Temperature is shown to have a huge influence on the electronic properties of nanometric spinel-type cobalt oxides precipitated at low temperature in alkaline media. The initial phase, with formula H x Li y Co 3−δ O 4 , contains hydrogen, lithium, cobalt vacancies, and a mixed valence Co 4+ /Co 3+ within the structure, leading to an electronic conductivity higher than that of stoichiometric Co 3 O 4 . Its structural evolution under thermal treatment was studied by X-ray diffraction and chemical analysis, which reveal modifications in structure and compositions, involving water release, increase of the Co/O atomic ratio, and modification of the Co 4+ /Co 3+ ratio. The RT to 300 °C range is particularly interesting as a single-phase domain and the materials obtained in this temperature range were investigated by chemical analysis, electronic conductivity and specific surface area measurements. Upon increasing temperature, the enhancement of the Co 4+ /Co 3+ ratio, together with cationic redistribution in the spinel framework, results in an improvement of the electronic conductivity (more than 2 orders of magnitude for materials heated above 150 °C). Finally, the systematic thermal study of electronic conductivity and specific surface area of the materials allows to determine an optimal heat-treatment temperature leading to an optimized active electrode material for electrochemical energy storage applications, especially in supercapacitors. Such a solid state chemistry approach combining many material characterization techniques to reach a complete knowledge of the material is quite rare in the literature concerning oxides for supercapacitors.
Co 3 O 4 spinel type nanoparticles (5-10 nm) were synthesized by oxidizing precipitation in various basic solutions. The materials prepared in lithium-containing hydroxide solutions exhibit high electronic conductivity (higher than 10 −2 S.cm −1 at room temperature), which can be increased (up to 3 S.cm −1 ) by electrochemical oxidation. This high conductivity is due to the presence of Co 4+ ions in the octahedral framework of the spinel structure, which entails electronic delocalization. In addition to high conductivity, the obtained powder exhibits a high surface area (around 200 m 2 .g −1 ), which should be suitable for applications in electrodes: as conductive additive in the positive electrode of alkaline Ni-MH cells, or directly as electrode material in hybrid C/metal oxide supercapacitors.Co 3 O 4 was intensively studied for its potential applications in numerous fields such as electrochemistry and energy storage, 1-5 catalysis, 6-9 sensors 10,11 or magnetism. 12 A classical synthesis procedure of Co 3 O 4 consists of thermal decomposition of cobaltous salts at high temperature (300 • C-900 • C) under oxidizing conditions. 13,14 But such methods lead often to large crystallites (0.1-1 μm), unsuitable to explore the full range of intrinsic properties. Various synthesis procedures of Co 3 O 4 nanoparticles were reported in the literature: oxidative precipitation from aqueous solutions, 15,16 thermal decomposition of cobalt alkoxides 3 or inorganic precursors. 8,17 Mesoporous Co 3 O 4 was also synthesized by hydrothermal method, 18,19 template removal 9,20,21 or decomposition of cobalt oxalate. 22 We reported recently the preparation and characterization, in our laboratory, of battery-related H x Li y Co 3−δ O 4 original spinel materials, exhibiting high electronic conductivity. 23-26 These materials were synthesized by electrochemical oxidation of CoO in alkaline electrolyte containing the three components: KOH, NaOH, LiOH. They were intended to be used as conductive additives in the positive electrode of Ni-MH batteries. Let us remind that the structure of ideal cubic Co 3 O 4 , which is a normal spinel with the general formula A[B 2 ]O 4 , can be described as a 3 dimensional -[Co 2 O 4 ] framework of Co 3+ ions in edge-sharing octahedral B sites, where Co 2+ ions are located in tetrahedral A sites that share only corners with the octahedra. An overlapping of the cobalt t 2g orbitals across the shared edges of the CoO 6 octahedra occurs. In the case of ideal Co 3 O 4 , the t 2g band is completely filled, due to the electronic configuration of Co 3+ (d 6 LS) ions, leading to a semi-conductor behavior with a very low electronic conductivity (about 10 −5 S.cm −1 at room temperature). The phases that we synthesized in the presence of lithium ions are different from the ideal Co 3 O 4 phase, by the presence of protons, of lithium and of cobalt vacancies within the spinel structure, and of Co 4+ in the octahedral [Co 2 O 4 ] framework. 23 The negative charge resulting from cobalt vacancies is compensated by lithi...
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