Ion Selectivity in Nickel Hexacyanoferrate Films on Electrode Surfaces

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We have examined the redox behavior of anodically formed nickel hexacyanoferrate complexes on electrode surfaces. Cyclic voltammetry in dry acetonitrile solutions containing various cations indicates a clear size dependence. Li+ and Na+ reversibly insert into the nickel hexacyanoferrate lattice while K+ suffers a kinetic inhibition, and larger ions are unable to insert into the lattice. This behavior is in agreement with our estimate of 0.77 A for the size of the diffusion bottleneck. A much larger range of cation sizes are reportedly accommodated in the AxNiFe(CN)6 lattice in aqueous electrolytes, and the role of water in the aqueous electrochemistry of this material is unclear. We have shown that protons (or hydronium ions) may play a role in effecting charge neutrality in these films. Sites in the cubic framework structure of nickel hexacyanoferrate, an analogue of Prussian blue, are occupied upon reduction of the film in reactions analogous to insertion or intercalation reactions, which occur for graphite or oxide framework structures.



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