Investigation Of Vanadium Alloys And Two-dimensional Materials For Onboard Hydrogen Storage

Compared with traditional fossil fuels,hydrogen is clean,efficient and renewable,thus it can be the next generation of fuels.A major obstacle in utilizing hydrogen as a fuel in vehicles is the difficulty of storing it.Solid-state hydrogen storage materials show potential for onboard hydrogen storage because of their own unique properties.Hydrogen can be efficiently stored in solid state materials in two different forms,atomic and molecular.Accordingly,two typical hydrogen storage material classes,vanadium based alloys and metal decorated two-dimensional(2D)materials,were studied in this paper.Vanadium-based alloys are interesting materials for reversible hydrogen storage with their high hydrogen storage capacity(4 wt.%)and fast(de)hydrogenation kinetics at moderate conditions.During(de)hydrogenation process,vanadium alloys have two plateaus,corresponding to the reaction between V and VH1-x,VH1-x and VH2-x.We applied a two-track approach that involved pressure-composition-temperature(PCT)measurements and density functional theory(DFT)calculations to study alloying effects on hydrogen storage properties of vanadium.1.V-3A binary alloys of V97Al3,V97Mn3,and V97Ru3 were prepared by vacuum arc melting and their hydrogen storage properties on the second platform of PCT curve were studied.Compared with pure vanadium,it was found that the addition of alloying elements increased hydrogen desorption plateau pressure.Among studied materials,V-3Ru showed the highest hydrogen desorption plateau pressures between T=373 and 433 K.During hydrogen loading,V-3Ru also reached the highest hydrogen to metal ratio between the alloys.Findings of hydrogen absorption and desorption measurements were supported with DFT calculations of hydride formation energies.The calculated density of states,electron localization function and atomic charges were used to study the effects of alloying on the electronic properties of hydrides.The bonding interactions in hydrides originated from the hybridization of H s electrons and the transition metal(V,Mn,and Ru)p and d electrons.As Al is missing d electrons,its interaction with nearby hydrogens operated mainly through the s and p states of the metal.In contrary to V and Al that were positively charged in all hydrides,both Mn and Ru were charged negatively in lower V-3A hydrides,but they became positively charged in higher hydrides.Additionally,the electron depletion of V atoms and ionicity in bonding increased with increasing amount of hydrogen inserted into V-3A alloys.2.The hydrogen absorption properties and diffusion behavior in V-3A alloys(V,V97Al3,V97Ru3)were further studied and it was found that addition of Al and Ru could increase first plateau pressure of PCT curve.With the largest surface area among these alloy powders,V-3Ru achieved the highest hydrogen diffusivity at 433 K under an initial pressure of 1500 Pa.Hydrogen diffusion behaviors were examined by DFT calculations of barriers in both surface and internal cases of V-3 A alloys.Through Cl-NEB analysis it was found that the hydrogen diffusion barriers from surface to interior of V-3 A were all around 1,10 eV,much higher than the corresponding internal(?0.10 eV)or surface(?0.30 eV)cases.By comparing diffusions with SEM micrographs,it was proved that surface morphology played a decisive role in initial hydrogen diffusivities.Metal-modified 2D materials have been extensively studied theoretically and experimentally due to their moderate hydrogen molecular adsorption energy,large hydrogen storage capacity and fast hydrogen absorption and desorption kinetics.Within the framework of DFT,hydrogen storage properties of two new 2D materials,Li-modified B2S structure and TiB4 monolayer,were predicted.1.Hydrogen storage properties of Li functionalized B2S honeycomb monolayers were studied using DFT calculations.The binding of H2 molecules to the clean B2S sheet proceeded through physisorption.Dispersed Li atoms on the monolayer surface increased both the hydrogen binding energies and the hydrogen storage capacities significantly.Additionally,ab initio molecular dynamics calculations showed that there was no kinetic barrier during H2 desorption from lithiated B2S.Among the studied B8S4Lix(x=1,2,4,and 12)compounds,the B8S4Li4 was found to be the most promising candidate for hydrogen storage purposes,with a 9.1 wt.%H2 content and 0.14 eV/H2 average hydrogen binding energy.Furthermore,a detailed analysis of the electronic properties of the B8S4Li4 compound before and after H2 molecule adsorption confirmed that the interactions between Li and H2 molecules were of electrostatic nature.2.Both CH4 and H2 were chosen as adsorption molecules and their interactions with TiB4 sheet were investigated.TiB4 attracted gas molecules through the open Ti sites and each Ti atom could adsorb two molecules to the most.Through electronic density of states and atomic charges analysis we found the mechanism of the gas adsorption was mainly electrostatic.For H2 adsorption cases,orbital interactions also made contributions.As one CH4 molecule has three times higher combustion energy than one H2,the TiB4-2CH4 compound could reach best equivalent gravimetric hydrogen density of 10.14 wt.%with an average adsorption energy of 0.38 eV.Ab initio molecular dynamics calculations on this compound showed that there was no kinetic barrier during CH4 desorption.Besides,the stacking of the TiB4 monolayers could weaken the energy storage capacity,so it should be avoided in practial usage.