Operando XES and HERFD-XANES studies of Prussian Blue analogue battery materials

Abstract

X-ray absorption spectroscopy has proved to be valuable tool in the characterisation of materials, enabling determination of the oxidation states of metal atoms and ions via analysis of the near edge XANES region and the local coordination geometry via analysis of the XANES and EXAFS regions of the spectra. One significant limitation of these methods is that the data represent the per-atom average of the state and structure of the element under investigation. When studying mixed oxidation state materials or materials in which the element is in more than one type of site this limitation decreases the value of the data. However, if the sites differ by spin-state, it is hypothesised that selectivity may be achieved by the use of high energy resolution fluorescence detection HERFD-XANES in which the spin-state selectivity arises by collecting the spectra using X-ray emission energies that discriminate between the two spin states. This may be achieved in the case of iron by using the Kb emission lines, rather than the more typical Kb emission lines. In this thesis the hypothesis of spin-state selectivity is tested by applying the technique to study the HERFD-XANES of a series of Prussian blue analogue (PBA) materials in which the spin-state of the iron atoms is determined by the bonding to either the C or N atom of the CN ligands and then Prussian blue (PB) itself, where iron atoms are present in both spin states. The ability of the technique to resolve changes in the oxidation states of each of the iron centres is furthermore tested in in situ studies of PB acting as a sodium ion battery material in both the aqueous and non-aqueous environments.

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Identifiers

ORCID for Oliver Frederick Blackman: orcid.org/0000-0002-7398-9521

ORCID for Andrea Russell: orcid.org/0000-0002-8382-6443

ORCID for Andrew Hector: orcid.org/0000-0002-9964-2163

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