ReaxFF simulations

Fig. 3 Snapshot of a ReaxFF simulation of benzene adsorption in Raney Ni. The model stmcture of Raney Ni was generated as described above for an initial 50 wt% Ni/Al composition...

Study Surface Activity with ReaxFF Simulation 100... 

Traditional MD simulations can investigate the different residence times due to the hydrophobic property of the surface of the pores and then obtain different surface fluid transport coefficients and provide a reliable basis for the experimental design. Quantum chemical calculations can consider reactive molecules with different reaction mechanisms for the active surfice of pores the reactivity will also affect the transport fluid to some extent. MD simulation cannot reflect the chemical interaction between the particles and cannot simulate a chemical reaction, while quantum chemical calculations cannot be apphed to large systems. ReaxFF simulation method is a new simulation method rising in recent years it can simulate not only the transport process but also chemical reactions and can simulate transport and reaction in a large system at the same time, which can fill the gap between quantum chemistry and classical force field (empirical force field). It simulates the process with more reahstic method because we can not only obtain the transport properties of the fluid but also reflect a chemical reaction of fluid and the surface. 

This chapter will focus on the modeling of MEA and its polymer electrolyte membrane. First, 3D modeling of PEMFC and its MEA will be discussed, and an example will be put forward. Then, dynamic modeling of PEM will be introduced. Further, this chapter will move on to the fault-embedded modeling of PEM. As an extension, application of membranes in other cases will be recommended, such as in lithium battery, vanadium redox flow battery (VRFB), chlor-alkali electrolysis, water electrolysis, and solar cell. Finally, several typical examples will be given, including Pt and Pt alloy simulation with density functional theory (DFT), water formation and Pt adsorption on carbon reactive force field (ReaxFF) simulation, and coarse-grained simulations. 

Water Formation and Pt Adsorption ON Carbon ReaxFF Simulation... 

The results of the generic ReaxFF simulations were then compared with theoretical predictions using classical reaction rate theory as governed by... 

Buehler et al. presented a preliminary study on formation of water from molecular oxygen and hydrogen using a series of atomistic simulations based on ReaxFF MD method.111 They described the dynamics of water formation at a Pt catalyst. By performing this series of studies, we obtain statistically meaningful trajectories that permit to derive the reaction rate constants of water formation. However, the method requires calibrations with either ab initio simulation results in order to correctly evaluate the energetics of OER on Pt. Thus, this method is system specific and less reliable than the ab initio methods and will not replace ab initio methods. Nevertheless, this work demonstrates that atomistic simulation to continuum description can be linked with the ReaxFF MD in a hierarchical multiscale model. 

In this chapter we review our recent efforts towards understanding many of the salient features of detonation using NEMD simulations. We will focus on large-scale NEMD simulations using a model interatomic potential (denoted REBO) to study generic, but complex, detonation phenomena and the use of a new, computationally more intensive, potential (denoted ReaxFF) that accurately describes a real nitramine energetic material. 

Figure 10. Snapshots of an MD shock simulation of RDX using ReaxFF with Up = 5 kin/s.

We have shown in Section 3.4 that NEMD simulations using ReaxFF allow one to study the initial chemical events induced by shockwaves in RDX. We have also seen that the interaction of voids in the crystals with shocks can lead to large local heating and that this localization of energy can initiate chemical reactions in the AB system. We now turn to analyze the effect of small voids (planar gaps) on the initial chemical events in RDX. 

Equilibrium MD simulations can provide valuable information about the thermal decomposition of energetic materials and can also enable the exploration of phenomena with time-scales much longer than in shockwaves. As an example, we studied the decomposition and subsequent reactions of RDX under various temperatmes (between T = 1200 K and T = 3000 K) and densities (at low density, 0.21 g/cm near normal density, 1.68 g/cm and under compression, 2.11 g/cm ), using MD with RDX interactions given by the reactive potential ReaxFF. 

Figure 18. Characteristic time vs. inverse temperature from ReaxFF MD simulations for three densities 2.11 g/cm 1.68 g/cm, and 0.21 g/cm. We also show the Arrhenius behavior obtained from experimental ignition times [42] in HMX.

