Prediction Of Hydrogen Storage Capacity In Quasi-2D System And Theoretical Study Of The 3D Graphene/h-BN Van Der Waals Heterostructures

The application of hydrogen energy has always been an important subject for scientific research.Here,we focus on the development of a theoretical method for estimating the hydrogen storage capacity in certain materials.In recent years,two-dimensional materials such as graphene and hexagonal boron nitride(h-BN)have been found to have enormous potential for application in various fields.In this work,the hydrogen storage capacities of certain graphene and h-BN systems are predicted with the aid of our fugacity method.At the same time,the properties of the 3D periodic grapheme/h-BN van de Waals heterosturcture(G/BN)are theoretically studied.This work involes the following research: 1.Fugacity coefficient methodThe fugacity is defined in thermodynamics as the effect pressure of a real gas,which is also the pressure of an ideal gas with the same chemical potential under the same conditions.The ratio of the fugacity to the pressure is the so-called fugacity coefficient.The DFT calculation in an effective continuous model is a cheap and accurate method for rationally designing gas storage materials.In order to make the method have full physical meaning,the new calculations for the fugacity and EOS insteat of the empirical expressions are nessecery.In this work,the virial expansion(VE)for EOS is adopted.After the theoretical calculation for the virial coefficient with our method,the fugacity coefficient of a non-polar particle fluid and its EOS can be determined.The hydrogen fluid can be handled with our VE method at the temperature in the range of 160-773 K.The hydrogen storage capacity and the detailed thermodynamic information of a designed novel material can thereby be estimated by using this method with relatively high accuracy and low computing cost.2.Hydrogen storage capacity of porous carbon materialsAn expanded bilayer graphene system is a good simplified model for the porous carbon materials.Assuming that the in-and outside phases of the hydrogen fluid are in equilibrium,we can now calculate the hydrogen storage capacity of the expanded bilayer graphene systems with the help of the fugacity coefficients and EOS which are determited above.The bilayer graphene system with a 9 ? interlayer distance possesses the relatively high hydrogen uptake rates,which are around 3.84 wt.% and 3.65 wt.% at T =243 K,P =10 MPa and T = 298 K,P =21 MPa according to our calculations,respectively.These results agree very well with the experimental data.It has been proved that our VE method can be used in predicting the hydrogen storage capacity.We believe that our VE method also has the vigorous potential for the predictions of the storage capacities or the select capabilities in certain material system for other non-polar particle fluids,e.g.,noble gases,methane,nitrogen,oxygen and so on.3.Hydrogen storage capacity of expanded h-BN systemsThe hydrogen storage capacity of the expanded hexagonal Boron Nitride(e eh-BN)systems has also been presented in this work.We have employed the new EOS for hydrogen fluid to figure out the hydrogen density distribution profiles in the eh-BN systems.In this regard,the environmental conditions(i.e.,temperature and pressure)are considered in the prediction procedure using DFT single point calculations.The eh-BN systems with different layer spacings are studied by PBE method with consideration of the long range dispersion corrections.On account of the in-plane polar bonds,a series of adsorption positions are considered.Additionally,the adsorption energy and hydrogen density profiles are reported.Furthermore,the relation between uptakes and the interlayer spacings with the effects of the environmental conditions are also provided.The limit of the physical hydrogen storage capacity in a perfect eh-BN system at 243 K and 10 MPa is founded to be 2.96 wt.%.4.Theoretical study of the 3D-G/BN van de Waals heterostucturesThe 3D periodic graphene/h-BN(G/BN)heterostuctures are studied in this work.The stacking forms of the graphene and h-BN layers are discussed.The varieties of the geometric and electronic configurations details at the interface between graphene and h-BN layers are also reported.The metal-semiconductor transition of the G/BN material can be achieved by adjusting the stacking form of the h-BN layers or changing the proportion of grapheme layers in the unit cell.An electrostatic potential well is found at the interface between the layers.Due to the potential well and the only dispersion correlation at the interface,the dielectric constant zz in vertical direction is independent of the variety of the thickness or the proportion of the compositions in one unit cell.