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 t(BM) = 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 H-2 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 % H-2 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 H-2 nor with the H-2 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 H-2) 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. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.