4. Conclusions

Lithium decoration and defects onto silicene were studied using Density Functional Theory (DFT) aiming to enhance its hydrogen storage capabilities. Silicene was chosen due to its much lower defect formation energies compared to graphene which can lead to more sites for viable hydrogen storage. Single vacancy and Stone-Wales defects were created onto pristine silicene and the defect formation energies were found to be 3.55 eV and 2.10 eV respectively which are much lower when compared to other 2D materials such as graphene. The defects were shown to enhance the Li binding energy of silicene with the energies high enough to prevent Li adatom clustering. It was observed that in the defective systems, the Li atom tends to bind to three silicon atoms each, with two of the Silicon atoms sharing the same Si-Li bond length. The slightly shorter Li-Si bond lengths coupled with a much greater region of overlap between the Li (p) and Si (p) orbitals in the SV silicene system indicated that SV silicene has the highest Li binding energy out of the three systems which was measured to be -3.44 eV. Hydrogen adsorption was subsequently studied on the defective systems and was found to be thermodynamically favourable. The H2 binding energy in the Li decorated SW silicene system was found to be slightly higher than the Li decorated SV silicene system measuring to be -0.261 eV/H2, with energies in both the systems lying above the DOE limit. The combined contribution of both Li (p) and Li (s) orbitals in the electronic interaction with hydrogen was observed via the pDOS plots which contributed to the improved hydrogen binding energies in the Li decorated defective systems. Charge differential diagrams were used to verify the electronic charge transfer between the Li and H atoms in the defective systems and it was seen that the silicene substrate led to an indirect improvement in lithium’s ability to effectively store hydrogen. The defective substrates were decorated on both sides with Li atoms to understand their efficiency as high gravimetric density hydrogen storage substrates. SV silicene was shown to bind the two Li atoms with an average binding energy of -3.14 eV while in the case of SW silicene, the average Li binding energy was measured to be -2.66 eV. Upon adsorption of multiple H2 molecules, it was seen that the double side Li decorated defective systems could effectively store up to 28 H2 molecules with the average H2 binding energies still being thermodynamically favourable, showing a maximum gravimetric density of 5.96 wt. %. AIMD simulations were performed to verify the room temperature stability of the Li adatom as well as the stored H2 molecule on the silicene surface. It was thus concluded that lithium decoration coupled with defect engineering on silicene can be used to create high-gravimetric density, physisorption based hydrogen storage systems with further applications in gas sensing and energy storage.