Electrochemical behavior of manganese oxides on flexible substrates for thin film supercapacitors

Highlights

• Amorphous MnO_x thin films were prepared on flexible substrates by electron beam evaporation.

• The electrochemical behavior of the MnO_x electrode was investigated using cyclic voltammetry.

• A large potential window was employed to examine the possibility of increased energy density.

• Potential window for supercapacitor electrodes should be carefully selected by considering the material properties.

ABSTRACT

Amorphous MnO_x thin films were prepared on flexible substrates by electron beam evaporation, with the aim of using them as electrodes in supercapacitors. The material properties of the MnO_x films were characterized by scanning electron microscopy, atomic force spectroscopy, X-ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy. Cyclic voltammetry was then employed to study the electrochemical reaction mechanism. A large potential window was employed (–0.4 to 1.0 V vs. SCE) to investigate the possibility of increased energy density. When the lower potential limit was expanded from 0.0 to –0.4 V vs. SCE, faradaic redox waves were observed in the expanded region, suggesting that the redox reaction was mainly controlled by cation diffusion into the bulk structure. The galvanostatic cycle test demonstrated that capacitance was retained up to 500 cycles for a potential window of –0.1 to 1.0 V vs. SCE.

Introduction

Electrochemical energy storage technology has continued to grow in importance given the increasing global energy demand and the rising popularity of portable electronic devices. Among potential devices, electrochemical capacitors, specifically supercapacitors, have attracted significant attention because of their high power capability, long cycle life, and relative safety. Pseudocapacitors, also known as redox capacitors, are a class of supercapacitors based on a fast and reversible redox reaction at or near the interface of the active storage material (such as a conducting polymer or metal oxide) and the electrolyte; this kind of interaction can lead to high energy densities. Meanwhile, electrical double layer capacitors store electricity by relying on non-faradaic charge separation at the interface. Among various pseudo-capacitor materials, hydrous RuO₂ exhibits the highest performance to date [1], [2], [3], [4], [5], but it is very expensive, which limits its commercial use. Therefore, there have been numerous efforts to find alternative electrode materials showing pseudocapacitive behavior, with research focused on manganese oxide [6], [7], [8], [9], [10], cobalt oxide [11], [12], [13], and nickel oxide [14], [15], [16], [17].

Among suitable oxides, manganese oxide is particularly promising as an electrode replacement for RuO₂ in that it shows a high theoretical specific capacitance, is of relatively low cost because of its natural abundance, and is environmentally friendly. Crystallinity, defect chemistry, morphology, and porosity play a crucial role in yielding this high specific capacitance, because energy storage only takes place on the surface and the inherent electrical conductivity of manganese oxide is very low.

The manganese oxide materials used in supercapacitors can take the form of either a powder or thin film [18], [19]. At a larger scale, bulky, thick electrodes should be used, which are best fabricated using powder-type active materials; these powders can be both amorphous and crystalline, and can be prepared by hydrothermal, sol-gel, solution combustion, and chemical coprecipitation techniques [18]. Composite electrodes that integrate other metals, conducting polymers, and various carbon materials are also often used to address the conductivity issue [20], [21], [22], [23], [24]. Thin-film or manganese oxide coated electrodes have also been studied to analyze more fundamental scientific phenomena and for their potential use as microscale power sources in integrated systems; these are of particular interest given increasing demand for thin, lightweight, flexible, and wearable devices. MnO_x thin film electrodes have been produced by a number of methods, including sol-gel dip coating [7], [25], anodic/cathodic electrodeposition [26], [27], [28], Mn sputtering/electrochemical oxidation [29], [30], pulsed laser deposition [31], plasma enhanced chemical vapor deposition and evaporation [32].

In this study, an amorphous MnO_x thin film layer was prepared on flexible polyethylene terephthalate (PET) substrates by electron beam evaporation. This technique is a physical vapor deposition method that decomposes the source materials, offering facile control of microstructure and morphology. However, to the best of our knowledge, there is only one previous example in the literature in which manganese oxide was prepared in this manner for use in supercapacitor electrodes; in that case, the primary focus was full cell design and fabrication [33]. Here, we fabricated the MnO_x thin layer and investigated the electrochemical properties of the thin film electrode. To investigate the potential of the materials in increasing energy density of supercapacitors, the cyclic voltammetry measurement was carried out in an extended potential window of 1.4 V and the electrochemical behavior was then discussed.