Draft version
1. Introduction
The electrolytic production of synthetic fuels provide a promising means to average fluctuant energy supply
from renewable sources. It is evident that the water oxidation reaction, known as oxygen evolution, is set to
play a key role in such transformations.1–4 Whether the goal is to produce pure hydrogen, reduce CO2 to
hydrocarbons or upgrade biomass it is crucial to find a suitable source of hydrogen. For this purpose, water is
ideal due to its abundance and the relative ease of water splitting, which leaves no harmful byproducts.
However, the electrochemical evolution of oxygen, OER, imposes a large overpotential, due to sluggish kinetics
at the electrode.5–7 More specifically, the difficulties in catalyzing the reaction arise from non-optimal binding
energies to the three reaction intermediates, even on the most active catalysts.5,8–10 This is because the binding
to the reaction intermediates, *O, *OH and *OOH, correlate linearly with each other: their relationships to
each other are known as the scaling relations. For all surfaces which obey scaling relations, no catalyst will
exhibit optimal binding to all three intermediate. For this reason, there is a need for novel strategies that
circumvent the scaling relations and lead to catalytic surfaces with lower overpotential.11
A possible strategy for improving catalyst properties is to modify a catalytic surface with another material.
There are several examples in the literature where such mixtures have been successful in achieving improved
performance. In acidic media ruthenium oxides mixed with either Ni or Co has been reported to be more active
than the pure oxide.12–15 Recently, Ti has also been shown to stabilize MnO2 thin films against anodic
dissolution in acidic media.16 In alkaline media Ni and Fe based oxides are currently utilized in commercialized
electrolyser systems and combinations of the two elements have been shown to increase activity
significantly.17–19 Furthermore, Mn and Co based oxygen evolution catalysts have shown good performance in
alkaline environment and various strategies have been proposed to improve the activities.20–28 Interestingly,
the presence of metallic particles or support has a profound influence on the activity of both Mn and Co based
oxide catalysts.29–35 Mn nanoparticles deposited above or below gold clusters can lead to a 20-fold increase in
turnover frequency.29 A possible explanation for the beneficial interaction was later proposed by two of the
authors of the current manuscript on the basis of stabilized *OOH adsorption on neighboring Mn and Au sites,
due to a proton transfer mechanism.30 Kuo et al. experimentally investigated the role of gold nanoparticles for
five different MnOx polymorphs: they argued that the increase in OER activity could be correlated to facile
formation of active Mn3+ sites in the presence of gold.33 This conclusion was reached primarily from ex-situ Xray
Absorption Near Edge Spectroscopy, XANES, which indicated a lower oxidation state of Mn sites when
combined with gold nanoparticles. However, judging from stability regions of the Mn-O system in aqueous
2