68 Benchmarking the stability of OER catalysts
Figure 4.3: Voltammogram showing current density of MnOx on both EQCM and
glassy carbon substrates for the rst anodic scan. The measurements were done with
5 mV/s in 1M KOH and the potential scale has been corrected for Ohmic losses. For
the sample supported on glassy garbon, the measurement was done with a rotating
disk electrode setup and the electrode was rotated at 1600 RPM.
activity 208. The onset potential is in practice dicult to nd since the true onset
of most electrochemical reactions are usually masked by pseudo-capacitance,
reactions with contaminants or other electrochemical processes. The overpotential
needed for running 10 mA/cm2 is a useful metric when OER catalysts are
combined with suitable photoabsorbers to achieve solar water splitting 109.
However, the exact same type of material can easily be reported to have very
dierent overpotentials due to dierent preparation methods, as discussed in
chapter 2. Another possibility is to compare turnover frequency, TOF, which is
the number of (O2) molecules produced per second per active site. The challenge
in nding a useful TOF is to evaluate how much of the catalyst that participated
in the reaction. The 40 nm MnOx thin lms reported here have an estimated
TOF of 0.007 s��1 at 400 mV overpotential, assuming a density of rutile MnO2
and that every Mn atom can participate in the electrochemical reaction. This
value is a lower bound but compares favorably to literature results, which are
typically in the range of 0.002 to 0.006 s��1 for pure Mn oxides 209.
4.3 Stability measurements
The protocol for the stability measurements in this study was focused on utilizing
the EQCM technique. For such measurements to be useful it is important
to isolate frequency changes due to the actual dissolution of catalyst from
changes that are due to the surroundings. For each sample the activity was