Controllable reduction of NiCoO2@NiCo core-shell nanospheres on CNTs for high-performance electrochemical energy storage

The performances of energy storage devices are strongly dependent on the electrode materials. Owing to the high theoretical capacity, NiCoO2 is a promising transition metal oxide for supercapacitors. Despite many efforts have been devoted, it still lacks of effective methods to overcome its shortcomings such as low conductivity and poor stability, in order to achieve its theoretical capacity. Herein, utilizing the thermal reducibility of trisodium citrate and its hy-drolyzate, a series of NiCoO2@NiCo/CNT ternary composites in which NiCoO2@NiCo core-shell nanospheres deposited on CNT surface with adjustable metal contents are synthesized. Benefiting from the enhanced synergistic effect of both metallic core and CNTs, the optimized composite exhibits an extremely high specific capacitance (2660 F g-1 at 1 A g-1, the effective specific capacitance of the loaded metal oxide is 4199 F g-1, close to the theoretical value), an excellent rate performance and stability, when the metal content is about 37%. After depolarized calcu-lation, the energy storage mechanism of the composite is reasonably analyzed. By controlling the contents of hexa-methylenetetramine, trisodium citrate and CNTs in the reactant, the roles of them are distinguished. This study reveals an efficient novel strategy for transition metal oxides to maximize the electrochemical performances.