Nowadays, electrochemical energy storage plays a major societal role due to its widespread technological applications. Host nanostructured materials have a crystal structure with insertion sites, channels and/or interlayer spacings allowing the rapid insertion and extraction of lithium ions with generally little lattice strain. Therefore they are used as electrode materials for batteries. Dynamic processes occurring in batteries can be studied by operando modality. Operando experiments provide a realistic representation of the reaction behavior occurring at electrodes, which permits to checking the structural and electronic reversibility of a battery system, while at least one full cycle is performed. For all these reasons, ex situ studies, which reflect a given state of charge (SOC) of electrode materials are now complemented by operando measurements using complementary tools such as X-ray diffraction (XRD) and spectroscopic techniques such as X-ray absorption spectroscopy (XAS).

X-ray absorption spectroscopy is a synchrotron radiation based technique that is able to provide information on local structure and electronic properties in a chemically selective manner. Operando synchrotron radiation x-ray powder diffraction (SR-XRPD) experiments allow monitoring the extended structure of a material during the intercalation/release process of ions.

The potentiality of the joint XAS-XRD approach in the newly proposed Prussian Blue-like cathodes materials for rechargeable batteries is here underlined.

Prussian blue analogous (PBAs) or metal hexacyanoferrates are bimetallic cyanides with a three-dimensional cubic lattice of repeating -Fe-CN-M-NC- units (where M=transition metals). Because of their peculiar structure exhibiting large ionic channels, interstices in the lattice and redox-active sites they have been proposed as active materials for electrodes in batteries. In our group, a series of PBAs have been synthesized, such as copper hexacyanoferrate (CuHCF), manganese hexacyanoferrate (MnHCF), titanium hexacyanoferrate (TiHCF), multi-metal doped hexacyanoferrate, as well as copper nitroprusside etc. In particular, this talk will be summarize results obtained in the case of copper hexacyanoferrate and copper nitroprusside, as well as the manganese hexacyanoferrate [1-5]. Sodium-rich manganese hexacyanoferrate (MnHCF) is gaining consideration as battery materials for the versatility toward several chemistries beyond lithium, the ease of synthesis, as well as their affordable cost of production. MnHCF is constituted only by earth-abundant elements, and it displays high operational voltages and high specific capacities. Since PBAs act as sponge-like materials towards water molecules, also in case of short time exposure to contamination, and both the electrochemical behavior and the reaction dynamics are affected by interstitial/structural water and adsorbed water, the effect of hydration is critical in determining the electrochemical performance. The electrochemical activity of MnHCF without extensive dehydration was investigated by varying the interstitial ion content through a joint approach using operando x-ray absorption fine structure (XAFS) spectroscopy conducted at the XAFS beamline in ELETTRA and multivariate curve resolution with alternating least squares algorithm (MCR-ALS), with the intent to assess the structural and electronic modifications occurring during sodium release and lithium insertion as well as the overall dynamic evolution of the system. The study is also complemented to the and operando XRPD. It was found that only a minor volume change (about 2%) is recorded upon cycling the electrode material against lithium.

[1] A. Mullaliu, G. Aquilanti, P. Conti, J. R. Plaisier, M. Fehse, L. Stievano, M. Giorgetti. Copper Electroactivity in Prussian Blue-Based Cathode Disclosed by Operando XAS. J. Phys. Chem. C, 122 (2018) 15868-15877.

[2] A. Mullaliu, M. Gaboardi, J. Rikkert Plaisier, S. Passerini, M. Giorgetti. Lattice Compensation to Jahn-Teller Distortion in Na-rich Manganese Hexacyanoferrate for Li-ion Storage: An Operando Study. ACS Appl. Energy Mater., 3, (2020) 5728–5733.

[3] A. Mullaliu, J. Asenbauer, G. Aquilanti, S. Passerini, M. Giorgetti. Highlighting the Reversible Manganese Electroactivity in Na-Rich Manganese Hexacyanoferrate Material for Li- and Na-Ion Storage. Small Methods, (2020) 1900529.

[4] M. Li, A. Mullaliu, S. Passerini, M. Giorgetti. Titanium Activation in Prussian Blue Based Electrodes for Na-ion Batteries: A Synthesis and Electrochemical Study. Batteries, 7 (2021) 5.

