Study of Structural, Thermodynamic and Electrochemical Properties of MmNi3.55Mn0.4Al0.3Co0.75-xFex (x = 0 and 0.75) Compounds

摘要

The structural, thermodynamic and electrochemical properties of the MmNi3.55Mn0.4Al0.3Fe0.75 compound, used as negative electrode of Ni – MH accumulator, are studied and compared to those of MmNi3.55Mn0.4Al0.3Co0.75 electrode. The thermodynamic results show that the total substitution of cobalt by iron leads to a decrease solid – gas capacity measured at 25°C from 5.5 H/mol to 3.93 H/ mol for MmNi3.55Mn0.4Al0.3Co0.75 and MmNi3.55Mn0.4Al0.3Fe0.75 compounds, respectively. The electrochemical discharge capacity for the MmNi3.55Mn0.4Al0.3Co0.75 compound reaches its maximum of 270 mAh/g after 12 cycles and then decreases to 200 mAh/g after 25 cycles. However, the electrochemical discharge capacity of the MmNi3.55Mn0.4Al0.3Fe0.75 compound reaches its maximum value of 200 mAh/g after 9 cycles and then decreases to 180 mAh/g after 25 cycles. The decrease of the discharge capacity for the two alloys is attributed to the corrosion and deprecipitation of the alloys in the KOH aqueous solution. The value of the hydrogen diffusion coefficient DH determined by the cyclic voltammetry are equal to 5.85 10-10 cm2 s-1 and 3.96 10-10 cm2 s-1 for MmNi3.55Mn0.4Al0.3Co0.75 and MmNi3.55Mn0.4Al0.3Fe0.75 compounds, respectively. The hydrogen diffusion coefficient determined by electrochemical impedance spectroscopy (EIS), corresponding to 10 and 100? of the charge state are equal to 6.64 10-10 cm2 s-1 (? phase) and 1.6 10-10 cm2 s-1 (? phase) for the MmNi3.55Mn0.4Al0.3Co0.75 alloy. However, for the MmNi3.55Mn0.4Al0.3Fe0.75, the hydrogen diffusion coefficient are equal to 7.27 10-10 cm2 s-1 (? phase) and 2.51 10-10 cm2 s-1 (? phase).