Ok guys, if you run across sources for some fuel cell membranes please post here.
Depending on editing time, i will edit this original post, with updated links as they are posted. I will start with 2 links that I have thus far, as this stuff is hard to find.
http://store.ion-power.com/SearchResults.asp?Cat=25
http://www.cleanfuelcellenergy.com/membrane.html
http://www.celgard.com/ <-- finallyme
THIS IS A GOOD READ REGARDING H2 AND FUEL CELL MEMBRANES!
To build an electroylis unit i feel it is essential to understand the membrane properties needed for ion exchange.
From Industrial Membrane Separation Technology, Written by Scott, K. and Hughes R., Published by Blackie Academic and Professional, an imprint of Chapman and Hall, located in London, UK. I choice certain excerpts that pertain to ion exchange membrane, the most crucial part of an electrolysis cell.
Membranes are an essential part of many electrochemical cells and devices, enabling the processes emanating from the cathode and anode to be segregated and to facilitate selective motion of certain charged species.
Range of applications of diaphragms and membranes in electrochemical processes:
Electrolysis,
Electrochemical separators for cells
In electrochemical processes it is frequently necessary to separate the processes occurring at the anode and cathode for three reasons:
(i) high yield and selectivity
(ii) product separation
(iii) safety in operation
The fact oxidation and reduction occur simultaneously in a cell generally dictates the use of a cell separator. If starting material or products are susceptible to reaction at the counter electrode then separation of anode and cathode is required to achieve a high yield of desired product. Furthermore if starting reagents or products of one electrode process can chemically react undesirably with species generated at the counter electrode, then again separators are required. For example in the case in the chlor-alkali industry without a cell separator the contact of brine solution saturated with chlorine with the hydroxide ion generated cathodic decomposition of water would form hypochlorous acid and other species. Cell separators are of the required to separate the products obtained at the two electrodes in a cell, e.g. in the electrolytic production of hydrogen and oxygen by water electrolysis where the requirement of safety in operation is important. Overall the material used as separators are varied and may simply be micro porous separators or may possess specific ion transport characteristics.
Cell separators
The separators used in cells are generally classified as permeable or semi-permeable. Desirable properties of separators:
Correct permeability and selectivity Good Current efficiency
Inert to cell environment: good temperature
And chemical stability long life
Uniform properties across its surface
Good current efficiency, good
Homogenous flow quality products
Low voltage drop Lower energy performance
Finite thickness Overcomes diffusion
Some physical strength: mechanical stability Long life: easy to install in cells
Resistance to gas blinding Low voltage: good energy
Low Cost performance
Environmentally acceptable Economic operation
Permeable membranes permit the bulk flow of liquors through their structure and are thus non-selective regarding transport of ions or neutral molecules. In electrochemical processes these are referred to as diaphragms.
Semi-permeable membranes permit the selective passage of certain species by virtue of molecular size or charge. In electrochemical processes these are termed membranes and separation is based on the charge carried by the molecule.
Although there has been a steady move away from the use of porous diaphragms towards membranes (ion exchange membranes) the former are still used in several industries. An effective separator material must exhibit a range of desirable properties listed previously.
The porous diaphragm represents a compromise between the demands of separation of anolyte and catholyte and effective electrical conductivity between anode and cathode via the ions in solution. A good degree of separation is achieve by using a uniformly fine porous structure which permits diffusion of material, but not mass flow. The conduction of electrical current is by the solution of ions in the porous structure, which gives rise to a higher electrical resistance than that of the bulk electrolyte solution. The higher the porosity (size and/or number of pores) the greater the electrical conductivity of the diaphragm, but the poorer the separation of the anolyte from the catholyte. The transport of species across a porous diaphragm will be greater the thinner the material and the higher the concentration gradient across it. Since a diaphragm is positioned in a voltage gradient the material it is made of should be an electrical non-conductor. . There is potentially a wide range of materials which can be used as cell separators. THe following table lists some of the materials which can be used as cell separators which are divided into three types, organic, inorganic, and composites.
