Electrodeposition of Prussian blue analogues as versatile cation hosts

Efficient energy storage solutions are quintessential for diverse applications - renewable energy harvesting, transportation, portable electronics, automation etc. Regardless of category specific requirements, high power and high energy are inevitable requirements. In general, the bottle neck is always the energy/power density of the cathode materials. Layered metal oxides suffer from issues like low redox potential, irreversible structural changes at high potentials (>3V), poor thermal stability and cationic disorders whereas polyanionic compounds exhibit low gravimetric capacity (owing to their large molecular weight) and low cycling stability.

In this regard, Prussian blue analogues (PBAs) are promising alternative high voltage host materials which have shown promising results for alkali metal ions Na⁺ ,K⁺ and even divalent metal cations like Ca²⁺ and Mg²⁺. They are coordination polymers having promising capacity (e.g. ~ 170 mAh/g for Na⁺), excellent rate capability (up to 50 C in certain cases), and high discharge potential (~3.5 – 4.2 V), along with synthetic ease for development of wide variety of compounds. PBA’s or metal hexacyanoferrates have the general formula, A_xM[Fe(CN)₆]_y which can reversibly store mobile cations like Li⁺, Na⁺, K⁺ etc. Since M can be replaced by many transition metals and even solid solutions of two different metals, it is possible to realize different types of PBAs depending upon the choice of the starting precursors. Depending on the choice of the transitional metal atoms, this can provide two redox active centers per formula unit of PBAs i.e M^+2/+3 and Fe^+2/+3, which can reversibly store alkali metal ions.

electrodeposit PBAs, where their precipitation can be triggered electrochemically on the surface of the electrodes by choosing appropriate reactants and electrochemical conditions. Solution based electrodeposition of PBA’s provides more synthetic control and possibility to engineer these open framework structures compared to a solution-based process.

This PhD project will focus on understanding and optimisation of electrodeposition process of PBAs with focus on material composition, structural integrity and crystal water content. Various parameters like solvent additives, current/potential, concentration of the reactants will be evaluated. This will be followed by evaluation of electrochemical properties in non-aqueous electrolytes for alkali metal ion storage.