DFG-RSF: Dotiertes Graphen für elektrochemische Energiespeicherung und .-umwandlung: Einfluss der elektronischen Struktur auf die elektrokatalytische Aktivität in Sauerstoffredoxreaktionen
Zusammenfassung der Projektergebnisse
Carbon-based materials are very attractive for use as electrodes in Li-Air batteries. Unfortunately, however, they exhibit instabilities under battery operating conditions caused by undesirable side processes which lead to formation of oxygenated species at the surface and passivate the electrode material. With electron spectroscopic experiments under UHV conditions, we first showed that vapour-deposited Li intercalates on graphene/Ni(111) and forms islands of Li2O2 at the surface upon oxygen exposure, but no oxidation of the graphene matrix takes place up to 500°C. However, such an oxidation occurs when Li is replaced by K, where besides K2O2 also the superoxide KO2 appears, which is apparently responsible for the oxidation. LiO2 is unstable, but might be formed on a short time scale during the electrochemical operation of the cell and could then behave even more reactive than KO2. Moreover, in addition to the carbon electrode, also the electrolyte, in mostl cases acetonitrile, could be attacked by this superoxide. We therefore performed in-operando experiments using an electrochemical model cell under near ambient conditions and observed that acetonitrile, which does not react with the Li2O2-covered graphene surface under UHV conditions, oxidizes and polymerizes under operating conditions and contaminates the graphene surface with these by-products. The by-products formed desorb or decay upon evacuation and can not be detected ex-situ. In further experiments, we have studied the oxidation of graphene using nascent oxygen as well as the intercalation of molecular oxygen. Oxidation with nascent oxygen shows that oxygen initially accumulates on the surface forming epoxy groups and breaks carbon bonds of the graphene matrix only at higher oxygen doses. As a possible alternative electrode material we have investigated single-crystalline TiC that leads to formation of titanium oxocarbides and carbonates upon Li2O2 coverage. Coating the surface with graphene only partially suppresses such processes, presumably because the graphene layer has too many defects. As an alternative, we investigated a possible passivation by TiO2, where TiO2 was generated by direct oxidation of the TiC surface. Hereby, oxide layers produced at elevated temperature at 10^-7 mbar revealed a much weaker passivating effect than those produced by exposing the sample to air for 30 minutes at 200 mbar and room temperature, so it is hoped that TiC surfaces can be passivated with TiO2 by suitable control of the oxidation process. Since our work dealt with Li/graphene systems and superconductivity had been predicted for such systems from DFT calculations, we studied the Li/graphene/Si/Co(0001) system, first intercalating Si under graphene and then depositing Li on the graphene surface. We observed an epitaxial layer-by-layer growth with very strong charge transfer from Li to graphene and pronounced phonon interactions within the graphene layer, as evidenced from corresponding kinks in the ARPES valence band spectra. Our DFT calculations for the system show in agreement with the ARPES data, that both the Si and Li layers are magnetized by the Co substrate, but there is no net moment on the carbon atoms. Whether superconductivity occurs under such conditions is an interesting question that cannot be answered, however, on the basis of the present data.
Projektbezogene Publikationen (Auswahl)
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“ Oxygen reduction by lithiated graphene and graphene-based materials” ACS NANO 9, 320 (2015)
E.Yu. Kataev, D.M. Itkis, A.V. Fedorov, B.V. Senkovsky, D.Yu. Usachov, N.I. Verbitskiy, A. Grüneis, A. Barinov, D.Yu. Tsukanova, A.A. Volykhov, K.V. Mironovich, V.A. Krivchenko, M.G. Rybin, E.D. Obraztsova, C. Laubschat, D.V. Vyalikh, and L.V. Yashina
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“Tuning surface chemistry of TiC electrodes for Li-air-batteries”, Chem. Mater. 28, 8248 (2016)
A.Ya. Kozmenkova, E. Yu. Kataev, A.L. Belova, M. Amati, L. Gregoratti, J. Velasgo Velez, A.K. Guericke, Boris Senkowski, D.V. Vyalikh, D.M. Itkis, Y.Shao- Horn, and L. Yashina
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“Laterally selective oxidation of large-scale graphene with atomic oxygen”, J. Phys. Chem. C 121, 27915 (2017)
O.O. Kapitanova, E.Yu. Kataev, D.Yu. Usachov, A.P. Sirotina, A.I. Belova, H. Sezen, M. Amati, M. Al-Hada, L. Gregoratti, A. Barinov, H.D. Cho, T.W. Kang, G.N. Panin, D.V. Vyalikh, D.M. Itkis, and Lada Yashina
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“Raman spectroscopy of lattice-matched graphene on strongly interacting metal surfaces”, ACS Nano 11, 6336 (2017)
D.Yu. Usachov, V. Yu. Davydov,V. S. Levitskii, V.O. Shevelev, D.E. Marchenko, B.V. Senkovskiy, O.Yu. Vilkov, A.G. Rybkin, L.V. Yashina, E.V. Chulkov, I.Yu. Sklyadneva, R. Heid, K.-P. Bohnen, C. Laubschat, and D.V. Vyalikh
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“Electron-phonon coupling in graphene placed between magnetic Li and Si layers on Cobalt”, Phys. Rev. B 97, 085132 (2018)
D.Yu. Usachov, A.V. Fedorov, O.Yu. Vilkov, I.I. Ogorodnikov, M.V. Kuznetsov, A. Grüneis, C. Laubschat, and D.V. Vyalikh
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“Notable reactivity of acetonitrile towards Li2O2/LiO2 probed by NAP XPS during Li-O2 battery discharge” Topics in Catalysis 61, 2114 (2018)
T.K. Zakharchenko, A.I. Belova, A.S. Frolov, O.O. Kapitanova, J.J. Velasgo Velez, A. Knop-Gericke, D.V. Vyalikh, D.M. Itkis, and L.V. Yashina
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“Photoelectron diffraction and holography studies of 2D materials and interfaces”, J. Phys. Soc. Japan 87, 061005 (2018)
M. Kuznetsov, I.I.Ogorodnikov, D.Yu.Usachov, C. Laubschat, D.V.Vyalikh, and L.V.Yashina
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“Oxygen intercalation and oxidation of atomically thin h-BN grown on a curved Ni crystal”, J. Phys. Chem. 123, 593 (2019)
A.A. Makarova, L. Fernandez, D.Yu. Usachov, A. Fedorov, K.A. Bokai, D.A. Smirnov, C. Laubschat, D.V. Vyalikh, F. Schiller , and J.E. Ortega