[5] A. Mullaliu, M.T. Sougrati, N. Louvain, G. Aquilanti, M.L. Doublet, L.Stievano, M. Giorgetti. The electrochemical activity of the nitrosyl ligand in copper nitroprusside: a new possible redox mechanism for lithium battery electrode materials? Electrochimica Acta, 257 (2017) 364–371.

The following researchers are kindly acknowledged: Angelo Mullaliu, Giuliana Aquilanti, Jasper R. Plaisier, Min Li, Stefano Passerini.

The magnetic response of a system is the result of several contributions to the magnetic anisotropy which come from a plethora of effects. In the case of thin ferromagnetic films, the local-scale and long-range structural details, including film thickness, and interface interactions (intermixing, alloying, oxidation etc.) have a deep impact on the magnetism. Once known, these features can be used to tailor the magnetic response of the system, however, to fully control the system response, a deep knowledge is required.

To contrast the high reactivity of some ferromagnetic films, a passivating overlayer can be used: in these cases, further degrees of freedom add up in the definition of the magnetic response due to the upper interface phenomena. A deep understanding of such complex systems requires to assess and isolate the fine details linked to the interactions at the interfaces, and to those occurring within the film itself. Also, the oxidation prevention provided by the capping layer should be verified in view of application purposes.

Such a challenging task can be pursued only by combining complementary, state of the art techniques. This work presents the results of in-situ synchrotron radiation techniques and Magneto-Optic Kerr Effect (MOKE) measurements on Gr/Co/Ir systems, i.e. Co films intercalated between Graphene and Ir(111) [1]. The contributions to the in-plane and out-of-plane magnetic response of the system were evaluated based on Grazing Incidence X-Ray Diffraction (GI-XRD), X-ray Absorption Near Edge Spectroscopy (XANES), and Extended X-ray Absorption Fine Structure (EXAFS). The Gr/Co/Ir system is particularly interesting as the understanding of its magnetic behaviour requires to explore the thickness and thermal dependencies of local-scale and long-range anisotropies, to assess interface intermixing phenomena, and to evaluate the evolution of Co oxidation states (especially upon exposure to ambient conditions).

The development of effective catalytic processes for the conversion of CO₂ into value-added chemicals or fuels, such as methanol synthesis or the dry reforming of methane (DRM) relies strongly on a rational catalyst design, which in turn requires an in-depth understanding of structure-activity relationships. Due to the inherent complexity of heterogeneous catalytic systems, an arsenal of complementary techniques is required to characterize the catalytic structure (and dynamics thereof) from the atomic-to-nanoscale (under reaction conditions). In this talk, we show how the application of combined X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (XAS) allows obtaining the oxidation state, the local and (nano)crystalline structure of the catalysts providing the basis for the formulation of structure-performance relationships in catalysts for CO₂ valorization reactions.

In the first example, we demonstrate how a combined operando XAS-XRD experiment allowed us to relate the evolution of the structure of In₂O₃ nanoparticles (NPs) to their activity for CO₂ hydrogenation to methanol.^[1] The experiments revealed a reductive amorphization of the In₂O_3−x nanocrystallites with time on stream (TOS), leading ultimately to an over-reduction of In₂O_3−x to (molten) In⁰, in a process that is linked to catalyst deactivation. When the In₂O₃ NPs were supported on a nanocrystalline monoclinic ZrO₂ support, we observed the stabilization of the oxidation state of In via the formation of a solid solution m-ZrO₂:In.^[2] In the second example, we explore a Ni-Fe-based catalyst for the DRM. Combined, operando XAS-XRD experiments allowed us to probe the dynamics of Ni-Fe alloying/dealloying with the formation of FeO to explain the superior stability of the NiFe catalysts compared to a Ni-based analogue, due to a Fe-FeO_x-based redox cycle.^[3] In the last example, combined XAS–XRD experiments are used to shed light on the formation of Ru⁰ nanoparticles (ca.1 nm) via their exsolution from defective, fluorite-type Sm₂Ru_xCe_2−xO₇ solid solutions. The resulting exsolved nanoparticles show a high activity and stability for the DRM.^[4]

