fuel cell technology and types

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Types of Fuel CellsPhosphoric AcidProton Exchange MembraneMolten CarbonateSolid OxideAlkalineDirect MethanolRegenerativeZinc AirProtonic CeramicMicrobial Fuel Cell

Click here to read an article by Fuel Cells 2000 in Earthtoyson the different types of fuel cells.

Phosphoric Acid fuel cell (PAFC) - Phosphoric acid fuel cellsare commercially available today. Hundreds of fuel cell systemshave been installed in 19 nations - in hospitals, nursing homes,hotels, office buildings, schools, utility power plants, landfills andwaste water treatment plants. PAFCs generate electricity at morethan 40% efficiency - and nearly 85% of the steam this fuel cellproduces is used for cogeneration - this compares to about 35%for the utility power grid in the United States. Phosphoric acidfuel cells use liquid phosphoric acid as the electrolyte andoperate at about 450°F. One of the main advantages to this typeof fuel cell, besides the nearly 85% cogeneration efficiency, isthat it can use impure hydrogen as fuel. PAFCs can tolerate a COconcentration of about 1.5 percent, which broadens the choice of fuels they can use. If gasoline is used, the sulfur must beremoved.

Proton Exchange Membrane fuel cell (PEM) - These fuel cellsoperate at relatively low temperatures (about 175°F), have highpower density, can vary their output quickly to meet shifts inpower demand, and are suited for applications, such as inautomobiles, where quick startup is required. According to theU.S. Department of Energy (DOE), "they are the primarycandidates for light-duty vehicles, for buildings, and potentiallyfor much smaller applications such as replacements forrechargeable batteries." This type of fuel cell is sensitive to fuelimpurities. Cell outputs generally range from 50 watts to 75 kW.

Molten Carbonate fuel cell (MCFC) - Molten carbonate fuelcells use an electrolyte composed of a molten carbonate saltmixture suspended in a porous, chemically inert matrix, andoperate at high temperatures - approximatelly 1,200ºF. Theyrequire carbon dioxide and oxygen to be delivered to thecathode. To date, MCFCs have been operated on hydrogen,carbon monoxide, natural gas, propane, landfill gas, marinediesel, and simulated coal gasification products. 10 kW to 2 MWMCFCs have been tested on a variety of fuels and are primarilytargeted to electric utility applications.

Solid Oxide fuel cell (SOFC) - Solid oxide fuel cells use a hard,non-porous ceramic compound as the electrolyte, and operate atvery high temperatures - around 1800°F. One type of SOFC usesan array of meter-long tubes, and other variations include acompressed disc that resembles the top of a soup can. TubularSOFC designs are closer to commercialization and are beingproduced by several companies around the world. SOFCs aresuitable for stationary applications as well as for auxiliary powerunits (APUs) used in vehicles to power electronics.

Fuel Cell

Developers

For a full listing of fuel celldevelopers, perusethe list of developer links.

Cells 2000 : Fuel Cell Basics : Types http://www.fuelcells.org/basics/t

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Alkaline fuel cell (AFC) - Long used by NASA on spacemissions, alkaline fuel cells can achieve power generatingefficiencies of up to 70 percent. They were used on the Apollospacecraft to provide both electricity and drinking water. Alkalinefuel cells use potassium hydroxide as the electrolyte and operateat 160°F. However, they are very susceptible to carboncontamination, so require pure hydrogen and oxygen.

Direct Methanol fuel cell (DMFC) - These cells are similar tothe PEM cells in that they both use a polymer membrane as theelectrolyte. However, in the DMFC, the anode catalyst itself draws the hydrogen from the liquid methanol, eliminating the

need for a fuel reformer. Efficiencies of about 40% are expectedwith this type of fuel cell, which would typically operate at atemperature between 120-190°F. This is a relatively low range,making this fuel cell attractive for tiny to mid-sized applications,to power cellular phones and laptops. Higher efficiencies areachieved at higher temperatures. Companies are also working onDMFC prototypes to be used by the military for poweringelectronic equipment in the field.

Regenerative fuel cell - Regenerative fuel cells are attractive asa closed-loop form of power generation. Water is separated intohydrogen and oxygen by a solar-powered electrolyzer. Thehydrogen and oxygen are fed into the fuel cell which generateselectricity, heat and water. The water is then recirculated back tothe solar-powered electrolyzer and the process begins again.These types of fuel cells are currently being researched by NASAand others worldwide.

Zinc Air fuel cell (ZAFC) - In a typical zinc/air fuel cell, there isa gas diffusion electrode (GDE), a zinc anode separated byelectrolyte, and some form of mechanical separators. The GDE isa permeable membrane that allows atmospheric oxygen to passthrough. After the oxygen has converted into hydroxyl ions andwater, the hydroxyl ions will travel through an electrolyte, andreaches the zinc anode. Here, it reacts with the zinc, and formszinc oxide. This process creates an electrical potential; when aset of ZAFC cells are connected, the combined electrical potentialof these cells can be used as a source of electric power. Thiselectrochemical process is very similar to that of a PEM fuel cell,but the refueling is very different and shares characteristics withbatteries. ZAFCs contain a zinc "fuel tank" and a zinc refrigeratorthat automatically and silently regenerates the fuel. In thisclosed-loop system, electricity is created as zinc and oxygen aremixed in the presence of an electrolyte (like a PEMFC), creatingzinc oxide. Once fuel is used up, the system is connected to thegrid and the process is reversed, leaving once again pure zincfuel pellets. The key is that this reversing process takes onlyabout 5 minutes to complete, so the battery recharging timehang up is not an issue. The chief advantage zinc-air technologyhas over other battery technologies is its high specific energy,which is a key factor that determines the running duration of abattery relative to its weight.

Protonic Ceramic fuel cell (PCFC) - This new type of fuel cellis based on a ceramic electrolyte material that exhibits highprotonic conductivity at elevated temperatures. PCFCs share thethermal and kinetic advantages of high temperature operation at700 degrees Celsius with molten carbonate and solid oxide fuelcells, while exhibiting all of the intrinsic benefits of protonconduction in PEM and phosphoric acid fuel cells. The high

operating temperature is necessary to achieve very highelectrical fuel efficiency with hydrocarbon fuels. PCFCs canoperate at high temperatures and electrochemically oxidize fossilfuels directly to the anode. This eliminates the intermediate stepof producing hydrogen through the costly reforming process.Gaseous molecules of the hydrocarbon fuel are absorbed on thesurface of the anode in the presence of water vapor, andhydrogen atoms are efficiently stripped off to be absorbed intothe electrolyte, with carbon dioxide as the primary reactionproduct. Additionally, PCFCs have a solid electrolyte so themembrane cannot dry out as with PEM fuel cells, or liquid can'tleak out as with PAFCs.

Microbial fuel cell (MFC) - Microbial fuel cells use the catalyticreaction of microorganisms such as bacteria to convert virtuallyany organic material into fuel. Some common compounds


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include glucose, acetate, and wastewater. Enclosed inoxygen-free anodes, the organic compounds are consumed(oxidized) by the bacteria or other microbes. As part of thedigestive process, electrons are pulled from the compound andconducted into a circuit with the help of an inorganic mediator.MFCs operate well in mild conditions relative to other types of fuel cells, such as 20-40 degrees Celsius, and could be capableof producing over 50% efficiency. These cells are suitable forsmall scale applications such as potential medical devices fueledby glucose in the blood, or larger such as water treatment plantsor breweries producing organic waste that could then be used tofuel the MFCs.

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