Electrochemical Preparation of Manganese Hexacyanoferrate/Carbon Nanotubes Composite Film on Graphite Substrate via a Two-Step Method

As the intermediate energy storage devices between conventional batteries and dielectric capacitors, supercapacitors have attracted significant attention because of its advantages, such as high power density, fast charging/discharging, long cycle life and superior reversibility, etc. Nowadays, exploiting an electrode material with excellent electrochemical properties and high conductivity is very important for the development of supercapacitors. Prussian blue (PB), namely iron hexacyanoferrate with FCC crystal structure, is an important kind of mixed-valence compoundsand considered as one of the oldest synthetic coordination compounds. Manganese hexacyanoferrate (MnHCF) has attracted attention in some field due to the abundant raw materials, unique tunnel structure, low cost and higher specific area, etc. Herein, MnHCF was electrochemically deposited on CNTs-coated graphite via a pulse galvanostatic electrodeposition technique at 100 μA cm−2 with both on-time and off-time equal to 0.5 s, to prepare MnHCF/CNT composite electrode. The CNTs-coated graphite was prepared by electrophoretic deposition (EPD) of functionalized CNTs onto the graphite substrate from ethanolic suspension (50 ml) containing 12.5 mg of functionalized CNTs, 50.0 mg of Al(NO3)3·6H2O as the charging agent and 1.0 mg of polyvinylpyrrolidone (PVP, 25000 g/mol) as the stabilizing agent. EPD was performed by applying a constant voltage of 30 V between graphite working electrode and 316 L stainless steel counter electrode spaced 10 mm apart for 30 min. After 14th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 12- 13 Dec, 2018 286 EPD, MnHCF was electrodeposited from the solution containing 2 mM K3Fe(CN)6, 2 mM MnSO4.H2O and 0.5 M KCl. The crystal structure of the composite film was characterized by XRD (Fig. 1). The diffraction peaks located at 17.5, 24.8, 35.5, 39.5 and 44 were assigned to planes (200), (220), (400), (420) and (422) of the face-centered cubic MnHCF, respectively (JCPDS Card no.04-0850). Fig. 1. XRD patterns of the MnHCF/CNT composite film. FESM images related to the obtained CNTs film and MnHCF-coated CNTs film are shown in Fig. 2 (a,b) and (c,d), respectively. As indicated, a uniform structure with an open nanoporous network and without any crack is obtained for CNTs film. Also, it can be seen that MnHCF nanoparticles were homogeneously distributed and coated on the CNTs matrix, to form a porous hybrid nanostructure. Fig. 2. SEM images of CNT (a,b) before and (c,d) after coating MnHCF.