Electrochemical Characterization of Metal Oxides for Supercapacitor Application

In recent years, supercapacitors have attracted a lot of attention as potential energy storage systems due to their promising properties for energy storage and power supply. They can provide much higher power density compared to batteries and higher energy density compared to conventional capacitors. It opens up the possibility for their usage in different applications ranging from portable devices to electric vehicles.

In the most cases, commercially available supercapacitors use activated carbon as active material. Its charge storage mechanism is based on electrochemical double layer charging/discharging and capacitance is limited by the active surface area. The maximum capacitances achieved for activated carbon are approximately 200 F g^-1. To overcome this limitation there are efforts to increase capacitance by using metal oxides or conductive polymers where charging/discharging mechanism includes fast redox reaction of material and, as a result, much higher specific capacitance is obtained. Considering their charge storage mechanism these materials are classified as ˝pseudocapacitors˝.

Different transition metal oxides show capacitive properties such as oxides of ruthenium, manganese, iridium, cobalt, nickel, iron etc. Ruthenium oxide is known for its superior capacitive properties especially in an amorphous hydrous form. However, the high cost of ruthenium oxide constrain scientist to investigate other metal oxides. Presently, one of the most promising replacements is manganese oxide because of low cost, low toxicity and it reaches capacitances exciding 200 F g^-1.

Capacitive properties of metal oxides depend on both electron and ion conductivity. To increase electronic conductivity the active material is usually mixed with a small amount of carbon. A higher percentage of crystalline structure also increases electronic conductivity. In contrast, ionic conductivity will increase with amount of amorphous hydrous form, as well as, with ability of material to incorporate cation within the crystalline structure. Thus, in the case of different polymorphs of MnO₂, capacitance depends strongly on the size of channels within the material and ability for cation intercalation. An important parameter for capacitive properties is a morphology and a thickness of the active material. Furthermore, different nano-structures open up possibility for the design of new, high-performance electrodes with maximum charge efficiency and power density.

Different electrochemical methods can be used in order to characterize metal oxide capacitive properties and reaction mechanism. Characteristic of the metal oxide charge transfer reaction is nearly rectangular response in cyclic voltammogram in wide potential range and linear potential change during charging/discharging process. The operative potential range is defined by reversibility of charge transfer reaction and deterioration of the electrode during cycling. The redox reaction usually involves protons or alkali metal cations exchange process that can be easily detected by Electrochemical Quartz Crystal Microbalance (EQCM) measurements. That process gives deeper insight into reaction mechanism and enables to find out optimal conditions in order to improve capacitive properties.