TYPES OF FUEL CELLS

Phosphoric Acid
Proton Exchange Membrane or Solid Polymer
Molten Carbonate
Solid Oxide
Alkaline
Direct Methanol Fuel Cells
Regenerative Fuel Cells
Zinc Air Fuel Cells
Protonic Ceramic Fuel Cell

Phosphoric Acid (PAFC). This type of fuel cell is commercially available today. More than 200 fuel cell systems have been installed all over the world - in hospitals, nursing homes, hotels, office buildings, schools, utility power plants, an airport terminal, landfills and waste water treatment plants. PAFCs generate electricity at more than 40% efficiency -- and nearly 85% of the steam this fuel cell produces is used for cogeneration -- this compares to about 35% for the utility power grid in the United States. Operating temperatures are in the range of 300 to 400 degrees F (150 - 200 degrees C). At lower temperatures, phosphoric acid is a poor ionic conductor, and carbon monoxide (CO) poisoning of the Platinum (Pt) electro-catalyst in the anode becomes severe. The electrolyte is liquid phosphoric acid soaked in a matrix. One of the main advantages to this type of fuel cell, besides the nearly 85% cogeneration efficiency, is that it can use impure hydrogen as fuel. PAFCs can tolerate a CO concentration of about 1.5 percent, which broadens the choice of fuels they can use. If gasoline is used, the sulfur must be removed. Disadvantages of PAFCs include: it uses expensive platinum as a catalyst, it generates low current and power comparably to other types of fuel cells, and it generally has a large size and weight. PAFCs, however, are the most mature fuel cell technology. Through organizational linkages with Gas Research Institute (GRI), electronic utilities, energy service companies, and user groups, the Department of Energy (DOE) helped in bringing about the commercialization of a PAFC, produced by ONSI (now UTC Fuel Cells). Existing PAFCs have outputs up to 200 kW, and 1 MW units have been tested.

Anode: H2(g) -> 2H+(aq)+ 2e-

Proton Exchange Membrane (PEM). These cells operate at relatively low temperatures (about 175 degrees F or 80 degrees C), have high power density, can vary their output quickly to meet shifts in power demand, and are suited for applications, -- such as in automobiles -- where quick startup is required. According to DOE, "they are the primary candidates for light-duty vehicles, for buildings, and potentially for much smaller applications such as replacements for rechargeable batteries." The proton exchange membrane is a thin plastic sheet that allows hydrogen ions to pass through it. The membrane is coated on both sides with highly dispersed metal alloy particles (mostly platinum) that are active catalysts. The electrolyte used is a solid organic polymer poly-perflourosulfonic acid. The solid electrolyte is an advantage because it reduces corrosion and management problems. Hydrogen is fed to the anode side of the fuel cell where the catalyst encourages the hydrogen atoms to release electrons and become hydrogen ions (protons). The electrons travel in the form of an electric current that can be utilized before it returns to the cathode side of the fuel cell where oxygen has been fed. At the same time, the protons diffuse through the membrane (electrolyte) to the cathode, where the hydrogen atom is recombined and reacted with oxygen to produce water, thus completing the overall process. This type of fuel cell is, however, sensitive to fuel impurities. Cell outputs generally range from 50 to 250 kW.

Anode: H2(g) -> 2H+(aq) + 2e-

Molten Carbonate (MCFC). These fuel cells use a liquid solution of lithium, sodium and/or potassium carbonates, soaked in a matrix for an electrolyte. They promise high fuel-to-electricity efficiencies, about 60% normally or 85% with cogeneration, and operate at about 1,200 degrees F or 650 degrees C. The high operating temperature is needed to achieve sufficient conductivity of the electrolyte. Because of this high temperature, noble metal catalysts are not required for the cell's electrochemical oxidation and reduction processes. To date, MCFCs have been operated on hydrogen, carbon monoxide, natural gas, propane, landfill gas, marine diesel, and simulated coal gasification products. 10 kW to 2 MW MCFCs have been tested on a variety of fuels and are primarily targeted to electric utility applications. Carbonate fuel cells for stationary applications have been successfully demonstrated in Japan and Italy. The high operating temperature serves as a big advantage because this implies higher efficiency and the flexibility to use more types of fuels and inexpensive catalysts as the reactions involving breaking of carbon bonds in larger hydrocarbon fuels occur much faster as the temperature is increased. A disadvantage to this, however, is that high temperatures enhance corrosion and the breakdown of cell components.

Solid Oxide (SOFC). Another highly promising fuel cell, this type could be used in big, high-power applications including industrial and large-scale central electricity generating stations. Some developers also see SOFC use in motor vehicles and are developing fuel cell auxiliary power units (APUs) with SOFCs. A solid oxide system usually uses a hard ceramic material of solid zirconium oxide and a small amount of ytrria, instead of a liquid electrolyte, allowing operating temperatures to reach 1,800 degrees F or 1000 degrees C. Power generating efficiencies could reach 60% and 85% with cogeneration and cell output is up to 100 kW. One type of SOFC uses an array of meter-long tubes, and other variations include a compressed disc that resembles the top of a soup can. Tubular SOFC designs are closer to commercialization and are being produced by several companies around the world. Demonstrations of tubular SOFC technology have produced as much as 220 kW. Japan has two 25 kW units online and a 100 kW plant being testing in Europe.

