Hydrogen Storage Performance Of Metal Decorated Two-dimensional Carbon Nitrides

New two-dimensional materials:two-dimensional carbon nitrides g-C₃N₄ and C₂N,due to their unique porous geometric structure and relatively light weight,have good application prospects in the fields of hydrogen production,hydrogen storage and nanoelectronic devices.However,there is very little research on its hydrogen storage characteristics.It is necessary to study further how to make its hydrogen storage mass density and hydrogen adsorption energy reach the standards.In order to study the hydrogen storage characteristics of two-dimensional g-C₃N₄ and C₂N,based on first-principles methods and metal decoration methods,the paper systematically studied the hydrogen storage characteristics and hydrogen storage mechanism of g-C₃N₄ and C2N.The main research results are as follows:(1)Based on the first-principles density functional theory,using the GGA-PBE exchange correlation function,the material structure,electrical properties and hydrogen storage characteristics of the two-dimensional intrinsic g-C₃N₄(2×2)and C2N(2×2)have been studied in depth.According to the energy band diagram and the density of states diagram,it is found that the single-layer g-C₃N₄ and the single-layer C₂N are both direct bandgap semiconductors,with band gaps of 1.47 e V and 1.65 e V,respectively.There are multiple resonance peaks for C atoms and N atoms.The p-orbital states of C and N are hybridized with each other.The above shows that there is a stable covalent interaction between C atoms and N atoms.The calculation of H2 at different adsorption sites of g-C₃N₄ and C₂N shows that the average hydrogen adsorption energy of intrinsic g-C₃N₄ is-0.059 e V/H2,and the average hydrogen adsorption energy of intrinsic C₂N is-0.070 e V/H2.In order to enhance the H2 adsorption energy,the intrinsic system needs to be further decorated.(2)In order to further enhance the hydrogen storage capacity of g-C₃N₄ and C₂N,based on first-principles density functional theory,the paper uses Li,Ca,Ti atoms to decorate g-C₃N₄ and Ca,Sc,Ti to decorate C₂N.Their geometric structure,electrical properties and bonding methods have been studied in depth.The results show that the most stable binding position for Li,Ca,Ti decorated g-C₃N₄ and Ca,Sc,Ti decorated C₂N is in the(off-center)center pore site.They all exhibit the characteristics of N-type semiconductors.Through the analysis of density of states and bader charge,it is clearly that the bonding mode of metal atoms with g-C₃N₄ and C₂N is ionic bond,and metal atoms have a strong hybridization effect with g-C₃N₄ and C₂N.In general,the metal decorated g-C₃N₄ and C₂N are very similar in structure,but have different effects on their electrical properties.(3)Based on the system of g-C₃N₄ and C₂N decorated by metal atoms at the most stable binding site,the paper systematically studied the hydrogen storage characteristics of Li,Ca,Ti decorated g-C₃N₄ and Ca,Sc,Ti decorated C₂N.The results show that: C₂N decorated with Ca atom can adsorb up to 3 hydrogen molecules;C2N decorated with Sc atom can adsorb up to 3 hydrogen molecules;C2N decorated with Ti atom can adsorb up to 5 hydrogen molecules,which can only adsorb stably 1 hydrogen molecule.The g-C₃N₄ decorated with Li atom can adsorb up to 3 hydrogen molecules;the g-C₃N₄ decorated with Ca atom can adsorb up to 5 hydrogen molecules;the g-C₃N₄ decorated with Ti atom can adsorb up to 4 hydrogen molecules,and it can meet DOE standard adsorption energy for hydrogen storage.Comparing the hydrogen storage performance of metal-decorated C₂N and g-C₃N₄,Ti-decorated g-C₃N₄ has better hydrogen storage capacity.(4)Take the metal-modified g-C₃N₄ hydrogen adsorption system as an example to study the hydrogen storage mechanism of two-dimensional carbon nitrides.According to the DOS diagram,it is found that there is an obvious resonance peak between the metal atom and the H atom,indicating that there is orbital hybridization between the metal atom and the H2 molecule.The differential charge diagram shows that the charge is concentrated between the metal atom and the H2 molecule.And the charge accumulation and depletion of H2 molecules indicate that H2 molecules are polarized.The results of the bard charge analysis further confirmed the charge transfer between the metal and H2.The results show that there is an obvious bonding effect between H2 and the metal-decorated g-C₃N₄ system,and the metal acts as a bridge between H2 and g-C₃N₄,which will be used for the metal-decorated two-dimensional carbon and nitrides for hydrogen storage applications.In general,electrostatic interaction and orbital hybridization are the keys to the hydrogen storage capacity of metal-decorated g-C₃N₄ and C₂N.