Figure 5. a) A schematic showing the layered thin film structure, deposited onto a gold-coated
quartz crystal oscillator. The thicknesses shown are based on deposition rates measured with
an in-chamber QCM. For the unmodified MnO2 samples the thin film structure is the same
except for the top layer being without Ti, making it 40 nm thick MnO2. b) Initial OER activity of
the MnO2 thin films measured with a RDE setup. The activity of the unmodified MnO2
measured in 1M KOH solution and in 0.05 M H2SO4 are shown in green and blue, respectively;
while the Ti modified MnO2 is shown in red. A pure TiO2 film was measured, as is shown in
purple. The results shown here are the first scan to anodic potentials, the rotation speed was
1600 RPM and scan rate 5 mV/s. In the inset, the Tafel plots of the unmodified films in alkaline
and acid media are illustrated.
It is striking that the OER activity in acid media drops by a factor of two relative to alkaline, to a
of 0.0035 ± 0.001 s-1 at 0.4 V overpotential. There is also a difference in Tafel slope, from 70
mV/dec in 1 M KOH to 170 mV/dec in 0.05 M H2SO4. This Tafel slope is nonetheless considerably
lower (i.e. steeper) than reported in recent literature in acid.35,36 Moreover, to the best of our
knowledge the activity we report is the highest reported for a non-precious metal oxide in acid on a
basis; see Figure 1 for direct comparison to literature references. The figure also shows that the
MnO2 has a turnover frequency only 20-40 times lower than the state-of-the-art RuO2 thin film.
The samples were tested at constant potential for two hours at 1.8 VRHE and subsequently at 1.9 VRHE.
Before and after each potential step, aliquots of electrolyte were sampled for ICP-MS analysis. To
compare the activity of modified and unmodified MnO2, we used the current obtained after one hour