2.5 Oxygen Evolution 31
tial could be decreased for surfaces that are limited by a too weak adsorption
of OOH compared to OH. Essentially, a selective stabilization of OOH by
immediate splitting into adsorbed H and O2 is proposed to occur on active Ru
sites with an oxidised Ni or Co site as neighbour.
Just as it can be complicated to compare Tafel slopes from dierent materials
prepared dierently, it can be quite dicult to document activity enhancements
from mixed oxides appropriately. First, it is important that the pure oxide is
actually performing as good as possible, which is why proper benchmarking
is critical 109, 110. Second, the reason for improvements can be manifold.
Depending on preparation method a big contributor is increase in surface area,
which changes the current measured directly. Other examples are increase in
conductivity and change in electrode-electrolyte interface 111, 112.
In order to better understand the inuence of surface structure on catalytic
activity, single crystal studies have proven very useful for investigations of the
oxygen reduction reaction 113119. However, this is not trivial for OER due
to diculties in preparation of oxide single crystals and reconstruction taking
place under reaction conditions 120. From a report by Trasatti and coworkers
in 1986, early works on geometric eects are analysed 121 and it was
argued that polycrystalline and single crystal studies often suer from poor
characterization of the surface structure during reaction conditions. From their
own electrochemical experiments the (110) surface of rutile RuO2 exhibited
a change in Tafel slope as a function of the overpotential and a roughening
of the surface during testing. Unfortunately, that study did not include any
surface science methods to conrm the expected crystalline orientation. The
overall goal of these early studies was to map out Tafel slopes as a function of
the surface structure. This phenomenological approach, although theoretically
valid, have achieved little success in OER due to diculties in reproducibility
for similar materials across dierent research groups. More recent studies
have focused on systematically preparing comparative surfaces, exemplied by
work from Shao-Horn and co-workers, who compared the activity of (110) and
(100) oriented surfaces on rutile RuO2 and IrO2 122. They found that the
more open (100) surfaces on both RuO2 and IrO2 exhibited increase in activity
over the (110) orientation. They argue that it could be related to a
higher surface density of coordinatively undersaturated metal sites, CUS, which
Rossmeisl et al proposed to be the active site 90. It should here be noted
that those experiments were performed in alkaline solution. There is much
to learn from studying well-dened surfaces of RuO2 and IrO2 which are the
state of the art catalysts. It is particularly valuable to use such experimental
evidence of active sites as input for more accurate and realistic DFT calculations.
If a close agreement between theory and experiments can be established
the insight gained could accelerate rational design of oxygen evolution catalysts.
In practical devices the catalyst has to be nanostructured to maximise the surface
area and minimise catalyst loading. For Ru and Ir based catalysts it is