Kinetic studies of metal hexacyanoferrate electrode using energy dispersive EXAFS

Abstract

Metal hexacyanoferrate (MHCF) compounds are candidate electrode materials for aqueous sodium and potassium ion batteries and supercapacitors. This is due to their large open structure, long cycling stability and high current density. In this work we have investigated the electron transfer kinetics during the (de)intercalation of cations into/from the MHCF lattice using energy dispersive EXAFS for the cases where M = Fe, Cu, Ni, Co, and Mn. Energy dispersive EXAFS enables the oxidation state (XANES, edge position) to be determined in real time during cycling or following a potential step applied electrochemically. By comparison of the EDE and electrochemical data we were able to explore the differences in the rates of ion and electron transfer in some cases. These differences were most apparent when the oxidation/reduction of the metal ions resulted in a change in the conductivity of the MHCF. For FeHCF we observed that the change from insulator to conductor when the high-spin, carbon bound Fe atom, is reduced, the rate of the process was limited by the electron transfer reaction, whilst the reoxidation (conductor to insulator) was limited by the rate of diffusion of the cations in the FeHCF lattice. In the case of CuHCF and NiHCF, only the low spin, nitrogen bound Fe, atom is electrochemically active and, as there was little change in the conductivity of the material upon oxidation from Fe2+ to Fe3+, we were unable to separate the rates of the ion and electron transfer. For CoHCF and MnHCF, both the Co or Mn and Fe atoms are electrochemically active and by using EDE we were able to monitor the rates of change of the oxidation states of both species in a truly simultaneous manner by collecting the data over an energy range that covered both absorption edges. As in the case of FeHCF, the rate determining process (ion or electron transfer) was found to be dependent on the oxidation state of the carbon bound metal atom, but not on the oxidation state of the nitrogen bound metal atom.

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

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