Large-scale synthesis of porous NiCo2O4 and rGO-NiCo2O4 hollow-spheres with superior electrochemical performance as a faradaic electrode

Synthesis of nanocomposites of metal oxide and reduced graphene oxide (rGO) in a hollow spherical form has been proved to be challenging due to the crumbling effect of rGO. In this paper, we report a simple, cost-effective and large-scale synthetic strategy for producing porous NiCo2O4 hollow spheres as well as rGO-NiCo2O4 hollow spheres through spray drying of respective metal ammonium carbonate complex solutions followed by calcination. During the synthesis process, ammonium carbonate played a pivotal role in hollow sphere formation through easy decomposition into CO2 and NH3 and enhancement of internal pressure of atomized droplets helped to overcome the crumbling effect of rGO. The synthesized hollow spheres are porous, made of 7-12 nm particles with an average diameter of 2-3 mm and a surface area of 76 m(2) g(-1) for pristine NiCo2O4 and 21 m(2) g(-1) for rGO-NiCo2O4. Observance of distinct redox peaks in cyclic voltammetry (CV) indicates that the electrochemical charge storage mechanism of NiCo2O4 is non-capacitive and somewhat battery type in nature. The synthesized rGO-NiCo2O4 hollow spheres showed a specific capacity as high as 427 C g(-1) (971 F g(-1)) at a current density of 0.5 A g(-1) which is much superior to that of pristine NiCo2O4 hollow spheres (183 C g(-1)). rGO-NiCo2O4 also exhibited an excellent rate performance with capacities of 385.3, 345.4, 256, 169 and 89 C g(-1) at current densities of 1, 2, 5, 10 and 20 A g(-1), respectively and 76% retention of capacity after 5000 cycles at 10 A g(-1). Furthermore, studies on AC//rGO-NiCo2O4 asymmetric cells show that the energy storage performance of rGO-NiCo2O4 hollow spheres obtained by the present scalable and cost-effective process is quite comparable, or even superior to those reported for NiCo2O4 synthesized through sophisticated and costly synthetic protocols.