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A stable film of zinc hexacyanoferrate is deposited on a GC (glassy carbon) substrate following a specific electrochemical protocol. The electrode maintains its characteristic even after dry and wet processes. SEM characterization confirms the cubic morphology of the materials and the IR suggests the presence of the FeII-C-N-ZnII structural unit. The electrochemical characterization indicates a very good stability of the film, thus opening application in ion exchange system. The focus is on monovalent, divalent, and trivalent ions. These results, the zinc low toxicity, and cost make zinc hexacyanoferrate films a promising candidate for many electrochemical applications.

Aqueous rechargeable Zinc-ion batteries (ARZIBs) have attracted extensive attention as one of the most promising post-lithium ion battery candidates for large-scale electrochemical applications, because of their low cost, intrinsic safety, and environmental friendliness. The metallic zinc, as an ideal anode material shows high theoretical gravimetric and volumetric capacity of 820 mAh g-1 and 5855 mAh cm-3, low electrochemical potential (-0.76 V vs. SHE) and high abundance.[1,2] Manganese hexacyanoferrate (MnHCF), one of the Prussian blue analogues (PBAs), has attracted widely attention as promising cathode material for Li-ion and post-Li ion batteries. MnHCF is composed of only highly abundant metals and displays large capacity and high discharge potential owning to the two-redox active sites [3-4]. Here, the electrochemical performance of MnHCF was studied in 3 M ZnSO4 aqueous electrolyte with Zn sheet as anode. The battery exhibits high specific capacity (176 mAhg-1) at C/20, and around 61% capacity retention after 50 cycles at C/5. In order to explain the capacity fading problems during cycling, the local geometric, electronic structures, as well as the framework structure change of MnHCF electrode were studied by means of ex-situ X-ray absorption spectroscopy (XAS) and synchrotron X-ray diffraction (XRD). Based on XAS data, no obvious change was observed at the Fe K-edge during cycling, and this indicates that there is no apparent change of the local Fe structural environment. However, the XAS spectra of Mn K-edge exhibit an apparent change after 10 cycles. The Zn K-edge shows a typical -Zn-NC-Fe- structural framework in the cycled samples that resembles the one of zinc hexacyanoferrate (ZnHCF), providing evidence for Zn-Mn partially replacement upon cycling, resulting in dissolution the Mn ion [5]. From the ex-situ XRD data, we found that this phase changes mainly concerns early cycles, because the XRD patterns of the 2nd and 10th cycle are almost identical. ZnHCF phase formed even during the first charge process. The more detailed phase transformation process is still under studying. By combing the ex-situ XAS and XRD data, the charge/discharge mechanism of MnHCF in aqueous Zn-ion battery can be more clearly illustrated. Trócoli, F. La Mantia, ChemSusChem. 8, 2015, 481–485. Tang, L. Shan, S. Liang, J. Zhou, Energy Environ. Sci. 12, 2019, 3288–3304. Mullaliu, J. Asenbauer, G. Aquilanti, S. Passerini, M. Giorgetti. Small Methods, 2019, 1900529. Mullaliu, M. Gaboardi, J. R. Plaisier, S. Passerini, and M. Giorgetti, ACS Applied Energy Materials 3, 2020, 5728. Li, R. Sciacca, M. Maisuradze, G. Aquilanti, J. Plaisier, M. Berrettoni, M. Giorgetti, Electrochim. Acta. 400 ,2021.

A key factor for the developing of post-lithium batteries is represented by suitable active materials to be used in the positive and the negative electrodes. Host nanostructured have insertion sites, channels and/or interlayer spacings allowing the rapid insertion and extraction of the required ions, occurring generally with little lattice strain, with concomitant reduction/oxidation of an active metal belonging to the host. In this context Prussian Blue Analogs (PBA) offer an interesting host material for a wide variety of cations, which is also characterized by an appropriate electroactive bimetallic network. Dynamic processes occurring in batteries can be studied by operando modality, which provide a realistic representation of the electrochemical reactions occurring at the electrodes, or in ex situ modality, which reflect a given state of charge (SOC) of the electrode material. Monitoring the local structure around the metallic sites as well as the periodic structure can be performed using spectroscopic methods and in particular X-rays. X-ray absorption spectroscopy (XAS) is a synchrotron radiation-based technique that provide both electronic and structural information on a selected atomic species in a sample. General guidelines are presented in order to give a useful guide for a correct interpretation of the EXAFS spectra of this class of compounds, which is characterized by close, but sufficiently separated, discontinuities of the absorption coefficient due to contiguous transition metal K-edges. The dynamic of the ions insertion/release electrochemical reaction can further be investigated by adopting a chemometric approach using a multivariate curve resolution with alternating least squares algorithm (MCR-ALS), with the intent to assess the number of species involved and their evolutions during the electrochemical process. X-ray powder diffraction (XRPD) experiments allow monitoring the periodic structure of a material following the intercalation/release process of the ions involved. The potentiality of the joint XAS-XRD approach in the newly proposed PBA cathodes materials for rechargeable batteries is here highlighted, giving emphases on the copper hexacyanoferrate, copper nitroprusside, and manganese hexacyanoferrate electrodes. 1. Conti, S. Zamponi, M. Giorgetti, M. Berrettoni, W.H. Smyrl, Anal. Chem. 82(2010)3629. 2. Giorgetti, ISRN Materials Science, 2013, 938625. 3. Mullaliu, M. Sougrati, N. Louvain, G. Aquilanti, M. Doublet, L. Stievano and M. Giorgetti, Electrochimica Acta 257(2017)364. 4. Mullaliu, G. Aquilanti, L. Stievano, P. Conti, J. R. Plaisier, S. Cristol, and M. Giorgetti, J. Phys. Chem. C 122(2018)15868. 5. Mullaliu, P. Conti, G. Aquilanti, J. Plaisier, L. Stievano, and M. Giorgetti, Condens. Matter 3(2018)36. 6. Mullaliu, G. Aquilanti, P. Conti, J. R. Plaisier, M. Fehse, L. Stievano, and M. Giorgetti, J. Phys. Chem. C 123(2019)8588. 7. Mullaliu, J. Asenbauer, G. Aquilanti, S. Passerini, and M. Giorgetti, Small Methods 4(2020)1900529. 8. Mullaliu, G. Aquilanti, P. Conti, M. Giorgetti, and S. Passerini, ChemSusChem 13(2020) 608. 9. Mullaliu, M. Gaboardi, J. R. Plaisier, S. Passerini, and M. Giorgetti, ACS Applied Energy Materials 3(2020)5728. 10. Li, A. Mullaliu, S. Passerini, and M. Giorgetti, Batteries 7(2021)Art. No. 5. 11. Li, R. Sciacca, M. Maisuradze, G. Aquilanti, J. Plaisier, M. Berrettoni, and M. Giorgetti, Electrochimica Acta 400(2021) Art. No. 139414.