76 Towards a stable and inexpensive catalyst for OER in acid
compared to Ru or Ir based catalysts, so the anodic formation of MnO��
4 above
1.7 VRHE is an issue that must be taken into account. In the previous chapter
the dissolution rates of MnOx thin lms were measured in alkaline environment
and an unsatisfactory lifetime was predicted. In this chapter, focus will be on investigating
the stability of the MnOx lms in acidic environment, together with
a novel strategy for improving the stability. This strategy will rst be explained
on a basis of Density Functional Theory calculations and then an experimental
approach will be presented. These ndings are also reported as the appended
paper III.
5.1 MnOx thin lms in sulfuric acid
To investigate the stability of MnOx in acid, a test protocol similar to the
one presented in chapter 4 was employed. The thin lms were prepared in
the same way and deposited on either quartz crystal microbalances or gold
polycrystalline samples. The main dierence from testing in alkaline stems
from the fact that MnOx dissolves as Mn2+ at potentials below 1.3 VRHE. It is
possible that in a real electrolyzer this could be solved by using a backup battery
that can maintain the potential above a critical value at all times. However, for
this early stage investigation the potentiostat is used instead of such a battery.
The electrode was rst immersed into the electrolyte under potential control at
1.4 VRHE, using a Pt mesh as auxiliary working electrode. Then initial cyclic
voltammetry, Electrochemical Impedance Spectroscopy, Ohmic drop correction
and chronoamperometry techniques are started. The protocol is similar to the
one presented in gure 4.4, but throughout the whole experiment the potential
never drops below 1.4 VRHE. The initial cyclic voltammetry is used to evaluate
the activity which can be compared to the experimental studies reported in the
literature, some of which are shown in gure 5.2. In gure 5.1 a rst anodic
scan is shown for the same type of thin lm tested in both sulfuric acid and
potassium hydroxide. There is a clear dierence in the activity, which is much
higher in alkaline solution. There is also a dierence in the Tafel slope which
in KOH is about 70 mV/decade and in H2SO4 170 mV/decade. The reason
for the lower activity in acid is currently not well established in the literature.
Nocera and co-workers proposed an explanation based on a disproportionation
reaction which minimizes the amount of Mn3+ species at the surface in acidic
solution 166. Their conclusions were primarily based on analysing Tafel slope
as function of the pH. However, these conclusions were for a catalyst with a Tafel
slope higher than 600 mV/dec in acidic media. Takashima et al. had previsously
reported a similar explanation based on UV-vis spectroelectrochemical detection
of Mn3+, which was observed to coincide with OER onset 164. However,
they tested the catalyst down to a pH of 4 and found the largest overpotential
in neutral solution. The hypothesis about Mn3+ as the active species is in
contrast to another report by Su et al. where MnO2, or Mn4+, was found