120 Conclusion and outlook
that mass loss analysis, even in the short term, can be used to predict lifetimes.
The established procedure was based on EQCM measurements combined with
ICP-MS analysis of the electrolyte. These two methods complement each other,
since the former gives information about the electrode mass and the latter gives
information about which elements are in the electrolyte. Specically for MnOx
thin lms the rates of mass loss were established at potentials anodic of the
dissolution potential and lifetimes were calculated.
MnOx is a unique material for oxygen evolution. In contrast to NiOx and CoOx,
it has a stable phase in acidic environment in the range of 1.3 to 1.7 VRHE and
it is therefore possible to use this material in acidic electrolyzers. However, as
it was established in chapter 4, the anodic dissolution constitutes a problem for
long term stability. In chapter 5 the focus has been on investigating a strategy
for stabilizing MnO2 surfaces. This could be done by selectively blocking sites
responsible for the dissolution process. It is likely that undercoordinated sites
are dominant in that process and from theoretical calculations it was found
that Ti atoms segregate to such sites. At the same time, according to current
understanding, the at terrace sites are most important for the OER activity.
Experimentally, co-sputtered thin lms of Ti-MnO2 were prepared and found
to exhibit a better compromise between stability and activity than pure MnO2.
As mentioned above, the catalytic performance of MnOx electrodes can not
match noble metal based electrodes at this stage. To that end, it has been
reported that MnOx electrodes mixed with Au exhibit remarkable activity enhancements.
While gold is not an ideal activity promoter from a practical point
of view, it is important to understand the benecial interactions. In chapter
6 the interactions between gold and manganese oxides were investigated with
a conceptual as well as experimental approach. It was shown that activity enhancements
could be explained by introducing a Au proton acceptor site next
to OER active Mn sites. The experimental approach encompassed mixed thin
lms, which were characterized thoroughly. The activity towards OER could
be increased more than ve fold by incorporating Au domains of approximately
3 nm, whereas 2 nm domains lead to modest enhancements. These thin lms
were found to behave dierently under reaction conditions by characterization
with in-situ XANES. Importantly, these measurements show that Mn atoms are
more oxidised in the presence of gold. It is important to combine the conceptual
understanding with experimental observations and the ndings presented here
provide a step in that direction.
As mentioned in chapter 5 and 6, important progress was made regarding both
stabilization of MnOx and understanding the activity enhancements observed.
However, it is also evident that both stability and activity must be improved
further in order for MnOx to be a practical electrode material for PEM elec