78 Towards a stable and inexpensive catalyst for OER in acid
Figure 5.2: Comparison of activities achieved with MnOx based catalysts for OER
in acid on a TOF basis. The lower TOF is found from normalising with all Mn atoms
in the catalyst, whereas the maximum is found from assuming a perfectly at surface,
where only the rst layer participates in the reaction. The catalyst from this work
is shown in red. The thick MnOx catalyst from Morita et al. is in blue 158, the
-MnO2 catalyst from Takashima et al. in green 164, the electrodeposited MnOx
from Huynh et al. in yellow 166 and the Ca-MnOx particles from Najafpour et al.
in magenta 211.
limit of the turnover frequency. It was calculated with the following equation:
TOFmin =
j MMnO2
4 NA e Vcat MnO2
(5.1)
where j is the current measured, MMnO2 the molar mass of MnO2, 4 electrons
are transferred per O2 molecule formed, NA is Avogadro's constant, e is the
elementary charge of an electron, Vcat the corresponding volume of the catalyst
and MnO2 is the density of MnO2. The TOFmax is instead based on the Mn
atoms of an atomically at surface assuming that the electrodes reported in the
respective papers have a roughness of 1. From the comparison in gure 5.2 the
MnOx thin lm from this work stands out as the more active, except for the
upper TOF limit of the electrode reported by Morita et al. It should be noted
that the thermal decomposition method tends to yield high roughness factors
99 and in fact the same group later reported roughness factors between 50 and
100 for similar lms 159. It is likely that the roughness factor is proportional to
the number of active sites available for the reaction. The stability in acid of the