4.3 Stability measurements 69
rst evaluated with cyclic voltammetry and then Ohmic losses were evaluated
with Electrochemical Impedance Spectroscopy, EIS. Following these two steps a
stabilization period was introduced, where the electrode was kept at 1.4 VRHE.
This potential is well within the stability range of MnO2 at pH 14 and almost
no current is owing 131, 138. In this period the frequency measurement was
allowed to stabilize so that eects from vibrations, temperature and cycling the
potential were minimized. When the frequency changed with less than 1 Hz per
15 minutes, the actual corrosion tests were initiated. Naturally, this criterion is
somewhat arbitrarily chosen but the important aspect is that drift in the frequency
due to the setup or temperature is signicantly lower than the changes
due to dissolution. For samples that are expected to be highly stable the EQCM
tests must be designed so that the drift is close to zero over extended periods.
The stability tests consisted of chronoamperometry, CA, at 1.8 and 1.9 VRHE or
chronopotentiometry, CP, at 20 mA/cm2. An overview of this stability protocol
can be seen in gure 4.4.
Figure 4.4: Schematic representation of the protocol for measuring the stability of
thin lms for oxygen evolution. This protocol is specically designed to probe the
anodic dissolution rate of MnOx.
As mentioned above, the EQCM tests allow for simultaneously monitoring
the catalytic activity, in terms of current or overpotential, and the change of
mass through the resonant frequency. Such time resolved information is not
easily available with any other methods. In gure 4.5a and b results of the