ИСТИНА

Post-lithium metal-ion battery technologies based on Na, K, Mg and Ca are attracting more and more attention from the battery community. Recently, the possibility of employing proton-intercalating materials for energy storage applications has been investigated due to the low cost of the materials, environmental benignity of aqueous solutions and exceedingly high transport rates of protons.[1] Prussian Blue Analogues (PBAs) have been acknowledged as prospective electrode materials for both sodium-ion and potassium-ion batteries due to high diffusivities of Na+ and K+ cations, which are achieved due to the presence of spacious cavities in PBA structures.[2] PBAs can also be considered as suitable materials for proton-intercalation batteries due to the hydrogen bond networks in the structures of hydrated PBAs, which could provide exceptionally high proton transport rates via Grotthuss-type mechanisms. Yet, PBAs exhibit different structures and different phase-transformation patterns[3] depending on the amount of water and intercalated alkali ions in the as-synthesized materials, which calls for the investigation of the effect of the PBA structure type and hydration level on the mobilities of charge-compensating species. In this work, we focus on comparing potassium-ion and proton diffusivities in nickel hexacyanoferrate structures. The electrochemical behavior of the PBA cathode materials with the compositions Ni1.5[Fe(CN)6]·7H2O and K2Ni[Fe(CN)6]·1.6H2O was investigated with respect to their charge storage properties in aqueous solutions containing K+ and H3O+ as charge compensating cations. The diffusion coefficients and charge transfer resistances were estimated based on chronoamperometry and electrochemical impedance spectroscopy data. Complementary information on the mechanism of redox transformations of PBA materials in aqueous solutions was obtained based on IR spectroscopy and electrochemical quartz crystal microbalance experimental data. Our results indicate that the rates of diffusional transport of cations in nickel hexacyanoferrate structure are strongly dependent on the PBA water content PBA. The presence of a hydrogen bond network in the PBA cavities leads to an increase in the proton transport rates and slows down the transport of potassium cations. References [1] X. Wu, J. J. Hong, W. Shin, L. Ma, T. Liu, X. Bi, Y. Yuan, Y. Qi, T. W. Surta, W. Huang, J. Neuefeind, T. Wu, P. A. Greaney, J. Lu, X. Ji, Nat. Energ. 2019, 4, 123. [2] K. Hurlbutt, S. Wheeler, I. Capone, M. Pasta, Joule 2018, 2, 1950. [3] E. E. Levin, A. A. Kokin, D. E. Presnov, A. G. Borzenko, S. Y. Vassiliev, V. A. Nikitina, K. J. Stevenson, ChemElectroChem 2020, 7, 761. Acknowledgements This work was supported by the Russian Foundation for Basic Research (grant # 20-33-70092).