(Invited) Influencing the Catalytic Activity for Oxygen Reduction and Evolution in Aqueous and Non-Aqueous Electrolytes: Support and Cations
Authors: Reinsberg Philip , Zan Lingxing , Amin Hatem M.A. , Mostafa Ehab , Baltruschat Helmut 
Published: February 2020

The idea of a battery employing a lightweight metal anode and an oxygen cathode has inspired many electrochemists during the last decades. This is mainly due to the theoretically high specific energies, which would allow for surpassing some of the shortcomings of conventional lithium–ion batteries. However, one of the main issues to address is the stability of the metal anode in contact with the electrolyte and possibly oxygen. This is why the oxygen reduction in organic electrolytes has recently attracted much attention [1]. Organic electrolytes, unlike aqueous electrolytes, generally exhibit an increased compatibility with metal anodes like lithium or sodium, which are unstable in aqueous electrolytes due to their low redox potential. Although lithium is the most promising metal energy-wise, other alkaline and earth alkaline metals would still yield superior specific energies in comparison with lithium-ion batteries. This is why this work aims at elucidating the role of the metal cation on the ORR and OER in organic solvents. Previous DEMS (differential electrochemical mass spectrometry)-studies imply that there might be a charge-density related effect within the alkaline metals: Monovalent Cations with high charge densities seem to foster the formation of the corresponding peroxide [2]. However, recent observations using Ca2+ as a cation show that the picture is obviously more complicated when using the earth alkaline metals. Despite its low charge density, Ca2+ fosters the peroxide at gold electrodes [3]. Within the earth alkaline series, the (average) number of electrons transferred per oxygen molecule increased in the order: z(Ca2+)<z(Sr2+)<z(Ba2+)~z(Mg2+). Furthermore, the solubility and stability of the products is also significantly influenced by the cation.
In aqueous electrolytes, oxides are the best bifunctional electrode materials. We have recently shown that the activity of silver for oxygen reduction is increased when Ag particles are in contact with Co3O4 particles. Also, the activity of Co3O4 for oxygen evolution is increased in such a mixed catalyst. [4] We will demonstrate here that to some extent such effects can also be observed when other support materials instead of silver are used, therefore, part of the effect may be due to conductivity effects. However, model experiments using smooth electrodes demonstrate that there is also a true synergistic effect between Ag and the Co-oxide. We will further demonstrate that this effect is also observed for various perovskites mixed with or deposited onto Ag. XPS analysis has shown that the presence of Ag+ in contact with Co3O4 facilitates the redox switching of Co3O4, which might be the reason for the enhanced catalytic activity of the mixture.[5]
Using differential electrochemical mass spectrometry (DEMS) and 18O- isotope labeling it has been shown that the lattice oxygen is participating in the OER at oxides.[6-9] This procedure is applied to Co3O4 and the mixed catalyst as well. The active area thus determined will be compared to that estimated from BET data and a simple ball model.
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