Molecular Simulation Studies Of The Impact Of External Electric Fields On Mass Transfer And Chemical Reactions At The Microscopic Level

Processes of mass transfer and chemical reactions have always been core problems in chemistry and chemical engineering.As external electric fields can change micro-scale motions of electrons,atoms and molecules,it may further affect the occurrence and proceeding of process of mass transfer and chemical reactions in macro-scales.As a result,researchers have paid attention to these related studies all the time.Meanwhile,many related results have been published on Nature,Science,JACS and other famous journals frequently.However,most research in this field is based on experimental method and quantum mechanics calculations.Research via the molecular simulation perspective for investigation of inherent mechanism in such field is still rarely reported.Recently,the development of molecular dynamic simulations especially for the OPLS-AA force field and the reactive force field(ReaxFF)methodology has opened a new door for understanding the mechanisms of diffusion and reactions from large molecular or atom scales.It can also provide us with some original views and carry out some studies that can never be done via experiment and quantum mechanics calculations to supplement them so that some innovation results can be got.To this end,we will choose two specific processes(pulse current charging for stable lithium-metal batteries and oxidation of toluene at high temperatures with external electric fields(Efields)to study the influence of Efields on processes of mass transfer and chemical reactions based on OPLS-AA force field and ReaxFF via molecular dynamics simulations,respectively.Meanwhile,simulation results will also be compared with corresponding experimental work to validate the correctness.The final results are as follows,respectively.(1)Significant difference of diffusion coefficients of lithium ions(Li+)could be found in different pulsed ratio and pulsed voltage.Totally,pulsed voltage would enhance diffusion of Li+ more remarkably than direct voltage did.Pulsed voltage dominate the diffusion of lithium ions when pulsed voltage at a high level((3×108V/m)and diffusion coefficients decreased against the increase of pulse duration.However,pulsed ratios affect the diffusion of lithium ions most remarkably when the pulsed voltage is limited(3×105 V/m).Li+ exhibit the best diffusion behavior at the pulsed ratio of 1:3 and 1:5.What's more,our simulation results were in good agreement with related experimental studies via SEM.Pulse current charging significantly suppressed growth of lithium dendrites and in contrast with direct current charging,life time of lithium metal batteries can be extended at least one time when they are charged via pulsed waves.(2)Efields might not only greatly enhance the oxidation rate of toluene but decreased the temperature that required in oxidation of toluene.Such simulation results are in good agreement with previous experimental works.Furthermore,a stronger Efield led to a faster oxidation rate of toluene.For instance,all cases of toluene oxidation in MD simulations were enhanced remarkably by Efield at the intensity of 2×109V/m.However,when the intensity of Efields decreased(2×105 V/m),the influence of Efields on oxidation of toluene might also be weaken.There were only limited cases which exhibited the enhancement of toluene oxidation by Efields.In conclusion,external electric fields can affect both processes of mass transfer and chemical reactions significantly.On the one hand,from the perspective of mass transfer,superior pulsed ratio in pulsed current charging could tune the structure of Li+ in bulk electrolyte and enhanced the combination of Li+ with solvent molecules.As a result,the diffusion of lithium ions got enhanced and caused dendrite suppression finally.On the other hand,Efields can enhance occurrence and proceeding of chemical reactions significantly.Like oxidation reaction of toluene,the inherent mechanism that external electric fields significantly enhance the occurrence of the initial radical generation for different pathways of toluene oxidation but they do not seem to favor any of the pathways were discovered to affect the oxidation reactions of toluene.