Synthesis of Co3O4/Fe2O3 double-shelled nanocages with enhanced pseudocapacitance

Supercapacitors (SCs) have garnered significant attention in recent years owing to their elevated power density, rapid charge-discharge characteristics and exceptional cycle durability. The large scale applications of frequently employed electrode material transition metal oxides (TMOs) are hindered by their low conductivity and huge volume expansion. Therefore, rational design and preparation of electrode materials with a multi-shell structure represent an effective approach to enhance the electrochemical performance of SCs. In this paper, a yolk-shell structure of cobalt-based zeolitic imidazolate framework material (ZIF-67)/Co-Fe Prussian blue analogue (PBA) is prepared via an [Fe(CN)6]3- anion exchange method using ZIF-67 as a template. The as-prepared ZIF-67/Co-Fe PBA is converted into Co3O4/Fe2O3 double-shelled nanocages (DSNCs) via a slow calcination method. In 2 M KOH electrolyte, the Co3O4/Fe2O3 DSNC electrode demonstrates a remarkable specific capacitance of 1229.7 F g-1 at 10 A g-1. The double-shell structure displays a more accessible active surface, which can augment the interaction between the electrode and electrolyte, shorten the OH- diffusion path, and furnish more reaction sites for the rapid redox reaction, thus resulting in excellent specific capacitance. The capacitance retention rate of the Co3O4/Fe2O3 DSNC electrode is 77% after 5000 cycles. The double-shell structure supplies enough space to accommodate volume expansion/contraction during cycling and ultimately achieves a stable long-term cycle life. This innovative approach presents a novel concept for advancing high-performance SCs based on PBAs.