Hydrogen Storage Properties Of Magnesium Hydride Doped With Novel Catalysts By Ball Milling

Magnesium-based hydrogen storage alloy is one of the most promising high-capacityhydrogen storage materials. However, the hydrogen storage applications of MgH₂are limitedbecause of its sluggish kinetics and high dehydriding stability for hydriding under moderatepressures and temperatures. Doping catalysts is an effective approach to improve thehydrogen storage properties of MgH₂. Various catalysts have been investigated to improve thehydrogen storage performances of MgH₂, but the problems of high desorption temperatureand slow kinetics remian. In this thesis, we explore some new catalysts for thehydriding/dehydrding of MgH₂, and the catalytic nature and the catalytic mechanism ofadditives were investigated in details.It was found that MgH₂doped with Al₄C₃showed poor improvement ofhydriding/dehydrding which might be due to large size and poor distribution of the ceramicmaterial milling. Ball-milling LaF₃with Mg under hydrogen atmosphere caused their reactionand the formation MgF₂and LaH₃phases, dramatically improved the dehydrogenationcapacity and kinetics. The LaH₃phase is believed to act as a hydrogen pump to transfer thehydrogen atom to surface of MgH₂grains. Dissolving MgF₂to MgH₂lattice by high-energyball milling also greatly improved the dehydrogenation performance of MgH₂. The catalyticmechanism is probably that MgF₂precipitated from MgH₂lattice in the dehydrogenation,destabilizing Mg-H bond and promoting the dehydrogenation of MgH₂. The catalytic effect ofZnF₂dissolving in the MgH₂is not as well as MgF₂. Therefore,in-situ formed MgF₂with afine grain size and uniform distribution have a good catalytic effect on MgH₂. In summary,the sequence of catalytic effect on the kinetic performance of MgH₂is MgF₂> LaF₃> ZnF₂.NaOH as new catalytic additive can greatly enhances the hydrogen storage properties ofMgH₂. The added NaOH actually acted as the catalyst precursor that readily reacted with theMgH₂to form the NaMgH₃and MgO. The improved hydrogen sorption kinetics for themodified mixture can be explained by the presence of NaMgH₃with perovskite structure,which enables fast motion of hydrogen and facilitates the hydrogen sorption kinetics of MgH₂.Similar catalytic effect were found in the KOH modified MgH₂. It is the first report of suchcheap hydroxides could be used to improve the hydrogen storage properties of high capacity metal hydrides and complex hydrides.Addition of NaH can also improve the hydrogen storageproperties of MgH₂, which correlates with the formation of ternary hydride NaMgH₃. TheNaMgH₃crystallizes in the perovskite structure that enables fast hydrogen motion at a broadtemperature range, and thereby could act as fast hydrogen diffusion pathways to enhance thehydriding/dehydriding of MgH₂. The NaMgH₃also shows fast hydrolysis reaction kineticswithout any passivation.