Effect of the oxygen vacancy electronic state on Ni migration in Li0.5(Ni0.8Mn0.1Co0.1)O2 cathode material.

Cation migration coupled with oxygen vacancy formation is known to drive the layered to disordered spinel/rock-salt phase transformation in the high-Ni layered oxide cathodes of Li-ion batteries. However, the effect of different electronic states of oxygen vacancies on the cation migration still remains elusive. Here, we investigate Ni migration in delithiated Ni-rich Li0.5Ni0.8Mn0.1Co0.1O2 (hence Li0.5NMC811) in the presence of neutral and charged oxygen vacancies by means of first-principles density functional theory (DFT) calculations coupled with the nudged elastic band (NEB) method. We find that oxygen vacancies with neutral or +2 charge favor the Ni migration to Li tetrahedral and/or octahedral sites, both thermodynamically and kinetically. As for the case of +1 charged oxygen vacancies, while they thermodynamicaly favor the Ni migration to the Li site, the relatively high migration barrier suggests that they kinetically prohibit the Ni migration. Our results suggest that controlling the formation of oxygen vacancies is the key to enhancing the Ni-rich NMC structural stability in particular in their charged states.

Revealing the incorporation of an NH2 group into the edge of carbon dots for H2O2 sensing via the C-N⋯H hydrogen bond interaction.

We investigated hydrogen peroxide (H2O2) sensing on NH2-functionalized carbon dots (Cdots) for three different -NH2 positions, and the N atom was found to be the active site using a quantum computational approach. B3LYP and 6-31G(d,p) were used for density functional theory (DFT) ground state calculations, whereas CAM-B3LYP and the same basis set were used in time-dependent density functional theory (TD-DFT) excited state calculations. Structural optimization showed that the H2O2 is chemisorbed on 1-sim via a C-N⋯H hydrogen bond interaction with an adsorption energy of -10.61 kcal mol-1. Mulliken atomic charge distributions and electrostatic potential (ESP) analysis were both used to determine reactivity of the molecules at the atomic level. For in-depth analysis of the ground states, we utilized Frontier molecular orbital (FMO) theory, quantum theory of atoms in molecules (QTAIM), and non-covalent interaction (NCI) index analysis. In addition, we also present UV-vis absorption spectra and charge transfer lengths to understand the mechanism of H2O2 sensing in excited states. Based on the molecular and electronic properties of the NH2-Cdots, it was shown that 1-sim is a potential candidate for use as an electrochemical sensor for H2O2 sensing. Whereas 3-sim is believed to be a potential candidate for use as an optical sensor of H2O2 based on the UV-vis characteristics via photoinduced charge transfer.

Molecular insight into the role of zeolite lattice constraints on methane activation over the Cu-O-Cu active site.

Understanding the factors that influence the activity of a catalyst toward CH4 activation is of high importance for tuning the catalyst performance or designing new, better catalysts. Here, we performed a set of density functional theory (DFT) calculations on the H-CH3 bond cleavage over the Cu-O-Cu active site in the MOR zeolite with various Al-pair arrangements to obtain molecular insight into the structure-activity relation and clarify key parameters that define the Cu-O-Cu reactivity toward CH4. We found that weakening of the Cu-O-Cu bond during CH4 activation is crucial for determining the O-H bond strength and thus the Cu-O-Cu reactivity. In this regard, the zeolite lattice constraints are found to play a significant role as, on the one hand, it strengthens the Cu⋯Cu interaction and consequently weakens the Cu-O-Cu bonds and, on the other hand, it forces the Cu-O-Cu bond elongation process to destabilize the active site structure. The non-planar Cu-O-Cu geometry, due to lattice constraints, is also found to make the CH4 adsorption site, whether positioned closer to the µ-O or the Cu atom, crucial in determining the C-H activation product, i.e., a ˙CH3 radical or a Cu2-CH3- ligand.

DFT + U study of H2O adsorption and dissociation on stoichiometric and nonstoichiometric CuO(1 1 1) surfaces.

