NiCo2O4 hollow microspheres with tunable numbers and thickness of shell for supercapacitors

We report a new resin formaldehyde (RF-COOH) microspheres hard templates based method to synthesize NiCo2O4 hollow microspheres with thin/thick single/double/triple-shell. Single-shell NiCo2O4 hollow microspheres are obtained with deionized water as solvent. Furthermore, ethylene glycol can decrease the aquo groups (H2O) number within the coordination sphere of Ni2+/Co2+, which leads to a decrease of hydrated metal ions size and more penetrated metal ions into RF microspheres that is essential to the formation of more shells. Thus, double and triple-shell NiCo2O4 hollow microspheres are harvested when ethylene glycol is solvent and the heating ramp rate are 2 and 5 degrees C/min, respectively. Larger heating ramp rate (2-5 degrees C/min) increases the temperature gradient of the infused RF microspheres, which favors the separation between adjacent NiCo2O4 layers and RF cores, and thus resulting in the formation of more shells. What's more, shells become thick and more compact when the precursor concentration is increased from 0.05 to 0.1 M due to more infused Ni2+/Co2+. The electrochemical performance for super capacitors of NiCo2O4 hollow microspheres with thin multi-shells is superior to those with thick single shell. The better performances are attributed to multi-shells with more electroactive sites and hollow structures as ion-buffering reservoir, while thin shell is beneficial to the diffusion of electrolyte/ions. (C) 2016 Published by Elsevier B.V.