Multiphase and Double-Layer NiFe₂O₄@NiO-Hollow-Nanosphere-Decorated Reduced Graphene Oxide Composite Powders Prepared by Spray Pyrolysis Applying Nanoscale Kirkendall Diffusion

Multicomponent metal oxide hollow-nanosphere decorated reduced graphene oxide (rGO) composite powders are prepared by spray pyrolysis with nanoscale Kirkendall diffusion. The double-layer NiFe₂O₄@NiO-hollow-nanosphere decorated rGO composite powders are prepared using the first target material. The NiFe-alloy-nanopowder decorated rGO powders are prepared as an intermediate product by post-treatment under the reducing atmosphere of the NiFe₂O₄/NiO-decorated rGO composite powders obtained by spray pyrolysis. The different diffusion rates of Ni (83 pm for Ni²⁺) and Fe (76 pm for Fe²⁺, 65 pm for Fe³⁺) cations with different radii during nanoscale Kirkendall diffusion result in multiphase and double-layer NiFe₂O₄@NiO hollow nanospheres. The mean size of the hollow NiFe₂O₄@NiO nanospheres decorated uniformly within crumpled rGO is 14 nm. The first discharge capacities of the nanosphere-decorated rGO composite powders with filled NiFe₂O₄/NiO and hollow NiFe₂O₄@NiO at a current density of 1 A g^-1 are 1168 and 1319 mA h g^-1, respectively. Their discharge capacities for the 100th cycle are 597 and 951 mA h g^-1, respectively. The discharge capacity of the NiFe₂O₄@NiO-hollow-nanosphere-decorated rGO composite powders at the high current density of 4 A g^-1 for the 400th cycle is 789 mA h g^-1.