Improved Hydrogen Storage Properties Of Magnesium Hydride By Catalyst Doping

Hydrogen is a clean energy carrier, and it’s also an important new energy in future. Theapplication of hydrogen needs an efficient and safe technology of solid hydrogen storage.Magnesium-based hydrogen storage alloys are one of the hydrogen storage material with thebest applied prospect due to its light weight, high capacity of hydrogen storage, low cost andabundant source. However, MgH2shows high thermodynamic stability and slow sorptionkinetic, which makes its dehydrogenation temperature higher than300oC, and limits itspractical application. Various methods such as mechanical alloying, adding catalyst,nano/nano-confinement and synthesizing complex hydride have been tried to improve thehydrogen storage properties of magnesium alloys, especially for the kinetic performance.However, there is few ways for thermodynamic adjustment. This paper studies the hydrogenstorage properties of Mg92.6Ni3.2Y4.2composite, Mg93Y7and the MoO3-catalyzed MgH2aiming to investigate the effect of catalysts on the thermodynamic and kinetic properties. Thefollowing conclusions have been drawn:The hydrogenated Mg92.6Ni3.2Y4.2alloy is composed of MgH2, Mg2NiH4and yttriumhydrides. The PCT curves of hydrogenated Mg92.6Ni3.2Y4.2alloy show two dehydrogenationplateaus, the lower one is due to the desorption of MgH2and the higher one is Mg2NiH4. Theplateau pressure of MgH2in Mg92.6Ni3.2Y4.2alloy is higher than that of pure magnesiumhydride, which shows the stability of MgH2in the Mg92.6Ni3.2Y4.2alloy is decreased. It isproposed that the unit cell shrinkage due to the dehydrogenation of Mg2NiH4causescompressive stress on the near MgH2, which explains the destabilization of MgH2. The kineticresults indicate that the hydrogen absorption rate of Mg92.6Ni3.2Y4.2alloy is slower than thedesorption rate. It is explained by the assumption that the Mg2NiH4is hydrogenated inadvance, which applies stress to magnesium nearby and prevents hydrogen from going intomagnesium lattice to form MgH2. Therefore, the hydrogen absorption kinetics andthermodynamics of magnesium is influenced by Mg2Ni.The hydrogenated Mg93Y7alloy is composed of MgH2, YH3and YH2, and the main dehydrogenation phase is MgH2. PCT testing results show that the dehydrogenation plateaupressure of MgH2in hydrogenated Mg93Y7alloy is higher than that pure MgH2. The kineticresults indicate that yttrium hydride can not improve the dehydrogenation kinetic properties ofMgH2greatlly. YH3content due to prolonged hydriding time causes few improvement in thedehydrogenation rate. After the accident treatment of oxidation,Y2O3and YH2in Mg93Y7alloy contributed to the obvious improvementof dehydrogenation kinetic properties of MgH2, the dehydrogenate rate is increased by threetimes.Doping with MoO3into MgH2by ball milling at300oCcan improve the kinetic propertiesof hydrogen storage greatly. The dehydrogenation activation energy of MgH2-2mol%MoO3is114.7kJ/mol, which is lower than pure magnesium hydride. The XRD results indicate that theMoO3transforms into MoO2in de/hydrogenation process, indicating that MoO3reacted withmagnesium to form MoO2and MgO. Therefore, the real catalyst is MoO2.It is proposed thatthe MoO2can provide a fast diffusion pathway for the diffusion of hydrogen. The resultshows oxides can be used as efficient catalyst for hydrogen absorption/desorption reaction ofmagnesium hydride.