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evolution at standard conditions of 1.23 V.9 Thus, the two
proton–electron transfers both proceed at 1.6 V, equivalent to
an overpotential of 0.37 V. By changing from one catalyst to
another and thereby changing the binding strength, there is
little hope to break this fundamental limitation for OER. For
catalyst surfaces that bind too strongly (which is the situation
on Mn, Co, Ir, and Ru oxides), the overpotential originates from
breaking a bond between the intermediates and the surface
Eq. (3).
For surfaces that bind too weakly, such as NiO or TiO2, the
overpotential is related to bonds forming to the surface
Eqs. (1) or (2). This simple relationship between catalytic activity
and binding of intermediates illustrates the Sabatier principle.
22 For the OER a suitable descriptor for the activity is the
reaction energy of the second step Eq. (2), DG*=ODG*OH.
Through the linear scaling relation between *OH and *OOH,
this single descriptor can describe the potential-determining
step for both strong and weak binding catalysts towards the
OER.10
For catalyst surfaces on which reaction (3) is potential-determining,
the activity could be enhanced by stabilizing the
*OOH intermediate relative to *=O and *OH, as reaction (3)
would then require a lower potential. A strategy and example
on this concept have recently been demonstrated for mixtures
with RuO2 and either Co or Ni.23 The idea is to introduce a hydrogen
acceptor on the RuO2 surface, in this case an oxygen
atom, near Co or Ni so that *OOH forms a strong hydrogen
bond to this acceptor or even donates the hydrogen, forming
*OHacceptor and O2 on the surface rather than the *OOH intermediate.
Experimentally, several studies have reported activities
that oxides based on RuCo or RuNi mixtures are more
active than pure RuO2.24–26
Decoupling the *OOH binding energy from the *OH binding
makes it possible to tune the catalytic properties by varying
the hydrogen acceptor. Reaction (3) is therefore changed
into reaction (5):
*¼O þ H2O þ *¼Oacceptor ! *O2 þ *OHacceptor þ Hþ þ e ð5Þ
In this case the thermodynamic restrictions owing to the
linear scaling relationships between *OH and *OOH binding
no longer hold, that is, formally the OER may proceed at potentials
closer to the thermodynamic limit.23 The desirable
property of the acceptor site is a suitable potential at which
*=Oacceptor!*OHacceptor can proceed and regenerate. For
oxygen evolution, the optimal potential for this hydrogen acceptor
process would be near 1.23 V.
Manganese and cobalt oxides have been studied extensively
in recent years6–8, 13, 27–31 as alternatives to the commonly used
Ru or Ir-based catalysts.11 Besides being abundant and benign
elements, they have been proven active in the OER32–35 and,
in the case of MnOx, also in the oxygen reduction reaction.36, 37
It has recently been shown that the activity of MnOx nanoparticles
towards oxygen evolution can be drastically increased in
the presence of Au.38 From those results, a combination of
MnOx and Au nanoparticles showed a 20-fold increase in turnover
frequency at 400 mV overpotential. The enhanced activity
was also obtained by adding Au as HAuCl4 to the electrolyte.
A similar effect was found earlier by El-Deab, Mohammad et al.
by using Au as substrate for nano-MnOOH, reducing the overpotential
by more than 200 mV compared to that found on Pt
or glassy carbon substrates.39, 40
Similarly, for CoOx it has been found that depositing
0.4 monolayers of CoOx on Au results in a higher activity than
pure CoOx.41 It was even shown that Au-supported CoOx was
more active than Pt, Pd, and Cu-supported CoOx. From those
results the authors suggested that the effect of the metal support
was related to the electronegativity affecting the binding
to oxygen.41 Another interpretation could be that the metal
support was directly involved in the OER mechanism, as the
effect was most pronounced for submonolayer films. Furthermore,
in a recent study Au nanoparticles embedded in mesoporous
Co3O4 were found to enhance the activity towards
OER.30 In Figure 1, the experimental observations from30, 38, 41
have been summarized in a Tafel plot. For MnOx nanoparticles,
the decrease in overpotential due to presence of Au varied
from 100 to 150 mV, whereas for CoOx the decrease varied
from 20 to 100 mV.
Figure 1. Experimental data summarized in a Tafel plot, showing recent reports
of OER activity enhancements due to the presence of Au. For MnOx/
Au and MnOx the data is taken from Ref. 38. For CoOx on Au and on bulk
Co the data is from Ref. 41, and for Au in mesoporous (m-) Co3O4 and
Co3O4 the data is from Ref. 30.
In this work we propose an explanation for these activity enhancements
on the basis of the recently proposed hydrogen
transfer from *OOH to an adjacent acceptor site.23 By using
DFT, the binding energies to the OER intermediates have been
calculated on both rutile MnO2 and Mn2O3 and the effect of Au
interaction is explored. These two oxides are chosen due to
their stability at OER relevant potentials.27, 42 Data for CoOx are
taken from Ref. 43.
Results and discussion
First, we focus on MnOx and later extend the conceptual understanding
to CoOx. Water oxidation on pure Mn2O3 (and
Co3O4) via *OOH is thermodynamically limited by the formation
of the *OOH intermediate, as seen in the free energy diagram
in Figure 2 a. The free energy diagrams in Figure 2 are all
shown at 0 V and depict the energy levels for each reaction
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