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.

The methanol economy (see below) is a suggested future economy in which methanol replaces fossil fuels as a mean of energy storage, fuel and raw material for synthetic hydrocarbons and their products. It offers an alternative to the proposed hydrogen economy or ethanol economy.

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 made of platinum particles, and deprotonized until carbon dioxide is formed. Usually, the catalyst consists of another metallic component, usually ruthenium, which is used to catalyze methanol oxidation (see last paragraph for more details).

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, a CO species is produced, which adsorbs strongly on the platinum catalyst, reducing the surface area for the catalyst reaction. 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: H₂O → OH• + H⁺ + e^-. The OH species from the oxidized water molecule oxidizes CO to produce CO₂ which can then be released as a gas: CO + OH• → CO₂ + H⁺ + e^-.

See also

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

External links

• Fuel Cell Today. An internet portal of news and articles of fuel cell developments
• Standards for Transportable Fuel Cell Power Units

The methanol economy is a suggested future economy in which methanol replaces fossil fuels as a mean of energy storage, fuel and raw material for synthetic hydrocarbons and their products. It offers an alternative to the proposed hydrogen economy or ethanol economy.

In 2005 Nobel prize winner George A. Olah advocated the methanol economy in an essay ^[1] and in 2006 he and two co-authors published a book around this theme ^[2] In these publications, they summarize the state of our fossil fuel and alternative energy sources, their availability and limitations before suggesting a new approach in the so called methanol economy.

Methanol is a fuel for heat engines and fuel cells. Due to its high octane rating it can be used directly as a fuel in cars (including hybrid and plug-in vehicles) using existing internal combustion engines (ICE). Methanol can also be used as a fuel in fuel cells, either directly in Direct Methanol Fuel Cells (DMFC) or indirectly after conversion into hydrogen by reforming.

Methanol is a liquid under normal conditions, allowing it to be stored, transported and dispensed easily, much like gasoline and diesel fuel nowadays. It can also be readily transformed by dehydration into dimethyl ether, a diesel fuel substitute with a cetane number of 55.

Methanol is already used today on a large scale (about 37 million tonnes per year)^[3] as a raw material to produce numerous chemical products and materials. In addition, it can be readily converted in the methanol to olefin (MTO) process into ethylene and propylene, which can be used to produce synthetic hydrocarbons and their products, currently obtained from oil and natural gas.

Methanol can be efficiently produced from a wide variety of sources including still abundant fossil fuels (natural gas, coal, oil shale, tar sands, etc.), but also agricultural products and municipal waste, wood and varied biomass. More importantly, it can also be made from chemical recycling of carbon dioxide. Initially the major source will be the CO₂ rich flue gases of fossil fuel burning power plants or exhaust of cement and other factories. In the longer range however, considering diminishing fossil fuel resources and the effect of their utilization on earth's atmosphere, even the low concentration of atmospheric CO₂ itself could be captured and recycled via methanol, thus supplementing nature’s own photosynthetic cycle. Efficient new absorbents to capture atmospheric CO₂ are being developed, mimicking plant life’s ability. Chemical recycling of CO₂ to new fuels and materials could thus become feasible, making them renewable on the human timescale.

Uses of methanol in a methanol economy

Fuel uses

In an economy based on methanol, methanol could be used as a fuel

• In ICEs:

Methanol has a high octane number (RON of 107 and MON of 92), which makes it a suitable gasoline substitute. It has a higher flame speed than gasoline, leading to higher efficiency as well as a higher latent heat of vaporization (3.7 times higher than gasoline), meaning that the heat generated by the engine can be removed more effectively, making it possible to use air cooled engines. Besides this methanol burns cleaner than gasoline and is safer in the case of a fire. However, methanol has only half the volumetric energy content of gasoline (8,600 BTU/lb).

• In compression ignition engines (diesel engine)

Methanol itself is not a good substitute for diesel fuels. Methanol can, however, be converted by dehydration to dimethyl ether, which is a good diesel fuel with a cetane number of 55-60 as compared to 45-55 for regular diesel fuel. Compared to diesel fuel, DME has much lower emissions of particulate matter, NO_x and CO and does not emit any SO_x. Methanol can also be used, and is in fact already used, to produce biodiesel via transesterification of vegetable oil.

• In advanced methanol powered vehicles

The use of methanol and dimethyl ether can be combined with hybrid and plug-in vehicle technologies allowing higher gas mileage and lower emissions. These fuels can also be used in fuel cells either via onboard reforming to hydrogen or directly in direct methanol fuel cells (DMFC).

