RESUMEN
In this work, we probed the lithium and sodium intercalation properties in monolayer-stacked NbSe2 bilayer homostructure configurations for their potential application as anode materials in lithium and sodium ion batteries. Similar to known monolayer transition metal dichalcogenides, such as VS2, the structural phase transition barrier of NbSe2 from 1H to 1T is strengthened by lithium and sodium adsorption, implying that it is robust under multiple charging and discharging processes. As multi-layer, stacked 2D materials are more relevant to experiments and their intended applications, four bilayer homostructure stackings were constructed based on the alignment of Nb and Se. All four bilayer homostructure stackings were found to significantly enhance the binding of lithium and sodium at the van der Waals interface, and thus potentially increase the theoretical specific energy capacity of NbSe2via bilayer stacking. In terms of ionic diffusion, it is observed that for all of the bilayer homostructure configurations the diffusion energy barrier for lithium and sodium generally increased compared to the monolayer case. The nature of the stacking appears to affect the diffusion energy barrier with a value of as high as 1.94 eV in the case of sodium for the AB full stacking (compared to 0.08 eV for the monolayer). It is inferred that although the bilayer homostructure stacking of NbSe2 could significantly increase the theoretical specific energy capacity for both lithium and sodium, its drawback is the slowing down of the ion kinetics at the van der Waals interface, which are also important in the charging and discharging processes of a battery system.
RESUMEN
We theoretically investigated hydrogen evolution reaction (HER) on the XRD observed (100), (110), (111), and (210) surfaces of pyrite structure CoS2. The random structure searching method was employed in this work to thoroughly and less-biasedly identify the active sites for each considered surface. We calculated the free energy of hydrogen adsorption, and found that (110) and (210) surfaces are more active than the conventionally assumed (100) facet. While the lowest energy active site on the (100) and (210) surfaces is the five-coordinated transition metal site that is commonly seen in other HER catalysts, the lowest energy active site on the (110) surface is the two-coordinated S site, which is a S tetrahedron with two corners missing. Besides those lowest energy active sites, both (110) and (210) have more than one species of active site on the surface, including not fully coordinated transition metals and sulfur. We further explored the reaction for MnS2, FeS2, and NiS2, and analyzed the density of states. Our results showed both CoS2 and NiS2 (110) and (210) surfaces are catalytically reactive for HER.
RESUMEN
We systematically investigated the potential of single-layer VS2 polytypes as Na-battery anode materials via density functional theory calculations. We found that sodiation tends to inhibit the 1H-to-1T structural phase transition, in contrast to lithiation-induced transition on monolayer MoS2. Thus, VS2 can have better structural stability in the cycles of charging and discharging. Diffussion of Na atom was found to be very fast on both polytypes, with very small diffusion barriers of 0.085 eV (1H) and 0.088 eV (1T). Ab initio random structure searching was performed in order to explore stable configurations of Na on VS2. Our search found that both the V top and the hexagonal center sites are preferred adsorption sites for Na, with the 1H phase showing a relatively stronger binding. Notably, our random structures search revealed that Na clusters can form as a stacked second layer at full Na concentration, which is not reported in earlier works wherein uniform, single-layer Na adsorption phases were assumed. With reasonably high specific energy capacity (232.91 and 116.45 mAh/g for 1H and 1T phases, respectively) and open-circuit voltage (1.30 and 1.42 V for 1H and 1T phases, respectively), VS2 is a promising alternative material for Na-ion battery anodes with great structural sturdiness. Finally, we have shown the capability of the ab initio random structure searching in the assessment of potential materials for energy storage applications.
RESUMEN
We investigated the application of 1T'-MX2 (M = Mo, W; X = S, Se, Te) 2D materials as hydrogen evolution reaction (HER) catalysts using density functional theory. Our results show that 1T'-MX2 have lower energies and are dynamically more stable than their 1T counterparts, therefore likely more relevant to previous experimental findings and applications. We found that sulfides are better catalysts, followed by selenides and tellurides. Specifically, 1T'-MoS2 and WS2 are the best HER catalysts among MX2. We proposed a mechanism, rather than the metallicity surmised previously, based on the calculated density of states. On the other hand, the effectively stretched (compressed) X site on the 1T' 2 × 1 reconstruction from 1T is shown to be more (less) active for the HER. We further exploited the application of external strain to tune and boost the HER performance. Our findings are of significance in the elucidation of previous experimental studies and exploration of potential materials for clean energy applications.
RESUMEN
Utilizing ab initio random structure searching, we investigated Li adsorption on MoS2 and hydrogen molecules on Li-decorated MoS2. In contrast to graphene, Li can be adsorbed on both sides of MoS2, with even stronger binding than on the single side. We found that high coverages of Li can be attained without Li clustering, which is essential for hydrogen storage and Li ion batteries. Moreover, regarding battery applications, Li diffusion was also found to be easy. The fully-lithiated MoS2 can then adsorb H2 with 4.4 wt%. Interestingly, our calculations revealed that hydrogen molecules can be dissociated at high Li coverage with a minimal energy barrier. We further showed that the dissociated hydrogen atom can readily diffuse on the surface, thus keeping the reaction site active. We therefore propose that Li-MoS2 could be an inexpensive alternative catalyst to noble metals in hydrogen dissociation reactions.
