Proton exchange membrane
Encyclopedia
A proton exchange membrane or polymer electrolyte membrane (PEM) is a semipermeable membrane
generally made from ionomer
s and designed to conduct protons
while being impermeable to gases such as oxygen
or hydrogen
. This is their essential function when incorporated into a membrane electrode assembly
(MEA) of a proton exchange membrane fuel cell
or of a proton exchange membrane electrolyser : separation of reactants and transport of protons.
PEMs can be made from either pure polymer
membranes or from composite
membranes where other materials are embedded in a polymer matrix. One of the most common and commercially available PEM materials is Nafion
, a DuPont
product.
While Nafion is an ionomer with a perfluorinated backbone like Teflon, there are many other structural motifs used to make ionomers for proton exchange membranes. Many use polyaromatic polymers while others use partially fluorinated polymers.
Proton exchange membranes are primarily characterized by proton conductivity
(σ), methanol
permeability (P), and thermal stability.
PEM fuel cells use a solid polymer membrane (a thin plastic film) as the electrolyte. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons.
Proton Exchange Membrane Fuel Cells (PEMFC) are believed to be the best type of fuel cell as the vehicular power source to eventually replace the gasoline and diesel internal combustion engines. They are being considered for automobile applications because they typically have an operating temperature of ~80oC and a rapid start up time. PEMFCs operate at 40-60% efficiency and can vary the output to match the demands. First used in the 1960s for the NASA Gemini program, PEMFCs are currently being developed and demonstrated for systems ranging from 1W to 2kW.
PEMFCs contain advantages over other types of fuel cells such as Solid Oxide Fuel Cells (SOFC). PEMFC are more easily sealed compared to the liquid electrolyte of the SOFC. In addition, PEMFCs operate at a lower temperature, are lighter, and more compact, which makes them ideal for applications such as cars.
However, some disadvantages are: the ~80oC operating temperature is too low for cogeneration like in SOFCs and that the electrolyte for PEMFCs must be water saturated.
The fuel for the PEMFC is hydrogen and the charge carrier is the hydrogen ion (proton). At the anode, the hydrogen molecule is split into hydrogen ions (protons) and electrons. The hydrogen ions permeate across the electrolyte to the cathode while the electrons flow through an external circuit and produce electric power. Oxygen, usually in the form of air, is supplied to the cathode and combines with the electrons and the hydrogen ions to produce water. The reactions at the electrodes are as follows:
Anode Reactions: 2H2 => 4H+ + 4e-
Cathode Reactions: O2 + 4H+ + 4e- => 2 H2O
Overall Cell Reactions: 2H2 + O2 => 2 H2O
Semipermeable membrane
A semipermeable membrane, also termed a selectively permeable membrane, a partially permeable membrane or a differentially permeable membrane, is a membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion".The rate of...
generally made from ionomer
Ionomer
An ionomer is a polymer that comprises repeat units of both electrically neutral repeating units and a fraction of ionized units...
s and designed to conduct protons
Proton conductor
A proton conductor is an electrolyte, typically a solid electrolyte, in which H+ are the primary charge carriers.-Composition:For practical applications, proton conductors are usually solid materials. Typical materials are polymers or ceramic. Typically the pores in practical materials are small...
while being impermeable to gases such as oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
or hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
. This is their essential function when incorporated into a membrane electrode assembly
Membrane electrode assembly
A membrane electrode assembly is an assembled stack of proton exchange membranes or alkali anion exchange membrane , catalyst and flat plate electrode used in a fuel cell.-PEM-MEA:...
(MEA) of a proton exchange membrane fuel cell
Proton exchange membrane fuel cell
Proton 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. Their distinguishing features include lower...
or of a proton exchange membrane electrolyser : separation of reactants and transport of protons.
PEMs can be made from either pure polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
membranes or from composite
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
membranes where other materials are embedded in a polymer matrix. One of the most common and commercially available PEM materials is Nafion
Nafion
Nafion 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...
