Structure and electrochemical hydrogen storage properties of spinel ferrites SmxZn1-xFe2O4 alloys (x=0, x=0.2, x=0.4, and x=0.6) for Ni-MH accumulator applications

In this article, we examine the effects of Sm substitution by Zn on the structures and electrochemical characteristics of SmxZn1-xFe2O4 alloys prepared by sol-gel technique. The structural properties of the electrodes are studied by X-ray diffraction and scanning electron microscope (SEM). The structural properties of these alloy are well influenced by the substitution rate. The SEM images of the powder of all samples (x = 0, 0.2, 0.4, and 0.6) shows the presence of agglomerates in the powder particles. Moreover, EDX investigation demonstrates that the chemical and theoretical compositions of all the prepared alloys are almost the same. The electrochemical performance of the oxide electrodes was analyzed at temperature 298 K using chronopotentiometry, chronoamperometry, and cyclic voltammetry techniques. During the first cycles, the electrochemical discharge capacity increases for all alloys. Obviously, this evolution is rapid for the unsubstituted alloy, but it becomes slower by increasing the substitution rate. The highest discharge capacity value was obtained for the ZnFe2O4 electrode of the order of 180 mAh/g while the highest retained capacity was observed for the Sm0.6Zn0.4Fe2O4 electrode. The kinetic results showed that the exchange current density I-0 and the hydrogen diffusion coefficient D-H of the anodes increased with increasing substitution rate and the best values were obtained for the Sm0.6Zn0.4Fe2O4 anode.