Solid-state electrochemistry of iron hexacyanoferrate (Prussian Blue type) powders: Evidence for redox transitions in mixed-valence ionically conducting microstructures

Abstract

The powders of Prussian Blue (KFe III[Fe II(CN) 6]) and Berlin Green (K 1− x {Fe III[Fe III(CN) 6] x }· {Fe III[Fe II(CN) 6]} 1− x , where x < 1), when sandwiched between glassy carbon slide electrodes, are electroactive in the absence of an external supporting electrolyte. The results are compared to the solid-state electrochemical responses of the metal substituted Prussian Blue analogues: indium(III) hexacyanoferrates(III,II) and iron(II) hexacyanoruthenates(II). The cyanometallate powders exhibit solid-state redox transitions when they are ionically conducting and mixed-valent, but they are not electroactive when they are composed of metals in one oxidation state only. The latter systems are characterized by insulating or semiconducting properties. The sandwich assembly permits investigation of electron self-exchange (redox conduction) in Berlin Green or Prussian Blue + Berlin Green mixture; the process is driven by transport of electrochemically produced concentration gradients of mixed-valence hexacyanoferrate(III,II) sites in these systems. Application of potentials exceeding the differences between the formal potentials for the material's redox transitions leads to simultaneous solid-state electrochemical reactions at the opposing sandwich electrodes; e.g. Prussian Blue is reduced at the negatively charged electrode and oxidized at the positive one. Cyclic voltammetry and chronocoulometry experiments are consistent with the predominantly diffusional character of redox processes in ca 0.2 mm thick powders. The apparent diffusion coefficient for charge propagation during redox conduction between hexacyanoferrate(III,II) sites is estimated ca 10 −10 to 10 −9 cm 2 s −1.