Rational synthesis of 3D coral-like ZnCo2O4 nanoclusters with abundant oxygen vacancies for high-performance supercapacitors.

ABSTRACT

Transition metal oxides with high theoretical capacitance are regarded as desired electrode materials for supercapacitors, however, the poor conductivity and sluggish charge transfer kinetics constrain their electrochemical performance. The three-dimensional (3D) coral-like ZnCo2O4 nanomaterials with abundant oxygen vacancies were synthesized through a facile hydrothermal method and chemical reduction approach. The introduced oxygen vacancies can provide more active sites and lower the energy barrier, thereby facilitating the kinetics of surface reactions. Furthermore, the abundant oxygen vacancies in metal oxides can function as shallow donors to facilitate charge carrier diffusion, resulting in a faster ion diffusion rate and superior electrochemical conductivity. The electrochemical performance of ZnCo2O4 was optimized by the introduction of oxygen vacancies. The ZnCo2O4 nanoclusters, reduced by 0.5 M NaBH4 (ZnCo2O4-0.5), exhibit a specific capacitance of 2685.7 F g-1 at 1 A g-1, which is nearly twice that of the pristine ZnCo2O4 (1525.7 F g-1 at 1 A g-1). The ZnCo2O4-0.5 exhibits an excellent rate capacity (81.9% capacitance retention at 10 A g-1) and a long cycling stability (72.6% specific capacitance retention after 10 000 cycles at 3 A g-1). Furthermore, the asymmetric supercapacitor (ASC, ZnCo2O4-0.5 nanoclusters//active carbon) delivers a maximum energy density of 50.2 W h kg-1 at the power density of 493.7 W kg-1 and an excellent cycling stability (75.3% capacitance retention after 3000 cycles at 2 A g-1), surpassing the majority of previously reported ZnCo2O4-based supercapacitors. This work is important for revealing the pivotal role of implementing the defect engineering regulation strategy in achieving optimization of both electrochemical activity and conductivity.