Theoretical Study On Hydrogen Storage Properties Of Superalkali NLi₄ Decorated Graphene

The over-dependence of our current energy infrastructure on fossil fuels leads to heavy greenhouse gas emissions and poses serious threats to our living environment.Providing sustainable clean energy sources is one of the key technological challenges facing mankind.Hydrogen is considered an ideal energy carrier due to its highest energy density per mass and the clean combustion products.Reversible hydrogen storage with high densities at ambient temperatures is the key obstacles to the wide use of hydrogen as an energy carrier,especially for the hydrogen fuel-cell powered vehicles.Relative to the conventional high-pressure gaseous（35–70 MPa at ambient temperature）and cryoliquid（0.1–1 MPa,-253°C）storage methods,solid state storage of hydrogen is potentially superior with regard to safety,energy efficiency,and the storage capacity.A practical storage system should be able to operate under ambient temperatures with high reversible delivery capacity and suitable uptake-release kinetics.According to the target settled by U.S.Department of Energy,the storage capacity should be at least 5.5 wt.%at operating temperatures-40 to 60°C with a charging/discharging rate of 1.5 kg H₂/min.Reversible hydrogen storage at ambient conditions requires an optimal hydrogen adsorption energy around-0.20 e V/H₂.This binding strength is between typical physisorption and chemisorption.Light alkaline metal decorated nanostructures are considered potential hydrogen storage materials with proper hydrogen binding strengths for ambient temperature storage.However,the aggregation of the metal atoms over the support materials is the key issue to solve.This paper offers a new idea by replacing the alkaline metals with superalkali cluster NLi₄,and investigate the decoration of NLi₄ on graphene and the hydrogen storage properties by using first principles calculations.The results show that the NLi₄ units can be stably anchored on the graphene monolayer while the Li atoms are strongly bound together in the superalkali cluster.The NLi₄ clusters transfer electrons to graphene and the Li atoms exist as cations.Decoration using the superalkali clusters not only solve the aggregation of metal atoms,it also provide more adsorption sites for hydrogen.Computations show that each NLi₄ unit can adsorb up to 10 H₂ molecules,and the adsorption energies are over-0.21 e V/H₂.We design two decoration configurations with different NLi₄ densities（the N:C ratios）of 1:16 and 1:12,and the investigation shows that the hydrogen storage capacity is 8.55 wt.%at N:C=1:16.When the the N:C=1:12,the hydrogen storage capacity reaches 10.75 wt.%.The average adsorption energies of H₂ molecules are-0.21 e V/H₂ for both of the configurations.We also compute the zero-point energies and the entropy change upon adsorption based on the harmonic frequencies.After considering the entropy effect,the adsorption strengths fall in the ideal window for reversible hydrogen storage at ambient temperatures.So NLi₄ decorated graphene can be promising hydrogen storage material with high storage capacities.