Self-assembled hollow and special core–shell microspheres with WO3 nanosheets for supercapacitor high-performance electrodes

Nanosheets self-assembled WO 3 microsphere with hollow and special core–shell structures was successfully synthesized using hydrothermal, acid etching, and heat treatment methods. The special core–shell WO 3 samples with oxygen vacancies (WN-O) have higher carrier concentration and specific capacitance. The symmetrical supercapacitors (SCs) assembled by WN-O show excellent potential in PVA/KOH gel electrolyte and 3 M KOH aqueous solution. The aqueous symmetric WN-O SCs demonstrated great capacitance (22 mAh g −1 , 1 A g −1 ), extraordinary stability in cycling (118% of the retained capacity after 20 000 cycles, 5 A g −1 ), and a huge energy storage capacity (8.8 Wh Kg −1 of energy density, 400 W Kg −1 of power density). Additionally, the all solid-state symmetric WN-O SCs displayed 52.2% retained capacitance even when the current density increases five times, to 5 A g −1 , and an outstanding capacity of 18.7 mAh g −1 at 1 A g −1 . The distinct core–shell structure and plenty of oxygen vacancies of the WN-O electrode are responsible for its exceptional electrochemical performance. The oxygen vacancies and loose core–shell structure provide more active sites, while the unique cavity structure on the core–shell provides channels for charge transfer and successfully prevents collapse caused by contraction and expansion during charge and discharge processes. The mechanism discovered in this study will contribute to the design of better performing WO 3 energy storage devices in the future.