Gas diffusion electrode
Encyclopedia
Gas diffusion electrodes (GDE) are electrode
s with a conjunction of a solid, liquid and gaseous interface, and an electrical conducting catalyst supporting an electrochemical reaction between the liquid and the gaseous phase.
and hydrogen
react at the gas diffusion electrodes, to form water, while converting the chemical bond
energy into electrical energy. Usually the catalyst is fixed in a porous foil, so that the liquid and the gas can interact. Besides these wetting
characteristics, the gas diffusion electrode has, of course, to offer an optimal electric conductivity, in order to enable an electron
transport with low ohmic resistance
.
An important prerequisite for the operation of gas diffusion electrodes is that both the liquid and the gaseous phase coexist in the pore system of the electrodes which can be demonstrated with the Young-Laplace equation:
The gas pressure p is in relation with the liquid in the pore system over the pore radius r, the surface tension σ of the liquid and the contact angle Θ. This equation is to be taken as a guide for determination because there are too many unknown, or difficult to achieve, parameters. When the surface tension is considered, the difference in surface tension of the solid and the liquid have to be taken into account. But the surface tension of catalysts such as platinum
on carbon
or silver
are hardly measurable. The contact angle on a flat surface can be determined with a microscope
. A single pore, however, cannot be examined so it is necessary to determine the pore system of an entire electrode. Thus in order to create an electrode area for liquid and gas, the path can be chosen to create different pore radius r, or to create different wetting angles Θ.
Most of the electrodes that were manufactured from 1950 to 1970 with the sintered method were for use in fuel cells. This type of production was dropped for economic reasons because the electrodes were thick and heavy, with a common thickness of 2 mm, while the individual layers had to be very thin and without defects. The sales price was too high and the electrodes could not be produced continuously.
and subsequent sintering or hot pressing
. To produce multi-layered electrodes a fine-grained material was scattered in a mold
and smoothed. Then, the other materials were applied in multiple layers and put under pressure. The production was not only error-prone but also time consuming and difficult to automate.
's are used to produce an electrode having both hydrophilic and hydrophobic properties while chemically stable and which can be used as binders. This means that, in places with a high proportion of PTFE, no electrolyte can penetrate the pore system and vice versa. In that case the catalyst itself should be non-hydrophobic.
The dispersion route is chosen mainly for electrodes with polymer
electrolytes, as successfully introduced in the PEM fuel cell
and in PEM
or HCL
membrane electrolysis
. When used in liquid electrolyte
, a dry process is more appropriate.
Also, in the dispersion route (through evaporation of water and sintering of the PTFE's at 340°C) the mechanical pressing is skipped and the produced electrodes are very porous. With fast drying methods, cracks can form in the electrodes which can be penetrated by the liquid electrolyte. For applications with liquid electrolytes, such as the zinc-air battery
or the alkaline fuel cell, the dry mixture method is used.
s are usually precious metals like platinum
, ruthenium
, iridium
and rhodium
. In alkaline electrolytes, like zinc-air batteries and alkaline fuel cell
s, it is usual to use less expensive catalysts like carbon
, manganese
, silver
, nickel foam or nickel
mesh
.
, Francis Thomas Bacon
was the first to use gas diffusion electrodes for the Bacon fuel cell
, converting hydrogen
and oxygen
at high temperature into electricity. Over the years, gas diffusion electrodes have been adapted for various other processes like:
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s with a conjunction of a solid, liquid and gaseous interface, and an electrical conducting catalyst supporting an electrochemical reaction between the liquid and the gaseous phase.
Principle
GDE's are used in fuel cells, where oxygenOxygen
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...
and 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...
react at the gas diffusion electrodes, to form water, while converting the chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
energy into electrical energy. Usually the catalyst is fixed in a porous foil, so that the liquid and the gas can interact. Besides these wetting
Wetting
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...
characteristics, the gas diffusion electrode has, of course, to offer an optimal electric conductivity, in order to enable an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
transport with low ohmic resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
.
An important prerequisite for the operation of gas diffusion electrodes is that both the liquid and the gaseous phase coexist in the pore system of the electrodes which can be demonstrated with the Young-Laplace equation:
The gas pressure p is in relation with the liquid in the pore system over the pore radius r, the surface tension σ of the liquid and the contact angle Θ. This equation is to be taken as a guide for determination because there are too many unknown, or difficult to achieve, parameters. When the surface tension is considered, the difference in surface tension of the solid and the liquid have to be taken into account. But the surface tension of catalysts such as platinum
Platinum
Platinum 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 a dense, malleable, ductile, precious, gray-white transition metal...
on carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
or silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
are hardly measurable. The contact angle on a flat surface can be determined with a microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
. A single pore, however, cannot be examined so it is necessary to determine the pore system of an entire electrode. Thus in order to create an electrode area for liquid and gas, the path can be chosen to create different pore radius r, or to create different wetting angles Θ.
Sintered electrode
In this image of a sintered electrode it can be seen that three different grain sizes were used. The different layers were:- top layer of fine-grained material
- layer from different groups
- gas distribution layer of coarse-grained material
Most of the electrodes that were manufactured from 1950 to 1970 with the sintered method were for use in fuel cells. This type of production was dropped for economic reasons because the electrodes were thick and heavy, with a common thickness of 2 mm, while the individual layers had to be very thin and without defects. The sales price was too high and the electrodes could not be produced continuously.
