Proteic sol-gel synthesis, structure and battery-type behavior of Fe-based spinels (MFe2O4, M = Cu, Co, Ni)

Nanocrystalline Fe-based spinels (MFe2O4, where M = Cu, Co or Ni) were synthesized by a proteic sol-gel method. The effect of metal cation swap on the battery-type behavior was evaluated at room temperature in a three-electrode cell configuration in alkaline medium (3 M KOH). Raman spectroscopy was performed to assess the cationic distribution of divalent Cu, Co and Ni at the tetrahedral and octahedral sites, establishing a correlation with oxygen vacancies. X-ray photoelectron spectroscopy (XPS) was used to confirm oxygen vacancies and oxidative states of metal elements. Rietveld refinement analysis combined with FESEM inspection reveals the attainment of mixed spinel ferrites with nanosized crystallites (39-77 nm) and particle sizes (44-92 nm). Cyclic voltammetry and discharging curves of spinel-based electrodes indicate an improved performance for CuFe2O4 (Q(s) = 183 C g(-1)), followed by CoFe2O4 (Q(s) = 79 C g(-1)) and NiFe2O4 (Q(s) = 32 C g(-1)) at a specific current of 0.5 A g(-1). The remarkable electrochemical stability for CuFe2O4 is confirmed by retention capacity of 98% after 1000 charge-discharge cycles at a specific current of 1 A g(-1). The enhanced electrochemical performance of CuFe2O4 is due to an increase in Faradaic reactions boosted by a higher fraction of surface defects (determined by Raman and XPS spectroscopies) combined with a grain boundary-dependent effect responsible for a smaller charge transfer resistance as measured by electrochemical impedance spectroscopy. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.