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Novel Electrode Materials Based Zn-Air Batteries for Energy Storage: From Fundamental Aspects to System Engineering

Fachliche Zuordnung Physikalische Chemie von Festkörpern und Oberflächen, Materialcharakterisierung
Förderung Förderung von 2017 bis 2022
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 339689134
 
Erstellungsjahr 2023

Zusammenfassung der Projektergebnisse

The application of rechargeable Zinc-air batteries, which are attractive because of their high energy density, low cost and high safety, is hindered not only by technical problems, but also by fundamental problems such as the lack of stable and highly active bifunctional cathode catalysts for both the oxygen reduction (ORR) and the oxygen evolution reaction (OER) or Zn dendrite formation at the Zn anode during charging. This project aimed at an improved molecular scale understanding of the electrode reactions in rechargeable Zn-air batteries, in particular the ORR/OER, the electro-oxidation/reduction of zinc species and Zn deposition/dissolution, employing operando spectro-electrochemical measurements and high-resolution microscopy and spectroscopy studies on structurally and chemically well-defined model systems under ultrahigh vacuum (UHV) conditions. For interpretation of the experimental data we closely collaborated with theory. The controlled preparation of thin film oxide electrodes under UHV conditions, by vapor deposition of the respective metal in an O2 atmosphere, was investigated by scanning tunneling microscopy and X-ray photoelectron spectroscopy. We prepared well-ordered, atomically flat CoO(111) and CO3O4(111) films with the expected stoichiometry. Also atomically flat binary Ni- and Fe-doped Co3O4(111) film surfaces were prepared, both surface decorated and mixed oxide films. While Ni,Co oxide films were truly mixed, with Nin+ increasingly replacing Con+ species, Fe,Co oxide films consist of CoFe2O4, overgrown by either CoO(111) or Fe2O3(111), depending on the stoichiometry. Electrochemical characterization showed that Fe oxides can be activated for the ORR/OER by Co modification, but not vice versa, leaving Co oxides as the most active bifunctional OER/ORR catalysts. Ni modification had little effect, allowing partial replacement of expensive Co by Ni. Detailed information on the different reactions occurring in the presence of O2 and Zn2+ in the electrolyte was obtained from differential electrochemical mass spectrometry, using an ionic liquid (IL) as promising alternative to aqueous electrolytes. Quantitative comparison of the Faradaic current and O2 consumption/evolution allowed us to distinguish between Zn deposition and ORR, which occur in the same potential range. While in only Zn2+- or O2-containing electrolyte the ORR/OER or Zn deposition/dissolution are reversible, ZnO2 is formed if both are present and reversibility is only obtained after cycling to very negative potentials (-1.4 V). The measurements were complemented by studies of the chemical stability/decomposition of different solvents in contact with well-defined oxide surfaces under UHV conditions. In summary, we could derive detailed molecular scale information on the electrochemical properties and ORR/OER activity of complex, but structurally and chemically well-defined oxide electrodes, on the interaction between O2 and Zn2+ in ionic liquid electrolyte and on the chemical stability/decomposition of relevant solvents in contact with oxide surfaces, which contributes significantly to the fundamental understanding of the processes in Zn-air batteries.

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