Scalable fabrication of NiCo2O4/reduced graphene oxide composites by ultrasonic spray as binder-free electrodes for supercapacitors with ultralong lifetime

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• NiCo 2 O 4 /rGO nanocomposites with hierarchical and laminated structure was fabricated by one-step ultrasonic spray. • In-situ grown NiCo 2 O 4 nanoparticles act as spacers to prevent re-stacking of graphene. • Tight adhesion between active materials and Ni foam guarantees efficient electron transfer and alleviate peeling of active materials. • NiCo 2 O 4 /rGO composite exhibits a high specific capacitance and excellent cycling stability. • The assembled asymmetric supercapacitor manifests an ultralong cycling lifetime up to 30,000 cycles. Durable and cost-effective electrode materials are essential for practical application of supercapacitors. Herein, large area NiCo 2 O 4 /reduced graphene oxide (NiCo 2 O 4 /rGO) composites with hierarchical structure were fabricated by a facile one-step ultrasonic spray on Ni foam and directly used as the binder-free electrodes for supercapacitors in aqueous KOH electrolyte. Owing to high electrical conductivity of rGO, hierarchical and layered structure of the electrode, as well as tight adhesion of active materials on the current collector, the as-obtained hybrid electrodes show a high specific capacitance of 871 F g −1 at current density of 1 A g −1 , good rate performance and remarkable cycling stability with a capacitance retention of 134% after 30000 cycles. Besides, the assembled NiCo 2 O 4 /rGO//AC asymmetric supercapacitor (ASC) displays the maximum energy density of 29.3 Wh kg −1 at a power density of 790.8 W kg −1 . Significantly, an ultralong cycling life of 102% capacitance retention is achieved for the ASC device after 30,000 charge/discharge cycles at 20 A g −1 . The scalable fabrication route and excellent electrochemical performance of the NiCo 2 O 4 /rGO composites open the door for making novel hybrid electrodes of advanced supercapacitors.