90 Towards a stable and inexpensive catalyst for OER in acid
a)b)0
1,81,9
40
30
20
10
0
Ti-MnO2
jgeo/mA.cm-2
U-iR/V(vs.RHE)
MnO2
MnO2
Activityafter1hour
1MKOH
Ti-MnO2
1,81,9
-400
-800
-1200
-1600
-2000
MnO2
Ti-MnO2
Ti-MnO2
MasslossfromICP-MS
2hourtests-1MKOH
MnO2
Masschange/ngOxide.cm-2
U-iR/V(vs.RHE)
Figure 5.12: a) Activity of MnO2 and Ti-MnO2 supported on EQCM crystals at
1.8 and 1.9 VRHE measured in 1 M KOH. The activity is compared as the current
density measured one hour into the test. b) Mass losses of MnO2 and Ti-MnO2 at
1.8 and 1.9 VRHE. The mass losses here are based on ICP-MS measurements before
and after each two hour test. For both graphs MnO2 results are shown in blue and
Ti-MnO2 in red. Error bars indicate 1 standard deviation from three independent
measurements.
matched with a sucient amount of Ti atoms. Another point worth discussing
is the reproducibility of these experiments. So far the averaged results have
been discussed and in the graphs error bars are shown based on standard deviation.
In some cases these error bars are rather large, which indicates a large
spread in results. This was especially true for the measurements in alkaline. At
this stage it is dicult to assess the exact reason. However, both the current
density measurements and mass losses are highly dependent on the electrochemical
potential. As mentioned earlier, the applied potentials were corrected for
Ohmic drops with a feature in the potentiostat but if the resistance of the system
somehow change during a test, the current method does not accommodate
for that. For just a few Ohms dierence a large change in current density can be
expected. In a few cases the Ohmic drop was measured before and after a two
hour test and sometimes there would be a change of 1-2 Ohm, while in other
instances it would be the same. These resistance changes could be a reason for
the spread in current and mass losses, however, it is not likely to have a large
impact on the overall results.
5.3.2 Titania overlayers for stabilized MnO2
While most of this thesis follows a chronological order, this small section will
be an exception. Initially, the stabilization predicted by the DFT calculations
was investigated by depositing thin overlayers of titania, TiO2, on top of the
MnO2 lms. This is in contrast to the physical mixing of Ti and Mn obtained by
co-sputtering as explained above. To indicate the dierence between overlayer