First Principles Studies Of The Preparation Of Proton Conductive Materials By Sulfonation And Corrosion Mechanism Of Metallic Materials By Active Components In Molten FLiBe

“Emission peak”and"carbon neutrality"are China's carbon pledge to deal with the challenges brought by global warming,and will also be our future development direction of energy technology.In order to realize"emission peak"and"carbon neutrality",we must develop carbon-free or low-carbon energy to replace the currently used high-carbon fossil energy,as well as apply the clean energy to mobile equipment.Nuclear energy,as the only realistic base-load power source that can replace fossil fuels at large scale,will certainly play an important role in the realization of“emission peak”and"carbon neutrality".And the proton exchange membrane fuel cells(PEMFC),which uses hydrogen as the energy carrier,will play an essential role for the application of clean energy to the transportation sector.Proton conductive material is a core material for PEMFC besides electrocatalyst.However,the large-scale commercialization of PEMFC in the transportation sector is impeded by the high costs and the degradation of proton conductivity at high temperature and dehydrated state of poly(perfluorosulfonic acid)material(PFSA)widely accepted for PEMFC.In this thesis,the preparation mechanisms,including the sulfonation and the chain scission mechanisms,of proton conductive materials based on aromatic polymers will be investigated using first principles calculations.In this thesis,the sulfonation mechanisms of aromatic polymers with various aryl-bridge-aryl linkages as well as the sulfonation activity influenced by bridges and solvation effects are investigated.It is shown that the ortho-position of aryl-aryl direct linkage possesses the lowest activation energy of 38.35?43.81 k J·mol^-1 in gaseous phase,and the ortho-position of ether bond linkage possesses the lowest activation energy of 2.22?19.93 k J·mol^-1 in H₂SO₄ solvent.The carbonyl linkage elevates activation energy in gaseous phase;however,the solvation effect of H₂SO₄ accelerates significantly the sulfonation reaction.Besides,the large steric hindrance of sulfonyl linkage and dimethyl-methylene linkage hinder the sulfonation,and the solvation effect of H₂SO₄ is insignificant.Using H₂SO₄ as sulfonation agent,the sulfonation proceeds in a tri-molecular mechanism including the substrate,the SO₃ and the H₂SO₄.Because of the steric hindrance and orientation effect,the sulfonation tends to attack the ortho-position of ether bond linkage,rarely to attack the ortho-position of sulfonyl linkage,and not to attack the ortho-position of dimethyl-methylene linkage.In addition,the solvation effect impacts greatly on the sulfonation of polar bridge.Therefore,the control of sulfonation position can be realized based on solvation effect of polar and non-polar bridges.In the sulfonation environment,chain scission may take place in ether bond bridge,carbonyl bridge,dimethyl methylene bridge,and sulfonyl bridge.The C1 position of ether bond bridge is attacked by the oxygen atom of SO₃;the sulfonyl bridge is activated by SO₂⁺in ortho-position and its C1 position is vulnerable to SO₃ attack;the carbonyl bridge is strong polarity and vulnerable to SO₃ attack with low activation energy;the C1 position of dimethyl-methylene bridge is attacked by hydronium ions under acidic conditions with high activation energy in comparison with other bridges.Thorium Molten Salt Reactor(TMSR)is an important reactor type of the fourth generation of nuclear energy systems with the excellent safety ratings,the utilization of thorium resources,and the closed cycle of nuclear resources.However,the corrosion of structural materials,attacked from molten salt system,especially from the highly active components of O₂ and H₂O,impedes the development of TMSR.In this thesis,the characteristics of molten FLiBe(Li F-Be F₂),which is used as liquid fuel medium in TMSR,are studied by first principles molecular dynamics simulations.Especially,the characteristics of active components of O₂ and H₂O and the mechanisms of corrosion attack to metallic materials will be investigated.The basic structure of molten FLiBe is the tetrahedral[Be F₄]^2–anion,and two of the vertexes(F^–)share a Li⁺forming complicated ion cluster,or multiple[Be F₄]^2–combine by sharing vertex forming more complicated polyanion.The introduction of a small amount of O₂,H₂O,and other components into molten FLiBe will not disrupt the overall structure of the molten salt,but only the local structure around the introduced molecules.Though the molecular components of O₂,O₃,H₂O and H₂O₂ exist in molecular form in molten FLiBe,these molecules can reach very close distances to Li⁺or Be²⁺attributing to the strong interactions.The ionic components,O^2–and O^–,interact strongly with Li⁺and Be²⁺forming stable coordination structures.In addition,O₂^2–tends to bond with Li⁺,and·OH free radical and OH^–tend to interact with Be²⁺in molten FLiBe.In molten FLiBe,the neutral molecules of O₂,O₃,and O atom attract electron from neighboring molten salt exhibiting high electron density difference.However,the negatively charged ionic components of O^2–,O^–,and O₂^2–attracts negligible electron showing low electron density differences,and the components composed of hydrogen possess low electron density differences.In molten FLiBe,the molecular O₂ transports fastest with a self-diffusion coefficient of 1.3×10^-4 cm² s^-1,and the O^2–slowest with self-diffusion coefficient of2.6×10^-5 cm² s^-1.In overall,the ionic components transport slowly because of the formation of coordination with the moieties of molten FLiBe.Amongst the neutral components,the interaction between H₂O and Li⁺or Be²⁺is weak,and the self-diffusion coefficient of H₂O is high,while the interaction between·OH free radical and Li⁺or Be²⁺is powerful,and the self-diffusion coefficient of·OH free radical is low.