5.1 MnOx thin lms in sulfuric acid 77
most stable at the surface of MnOx at OER relevant potentials, based on DFT
calculations 131. The binding energies for MnO2 to OER intermediates are
also more favorable than the ones found for Mn2O3 98. The thermodynamic
analysis from Pourbaix also shows MnO2 as the most stable phase in both acid
and alkaline under OER conditions 138.
Figure 5.1: Cyclic voltammetry measurements of MnO2 in 0.05 M H2SO4, blue
line, and 1M KOH, dashed green line. Both measurements were taken with 5 mV/s in
an RDE setup using 1600 RPM and the rst anodic scan is shown. The inset shows
the same data as a Tafel plot in a smaller potential range, from which the Tafel slopes
can be found.
The question is how the activity of the thin lms presented here compares to
other reports of Mn based oxides tested in acid. The rst of these reports
date back to 1977 with the work done by Morita et al. 158 who prepared
Mn oxide electrodes by thermal decomposition. More recent reports include
electrodeposited thin lms prepared by Huynh et al. 166, layered Mn-Ca oxides
by Najafpour et al. 211 and -MnO2 particles from Takashima et al. 164. Due
to the varying experimental conditions they will be compared on a turnover
frequency, TOF, basis, taking into account an estimate of the amount of active
sites contributing to the current. In gure 5.2 this comparison is shown, where
a TOF range is given for each material.
The TOFmin is based on the total amount of Mn atoms used and gives a lower