Free Enthalpy and Cell Voltage

When a system is transferred from one state to another at constant pressure and temperature, it is subject to a change of free enthalpy (or Gibbs free energy) given by 

AG= -(maximum work available from the system through the state transition), when this work is given a positive sign. 

We know already that the system tends to transform to a lower energy level by itself i.e., a net state transition (a net reaction) that occurs without any external supply of energy, is accompanied by a negative AG. Such a reaction is called a spontaneous reaction. Thus we have  

AG negative Spontaneous reaction, the system can carry out work, 

AG positive Energy must be supplied to obtain this. 

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