Countercation‐Sensitive Electrochromism of Cobalt Hexacyanoferrate Films

Cobalt(II) hexacyanoferrate(III,II), a system analogous to prussian blue, is a unique electrochromic material: its color is not only dependent on the oxidation potential, but also on the nature of the countercations sorbed from electrolyte during reduction. The electrodeposition of cobalt hexacyanoferrate thin films, their voltammetric behavior and spectroelectrochemical identity are reported here in potassium and sodium electrolytes. The oxidized film is purple brown in both electrolytes, but following reduction, the system turns olive‐brown in 1 M KCl and becomes green in 1 M NaCl.