In this work, we present new geochemical and structural data in order to document a new occurrence of andradite garnet (garnet general formula {X}₃Y₂(Z)₃O₁₂) “demantoid” variety” in Sardinia, Italy [1]. The crystal structure consists of a framework of alternating ZO₄ tetrahedra and YO₆ octahedra that share corners, with cavities in the X cations coordinated by 8 oxygen atoms in the form of a triangular dodecahedron. The yellowish-green to intense green variety of andradite, called “demantoid”, is a precious and greatly appreciated gemstone, mainly found in Russia, Namibia, Madagascar and Italy (Valmalenco, Lumbardy) [2]. The studied samples come from a new deposit from Domus de Maria municipality, nearby “Sa Spinarbedda” mine. This found is very peculiar, because although the beauty of these samples, it has not been previously described from Sardinia region. In this work we investigated the structural and chemical features of these new demantoid samples by combining electron microprobe analyses (EMPA), laser ablation-inductively coupled mass spectrometry (LA-ICP-MS), single crystal X-ray diffraction (XRSD), IR and X-ray absorption (XAS) spectroscopies. Chemical analyses revealed an enrichment of Ca and Fe and a low content of Ti, Mn and Al, thus confirming the andradite nature of the garnet. The Cr content (~8.73 ppm, mean) has been useful to confirm the demantoid variety of andradite. Fe and Mn K-edge XAS data were collected at the XAFS beamline (ELETTRA, Trieste, Italy) both in transmission (Fe) and fluorescence mode (Mn), using fixed exit Si (111) monochromator [4] to better understand the coordination number of both ions. Position of the absorption edge together with the pre-edge peaks analysis, point to the presence of only Fe³⁺ in octahedral coordination, confirming that the whole Fe content can be allocated in the Y crystallographic site. A more complex situation has been found for Mn where pre-edge peaks analysis on our spectrum indicate that Mn should be mainly in the form of Mn²⁺ and 8-fold coordination, occupying the X crystallographic site, beside a small amount of Mn³⁺ is probably present in octahedral Y site. The infrared spectra of andradite crystals investigated at the SISSI beamline (ELETTRA, Trieste, Italy) show a prominent absorption band at about 3560 cm^-1, suggesting the well-known hydrogarnet substitution of (SiO₄)₄ with (O₄H₄)₄ [5][6]. These absorption features are related to hydroxide, which can be incorporated in the andradite structure in the form of structurally bonded OH groups, according to previous experimental findings [5]. X-ray single-crystal diffraction experiments refinement (Ia3̅d space group) highlighted a unit cell volume (1757.15(2)ų) larger than that reported usually in the literature thus confirming the presence of a slight water content [5]. The dodecahedral site X resulted to be partially occupied in a proportion of ≈96.2% Ca and ≈3% of Mn⁺². The octahedral site Y also resulted to be partially occupied in a proportion of ≈95.6% Fe⁺³ and ≈4.5% Al, while the Mn⁺³ content was too low to be estimated. Then, according to Adamo et al. (2011) [6] the potential partial occupation of tetrahedral site has been checked. Actually, the site resulted occupied only for ≈98%, the other 2% has been refined for O (same position and same thermal factor), suggesting the presence of structural water. Refining the site occupancy factor (s.o.f.) at the Si-site, modelled with the scattering curve of silicon alone in the X-ray structure refinement, we obtained s.o.f. value ≈98%, which barely confirmed potential hydrogarnet substitution [i.e., ((SiO₄)₄ with (O₄H₄)₄].

[1] Grew, Edward & Locock, A. & Mills, S.J. & Galuskina, Irina & Galuskin, Evgeny & Hålenius, Ulf. (2013). Am. Min. 98, 785-811.

[2] Štubňa, J., Bačík, P., Fridrichová, J., Hanus, R., Illášová, Ľ., Milovská, S., ... & Čerňanský, S. (2019). Minerals, 9(3), 164

[3] Geiger, C. A., & Rossman, G. R. (2020). Am. Min. 105(4), 455–467

[4] Di Cicco, A., Aquilanti, G., Minicucci, M., Principi, E., Novello, N., Cognigni, A., & Olivi, L. (2009). J. Phys. Conf. Ser. 190(1), 012043

[5] Amthauer, G. & Rossman, G.R. (1998): The hydrous component in andradite garnet. Am. Mineral., 83, 835–840.

