108 The benecial interaction between Au and MnOx
higher than for the pure Mn3O4 lm. The increase in capacitance over the pure
Mn oxide lm is plotted in gure 6.11 together with the improvement in current
density at 400 mV overpotential. Assuming an increase in capacitance is
proportional to an increase of the OER active sites, the increase in capacitance
is not enough to explain the activity improvement observed for 30 or 50 % gold.
6
5
4
3
2
1
0
Auconcentration/%
Normalisedcurrent@400mV
6
4
2
0
+50%
Au
+30%
Au
NormalisedCapacitance@1.3VRHE
Mn3O4
Figure 6.11: Current density increase (in blue) and pseudo capacitance increase (in
green) for the thin lms as function of gold concentration. Both scales are normalised
to the value obtained for Mn3O4.
With such an improvement in activity it is interesting to further analyse the
dierences in behavior for these lms. Especially the changes of Mn and Au
under reaction conditions are of interest. Such a study can be done with in situ
X-ray Absorption Near Edge Spectroscopy, XANES.
6.2.3 In situ X-ray Absorption Near Edge Spectroscopy
The three types of lms, Mn3O4, Au(30%)-MnOx and Au(50%)-MnOx, were
investigated with XAS. Specically, the Mn K-edge and Au L3-Edge were investigated
at the Stanford Synchrotron Radiation Lightsource using a setup with
high energy resolution uorescence detection which allowed for in-situ measurements.
This detection method is a bulk sensitive technique and gives information
about the average state of all Mn and Au atoms in the sample. The great advantage
is that this setup allows for studying the Mn and Au atoms in the lms
while working as oxygen evolution catalysts. As mentioned in section 3.2.3, the
measurements were carried out in collaboration with Prof. Thomas Jaramillo's
group at Stanford University Chemical Engineering and in particular with Linsey
Seitz and beam scientists Dimosthenis Sokaras, Tsu-Chien Weng and Dennis