Efficiency of Hydrogen Fuel Cell, Diesel-SOFC-Hybrid and
Battery Electric Vehicles
European Fuel Cell Forum
October 20, 2003
Hydrogen is a synthetic fuel. At least the heat (enthalpy) of formation (∆fH0 = 286 kJ
mol-1) must be invested for its "fabrication" from water by electrolysis. This number
corresponds to the Higher Heating Value HHV (= 142 MJ kg-1) of hydrogen.
According to the energy conservation principle, this is the true energy carried by
hydrogen gas at 25°C. Consequently, for any known process of recombination of
hydrogen and oxygen to water, the energy efficiency must be related to the original
energy input or the Higher Heating Value HHV of the synthetic fuel. The widespread
use of the Lower Heating Value LHV may be a convenient convention, but it is not
supported by physics. In fact, the use of the Lower Heating Value for hydrogen
produced by electrolysis (and other means) violates the energy conservation
principle. Comparative studies of competing fuel options including hydrogen are
meaningful and fair only if the analyses are based on the Higher Heating Values HHV
of all energy carriers considered.
According to Faraday's Law the heat of formation ∆fH0 of hydrogen can also be
expressed as an electrochemical potential ("standard potential")
U00 = - ∆fH0 / ne F = 1.48 Volt
with ne = 2 being the number of electrons participating in the conversion and F =
96,485 Coulomb mol-1 the Faraday constant.
Only a fraction of the heat of formation ∆fH0 is available for reversible energy
conversion in fuel cells. This fraction is given by the Gibbs Free Energy ∆fG0 = 237 kJ
mol-1 for water at 25°C. Consequently, the theoretical voltage required to split water
at 25°C by electrolysis is 1.23 Volts.
In fuel cells gaseous hydrogen is combined with oxygen to water. This process is the
reversal of the electrolysis of liquid water and should provide an open circuit voltage
of 1.23 Volts per cell. Furthermore,