Transition Metal Oxides/Hydroxides
Although noble metal oxides and their composites can increase the specific capacitance in supercapacitors, their high cost greatly limits their application in commercial production. Therefore, researchers are working hard to explore replacing noble metal oxides with other transition metal oxides/hydroxides as electrode materials. Some cheap metal oxides such as NiO, MnO2, Co3O4, SnO2, V2O5, etc. have similar properties to RuO2, and are rich in resources and cheap, and have attracted extensive attention from researchers. Therefore, transition metal oxides with supercapacitive properties have good development prospects as an electrode material for supercapacitors with low price and good electrochemical performance. The commonly used transition metal oxide electrode materials mainly include NiO/Ni(OH)2, MnO2, Co3O4, V2O5 and so on.
Cobalt Oxide and Cobalt Hydroxide
Among various metal oxides, Co3O4 has been regarded as an advanced material with potential to replace RuO2 because of its low cost, high redox activity, theoretical specific capacitance up to 3560 F·g-1, good reversibility and environmental friendliness.
Lin et al. prepared ultrafine Co2O3 electrode active materials using the alkoxide hydrolysis method, and the specific capacity of the single electrode reached 246 F·g-1. The CoOx xerogel synthesized by the alkoxide sol-gel method can obtain a maximum specific capacity of 291 F·g-1 at 150 oC, which is very close to the theoretical value of 335 F·g-1, and the cycle performance is stable.
In addition, scientists have also been working on the synthesis of Co3O4 nanostructures with different morphologies, such as nanolayers, nanowires, nanotubes, nanorods, gels, and microspheres. The specific capacitance of Co3O4 nanolayer array can reach 2735 F·g-1, and it has fast ion and electron transport characteristics due to its unique three-dimensional layered structure. The mesoporous Co3O4 nanowire array can achieve a specific capacitance of 1160 F·g-1, coated on nickel foam, with a retention rate of 90.4% after 5000 cycles. Cobalt oxide nanotubes also show good capacitive properties due to their unique structure and large specific surface area.
In order to improve the conductivity of Co3O4 electrodes, it is combined with various carbonaceous materials and applied in supercapacitors. Compared with pure Co3O4, the cobalt oxide/carbon nanotube composite material synthesized by co-precipitation method has a significant increase in specific capacitance to 418 F·g-1, which is due to the synergistic effect between Co3O4 and nanotubes. The aqueous solution of graphene/Co3O4 composite can reach a maximum specific capacitance of 243.2 F·g-1. The three-dimensional graphene foam-based Co3O4 nanowires have a specific capacitance of 1100 F·g-1 and excellent cycle stability. The flexible Co3O4/graphene/carbon nanotube paper composite electrode exhibits a specific capacitance of 378 F·g-1.
The specific capacitance produced by potential deposition of stainless steel on cobalt hydroxide nanolayer is 890 F·g-1. The porous cobalt hydroxide/nickel composite material has improved conductivity due to the introduction of nickel, and the specific capacitance can reach 1310 F·g-1. The sea urchin-like mesoporous cobalt hydroxide nanowires exhibit a specific capacitance of 421 F·g-1, and the higher specific capacitance of the material is attributed to its ordered structure, hierarchical porosity, and good electrical conductivity. In order to further improve its electrochemical performance, some people use conductive carbon materials, such as carbon nanotubes and graphene, to construct cobalt hydroxide composite nanostructures. The unbonded cobalt hydroxide/carbon nanotube array electrode yields high capacitance (12.74 F·cm-3) and excellent high-rate performance. Graphene/cobalt hydroxide composite exhibits high specific capacitance (972.5 F·g-1) compared with pure cobalt hydroxide (726.1 F·g-1). Although the composites of cobalt hydroxide and its derivatives show high specific capacitance, the low content of active materials and narrow potential range will greatly limit their practical application in supercapacitors.
