Types of fuel cells

Fuel cells can be divided into five major categories named after the electrolyte used in each. The five types resulted from the knowledge that heat accelerates chemical reaction rates and thus the electrical current. The materials used for electrolytes have their best conductance only within certain temperature ranges and thus other materials and thus other materials must be used in order to take advantage of the temperature increase. The main characteristics of the fuel cell types are described below

Source: European Commission, Version 1998

Solid Oxide Fuel Cell (SOFC)

They operate at high temperatures (1000-1100°C) and a practical efficiency 50-60%. They are not the most reactive because of the low conductivity of its ionic conducting electrolyte (yttria-stabilised zirconia). Because of the conductivity and heat, They have been used in large power plants which can use the cogeneration of steam for additional power. The primary drawback to this type of fuel cell is the cost of containment which requires ceramics which are difficult to fabricate in forms and shapes that can accommodate the high thermal stresses. They can be used in power back for ouitdoor recreation (small tubular system) and in micro CHP systems in residences

Electrochemical Equation:

                     Anode:        H₂ + O₂  ®H₂O + 2 e^-

                     Cathode:     ½ O₂ + 2e^-  ® O^2-

Molten Carbonate Fuel Cell (MCFC)

They operate at 600°C and can use CO as a fuel input on the cathode side but need hydrogen on the anode. The temperature is high enough to be used for additional power production through cogeneration of steam. The efficiency of these type of fuel cells has risen to 50% in a combined (electrical and steam) cycle. They can also be used in mega-watt size power plants because of their heat. 

  Electrochemical Equation:

             Anode:        H₂ + CO₃^2-  ®H₂O +CO₂ +  2 e^-

              Cathode:     ½ O₂ + CO₂ + 2e^-  ® CO₃^2-

Phosphoric Acid Fuel Cell (PAFC)

PAFCs have an operating temperature of 200 °C. The efficiency of this system is much lower than that of the other systems at  40%. It is the FC that has mostly been exploited, mainly due to its high grade heat, which can be used in small-scale CHP especially at military sites and UPS systems fuelled with hydrogen, natural gas, LPG and methane from waste water purification plants.. The power output varies from 200 kW to 20 MW. The main disadvantage is that it has no self-starting capability, because at lower temperatures (40-50 °C) freezing of concentrated Phosphoric Acid occurs. In order to reduce losses, the cathode catalyst and the reformer need to be improved.

Electrochemical Equation:

Anode:        H₂  ®2H⁺ + 2e^-

Cathode:     ½ O₂ + 2H⁺ + 2e^-  ® H₂O

Proton Exchange Membrane or Solid Polymer Membrane Fuel Cell (PEMFC or SPMFC)

PEM fuel cells operate at around 80°C and a practical efficiency of 60%. Power output is in the range of 5-200 kW.  They are ideal for transportation and portable power. Additional advantages are their high response, the small size and low cost.

An attractive future development is the Direct Methanol Fuel Cell (DMFC). This uses methanol as a fuel for fuel cells by reforming it into hydrogen because of the capacity of safe hydrogen storage and transportation that methanol provides. DMFC is basically used in transportation.

 Electrochemical Equation:

   Anode:        H₂  ®2H⁺ + 2e^-

   Cathode:     ½ O₂ + 2H⁺ + 2e^-  ® H₂O

Alkaline Fuel Cell (AFC)

The operating temperature of AFCs is about 70°C and their power output is 10-100 kW. They have been widely used for space and defense applications, where pure hydrogen is used. Their excessive cost and sensitivity to CO₂ , have restricted their research and development, no matter their high efficiency and power density.

Electrochemical Equation:

Anode:        H₂ + 2(OH)^-  ® 2H₂O  +  2 e^-

Cathode:     ½ O₂ + HO₂ + 2e^-  ® 2(OH)^-

                                                                           Advantages/Disadvantages of Fuel Cells