Direct-methanol fuel cells or
DMFCs are a subcategory of
proton-exchange fuel cellsProton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells , are a type of fuel cell being developed for transport applications as well as for stationary fuel cell applications and portable fuel cell applications...
in which
methanolMethanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with formula CH3OH . It is toxic: drinking 10 ml will cause blindness, and as little as 100 ml will cause death...
is used as the fuel. Their main advantages are the ease of transport of methanol, an energy-dense yet reasonably stable liquid at all environmental conditions, and the lack of complex
steam reformingFossil fuel reforming, hydrogen reforming or catalytic oxidation, is a method of producing useful products, such as hydrogen or ethylene from fossil fuels. Fossil fuel reforming is done through a fossil fuel processor or reformer. At present, the most common fossil fuel processor is a steam...
operations.
Efficiency is quite low for these cells, so they are targeted especially to portable applications, where energy and power density are more important than efficiency.
In contrast to indirect methanol fuel cells, where methanol is reacted to
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
by steam reforming, DMFCs use a methanol solution (usually around 1M) to carry the reactant into the cell; common operating temperatures are in the range 50–120 °C, where high temperatures are usually pressurised.
DMFCs themselves are more efficient at high temperatures and pressures, but these conditions end up causing so many losses in the complete system that the advantage is lost; therefore, atmospheric-pressure configurations are preferred nowadays.
Because of the methanol cross-over, a phenomenon by which methanol diffuses through the membrane without reacting, methanol is fed as a weak solution: this actually improves efficiency significantly, since crossed-over methanol, after reaching the air side (the cathode), immediately reacts with air: whereas the exact kinetics is debated, the end result is a reduction of the cell voltage.
Cross-over remains a major factor in inefficiencies, and often half of the methanol is lost to cross-over.
Other issues include the management of
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
created at the
anodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID . Electrons flow in the opposite direction to the electric current...
, the sluggish dynamic behaviour, and the ability to maintain the solution water.
The only waste products with these types of fuel cells are
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
and water.
Current DMFCs are limited in the power they can produce, but can still store a high energy content in a small space.
Direct-methanol fuel cells or
DMFCs are a subcategory of
proton-exchange fuel cellsProton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells , are a type of fuel cell being developed for transport applications as well as for stationary fuel cell applications and portable fuel cell applications...
in which
methanolMethanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with formula CH3OH . It is toxic: drinking 10 ml will cause blindness, and as little as 100 ml will cause death...
is used as the fuel. Their main advantages are the ease of transport of methanol, an energy-dense yet reasonably stable liquid at all environmental conditions, and the lack of complex
steam reformingFossil fuel reforming, hydrogen reforming or catalytic oxidation, is a method of producing useful products, such as hydrogen or ethylene from fossil fuels. Fossil fuel reforming is done through a fossil fuel processor or reformer. At present, the most common fossil fuel processor is a steam...
operations.
Efficiency is quite low for these cells, so they are targeted especially to portable applications, where energy and power density are more important than efficiency.
The Cell
In contrast to indirect methanol fuel cells, where methanol is reacted to
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
by steam reforming, DMFCs use a methanol solution (usually around 1M) to carry the reactant into the cell; common operating temperatures are in the range 50–120 °C, where high temperatures are usually pressurised.
DMFCs themselves are more efficient at high temperatures and pressures, but these conditions end up causing so many losses in the complete system that the advantage is lost; therefore, atmospheric-pressure configurations are preferred nowadays.
Because of the methanol cross-over, a phenomenon by which methanol diffuses through the membrane without reacting, methanol is fed as a weak solution: this actually improves efficiency significantly, since crossed-over methanol, after reaching the air side (the cathode), immediately reacts with air: whereas the exact kinetics is debated, the end result is a reduction of the cell voltage.
Cross-over remains a major factor in inefficiencies, and often half of the methanol is lost to cross-over.
Other issues include the management of
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
created at the
anodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID . Electrons flow in the opposite direction to the electric current...
, the sluggish dynamic behaviour, and the ability to maintain the solution water.
The only waste products with these types of fuel cells are
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
and water.
Application
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 phoneA mobile phone or mobile is a long-range, electronic device used for mobile telecommunications...
s,
digital cameraA digital camera is a camera that takes video or still photographs, or both, digitally by recording images via an electronic image sensor....
s or
laptopA laptop is a personal computer designed for mobile use and small and light enough to sit on one's lap while in use. A laptop integrates most of the typical components of a desktop computer, including a display, a keyboard, a pointing device , speakers, and often including a battery, into a single...
s. Military applications of DMFCs are an emerging application since they have low noise and thermal signatures and no toxic effluent. These applications include power for soldier-carried tactical equipment, battery chargers, and autonomous power for test and training instrumentation. Units are currently available with power outputs between 25 and 250 Watts with durations up to 100 hours between refuelings.
Methanol
Methanol is a liquid from -97.0 °C to 64.7 °C at atmospheric pressure: this covers
all environmental conditions ever recorded on earth.
The
energy densityEnergy density is a term used for the amount of energy stored in a given system or region of space per unit volume, or per unit mass, depending on the context. The latter is more formally known as specific energy...
of methanol is an order of magnitude greater than even highly
compressed hydrogenCompressed hydrogen is the gaseous state of the element hydrogen which is kept under pressure. Compressed hydrogen in hydrogen tanks at 350 bar and 700 bar is used for mobile hydrogen storage in hydrogen vehicles...
