Mg/MgH 2 as a thermochemical energy storage (TCES) material suffers from a slow hydrogenation-dehydrogenation kinetic and poor reversibility. Addition of metal oxide is an effective way to improve the kinetic properties of Mg/MgH 2 . However, there is little study on the effects of complex oxides . Thus, the catalytic effects of complex metal oxides and its catalytic mechanism were investigated in this paper. Results show that the dehydrogenation peak temperature and apparent activation energy of MgH 2 -Ta 2 O 5 are 320.8 °C and 128.54 kJ/mol. Combined with complex metal oxide, MgH 2 -Ta 2 O 5 -Nd 2 O 3 could further reduce the dehydrogenation peak temperature (307.2 °C) and apparent activation energy (95.33 kJ/mol). MgO and NdH 2.61 are found in the dehydrogenated MgH 2 -Ta 2 O 5 -Nd 2 O 3 , indicating that n-typed oxide Ta 2 O 5 and p-typed oxide Nd 2 O 3 are formed and improved the kinetics properties of MgH 2 . Furthermore, Nd 2 O 3 /Ta 2 O 5 interface is formed and acted as “p-n junction”. When heating, the “p-n junction” is exposed to thermal radiation and produce electrons and holes, and then the kinetics properties of MgH 2 are further enhanced.
Zhichao Yu,Wei Zhang,Yongxi Zhang,Yaokun Fu,Ying Cheng,Sanyang Guo,Yuan Li,Shumin Han
Magnesium hydroxide (MgH 2 ) has excellent reversibility and high capacity, and is one of the most promising materials for hydrogen storage in practical applications. However, it suffers from high dehydrogenation temperature and slow sorption kinetics. Rare earth hydrides and transition metals can both significantly improve the de/hydrogenation kinetics of MgH 2 . In this work, MgH 2 –Mg 2 NiH 4 –CeH 2.73 is in-situ synthesized by introducing Ni@CeO 2 into MgH 2 . The unique coating structure of Ni@CeO 2 facilitates homogeneous distribution of synergetic CeH 2.73 and Mg 2 NiH 4 catalytic sites in subsequent ball milling process. The as-fabricated composite MgH 2 -10 wt% Ni@CeO 2 powders possess superior hydrogenation/dehydrogenation characteristics, absorbing 4.1 wt% hydrogen within 60 min at 100 °C and releasing 5.44 wt% H 2 within 10 min at 350 °C. The apparent activation energy of MgH 2 -10 wt% Ni@CeO 2 is determined to be 84.8 kJ/mol and it has favorable hydrogen cycling stability with almost no decay in capacity after 10 cycles.
Liuting Zhang,Farai Michael Nyahuma,Haoyu Zhang,Changshan Cheng,Jiaguang Zheng,Fuying Wu,Lixin Chen
Nb 2 O 5 nanoparticles with an average particle size of 10 nm supported on a rhombic dodecahedral metal organic framework (MOF) were successfully synthesized by a facile one-pot hydrothermal reaction and subsequent calcination process. Experimental results demonstrated that the prepared catalyst drastically improved the hydrogen storage behavior of MgH 2 . 7 wt% Nb 2 O 5 @MOF doped MgH 2 started to desorb hydrogen at 181.9 °C and 6.2 wt% hydrogen could be released within 2.6 min and 6.3 min at 275 °C and 250 °C, respectively. The fully dehydrogenated composite also displayed excellent hydrogenation by decreasing the onset absorption temperature to 25 °C and taking up 4.9 wt% and 6.5 wt% hydrogen within 6 min at 175 °C and 150 °C, respectively. Moreover, the corresponding activation energy was calculated to be 75.57 ± 4.16 kJ mol −1 for desorption reaction and 51.38 ± 1.09 kJ mol −1 for absorption reaction. After 20 cycles, 0.5 wt% hydrogen capacity was lost for the MgH 2 +7 wt% Nb 2 O 5 @MOF composite, much lower than 1.5 wt% of the MgH 2 +7 wt% Nb 2 O 5 composite. However, the addition of Nb 2 O 5 @MOF had limited effect on reducing the dehydrogenation enthalpy of MgH 2 . Microstructure analysis revealed that Nb 2 O 5 particles were uniformly distributed on surface of the MgH 2 matrix and synergistically improved the hydrogen storage property of MgH 2 with MOF.
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