Interatomic potentials that can accurately describe real materials (such as ReaxFF) provide valuable information on energetic materials, such as i) the initial chemical events under shock loading, ii) decomposition and reactions under thermal loading, and iii) characterization of the C-J state. Unfortunately, the simulation size required to capture the entire reaction zone for most compounds of interest (such as HMX, RDX, and TATB) via NEMD is forbiddingly high. 

In addition to the classical force fields above, many other force fields have been developed for small drug molecules or macromolecules. The MM2, MM3, and MM4 force fields were developed by Norman L. Allinger for a broad range of chemicals, and CFF is a family of force fields adapted to a broad variety of organic compounds, polymers, metals, and so on. The MMFF force field was developed at Merck for a broad range of chemicals. ReaxFF is a reactive force field, developed by William Goddard and coworkers, is fast, transferable, and the computational method of choice for atomistic-scale dynamics simulations of chemical reactions. 

The results presented in Fig. 4 show the evolution of the number of adsorbed benzene in the different system as a function of the simulation time. The ReaxFF force field allows the creation and breaking of covalent bonds between the different atoms of the system during the molecular dynamics simulation. In fliis work, we considered that a benzene molecule was adsorbed, if it formed at least one bond with the Ni (100), Ni (111), or Raney-Nickel surface, respectively. In the first 5 ps, the benzene adsorption is comparable for all three systems evaluated, i.e. both clean Ni surfaces and Raney Ni model. After the first 5 ps, about 6-7 % of benzene molecules have been adsorbed. In the very beginning of the simulation time, the adsorption process is even faster on Ni (100) and Ni (111) surface (blue and green line in Fig. 4) compared to the catalyst (purple line). After the first few ps, the adsorption on Ni (100) surface (blue line in Fig. 4) remains rather constant and does not increase much. After 25 ps, only about 12 % of benzene have been adsorbed. In contrast to this finding, the benzene adsorption on the Ni (111) surface and the Raney Ni model system (green and purple line in Fig. 4) increases more... 

Besides electrode surface structure, the nature of the electrode and solvent also affect the evolution of the SEI. An interesting and detailed simulation of formation and growth of SEI on Li metal surface in EC, DMC, and EC mixed with DMC electrolyte was done by Kim et al., using reactive force field (ReaxFF) molecular simulations [61]. The SEI film was found to grow faster in EC-based electrolyte compared to DMC, generating thicker SEI film, and EC mixed with DMC electrolyte came in between, as shown in Fig. 5.18. This simulation result agrees with the... 

Bedrov, D. Smith, G. D. van Duin, A. C., T. Reactions of Singly-Reduced Ethylene Carbonate in Lithium Battery Electrolytes A Molecular Dynamics Simulation Study Using the ReaxFF, J. Phys. Chem. A, 2012,116,2978-2985. 

ReaxFF. ReaxFF allows for bond breaking and bond formation in MD simulation so that thermal decomposition can be modeled as has been shown recently for polydi-methylsiloxane [9]. ReaxxFF includes terms for traditional bonded potentials as well as nonbonded potentials (i.e., van der Waals and Coulombic). Bond breaking and bond formation are handled through a bond order/bond distance relationship. Parameterization is through high-level DFT calculations (B3LYP/6-311++G ). 

In addition to QC studies, reactive molecular dynamics (RMD) simulations using the reactive force field ReaxFF have been used to gain insight into reactions of singly reduced EC in the condensed (solution) phase [31]. In this study the reaction of Li /o-EC with both LiVo-EC and LiVc-EC has been studied in a solution of EC molecules. A snapshot of the system is shown in Fig. 7.5. RMD simulations were used to determine the free energy as a function of reaction coordinate (see below) and to examine the propensity of various radical combination reactions to occur in the condensed phase of an EC solvent. 

Bedrov D, Smith GD, van Duin A (2010) Reactions of singly-reduced ethylene carbonate in lithium battery electrolytes. A molecular dynamics simulation study using the reaxFF. J Phys Chem B (accepted)... 

ReaxFF [50] provides a generally valid and accurate way to capture the barriers for various chemical reaction processes (allowed and forbidden reactions) into the force fields needed for large-scale MD simulation. ReaxFF is parameterized exclusively from QM calculations, and has been shown to reproduce the energy surfaces, structures, and reaction barriers for reactive systems at nearly the accuracy of QM but at costs nearly as low as conventional FFs. 

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