Synergistic dual electrolyte additives for fluoride rich solid-electrolyte interface on Li metal anode surface: Mechanistic understanding of electrolyte decomposition.

Improving the quality of the solid-electrolyte interphase (SEI) layer is highly imperative to stabilize the Li-metal anodes for the practical application of high-energy-density batteries. However, controllably managing the formation of robust SEI layers on the anode is challenging in state-of-the-art electrolytes. Herein, we discuss the role of dual additives fluoroethylene carbonate (FEC) and lithium difluorophosphate (LiPO2F2, LiPF) within the commercial electrolyte mixture (LiPF6/EC/DEC) considering their reactivity with Li metal anodes using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Synergistic effects of dual additives on SEI formation mechanisms are explored systematically by invoking different electrolyte mixtures including pure electrolyte (LP47), mono-additive (LP47/FEC and LP47/LiPF), and dual additives (LP47/FEC/LiPF). The present work suggests that the addition of dual additives accelerates the reduction of salt and additives while increasing the formation of a LiF-rich SEI layer. In addition, calculated atomic charges are applied to predict the representative F1s X-ray photoelectron (XPS) signal, and our results agree well with the experimentally identified SEI components. The nature of carbon and oxygen-containing groups resulting from the electrolyte decompositions at the anode surface is also analyzed. We find that the presence of dual additives inhibits undesirable solvent degradation in the respective mixtures, which effectively restricts the hazardous side products at the electrolyte-anode interface and improves SEI layer quality.

Eliminating ciprofloxacin antibiotic contamination from water with a novel submerged thermal plasma technology.

Thermal plasma is successfully used to degrade the model pharmaceutical wastewater ciprofloxacin (CIP) under submerged operating conditions at atmospheric pressure. The model aqueous solution is prepared for two different concentrations (10 and 25 mg/L) and treated separately at 7 kW discharge power with two different plasma-forming gas compositions, Ar/Air and Ar/CO2. A direct current (DC) hollow cathode plasma torch produces a thermal plasma jet inside the solution. The effect of plasma gas compositions on the CIP degradation process is investigated, and the corresponding degradation and mineralisation efficiencies for different treatment times are systematically compared using high-performance liquid chromatography (HPLC) and total organic carbon (TOC) analysis, respectively. Submerged Ar/CO2 plasma shows higher degradation and mineralisation efficiency than the Ar/Air plasma. Energy yields of 74.32 mg/kWh and 176.98 mg/kWh are achieved for a 5-min treatment by Ar/CO2 submerged thermal plasma at concentrations of 10 mg/L and 25 mg/L, respectively. The degradation of CIP by submerged plasma shows a resemblance with first-order reaction kinetics having reaction rates 0.149 min-1 and 0.073 min-1 for Ar/CO2 and Ar/Air, respectively. Density Functional Theory (DFT) calculations are used to identify the various reactive sites on CIP, and the results are consistent with the formation of various intermediates detected through liquid chromatography-mass spectrometry (LC-MS) analysis. These findings suggest that reactive species formed through thermal and photochemical processes in submerged thermal plasma play a significant role in the degradation of CIP. This study also offers a possible way of using CO2 gas in wastewater treatment using submerged thermal plasma.

Combined Density Functional Theory and Microkinetics Study to Predict Optimum Operating Conditions of Si(100) Surface Carbonization by Acetylene for High Power Devices.

The Si(100) surface carbonization mechanisms by acetylene are explored using density functional theory calculations combined with microkinetic simulations. The most stable acetylene adsorption geometries and their subsequent decomposition mechanisms to form a carbon dimer on the Si surface are investigated. Microkinetics simulations are further used to examine the optimal reaction conditions for obtaining a single-crystalline silicon carbide (SiC). We find that the carbon dimer (C2*) as an end-bridge structure can be formed at 560 K, and the maximum of C2* can be obtained near 640 K. The acetylene adsorbed via the di-σ configuration starts to dehydrogenate when the heating rate is too fast and will form two possible carbon dimers (di-C2* and C2*), which will lead to a polycrystalline SiC buffer layer. We predict that 750 K and 10-6 bar will be the optimum temperature and pressure for obtaining a single-crystalline SiC buffer layer, respectively.

