Sodium Charge Storage in Thin Films of MnO₂Derived by Electrochemical Oxidation of MnO Atomic Layer Deposition Films

Abstract

Sodium charge storage in ultrathin MnO2 films was studied using cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) measurements. The MnO2 films were fabricated by electrochemical oxidation of MnO films grown by atomic layer deposition (ALD). CV analysis confirmed that oxidation of MnO to MnO2 involved two moles of electrons per mole of Mn in the MnO ALD film. Scanning electron microscopy (SEM) images revealed that electrochemical oxidation of MnO led to the formation of MnO2 nanosheets. EQCM measurements suggested that Na+ cations participate in charge storage in MnO2. X-ray photoelectron spectroscopy (XPS) experiments measured sodium in MnO2 after both positive/anodic and negative/cathodic voltage sweeps. The stoichiometry was Na0.25MnO2 after negative/cathodic voltage sweeps. Approximately one-half of the sodium was removed after positive/anodic voltage sweeps. The areal capacitance increased progressively with initial MnO ALD film thickness. This increase in areal capacitance is consistent with bulk charge storage in MnO2 or higher surface area of MnO2 nanosheets resulting from larger MnO ALD film thicknesses. Experiments at varying scan rates indicated that charge storage in MnO2 originates from a combination of capacitive and diffusive processes. Bulk charge storage makes a significant contribution to total charge storage in the thicker MnO2 films.