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.

Sputtered manganese oxide thin film on carbon nanotubes sheet as a flexible and binder-free electrode for supercapacitors

“…The CV of CNTs/MnO 2 in 1‐M Na 2 SO 4 electrolyte displayed shape without redox peaks, demonstrating pseudocapacitive behavior with fast and reversible surface reactions indicating good capacitive characteristics . However, the CV curve of the supercapacitor based on CNTs/MnO 2 in 1‐M H 2 SO 4 electrolyte revealed shapes with one pair of noteworthy redox peaks appearing at around 0.24 V (cathodic) and 0.6 V (anodic), typical behavior of the integration of the EDLC of CNTs and the pseudocapacitance of MnO 2 . Additionally, the area of the CV loop of CNTs sheet is much smaller than that of CNTs/MnO 2 composite.…”

“…This process is also transferred to the charge‐discharge curves at low scan rates (5 and 10 mV/s) because these curves have no sawtooth shape due to solubility of MnO 2 . The higher capacitance than the theoretical value (1370 F/g) (especially for low scan rates) is linked to reversible redox reactions due to solubility of MnO 2 in solution and accompanied by the insertion/deinsertion of alkali Na + cation or H + protons from the electrolyte . It is then recommended to use Na 2 SO 4 as electrolyte to minimize the dissolution of MnO 2 film.…”

“…On another hand, transition metal oxides (eg, Co 3 O 4 , MnO 2 , and RuO 2 ) and conducting polymers (polyaniline, polypyrrole, etc) were largely used in pseudocapacitors as electrode materials. Among the several pseudocapacitive transition metal oxides materials, manganese dioxide (MnO 2 ) has displayed a lot of interesting properties for its natural abundance, excellent electrochemical capacitive properties, environmental friendliness, wide potential range, low cost, and high theoretical specific capacitance 1370 F/g . However, MnO 2 suffers from poor ionic and electronic conductivity (10 −5 −10 −6 S/cm) and can dissolve, which limit its practical utilization in supercapacitors …”

High‐Performance 2.6 V Aqueous Asymmetric Supercapacitors based on In Situ Formed Na0.5MnO2 Nanosheet Assembled Nanowall Arrays

“…Several groups found that preinsertion of cations (such as Na + and K + ) in MnO 2 can effectively improve supercapacitive performance. [25] In most of previous works on cation preinserted MnO 2 , the cation content is low and the cation/Mn atomic ratio is usually less than 0.3, [26,27] which limits pseudocapacitance contribution from MnO 2 . [25] In most of previous works on cation preinserted MnO 2 , the cation content is low and the cation/Mn atomic ratio is usually less than 0.3, [26,27] which limits pseudocapacitance contribution from MnO 2 .…”

Band Diagram and Rate Analysis of Thin Film Spinel LiMn2O4 Formed by Electrochemical Conversion of ALD‐Grown MnO

“…The fraction of diffusive charge storage, f d , is determined using the expression to separate capacitive and diffusive charge storage using expressions of current versus sweep rate, [40,41] as described in more detail in the Supporting Information of ref. [14].…”