Ultrasmall NiMoO4 robust nanoclusters-active carbon composite for high performance extrinsic pseudocapacitor

Highlights: • Ultrasmall NiMoO₄ nanoclusters of size 2–3 nm for charge storage application. • Faster kinetics due to surface governed electrochemical reactions. • Achieved specific capacity of 665C/g @ 1 A/g for half-cell. • Achieved specific capacity of 131.5C/g @1 A/g for full-cell device. • Retains 92% capacity after 4000 cycles. -- Abstract: NiMoO₄ is one of the potential candidates for pseudocapacitor application because of its high theoretical capacitance (∼2500 F/g) and conductivity due to the presence of NiO and molybdenum oxide, respectively. However, the currently available NiMoO₄ morphologies possess limited capacitance and recyclability which is mainly due to the reduced surface to volume ratio affecting the expansion volume during charging/discharging process and low mass loading of active material. This limitation can be circumvented by synthesizing NiMoO₄ in ultra-small size nanoclusters. Herein, we report a facile synthesis of ultrasmall NiMoO₄ nanoclusters on active carbon with high specific capacity and long-term cycle stability. The ultrasmall NiMoO₄-active carbon composite (NiMo@AC) sample has displayed enhanced specific capacity of 665 C/g (1662 F/g or 184.7 mAh/g) at a current density of 1 A/g which is higher than the reported values for NiMoO₄. An asymmetric supercapacitor fabricated from NiMo@AC nanocomposite displayed a high specific capacity of 131.5 C/g (87.7 F/g or 36.5 mAh/g), a high energy density of 27.4 Wh/Kg and a power density of 184 W/Kg at a current density of 1 A/g. The fabricated supercapacitor retains ∼92% of initial capacity after 4000 cycles. This high performance can be attributed to the ultrasmall size which has maximized surface to volume ratio of the electroactive material. Further, active carbon not only provides better dispersion to nanocluster but also contributes towards the structural stability due to embedment of the particles into the porous carbon matrix, accommodating the volume expansion during ion intercalation. The results indicate that NiMo@AC nanocomposite could be a promising candidate for electrochemical energy storage.