Electrochemical properties of nanostructured cobalt hexacyanoferrate containing K+ and Cs+ synthesized in water-in-oil AOT reverse microemulsions

Herein a series of cobalt–iron Prussian blue analogs (PBAs) containing K+ and Cs+ were synthesized in water-in-oil AOT reverse microemulsions. The size, morphology and composition of the as-prepared cobalt hexacyanoferrate (CoHCF) can be fine-tuned by conveniently varying the water-to-surfactant molar ratio (w) of the microemulsion, as confirmed by various techniques such as infrared spectroscopy (IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). These nanostructured PBA were thereafter employed to chemically modify electrodes to thoroughly investigate their electrochemical properties. Oxidized and reduced cobalt hexacyanoferrates were fabricated and characterized in the presence of alkali metal (Li+, Na+, K+, Rb+, Cs+) and NH 4 + counteractions by cyclic voltammetry (CV). In addition, it was demonstrated that formal potentials of hexacyanoferrate (III, II) redox reactions were sensitive to the choice of electrolyte cations, and they correlated well with the sizes of hydrated Na+, K+, Rb+, Cs+ and NH 4 + except for Li+. Furthermore, the samples are found to vary significantly in permeability, probably due to the difference of composition, structure and morphology of products, and the changes during charging/discharging. Finally, it is noteworthy to highlight that the nanostructures of cobalt hexacyanoferrate synthesized at w = 5–40 had much more electrochemistry stability than that synthesized in aqueous solution in all electrolytes. The potential application of the modified electrode of CoHCF films in electrochromic display was also noticed.