Zn Ion-Doped Amorphous NiWO4 Nanospheres as Cathode Material for High-Performance Asymmetric Supercapacitors

Zn ion-doped amorphous NiWO4 nanospheres (named ZNWO) were successfully synthesized using a simple chemical coprecipitation method. The specific surface area and pore size of NiWO4 were significantly improved owing to the Zn atom dopant. Compared with NiWO4, the conductivity of ZNWO increases and, more importantly, the energy gap decreases. This is mainly because Zn atoms are used as dopants to generate energy levels in the forbidden band of NiWO4, which decreases the energy absorption of electronic transitions. When tested under a traditional three-electrode system, the obtained Zn-doped NiWO4 with 10 at.% Zn (labeled ZNWO-10) delivered an impressive specific capacitance of 653.75 F g-1 at a current density of 1 A g−1, with a capacitance retention of 64.1% at 1 A g-1 over 1000 cycles. However, pristine NiWO4 displayed an inferior capacitance retention of 6.4% under the same conditions. Studying confirms the availability of electronic structure control in NiWO4 by introducing Zn atoms for pseudocapacitance (PC). When it is combined with an activated carbon (AC) electrode, the ZNWO//AC device exhibits a maximum energy density of 34.4 W h kg-1 at a power density of 525 W kg-1 and excellent cycling stability, with 116.3% retention after 1000 cycles. This study provides new options and an important foundation for application in asymmetric supercapacitors.