NANOSTRUCTURED NI 0 . 9 CO 2 . 1 O 4 SPINEL OXIDE : ELECTROCHEMICAL , SPECTROSCOPIC AND MORPHOLOGICAL INVESTIGATION

Mamadou Guèye 1 , Papa Charles Harris Mandiamy 1 , * Makhtar Guène 1 and Abdou Aziz Diagne 2 . 1. Laboratoire de Chimie Physique Organique et d’Analyse Environnementale, Département de Chimie, Faculté des Sciences et Techniques, Université Cheikh Anta Diop, Dakar, Sénégal. 2. Département de chimie, UFR SATIC, Université Alioune Diop, Bambey, Sénégal. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

817 material. It can be enhanced by reducing the size and uniformly dispersing the nanoparticules, which can provide high surface area (Chang et al., 2010). This field has prompted extensive research because it can provide clean and renewable energy sources, one of the major challenges in response to the needs of modern society and increased pollution. Water electrolysis is one of the most promising and most progressive methods of hydrogen (H 2 ) production. Hydrogen is expected to be one of the most promising energy sources in the future. Therefore, development of an electrocatalyst having high stability, high efficiency, low cost and low overpotential for oxygen and hydrogen evolution reaction is required. Keeping the above facts in mind, herein, we synthesized Ni 0.9 Co 2.1 O 4 by using sol-gel via propionic acid method and studied its morphological, optical and electrochemical properties and results of the study are presented in this paper.

Physical characterization:
The morphology of Ni 0.9 Co 2.1 O 4 material was examined using a scanning electron microscopy (SEM, Hitachi model S-520). The Raman spectrum was recorded in the range of 200 -800 cm -1 at room temperature using a Raman microscope Renishaw model 1000. The optical absorption spectrum of the mixed oxide powders is carried out at ambient temperature using a SAFAS 2000 UV-visible spectrophotometer whose spectral range extends over a range ranging from from 250 nm to 2000 nm.

Electrochemical characterization:
The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out at room temperature in a three-electrode glass cell in a solution of 3 M KOH aqueous electrolyte using a Potentiostat/Galvanostat Fra 2 µAutolab Type III. A silver wire and an Hg/Hg 2 Cl 2 (Saturated KCl) were used as the counter and reference electrodes, respectively. The oxide powder in a form of disk pastille (1.33 cm 2 surface area) were used as working electrode. The impedance spectra were recorded in a range of frequency from 10 kHz to 0.1 Hz by applying an AC voltage of 0.5 V.

Preparation of working electrode:
For electrodes with physical characteristics easily reproducible, with good electronic conductivity and good mechanical properties we were interested to rigid electrodes impregnated with electrolyte. They were made from a homogeneous mixture of cobalt and nickel mixed oxide as active material and a small amount of Teflon (5 wt %) which serves as a mechanical binder. Each of these components was weighed with an electronic balance. After weighing, the constituents were homogenized using a mortar and then introduced into a tabletting mold of Beckam type of 13 mm diameter. A pressure of 3 ton / cm 2 (Touré et al., 1994) is applied to finally obtain pellet of 300 mg weight.  Fig. 1 shows the SEM images of Ni 0.9 Co 2.1 O 4 at two different magnifications. It is revealed from Fig. 1 (a & b), that Ni 0.9 Co 2.1 O 4 oxide powders are constituted of aggregates of grains in spherical shape with diameters up to around 1,6 μm, and nanoplates with lengths up to around 12 μm, an thickness of about 465 nm. Some singular spherical grains have also been observed in the morphology of the powders (Fig. 1b). The SEM micrographs also reveal that the samples have good dispersibility.

Conclusion:-
Ni 0.9 Co 2.1 O 4 oxide was fabricated through a sol-gel via propionic acid method. The sample morphology of the oxide powders is constituted of aggregates of grains in spherical-like shape with good dispersibility. The UV-visible analysis reveals the presence of Co (III) cations in tetrahedral sites and the replacement of Ni ions by the Co ions in the octahedral and tetrahedral sites. The Ni 0.9 Co 2.1 O 4 electrode has good reversibility revealed by the cyclic voltammetry study, which also show that Faradic redox reactions associated with the surface redox reactions of Co 2+ /Co 3+ and Ni 2+ /Ni 3+ . Furthermore, it is indicated that the diffusion of the hydroxide ions rate controlling process. EI-spectrum further exhibit enhanced capacitive behavior of the Ni 0.9 Co 2.1 O 4 electrode.