ESRF Highlights 2021

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PRINCIPAL PUBLICATION AND AUTHORS

Molecular Understanding of the Impact of Saline Contaminants and Alkaline pH on NiFe Layered Double Hydroxide Oxygen Evolution Catalysts, S. Dresp (a), F. Dionigi (a), M. Klingenhof (a), T. Merzdorf (a), H. Schmies (a), J. Drnec (b), A. Poulain (b), P. Strasser (a), ACS Catal. 11, 12, 6800-6809 (2021); https:/doi.org/10.1021/acscatal.1c00773 (a) Technical University Berlin (Germany) (b) ESRF

REFERENCES

[1] W. Tong et al., Nat. Energy 5(5), 367-377 (2020). [2] S. Dresp et al., Adv. Energy Mater. 8, 1800338 (2018).

Fig. 125: Lattice parameter and crystallite size trends under catalytic OER conditions obtained by Rietveld refinement.

a) Interlayer distances, (b) lattice parameter a. The two horizontal yellow bands in panels (a) and (b) show the expected

value range of α- and γ-NiFe LDH. Adapted with permission from S. Dresp et al., ACS Catal. 11, 12, 6800-6809 (2021).

catalytically active phase. Definitive conclusions as to the significance of this correlation will need further studies.

While this work provided a molecular picture and answered many questions related to the effect of salt additives and pH on the activity of NiFe LDH OER catalysts in seawater, it also raised further questions regarding the effect of the salt on the morphology, strain and the dynamics of the phase transformations, which are important for the practical use of the catalyst. Given their large abundance and ease of preparation, these materials, once fully understood, can hold on the promise of a decarbonised society and a hydrogen economy.

A new view of defects at the cusp of melting

Imperfections or defects often dictate a material s mechanical properties; an understanding of how to tune defects therefore allows unprecedented levels of control over materials. Dark-field X-ray microscopy is used to characterise how dislocations evolve at the cusp of melting, offering new insights into the connection between microscopic defects and the bulk properties they cause.

At the macroscopic scale, dislocations are responsible for propagating shear through a lattice, and their interactions set the mechanical properties in most metals. While we know the importance and many phenomena of how dislocations behave, their microscopic motion deep inside macroscopic materials has remained elusive especially

Fig. 126: With stationary optics, the dark-field X-ray microscope at ID06 can directly resolve the

subtle distortions from individual dislocations as real-time movies.

under exotic conditions. At the edge of a phase transition, as the inherent stability of a crystal breaks down, the behaviour of dislocations becomes much less certain. In this exotic regime of phase space, the well-defined mechanisms for dislocation motion and interactions begin