• For electricity production:

Methanol and DME can be used in existing gas turbines to generate electricity. Fuel cells (PAFC, MCFC, SOFC) can also be used for electricity generation

• As a domestic fuel

Methanol and DME can be used in commercial buildings and homes to generate heat and/or electricity. DME can be used in a commercial gas stove without modifications. In developing countries methanol could also be used as a cooking fuel, burning much cleaner than wood, thus mitigating indoor air quality problems.

Raw material for chemicals and materials

Methanol is already used today on a large scale as raw material to produce a variety of chemicals and products. Through the methanol to gasoline (MTG) process, it can be transformed into gasoline. Using the methanol to olefin (MTO) process, methanol can also be converted to ethylene and propylene, the two largest chemicals produced by the petrochemical industry. These are important building blocks for the production of essential polymers (LDPE, HDPE, PP) and other chemical intermediates are currently produced mainly from petroleum feedstock. Their production from methanol could therefore reduce our dependency on petroleum. It would also make it possible to still produce these chemicals when fossil fuels reserves will be depleted.

Methanol production

The methanol needed in the methanol economy can be synthesized from a wide array of carbon sources including still available fossil fuels and biomass but also CO₂ emitted from fossil fuel burning power plants and other industries and eventually even the CO₂ contained in the air.

Today methanol is produced exclusively from syngas, a mixture of H₂, CO and CO₂ obtained by partial oxidation of fossil fuels, mainly natural gas and coal. This technology is well developed and operated on a large scale.

Although conventional natural gas resources are currently the preferred feedstock for the production of methanol, unconventional gas resources such as coalbed methane, tight sand gas and eventually the very large methane hydrate resources present under the continental shelves of the seas and Siberian and Canadian tundra could also be used. Besides methane all other conventional or unconventional (tar sands, oil shale,etc.) fossil fuels could be utilized to produce methanol.

Besides the conventional route to methanol from methane passing through syngas generation by steam reforming combined (or not) with partial oxidation, new and more efficient ways to produce methanol from methane are being developed. These include:

• methane oxidation with homogeneous catalysts in sulfuric acid media
• methane bromination followed by hydrolysis of the obtained bromomethane
• direct oxidation of methane with oxygen
• Microbial or photochemical conversion of methane

The use of methane (and other fossil fuel) for the production of methanol using all the above mentioned synthetic routes has however a major drawback of growing concern: the emission of the greenhouse gas CO₂, its accumulation in the atmosphere and detrimental effect on the climate.

To address this problem methanol will have to be made increasingly through ways minimizing the emission of CO₂. One solution is to produce it from syngas obtained by biomass gasification. For this purpose any biomass can be used including wood, wood wastes, grass, agricultural crops and their by-products, animal waste, aquatic plants and municipal waste. There is no need to use food crops as in the case of ethanol from corn, sugar cane and wheat.

  Biomass → Syngas (CO, CO2, H2) → CH3OH 

More importantly, methanol can also be produced from CO₂ by catalytic hydrogenation of CO₂ with H₂ obtained from water electrolysis or through CO₂ electrochemical reduction. The energy needed for these reactions in order to be carbon neutral would come form renewable energy sources such as wind, hydroelectricity and solar as well as nuclear power.

  CO2 + 3H2 → CH3OH + H2O

  CO2 +2H2O + electrons → CO + 2H2 (+ 3/2 O2) → CH3OH

The necessary CO₂ would be captured from fossil fuel burning power plants and other industrial flue gases including cement factories. With diminishing fossil fuel resources and therefore CO₂ emissions, the CO₂ content in the air could also be used. Considering the low concentration of CO₂ in air (0.037%) improved and economically viable technologies to absorb CO₂ will have to be developed. This would allow the chemical recycling of CO₂, thus mimicking nature’s photosynthesis.

Advantages over other energy storage media

Advantages over hydrogen

Methanol economy advantages compared to a hydrogen economy:

• efficient energy storage (by volume) and also by weight as compared with compressed hydrogen, when hydrogen pressure-confinement vessel is taken into account. The volumetric energy density of methanol is considerably higher than liquid hydrogen, in part because of the low density of liquid hydrogen of 71 grams/liter. Hence there is actually more hydrogen in a liter of methanol (99 grams/liter) than in a liter of liquid hydrogen, and methanol needs no cryogenic container maintained at a temperature of -253°C.
• required hydrogen infrastructure would be prohibitively expensive. Methanol can use existing gasoline infrastructure with only limited modifications.
• can be blended with gasoline (for example in M85, a mixture containing 85% methanol and 15% gasoline).
• user friendly. Hydrogen is volatile and requires high pressure or cryogenic system confinement.
• methanol can also serve as a raw material for the chemical industry

Methanol economy advantages compared to ethanol

• can be made from any organic material using the proven Fischer Tropsch method going through syngas. No need to use food crops and compete with food production. Amount of methanol that can be generated from biomass much greater than ethanol.
• can compete with and complement ethanol in a diversified energy marketplace. Methanol obtained from fossil fuels has a lower price than ethanol.
• can be blended in gasoline like ethanol. This year already China blended more than 1 billion gallons of methanol into fuel and will introduce methanol fuel standard by mid-2008.^[4] M85,a mixture of 85% methanol and 15% gasoline can be used much like E85 sold in some gas station today.

