2.3 Water splitting in electrolyzers 21
in both directions when a reaction is in equilibrium. From equation 2.5 it
is evident that a high i0 results in higher currents for a given . It is also
noteworthy that the current is exponentially dependent on overpotential, .
At high overpotentials one of the two exponentials in equation 2.12 becomes
negligible and a simpler version can be formulated. In semilogarithmic form
this version is known as Tafel lines. Briey, Tafel slopes indicate the change in
potential for a ten-fold increase in current. From this denition it is obvious
that lower Tafel slopes are desirable. Traditionally, electrochemical experiments
have been conducted with the aim of determining the Tafel slope of reactions
on specic electrode materials. It is commonplace in the literature to use Tafel
slopes to phenomenologically deduce a reaction mechanism 51. However, this
requires several loaded assumptions, including the value of a symmetry factor,
an adsorption isotherm, etc. Consequently I chose not to use such an approach
for this thesis.
2.3 Water splitting in electrolyzers
Electrolysis is a general term describing the process of driving a non-spontaneous
electrochemical reaction by applying a potential over two electrodes. This is in
contrast to a potential dierence generated from a spontaneous electrochemical
reaction like in the case of fuel cells. In water electrolysis hydrogen and oxygen
gasses are generated from water.
2H2O (liquid) ! 2H2 (gas) + O2 (gas) (2.6)
This is an energy demanding reaction with a change in Gibbs free energy of
237.2 kJ/mol at standard conditions. If this is done in an electrochemical cell,
an electrolyzer, a potential dierence of 1.23 V is required at room temperature
and standard pressure. As discussed above, for any signicant current a higher
potential has to be applied in terms of overpotential and resistance losses. The
eciency of such a process is typically dened from the change in enthalpy of
the reaction. The enthalpy can be taken for either water as a liquid (higher
heating value, HHV, equivalent of 1.48 V) or as a gas (lower heating value,
LHV, equivalent of 1.25 V) 50.
where Vc is the cell voltage. Note that for electrolyzers using the HHV results
in higher eciencies while for fuels cells in lower. The overall reaction can be
split into half reactions, formulated in 2.9 and 2.8 for an acidic electrolyte.
4H+ + 4e ! 2H2 (gas) Hydrogen Evolution (2.8)
2H2O (liquid) ! O2 (gas) + 4H+ + 4e Oxygen Evolution (2.9)