[6] Adamo, I., Gatta, G. D., Rotiroti, N., Diella, V., & Pavese, A. (2011). Eur. J. Mineral. 23(1), 91-100

LiFePO₄ (LFP) as a cathode material in lithium ion batteries has a number of advantages including a long cycle life, a long calendar life and can be used at high discharge currents. A low cost, low energy hydrothermal synthetic route is being investigated where homemade Teflon bombs are used in an oven at 120^oC to synthesise LFP. During synthesis an aqueous LiOH solution is added dropwise to a FeSO₄–H₃PO₄ solution. All solutions are constantly purged with nitrogen during this step to prevent any oxidation. Interestingly it was determine that the rate at which the Li⁺ solution was added to the Fe²⁺ solution (while being stirred at a constant speed) influenced the final product. The addition rate was set to one drop every 1, 2, 3, 4 and 5 seconds. If the Li⁺ was added too slowly a mixture of phases (LFP and Li₃PO₄) was formed and when it was added too fast a completely different final phase was formed. In the latter case no LFP was identified using PXRD, but raman spectroscopy (RS) showed that non-crystalline LFP was present in the sample together with other phases.

A range of different techniques have been combined to probe the effect of the different addition rates on the local and average environments. It was determined that the 3sec addition rate was the optimum rate. Synchrotron X-ray diffraction (SXRD) with Rietveld refinement was used to characterize the average structures of the different environments (Figure 1). Mössbauer spectroscopy (MS) was used to probe the effect of Li⁺ addition on the local environment. Although no impure phases were identified using SXRD in the samples synthesised with the optimum addition rate, MS indicated that there was amorphous phases present. MS also showed that there was more than one Fe environment present in the sample. The major phase was Fe²⁺ in a distorted octahedral environment (LiFePO₄). The other 3 contributions to the total Fe in the sample are due to either structural defects, distortions or disorder [1]. X-ray absorption spectroscopy (XAS), in particular extended X-ray absorption fine structure (EXAFS) (Figure 2) was used to determine what the effect of the different addition rates on the local structure is.

Noble metal nanoparticles, due to their relatively high stability and wide scope of application, attract a lot of researchers’ attention. The catalyst effectiveness directly depends on the dispersion rate and the presence of active catalytic sites, since the higher the dispersion, the greater the surface area available for catalytic reactions. The substrate material also plays a large role in the efficiency of the final product. One of the new and effective methods for the synthesis of the noble metals ultrafine nanoparticles is UV irradiation of their precursor salts. The main advantages of this method are relative simplicity, high recovery rate and environmental friendliness. The nanoparticles synthesized in this way are less susceptible to agglomeration, which eliminates the need for introducing various surfactants and toxic solvents into the system, in contrast to standard methods. Due to the relatively high value of the electrode potential of the Pd²⁺/Pd⁰ pair, as well as low photostability, complex palladium salts are quite easily restored, and the selection of the optimal salt and radiation power allows the process to be rapidly carried out.

In this work, palladium nanoparticles were synthesized in an aqueous solution by UV irradiation using complex palladium oxalate as a precursor. The synthesis consists of UV irradiation of an aqueous dispersion of CeO₂ containing the [Pd(C₂O₄)₂]^2– complex as one of the most photoactive non-toxic precursors. Cerium dioxide was synthesized by a simple one-step method and was selected due to its high thermal stability and relative chemical inertness, as well as its large oxygen storage capacity due to the formation of the Ce⁴⁺/Ce³⁺ redox pair, which allows CeO₂ to efficiently release catalytically active oxygen species. Samples were studied by various laboratory methods, such as TEM, XRF, XRPD, XAFS spectroscopy, and diffuse reflection IR spectroscopy of CO probing molecules.

TEM images did not allow to distinguish Pd nanoparticles from the substrate material but showed the absence of the UV radiation influence on the sizes of CeO₂ nanoparticles. XRF data showed the presence of cerium and palladium atoms in the material. X-ray diffraction patterns indicate the presence of both a cerium dioxide phase and a phase of metallic palladium, while the analysis of XAFS spectra beyond the K edge of palladium also showed the presence of a PdO phase in the system (Fig. 1). The approximate size of palladium nanoparticles was estimated from the infrared spectra after CO adsorption (Fig. 2) and it was less than 2 nm, which is significantly smaller than the average size of Pd nanoparticles obtained by a similar method without a CeO₂ substrate (1.5–9.5 nm) [1].

[1] Navaladian, S., Viswanathan, B., Varadarajan, T. K., & Viswanath, R. P. (2008). Nanoscale research letters 4(2), 181.

The work was supported by grant of President of Russia for young scientists (MK-2730.2019.2).