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 μm and a surface area of 76 m² g⁻¹ for pristine NiCo₂O₄ and 21 m² 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.