WS2-Graphene van der Waals Heterostructure as Promising Anode Material for Lithium-Ion Batteries: A First-Principles Approach.

In this work, we report the results of density functional theory (DFT) calculations on a van der Waals (VdW) heterostructure formed by vertically stacking single-layers of tungsten disulfide and graphene (WS2/graphene) for use as an anode material in lithium-ion batteries (LIBs). The electronic properties of the heterostructure reveal that the graphene layer improves the electronic conductivity of this hybrid system. Phonon calculations demonstrate that the WS2/graphene heterostructure is dynamically stable. Charge transfer from Li to the WS2/graphene heterostructure further enhances its metallic character. Moreover, the Li binding energy in this heterostructure is higher than that of the Li metal's cohesive energy, significantly reducing the possibility of Li-dendrite formation in this WS2/graphene electrode. Ab initio molecular dynamics (AIMD) simulations of the lithiated WS2/graphene heterostructure show the system's thermal stability. Additionally, we explore the effect of heteroatom doping (boron (B) and nitrogen (N)) on the graphene layer of the heterostructure and its impact on Li-adsorption ability. The results suggest that B-doping strengthens the Li-adsorption energy. Notably, the calculated open-circuit voltage (OCV) and Li-diffusion energy barrier further support the potential of this heterostructure as a promising anode material for LIBs.

Effective Formation of a Mn-ZIF-67 Nanofibrous Network via Electrospinning: An Active Electrocatalyst for OER in Alkaline Medium.

Finding the active center in a bimetallic zeolite imidazolate framework (ZIF) is highly crucial for the electrocatalytic oxygen evolution reaction (OER). In the present study, we constructed a bimetallic ZIF system with cobalt and manganese metal ions and subjected it to an electrospinning technique for feasible fiber formation. The obtained nanofibers delivered a lower overpotential value of 302 mV at a benchmarking current density of 10 mA cm-2 in an electrocatalytic OER study under alkaline conditions. The obtained Tafel slope and charge-transfer resistance values were 125 mV dec-1 and 4 Ω, respectively. The kinetics of the reaction is mainly attributed from the ratio of metals (Co and Mn) present in the catalyst. Jahn-Teller distortion reveals that the electrocatalytic active center on the Mn-incorporated ZIF-67 nanofibers (Mn-ZIF-67-NFs) was found to be Mn3+ along with the Mn2+ and Co2+ ions on the octahedral and tetrahedral sites, respectively, where Co2+ ions tend to suppress the distortion, which is well supported by density functional theory analysis, molecular orbital study, and magnetic studies.

Constructing electrospun spinel NiFe2O4 nanofibers decorated with palladium ions as nanosheets heterostructure: boosting electrocatalytic activity of HER in alkaline water electrolysis.

The development of efficient electrocatalysts for the water splitting process and understanding their fundamental catalytic mechanisms are highly essential to achieving high performance in energy conversion technologies. Herein, we have synthesised spinel nickel ferrite nanofibers (NiFe2O4-NFs) via an electrospinning (ES) method followed by a carbonization process. The resultant fiber was subjected to electrocatalytic water splitting reactions in alkaline medium. The catalytic efficiency of the NiFe2O4-NFs in OER was highly satisfactory. But it is not high enough to catalyse the HER process. Hence, palladium ions were decorated as nanosheets on NiFe2O4-NFs as a heterostructure to improve the catalytic efficiency for HER. Density functional theory (DFT) confirms that the addition of palladium to NiFe2O4-NFs helps to reduce the effect of catalyst poisoning and improve the efficiency of the catalyst. In an alkaline hybrid electrolyser, the required cell voltage was observed as 1.51 V at a fixed current density of 10 mA cm-2.

Enhancing Hydrogen Evolution Reaction Activities of 2H-Phase VS2 Layers with Palladium Nanoparticles.

Effective hydrogen (H2) production with surface engineering of less active catalysts by an innovative approach is followed here. In this work, a non-noble 2H phase of VS2 layers, which showed poor activity for hydrogen evolution reaction (HER) in 0.5 M H2SO4, was made highly active by decorating palladium (Pd) nanoparticles (NPs) over VS2 layers. A density functional theory (DFT) study confirmed the successful binding of Pd with VS2, and the bond length in a Pd4 tetrahedron was measured to be 2.60 Å. In VS2-Pd, Pd as a Pd4 tetrahedron is pointed toward the VS2 layer, and the calculated Pd-S bond distance is 2.42 Å with some expansion of three Pd-Pd bonds (2.85 Å). From the density of states, it was confirmed that the band gap was too high for VS2 (0.2 eV; 2H phase) and was reduced to nearly zero in VS2-Pd (0.05 eV). In the electrocatalytic HER part, the obtained ΔGH values from DFT were 0.05, -0.45, and 0.22 eV for VS2/Pd4, Pd4, and VS2, respectively, which imply that VS2-Pd4 had improved HER activity compared to pristine VS2 and Pd4. A concentration-dependent study was carried out with molar ratios of Pd at 0.01, 0.05, and 0.1 M with VS2 layers. From the HER polarization study, VS2-Pd (0.05 M) showed an overpotential of 157 mV at 20 mA cm-2, which is 373 mV less than only VS2 with a Tafel slope of 75 mV dec-1 with overwhelming stability. These highly promising results will be interesting to make less active stable phases by incorporating metal NPs for efficient and stable H2 production.