Methanol economy disadvantages

• high energy costs associated with generating hydrogen (when needed to synthesize methanol)
• depending on the feedstock the generation in itself can be not clean
• presently generated from syngas still dependent on fossil fuels (although in theory any energy source can be used).
• energy density (by weight or volume) one half of that of gasoline
• corrosive to to some metals including aluminum, zinc and manganese. Parts of the engine fuel-intake systems is made from aluminum. Similar to ethanol, compatible material for fuel tanks, gasket and engine intake have to be used.
• hydrophilic: attracts water: in mixture with gasoline this could lead to phase separation and difficulty to start the engine or make it run smoothly
• methanol, as an alcohol, increases the permeability of some plastics to fuel vapors (e.g. high-density polyethylene). ^[5] This property of methanol has the possibility of increasing emissions of volatile organic compounds (VOCs) from fuel, which contributes to increased tropospheric ozone and possibly human exposure.
• low volatity in cold weather: pure methanol-fueled engines can be difficult to start, and they run inefficiently until warmed up. This is why, a mixture containing 85% methanol and 15% gasoline called M85 is generally used in ICEs. The gasoline allows the engine to start even at lower temperatures.
• Methanol is generally considered toxic^[6].Methanol is in fact toxic and eventually lethal when ingested in larger amounts (30 to 100 mL).^[7] But so are most motor fuels, including gasoline (120 to 300 mL) and diesel fuel. Gasoline also contains many compounds known to be carcinogenic (e.g. benzene). Methanol is not a carcinogen.
• methanol is a liquid: this creates a greater fire risk compared to hydrogen in open spaces. Methanol leaks do not dissipate. Compared to gasoline, however, methanol is much safer. It is more difficult to ignite and releases less heat when it burns. The EPA has estimated that switching fuels from gasoline to methanol would reduce the incidence of fuel related fires by 90%.^[8]
• methanol accidentally released from leaking underground fuel storage tanks may undergo relatively rapid groundwater transport and contaminate well water, although this risk has not been thoroughly studied. The history of the fuel additive methyl t-butyl ether (MTBE) as a groundwater contaminant has highlighted the importance of assessing the potential impacts of fuel and fuel additives on multiple environmental media. ^[9]. An accidental release of methanol in the environment would, however, cause much less damage than a comparable gasoline or crude oil spill. Unlike these fuels, methanol, being totally soluble in water, would be rapidly diluted to a concentration low enough for microorganism to start biodegradation. Methanol is in fact used for denitrification in water treatment plant as a nutrient for bacterias.^[10]

See also

• Alcohol fuel
• Methanol
• Methanol fuel
• Synthetic fuel

References

1. ^ Beyond Oil and Gas: The Methanol Economy , George A. Olah, Angewandte Chemie International Edition Volume 44, Issue 18, Pages 2636-2639, 2005
2. ^ Beyond Oil and Gas: The Methanol Economy , George A. Olah, Alain Goeppert, G. K. Surya Prakash, Wiley-VCH, 2006
3. ^ Product Focus: Methanol, Chemical Week May 23, 2007, Page 29
4. ^ Methanol's Allure, Kemsley, J., Chemical & Engineering News, December 3, 2007, pages 55-59
5. ^ Abstract
6. ^ Methanol is a developmental and neurological toxin, though typical dietary and occupational levels of exposure are not likely to induce significant health effects. The a National Toxicology Program panel recently concluded that blood concentrations below approx. 10 mg/L there is minimal concern for adverse health effects.[1] Other literature summaries are also available (see, for instance, Reproductive Toxicology 18 (2004) 303–390).
7. ^ http://www.methanol.org/pdfFrame.cfm?pdf=Methanol_humantox_rev.pdf, Methanol in fuel cell vehicles Human toxicity and risk evaluation (Revised), Statoil, 2001
8. ^ http://www.epa.gov/otaq/consumer/08-fire.pdf, Methanol Fuels and Fire Safety, EPA 400-F-92-010
9. ^ Abstract
10. ^ http://www.methanol.org/pdf/evaluation.pdf, Evaluation of the fate and transport of methanol in the environment, prepared by Malcolm Pirnie, Inc. for the Methanol Institute, 1999

External links

• A discussion of the Methanol Economy with George Olah Recording of a program broadcast on NPR.
• Methanol Institute
• Recent information about methanol and its uses Greencarcongress.com
• Recent information about DME Greencarcongress.com