SILAR Grown K⁺ and Na⁺ Ions Preinserted MnO₂ Nanostructures for Supercapacitor Applications: A Comparative Study

Abstract

Herein we have demonstrated facile synthesis approach to grow K+ and Na+ ions preinserted MnO2 (KxMnO2 and NaxMnO2) nanostructured thin films with the help of successive ionic layer adsorption and reaction (SILAR) method. KxMnO2 and NaxMnO2 thin films were grown directly on the stainless-steel substrate at room temperature using different reducing agents like manganese sulfate (MnSO4) and sodium borohydride (NaBH4). Reducing agents control reduction of MnO4– ions, morphological evolution, and preinsertion of Na+ ions in MnO2 matrix. The plausible growth mechanism was proposed behind the occurrence of different morphologies. Due to preinsertion of alkali ions, nonstoichiometric MnO2 phase was formed and confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy studies. The electrochemical measurements of NaxMnO2 thin films demonstrated and 1.45 times superior specific capacitance at a higher scan rate (100 mV/s) and current density (32 A/g) than those of KxMnO2 thin films. Enhanced surface capacitance of NaxMnO2 thin films corresponds to superior electric double layer capacitance (EDLC) and surface redox reactions between Mn3+/Mn4+ ions. On the contrary, KxMnO2 thin films showed higher diffusion controlled reactions, which were supported by electrochemical impedance spectroscopy (EIS) data. Insertion of Na+ ions was found to be advantageous over K+ ions, because it provides higher specific capacitance values, specific morphology, improved capacitive retention, accessibility of more electroactive area, stable salt concentration and electrical conductivity of the electrolyte, fast and dynamic utilization of electrode material and higher conservation of energy.