Materials used as separators:
Organic Inorganic Composites
Porous Plastics Asbestos Asbestos fibers and glass fibers
PTFE Paper, felt, fiber
Polypropylene Asbestos sheet and composite fiber sheet
PVC Asbestos on metal screen
Copolymers Ceramics Coated asbestos
Styrene AL2O3,SiO2
Nafion ZrO2
porous PTFE Glass fibers
Membrane and diaphragm materials
One of the largest electrochemical industries is for the production of chlorine and caustic soda, i.e. the chlor-alkali industry. One cell design uses a diaphragm constructed onto the electrode itself. Asbestos fibers are widely used in the cholor-alkali industry (and in water electrolysis) because they possess the required chemical and physical stability in alkaline media and can be engineered to give the required permeability. Chrysotile asbestos approximate formula Mg3(Si2O5)(OH)4), fibers are slurried with caustic soda solution and the slurry sucked on to a mild steel mesh cathode which collects the asbestos fiber. The quantity (wt) and quality of fiber is controlled to give a required thickness and permeability of diaphragm.
As a general diaphragm asbestos is not an ideal material as it is not resistant to very acid conditions, not robust physically and environmentally unacceptable. Thus for general application alternative materials with the required chemical resistance are required. This generally restricts the choice to perflourinated plastics of ceramics. For a cholr-alkali cell the choice is restricted to perflourocarbon plastics, e.g. PTFR (polytetraflouroehtlene), and certain ceramics, e.g. TiO2, ZrO2.
In the case of ceramics, processing the material into a porous structure suitable for a diaprhagm can be difficult with the resultant material quite brittle. Installation in parallel plate electrolyzes presents mechanical difficulties which outweigh advantages of mechanical strength, and temperature and chemical stability. In the case of alkaline water electrolysis some flexibility in the structure is introduced y sintering the porous ceramic onto a supporting metal net. Ceramic materials are available as membranes for micro filtration and ultra filtration typically in tubular form and thus only few cells could capitalize on this technology. One example of their use, however, is a design for a fluidized bed electrode, used in metal recovery. Differential pressure is used hydraulically to limit the transport of species. The fabrication of polymers for diaphragms does not create unusual problems. Operation conditions in electrolysis usually involve extremes of pH and/ or organic solvents and the material are limited to polymers of ethylene, propylene, vinyl chloride and tetraflouroethylene etc. IN manufacture an open porous structure can either be created at the time of fabrication of the sheet or by the incorporation of leach able filler. Several polymer cells. In these applications the materials used must be hydrophilic so that they are completely wetted and thus not blocked by gas bubbles, In the case of flouropolymers this can be achieved by adding suitable wetting agents e.g. ZrO2, into the diaphragm structure. A major limitation is the operating temperatures of organic materials, generally less than 120 degrees Celsius, although materials such as Ryton and PTFE are stable at temps up to 160 C.
Semi-permeable membranes: ion-exchange membranes
The disadvantage and limitations of permeable separators in electrochemical cells has forced interest on one range of semi-permeable separators, i.e. ion-exchange membranes. Ion-exchange membranes have the characteristic property of being able to distinguish between cations and anions. The interest in ion-exchange membranes has arisen because they can be used to keep, selectively, either anions or cations from transferring from one cell compartment to the other and they allow electrolysis to be carried out under close control of pH. For example the use of anion-exchange membrane will prevent the transfer of H ions, generated at an anode during oxygen evolution, into the catholyte chamber and thus allow a pH differential to be set up in the cell.
The main properties required of ion exchange membranes for them to be successful in technical processes are:
1. Low electrical resistance. The permeability for the counter-ions under the driving force of an electrical potential gradient should be height to minimize the membrane IR lasses.
2. High perm selectivity. It should be highly permeable for counter-ions, but should be highly impermeable to co-ions, and to non-ionized molecules and solvents.
3. Good mechanical stability. I should be mechanically strong, to prevent high degrees of welling or shrinking due to osmotic effects, when transferred from concentrated to diluted salt solutions and vice versa and to be dimensionally stable.
4. Good chemical stability. It should be stale over a wide pH0range and in the presence of oxidizing agents.
5. Good operating characteristics. It should be capable of operation over a wide range of current densities and under varying conditions of temperature, current density, pH, etc.
Commercial manufacturers of ion-exchange membranes.
Asahi Chemical LTd.
DU Pont
Tokuyama Soda
Tosoh
WSI Tech
Solvay-Morange
Aquatech
Stantech
Ionics, Inc
Membranes International
RAI Research Corp
Sybron Chemicals Inc
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