22 Electrocatalysis and the splitting of water
These two half reactions will be treated more carefully in the following subsections.
2.4 Hydrogen Evolution
The electrolytic evolution of hydrogen gas is one of the most, if not the most,
studied electrochemical reactions throughout the history of electrochemistry as
a research eld. In fact, fundamental studies on hydrogen evolution has helped
shape the modern understanding of electrochemical processes. It is a rather
simple two electron transfer process where protons are reduced to form molecular
hydrogen. The overall reaction in acid can be written as:
2H+ (aq) + 2e�� ! H2 (gas) (2.10)
The equilibrium potential of this reaction is 0 V using the conventional electrochemical
potential scale. In acidic environment, the reaction can be further
split up into the following reaction steps:
H+ (aq) + e��+ ! H (Volmer) (2.11)
2H ! H2 (gas) + 2 (Tafel) (2.12)
H + e�� + H+ ! H2 (gas) + (Heyrovsky) (2.13)
While it is commonly accepted that the rst step is the Volmer process, it
has proven dicult to understand whether the Tafel or Heyrovsky mechanism
dominates 52. Studies from Markovic et al. have shown experimentally that
the surface structure plays a large role in which mechanism dominates. The
Tafel step takes place on Pt(110) and Heyrovsky on Pt(100) 53. However, for
polycrystalline surfaces it is dicult to assess experimentally through kinetic
measurements alone 54. Despite this ambiguity in the understanding of hydrogen
evolution there is only one intermediate, which is adsorbed hydrogen, H.
The binding between a surface and this intermediate has turned out to be an
exceptionally good descriptor for identifying active catalysts 48, 49, 52, 5557.
The best known catalyst for the reaction is platinum which can catalyse the
reaction at negligible overpotentials. Work done by Neyerlin et al. shows that
with overpotentials down to 50 mV a current density of more than 1 A/cm2 can
be reached even for a loading as low as 50gPt/cm2 58. It should also be noted
that hydrogen evolution is signicantly faster in acid compared to alkaline. This
was shown in a study by Sheng et al., where systematic rotating disk electrode
measurements were used to elucidate the dierences between HER in acid and
alkaline 36. In alkaline the overpotential needed to reach around 1.5 A/cm2
is at least 100 mV higher than the same reaction in acid. An overview of the