Anode: H2(g) + O2- -> H2O(g) + 2e-

Alkaline. Long used by NASA on space missions, these cells can achieve power generating efficiencies of up to 70 percent. They were used on the Apollo spacecraft to provide both electricity and drinking water. Their operating temperature is 150 to 200 degrees C (about 300 to 400 degrees F). They use an aqueous solution of alkaline potassium hydroxide soaked in a matrix as the electrolyte. This is advantageous because the cathode reaction is faster in the alkaline electrolyte, which means higher performance. Until recently they were too costly for commercial applications, but several companies are examining ways to reduce costs and improve operating flexibility. They typically have a cell output from 300 watts to 5 kW.

Anode: H2(g) + 2(OH)-(aq) -> 2H2O(l) + 2e-

Direct Methanol Fuel Cells (DMFC). These cells are similar to the PEM cells in that they both use a polymer membrane as the electrolyte. 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 expected with this type of fuel cell, which would typically operate at a temperature between 120-190 degrees F or 50 -100 degrees C. 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 are achieved at higher temperatures. A major problem, however, is fuel crossing over from the anode to the cathode without producing electricity. Many companies have said they solved this problem, however. They are working on DMFC prototypes used by the military for powering electronic equipment in the field.

Anode: CH3OH(aq) + H2O(l) -> CO2(g) + 6H+(aq) + 6e-

Regenerative Fuel Cells. Still a very young member of the fuel cell family, regenerative fuel cells would be attractive as a closed-loop form of power generation. Water is separated into hydrogen and oxygen by a solar-powered electrolyser. The hydrogen and oxygen are fed into the fuel cell which generates electricity, heat and water. The water is then recirculated back to the solar-powered electrolyser and the process begins again. These types of fuel cells are currently being researched by NASA and others worldwide.

Zinc-Air Fuel Cells (ZAFC). In a typical zinc/air fuel cell, there is a gas diffusion electrode (GDE), a zinc anode separated by electrolyte, and some form of mechanical separators. The GDE is a permeable membrane that allows atmospheric oxygen to pass through. After the oxygen has converted into hydroxyl ions and water, the hydroxyl ions will travel through an electrolyte, and reaches the zinc anode. Here, it reacts with the zinc, and forms zinc oxide. This process creates an electrical potential; when a set of ZAFC cells are connected, the combined electrical potential of these cells can be used as a source of electric power. This electrochemical process is very similar to that of a PEM fuel cell, but the refueling is very different and shares characteristics with batteries. Metallic Power is working on ZAFCs containing a zinc "fuel tank" and a zinc refrigerator that automatically and silently regenerates the fuel. In this closed-loop system, electricity is created as zinc and oxygen are mixed in the presence of an electrolyte (like a PEMFC), creating zinc oxide. Once fuel is used up, the system is connected to the grid and the process is reversed, leaving once again pure zinc fuel pellets. The key is that this reversing process takes only about 5 minutes to complete, so the battery recharging time hang up is not an issue. The chief advantage zinc-air technology has over other battery technologies is its high specific energy, which is a key factor that determines the running duration of a battery relative to its weight. When ZAFCs are used to power EVs, they have proven to deliver longer driving distances between refuels than any other EV batteries of similar weight. Moreover, due to the abundance of zinc on earth, the material costs for ZAFCs and zinc-air batteries are low. Hence, zinc-air technology has a potential wide range of applications, ranging from EVs, consumer electronics to military. Powerzinc in southern California is currently commercializing their zinc/air technology for a number of different applications.

Protonic Ceramic Fuel Cell (PCFC). This new type of fuel cell is based on a ceramic electrolyte material that exhibits high protonic conductivity at elevated temperatures. PCFCs share the thermal and kinetic advantages of high temperature operation at 700 degrees Celsius with molten carbonate and solid oxide fuel cells, while exhibiting all of the intrinsic benefits of proton conduction in polymer electrolyte and phosphoric acid fuel cells (PAFCs). The high operating temperature is necessary to achieve very high electrical fuel efficiency with hydrocarbon fuels. PCFCs can operate at high temperatures and electrochemically oxidize fossil fuels directly to the anode. This eliminates the intermediate step of producing hydrogen through the costly reforming process. Gaseous molecules of the hydrocarbon fuel are absorbed on the surface of the anode in the presence of water vapor, and hydrogen atoms are efficiently stripped off to be absorbed into the electrolyte, with carbon dioxide as the primary reaction product. Additionally, PCFCs have a solid electrolyte so the membrane cannot dry out as with PEM fuel cells, or liquid can't leak out as with PAFCs. Protonetics International Inc. is primarily researching this type of fuel cell.