Manganese dioxide (MnO2) is considered to be one of the most promising
materials for energy storage systems including batteries and supercapacitors.32,
58-61 Compared to batteries, supercapacitors have much higher power density
(10-100 times higher than batteries), fast charge rate (charge a supercapacitor
takes only a few seconds to minutes) and excellent cycling stability (cycle
number of more than 1 million cycles). This makes supercapacitors ideal devices
for many critical applications such as consumer electronics, hybrid electric
vehicles, and smart grid storage. 2, 62 However, their relatively lower energy
density (~10 times lower than that of batteries) has thus far limited their widespread application as energy storage devices. 5 Carbon based supercapacitors
show low charge storage capacity due to the limited electrolyte ion adsorption
and desorption on the surface of the carbon. Therefore, replacing carbon
materials in commercial supercapacitors by high capacity materials like MnO2
can be an effective strategy to increase the energy density of supercapacitors,
thus paving the way for their adoption in a variety of applications. However, oxide
50
electrodes have the fatal disadvantage of poor cycling performance, an issue that
so far has not been sufficiently investigated.