Types of Fuel Cells

Fuel cells types are determined by their electrolyte.  Different electrolytes define the specific properties of a fuel cell, like fuel type, temperature or efficiency.  There are benefits to each type of fuel cell, enabling applications of all sorts.

●    Proton Exchange Membrane or Polymer Electrolyte Membrane Fuel Cell (PEMFC) 

A membrane separates the anode and cathode serving as both the electrolyte and a catalyzer for the conduction of protons. PEMFCs can use hydrogen gas or some alcohols such as ethanol or methanol as fuels.  This type of fuel cell runs at low temperatures, under 200 degrees Fahrenheit and has a wide variety of applications, including automotive vehicles (including busses and forklifts), unpiloted military vehicles, residential primary or backup power, portable charging for electronics (laptops, cell phones), and distributed generation power plants.

●   Direct Methanol Fuel Cell (DMFC)

DMFCs are a specific type of PEMFC.  They operate on pure methanol (CH3OH) combined with steam as a fuel.  DMFCs run at low temperatures between 85 and 200 degrees Fahrenheit.  Using a liquid fuel and operating at low temperatures makes them best suited for powering portable electronics and micro power applications.  Examples include cell phones, laptops, and portable battery rechargers.

●    Solid Oxide Fuel Cell (SOFC)

The electrolyte in a SOFC is a solid, ceramic compound of metal oxides. SOFCs operate at temperatures up to 1832 degrees Fahrenheit, making it an ideal candidate for combined heat and power (CHP) or combined cycle electrical generation.  In systems where the waste heat is captured and used, the system efficiencies can be over 85%.  SOFCs can reform hydrocarbon fuels internally, enabling a wider variety of fuels to be used and reducing the amount of activity prior to electrical generation.  SOFCs are currently being used for stationary applications.  SOFCs have also been applied to use in military transportation vehicles and truck APUs.

●    Phosphoric Acid Fuel Cell (PAFC)

The electrolyte used in PAFCs is liquid phosphoric acid.  PAFCs operate at temperatures of 300-400 degrees Fahrenheit.  The fuel used in PAFCs is hydrogen which must be reformed outside of the fuel cell.  Electrical generating efficiencies range between 37 and 42% but when used as a CHP system, the efficiencies increase to 90%.  Applications of PAFCs include buses and stationary power generation.

●    Molten Carbonate Fuel Cell (MCFC)

In a MCFC, molten lithium-potassium carbonate salts are used for the electrolyte.  These salts are heated to 1200 degrees Fahrenheit where they melt into a molten state inside the fuel cell.  Since MCFCs operate at high temperatures, they do not have a need for outside fuel reforming.  This means they can use fuels such as natural gas directly.  Efficiencies average 50% for fuel to electricity generation from the fuel cell alone.  The MCFC can be combined with a steam turbine to create more electricity increasing efficiency to 65% and can be increased up to 85% when used as a CHP device.  MCFCs are best applied to large-scale stationary CHP applications.

●    Alkaline Fuel Cell (AFC)

These fuel cells use a solution of potassium hydroxide in water as the electrolyte.  High temperature AFCs operate at temperatures between 100 and 250 degrees Celsius but through advancements in technology, some AFCs can operate at temperatures between 23 and 70 degrees Celsius.  AFCs require pure hydrogen and pure oxygen as the reactants due to possible CO2 pollution that could affect the cell’s operation.  Although they require pure inputs of fuel, AFCs operate at efficiencies of nearly 60%.  These fuel cells have mostly been used in remote locations, like in underwater vehicle and in outer space.

●    Other Fuel Cells

Recent advancements in the fuel cell industry have produced other types of fuel cells that are relatively new to the family of fuel cells. For instance, a Regenerative Fuel Cell contains a membrane that can produce electricity and steam like other fuel cells, but can also work in reverse and use electricity to split water into hydrogen and oxygen.  Regenerative fuel cells can be used in combination with solar or wind power to utilize excess electricity.  The excess electricity can be used in the regenerative fuel cell to produce hydrogen which can be stored and later power the fuel cell, or used as fuel for a fuel cell electric vehicle.