Growth of cobalt hexacyanoferrate particles through electrodeposition and chemical etching of cobalt precursors on reticulated vitreous carbon foams for Na-ion electrochemical storage

In the present work, particles of cobalt hexacyanoferrate (CoHCF) were deposited on reticulated vitreous carbon (RVC) foam surface to overcome the lack of conductivity of CoHCF and take advantage of their capacity for Na⁺ ions storage. The deposition was carried out through direct formation of the CoHCF by cyclic voltammetry, and the chemical transformation of cobalt hydroxide and cobalt hydroxide carbonate films obtained by electrochemical and hydrothermal synthesis, respectively, to CoHCF by immersion in an aqueous solution containing [Fe(CN)₆]^3- ions. The prepared composites were characterized through FTIR, Raman, XRD and SEM-EDS techniques to obtain information about their morphology, composition and structure. The results indicated that the employ of RVC foam leads to well-distributed CoHCF particles connected to its surface, offering abundant sites for ion-storage. Moreover, their electrochemical performance was evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). Benefiting from the highly conductive substrate, the charge storage mechanism of CoHCF and RVC, and the lower impedance originated at the CoHCF/RVC interface, the composites exhibited an enhanced performance at high potential scan rates or charge and discharge rates. Notably, improved electrochemical utilization of CoHCF was reached through the methods based on cobalt precursors, such composites displayed higher storage capacity. The CoHCF(-1.1 V)/RVC electrode synthesized by electrochemical-chemical synthesis exhibits a high specific capacity of 816 mC cm⁻³ at 0.25 mA cm⁻³ and 36% (296 mC cm⁻³) of its initial capacity was maintained at 5 mA cm⁻³, which improves 2.8 times the capacity achieved by CoHCF(50 mVs⁻¹)/RVC through electrochemical strategy and the 19% of capacity retention from CoHCF(1 mmol)/RVC composite prepared by the hydrothermal-chemical method. Therefore, the CoHCF(-1.1 V)/RVC electrode can be potentially employed as a cathode in the recent sodium-ion batteries technology.