| Since the beginning of the 21 st century,the energy problem has become increasingly serious,and there have been serious problems such as excessive consumption of energy resources,surge in oil prices,and slow development of non-renewable resources around the world.In order to deal with the above problems,countries around the world have actively adopted a variety of coping strategies focusing on clean energy.As a major energy country,energy issues need special attention.In 2020,my country proposes to achieve "carbon neutrality" and "carbon peak" as soon as possible.In order to achieve the above goals at an early date,it is necessary to vigorously develop key technologies such as clean energy,high energy density storage,and carbon recycling.Theoretical simulations based on first principles can play an important role in energy chemistry simulations.In this paper,two important electrochemical energy problems are studied by means of quantum mechanics.The first is the battery as the core electrochemical energy storage,including:the chemical reaction and microstructure of the lithium metal battery interface,and the basic reduction mechanism in the lithiumsulfur battery.The second is the energy conversion with electrocatalysis as the core,including:the microscopic reaction mechanism of CO2 electroreduction to ethanol in the copper-silver system,and the Oxygen Evolution Reaction(OER)using metal-organic frameworks as catalysts.In the above research,a series of findings and achievements have been obtained from the micro level,as follows:1.Titanium hydrogen nanodots catalyze the reduction of polysulfur in lithium-sulfur batteries.Lithium-sulfur(Li-S)batteries have attracted much attention due to their low cost,environmental friendliness,and high theoretical energy density.However,due to various technical difficulties,Li-S batteries have not been commercialized so far.Among them,the shuttle effect of lithium polysulfide(LiPSs)is a fatal problem that leads to rapid capacity decay and short lifespan of Li-S batteries.Existing experimental results show that TiH2 nanodots(hereinafter referred to as THNDs)can reduce the shuttle effect of lithium polysulfides(LiPSs).However,its microscopic reaction mechanism is still unclear.In order to elucidate the reaction mechanism,we systematically studied the reduction process of polysulfides catalyzed by THND using Density Functional Theory(DFT)and calculated the reaction free energies of the elementary reactions.Intermediates include S8,Li2S8,Li2S6,Li2S4,Li2S2,and Li2S.The DFT calculation results show that the whole process of reducing S8 to Li2S,from Li2S2 to Li2S is the rate-determining step.Taking this as a criterion,THND can significantly improve the conversion efficiency of S8,and its performance is significantly better than that of conventional graphene systems.The above results are consistent with the collaborative experimental results,indicating that THND is a high-performance material that can significantly reduce the shuttle effect of lithium polysulfide(LiPSS).2.Theoretical simulation of Li-metal interface structure and reaction mechanism.Lithium metal batteries have extremely high energy density and are considered as a nextgeneration battery technology with important application potential.However,there are fatal problems such as uncontrollable growth of dendrites and serious dead lithium in lithium metal batteries,which have not been commercialized so far.In this paper,an indepth study of a two-solvent electrolyte is carried out.The experimental results show that the electrolyte can significantly improve the cycle stability of lithium metal batteries.In order to reveal the structure of its solid electrolyte interface(Solid Electrolyte Interphase,SEI).We systematically investigate the reaction mechanisms and structural evolution during SEI formation using an independently developed hybrid algorithm,Hybrid Molecular Dynamics(HAIR)method that combines ab initio and reaction force fields.HAIR simulations provide key information on the initial reduction mechanism of solvents(FDMA and FEC)and salts(LiTFSI),including:rapid decomposition of FDMA and FEC to F-,forming LiF-inorganic SEI inner layer to protect the lithium anode;FDMA decomposition leading to nitrogen-containing organic components,such as Li-N-C,LixN,etc.,can increase the ionic conductivity and improve the performance of the battery,etc.XPS analysis confirmed that the simulation results of the evolution of SEI morphology were consistent with the existing experiments.These results provide a profound understanding of the microscopic mechanism of the formation process of SEI and provide a theoretical basis for the development of commercial lithium metal batteries.3.Study on the reaction mechanism of copper-silver catalyst for CO2 reduction to ethanol.Ethanol(EtOH)has a wide range of applications and high energy density,is an important liquid fuel,and is therefore an ideal product for CO2 reduction.However,the existing catalysts cannot achieve high selectivity of CO2 reduction to ethanol.In this work,we worked closely with our experimental collaborators to develop a Cu-Ag-O three-way catalyst that can achieve high selectivity to ethanol.By calculating the potential energy surface of the reduction pathway of CO on dCu2O and dCu2O/Ag2.3%surfaces,we found that compared with pure copper surfaces,Ag-doped Cu-Ag-O surfaces has higher catalytic efficiency,easier to obtain ethanol products,and lower product formation energy,which is thermodynamically advantageous.Since the ethanol product branching occurs after*HCCOH,the formation of*OCCOH determines the ethanol selectivity.The simulation results show that doping Ag reduces the formation energy(rate-determining step,RDS)of*OCCOH by 0.1 eV compared to pure Cu(1 1 1).On Cu-Ag-O(11 1),EtOH is thermodynamically favorable,which indicates that the formation of EtOH is easier than that of C2H4 on the surface of Cu-Ag-O.Compared with pure copper,the formation energies of the EtOH pathway of*HCCHOH and*H3CCHOH on Cu-Ag-O were significantly reduced by 0.09 eV and 0.17 eV.The above predicted trends are consistent with the experimentally observed high selectivity to ethanol.The above theoretical predictions provide important guidance for the further development of highly selective ethanol catalysts.4.Metal site effect on MOF-catalyzed oxygen evolution reaction.Organic frameworks(MOFs)can be used as catalysts for Oxygen Evolution Reaction(OER),but the performance has not yet met the requirements for commercialization.In this work,we cooperated with our experimental collaborators to develop a high-performance catalyst for OER based on Hofmann-type MOFs through metal site modulation.Using density functional theory,we investigated the effects of three cationic sites,Fe(Ⅱ),Co(Ⅱ),and Ni(Ⅱ),on the OER catalytic performance.The simulation results show that the formation of*O(*OH to*O)is the Potential Determing Step(PDS)in the MOF catalytic system.However,the formation energy of*O in Fe-MOF is much lower than that of Ni-MOF(2.42 eV)and Co-MOF(2.45 eV),which is only 2.25 eV,which indicates that Fe-MOF is more favorable for the reaction than the other two MOFs.In addition,Fe-MOF has a relatively high energy in the first step to obtain*OH,and other intermediates have low energy,especially in the RDS step.These simulation results indicate that Fe-MOF is more favorable compared with Co-MOF and Ni-MOF in steps such as initial water adsorption to generate surface*OH species and subsequent dissociation of*OH to*O,which reasonably explains the superiority of Fe-MOF in OER performance.The theoretical predictions are in good agreement with the experimental results. |
