98 The benecial interaction between Au and MnOx
to the two others. The scaling relations therefore implies a constant oset in
binding energy between *OH and *OOH of approximately 3.2 eV for a large
range of materials. Since the optimal catalyst should have an oset of 2.46 eV
the scaling relations predict that the best catalysts will need 3:2��2:46eV
2e = 0:37
V in overpotential to drive the reaction at high current densities 98. To nd
better catalysts it is necessary to design surfaces that do not obey the scaling
relations. However, an interesting starting point for such a venture is to look for
materials that exhibit activity improvements beyond what is predicted from the
binding energies. For this project it is particularly interesting that the activity
of MnOx based materials have been reported improve in OER activity when
Au is present either as a substrate or as nanoparticles 225227. The same has
been found for CoOx and Ni hydroxides 228230. In gure 6.1 a few selected
examples can be seen.
mCo3O4
MnOxonGC
0,40,50,60,7
100
10
1
0,1
a-MnO2
CooxideonAu
CooxideonbulkCo
MnOx/AuonGC
jgeo/mA.cm-2
/V
a-MnO2-AuNP
Figure 6.1: Examples from literature combined in a Tafel plot, where improvements
in activity towards OER are obtained for Au-MnOx and Au-CoOx systems. -MnO2-
AuNP results are from Kuo et al. 226 and are shown in purple. MnOx/Au on GC is
from Gorlin et al. 225 and are shown in red. Co oxide on Au are from Lu et al. 228
and are shown in blue.
The improvements for the mixed Au-MnOx systems are signicant with decreasing
overpotentials ranging from 100 - 200 mV for the same current density.
Currently, there is no established explanation for the activity enhancements.
From the work by Kuo et al. it is proposed that Au facilitates the formation
of more active Mn3+ species on the surface, which they state have a more labile
Mn-O bond compared to Mn2+ and Mn4+. This statement was supported