Synthesis and electrochemical properties of binary MgTi and ternary MgTiX (X = Ni, Si) hydrogen storage alloys

Abstract

Mg-based hydrogen storage alloys are promising candidates for many hydrogen storage applications because of the high gravimetric hydrogen storage capacity and favourable (de)hydrogenation kinetics. In the present study we have investigated the synthesis and electrochemical hydrogen storage properties of metastable binary MgyTi1−y (y = 0.80–0.60) and ternary Mg0.63Ti0.27X0.10 (X = Ni and Si) alloys. The preparation of crystalline, single-phase, materials has been accomplished by means of mechanical alloying under controlled atmospheric conditions. Electrodes made of ball-milled Mg0.80Ti0.20 powders show a reduced hydrogen storage capacity in comparison to thin films with the same composition. Interestingly, for a Ti content lower than 30 at.% the reversible storage capacity increases with increasing Ti content to reach a maximum at Mg0.70Ti0.30. The charge transfer coefficients (α) and the rate constants (K1 and K2) of the electrochemical (de)hydrogenation reaction have been obtained, using a theoretical model relating the equilibrium hydrogen pressure, electrochemically determined by Galvanostatic Intermittent Titration Technique (GITT), and the exchange current. The simulation results reveal improved values for Mg0.65Ti0.35 compared to those of Mg0.80Ti0.20. The addition of Ni even more positively affects the hydrogenation kinetics as is evident from the increase in exchange current and, consequently, the significant overpotential decrease.