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stabilised at 1.4 VRHE; in both cases these potentials were
chosen on the basis that the dioxide phases would be
stable.59 After the initial period of cycling, impedance measurement
and stabilisation, samples for ICP–MS analysis were
taken. From four measurements on each catalyst we found an
average of (12360) ngoxidecm2 RuO2 and (472
240) ngoxidecm2 MnO2 present in the solution prior to further
testing. These results indicate that the initial cyclic voltammetry
induces mass losses for both materials, these losses being
more significant for MnOx. During subsequent corrosion measurements,
to assess the amount of Ru or Mn dissolved during
the test, the initial amount of dissolved Ru or Mn at the start
of the measurement was subtracted from the final amount.
Once the frequency had stabilised, first chronoamperometry,
CA, and then chronopotentiometry, CP, measurements were
started. For RuO2, we measured CA at 1.8 VRHE and CP at
30 mAcm2. These parameters were chosen to ensure that the
potential was positive of the reversible potential for RuO4 formation,
1.39 VRHE,60 under standard conditions. For MnOx, potentials
at 1.8 and 1.9 VRHE and a current density of 20 mAcm2
were chosen to be positive of the potential for MnO4
formation.
60 All stability tests were carried out for 2 h. We have observed
that longer term tests tend to yield a poorer reproducibility;
this could be a result of thickness gradients giving rise
to local conductivity issues, exposed substrate, redeposition of
dissolved species or precipitation. It should also be noted that
corrosion mechanisms are highly dependent on the material.39
The degradation of some materials may actually be accelerated
by potential cycling (as shown by Mayrhofer and co-workers to
be the case for Pt, by combining cyclic voltammetry with
online ICP measurements61). Consequently, to study the resistance
to corrosion of such materials, potentiodynamic—rather
than potentiostatic—tests would be necessary.
2.1. Stability of RuO2
In the case of RuO2, the results from chronopotentiometry and
chronoamperometry can be seen in Figure 4 a,b. Chronopotentiometry,
as a technique, should correspond to the performance
for a constant hydrogen production load on an electrolyser.
The extra overpotential needed to sustain the hydrogen
production is directly correlated to energy loss. In Figure
4 a, it can be seen that RuO2 can maintain a stable performance,
which changes only slightly during 2 h. However,
looking at the mass change associated with the test, it is clear
that there is a constant mass loss. This mass loss is equivalent
to 4.8 monolayers (ML) per hour assuming the density and lattice
parameters of (110) RuO2 layers.54 Figure 4b shows the results
of the chronoamperometry measurement. The potential
is held constant throughout the measurement; since the current
depends exponentially on the potential, any deactivation
shows up more clearly than in a chronopotentiometry measurement.
A constant potential at 1.8 VRHE yields a mass loss
equivalent of 4.4 ML per hour. With this rate, it would take approximately
29 h to corrode all of the 40 nm film. Assuming
that the corrosion proceeds in accordance to RuO2+2H2O!
RuO4(aq)+4H+ +4e,36 a dissolution rate of 4 ML per hour
would be equivalent to 0.6 mAcm2, that is, more than four
orders of magnitude lower than the oxygen evolution current
density. In principle, the transient formation of RuO4
36 could
be detected with a rotating ring disk electrode setup; however,
the necessary current sensitivity would be unrealistic. We also
analysed the solution by ICP–MS after the electrochemical
tests; Figure 4 c compares the EQCM and ICP–MS results. The
mass losses found from the ICP–MS measurements shown in
Figure 4 c are converted to RuO2 equivalent since ICP–MS is
only sensitive towards single elements and not the initial
oxides. Comparing the two methods, there are differences for
Figure 5. a) Chronopotentiometry at 20 mAcm2 for 40 nm MnOx by using
EQCM in 1m N2-saturated KOH 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 1.9 VRHE for 40 nm MnOx by using EQCM in 1m N2-saturated KOH 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 MnO2 mass (rather
than the Mn mass), for more direct comparison to EQCM measurements.
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