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PRINCIPAL PUBLICATION AND AUTHORS

Exploiting Confinement to Study the Crystallization Pathway of Calcium Sulfate, C. Anduix-Canto (a), M.A. Levenstein (a,b), Y-Y. Kim (a), J.R.A. Godinho (c), A.N. Kulak (a), C. González Niño (b), P.J. Withers (c), J.P. Wright (d), N. Kapur (b), H.K. Christenson (e), F.C. Meldrum (a), Adv. Funct. Mater. 31 2107312, (2021); https:/doi.org/10.1002/adfm.202107312. (a) School of Chemistry, University of Leeds, Leeds (UK) (b) School of Mechanical Engineering, University of Leeds, Leeds (UK) (c) Henry Royce Institute, Department of Materials, University of Manchester (UK) (d) ESRF (e) School of Physics and Astronomy, University of Leeds, Leeds (UK)

that these phases could be readily distinguished, where the denser, spheroidal bassanite crystals appear brighter than the dendritic gypsum crystals due to their difference in X-ray absorption (Figure 115).

That both bassanite and gypsum formed within the functionalised CPG rods provided a unique opportunity to study the influence of adjacent crystals on each other. Gypsum crystals failed to induce dissolution or transformation of bassanite crystals in their vicinity, where this is very different behaviour to bulk solution. However, the transformation of bassanite to gypsum could be triggered by exchanging the reagent solutions for water. In-situ μ-CT revealed a direct transformation of bassanite

to the more hydrated gypsum, where this occurred from the outer surface towards the centre. Notably, the morphology of the original bassanite was retained, as was the integrity of the CPG matrix, suggesting that the transformation occurred by a local dissolution/ reprecipitation mechanism.

It is envisaged that this experimental approach can be used to study processes such as weathering and biomineralisation, and that it will make it possible to predict and control crystallisation within porous media. Continued development of tomographic techniques will also enable measurements at enhanced spatial resolution and facilitate study of the early stages of crystallisation.

Shedding synchrotron light on noble metal nanocatalyst strain dynamics

X-ray diffraction has revealed the interactions between nanocatalysts and electrochemical species during operation in liquid electrolytes. The strain in nanocatalysts is shown to describe both their absorption and adsorption trends, which are critical for understanding their performance and stability.

The eventual future of a hydrogen economy relies strongly on the development of key electrochemical conversion devices, such as proton exchange membrane water electrolysers (PEMWE, for the production of hydrogen from electricity) and proton exchange membrane fuel cells (PEMFC, for the production of electricity from hydrogen). In both cases, catalyst materials play a major role in device cost, performance and lifetime. Consequently, there is an urgent need to design more effective catalysts.

In the case of heterogeneous catalysis (where the reactions occur on the catalyst surface), the most promising approach in materials design follows the Sabatier principle: the ability of the surface to bind adsorbates, and the strength of the bonds, define the reaction thermodynamics and kinetics. In this respect, the catalyst surface chemistry and

structure (crystallographic orientation of the facets and/or strain) are used as levers to optimise performance.

However, it is largely accepted that the structure of PEMWE anode and PEMFC cathode catalysts is altered during long-term operation due to harsh (electro)chemical conditions. Pioneering work conducted at beamline ID31 has previously revealed that platinum (Pt) nanoparticle strain is also generally a function of the electrode potential [1]. This was qualitatively rationalised as a consequence of the adsorption of various molecules on the Pt surface, and triggered two fundamental questions: do the observed structural modifications of the catalyst provide critical information about catalyst adsorption trends, and do they impact the properties of the catalyst?

This study investigated strain dynamics for Pt and palladium (Pd) nanocatalysts in liquid electrolytes. The work demonstrates the power of the new ESRF-EBS source to reveal the microstructural information of device-relevant nanomaterials in a liquid electrochemical environment under operando conditions with unprecedented quality and time resolution (Figure 116a).

Beyond the technical showcase, the study confirms that the bulk microstructures (lattice constant, crystal phase or

REFERENCES

[1] F.C. Meldrum & C. O Shaughnessy, Adv. Mater. 32, 2001068 (2020). [2] Y-W. Wang et al., Chem. Commun. 48, 504-506 (2012).