Surfactant-free hydrothermal synthesis of hierarchically structured spherical CuBi2O4 as negative electrodes for Li-ion hybrid capacitors.

Hierarchically structured spherical CuBi2O4 particles were prepared using a facile hydrothermal method without using a surfactant over various hydrothermal reaction periods. The prepared CuBi2O4 samples were examined via X-ray diffraction (XRD), which confirmed the formation of a tetragonal crystal structure. The morphological features were analyzed using field emission scanning electron microscopy (FESEM), which elucidated the construction of the hierarchical microspherical CuBi2O4 particles. The plausible growth mechanism of the hierarchical structure was explained in terms of a time-dependent synthesis process and its crystal structure. The uniform hierarchical CuBi2O4 microspheres were used to fabricate a Li-ion hybrid capacitor (Li-HC) along with activated carbon (AC), the generated device delivers a stable specific capacitance of 26.5 F g(-1) over 1500 cycles at a high current density of 1000 mA g(-1) and a capacity retention of ∼86%. The AC/CB2 Li-ion hybrid cell exhibits high energy density and power density values of 24 W h kg(-1) and 300 W kg(-1), respectively.