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Mg Ni alloys are produced by molten salt electrolysis and diffusion method on a nickel plate in KCl NaCl based melts. Structure, alloy composition and electrochemical properties of these alloys are evaluated. Electrolyte composition and applied current density parameters are studied for the characterization of the alloys. It is found that Mg Ni alloys with 45 wt% to 49 wt% Ni content which can be defined as MgNi–type alloys, have both higher discharge capacity and good retaining rate (72%). These alloys are mainly composed of Mg 2 Ni phases and displayed a maximum discharge capacity of 384 mAhg −1 . Increased Ni content is advantageous for the enhancement of the cycle life. At the same time, the discharge capacity of Mg Ni alloy electrode is found to be decreasing. The highest capacity retaining (85%) rate is observed in the highest Ni content alloys (72 wt% Ni). Electrochemical impedance spectroscopy and potentiodynamic polarization measurements show that the controlling–step of the discharge process changed from a mixed rate–determining process at lower depth of discharge to a mass transfer controlled process at higher depth of discharge. The charge transfer resistance increases from 0.5 Ω to 23 Ω with increasing depth of discharge. [ABSTRACT FROM AUTHOR]

Mg–Ni hydrogen storage alloy electrodes with composition of Mg–33, 50, 67 Ni at. % in amorphous phase were prepared by means of mechanical alloying (MA) process using a planetary ball mill. The electrochemical hydrogen storage characteristics and mechanisms of these electrodes were investigated by electrochemical measurements, X–ray diffraction (XRD) and scanning electron microscope (SEM) analyses. The relationship between alloy composition and electrochemical properties was evaluated. In addition, optimum milling time and composition of Mg–Ni hydrogen storage alloy with acceptable electrochemical performance were determined. XRD results show that the alloys exhibit dominatingly amorphous structures after milling of 20 h. The electrochemical measurements revealed that the discharge capacity of Mg 33 Ni 67 and Mg 67 Ni 33 alloy electrodes reached a maximum when alloys were prepared after 20 h of milling time (260 and 381 mAhg −1 , respectively). The maximum discharge capacity of Mg 50 Ni 50 alloy was observable after 40 h milling (525 mAhg −1 ). It was also found that the cyclic stability of the alloys increased with increasing Ni content. Among these alloys, the amorphous Mg 50 Ni 50 alloy presents the best overall electrochemical performance. In this paper, electrode process kinetics of Mg 50 Ni 50 alloy electrode was also studied by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. The impedance spectra of electrodes were measured at different depths of discharge (DODs). The observed spectra were fit well with the equivalent circuit model used in the paper. The electrochemical parameters calculated from electrochemical impedance were also compared. The electrochemical discharge and cyclic performance of 20, 40 and 60 h milled Mg 50 Ni 50 alloy electrodes were demonstrated by the fitted charge transfer resistance and Warburg impedance obtained at various DODs. It was further observed that the controlling-step of the discharge process changed from a mixed rate-determining process at lower DODs to a mass-transfer controlled process at higher DODs. The fitted results demonstrated that charge–transfer resistance (R ct ) increased with DOD. The R ct of 40 h milled Mg 50 Ni 50 alloy (29.27 Ω) was lower than that of 20 h (41.89 Ω) and 60 h milled alloys (92.43 Ω) at fully discharge state. [ABSTRACT FROM AUTHOR]