There are many types of fuel cells designed for different applications. Essentially they consist of two electrodes, a negative anode and a positive cathode separated by a solid or liquid electrolyte which moves particles from the anode to the cathode. A catalyst, such as platinum, is used to speed up the reaction. Typically, a fuel cell is classified by the type of electrode. The following descriptions are most common fuel cells:

• Proton Exchange Membrane or Polymer Electrolyte Membrane (PEMFC) Fuel Cells are capable of producing a great deal of power relative to their size, weight and operating temperature as compared to other fuel cells. Typically, platinum is used as a catalyst which adds a great deal to the cost. PEM fuel cells are easily poisoned by carbon monoxide which may hurt performance. The chemical reaction is as follows:
  Anode: 2H₂ 4H⁺ + 4e^-
  Cathode: O₂ + 4H⁺ + 4e^- 2H₂O
  Overall: 2H₂ + O₂ 2H₂O + energy

• Alkaline Fuel Cells (AFC) – Represents some of the earliest fuel cell technology developed by NASA to provide water and power to astronauts while in space. AFCs use a solution of potassium hydroxide in water as an electrolyte and combination of non-precious metals on the anode and cathode as catalysts. A disadvantage of this fuel cell is that they are easily poisoned by carbon dioxide and need purified oxygen and hydrogen to operate properly. The chemical reaction that takes place is somewhat different from that of a PEM fuel cell because hydroxyl ions (OH^-) migrate from the cathode to the anode where they react with hydrogen to produce water and electrons:
  Anode Reaction: 2H₂ + 4OH^- 4H₂O + 4e^-
  Cathode Reaction: O₂ + 2H₂O + 4e^- 4OH^-

• Phosphoric Acid Fuel Cells (PAFC) – Use phosphoric acid as an electrolyte and porous carbon electrodes containing platinum as catalysts. Electricity produced by a PAFC is between 37% to 42% efficiency although when combined heat and energy output is accounted for the efficiency reaches 80%.

• Molten Carbonate Fuel Cell (MCFC) – Are high temperature fuel cells that are about 60% efficient in converting hydrogen to electricity. Operating at about1200 degrees Fahrenheit allow these fuel cells to utilize less pure hydrogen as well as less expensive catalysts. The disadvantage of these fuel cells is that the high temperatures and corrosive electrolyte decrease MCFCs operating time.
  Anode Reaction: CO₃^2- + H₂ H₂O + CO₂ + 2e^-
  Cathode Reaction: CO₂+ 1/2O₂ + 2e^- CO₃^2-

• Solid Oxide Fuel Cells (SOFC)– Operate at the highest temperature of all the fuel cells 1,000°C or 1,830°F. These high temperatures allow for efficient electrical production from impure sources of hydrogen without being assisted by a catalyst. These fuel cells use a non-porous ceramic compound as an electrolyte. SOFC have potential application in industrial settings where fossil fuels may be used to generate electricity.
  Anode Reactions: H₂ + O^2- H₂O + 2e^-
  CO + O^2- CO₂ + 2e^-
  Cathode Reaction:O₂ + 4e^- 2O^2-

• Direct Methanol Fuel Cells (DMFC) - The advantage of this system is that it pulls hydrogen directly from methanol which is easier to store onboard a vehicle. This fuel cell technology is relatively new and research and development is about 3 to 4 years behind its hydrogen powered cousins.
  Anode Reaction: CH₃OH+ H₂O CO₂ + 6H⁺ + 6e^-
  Cathode Reaction: 3/2O₂ + 6H⁺ + 6e^- 3H₂O
  Cell Reaction: CH₃OH+ 3/2O₂ CO₂ + 2H₂O