34 Electrocatalysis and the splitting of water
calculating the eg band lling, Grimaud et al. later reported a similar nding
where the O p-band center was used to describe the OER activity of double
perovskites such as Pr0:5Ba0:5CoO3�� 140. This descriptor could be found from
DFT calculations and compared to oxidation state found with chemical titration
techniques. In gure 2.10 the results of this work can be seen as a volcano shaped
plot, indicating an optimum O p-band center of -1.7 eV relative to the Fermi
level, EF . With a higher O p-band center the oxides tend to become amorphous
during OER testing, which was the case for Ba0:5Sr0:5Co0:8Fe0:2O3��x 141.
Figure 2.10: Volcano shaped activity plot for a range of perovskites. The activity
is shown as the potential needed to reach 0.5 mA/cm2 as function of the computed
O p-band center. In the left side of the gure there are stable perovskite materials,
which improve in activity as the O p-band center inceases. For high O p-bands some
perovskite have been observed to become amorphous. The data is reproduced from
140.
Parallel to these eorts of relating bulk oxide properties to OER activity,
other groups have focused on nickel or cobalt based catalysts 142 where especially
Ni hydroxides mixed with iron have proven relevant. Boettcher and
co-workers have measured on a range of electrodeposited transition metal oxides
and found 2 nm thick Ni0:9Fe0:1Ox able to reach 10 mA/cm2 at only 340
mV overpotential. The advantage of measuring such thin lms is that almost all
the material can be considered active and roughness will not play a large role. A
disadvantage is that it is challenging to characterize such thin layers and there
could be a large dependence on interaction with the substrate. However, similar
activities were reported by other groups 109,143146. Interestingly, Boettcher
and co-workers later reported that pure NiOx is not very active on its own but,