All fuel cells consist of an electrolyte and two electrodes (an anode and a cathode), just like a cell in an ordinary battery. There are five main types of fuel cells, named after their electrolyte: AFC, PAFC, PEMFC, MCFC and SOFC.
AFC
The Alkaline Fuel Cell, AFC, has an electrolyte consisting of aqueous potassium hydroxide (KOH). Both anode and cathode consist of porous nickel. The operating temperature is 100 °C. The fuel must be very pure hydrogen, and the air also needs cleaning before the oxygen in it can be utilized. This is due to the fact that AFC cannot tolerate CO2 because it reacts with OH− to form CO32−, destroying the electrolyte. It is therefore necessary for the CO2 content in both hydrogen and oxygen to be below a few ppm (parts per million). Gasses of that purity are expensive to manufacture, and this has significantly limited the use of AFC. Today the major application is in space where the price is not crucial.
Principle of an AFC with hydrogen as fuel
PAFC
The Phosphoric Acid Fuel Cell, PAFC, has concentrated ortho-phosphoric acid, H3PO4, for its electrolyte. Anode and cathode are made of graphite with a platinum catalyst, and the operating temperature is 150-200 °C. In contrast to AFC, the PAFC tolerates CO2 but can only tolerate CO (and hydrocarbons) in low concentrations (below 100 ppm). If fossil fuel, e.g. natural gas, should be utilized it is consequently necessary to reform it and afterwards clean the hydrogen. PAFC systems are commercially available today and are used in hospitals, schools and hotels as an independent power supply or emergency power; the size is typically a few hundred kilowatts. However, for reasons of cost a wider range of applications for PAFC seems unlikely.
Principle of a PAFC with hydrogen as fuel
PEMFC
The Proton Exchange Membrane Fuel Cell (or Polymer Electrolyte Membrane Fuel Cell), PEMFC or just PEM, has an electrolyte consisting of a proton conducting polymer. Often the material Nafion® from the company DuPont is used. It is a perfluorosulfonic polymer related to Teflon®. In appearance it resembles a transparent plastic film (in operation it turns brown or black) with a typical thickness of 50-200 μm. Anode and cathode consists of graphite with a platinum catalyst. To work, the electrolyte membrane must be saturated with water. This limits the available span of operation temperatures; a typical temperature is 80 °C. At this relatively low temperature the platinum catalyst in the anode is poisoned by CO so that hydrogen made by reforming must cleaned which adds to the cost. A great amount of research and development is carried out on PEMFC since this is the type of fuel cell thought to be most suited for use in cars.
Principle of a PEMFC with hydrogen as fuel
The Direct Methanol Fuel Cell, DMFC, is a type of PEMFC which can utilize methanol as a fuel without having to reform it first. The anode catalyst in a DMFC contains in addition to platinum an element such as ruthenium which can break the methanol bond. One of the advantages of DMFC is that the fluid fuel is easy to handle. A drawback is a lower efficiency. Many portable systems use DMFC.
MCFC
The Molten Carbonate Fuel Cell, MCFC, has an electrolyte made of a molten carbonate salt. Often KLiCO3 is used. The anode is porous nickel while the cathode consists of nickel oxide (NiO) and lithium oxide (Li2O). The high operating temperature, 650 °C, means that MCFC can use both natural gas and CO. In an MCFC it is necessary to supply both oxygen and CO2 to the cathode to produce the CO32− ions which are transported through the electrolyte. If natural gas is used, CO2 is also produced but at the anode. It can be separated from the anode exhaust and fed to the cathode.
The salt melt, saturated with oxygen and CO2, is extremely corrosive. This limits the lifespan of the cell which today is the single largest problem for MCFC. In spite of this there are still development activities, in particular in Japan, aiming to commercialize their use. They will mostly be used for large, centralized power plants.
Principle of an MCFC with hydrogen as fuel
SOFC
The Solid Oxide Fuel Cell, SOFC, has a ceramic electrolyte which conducts oxygen ions. Most often it is made from YSZ (yttria stabilized zirconia), i.e. ZrO2 doped with a sufficient amount of Y2O3 to stabilize it in a cubic crystalline structure which gives the highest oxygen conductivity. The anode is a nickel cermet, i.e. a fine mixture of metallic nickel and ceramic YSZ. The cathode may be LSM, LaMnO3 doped with SrO. The operating temperature is 500-1000 °C. Like MCFC, the other high-temperature fuel cell, SOFC can run on both natural gas and CO in addition to hydrogen. In many ways SOFC seem to have the largest potential. The solid electrolyte is easy to handle, and high efficiencies (above 50%) and long lifetimes (above 10,000 hours) have been demonstrated.
SOFC may be used in systems from a few kW to MW power plants. The waste heat may be used for heating or, alternatively, be used to run a gas turbine to generate further electricity. In such a combined cycle system efficiencies may be as high as 80-85%.
At Risø the main thrust of our research is within SOFC. You can read more about our SOFC activities here.
Principle of an SOFC with hydrogen as fuel
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