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 MnO 2 (K x MnO 2 and Na x MnO 2 ) nanostructured thin films with the help of successive ionic layer adsorption and reaction (SILAR) method. K x MnO 2 and Na x MnO 2 thin films were grown directly on the stainless-steel substrate at room temperature using different reducing agents like manganese sulfate (MnSO 4 ) and sodium borohydride (NaBH 4 ). Reducing agents control reduction of MnO 4− ions, morphological evolution, and preinsertion of Na + ions in MnO 2 matrix. The plausible growth mechanism was proposed behind the occurrence of different morphologies. Due to preinsertion of alkali ions, nonstoichiometric MnO 2 phase was formed and confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy studies. The electrochemical measurements of Na x MnO 2 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 K x MnO 2 thin films. Enhanced surface capacitance of Na x MnO 2 thin films corresponds to superior electric double layer capacitance (EDLC) and surface redox reactions between Mn 3+ /Mn 4+ ions. On the contrary, K x MnO 2 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.

Green Strategy for the Synthesis of K+ Pre-inserted MnO2/rGO and Its Electrochemical Conversion to Na-MnO2/rGO for High-Performance Supercapacitors

“…The Raman spectra of all the samples show three intense peaks at about 514, 580, and and ∼406 cm −1 were found to be enhanced due to the intercalation of alkali metal ions such as Na + within the crystal structure. 46,64 A small peak at 735 cm −1 was also observed, which could be attributed to the symmetric vibrational mode of MnO 2 . 46 The D band located in K-MnO 2 /rGO and Na-MnO 2 /rGO samples at 1355 cm −1 and the G band at 1598 cm −1 are the characteristic Raman shifts of rGO.…”

“…46,64 A small peak at 735 cm −1 was also observed, which could be attributed to the symmetric vibrational mode of MnO 2 . 46 The D band located in K-MnO 2 /rGO and Na-MnO 2 /rGO samples at 1355 cm −1 and the G band at 1598 cm −1 are the characteristic Raman shifts of rGO. The D band corresponds to the presence of defects and disordered carbon in graphite layers, and the G band represents the in-plane stretching bond motion of the carbon pairs of sp 2 -hybridized atoms.…”

“…Previously, we proved that when KMnO 4 reacted with a reducing agent, it leads to pre-insertion of K + ions to the MnO 2 structure. 14,46 Similarly, during the green synthesis of K-MnO 2 , KMnO 4 dissociates into K + and MnO 4 − in water, and thereafter, reduction of MnO 4 − ions occurs by aloe vera leaf extract. Simultaneously, the K + ions present in the vicinity of the reacting species are inserted inside the MnO 2 matrix.…”

“…The CV and GCD curves were used to determine various supercapacitive parameters. Using CV curves, the C sp values of the prepared electrodes were calculated using eq 1 46,50…”

“…The other minor XPS peaks of MnO 2 positioned at 641.6 and 653.1 eV could be responsible for the existence of Mn 3+ ions and/or the pre-insertion of alkali metal ions. 46,70 ■ MORPHOLOGICAL ANALYSIS Scanning Electron Microscopy. The surface morphology of all the samples was investigated using SEM (Figure 3).…”