Copper Electroactivity in Prussian Blue Based Cathode Disclosed by

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Copper Electroactivity in Prussian Blue Based Cathode Disclosed by Operando XAS Angelo Mullaliu, Giuliana Aquilanti, Paolo Conti, Jasper Rikkert Plaisier, Marcus Fehse, Lorenzo Stievano, and Marco Giorgetti J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03429 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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The Journal of Physical Chemistry

Copper Electroactivity in Prussian Blue Based Cathode Disclosed by Operando XAS

Angelo Mullaliu(a), Giuliana Aquilanti(b), Paolo Conti(c), Jasper R. Plaisier(b), Marcus Fehse(d), Lorenzo Stievano(e,f)* and Marco Giorgetti(a)*

a

Department of Industrial Chemistry “Toso Montanari”, University of Bologna, Viale Risorgimento

4, 40136 Bologna, Italy. b

c

Elettra – Sincrotrone Trieste, ss 14, km 163.5, 34149 Basovizza, Trieste, Italy School of Science, Chemistry Division, University of Camerino, Via S. Agostino 1, 62032

Camerino (MC), Italy d

Dutch-Belgian (DUBBLE), ESRF-The European Synchrotron, CS 40220, Grenoble Cedex 9,

France e

Institut Charles Gerhardt Montpellier, CNRS UMR 5253, Université de Montpellier, Montpellier,

France f

Réseau sur le Stockage Electrochimique de l’Energie (RS2E), CNRS FR3459, Amiens, France

*Corresponding authors Email: [email protected]; Phone number: +39 051 20 93 666; Fax: +39 051 20 93 690. Email: [email protected]; Phone number: +33 4 67 14 33 46.

1 Environment ACS Paragon Plus

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Abstract The electronic and structural evolution of copper hexacyanoferrate (CuHCF) cathode material was studied by operando X-Ray Absorption spectroscopy (XAS) simultaneously at both Fe and Cu K-edges during a full galvanostatic cycle. The full set of XAS data collected during the electrochemical process was analyzed by a combined chemometric approach using the Multicurve Resolution Analysis (MRC) with the Alternate Least Squares (ALS) algorithm. Using this joint approach, and by applying a simultaneous multiple-edge fitting procedure, it was possible to clarify the participation of both copper and iron centres to the redox processes, and to analyze their local environment. The structural modifications occurring in CuHCF along with the redox processes are entirely reversible, with the steady multiplicity of Fe-C-N-Cu linear chains evidencing the structural stability of the material during cycling.


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1. Introduction Prussian blue analogues (PBAs), and in particular metal hexacyanoferrates, have gained considerable attention among insertion materials due to the ease of preparation and separation, effectiveness as electrode materials, and wide versatility towards several ions, ranging from monovalent ions such as lithium 1, sodium

2–6

, potassium

7,8

, to divalent and trivalent ions, for

instance calcium 9, magnesium 10 and aluminum 11. The ability of accommodating such a variety of ions originates in the structure of PBAs, which is characterized by a three-dimensional cubic network (although other crystal symmetries are found) of repeating -NC-Fe-CN-M-NC- units, where iron and M sites are typically octahedral in the so-called “soluble” structure, displayed in Sketch 1. The sites at the cube centre (8c positions) can be occupied by countercations and water molecules to achieve charge neutrality. The cube size, which is twice the Fe–M distance, is about 10 angstroms and guarantees enough space for insertion/release of ions inside the zeolitic channels of roughly 3.2 angstroms, beyond cavities of around 5 angstroms arising from vacancies. Our group has determined selectivity constants for alkali metals insertion using the nickel hexacyanoferrate analogue 12 and has studied the electroactivity of both metals in hexacyanoferrates, for instance for H2O2 sensing 13. Although the literature reports that both metals in copper hexacyanoferrate can be electroactive 14, the belief that iron is in fact the only active species is widely spread in the battery community 1,15–18. In some cases, not only the metal sites, but also the ligands are able to participate to the redox processes, as already observed for copper nitroprusside, Cu[Fe(CN)5(NO)], an analogue of copper hexacyanoferrate 19. In copper nitroprusside, the nitrosyl ligand originally in the oxidized –NO+ form can accept one electron, be reduced to the radical –NO, allowing the reaction of one extra lithium equivalent per unit formula. In this work we demonstrate that also copper is active in copper hexacyanoferrate during the electrochemical reaction with lithium, and that it plays a remarkable role in the redox process. To reach this goal, operando X-ray Absorption spectroscopy (XAS)

20,21

was carried out at both metal

centres. Indeed, XAS is a powerful tool that can be tuned to a chosen element and performed in


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operando mode, i.e., by collecting several spectra during electrochemical cycling in ad hoc developed in situ cells 21. In this way, the physico-chemical properties and the local structure of the selected element can be monitored at all moments during the charge and discharge processes. Moreover, the data treatment on the extended fine-structure part of the spectrum (EXAFS) can be accomplished by using both single edge and multiple edge approaches

22

, in the latter case by

refining simultaneously the spectral data at two or more edges, which increases the reliability of the structural results. Combined with operando XAS, MCR-ALS (Multivariate Curve Resolution using a constrained Alternating Least Squares algorithm) was used to recover the spectra of the pure chemical species. This chemometric approach is able to determine the number of “pure” components present in the system under study and to identify the evolution of their concentration. This analysis was first employed for battery systems by Conti et al. in 2010

23

, where a joint

chemometric-dynamic XAS permitted a complete understanding of the cell under investigation.

2. Experimental 2.1.Synthesis, Characterization, and Electrode Material Preparation Following a reported synthesis

1

and adapting it slightly, copper hexacyanoferrate was

synthesized by co-precipitation at 40 °C, by mixing 250 mL of a 0.04 M aqueous solution of CuSO4·5H2O and 250 mL of aqueous 0.02 M K4[Fe(CN6)]. Both solutions were prepared using double-distilled water (ddH2O) and previously thermostated at 40°C. Chemicals were purchased from Carlo Erba and used without further purification. The batch was aged for two days in the dark at room temperature. The red precipitate was separated by filtration on a Whatman 42 paper filter, washed twice with ddH2O and dried at 30-40°C under vacuum. The obtained solid (2.98 g) was finally ground in an agate mortar. X-ray fluorescence (XRF) was performed by means of a PANalytical AxiomAX spectrometer on boric acid pellets with a 26% mass concentration of active material to determine K:Cu:Fe ratio. Powder X-ray Diffraction (PXRD) data were recorded on the synthesized powder using a monochromatic X-ray beam with a wavelength of 1 Å at the MCX


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beamline in Elettra Sincrotrone Trieste, Basovizza (Italy) 24. Data were collected on the sample in a capillary geometry, setting the spinner at 300 revolutions per minute. The XRD pattern was collected consecutively in the range 5°

Copper Electroactivity in Prussian Blue Based Cathode Disclosed by

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