Effects of different carbon materials on MgH<inf>2</inf> decomposition

Abstract

Hydrogen sorption properties were investigated for selected mixtures of MgH2and carbon materials, including graphite, activated carbon, multi-walled carbon nanotubes (MWCNTs), carbon nanofibres (CNFs) and activated carbon fibres. The introduction of such carbon materials decreases the decomposition temperature of MgH2. The best results were achieved in the mixtures involving CNFs and MWCNTs, with metallic impurities, particularly nickel and iron, inherited from the original synthesis. The peak temperatures of MgH2decomposition were reduced to 322 and 341 °C, respectively, compared with 360 °C for simply milled MgH2. Substantial improvement in the decomposition kinetics of MgH2was achieved, particularly with CNF additions. The MgH2decomposition was completed within 20 min at 300 °C with a 5 wt.% CNF addition, compared to 240 min for simply milled MgH2. The improved kinetics was maintained even after several hydriding-dehydriding cycles. X-ray diffraction and transmission electron microscopy reveal that some structural changes occur after cycling: there is an increase in particle size of the MgH2phase, with separation between the magnesium and carbon phases, indicating a clear relationship between the decomposition temperature of MgH2and its structure. It has also been shown that the presence of carbon materials prevents MgH2particle growth which, in turn, enhances its decomposition. © 2007 Elsevier Ltd. All rights reserved.