2.5 Oxygen Evolution 35
due to inherent iron contamination from alkaline solutions and glassware, most
Ni electrodes are in fact doped with iron 147. This has spurred further investigations
into mixed nickel-iron oxides. Friebel et al. used a combination
of X-ray Absorption Spectroscopy, electrochemical measurements and DFT calculations
to propose a model explaining the activity of mixed NiFeOx 148.
Contrary to the common understanding that Ni provide the active site, they
argue that highly active Fe-sites in a Ni hydroxide matrix are responsible for
the low overpotential of these mixed catalysts. The X-ray Absorption technique
is particularly useful in combination with DFT calculations since it allows for a
very accurate determination of both oxidation state and local structure of the
metal atoms, which can then be used as input in the calculations. Furthermore,
the bad performance of pure iron hydroxides is argued to be due to bad conductivity
of -FeOOH. A similar conclusion was reached in a study on mixed
cobalt-iron hydroxides, from Burke et al. For this material addition of iron leads
to improvements in activity up to around 50 % Fe content, after which phase
separation and bad conductivity seemingly lead to a decrease. The primary
methods employed by Burke et al. are electrochemical measurements on electrodeposited
samples combined with in-situ studies of the potential dependent
resistivity.
These studies mark an interesting new era in electrocatalysis, where new
understanding arises from combinations of methods, rather than phenomenologically
relying on pure electrochemical parameters such as Tafel slope and
exchange current density. An additional focus point in the eld of OER is the
benchmarking of a large range of materials to allow for better comparison. Especially
the Joint Center for Articial Photosynthesis, JCAP, stands out in this
aspect with dedicated groups working on comparing catalysts for both HER
and OER 109, 110. In two comprehensive studies McCrory et al. test a large
number of catalysts with a simple electrochemical protocol. The primary parameters
resulting from the protocol are overpotential needed to reach 10 mA/cm2,
roughness factor based on pseudocapacitance, Faradaic eciency and change in
activity after 2 hours. It should be noted that even though almost all known
catalysts are tested, only Ru, Ir and Pt are active in acidic environment. To
sum up this section of the most active catalysts for OER in alkaline environment
a unied Tafel is shown in gure 2.11.
2.5.2.2 Manganese based OER catalysts
Manganese based catalysts for OER represent another interesting case, which
in many studies has been inspired by nature 151, 153155. This is due to the
manganese-calcium complex responsible for the turnover of molecular oxygen in
photosystem II. Manganese is also an extremely abundant element, among the
most available transition metals, and it is considered environmentally friendly
102, 156, 157. Finally, it is one of the few transition metal oxides with a stable
solid phase in the OER relevant potential region in acidic conditions that is also