Effect of C (graphite) doping on the H2 sorption performance of the Mg – Ni storage system.

Binary Mg – Ni mixtures and ternary Mg – Ni – C (graphite) samples with fixed proportions of metals (Mg 85 %: Ni 15 % by weight) and amount of C increasing in increments of 5 wt % from 5 wt % to 15 wt % were prepared by high energy ball milling (BM) in Ar for tBM = 2 h. The purpose of the study was to evaluate the effect of C addition on the reactivity, the sorption activation and the storage performance of the Mg - Ni system. Increasing the amount of C had the effect of decreasing (from 10 to 3) the number of cycles needed for activation (performed at 623 K and 40 bar/0.9 bar charging/discharging H2 pressure). After full activation, the 5 wt % C-containing sample exhibited the best absorption kinetics performance: the average rate to charge up to 5 wt % H2 was about 3 times higher than that observed for the undoped sample. Unfortunately, increasing the amount of C had a negative impact on the desorption behaviour, causing an increase in the dehydrogenation activation energy and a decrease in the discharging rates. Within the present study, C reacted neither with H2 nor with the H2 active phases (the two discharged phases Mg and Mg2Ni and the related hydrides) and consequently did not lead to variation in the sorption enthalpies of the Mg-Ni system. But, its presence did cause a small increase (4 K at 0.9 bar H2) in the minimum desorption temperatures of the hydrides and a consequent minor decrease (0.2 bar) in the equilibrium pressures. The best sorption properties were obtained for the 5 wt % C-sample, that on the whole worked better than the binary mixture.