Tuning intercalation sites in nickel hexacyanoferrate using lattice nonstoichiometry

Reversible intercalation/deintercalation of K+ in nickel hexacyanoferrate (NiHCF) thin films occurs in two energetically distinct, largely noninteracting, sites whose proportions are set by lattice nonstoichiometry. Principal component analysis (PCA) is used to correlate the stoichiometry of NiHCF, measured using energy-dispersive X-ray spectroscopy (EDS), with intercalation processes measured via cyclic voltammetry (CV). NiHCF voltammograms are well-known to exhibit two reversible peaks for K+-containing electrolytes; each peak represents a different local cation intercalation/deintercalation environment. PCA of 59 different CV-EDS data sets shows (to first order) that the relative sizes of the two CV peaks are proportional to the lattice nonstoichiometry, but the peak locations (energies) are largely independent of stoichiometry. Intercalation sites in stoichiometric lattices are energetically favored compared to sites associated with defects (Delta G approximate to 10 kJ/mol K+). The proportionality of CV peak sizes, and lack of peak shifting with lattice nonstoichiometry, indicates the different intercalation sites are largely noninteracting. Weak site-site interactions allow easy tuning of intercalation properties via lattice stoichiometry, though the best sensing or separation performance will likely be achieved in either defect-free lattices or highly nonstoichiometric lattices where, in each case, a single type of intercalation site is dominant. Processing conditions for creating a wide range of NiHCF lattice stoichiometries are presented.