N. Andes, C. Hydrean, I. Rozansky
Overview
A Fuel cell is an electrochemical energy conversion device. It does this by producing a continuous electric current directly from the oxidation of a fuel. There is a difference though between Fuel Cells and Batteries. Fuel Cells consume a reactant that must be replenished. Batteries on the other hand store electrical energy in a closed system. Batteries are expensive and only last a certain amount of time. There are however rechargeable batteries but these are even more expensive and is governed by the number of charge/discharge cycles a battery can endure. The development of Miniature Fuel Cells can solve this problem by providing cheap, efficient, and reusable batteries. These fuel cells have shown the capability of powering a laptop for 20 hours and phones for up to 3 days. The two most prominent fuel cells being utilized are hydrogen and methanol. Although both types have their positive and negative aspects, this paper will focus more on the Direct Methanol Fuel cell.
Researchers have found a trick with Hydrogen Fuel Cells that allow for a 50% boost in power. Due to the increased functionality of phones, laptops, blackberry handhelds, and other forms of portable technology there is a huge draw in battery power. This power boost effect only works with Hydrogen Fuel Cells. Big consumer electronics firms such as Motorola and NEC have chosen to use liquid methanol because it releases more energy than hydrogen, volume for volume. Since electronic gadgets keep drawing more power as they decrease, hydrogen fuel cells would hamper the size that gadgets could be made. Using methanol fuel cells, companies are able to manufacture smaller fuel cells, allowing for smaller consumer electronics.
The most promising miniature fuel cell is the direct-methanol fuel cell (DMFC). They are the most promising because instead of having to reform the CH3OH, it is fed directly into the fuel cell. By doing so no catalytic reforming is needed. Additionally, methanol is able to be stored much easier then hydrogen because it does not require high pressures or low temperatures. The only issue with the direct-methanol fuel cells is that the efficiency is low due to the high permeation of methanol. They are able to store a large amount of energy in a small space, which makes them ideal for devices such as cell phones, laptops, and cameras. The only downfall to the methanol fuel cell is that it methanol is toxic and its fuel cells produce CO2 emissions. Currently, advancements are being made to eliminate the risks associated with the DMFC.
Background and applications of Miniature Fuel Cells
Miniature Fuel Cells have and can be used for many different applications, including personal, industrial and military use.
In one instance, developed at Pacific Northwest National Laboratory, the fuel cells developed are only about the size of a dime and can power personal cooling systems to increase comfort in hot climates. Also, these cells can be used to power communication devices that are typically bulky and heavy, also providing and prolonging comfort. [3] The miniature fuel cells produce an equivalent 20mW of power to batteries and weigh a third of the weight. These fuel cells can also be used in civilian applications for construction workers and hikers/campers with the same desires for comfort. Shown below in Figure 2 is the soldier concept for the portable device. [4]
Picture 1. Concept for soldier-portable power [4]
Picture 2. Size of power supply for soldier concept. [4]
Not only military applications can benefit from the concept of miniature fuel cell power. Personal electronics can also use miniature fuel cells to increase battery life while reducing size and bulkiness. In collaboration between Lawrence Livermore National laboratory and UC Berkeley Laboratories, miniature fuel cells using methanol and water were found to be able to increase battery power by 300 percent. These can be used for anything ranging from personal music devices to laptops and cellular phones. [5]
Miniature Direct Methanol Fuel Cells: How they work
In a Direct Methanol Fuel Cell (DMFC), methanol is oxidized directly at the surface of an electrode to generate electricity without need for extra fuel processing in intermediate steps of the process. [1]
The electrodes are typically made from a silicon substrate, and the catalyst from platinum. Platinum is a good choice because it has good adhesive compatibility with silicon. A porous proton exchange membrane (PEM) is placed between the electrodes and acts as both a cathode and anode. The reactions for the anode and cathode are shown below:
CH3OH +H2O —> CO2 + 6H+ + 6e- (anode)
3/2 O2 + 6H+ +6e- —> 3H2O (cathode)
Overall, the reaction becomes:
CH3OH + 3/2 O2 —> CO2 + 2H20
Figure 1. One schematic of the Direct Methanol Fuel Cell.
To fabricate these fuel cells, silicon wafers were patterned with pores. Each pore is about 7μm in diameter and 30 μm in depth. This pattern varies depending on the type and power requirements of the fuel cell. The material of the PEM also varies. In one particular cell, a Nafion sheet treated with various solutions is used for the PEM. [2]
Here’s a link to an interactive demo on how fuel cells work
http://www.msnbc.com/modules/fuelcells/default.asp
Future Applications
There are many applications in which these fuel cells can be utilized. Many of the applications are for consumer electronics including but not limited to laptops, cell phones, and music players. Due to the high functionality of these devices there is a huge draw on power. This reduces the time that these electronics can go without needing to be recharged. Many companies have stepped up and have started to build small prototype fuel cells that can power these devices. Toshiba is one of the leading companies on the fuel cell front. In 2004 they unveiled the world’s smallest direct-methanol fuel cell (DMFC) for powering a laptop. This fuel cell as well as a prototype of a laptop powered by this fuel cell is shown in the figures below.
Picture 3: Prototype of DMFC powered laptop by Toshiba [p1]
Picture 4: Toshiba DMFC fuel cell [p2]
Portable music players have also been targeted for the usage of power by fuel cells. This would allow users to listen to music for a much longer period of time. The prototype portable music player shown below in Figure 3 is currently able to play music for up to 10 hours.
Picture 5: Toshiba prototype of portable music player powered by DMFC [p3]
Fuel cells being developed for consumer electronics are not the only types. The future of miniature fuel cells is emerging and has the potential to become a very beneficial technology.
References:
[1] Shi-Chune Yao et al. Micro-electro-mechanical systems (MEMS)-based micro-scale direct methanol fuel cell development. Elsevier Energy, volume 31, 2006. www.sciencedirect.com, http://www.mtl.mit.edu/researchgroups/mems-salon/Amon_2006.pdf
[2] Onoe, Shinsuke et. al. Miniature Fuel Cell with Conductive Silicon Electrodes. The University of Tokyo, Japan. IEEE Magazine, 2005.
[3] Parker, Randall. Mini Fuel cells for personal lightweight cooling systems. Energy Tech, April 7, 2003. http://www.futurepundit.com/archives/001111/html
[4] Holladay, Jamelyn et. al. Miniature Fuel Processors cor Portable Fuel Cell Power Supplies. Battelle Pacific Northwest Division, Richland, WA. http://www.pnl.gov/microcats/aboutus/publications/microchemical/MRS2002pnwd-sa-5862.pdf
[5] Hillman, Tyler. Miniature Fuel Cells to Extend Life of Electronics. The Daily Californian, March 6, 2002. http://www.dailycal.org
Pictures:
[p1] http://www.tribuneindia.com/2006/20061215/biz.htm
[p2] http://www.theregister.co.uk/2004/06/25/tosh_fuel_cell/
[p3] http://www.digitalworldtokyo.com/index.php/digital_tokyo/articles/toshibas_future_vision_sees_methanol_fuel_cells_on_our_heads/
More Information:
http://www.brighthand.com/default.asp?newsID=8974
http://freeenergynews.com/Directory/FuelCell/index.html
http://www.batteryuniversity.com/parttwo-52A.htm
http://arstechnica.com/news.ars/post/20040308-3505.html
