The next step would be to characterize the Ti-MnO2 layer with microscopic techniques such as
Transmission Electron Microscopy and Scanning Tunneling Microscopy, with a focus on identifying the
exact location of Ti in the MnO2 structure. It would be of great interest to combine a very accurate
characterisation with optimization of the deposition technique. To that end, STM studies of the
surface while depositing low amounts of Ti would be particularly fruitful.84,85 These studies are
outside the scope of the current study, where we develop and experimentally prove a key concept for
stabilizing non-precious metal oxygen evolution catalysts.
The activity of the catalyst could, in principle, be improved beyond what we reported herein. For
instance, in alkaline solution, when the surface of MnOx is in intimate contact with Au, its activity is
increased by an order of magnitude.75,77,86,87 Should this phenomenon be exploited in acid, ideally
using an element more abundant than Au, it would boost the efficiency of electrolysis with Mn-based
catalysts.
4. Conclusion
The search for non-precious OER catalysts for acidic media calls for new strategies for stabilizing oxide
surfaces, without compromising catalytic activity. Herein, we demonstrate such a strategy, by
selectively terminating under-coordinated sites on MnO2. Using DFT-based simulations, we showed
that the termination energy correlates with the surface formation energy of the guest oxides in their
pure form. Consequently, the termination of under-coordinated sites on MnO2 is favorable for guest
oxides with lower surface formation energies than MnO2. Our calculations predicted that GeO2- and
TiO2-terminated MnO2 should improve the stability of MnO2.
Based on this theoretical insight, we tested the OER activity and stability of sputter-deposited thin
films of pure MnO2 and Ti-MnO2 in 0.05 M H2SO4. The thin films exhibit unprecedented activity for
non-noble metal oxides in acidic solution. Using ICP-MS and EQCM measurements, we showed that
the mass losses could be moderately decreased by incorporating Ti into the catalyst, with only a small
drop in catalytic activity towards OER. Therefore, the experimental results shown here support the
DFT-based model, which suggested that a more optimal balance between stability and activity could