2.6 Conclusion 41
the stability results are not consistent with an inverse relationship. It should
be noted that in both of these analyses the metal stabilities are evaluated at
a constant current and therefore at dierent potentials. For instance Au is
evaluated at potentials around 2 VRHE whereas Ru is evaluated at 1.5 VRHE.
These contradicting results suggest that a more in-depth understanding of the
dissolution phenomena is needed before an established model can be reached.
In terms of stability analyses for non-noble metal oxides there are less studies
available at this time. However, one of most active catalysts based on nonprecious
metals, Ba0:5Sr0:5Co0:8Fe0:2O3��x, or in short BSCF, has been subjected
to Transition Electron Microscopy, TEM, analysis before and after electrochemical
tests 141. These tests consisted of cyclic voltammetry and from
the TEM images it was evident that the BSCF particles change from a crystalline
phase to an amorphous one. At the same time the catalytic activity
increases, which is likely due to a roughening of the surface causing more active
sites to be available. The authors therefore argued that the leaching of Ba2+
and Sr2+ ions was related to the position of the O p-band center. BSCF has an
O p-band center positioned relatively high compared to other catalyst materials,
like La0:4Sr0:6CoO3��, LaCoO3 and LaMnO3, all of which retain crystallinity
after electrochemical testing.
2.6 Conclusion
In summary, investigating catalysts for the oxygen evolution has been an active
research eld for more than six decades. Important progress has been made in
terms of understanding the reaction and nding suitable descriptors that can
aid in designing better catalysts. From theoretical analyses, scaling relations
have been used to predict and describe trends in activity for a large group of
oxides. Unfortunately, the scaling relations between intermediates also mean
that it is complicated to surpass the performance of existing RuO2 catalysts.
Nevertheless, a large amount of work has been dedicated to optimizing Ni, Fe
and Co based catalysts for use in alkaline solution and new materials are being
reported frequently. The stability of new and active catalysts are not often
reported in a very transparent way. This is one of the main issues addressed
in this thesis. Furthermore, there is a lack of progress for identifying nonnoble
metal based catalysts that can work in acidic solution. The new catalysts
reported in acidic solutions are instead based on Ru and Ir. It has therefore
been a main goal of this project to search for a compound based on abundant
elements, that can catalyse the oxygen evolution reaction in acidic environment.