Hybrid hollow spheres of carbon@CoxNi1−xMoO4 as advanced electrodes for high-performance asymmetric supercapacitors

Abstract

Combining pseudocapacitive materials with conductive substrates is an effective approach to enhance the overall performance of electrodes for supercapacitors. Herein, NiMoO₄ nanosheets were grown on the surface of porous carbon nanospheres (PCNS) that were derived from cyclodextrin, resulting in PCNS@NiMoO₄ hollow nanospheres. Co was further doped into NiMoO₄ which gave rise to a composite PCNS@Co_xNi_1−xMoO₄. The capacitive performance of these materials was systematically examined. Compared with pure NiMoO₄ and PCNS@NiMoO₄, PCNS@Co_0.21Ni_0.79MoO₄ showed the highest specific capacitance of 954 F g⁻¹ at 1 A g⁻¹ and an extraordinary rate performance of 92.8% retention at 40 A g⁻¹, which are significantly higher than those of PCNS@NiMoO₄ and pure NiMoO₄. This enhancement was due to the fact that PCNS provides high electrical conductivity, the hollow structure enables excellent contact and facile penetration of the electrolyte into the active material, and Co doping further improves the electrical conductivity and provides extra redox reaction sites. By using PCNS@Co_0.21Ni_0.79MoO₄ as the positive electrode and activated carbon (AC) as the negative electrode, an asymmetric supercapacitor was fabricated. Such a device delivered an energy density of 36.7 W h kg⁻¹ at a power density of 346.4 W kg^–1, and an outstanding cycling stability with 90.2% retention of its initial capacitance after 5000 cycles of charge and discharge.