2.5 Oxygen Evolution 37
relations 153, 156, 161163. In some reports Mn in a +3 state was concluded
superior in terms of activity 162166. This view is supported by characterizations
of Mn oxides before and in some instances after electrochemical testing.
In a study from Su et al. surface stability of Mn oxides together with binding
energies to the OER intermediates were evaluated using a combination of DFT
and electrochemical measurements 131. The DFT calculations suggested that
at OER relevant potentials the MnO2 phase was most stable, which is consistent
with the bulk stability regions found in the Pourbaix diagram 138. It is
therefore unlikely that a +3 state of Mn would persist at the electrode surface
at highly anodic potentials. Instead, it is likely that the preparation method
and the resulting roughness or conductivity of the catalyst play a large role
in determining an activity hierarchy. However, to elucidate the dependence on
oxidation state in-situ techniques must be used so that the surface can be characterised
under reaction conditions. Such studies are at this time not available
for Mn based OER catalysts. A selection of active Mn based OER catalysts is
shown in gure 2.12 as a Tafel plot. From that plot it is evident that even with
one type of material, e.g. Mn oxides, the range of measured activities is rather
large.
Figure 2.12: Overview plot of recent experiments with Mn based OER catalysts
tested in various electrolytes. The Rough MnO2 measurements in pH 0 (red) and 14
(magenta) are from 158, the MnOx ALD (teal) is from 167, the Mn2O3 (purple) is
from 151, the MnOx (green) tested in pH 7 is from 168 and the MnOx tested in pH
4 (blue) is from 165. A RuO2 thin lm from 137 is shown as a black star.