Tailoring the Performance of Oxides for the Oxygen Evolution Reaction
Authors: Tripkovic Vladimir , Hansen Heine A. , Garcia Lastra Juan Maria , Vegge Tejs 
Published: February 2020

The intermittency of renewable energy sources calls for smart energy storage and conversion solutions. Storing energy in chemicals with high energy density can bridge daily fluctuations between energy production and demand. The two most advanced energy storage and conversion devices are batteries and water electrolyzers/fuel cells. However, their efficiency is usually hampered by sluggish oxygen redox reactions.
We explore four different classes of oxides comprising 1st row transition metals as catalysts for the oxygen evolution reaction in neutral and alkaline media: 1) pristine and doped spinel Co3O4, 2) pristine and doped Co and Ni oxyhydroxides (CoOOH and NiOOH), 3) stoichiometric and non-stoichiometric MnO2 polymorphs (α, β, γ) and MnO2 containing minerals, and 4) pristine and doped perovskite oxides ABO3, where A is the earth-alkali or lanthanide metal and B the 1strow transition metal. The emphasis in the analysis is on the four most important performance parameters: stability, catalytic activity, conductivity and selectivity.
From adsorption isotherms we determine stable surface terminations at potentials of interest. For CoOOH we find that terraces on exfoliated nanosheets are somewhat less active than terraces of the bulk material. However, activity and conductivity improves drastically at terrace sites adjacent to nanosheet edges, which might explain why nanosheets outperform bulk materials in electrochemical experiments.1,2We assign this beneficial effect to different local environments and oxidation states of the two types of terrace atoms. Similarly, the performance can be tuned by means of doping, although in this case the improvement depends on the choice of the dopant atom. We investigated 10 different dopants making sure these are stable at OER pertinent conditions.
For NiOOH we find that terraces are more active than edges or near-edge terraces, and similarly as in CoOOH, activity and conductivity can be further tailored by doping.
We expect that partial oxidation of water to peroxide is suppressed at large overpotentials because the deprotonation of OOH to O2is not a thermochemical limiting step, and furthermore the calculated proton transfer barrier is found to be very small.
As for the MnO2 compounds, we find that activity reduces in the following order CaMnO3 > αMnO2 > βMnO2 > Birnessite mineral. Even being catalytically quite active CaMnO3, αMnO2 and βMnO2 are semiconductors; hence the conductivity will limit their efficiency. The conductivity can be vastly improved by creating Mn3+ sites. This can be achieved by intercalating electrolyte ions (e.g. Na+ or K+) in large αMnO2 cavities or introducing oxygen defects, that is, non-stoichiometry in the structure. Mn3+ will not improve the activity per se because it is highly unlikely that there will be any Mn3+ present on the surface at highly oxidizing conditions, in contrast to what has previously been proposed and modelled.3,4

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Acknowledgements
This study is a part of ZAS-project (Zinc Air Secondary innovative nanotech based batteries for efficient energy storage) sponsored by the European Commission under Horizon 2020-research and innovative framework program