Atomistic Scale Modeling of Anode/Electrolyte Interfaces in Li-Ion Batteries.

A key issue in lithium-ion batteries is understanding the solid electrolyte interphase (SEI) resulting from a reductive reaction on the anode/electrolyte interface. The presence of the SEI layer affects the transport behavior of the ions and electrons between the anode and electrolyte. Despite the influence on interfacial properties, the formation and evolution mechanism of the SEI layer are unclear owing to their complexity and dynamic nature. Atomistic-scale simulations have promoted the understanding of the reaction mechanism on the anode/electrolyte interface, the formation and evolution of the SEI layer, and their fundamental properties. This Perspective discusses the modeling and interpretations of anode/SEI/electrolyte interfaces through computational methods at the atomic-scale and highlights interfacial modeling techniques for a realistic interface design, which can overcome the limited time and length scale with high accuracy.

Reactive Molecular Dynamics Simulations and Quantum Chemistry Calculations To Investigate Soot-Relevant Reaction Pathways for Hexylamine Isomers.

Sooting tendencies of a series of nitrogen-containing hydrocarbons (NHCs) have been recently characterized experimentally using the yield sooting index (YSI) methodology. This work aims to identify soot-relevant reaction pathways for three selected C6H15N amines, namely, dipropylamine (DPA), diisopropylamine (DIPA), and 3,3-dimethylbutylamine (DMBA) using ReaxFF molecular dynamics (MD) simulations and quantum mechanical (QM) calculations and to interpret the experimentally observed trends. ReaxFF MD simulations are performed to determine the important intermediate species and radicals involved in the fuel decomposition and soot formation processes. QM calculations are employed to extensively search for chemical reactions involving these species and radicals based on the ReaxFF MD results and also to quantitatively characterize the potential energy surfaces. Specifically, ReaxFF simulations are carried out in the NVT ensemble at 1400, 1600, and 1800 K, where soot has been identified to form in the YSI experiment. These simulations account for the interactions among test fuel molecules and pre-existing radicals and intermediate species generated from rich methane combustion, using a recently proposed simulation framework. ReaxFF simulations predict that the reactivity of the amines decrease in the order DIPA > DPA > DMBA, independent of temperature. Both QM calculations and ReaxFF simulations predict that C2H4, C3H6, and C4H8 are the main nonaromatic soot precursors formed during the decomposition of DPA, DIPA, and DMBA, respectively, and the associated reaction pathways are identified for each amine. Both theoretical methods predict that sooting tendency increases in the order DPA, DIPA, and DMBA, consistent with the experimentally measured trend in YSI. This work demonstrates that sooting tendencies and soot-relevant reaction pathways of fuels with unknown chemical kinetics can be identified efficiently through combined ReaxFF and QM simulations. Overall, predictions from ReaxFF simulations and QM calculations are consistent, in terms of fuel reactivity, major intermediates, and major nonaromatic soot precursors.

Universality in surface mixing rule of adsorption strength for small adsorbates on binary transition metal alloys.

Understanding the adsorption phenomena of small adsorbates involved in surface reactions on transition metals is important because their adsorption strength can be a descriptor for predicting the catalytic activity. To explore adsorption energies on a wide range of binary transition metal alloys, however, tremendous computational efforts are required. Using density functional theory (DFT) calculations, here we suggest a "surface mixing rule" to predict the adsorption energies of H, O, S, CO and OH on bimetallic alloys, based on the linear interpolation of adsorption energies on each pure surface. As an application, the activity of CO oxidation on various bimetallic alloys is predicted from the adsorption energies of CO and O easily obtained by the surface mixing rule. Our results provide a useful tool for rapidly estimating adsorption energies, and furthermore, catalytic activities on multi-component metal alloy surfaces.

Ab Initio Thermochemistry of Highly Flexible Molecules for Thermal Decomposition Analysis.

