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further characterise the composition, XPS analyses were carried
out. The results can be seen in Figure 2b–d for MnOx and
RuO2, respectively. The MnOx film was evaluated using the Mn
3 s multiplet splitting and Mn 2p1=2 distance between the main
peak and its satellite.55, 56 The former was found to be 4.7 eV
and the latter 11.4 eV, suggesting that the stoichiometry is consistent
with MnO2, which is also the most stable surface for Mn
under OER conditions.57 The RuO2 film was evaluated based
on the position and area of the Ru 3d lines, which matched
with RuO2 literature references.58
The electrochemical results are described in experimental
order to ensure ease of reproducibility. As a first step, the activity
of each sample was evaluated by using standard cyclic voltammetry
at 5 mVs1. Representative results can be seen in
Figure 3 for both RDE and EQCM setups. A useful figure of
merit is the overpotential needed to sustain 10 mAcm2.3 For
RuO2 in 0.05m H2SO4 it is (3548) mV whereas for MnOx it is
(4946) mV, using the results from the RDE setup, based on
two independent measurements for each oxide. These overpotentials
are comparable to earlier reports in the literature, even
though the films are only 40 nm thick and deposited onto
smooth substrates, likely resulting in lower surface areas compared
to samples made by electrodeposition or thermally prepared
oxides. The activity obtained in the EQCM setup is
slightly lower at high overpotentials for both samples; this discrepancy
could result from a less facile bubble removal, compared
to the rotating disk. However, the onsets of OER are the
same in the two setups. Next, the Ohmic loss was evaluated
by using impedance spectroscopy, followed by a stabilisation
period during which the measured resonance frequency for
the EQCM settled at a constant level. We observed that the frequency
reading became stable after approximately 30 to
60 min, presumably due to temperature equilibration or equipment
vibrations from cell assembly. Our criterion for establishing
the stability was that the frequency would change less
than 1 Hz over 15 min, which corresponds to a lower change
than for any subsequent OER test. In the case of the RuO2, the
films were stabilised at 1.23 VRHE whereas the MnOx films were
Figure 3. Cyclic voltammetry curves at room temperature for: a) 40 nm RuO2
in 0.05m N2-saturated H2SO4. b) 40 nm MnOx in 1m N2-saturated KOH. The
scan rate was 5 mVs1 and 1600 RPM (revolutions per minute) were used in
the RDE tests. The current was normalised to the geometric area. The first
anodic sweeps are shown.
Figure 4. a) Chronopotentiometry at 30 mAcm2 for 40 nm RuO2 by using
EQCM in 0.05m N2-saturated H2SO4 at room temperature. The black line indicates
the measured potential and the blue line indicates the change in mass
based on in situ resonance frequency measurements. b) Chronoamperometry
at 1.8 VRHE for 40 nm RuO2 by using EQCM in 0.05m N2-saturated H2SO4
at room temperature. c) Comparison of the mass change found from EQCM
and ICP–MS based on four separate experiments. The mass loss from the
ICP–MS measurements was adjusted to the equivalent RuO2 mass (rather
than the Ru mass) for more direct comparison to EQCM measurements.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemElectroChem 0000, 00, 1–8 &4&
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