24 Electrocatalysis and the splitting of water
The rst attempts at understanding OER were made in the 1940's and 50's
with, among others, Hickling 76, Rütschi 77 and later Bockris 78 reporting
experiments with various electrode materials. The conclusions from the work of
Hickling and Hill describe the diculties in comparing the activities of dierent
materials due to large variance in Tafel slope dependent on current density and
measurement procedure. These issues are still haunting researchers today where
the comparison of electrode materials from dierent groups is complicated by
dierent preparation and experimental methods. Hickling and Hill tested twelve
metals as anodes and for all of them substantial overpotentials were needed to
drive the reaction. Some of these results can be seen in gure 2.4, where the
overpotentials needed to reach 10 mA/cm2 for seven electrode materials are
shown.
CoCuAuCFeNiPt
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
Overpotential /V
OERonelectrodesinKOH
10mA/cm2
Figure 2.4: Comparison of overpotentials needed to drive 10 mA/cm2 dependent
on the choice of electrode material. The data is taken from 76 and represents one
of the rst attempts at systematically identifying a good catalyst for OER. The measurements
were conducted at room temperature and in 1N KOH.
With the results from Hickling and Hill in 1946 the scene was set to understand
why some metals were much more active than others. In the 1950's the rst
reports were therefore focused on plotting the activity as a function of various
material properties such as lattice spacing or bond strength between metal and
hydroxide ions, M-OH 77. The latter descriptor was somewhat successful but
it largely depended on the way the M-OH binding was calculated. However, it
seemed evident that the interaction between an oxygen species and the metal was
key to understanding the catalytic activity. In the 1960's, work by Winter 79,
Boreskov 80 and Klier 81 added to the understanding of the metal-oxygen
interaction, oxide to higher oxide formation and oxygen exchange on oxides.
Nevertheless, it wasn't until 1980 that the knowledge of oxides was successfully
combined with measurements of OER activity. This approach was famously