Influence Of Carbon-Supported Rare Earth Oxide Additives On The Hydrogen Storage Performance Of Mg-La-Ni Alloys

With the rise of the new industrial revolution,energy development and utilization are undergoing profound changes.Green and low-carbon energy have become the dominant trends in global energy development.Compared with traditional fossil fuels such as coal,oil and natural gas,hydrogen energy does not emit greenhouse gases and pollutants during use.It is a truly green and low-carbon energy source.By replacing fossil fuels,hydrogen energy has the potential to reduce carbon emissions and contribute to green and low-carbon development,achieving China’s"dual carbon"goals of carbon peaking and carbon neutrality.Among many hydrogen storage material systems,Mg H₂ is widely regarded as one of the most promising hydrogen storage materials,with its high hydrogen storage capacity,low cost,abundant resources,and environmental friendliness.The hydrogen absorption and desorption reaction kinetics of Mg H₂ are slow,and relatively high temperatures and pressures are required for the hydrogen absorption and desorption processes to proceed effectively.This limits its efficiency and response speed in practical applications.In view of the above problems,this paper summarizes the research status of magnesium-based hydrogen storage materials and their hydrogen absorption and desorption mechanisms.Taking the Mg₉₆La₃Ni ternary alloy as the research object,a series of modification studies were carried out by introducing carbon-loaded different lanthanide rare earth oxide catalysts.The aim is to explore feasible ways to improve the hydrogen absorption and desorption dynamics and thermodynamics of magnesium-based hydrogen storage materials.performance.This article first utilizes the vacuum melting method to prepare the Mg₉₆La₃Ni ternary alloy.Six composite catalysts,including rare earth oxides(La₂O₃@C,Ce O₂@C,Pr O_1.83@C,Eu₂O₃@C,Gd O@C,and Yb₂O₃@C),are synthesized using an improved chemical blowing carbonization method.The samples are characterized by X-ray diffraction,transmission electron microscopy,Raman spectroscopy,scanning electron microscopy,energy-dispersive spectroscopy,and specific surface area testing.The research reveals that the rare earth oxide-carbon composite catalyst possesses a porous structure composed of micropores and mesopores.The rare earth oxide nanoparticles are uniformly dispersed on the porous carbon matrix,resulting in a high surface area and defect density.This unique structure provides additional catalytic active sites for hydrogen adsorption and desorption,thereby reducing the activation energy during the absorption and desorption processes.Simultaneously,the porous structure facilitates the rapid diffusion of hydrogen atoms.The synergy between rare earth hydride nanoparticles and the carbon matrix plays a crucial role:the former primarily acts on nucleation,while the latter promotes hydrogen diffusion.This study offers new insights for designing magnesium-based hydrogen storage materials.After alloy hydrogenation,rare earth oxides(La₂O₃@C,Ce O₂@C,Pr O_1.83@C,Eu₂O₃@C,Gd O@C,Yb₂O₃@C)react with hydrogen to form corresponding rare earth hydrides（La H₃,Ce H₂,Pr H₂,Eu H₂,Gd H₃,Yb H₂）.In Mg₉₆La₃Ni,the lanthanum element reacts with hydrogen to produce La H₃.These rare earth hydrides exhibit high thermal stability and do not decompose during hydrogen adsorption or desorption experiments.The in-situ formation of nanoscale rare earth hydrides enhances grain boundary density,adsorbs hydrogen atoms,and transfers them to the magnesium-metal interface,acting as active nucleation sites for Mg H₂.Additionally,they facilitate hydrogen atom diffusion.The reversible alloy hydrogenation reactions are Mg H₂?Mg+H₂ and Mg₂Ni+H₂?Mg₂Ni H₄.After the decomposition of Mg₂Ni H₄,the in-situ formed Mg₂Ni weakens the Mg-H bond,serving as a“hydrogen pump”for dehydrogenation kinetics improvement.Using the ball milling process,Mg₉₆La₃Ni was successfully prepared as a composite material with carbon-loaded various lanthanide rare earth oxide catalysts,each doped with different amounts of the catalyst.The study investigates the catalytic mechanisms of different composite catalysts on the Mg₉₆La₃Ni alloy,with a particular focus on their kinetic and thermodynamic performance during hydrogen adsorption and desorption processes.The research results indicate that the introduction of carbon-loaded lanthanide rare earth oxide catalysts contributes to enhancing the hydrogen adsorption and desorption kinetics and hydrogen storage capacity in the alloy,thereby improving its hydrogen storage performance.However,when the content exceeds a certain threshold,inhibitory effects on catalytic performance are observed.Considering both hydrogen adsorption rate and storage capacity,the alloy exhibits optimal kinetic performance when the addition amounts of La₂O₃@C,Ce O₂@C,Pr O_1.83@C,Eu₂O₃@C,Gd O@C,and Yb₂O₃@C are 1 wt.%,5 wt.%,3 wt.%,2 wt.%,1 wt.%,and 3 wt.%,respectively.At 360°C,the hydrogen saturation ratio(R_2min)after 2 minutes is77.3%,80.7%,78.1%,77.3%,84.4%,and 79.7%,while the corresponding time required for releasing 3%hydrogen is 1.6 minutes,1.5 minutes,1.3 minutes,1.4 minutes,1.6 minutes,and1.3 minutes.The associated hydrogen desorption activation energies are 113.1 k J/mol H₂,107.3k J/mol H₂,103.7 k J/mol H₂,103.6 k J/mol H₂,106.3 k J/mol H₂,and 103.3 k J/mol H₂.From the parameters of activation energy and the time required for releasing 3%H₂(t_3wt.%),the addition of 3%Yb₂O₃@C exhibits the best hydrogen storage performance.However,from the parameters of hydrogen storage capacity and activation energy,5wt.%Ce O₂@C and 3wt.%Pr O_1.83@C perform the best.An appropriate loading of carbon-supported lanthanide rare earth oxides introduces numerous active sites into the magnesium matrix,effectively reducing the activation energy barrier for hydrogen release.However,excessive carbon content completely envelops the metal particles,hindering further increases in active sites and blocking hydrogen diffusion channels,thus suppressing the promotion of dehydrogenation activation.Therefore,controlling the suitable loading of carbon-supported rare earth oxides is crucial for maximizing their catalytic effects and avoiding diminishing returns due to excessive addition.This strategy requires a delicate balance between introducing active sites and facilitating gas diffusion to achieve optimal dehydrogenation performance improvements.The addition of catalysts can significantly enhance the kinetics of hydrogen adsorption and desorption in alloys,but their impact on thermodynamics is limited.This limitation arises because catalysts merely accelerate the rate of hydrogen reactions without altering the overall reaction pathway.Therefore,the catalytic effect on thermodynamics remains constrained.Regarding the cyclic stability assessment of Mg₉₆La₃Ni and carbon-loaded rare earth oxide composite materials,after 10 cycles of hydrogen adsorption and desorption,the composite material exhibits excellent cyclic stability.Neither the hydrogen adsorption/desorption kinetics nor the capacity significantly deteriorates.The carbon effectively suppresses lattice expansion in Mg H₂/Mg particles,thereby enhancing the cyclic stability of Mg H₂.This suggests that the introduction of carbon-supported rare earth oxides not only improves the hydrogen adsorption kinetics of Mg₉₆La₃Ni but also maintains favorable cyclic stability in the composite system.