Principle
The principle of gas diffusion is illustrated in this diagram. The so-called gas distribution layer is located in the middle of the electrode. With only a small gas pressure, the electrolyte is displaced from this pore system. A small flow resistance ensures that the gas can freely propagate along the electrode. At a slightly higher gas pressure the electrolyte in the pore system is suppressed of the work layer. The surface layer itself has such fine pores that, even when the pressure peaks, gas can flow through the electrode into the electrolyte. Such electrodes were produced by scatteringScattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
and subsequent sintering or hot pressing
Hot pressing
Hot pressing is a high-pressure, low-strain-rate powder metallurgy process for forming of a powder or powder compact at a temperature high enough to induce sintering and creep processes. This is achieved by the simultaneous application of heat and pressure....
. To produce multi-layered electrodes a fine-grained material was scattered in a mold
Molding (process)
Molding or moulding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern....
and smoothed. Then, the other materials were applied in multiple layers and put under pressure. The production was not only error-prone but also time consuming and difficult to automate.
Bonded electrode
Since about 1970, PTFEPolytetrafluoroethylene
Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....
's are used to produce an electrode having both hydrophilic and hydrophobic properties while chemically stable and which can be used as binders. This means that, in places with a high proportion of PTFE, no electrolyte can penetrate the pore system and vice versa. In that case the catalyst itself should be non-hydrophobic.
Variations
There are two technical variations to produce PTFE catalyst-mixtures:- Dispersion of water, PTFE, catalyst and emulsifiers, thickening agents ...
- Dry mixture of PTFE powder and catalyst powder
The dispersion route is chosen mainly for electrodes with polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
electrolytes, as successfully introduced in the PEM 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...
and in PEM
Proton exchange membrane
A proton exchange membrane or polymer electrolyte membrane is a semipermeable membrane generally made from ionomers and designed to conduct protons while being impermeable to gases such as oxygen or hydrogen...
or HCL
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
membrane electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
. When used in liquid electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
, a dry process is more appropriate.
Also, in the dispersion route (through evaporation of water and sintering of the PTFE's at 340°C) the mechanical pressing is skipped and the produced electrodes are very porous. With fast drying methods, cracks can form in the electrodes which can be penetrated by the liquid electrolyte. For applications with liquid electrolytes, such as the zinc-air battery
Zinc-air battery
Zinc–air batteries , and zinc–air fuel cells, are electro-chemical batteries powered by oxidizing zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce...
or the alkaline fuel cell, the dry mixture method is used.
Catalyst
Last, but not least, the right choice of catalyst is important too. In acidic electrolytes the catalystElectrocatalyst
An electrocatalyst is a catalyst that participates in electrochemical reactions. Catalyst materials modify and increase the rate of chemical reactions without being consumed in the process. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the...
s are usually precious metals like platinum
Platinum
Platinum 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 a dense, malleable, ductile, precious, gray-white transition metal...
, ruthenium
Ruthenium
Ruthenium is a chemical element with symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most chemicals. The Russian scientist Karl Ernst Claus discovered the element...
, iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
and rhodium
Rhodium
Rhodium is a chemical element that is a rare, silvery-white, hard and chemically inert transition metal and a member of the platinum group. It has the chemical symbol Rh and atomic number 45. It is composed of only one isotope, 103Rh. Naturally occurring rhodium is found as the free metal, alloyed...
. In alkaline electrolytes, like zinc-air batteries and alkaline fuel cell
Alkaline fuel cell
The alkaline fuel cell , also known as the Bacon fuel cell after its British inventor, is one of the most developed fuel cell technologies. NASA has used alkaline fuel cells since the mid-1960s, in Apollo-series missions and on the Space Shuttle. AFCs consume hydrogen and pure oxygen producing...
s, it is usual to use less expensive catalysts like carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
, silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, nickel foam or nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
mesh
Mesh
Mesh consists of semi-permeable barrier made of connected strands of metal, fiber, or other flexible/ductile material. Mesh is similar to web or net in that it has many attached or woven strands.-Types of mesh:...
.
Application
At first solid electrodes were used in the Grove cellGrove cell
The Grove cell was an early electric primary cell named after its inventor, British chemist William Robert Grove, and consisted of a zinc anode in dilute sulfuric acid and a platinum cathode in concentrated nitric acid, the two separated by a porous ceramic pot.-Cell details:The Grove cell voltage...
, Francis Thomas Bacon
Francis Thomas Bacon
Francis Thomas Bacon OBE FREng F.R.S. was an English engineer who developed the first practical hydrogen–oxygen fuel cell.- Life and works :...
was the first to use gas diffusion electrodes for the Bacon fuel cell
Alkaline fuel cell
The alkaline fuel cell , also known as the Bacon fuel cell after its British inventor, is one of the most developed fuel cell technologies. NASA has used alkaline fuel cells since the mid-1960s, in Apollo-series missions and on the Space Shuttle. AFCs consume hydrogen and pure oxygen producing...
, converting 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...
and 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...
at high temperature into electricity. Over the years, gas diffusion electrodes have been adapted for various other processes like:
- Zinc-air batteryZinc-air batteryZinc–air batteries , and zinc–air fuel cells, are electro-chemical batteries powered by oxidizing zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce...
since 1980 - Nickel-metal hydride battery since 1990
- ChlorineChlorineChlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
production by electrolysis of waste hydrochloric acid - Chloralkali processChloralkali processThe chloralkali process is an industrial process for the electrolysis of sodium chloride solution . Depending on the method several products beside hydrogen can be produced. If the products are separated, chlorine and sodium hydroxide are the products; by mixing, sodium hypochlorite or sodium...
See also
- 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...
- 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:...
- Glossary of fuel cell termsGlossary of fuel cell termsThe 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. –...