Direct methanol fuel cell 1049602 224989204 2008-07-11T09:55:32Z Wogone 6352430 Grammar changes throughout '''Direct-methanol fuel cells''' or '''DMFCs''' are a subcategory of [[proton exchange membrane fuel cell|proton-exchange fuel cells]] where the [[methanol]] (CH<sub>3</sub>OH) fuel is not reformed, but fed directly to the [[fuel cell]]. Because methanol is fed directly into the fuel cell, complicated [[catalytic reforming]] is not required. Storage of methanol is much easier than for [[hydrogen]] as it does not need high pressures or low temperatures, because 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 - is an order of magnitude greater than even highly compressed hydrogen. The [[fuel efficiency|efficiency]] of current direct-methanol fuel cells is low due to the high [[permeation]] of methanol through the [[artificial membrane|membrane]] materials used, which is known as methanol crossover. A new kind of membrane ([http://www.materialstoday.com/archive/2008/11-06/news03.html polymer electrolyte thin films, assembled "layer by layer"]) has been shown to reduce this problem dramatically. Other problems include the management of [[carbon dioxide]] created at the [[anode]] and the sluggish dynamic behaviour. 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 presently ill-suited for powering vehicles (at least directly), but ideal for consumer goods such as [[mobile phone]]s, [[digital camera]]s or [[laptop]]s. 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. [http://www.fuelcelltoday.com/online/news/articles/2007-09/US-Department-of-Transportation-moves-to-approve-fuel-cells-for-aircraft-use 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 Department of Transportation issued a final ruling on [http://hazmat.dot.gov/regs/rules/final/73fr/73fr-23362.htm April 30, 2008], permitting passengers and crew to carry an approved fuel cell with an installed methanol cartridge and up to two additional spare cartridges. It is worth noting that 200 ml maxium methanol cartridge volume allowed in the final ruling is double the [http://www.tsa.gov/press/happenings/311_intl_acceptance.shtm 100 ml limit] on liquids allowed by the Transportation and Security Administration in carry-on bags. ==Reaction== The DMFC relies upon the [[redox|oxidation]] of [[methanol]] on a [[catalyst]] layer to form [[carbon dioxide]]. Water is consumed at the [[anode]] and is produced at the [[cathode]]. Positive [[ion]]s (H<sup>+</sup>) are transported across the proton exchange membrane - often made from [[Nafion]] - to the cathode where they react with [[oxygen]] to produce water. [[Electron]]s are transported through an external circuit from anode to cathode, providing power to connected devices. The [[half-reaction]]s are: '''Anode:''' CH<sub>3</sub>OH + H<sub>2</sub>O → CO<sub>2</sub> + 6H<sup>+</sup> + 6e<sup>-</sup> '''Cathode:''' (3/2)O<sub>2</sub> + 6H<sup>+</sup> + 6e<sup>-</sup> → 3H<sub>2</sub>O '''Overall reaction:''' CH<sub>3</sub>OH + (3/2)O<sub>2</sub> → CO<sub>2</sub> + 2H<sub>2</sub>O Methanol and water are adsorbed on a catalyst usually made of [[platinum]] and [[ruthenium]] particles, and lose protons until carbon dioxide is formed. As 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 fuel cells. During the methanol oxidation reaction [[carbon monoxide]] (CO) is formed, which strongly adsorbs onto the platinum catalyst, reducing the surface area and thus the performance of the cell. The addition of another components, such as ruthenium or [[gold]], to the catalyst tends to ameliorate this problem because, according to the most well-established theory in the field, these catalysts oxidize water to yield OH radicals: H<sub>2</sub>O → OH• + H<sup>+</sup> + e<sup>-</sup>. The OH species from the oxidized water molecule oxidizes CO to produce CO<sub>2</sub> which can then be released as a gas: CO + OH• → CO<sub>2</sub> + H<sup>+</sup> + e<sup>-</sup>. == See also == * [[Liquid fuels]] * [[Methanol (data page)]] * [[Methanol economy]] ==External links== *[http://www.fuelcelltoday.com Fuel Cell Today. An internet portal of news and articles of fuel cell developments] *[http://www.fuelcellstandards.com/2.1.9.4.htm Standards for Transportable Fuel Cell Power Units] {{FuelCellGroup}} [[Category:Fuel cells]] [[Category:Sustainable technologies]] [[de:Direktmethanolbrennstoffzelle]] [[fr:Pile à combustible à méthanol direct]] [[ja:直接メタノール燃料電池]] [[nl:DMFC]] [[pl:Ogniwo paliwowe zasilane bezpośrednio metanolem]] [[ru:Прямой метанольный топливный элемент]] [[th:Direct methanol fuel cell]] [[vi:DMFC]] [[tr:Doğrudan metanol yakıt hücresi]]