Capacitive Behavior of Sodium Ion Pre-Intercalation Manganese Dioxide Supported on Titanium Nitride Substrate

Abstract

The sodium ion pre-intercalation manganese dioxide (Na[Formula: see text]MnO[Formula: see text] is supported on titanium nitride (TiN) substrate to form electroactive Na[Formula: see text]MnO2/TiN electrode through an electrodeposition process in Mn(CH3COOH)2/Na2SO4 precursors with high Mn/Na ratio. MnO2 has a tiled leaf-like structure with a wrinkling morphology. Na[Formula: see text]MnO2 has a cross-linking nanorod structure with a nanoporous morphology, which is beneficial for electrolyte ion diffusion. The density functional theory (DFT) calculation results indicate that Na[Formula: see text]MnO2 reveals the enhanced density of states (DOS) and the lowered band gap than MnO2, which is consistent with higher cyclic voltammetry current response due to superior electroactivity of Na[Formula: see text]MnO2. The Faradaic process involves Na[Formula: see text] adsorption/desorption on the surface of MnO2 by contributing to electrochemical capacitance and Na[Formula: see text] intercalation/deintercalation on the deep interlayer of pre-intercalation Na[Formula: see text]MnO2 by contributing to pseudocapacitance. Concerning the electrolyte ion size effect, both MnO2/TiN and Na[Formula: see text]MnO2/TiN electrodes have higher capacitive performance in Li2SO4 electrolyte than that in Na2SO4 and K2SO4 electrolyte due to more feasible Li[Formula: see text] diffusion. When MnO2 is converted into Na[Formula: see text]MnO2, the capacitance at 2.5 mA cm[Formula: see text] increases from 351.3 mF cm[Formula: see text] to 405.6 mF cm[Formula: see text] in Na2SO4 electrolyte and from 376.3 mF cm[Formula: see text] to 465.1 mF cm[Formula: see text] in Li2SO4 electrolyte. The conductive TiN substrate leads to high rate capacity retention ratio of 50.7% for MnO2/TiN and 49.5% for Na[Formula: see text]MnO2/TiN when current density increases from 0.5 mA cm[Formula: see text] to 5 mA cm[Formula: see text]. So, Na[Formula: see text]MnO2/TiN with sodium ion pre-intercalation exhibits the improved capacitive performance in Li2SO4 electrolyte to act well as the promising supercapacitor electrode.