Pyrolysis is a promising technology for chemical recycling of waste plastics, since it enables the generation of high-value chemicals with low capital and operating cost. The calculation of thermodynamic equilibrium composition using the Gibbs free energy minimization approach can determine pyrolysis operating conditions that produce desired products. However, the availability of thermochemical data can limit the application of equilibrium calculations. While density functional theory (DFT) calculations have been commonly used to produce accurate thermochemical data (e.g., enthalpies of formation) of small molecules, the accuracy and computational cost of these calculations are both challenging to handle for large, flexible molecules, exhibiting multiple conformations at elevated (i.e., pyrolysis) temperatures. In this work, we develop a computational framework to calculate accurate, temperature-dependent thermochemistry of large and flexible molecules by combining force field based conformational search, DFT calculations, thermochemical corrections, and Boltzmann statistics. Our framework produces accurately calculated thermochemistry that is used to predict equilibrium thermal decomposition profiles of octadecane, a model compound of polyethylene. Our thermochemistry results are compared against literature data demonstrating a great agreement, and the predicted decomposition profiles rationalize a series of pyrolysis experimental observations. Our work systematically addresses entropic contributions of large molecules and suggests paths for accurate and yet computationally feasible calculations of Gibbs free energies. The first-principles-based thermodynamic equilibrium analysis proposed in this work can be a significant step toward predicting temperature-dependent product distributions from plastic pyrolysis and guide experimentation on chemical plastic recycling.

In situ temperature measurements in sooting methane/air flames using synchrotron x-ray fluorescence of seeded krypton atoms.

Synchrotron x-ray fluorescence has been used to measure temperatures in optically dense gases where traditional methods would fail. These data provide a benchmark for stringent tests of computational fluid dynamics models for complex systems where physical and chemical processes are intimately linked. The experiments measured krypton number densities in a sooting, atmospheric pressure, nonpremixed coflow flame that is widely used in combustion research. The experiments not only form targets for the models, but the simulations also identify potential sources of uncertainties in the measurements, allowing for future improvements.

2-(N-Methylbenzyl)pyridine: A Potential Liquid Organic Hydrogen Carrier with Fast H2 Release and Stable Activity in Consecutive Cycles.

The liquid organic hydrogen carrier (LOHC) 2-(N-methylbenzyl)pyridine (MBP) shows good potential for H2 storage based on reversible hydrogenation and dehydrogenation, with an H2 storage density of 6.15 wt %. This material and the corresponding perhydro product (H12 -MBP) are liquids at room temperature. Remarkably, H2 release is much faster from H12 -MBP over Pd/C than from the benchmark perhydro benzyltoluene over Pt/C at lower temperatures than 270 °C, owing to the addition of N atom into the benzene ring. Since this positive effect is unfavorable to the hydrogenation reaction, more Ru/Al2 O3 catalyst or prolonged reaction time must be applied for complete H2 storage. Experiments with repeated hydrogenation-dehydrogenation cycles reveal that reversible H2 storage and release are possible without degradation of the MBP/H12 -MBP pair. The prepared MBP satisfies the requirements for chemical stability, handling properties, and cytotoxicity testing.

Suppression of Cation Segregation in (La,Sr)CoO3-δ by Elastic Energy Minimization.

Strontium segregation at perovskite surfaces deteriorates the oxygen reduction reaction kinetics of cathodes and therefore the long-term stability of solid oxide fuel cells (SOFCs). For the systematic and quantitative assessment of the elastic energy in perovskite oxides, which is known to be one of the main origins for dopant segregation, we report the fractional free volume as a new descriptor for the elastic energy in the perovskite oxide system. To verify the fractional free volume model, three samples were prepared with different A-site dopants: La0.6Sr0.4CoO3-δ, La0.6Sr0.2Ca0.2CoO3-δ, and La0.6Ca0.4CoO3-δ. A combination of the theoretical calculations of the segregation energy and oxide formation energy and experimental measurements of the structural, chemical, and electrochemical degradation substantiated the validity of using the fractional free volume to predict the dopant segregation. Furthermore, the dopant segregation could be significantly suppressed by increasing the fractional free volume in the perovskite oxides with dopant substitution. Our results provide insight into dopant segregation from the elastic energy perspective and offer a design guideline for SOFC cathodes with enhanced stability at elevated temperatures.