Surface interaction through adsorption and dissociation between H2O and metal oxides plays an important role in many industrial as well as fundamental processes. To gain further insights on the interaction, this study performs dispersion-corrected Hubbard-corrected density functional theory calculations in H2O adsorption and dissociation on stoichiometric and nonstoichiometric CuO(1 1 1) surfaces. The nonstoichiometric surfaces consist of oxygen vacancy defect and oxygen-preadsorbed surfaces. This study finds that H2O is chemically adsorbed on the top of Cusub and Cusub-Cusub bridge due to the interaction of its p  orbital with d orbital of Cu. The adsorption is found to be the strongest on the surface with the oxygen vacancy defect, followed by the stoichiometric surface, and the oxygen-preadsorbed surface. The oxygen vacancy increases the reactivity for H2O adsorption and reduces the reaction energy required for H2O dissociation on the surface. However, the surface modification by the oxygen-preadsorbed significantly reduces the barrier energy for H2O dissociation when compared with the other surfaces.

Holey Assembly of Two-Dimensional Iron-Doped Nickel-Cobalt Layered Double Hydroxide Nanosheets for Energy Conversion Application.

Layered double hydroxides (LDHs) containing first-row transition metals such as Fe, Co, and Ni have attracted significant interest for electrocatalysis owing to their abundance and excellent performance for the oxygen evolution reaction (OER) in alkaline media. Herein, the assembly of holey iron-doped nickel-cobalt layered double hydroxide (NiCo-LDH) nanosheets ('holey nanosheets') is demonstrated by employing uniform Ni-Co glycerate spheres as self-templates. Iron doping was found to increase the rate of hydrolysis of Ni-Co glycerate spheres and induce the formation of a holey interconnected sheet-like structure with small pores (1-10 nm) and a high specific surface area (279 m2 g-1 ). The optimum Fe-doped NiCo-LDH OER catalyst showed a low overpotential of 285 mV at a current density of 10 mA cm-2 and a low Tafel slope of 62 mV dec-1 . The enhanced OER activity was attributed to (i) the high specific surface area of the holey nanosheets, which increases the number of active sites, and (ii) the improved kinetics and enhanced ion transport arising from the iron doping and synergistic effects.

DFT and microkinetic investigation of methanol synthesis via CO2 hydrogenation on Ni(111)-based surfaces.

A DFT study of methanol production via CO2 hydrogenation reactions on clean Ni(111) and Ni(111)-M (M = Cu, Pd, Pt, or Rh) surfaces has been performed. The reaction network of this synthesis reaction has been determined using energy profiles. The competing reaction network between the formate-mediated route and the carboxyl-mediated route is also presented. Both routes are equally possible in mediating the overall synthesis reactions. A simple selectivity analysis based on the energy barrier shows that methanol synthesis is more preferred rather than formic acid (HCOOH) or carbon monoxide (CO) production. A mean-field kinetic analysis is also employed to determine the kinetic performance of all catalytic surfaces. The formate-mediated route is found to be energetically and kinetically more dominant than the carboxyl-mediated route. Cu, Pd, and Pt dopants are successful in increasing the kinetic performance of the clean Ni(111) surface in the formate route and Cu, Pt, and Rh dopants in the carboxyl route.

Density functional study of methyl butanoate adsorption and its C-O bonds cleavage on MoS2-based catalyst with various loads of Ni promoters.