, a DuPont
DuPont
E. I. du Pont de Nemours and Company , commonly referred to as DuPont, is an American chemical company that was founded in July 1802 as a gunpowder mill by Eleuthère Irénée du Pont. DuPont was the world's third largest chemical company based on market capitalization and ninth based on revenue in 2009...
product.
While Nafion is an ionomer with a perfluorinated backbone like Teflon, there are many other structural motifs used to make ionomers for proton exchange membranes. Many use polyaromatic polymers while others use partially fluorinated polymers.
Proton exchange membranes are primarily characterized by proton conductivity
Conductivity (electrolytic)
The conductivity of an electrolyte solution is a measure of its ability to conduct electricity. The SI unit of conductivity is siemens per meter ....
(σ), methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
permeability (P), and thermal stability.
PEM fuel cells use a solid polymer membrane (a thin plastic film) as the electrolyte. This polymer is permeable to protons when it is saturated with water, but it does not conduct electrons.
Proton Exchange Membrane Fuel Cells (PEMFC) are believed to be the best type of fuel cell as the vehicular power source to eventually replace the gasoline and diesel internal combustion engines. They are being considered for automobile applications because they typically have an operating temperature of ~80oC and a rapid start up time. PEMFCs operate at 40-60% efficiency and can vary the output to match the demands. First used in the 1960s for the NASA Gemini program, PEMFCs are currently being developed and demonstrated for systems ranging from 1W to 2kW.
PEMFCs contain advantages over other types of fuel cells such as Solid Oxide Fuel Cells (SOFC). PEMFC are more easily sealed compared to the liquid electrolyte of the SOFC. In addition, PEMFCs operate at a lower temperature, are lighter, and more compact, which makes them ideal for applications such as cars.
However, some disadvantages are: the ~80oC operating temperature is too low for cogeneration like in SOFCs and that the electrolyte for PEMFCs must be water saturated.
The fuel for the PEMFC is hydrogen and the charge carrier is the hydrogen ion (proton). At the anode, the hydrogen molecule is split into hydrogen ions (protons) and electrons. The hydrogen ions permeate across the electrolyte to the cathode while the electrons flow through an external circuit and produce electric power. Oxygen, usually in the form of air, is supplied to the cathode and combines with the electrons and the hydrogen ions to produce water. The reactions at the electrodes are as follows:
Anode Reactions: 2H2 => 4H+ + 4e-
Cathode Reactions: O2 + 4H+ + 4e- => 2 H2O
Overall Cell Reactions: 2H2 + O2 => 2 H2O
See also
- Electrolysis of waterElectrolysis of waterElectrolysis of water is the decomposition of water into oxygen and hydrogen gas due to an electric current being passed through the water.-Principle:...
- Alkali anion exchange membraneAlkali anion exchange membraneAn 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...
- Isotope electrochemistryIsotope electrochemistryIsotope electrochemistry is a field within electrochemistry concerned with various topics like electrochemical separation of isotopes, electrochemical estimation of isotopic exchange equilibrium constants , electrochemical kinetic isotope effect, electrochemical isotope sensors, etc.It is an active...
- Membrane electrode assemblyMembrane electrode assemblyA membrane electrode assembly is an assembled stack of proton exchange membranes or alkali anion exchange membrane , catalyst and flat plate electrode used in a fuel cell.-PEM-MEA:...
- Artificial membraneArtificial membraneAn artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of twentieth century. A...
- Dry electrolyte
- Dynamic mechanical analysisDynamic mechanical analysisDynamic mechanical analysis is a technique used to study and characterize materials. It is most useful for studying the viscoelastic behavior of polymers. A sinusoidal stress is applied and the strain in the material is measured, allowing one to determine the complex modulus...
- Electroosmotic pumpElectroosmotic pumpAn electroosmotic pump , or EO pump, is used for generating flow or pressure by use of an electric field. One application of this is removing liquid flooding water from channels and gas diffusion layers and direct hydration of the proton exchange membrane in the membrane electrode assembly of the...
- Roll-to-roll