Molten-state cubic cage-templated uniform C3N4 hollow nanocubes and improved electrochemical performance of asymmetric micro-supercapacitor

Controllable synthesis of hollow structure still remains a huge challenge. g-C3N4 hollow nanocubes with (002) facets exposed (c-CN) are for the first time prepared by calcining cyanuric acid-melamine in the presence of KCl. It is proposed that the c-CN growth is templated by cubic-phase KCl cage at molten state, then Oswald ripening leads to the hollowing of nanocubes. Moreover, silver is loaded on c-CN and its electrochemistry properties are investigated. It is found that at 0.05 mA/cm2, Ag/c-CN electrode shows a higher specific capacitance (10.44 mF/cm2) than that (3.79 mF/cm2) of Ag/p-CN (Ag loading on particle-like g-C3N4). The higher capacitance of Ag/c-CN is mainly attributed to the unique hollow nanocube morphology. On the one hand, c-CN has a larger BET surface area (19.8 m2/g) and a smaller electrical resistance (121.4 Ω), compared to p-CN (4.4 m2/g, 530.8 Ω). On the other hand, theory calculation indicates that the electrolyte cation (e.g., Na+, K+) is easier to desorb from (002) facet than from (100) facet. Hence, the exposed (002) facet of c-CN benefits the cation desorption, improving charging-discharging kinetic processes. Furthermore, after 2500 cycles, ~100% capacitance of Ag/c-CN is retained. It is proposed that the internal and external surfaces of hollow nanocubes can provide more accommodation sites for Ag, favoring the uniform distribution of Ag. Therefore, Ag/c-CN exhibits excellent electrochemical capacitance and cycle stability. Additionally, Ag/c-CN@CF//AC@CF asymmetric supercapacitor (ASC) displays a high energy density of 25 μWh/cm3, a high power density of 17 mW/cm3, and a long cycling lifetime (86.1% capacitance retention after 2500 charge/discharge cycles). The ASC is flexible and can be embedded into other power supply devices. Molten-state cubic cage of KCl is developed to obtain g-C3N4 with a novel cubic nanostructure and Oswald ripening leads to hollowing process. [Display omitted] •Novel C3N4 hollow nanocubes are templated by molten KCl cubic cage for the first time.•Internal and external surfaces of hollow nanocube provide more accommodation sites for ions.•(002) facet benefits the cation desorption, which improves charging-discharging kinetic processes.•Hollow nanocubes-based supercapacitor has a high energy density and cycling stability.