Fiber Mesoporous Spherical Ni(OH) 2 @ g-C 3 N 4 Heterostructures with Amorphous Shell as Battery- type Capacitive Cathode Enable High-performance Hybrid Supercapacitor

Ni(OH) 2 -based materials are widely concerned as advanced energy storage materials, but their poor cycle performance makes their practical application and commercialization slow. Herein, Ni(OH) 2 @g-C 3 N 4 core-shell (crystal and amorphous) heterojunctions are synthesized. These heterojunctions enhance the specific surface area and ionic diffusion, especially the heterojunction formed by the amorphous shell shows more robust performance. It is found that this heterojunction mainly uses the fast reversible Faraday reaction of battery type as the primary energy storage mechanism. The as-prepared core-shell heterojunction showed nearly double specific capacitance and excellent cycle stability (578.54 F g -1 and 109.30% retentions after 5,000 cycles) over pristine Ni(OH) 2 (285.75 F g -1 and 101.55% retentions after 5,000 cycles). Finally, an efficient hybrid supercapacitor is developed via Ni(OH) 2 @g-C 3 N 4 //AC, which combines the advantages of the battery and supercapacitors. The hybrid supercapacitor device shows sparkled capacitance (311.83 F g -1 ) and striking cycle performance (84.33% retentions after 10,000 cycles). In particular, the energy density of 77.97 Wh kg -1 can still be provided while maintaining the power density of 1201.08 W kg -1 . This emerging heterostructure offers new insight into Ni(OH) 2 -based materials in energy applications.