“…In the context of the analysis about which materials could serve as optimal electrodes for pseudocapacitors, four key factors need to be taken into consideration: (1) high surface area for the redox reactions, (2) doping of the materials to increase the redox state and their conductivity, (3) a wide potential window, and (4) a high charge/discharge rate. − About the different materials, conducting polymers and transition-metal oxides can be found to meet these requirements; however, metal oxides are generally considered the main candidates that can be used as electrode materials in supercapacitors because of the wide variety of oxidation states that allows for redox charge transfer, together with other excellent properties to be used as electrodes in supercapacitor devices for energy storage applications (such as high specific surface area, high conductivity, abundant reserves, environmental geniality, etc.). Therefore, among the different metal oxides, the most widely used oxides as electrodes for pseudocapacitors include tin oxide (SnO 2 ), ruthenium oxide (Ru 2 O), cobalt oxide (Co 3 O 4 ), nickel oxide (NiO), iron oxide (Fe 2 O 3 ), vanadium oxide (V 2 O 5 ), copper oxide (CuO), tungsten oxide (WO 3 ), iridium oxide (IrO 2 ), molybdenum oxide (MoO 3 ), manganese oxide (MnO 2 ), and so forth. − Among these metal oxides, ruthenium oxide has attracted a lot of attention due to its high conductivity, good thermal stability, and excellent electrochemical performance; however, its high environmental toxicity and high cost hinder its commercialization as an electroactive material for supercapacitor applications.…”