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.