66 Benchmarking the stability of OER catalysts
and oxygen in a ratio of 5:1 and with a substrate temperature of 200 oC. The
substrates used are quartz crystal microbalances, QCMs, which can be used to
measure the mass changes of the catalyst, as described in section 3.4.1. The
QCMs have a gold lm on the front side which serves as the contact for electrochemical
measurements. On the back side another gold lm, with a smaller
area, serves as contact for the frequency measurement. The overlap between
these two lms is sensitive to changes in frequency and therefore only this area
was covered with MnOx. TiO2 was deposited on the remaining gold using a
mask. This was done to avoid having gold in contact with the electrolyte, which
could aect the electrochemical measurements. By only depositing manganese
where the QCMs are sensitive the comparison between mass losses measured
with EQCM and ICP-MS could be improved. The TiO2 at the same time contributes
with negligible currents and no frequency changes.
Manganese oxides exist in many valence states and therefore numerous stoichiometries
can be formed, including MnO, Mn3O4, Mn2O3 and MnO2. Furthermore,
some of these oxides have several polymorphs 207. As a rst step,
the deposited thin lms were therefore characterized with X-ray Photoelectron
Spectroscopy, XPS, and X-ray Diraction, XRD. The data from XPS can be
seen in gure 4.1 and from XRD in gure 4.2.
Satellitedistance
11.4eV
Mn2p1/2
64064565065566066567080859095
Intensity/a.u.
BindingEnergy/eV
Mn2p3/2
b)
Intensity/a.u.
BindingEnergy/eV
Mn3s
Multiplet4.7eV
fromMn3speak
a)
Figure 4.1: XPS spectra for a 40 nm MnOx thin lm. a) Mn2p peaks. b) Mn3s
peaks. For both spectra the green areas indicate tting peaks. The ts were used to
evaluate the distance from main peak satellite, in case of Mn2p, and multiplet splitting,
in the case of Mn3s.