94 Towards a stable and inexpensive catalyst for OER in acid
insucient coverage of undercoordinated sites.
For these reasons the TiO2/MnO2 approach was put on hold and mixed oxide
lms, Ti-MnO2 were investigated instead. It would, however, be relevant to
conrm the structure of the overlayers regarding particle formation and especially
selective blocking of undercoordinated sites. Furthermore, varying the
substrate temperature either during deposition or as an extra annealing treatment
could lead to better results. Due to the time limit of my Ph.D. project
these investigations were not pursued.
5.3.3 Cathodic dissolution in acid
The mixed Ti-MnO2 lms were also tested for cathodic dissolution in acid with
and without titanium added to the surface. The cathodic dissolution process
is dierent from the anodic since it is a reduction of the MnO2 surface instead
of an oxidation but it is possible that the TiO2 can have a stabilizing eect
towards that reaction as well. The dissolution reaction can be formulated as the
following 138:
MnO2 + 4H+ + 2e�� ! Mn2+ + 2H2O (5.4)
The equilibrium potential of this reduction reaction is 1.23 V at pH 0. This
process is fast compared to anodic dissolution and therefore a slightly dierent
experimental approach was followed. The MnO2 and Ti-MnO2 lms were
deposited on EQCM samples as before and the electrodes are again immersed
under potential control at 1.4 VRHE. Three initial potential cycles were then
initiated, followed by a stabilization period at 1.4 VRHE. After reaching close to
zero drift in the frequency, the potential was cycled towards 1 VRHE with a scan
rate of 0.5 mV/s. This allowed for recording a dissolution prole, on the basis
of the frequency change, as a function of applied potential. These proles can
be seen in gure 5.16 for both Ti-MnO2 and MnO2, based on three independent
tests for each. The loss of mass observed with this experimental procedure is
more drastic than what was observed for the anodic dissolution. At 1.15 VRHE,
80 mV cathodic of the equilibrium potential, the pure MnO2 lms have lost 58
% of the original mass. In comparison the amount of material dissolved from
the MnO2 lms after two hours at 1.9 VRHE, 200 mV anodic of the MnO��
4
equilibrium potential, was approximately 7 %.
For the Ti modied lms the mass loss prole is dierent from the MnO2 lms.
At 1.15 VRHE the mass loss of Ti-MnO2 is 26 %, less than half of the mass lost
in the pure lms. This clearly suggests a stabilizing eect from the titanium,
similar to what was observed for the anodic dissolution. The overall shift of
the dissolution prole due to Ti is approximately 30 mV at 10 % of the mass
dissolved and 40 mV at 90 %. For an actual electrolyzer such a potential shift
is not likely to make a big dierence. A battery solution will still be needed for
providing potential control when the electrolyzer is not running. Nevertheless,