Effect And Mechanism For Nano Cobalt Nickel-Based Catalyst On Hydrogen Absorption And Desorption Kinetics Of Magnesium Hydride

In recent years,extreme weather has occurred frequently in various countries.In order to limit the global temperature rise to 2℃,people have accelerated the pace of shifting the energy system to low carbon.As an environmentally friendly,efficient and renewable green energy,hydrogen energy is an important strategy for achieving energy structure adjustment and carbon neutrality.However,how to safely and efficiently store hydrogen is the key problem hindering the industrialization of hydrogen energy.According,possessing the advantages of high capacity（7.6 wt%）,abundant resources and stable cycling ability,MgH₂is acted as a comprehensive hydrogen storage material by researchers worldwide.However,slow kinetics and stable thermodynamic properties limit its practical application.Based on the latest research at home and abroad,series of Ni Co-based nanocatalysts were designed and prepared,and their catalytic effect and mechanism of MgH₂hydrogen storage performance were explored.Firstly,the particle size of the commercial medium-entropy alloy CrCoNi was reduced by wet chemical ball-milling,and then CrCoNi nanosheets were successfully prepared.The experimental results show that CrCoNi significantly improves the kinetic performance of MgH₂.5 wt%CrCoNi significantly reduced the initial dehydrogenation temperature of MgH₂from 325℃ to 195℃.At 285℃,the composite released 5.8 wt%H₂within 1000 s.In addition,at 130℃,the fully dehydrogenated sample could absorb 4.4 wt%H₂in 2000 s.After 20 cycles,MgH₂+9 wt%CrCoNi only lost 0.36 wt%hydrogen capacity.Microstructure characterization shown that CrCoNi remained stable in the form of a ternary alloy throughout the reaction.Moreover,CrCoNi nanosheets are uniformly distributed on the surface of MgH₂,which can provide a large number of catalytic active sites and low-energy potential barrier diffusion channels in the hydrogen release process,therefore accelerating the diffusion of hydrogen atoms.In the process of hydrogen absorption,hydrogen easily dissociates into hydrogen atoms on the surface of CrCoNi,and H atoms only need less activation energy to form new MgH₂phases along the low-energy barrier diffusion channel.Secondly,a MOF-derived bimetallic Co@NiO catalyst were successfully prepared by hydrothermal method and annealing,and the Co@NiO was doped with MgH₂to improve the hydrogen desorption and resorption kinetics.The introduction of Co@NiO reduced the initial dehydrogenation temperature of MgH₂to 190℃.At 300℃,this composite material could completely release hydrogen within 400 s.In addition,the desorption activation energy of MgH₂+9 wt%Co@NiO was decreased to 93.8±8.4 k J/mol.Noteworthy,the in-situ transformation from Co@NiO precursor into symbiotic Mg₂Ni H₄/Mg₂Co H₅on MgH₂surface.The nano-sized Mg₂Ni H₄/Mg₂Co H₅clusters act as“the multi-step hydrogen pump”to facilitate the hydrogen spillover and absorption.At the same time,plentiful interfaces would form.The more complicated interfaces provide numerous low energy barrier channels for hydrogen diffusion,therefore accelerating the hydrogen release and uptake.Overall,it can be concluded that the synergistic catalytic effect between the multi-step hydrogen pumps and active interfaces in MgH₂+9 wt%Co@NiO composite results in its enhanced hydrogen ab/desorption performances.Finally,amorphous Ni Co B spherical nanoparticles with a particle size of 10-50 nm were successfully prepared by chemical reduction method,and employed them as high-activity catalysts to considerably improve the hydrogen storage properties of MgH₂.After the introduction of Ni Co B into MgH₂,the fully dehydrogenated sample could rapidly absorb 4.9 wt%H₂within 600 s at 130℃.Compared with the previously studied metal Co@NiO system,the hydrogen absorption rate at low temperature is significantly improved.Significantly,the MgH₂+9 wt%Ni Co B composite still maintained 6.0 wt%H₂after 20cycles,which shows fantastic reversibility of hydrogen storage ability.Further XRD and TEM analysis showed that superficial Ni Co B would react with MgH₂to form Mg B₂,Mg₂Ni/Mg₂Ni H₄and Mg₂Co/Mg₂Co H₅during the hydrogen de/absorption cycles.This phase transformation,on one hand,is beneficial in forming more catalytic action areas and preventing Ni Co B particles from aggregation;on the other hand,it provides lots of boundary interfaces such as Ni Co B/MgH₂,Mg₂Ni/MgH₂and Mg₂Co/MgH₂,which can be served as hydrogen reservoir to accelerate the hydrogen storage reactions of MgH₂.