Enhanced moisture stability of cesium lead iodide perovskite solar cells - a first-principles molecular dynamics study.

An understanding of the interaction of water with perovskite is crucial in improving the structural stability of the perovskite. Hence, in this study, the structural and electronic properties of the γ-CsPbI3(220) perovskite surface upon the adsorption of water molecules have been investigated based on density functional theory calculations. Also, we perform first-principles ab initio molecular dynamics simulations (AIMD) to explore the structural stability of the γ-CsPbI3(220) perovskite surface in the presence of water molecules, and the results are compared with the conventional cubic CH3NH3PbI3(100) perovskite surface. The water molecules show stronger interactions with the (220) surface of γ-CsPbI3 than the (100) surface of CH3NH3PbI3. However, AIMD results demonstrate that the former is much more stable, and no trace of surface degradation was observed upon the adsorption of water molecules.

Understanding the Role of Dopant Metal Atoms on the Structural and Electronic Properties of Lithium-Rich Li1.2Ni0.2Mn0.6O2 Cathode Material for Lithium-Ion Batteries.

Improving the stability of lithium-rich cathode materials is important in refining the overall performance of lithium-ion batteries. Here, we have proposed doping of different metal atoms such as K+, Ca2+, Cd2+, and Al3+ in different sites of Li1.2Ni0.2Mn0.6O2, and we have investigated their structural and electronic properties using first-principles calculations. We found that the Ni ions in the pristine Li1.2Ni0.2Mn0.6O2 structure maintained the +3 oxidation state for a longer time and resulted in the structural deformation during the long cycling process. Whereas, the Ni ions in the Cd-, K-, and Ca-doped Li1.2Ni0.2Mn0.6O2 structure are in the +3 oxidation state for a very short time, compared to the pristine system. Our density functional theory (DFT) results show that the doping of the Cd ion in the Ni site of Li1.2Ni0.2Mn0.6O2 is the most suitable one, because it inhibits structural change, decreases the formation energy, and suppresses the Jahn-Teller distortion, compared with the pristine system and other dopant atoms. This theoretical study gives new insight about doping strategy and will help in improving the electrochemical performance of Li-rich cathode materials.

Theoretical Study of Electrochemical and Electrochromic Properties of Novel Viologen Derivatives: Effects of Donors and π-Conjugation.

We propose a linkage approach by merging ambipolar electrochromic (EC) materials in both π-acceptor-π (π-A-π) and donor-acceptor-donor (D-A-D) configurations and investigated their electrochemical and spectroelectrochemical properties using density functional theory calculations. Here, we considered anthracene, toluene, and pyrene as π-conjugated molecules, triphenylamine (TPA) as a donor, and viologen as an acceptor moiety for π-A-π and D-A-D configurations. We have also explored the substitutional effects in the donor moiety on the overall electrochromism during both oxidation and reduction processes. Here, we mainly focused on the relationship between the structure, substitution of functional groups, electronic and spectral properties, as well as redox potential of the designed monomers. Our calculations indicate that the designed monomers have attractive absorption properties and show clear color switching upon the redox process. We find that the substitution of stronger electron-donating and π-spacer groups create new absorption peaks in the oxidation states. These designed viologen derivatives will be potential candidates, which can be used in both oxidation and reduction processes for upcoming EC devices.

Temperature-programmed desorption studies of NH3 and H2O on the RuO2(110) surface: effects of adsorbate diffusion.

Temperature-programmed desorption (TPD) is one of the most straightforward surface science experiments for the determination of the thermodynamic and kinetic parameters of a reaction. In our previous study (J. Phys. Chem. C, 2013, 117, 6136-6142), we proposed a model combining DFT methods with microkinetics to investigate the TPD spectra of NH3 and H2O on the RuO2(110) surface. Although our model predicted both the physisorption and chemisorption peaks of both adsorbates in agreement with the experimental TPD spectra, it failed to explain the region between the physisorption and chemisorption areas and underestimated the intensity of the adsorbate in these areas. Hence, to improve our model, in this study, we considered the diffusion of adsorbates from the sub-monolayer to the second layer. Accordingly, we simulated the TPD spectra of both NH3 and H2O on the RuO2(110) surface using condensation approximation. Our results indicate that the diffusion barriers of the adsorbates at high coverage are smaller than their direct desorption energies. Hence, the diffusion of the adsorbates to the second layer from the sub-monolayer at higher coverage is kinetically favorable, which then desorb directly even at low temperatures. Furthermore, the simulated TPD spectra clearly depict the previous experimental results of both adsorbates after considering the diffusion.