, and 15 times higher than Lithium-ion batteries.
Methanol is
toxicToxicity is the degree to which a substance is able to damage an exposed organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...
and
flammableFlammability is defined at how easily something will burn or ignite, causing fire or combustion. The degree of difficulty required to cause the combustion of a substance is subject to quantification through fire testing. Internationally, a variety of test protocols exist to quantify flammability...
.
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 24, 2007, the US Department of Transportation issued a proposed to allow airline passengers to carry fuel cell cartridges on board.
The Department of Transportation issued a final ruling on 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 maximum methanol cartridge volume allowed in the final ruling is double the 100 ml limit on liquids allowed by the Transportation and Security Administration in carry-on bags.
Reaction
The DMFC relies upon the
oxidationRedox describes all chemical reactions in which atoms have their oxidation number changed....
of
methanolMethanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with formula CH3OH . It is toxic: drinking 10 ml will cause blindness, and as little as 100 ml will cause death...
on a catalyst layer to form
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
. Water is consumed at the
anodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID . Electrons flow in the opposite direction to the electric current...
and is produced at the
cathodeA cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .A widespread misconception is that cathode polarity is always negative...
. Positive
ionAn ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge...
s (H
+) are transported across the proton exchange membrane - often made from
NafionNafion is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer discovered in the late 1960s by Walther Grot of DuPont. It is the first of a class of synthetic polymers with ionic properties which are called ionomers...
- to the cathode where they react with
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
to produce water.
ElectronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
s are transported through an external circuit from anode to cathode, providing power to connected devices.
The
half-reactionA half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction.-Example:...
s are:
!
!Equation
|-
!Anode
|
oxidation
|-
!Cathode
|
reduction
|-
!Overall reaction
|
redox reaction
|-
|}>
Methanol and water are adsorbed on a catalyst usually made of
platinumPlatinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is in Group 10 of the periodic table of elements...
and
rutheniumRuthenium is a chemical element that has the symbol Ru and atomic number 44. A rare transition metal of the platinum group of the periodic table, ruthenium is found associated with platinum ores and used as a catalyst in some platinum alloys....
particles, and lose protons until carbon dioxide is formed. As water is consumed at the
anodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID . Electrons flow in the opposite direction to the electric current...
in the reaction, pure methanol cannot be used without provision of water via either passive transport such as back
diffusionMolecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material...
(
osmosisOsmosis is the diffusion of water through a semi-permeable membrane. More specifically, it is the movement of water across a semi-permeable membrane from an area of high water potential to an area of low water potential...
), or
active transportActive transport is the mediated process of moving particles across a biological membrane against a concentration gradient. If the process uses chemical energy, such as from adenosine triphosphate , it is termed primary active transport. Secondary active transport involves the use of an...
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 these cells.
During the methanol oxidation reaction
carbon monoxideCarbon monoxide is a colorless, odorless and tasteless gas, yet very toxic to humans. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond...
(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
goldGold is a chemical element with the symbol Au and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Gold is...
, 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
2O → OH• + H
+ + e
-. The OH species from the oxidized water molecule oxidizes CO to produce CO
2 which can then be released as a gas: CO + OH• → CO
2 + H
+ + e
-.
Cross-over Current
Methanol on the anodic side is usually in a weak solution (from 1M to 3M), because methanol in high concentrations has the tendency to diffuse through the membrane to the cathode, where its concentration is about zero because it is rapidly consumed by oxygen. Low concentrations help reducing the cross-over, but also limit the maximum attainable current.
The practical realisation is usually that a solution loop enters the anode, exits, is refilled with methanol, and returns to the anode again.
Water Drag
The water in the anodic loop is lost because of the anodic reaction, but mostly because of the associated water drag: every proton formed at the anode drags a number of water molecules to the cathode. Depending on temperature and membrane type, this number can be between 2 and 6.
Ancillary Units
A direct methanol fuel cell is usually part of a larger system including all the ancillary units that permit its operation. Compared to most other types of fuel cells, the ancillary system of DMFCs is relatively complex. The main reasons for its complexity are:
- providing water along with methanol would make the fuel supply more cumbersome, so water has to be recycled in a loop;
- CO2 has to be removed from the solution flow exiting the fuel cell;
- water in the anodic loop is slowly consumed by reaction and drag; it is necessary to recover water from the cathodic side to maintain steady operation.
See also
- Alkali anion exchange membrane
An alkali anion exchange membrane is a semipermeable membrane generally made from ionomers and designed to conduct anions while being impermeable to gases such as oxygen or hydrogen...
- Glossary of fuel cell terms
The Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few. -...
- Portable fuel cell applications
Portable fuel cell applications are portable fuel cell applications that are either used as micropower in consumer electronic devices to provide power or as portable power in emergency power systems for critical areas.-Codes and standards:Portable fuel cell applications is a classification in FC...
- Liquid fuels
Liquid fuels are those combustible or energy-generating molecules that can be harnessed to create mechanical energy, usually producing kinetic energy; they also must take the shape of their container...
- Methanol (data page)
- Material Safety Data Sheet : The handling of this chemical may incur notable safety precautions. It is highly recommend that you seek the Material Safety Datasheet for this chemical from a reliable source such as , and follow its directions...
- Methanol economy
The methanol economy is a suggested future economy in which methanol replaces fossil fuels as a means 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...
External links