(Invited) Homo- and Heteromatallic Cyanide Bridged Networks and Derived Materials for Selected Electrochemical Applications Involving Enhanced Charge Transport and Storage
Authors: Kulesza Pawel J. , Rutkowska Iwona A. 
Published: November 2022

Hexacyanoferrates and related cyanometallates exhibit model electron transfer properties that are of importance to many electrochemical and related applications. In particular homo- and heterometallic cyanide bridged networks and derived materials have proven to exhibit very rich and diverse electrochemical properties. Their redox properties can be tuned by adjusting stoichiometry and oxidation state of the constituent metal centers, incorporation of interstitial ions, or preparation methods. Polynuclear cyanometallates are promising open-framework systems for low-cost electrochemical energy storage applications. Both soluble and insoluble analogues of Prussian blue have been explored as cathode materials with lithium, sodium, potassium, magnesium, calcium, and even zinc intercalated ionic carriers. Electrochromic devices are another promising area for employing the unique properties of cyanometallates. Prussian blue itself exhibits electrocatalytic properties toward hydrogen peroxide, and it has been used for biosensing and amperometric detection of glucose, L-cysteine, and glutamate. Also removal of radioactive cesium ions from contaminated water has been successfully achieved with Prussian blue and its metal substituted analogues.

It is well-established that the choice of redox-active charge-storage material has a significant impact on the performance of a redox flow battery. The concentration of redox centers and their reaction kinetics have an influence on the available current densities and, thus, the power of the device. Remembering the requirement of good solubility of redox species, the semi-solid slurry approach (provided that the dispersion is homogeneous) represents another effective way to improve the volumetric capacity of the redox electrolyte (i.e. of the electrolyte with dissolved redox couples). An interesting approach to improve current densities involves application of circulating suspensions of electroactive materials. Prussian blue and its metal (Fe, Co, Ni, etc.) substituted analogues, which are electroactive mixed-valence inorganic systems, exhibit very rich and diverse electrochemistry. Their electrochemical properties that can be tuned through the variation of the material composition. Special attention will be paid to the formation of stable colloidal suspensions of truly mixed-valence fast-conducting Berlin Green, iron(III) hexacyanoferrate(III,II), together with multi-layered clay-like nickel(II) hexacyanoferrate(III) structures characterized by fast potassium counter-cation motion. It can be hypothesized that the proposed system could serve as the catholyte redox suspension containing large population of mixed-valence redox centers and capable of fast charge propagation and, consequently, yielding fairly large current densities. The materials can also be explored for sorption of large concentrations of Zn2+ ions and for the improvement of the Zn2+/Zn chemistry. While the application of Zn2+/Zn as the anode active system is well established in redox flow batteries, we are going to address and minimize limitations that include the efficiency of zinc deposition and the hydrogen evolution reaction taking place at the potentials where Zn is electrodeposited. The increase of current density could be achieved not only by reducing the viscosity of the electrolyte, thus accelerating charge-carrier transport, but also – by referencing to our experience with mixed-valent nickel hexacyanoferrate system as charge relay for dye-sensitized solar cells – through improvement of the dynamics of charge propagation by improving mobility of charge compensating counter-ions.