Revealing the influence of Cyano in Anchoring Groups of Organic Dyes on Adsorption Stability and Photovoltaic Properties for Dye-Sensitized Solar Cells.

Determining an ideal adsorption configuration for a dye on the semiconductor surface is an important task in improving the overall efficiency of dye-sensitized solar cells. Here, we present a detailed investigation of different adsorption configurations of designed model dyes on TiO2 anatase (101) surface using first principles methods. Particularly, we aimed to investigate the influence of cyano group in the anchoring part of dye on its adsorption stability and the overall photovoltaic properties such as open circuit voltage, electron injection ability to the surface. Our results indicate that the inclusion of cyano group increases the stability of adsorption only when it adsorbs via CN with the surface and it decreases the photovoltaic properties when it does not involve in binding. In addition, we also considered full dyes based on the results of model dyes and investigated the different strength of acceptor abilities on stability and electron injection ability. Among the various adsorption configurations considered here, the bidentate bridging mode (A3) is more appropriate one which has higher electron injection ability, larger VOC value and more importantly it has higher dye loading on the surface.

First principles study of organic sensitizers for dye sensitized solar cells: effects of anchoring groups on optoelectronic properties and dye aggregation.

We have designed a new set of D-π-A type organic dye sensitizers with different acceptor and anchoring groups, and systematically investigated their optoelectronic properties for efficient dye sensitized solar cell applications. Particularly, we have focused on the effects of anchoring groups on the dye aggregation phenomenon. TDDFT results indicate that the dyes with CSSH anchoring groups exhibit improved optoelectronic properties compared to other dyes. Further, molecular dynamics simulations have been performed to describe the formation of dye aggregation due to intermolecular hydrogen bonding. The observed results indicate that dyes with CSSH anchoring groups are less prone to aggregate because of their very weak intermolecular interactions.

A First Principles study on Boron-doped Graphene decorated by Ni-Ti-Mg atoms for Enhanced Hydrogen Storage Performance.

We proposed a new solid state material for hydrogen storage, which consists of a combination of both transition and alkaline earth metal atoms decorating a boron-doped graphene surface. Hydrogen adsorption and desorption on this material was investigated using density functional theory calculations. We find that the diffusion barriers for H atom migration and desorption energies are lower than for the previously designed mediums and the proposed medium can reach the gravimetric capacity of ~6.5 wt % hydrogen, which is much higher than the DOE target for the year 2015. Molecular Dynamics simulations show that metal atoms are stably adsorbed on the B doped graphene surface without clustering, which will enhance the hydrogen storage capacity.

Theoretical study on molecular design and optical properties of organic sensitizers.

A series of organic sensitizers based on different configurations such as D-π-A, D-[π]n-A, D-π-[A]n, [D]n-π-A, D-π-A-π-D, D-π-[A]n-π-D and D-[π-A]n-π-D (where n = 1-4) are designed using theoretical methods. The effects of repeating π-linker, donor-acceptor moieties and the substitution of additional donor-acceptor moieties on the optoelectronic properties have been addressed. Our results show that the strength of the acceptor units changes the mono band absorption into dual band absorption in all the considered strategies. We found that repeating π-linker/donor moieties in the D-π-A series enhances the intensity of the absorption and can tune their absorption spectra from UV-to-visible and visible to near IR regions by repeating acceptor units. Also, the present results indicate that the D-π-A-π-D configuration shows improved optical properties than the conventional D-π-A structure. This theoretical study explores the new configurations and design strategies of organic dyes for developing efficient light harvesting devices working in the whole visible and near IR regions.

