Fuel cells convert the energy of combustion directly to electrical energy. A fuel cell can be represented as
Fuel Electrode | Electrolyte | Electrode Oxidant
The common fuels are H 2 , N2H4 and Hydrocarbons. The major oxidants are Oxidants O2 , H2O2 , HNO3, etc
A fuel cell is a galvanic cell in which the chemical energy associated with the oxidation of reducing agents (fuels) is directly converted into electrical energy. The conventional method of utilizing the chemical energy of the fuel to produce electrical energy and the direct conversion process used in the fuel cell can be represented as shown below.
Chemical energy
Electrical energy
(a)
(c)
Thermal energy
Mechanical energy
Any losses in energy in steps (a) and (c) can be minimized. However the efficiency of the process (b) is limited by the second law of thermodynamics. Hence the process (d) might be expected to have a higher efficiency. If
H is the enthalpy change of the reaction, the amount of useful work that can be obtained out of this is
G. The rest of it. i.e.,
H -
G = T S is unavailable for work.
Efficiency = E = Work obtained from the cell reaction /
Heat change accompanying the reactIon
= G /H
Electrode | Electrolyte | Electrode 
Oxidant 
Electrical energy 
Mechanical energy 
H is the enthalpy change of the reaction, the amount of useful work that can be obtained out of this is
G. The rest of it. i.e.,
H -
G = T 
S is unavailable for work. 
