International Journal of Hydrogen Energy 31 (2006) 701 – 707 www.elsevier.com/locate/ijhydene Effect of partial substitution of Cr on electrocatalytic properties of CoFe 2 O 4 towards O 2 -evolution in alkaline medium  R.N. Singh a , ∗ , N.K. Singh a , J.P. Singh a , G. Balaji b , N.S. Gajbhiye b a Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi-221 005, India b Department of Chemistry, Indian Institute of Technology, Kanpur-208 016, India Available online 22 August 2005 Abstract Some nanocrystalline ternary ferrites (∼30–40 nm) having molecular formula, CoFe 2-x Cr x O 4 (0  x  1.0) and face cen- tered cubic structure were investigated by electronic, EPR, magnetization, impedance and Tafel polarization techniques. Re- sults indicate that the combination of antiferromagnetic superexchange interactions of the spinel lattice determines the Curie temperature (T c ) value. The AB interaction dominates over the sublattice interactions, and Fe 3+ occupies A sites, and Cr 3+ and Co 2+ ions occupy octahedral sites because of large crystal field stabilization energy. The saturation magnetization ( s ) coercivity (H c ), remnant field (H r ) and electrical resistivity are observed to decrease with the progressive replacement of Fe 3+ ions (d 5 , five unpaired electrons) by Cr 3+ ions (d 3 , three unpaired electrons). However, the electrocatalytic activity of the oxides towards the oxygen evolution reaction (OER) in 1M KOH at 25 ◦ C increases with the increase in x ; the optimum improvement in the apparent electrocatalytic activity being with 1.0mol Cr. At low overpotentials, the OER on substituted compounds displayed a Tafel slope of b = 50 ± 5 mV decade -1 and the reaction order with respect to OH - concentration as unity, regardless of the composition of the oxide catalyst. It seems that the electrocatalytic activity and saturation magnetiza- tion/Curie temperature for the oxide are inversely related.  2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Spinel ferrite; Electrocatalysis; Oxygen evolution; Cation distribution; Saturated magnetization 1. Introduction Fe 3 O 4 and its substituted products with the spinel struc- ture of the type, AB 2 O 4 , where A and B are transition met- als, possess relatively high overpotential [1] compared to cobaltites [2–8], for the oxygen evolution reaction (OER) in alkaline solution, and are scantly investigated for their use as oxygen anodes [2,9,10]. However, recent studies have shown  Presented at the Fourth International Symposium on Electrocatalysis—“From Theory to Industrial Applications” held in Como, Italy, 22–25 September, 2002. ∗ Corresponding author. Fax: +91 542 2369951. E-mail addresses: rnsbhu@rediffmail.com (R.N. Singh), nsg@iitk.ac.in (N.S. Gajbhiye). 0360-3199/$30.00  2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.07.003 [11–13] that the oxygen overpotential on ferrospinels can be reduced considerably by using low temperature prepara- tion methods and substituting suitable metal ions for Fe in the Fe 3 O 4 -matrix partially. For instance, at 100 mA cm -2 in 1M KOH at 25 ◦ C, Fe 3 O 4 , MnFe 2 O 4 , NiFe 2 O 4 and CoFe 2 O 4 recently obtained by a hydroxide precipitation method at controlled pH 11, produced the oxygen overpo- tentials 524, 338, 379 and 395mV, respectively. These val- ues of overpotentials are much lower than those obtained for similar anodes prepared by ceramic methods [9,10], but are comparable to those recently reported for Co 3 O 4 [14–16] and NiCo 2 O 4 [17–19]. Further, the overpotential data indi- cate that the effect of a d-electron deficient transition metal (i.e. Mn, 3d 5 ) substitution in Fe 3 O 4 is more pronounced compared to that of a d-electron rich transition metal