Theoretical studies on effective metal-to-ligand charge transfer characteristics of novel ruthenium dyes for dye sensitized solar cells.

The development of ruthenium dye-sensitizers with highly effective metal-to-ligand charge transfer (MLCT) characteristics and narrowed transition energy gaps are essential for the new generation of dye-sensitized solar cells. Here, we designed a novel anchoring ligand by inserting the cyanovinyl-branches inside the anchoring ligands of selected highly efficient dye-sensitizers and studied their intrinsic optical properties using theoretical methods. Our calculated results show that the designed ruthenium dyes provide good performances as sensitizers compared to the selected efficient dyes, because of their red-shift in the UV-visible absorption spectra with an increase in the absorption intensity, smaller energy gaps and thereby enhancing MLCT transitions. We found that, the designed anchoring ligand acts as an efficient "electron-acceptor" which boosts electron-transfer from a -NCS ligand to this ligand via a Ru-bridge, thus providing a way to lower the transition energy gap and enhance the MLCT transitions.

Nitrogen and Sulfur Compounds in Atmospheric Aerosols: A New Parametrization of Polarized Molecular Orbital Model Chemistry and Its Validation against Converged CCSD(T) Calculations for Large Clusters.

The parametrization of the polarized molecular orbital (PMO) method, which is a neglect-of-diatomic-differential-overlap (NDDO) semiempirical method that includes polarization functions on hydrogens, is extended to include the constituents that dominate the nucleation of atmospheric aerosols, including ammonia, sulfuric acid, and water. The parametrization and validation are based mainly on CCSD(T)/CBS results for atmospheric clusters composed of sulfuric acid, dimethylamine, and ammonia and on M06-2X exchange-correlation functional calculations for other constituents of the atmospheric aerosols. The resulting model, called PMO2a, is parametrized for molecules containing any type of H, C, or O, amino or ammonium N, and S atoms bonded to O. The new method gives greatly improved electric polarization compared to any other member of the family of NDDO methods. In addition, PMO2a is shown to outperform previous NDDO methods for atomization energies and atmospheric aerosol reaction energies; therefore, its use can be recommended for realistic simulations.

Polarized Molecular Orbital Model Chemistry 3. The PMO Method Extended to Organic Chemistry.

The polarized molecular orbital (PMO) method, a neglect-of-diatomic-differential-overlap (NDDO) semiempirical molecular orbital method previously parameterized for systems composed of O and H, is here extended to carbon. We modified the formalism and optimized all the parameters in the PMO Hamiltonian by using a genetic algorithm and a database containing both electrostatic and energetic properties; the new parameter set is called PMO2. The quality of the resulting predictions is compared to results obtained by previous NDDO semiempirical molecular orbital methods, both including and excluding dispersion terms. We also compare the PMO2 properties to SCC-DFTB calculations. Within the class of semiempirical molecular orbital methods, the PMO2 method is found to be especially accurate for polarizabilities, atomization energies, proton transfer energies, noncovalent complexation energies, and chemical reaction barrier heights and to have good across-the-board accuracy for a range of other properties, including dipole moments, partial atomic charges, and molecular geometries.

A Benchmark Test Suite for Proton Transfer Energies and its Use to Test Electronic Structure Model Chemistries.

We present benchmark calculations of nine selected points on potential energy surfaces describing proton transfer process in three model systems, H(5)O(2) (+), CH(3)OH…H(+)…OH(2), and CH(3)COOH…OH(2). The calculated relative energies of these geometries are compared to those calculated by various wave function and density functional methods, including the polarized molecular orbital (PMO) model recently developed in our research group and other semiempirical molecular orbital methods. We found that the SCC-DFTB and PMO methods (the latter available so far only for molecules consisting of only O and H and therefore only for the first of the three model systems) give results that are, on average, within 2 kcal/mol of the benchmark results. Other semiempirical molecular orbital methods have mean unsigned errors (MUEs) of 3 to 8 kcal/mol, local density functionals have MUEs in the range 0.7 to 3.7 kcal/mol, and hybrid density functionals have MUEs of only 0.3 to 1.0 kcal/mol, with the best density functional performance obtained by hybrid meta-GGAs, especially M06 and PW6B95.