Definition of fuel cells

The definition of fuel cells is similar to the definition of batteries, but an important distinction is that in fuel cells the chemical reaction takes place between gaseous liquid and/or liquid reactants. The definition of compound batteries is also similar to that of batteries, the only difference being that in compound batteries the chemical reaction takes place between a solid reactant on one of the electrodes and a gaseous and/or liquid reactant on the other electrodes. 

However, this may not be the best representation of fuel cell efficiency since the available chemical energy content of a fuel for power conversion is the enthalpy of formation or the heating values of the fuel. Thus, a more practical definition of fuel cell efficiency is given as the ratio of electrical work produced to the enthalpy of formation of the fuel... 

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