Rational Design of Highly Efficient Perovskite Hydroxide for Electrocatalytic Water Oxidation.

The production of hydrogen from ecofriendly renewable technologies like water electrolysis and fuel cells involves oxygen evolution reaction (OER), which plays a major role, but the slow kinetics of OER is a bottleneck of commercialization of such technologies. Herein, we have reported the formation of an efficient OER catalyst from SnCo(OH)6 (SCH) by leaching of Sn atoms during electrochemical OER studies. According to density functional theory calculations, adsorption of OH* species on Sn atoms is energetically more favorable than that of Co atoms, and as a result, highly active CoOOH is generated by leaching of Sn atoms from surface layers. We observed enhanced OER performance with superior mass activity by blending SCH with activated charcoal, which displays a low overpotential of 293 mV and higher mass activity than that of pristine SCH. More importantly, it outperforms Co(OH)2 and RuO2 having the same carbon composition because of the formation of thermodynamically stable and amorphous CoOOH on the surface of single-crystalline SCH and strong tethering ability of activated charcoal.

Atomic and electronic structure of solids of Ge2Br2PN, Ge2I2PN, Sn2Cl2PN, Sn2Br2PN and Sn2I2PN inorganic double helices: a first principles study.

We report the results of density functional theory calculations on the atomic and electronic structure of solids formed by assembling A2B2PN (A = Ge and Sn, B = Cl, Br, and I) inorganic double helices. The calculations have been performed using a generalized gradient approximation for the exchange-correlation functional and including van der Waals interactions. Our results show that the double helices crystallize in a monoclinic lattice with van der Waals type weak interactions between the double helices. In all cases except Ge2Cl2PN, the solids are stable with a binding energy between the double helices ranging from 0.06 eV per atom to 0.09 eV per atom and inter-double helices separation of more than 3.33 Å. All the solids are semiconducting. Further calculations have been done by using meta-GGA with a modified Becke-Johnson functional to obtain better band gaps, which are found to lie in the range of 0.91 eV to 1.49 eV. In the case of Ge2Br2PN the solid is a direct band gap semiconductor although the isolated double helix has an indirect band gap and it is suggested to be interesting for photovoltaic, and other optoelectronic applications. The charge transfer between the atoms has been studied using Bader charge analysis and the DDEC6 method in the CHARGEMOL program, which suggests charge transfer from the outer helix to the inner helix.

Computational Approach To Reveal the Structural Stability and Electronic Properties of Lithiated M/CNT (M = Si, Ge) Nanocomposites as Anodes for Lithium-Ion Batteries.

This work is motivated to explore the structural stability and electronic and electrochemical properties of nanocomposites of M4Li n (M = Si and Ge)-carbon nanotube (CNT) by employing first-principles density functional theory calculations. By analyzing the structural stability of various M4Li n (n = 0-10) clusters, it is revealed that a tetrahedron-shaped M4Li4 Zintl cluster is found to be highly stable. Our study on the interaction between the lithiated clusters and CNT illustrates that the charge transfer from the former to latter plays a pivotal role in stabilizing these nanocomposites. The structural stability of those nanocomposites arises as a consequence of bonding between lithiated clusters and CNT, which is mediated through the cation-π interaction. The strength of the interaction between them is well reflected in electronic structure calculations by shifting the energy levels with respect to the Fermi energy. Further, the electrochemical properties of these nanocomposites are explored by forming an assembly of the cluster-inserted CNT. The calculated average intercalation voltage of the systems is found to be low (maximum ∼1.0 V for M = Si and 1.05 V for M = Ge), which demonstrates their anodic behavior.

Atomic structure and electronic properties of A2B2XY (A = Si-Pb, B = Cl-I, and XY = PN and SiS) inorganic double helices: first principles calculations.

We study the structural stability and electronic properties of new classes of DNA-like inorganic double helices of the type A2B2XY (A = Si-Pb, B = Cl-I, and XY = PN and SiS) by employing first principles density functional theory (DFT) calculations including van der Waals interactions. In these quaternary double helices PN or SiS forms the inner helix while the AB helix wraps around the inner helix and the two are interconnected. We find that the bromides and iodides of Ge, Sn, and Pb as well as Pb2Cl2PN form structurally stable double helices while Ge2I2SiS as well as bromides and iodides of Sn and Pb have stable double helices. The atomic structures of different double helices have been analyzed in detail to understand the stability of these systems as there is up to about 80% difference in the interatomic distances in the two helices which is remarkable. Also in these new classes of double helices there is polar covalent bonding in the inner helix due to heteroatoms. We have calculated the DDEC6 partial atomic charges and bond orders which suggest strong covalent bonding in the inner helix. The electronic structure reveals that these double helices are semiconducting and in many cases the band gap is direct. Furthermore, we have studied the effects of doping and found that hole doping is the most appropriate way to tuning their electronic properties.