Designed Formation of Co₃O₄/NiCo₂O₄ Double-Shelled Nanocages with Enhanced Pseudocapacitive and Electrocatalytic Properties.

Hollow structures with high complexity in shell architecture and composition have attracted tremendous interest because of their great importance for both fundamental studies and practical applications. Herein we report the designed synthesis of novel box-in-box nanocages (NCs) with different shell compositions, namely, Co3O4/NiCo2O4 double-shelled nanocages (DSNCs). Uniform zeolitic imidazolate framework-67/Ni-Co layered double hydroxides yolk-shelled structures are first synthesized and then transformed into Co3O4/NiCo2O4 DSNCs by thermal annealing in air. Importantly, this strategy can be easily extended to prepare other complex DSNCs. When evaluated as electrodes for pseudocapacitors, the Co3O4/NiCo2O4 DSNCs show a high specific capacitance of 972 F g(-1) at a current density of 5 A g(-1) and excellent stability with 92.5% capacitance retention after 12 000 cycles, superior to that of Co3O4 NCs with simple configuration and Co3O4/Co3O4 DSNCs. Besides, the Co3O4/NiCo2O4 DSNCs also exhibit much better electrocatalytic activity for the oxygen evolution reaction than Co3O4 NCs. The greatly improved electrochemical performance of Co3O4/NiCo2O4 DSNCs demonstrates the importance of rational design and synthesis of hollow structures with higher complexity.