01 International Mechanical Engineering Congress and Exposition (IMECE) New York, New York USA November 11-16, 2001.
Wu, Y.; Tseng, F.; Tsai, C.; Chieng, C. (2008). “Micro and Nano Structured Reaction Device for Micro DMFC.” Proceedings of the 3rd IEEE Int. Conf. On Nano/Micro Engineered and Molecular Systems January 6-9, 2008, Sanya, China, pp. 816-9.
