5.2 A concept for improving stability of MnO2 79
MnOx thin lms used in this project will be evaluated from chronoamperometry
experiments at 1.8 and 1.9 VRHE, which are more anodic than the equilibrium
potential of MnO��
4 formation. In 1 M KOH the dissolution rates were found
to be 332 and 1570 ng/cm2 over two hours respectively, using the results from
ICP-MS. In gure 5.3 these results are compared to the rates in 0.05 M sulfuric
acid.
0
-500
-1000
-1500
-2000
CA1.8VCA1.9V
Masschange/ngOxide.cm-2
0.05M
H2SO41MKOH
0.05M1MKOH
H2SO4
Figure 5.3: Comparison between the rates of mass loss of MnO2 thin lms in 0.05
M H2SO4 and 1 M KOH. The mass loss is for two hour tests at either 1.8 or 1.9 VRHE
and the error bars are based on at least three independent measurements.
The mass loss rates are similar in the two electrolytes and the predicted lifetimes
are certainly not satisfactory for real devices, as discussed in chapter 4.
However, if the MnO2 surface could be meta-stabilized it could lead to development
of inexpensive, active and stable catalysts for acidic water electrolysis.
An investigation of a method for such a stabilization will be described in the
next section.
5.2 A concept for improving stability of MnO2
As mentioned in the previous chapter the anodic dissolution of MnO2 occurs
through formation of the permanganate ion, MnO��
4 , as described in equation
4.2. This process takes place at the surface of the thin lm where the exposed
Mn atoms can be oxidised. Interestingly, it has been reported that the sites that
dissolve with the lowest energy barrier are undercoordinated sites, such as steps
and kinks 212, 213. The same conclusion can also be reached from a simple
argument based on the surface formation energy. In a dissolution process an
atom is removed from the surface, leaving behind a new surface structure with