5.3 Experimental validation of the concept 87
Ti-MnO2
1,81,9
10
8
6
4
2
0
Ti-MnO2
MnO2
MnO2
Activityafter1hour
0.05MH2SO4
jgeo/mA.cm-2
U-iR/V(vs.RHE)
MnO2
1,81,9
0
-400
-800
-1200
-1600
-2000
MnO2
Ti-MnO2
Ti-MnO2
MasslossfromICP-MS
2hourtests
Masschange/ngOxide.cm-2
U-iR/V(vs.RHE)
a)b)
Figure 5.9: a) Activity of MnO2 and Ti-MnO2 supported on EQCM crystals at
1.8 and 1.9 VRHE measured in 0.05 M sulfuric acid. The activity is expressed 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.
The comparison in gure 5.9b is based on the ICP-MS measurements and these
can be compared to the EQCM frequency measurements. In gure 5.10 the
mass losses at 1.9 VRHE as a function of time are shown, again based on three
independent measurements for both Ti-MnO2 and MnO2. The mass losses start
out in a similar pace but towards the end the rate of mass loss is in fact 40 %
lower for the Ti modied lms. It should be noted that the absolute mass loss
measured with the EQCM is slightly lower than what is estimated from the
ICP-MS, but the trends are the same.
Finally, the same type of experiments were carried out in a RDE setup using Au
polycrystalline samples as disks. The RDE tests were conducted in a standard
three electrode glass cell with rotation speed of 1600 RPM. In gure 5.11 the
results of those tests can be seen. It is again evident that the mass loss decrease
is more signicant than the decrease in current density. It should be noted
however, that both the current density and the mass loss after Ti modication
are decreased by a larger percentage for the RDE tests compared to what is
seen for the EQCM tests. At 1.9 VRHE the current is decreased with 30 % and
the mass loss 60 %. Nevertheless, there is still a signicant dierence between
the two values with the mass loss decrease being the larger.
Without any selective termination eects a MnO2 surface with 20 % TiO2 is
expected to have a 20 % decrease in activity towards OER combined with a
20 % slower dissolution rate. Judging from the results presented here this is
not the case. For the EQCM tests the activity of Ti-MnO2 approaches the one
for pure MnO2 after prolonged testing. At the same time, the mass losses are