Direct methanol fuel cell
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Direct-methanol fuel cells or DMFCs are a subcategory of proton-exchange fuel cells where, the fuel, methanol (CH3OH), is not reformed, but fed directly to the fuel cell. Because methanol is fed directly into the fuel cell, complicated catalytic reforming is unneeded, and storage of methanol is much easier than that of hydrogen because it does not need to be done at high pressures or low temperatures, as methanol is a liquid from -97.0 °C to 64.7 °C (-142.6 °F to 148.5 °F). The energy density of methanol, the amount of energy released by using a given volume of methanol, is orders of magnitude greater than even highly compressed hydrogen.
However, the efficiency of direct-methanol fuel cells is low due to the high permeation of methanol through the membrane, which is known as methanol crossover, and the dynamic behaviour is sluggish. Other problems include the management of carbon dioxide evolved at the anode. At the current level of the technology, DMFCs are limited in the power they can produce, but can still store much energy in a small space. This means they can produce a small amount of power over a long period of time. This makes them ill-suited for powering vehicles, but ideal for consumer goods such as mobile phones, digital cameras or laptops.
Another issue is methanol's chemical properties. It is toxic and flammable. However, the International Civil Aviation Organization's (ICAO) Dangerous Goods Panel (DGP) voted in November 2005 to allow passengers to carry and use micro fuel cells and methanol fuel cartridges when aboard airplanes to power laptop computers and other consumer electronic devices. The formal regulation is still waiting to be implemented.
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[edit] Reaction
The DMFC relies upon the oxidation of methanol on a catalyst layer to form carbon dioxide. Water is consumed at the anode and is produced at the cathode. Positive ions (H+) are transported across the proton exchange membrane (often Nafion) to the cathode where they react with oxygen to produce water. Electrons are transported via an external circuit from anode to cathode providing power to external devices.
The half-reactions are:
Anode: CH3OH + H2O → CO2 + 6H+ + 6e-
Cathode: (3/2)O2 + 6H+ + 6e- → 3H2O
Net reaction: CH3OH + 1.5O2 → CO2 + 2H2O
Because water is consumed at the anode in the reaction, pure methanol cannot be used without provision of water via either passive transport such as back diffusion (osmosis), or active transport such as pumping. The need for water limits the energy density of the fuel.
Currently, platinum is used as a catalyst for both half-reactions. This is what causes the problem of methanol crossover, as any methanol that is present in the cathode chamber will oxidize. If another catalyst could be found for the reduction of oxygen, the problem of methanol crossover would likely be significantly lessened. Also, platinum is very expensive, which inhibits commercial production of the DMFC.
[edit] History
(Inverse timeline)
December 29, 2006. Samsung recently announced new breakthrough in this tech. Expect commercial sales of laptop batteries by end of 2007 (i.e. in Q35 laptop). [1]
As of 2005, the record for the smallest commercially available fuel cell is held by Toshiba, at 22 x 56 x 4.5 millimeters. This device outputs 100 milliwatts at 10 hours per milliliter of fuel, and takes advantage of new technology allowing the use of undiluted (99.5%) methanol.
| Fuel Cells |
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| Types: AFC | BE | DBFC | DEFC | DMFC | EGFC | FAFC | MCFC | MFC | MHFC | PAFC | PCFC | PEC | PEMFC | RFC | rfc | SOFC | ZFC |
| Other: Hydrogen Economy | Hydrogen storage | Hydrogen station | Hydrogen Vehicles |
