Nanocrystal-iron oxide fiber yarn electrodes with ultralong cycling life for flexible supercapacitors

Aqueous quasi-solid-state flexible supercapacitors (FSCs) as intelligent energy storage devices offer more options for wearable electronics design. Among many electrode materials, nanocrystalline transition metal oxides have attracted extensive attention because of their high electrochemical capacity and good charge-discharge cycle stability, which is conducive to energy storage devices. Herein, a flexible nanocrystal-Fe3O4@carbon fiber yarn electrode (NC-Fe3O4@CFY) with unique crystallite size, lattice parameter, electrical conductivity, and electrochemical properties is reported. NC-Fe3O4@CFY electrode has excellent electrochemical performance and cycle stability. Meanwhile, the FSCs were made of the NC-Fe3O4@CFY as an anode, and NC-Fe2O3@CFY (derived from anode) as a cathode in PAM-Li2SO4 hydrogel electrolyte. It demonstrates a high working voltage plateau and specific capacitance (2.4 V, 3790 mF cm(-3) at 2 mV s(-1)), ultrahigh coulombic efficiency (similar to 99.6 % after 50,000 cycles), and impressive energy density of 3.03 mWh cm(-3). Moreover, the FSCs exhibit ultralong cycling life with 100.3 % capacity retention after 50,000 cycles, as well as outstanding mechanical flexibility (105.2 % after 10,000 bending cycles), significantly outperforming most flexible supercapacitors. The superior performance benefited from the synergistic effect of the porous structure of the NC-iron oxide electrodes and the spatial confinement effect of the PAM-Li2SO4 polymer hydrogel network. This work opens a new door for the practical applications of nanocrystalline iron oxide-based materials in wearable electronic devices.