Theoretical Study On The Hydrogen Storage Properties Of Superalkali NLi₄ Decorated H-BN

The over-exploitation of fossil fuels raises serious energy crisis and the greenhouse gas emissions lead to ever-worsening global environments.Developing non-pollution and renewable new energy sources is an urgent task facing mankind.Due to its environmental friendliness,abundance and high energy density,hydrogen is deemed as one of the most promising energy carriers in the future.However,a safe and efficient storage technique is a substantial challenge to overcome before hydrogen can be used in large scale,especially for on-board vehicle applications.Due to the safety,energy efficiency and high hydrogen storage capacity,solid state storage of hydrogen is an alternative approach and it is divided into physical storage and chemical storage.The storage capacity and binding strength are the key criteria to evaluate the hydrogen storage materials.As far as the storage capacity is concerned,it should be as high as possible.The charging/discharging dynamics are determined by the binding strength,and reversible hydrogen storage at ambient conditions requires an optimal hydrogen adsorption energy around-0.20 e V/H₂ which is between typical physisorption and chemisorption.Up to now,no systems have been found to reach a satisfactory level of performance because hydrogen molecules interact either too weakly or too strongly with the host materials.Developing an efficient storage medium is a substantial challenge for the utilization of hydrogen as an energy carrier,and designing materials with intermediate holding strength for hydrogen is the key to the solution.In this paper we investigate the decoration of NLi₄ on h-BN and the hydrogen storage properties by using density functional theory.NLi₄ clusters can be anchored stably on BN sheets with a binding energy-1.43 e V.As the NLi₄ species has enhanced stability,decoration using the superalkali effectively improve the aggregation of the metal atoms on substrate.Replacing alkali metal atoms by superalkalis also offer more spaces for H₂ adsorption.The H₂ molecules adsorbed on the bottom Li⁺cations are strongly polarized due to the small radius of Li⁺,and the H₂ molecules attached upon the top Li⁺are adsorbed by feeding back the excess electron upto the antibondingσ^*orbitals of the H₂ molecules.Each NLi₄ can absorb 9 H₂ molecules with adsorption energies about-0.20 e V/H₂.Then,we designed two different configurations with the decoration densities（NLi₄/BN）of 1:8and 1:6.The calculation shows that the average adsorption energy of H₂ molecules is-0.19 e V/H₂ and the HSC is 7.44 wt%at NLi₄/BN=1:8.At NLi₄/BN=1:6,the average adsorption energy of H₂ molecules is-0.18 e V/H₂,a high HSC 9.40 wt%is obtained.We ever studied the optimal storage conditions for reversible H₂ adsorption on decorated metal cations by considering the entropy effects.The results show that superalkali NLi₄ decorated h-BN can be promising hydrogen storage material with high storage capacities.