Direct methanol fuel cell

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. 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 contained in a given volume of methanol, is an order of magnitude greater than even highly compressed hydrogen.

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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 created at the anode. Current DMFCs are limited in the power they can produce, but can still store a high energy content 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.

Methanol 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. On September 24th, 2007, the US Department of Transportation issued a proposed rulemaking to allow airline passengers to carry fuel cell cartridges on board. The rule will likely be finalised before the end of 2007 and take effect early in 2008.

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^-

The methanol is adsorbed on a catalyst, usually Pt, and deprotonized, until a CO species is left. Another catalyst, usually Ru is used to oxidize water, producing an OH species which reacts with the CO to form carbon dioxide.

Cathode: (3/2)O₂ + 6H⁺ + 6e^- → 3H₂O

Net reaction: CH₃OH + (3/2)O₂ → 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 contributes to the loss of cell voltage potential, 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. Furthermore, platinum is very expensive and contributes to the high cost per kilowatt of the fuel cell.

In one of the steps of the methanol oxidation reaction, CO is produced, which adsorbs strongly on the platinum catalyst reducing the surface area for the catalyst reaction. The addition of other catalysts such as Ruthenium or Gold tends to ameliorate this problem because these catalysts oxidize water. The oxygen atom from the oxidized water molecule combines with the CO to produce CO₂ which can then be released as a gas.

See also

• Liquid fuels
• Methanol (data page)
• Methanol economy