text: Hydrogen and oxygen can be combined through by the use of a fuel cell. In this process, the reaction is broken into two half reactions which occur at separate electrodes. In this situation the reactants energy is directly converted to electricity. Useful energy can be obtained from the thermal heat of this reaction through an internal combustion engine with an upper efficiency of 60% (for compression ratio of 10 and specific heat ratio of 1.4) based on the Otto thermodynamic cycle.  It is also possible to combine the hydrogen and oxygen through redox mechanism as in the case of a fuel cell.  In this process, the reaction is broken into two half-reactions which occur at separate electrodes.  In this situation the reactants energy is directly converted to electricity.
  The standard reduction potential of hydrogen is defined as 0V, and frequently referred to as the standard hydrogen electrode (SHE).
   HER can be promoted by many catalysts.