STRUCTURE OF MATERIALS
The unique interplay between copper and zinc during catalytic carbon dioxide hydrogenation to methanol, M. Zabilskiy (a), V.L. Sushkevich (a),
D. Palagin (a), M.A. Newton (b), F. Krumeich (b) and J.A. van Bokhoven (a,b), Nat. Commun. 11, 2409 (2020); https:// doi.org/10.1038/s41467-020-16342-1.
(a) Paul Scherrer Institute, Villigen (Switzerland) (b) Institute for Chemical and Bioengineering, ETH Zurich (Switzerland)
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A NOVEL REACTION MECHANISM FOR LITHIUM- ION BATTERY MATERIALS
Ex-situ X-ray absorption spectroscopy and in-situ X-ray diffraction, in combination with other techniques, were used to investigate the reaction mechanism of Fe-doped CeO2 as new electrode material for lithium-ion batteries. This comprehensive investigation reveals a new mechanism, involving the reversible selective reduction of the Fe dopant at the atomic level.
PRINCIPAL PUBLICATION AND AUTHORS
virtually identical catalyst activity, suggesting that no correlation exists between the observed methanol productivity and the amount of copper-zinc alloy in the system. During transient experiments by switching different gas compositions (from hydrogen to CO2/H2 mixture and back), it was found that copper-zinc alloy, which for a long time was considered the active site for methanol synthesis, is only stable under a highly reductive atmosphere (Figure 121). However, in the presence of carbon dioxide, this phase undergoes oxidation with the formation of zinc oxide and zinc formate. Combining this
finding with results obtained from the time- resolved isotope labeling experiment coupled with infrared spectroscopy, it was revealed that carbon dioxide hydrogenation to methanol occurs via a zinc formate intermediate route.
Ultimately, the role of copper-zinc alloy formation and its decomposition in forming the active interface represents a novel paradigm in this much-researched system. It can be considered as an exemplar model for structural changes in multicomponent materials and catalysts that occur during reaction.
With the steadily increasing demand for enhanced energy and power densities for the next generation of rechargeable lithium-ion batteries, the development of new electrode materials that are capable of hosting more lithium ions per unit weight and/or volume is one of the main research directions in this field [1,2]. This study reports a completely new active material, which generally follows an insertion-type mechanism: Fe-doped CeO2 (Ce0.9Fe0.1O2). While the reversible capacity of pure CeO2 is limited to about 100 mAh g-1, the introduction of carefully selected dopants into the crystal structure, herein exemplified for iron, allows for an increase in capacity by up to 200%, i.e., up to about 300 mAh g-1 (Figures 122a,b). To explain this dramatic increase, an in-situ X-ray diffraction (XRD) analysis of the first two dis-/charge cycles for both active materials CeO2 and Ce0.9Fe0.1O2 (Figures 122c,d) was conducted. The recorded XRD patterns are presented as waterfall diagrams, along with the corresponding dis-/charge profile.
CeO2 (Figure 122c) reveals a typical solid- solution de-/insertion mechanism with a continuous shift of the fluorite-related reflections to lower 2θ values as a result of the unit cell volume expansion due to the insertion of Li+. Upon delithiation, the reflections shift back and gradually increase in intensity, reflecting the corresponding unit cell volume contraction and Li+ deinsertion. For Ce0.9Fe0.1O2 (Figure 122d), the evolution of the XRD patterns shows essentially the same trend, though the shift of the reflections is much more pronounced. This indicates that the reaction mechanism is essentially the same, while more lithium cations are reversibly inserted. But why does the introduction of iron allow for much more Li+ to be reversibly inserted?
To address this question, an ex-situ X-ray absorption spectroscopy (XAS) investigation was performed at beamline BM08. XANES spectra were recorded at the Ce LIII-edge of