Effect Of Transition Metal/carbon-based Composite Material On Hydrogen Storage Performance Of MgH₂

Mg-based hydrogen storage materials stand out among many hydrogen storage materials due to their high theoretical hydrogen storage capacity and low price,and are considered to be one of the most potential hydrogen storage materials.However,the stable thermodynamic properties and slow kinetic performance restricts their practical application in production practice.In this work,the tubular g-C₃N₄（TCN）was prepared by high-temperature pyrolysis.Subsequently,two additive materials（Ni/TCN and Co/TCN）were further synthesized by a simple chemical reduction method.Then the above three materials were separately combined with pure Mg powder by hydrogenation combustion and high-energy ball milling to prepare MgH₂-TCN,MgH₂-Ni/TCN and MgH₂-Co/TCN composite materials.And finally,the hydrogen storage performance of the three composite mateirals was systematically explored.The results show that after the introduction of Ni/TCN and Co/TCN additives,the initial hydrogen release temperature of the prepared composite materials was reduced by116 K and 83 K,respectively,compared with pure MgH₂.At the same time,the apparent activation energy（E_a）values of the two composite materials are 82.6 and 103.8 k J/mol,respectively,which are both lower than the value of pure MgH₂（161.1 k J/mol）.In the hydrogen absorption and desorption kinetic test of the composite materials,the MgH₂-Ni/TCN and MgH₂-Co/TCN composite materials also showed better hydrogen absorption and desorption behavior than the MgH₂-TCN composite material and pure MgH₂ under relatively low temperature conditions.Under the test conditions of 3 MPa hydrogen pressure and 373 K,the hydrogen absorption of MgH₂-Ni/TCN composite material within 3500 s is 3.56 wt.%;and at 423 K,the hydrogen absorption of the MgH₂-Co/TCN composite material also reaches 3.89 wt.%under the same conditions.At573 K,the MgH₂-Ni/TCN composite material can release 1.72 wt.%of hydrogen in 3500s.In addition,the hydrogen absorption capacity retention rate of the MgH₂-Ni/TCN composite material is 96.6%after 10 cycles of hydrogen adsorption and desorption.The microscopic morphology and phase composition of the composite before and after hydrogen absorption and desorption are analyzed.The results show that the presence of TCN tends to equalize the particle size distribution of the composite system,which makes the hydrogen storage performance of MgH₂-Ni/TCN and MgH₂-Co/TCN composite materials better than that of pure MgH₂.At the same time,the induction and driving effect of the composite intermediate phases（Mg₂NiH₄ phase and Mg₂CoH₅ phase）generated by the transition metal particles during the composite process also accelerates the reaction.The above two reasons work together to lead to more excellent hydrogen storage behavior of composite materials.