Due to the increasing demands of new and renewable energy sources by utilising plant oils, uncovering the underlying physico-chemical phenomena at the atomic level responsible for the effective deoxygenation plays a vital role in improving the performance of well-known as well as in looking for the possible new catalysts. This study aims at investigating the adsorption and C-O bonds cleavage of methyl butanoate (MB) over MoS2-based catalyst with various loads of Ni promoters by using first-principles density functional theory (DFT). This study employs surface model that never been used by previous researchers for their investigations of adsorption and bonds cleavage on Ni promoted MoS2-based catalysts. The introduction of nickel into MoS2-based catalyst allows the surface charges when interacts with MB to redistribute in such a way that induces stronger Coulombic attractive forces. This in turn could result in a more stable adsorption configuration. However only in certain Ni-loads will results in the most stable adsorption. Nevertheless the most stable adsorption of MB occurs on M-edge configuration which consists of two Ni atoms, i.e. M-2-Ni-A with adsorption energy at about -2.96 eV. As a comparison, the adsorption energy of MoS2 with the absent of Ni, i.e. M-0-Ni is just -2.79 eV. Since there are three C-O bonds in MB, this study proposes three possible reactions for these bonds to cleave. By using CI-NEB method, the activation energies of those three reaction are calculated. It shows that the presence of Ni with appropriate load could promote C-O bond cleavage, especially in one reaction C-O bond is weaken considerably. Further evaluation on bond dissociation energies of the closest C-C bond to the catalyst surface, M-2-Ni-A shows better reactivity on C-C bond cleavage than M-0-Ni, disregarding of those three reaction routes.

A combined spectroscopic and TDDFT study of natural dyes extracted from fruit peels of Citrus reticulata and Musa acuminata for dye-sensitized solar cells.

This study reports the novel spectroscopic investigations and enhanced the electron transfers of Citrus reticulata and Musa acuminata fruit peels as the photosensitizers for the dye-sensitized solar cells. The calculated TD-DFT-UB3LYP/6-31+G(d,p)-IEFPCM(UAKS), experiment spectra of ultra-violet-visible spectroscopy, and Fourier transform infrared spectroscopy studies indicate the main flavonoid (hesperidin and gallocatechin) structures of the dye extracts. The optimized flavonoid structures are calculated using Density functional theory (DFT) at 6-31+G(d,p) level. The rutinosyl group of the hesperidin pigment (Citrus reticulata) will be further investigated compared to the gallocatechin (Musa acuminata) pigment. The acidity of the dye extract is treated by adding 2% acetic acid. The energy levels of the HOMO-LUMO dyes are measured by a combined Tauc plot and cyclic voltammetry contrasted with the DFT data. The electrochemical impedance spectroscopy will be performed to model the dye electron transfer. As for the rutinosyl group presence and the acidic treatment, the acidified Citrus reticulata cell under continuous light exposure of 100mW·cm-2 yields a short-circuit current density (Jsc) of 3.23mA/cm2, a photovoltage (Voc) of 0.48V, and a fill factor of 0.45 corresponding to an energy conversion efficiency (η) of 0.71% because the shifting down HOMO-LUMO edges and the broadening dye's absorbance evaluated by a combined spectroscopic and TD-DFT method. The result also leads to the longest diffusion length of 32.2µm, the fastest electron transit of 0.22ms, and the longest electron lifetime of 4.29ms.

First principles calculation on the adsorption of water on lithium-montmorillonite (Li-MMT).

The interaction of water molecules and lithium-montmorillonite (Li-MMT) is theoretically investigated using density functional theory (DFT) based first principles calculation. The mechanism of water adsorption at two different water concentrations on Li-MMT as well as their structural and electronic properties are investigated. It is found that the adsorption stability in Li-MMT is higher in higher water concentration. It is also found that an adsorbed water molecule on Li-MMT causes the Li to protrude from the MMT surface, so it is expected that Li may be mobile on H(2)O/Li-MMT.

Absorption of lithium in montmorillonite: a density functional theory (DFT) study.

The absorption of lithium in montmorillonite [LiSi8(Al3Mg)O20(OH)4] was investigated using Density Functional Theory (DFT). The final position of lithium after absorption was found to be in good agreement with an experimental observation where lithium atom migrated from the interlayer into the vacant octahedral site of montmorillonite. The lithium absorbed on montmorillonite was held together by a very strong attraction between ions and exhibited an insulating behavior as depicted from the density of states curve. Due to the presence of lithium in the octahedral site of montmorillonite, the OH group reoriented itself perpendicular to the ab plane and an electron of lithium was transferred in order to compensate the existing net charge of montmorillonite caused by isomorphous substitutions. Relative small charge transfer was observed between lithium and montmorillonite.