Optimized Double Manganese Oxide Deposition for Enhanced Electrochemical Capacitor Performance

Abstract

Manganese oxide pseudocapacitive materials are deposited using a novel procedure involving first depositing a heat-treated base layer followed by a hydrous top layer. The ratio of heat-treated to hydrous film is optimized to generate films that excel across a wide range of electrochemical capacitor (EC) properties and to elucidate the mechanisms underpinning the film performance. We show that a thin heat-treated base layer imparts low resistance, high energy efficiency and power-capability and enhanced film stability in a large potential window. These benefits are due to an improved oxide-current collector connection; however, if the layer is too thin (<25 nm), the stability is lost. Conversely, heat-treatment causes more parasitic oxidation reactions during initial film cycling, though these reactions are mitigated with a thick hydrous top layer. This hydrous film also affords high capacitance, capacity, coulombic efficiency and energy density due to an abundance of hydrated sites in the oxide to facilitate the cation insertion/removal needed for pseudocapacitance. The double-deposited films also show less self-discharge. We find that a dry:wet film ratio of 5:95 results in optimal film performance. While this novel dry-wet double-deposition has been demonstrated with manganese oxide, we anticipate similar performance benefits with other pseudocapacitive materials.

Potential-Dependent Electrochemical Impedance Spectroscopy as a Powerful Tool for Evaluating Supercapacitor Electrode Performance

“…The changes in R CT are due to the thermodynamic potential that allows the reactions to occur and was also reported for Nb 2 O 5 and LiFePO 4 electrodes. 4 Although some works have considered the HF loop as only an effect of contact resistance between current collector and active material, 15,56 the electrode resistance is included in solution resistance (R S ) and an increase in electrode resistance should also reflect in shifting the Z′ intercept to higher values according to Liu et al 57 The modeling of equivalent electrical circuits (EEC) with R CT in parallel with a capacitor or a constant phase element (CPE) is often used for quantitative evaluation of the process, where the capacitor-like element is associated to the double layer capacitance. 58,59 Therefore, if considering the double layer capacitance as a HF response the C′ obtained at LF should be the contributions of the pseudocapacitance summed with C DL.…”