Effect of Mg substitution for La on microstructure, hydrogen storage and electrochemical properties of La1−xMgxNi3.5 (x=0.20, 0.23, 0.25 at%) alloys

The effect of Mg substitution for La on microstructure, hydrogen storage and electrochemical properties of the annealed La1−xMgxNi3.5 (x=0.20, 0.23, 0.25 at%) alloys have been studied. All the samples were mainly composed of (LaMg)2Ni7, (LaMg)Ni3, and LaNi5 phases. Mg substitution for La changed the phase abundance, but did not change the constitution of all phases, which is confirmed by the results of back-scattered SEM images and EDS analysis. The P–C isotherms indicated that the La1−xMgxNi3.5 alloys reversibly absorbed and desorbed hydrogen smoothly at 298 K. The hydrogen absorption cyclic stabilities of La0.77Mg0.23Ni3.5 alloy after 5 hydrogen absorption/desorption cycles reached the maximum values of 91.9% and 96.0% at 298 K and 323 K, respectively. The hydrogen desorption capacity and plateau pressure for the La0.77Mg0.23Ni3.5 alloy reached the maximum values of 1.055 H/M and 0.074 MPa, respectively. The desorption capacities of La0.77Mg0.23Ni3.5 reached 0.193 H/M and 0.565 H/M in the first minute at 298 K and 323 K, respectively. Electrochemical property measurement indicated that La1−xMgxNi3.5 (x=0.20, 0.23, 0.25 at%) alloys possessed excellent activation capability and were completely activated within 3 cycles. Discharge capacities of La1−xMgxNi3.5 alloys reached 378.2 mA h/g (x=0.20 at%), 342.7 mA h/g (x=0.23 at%), and 369.6 mA h/g (x=0.25 at%), respectively. Moreover, energy density of La0.77Mg0.23Ni3.5 alloy was much larger than that of La0.80Mg0.20Ni3.5 alloy and nearly approaches the maximum value of La0.75Mg0.25Ni3.5. Thus, the La0.77Mg0.23Ni3.5 alloy exhibits optimum comprehensive properties of hydrogen storage and electrochemistry.