DFT Study On Cathode Materials For Lithium-Sulfur Batteries And PDH_x Catalysts For Fuel Cell

Rechargeable batteries are expected to replace traditional fossil fuels as a new type of energy source.In recent years,Lithium-Sulfur batteries and Fuel Cells have attracted attention because of their high theoretical energy and environmentally friendly.At this stage,the"shuttle effect"of lithium-sulfur batteries is the key to affecting the performance of batteries.Finding positive electrode composite materials to suppress the shuttle effect has always been a matter of great concern.Covalent Organic Frameworks?COFs?have attracted attention because of their polar functional groups which are advantageous for anchoring and good electrical conductivity.In this paper,we selected four COFs structures with different functional groups as the positive electrode composites,namely:the B and O six-membered ring linked COF-1 structure,the triazine ring linked CTF-1structure,the imine linked COF-LZU1 and hydrazone linked COF-42 structure.The mechanism of action of lithium sulphide?Li₂S_n?and COFs was studied by density functional theory?DFT?.The adsorption structures,binding energies,charge density difference and density of states indicate the electrostatic nature of Li-N/O ionic bond interaction between COFs and Li₂S_n.For COF-42/Li₂S_n complex,two Li atoms tend to directly bind to multiple N and O atoms and afford strong Li-N/O ionic bond interaction.Charge density difference and Bader charge difference indicate that the amount of charge transfer between Li₂S and COF-42 is significantly greater than that of COF-1,CTF-1 and COF-LZU1.The binding energies of Li₂S_n adsorbed on COF-42 exceed that of DOL or DME electrolytes,indicating COF-42 can resist the solvation effect and alleviate the shuttle of Li₂S_n.At the same time,according to the band gap value,the conductivity of COF-42 is also good.Therefore,we conclude that COF-42 has good electrical conductivity and anchoring advantages and may be an ideal anchoring material.The cathode reaction of a fuel cell is slow,and catalysts are required to accelerate the reaction,whereas the conventional cathode catalyst metal Pt is expensive and is not suitable for mass production.The properties of the metal Pd are similar to those of Pt,and the use of Pd as a catalyst can reduce the cost.Previous studies have shown that the PdHx structure doped with H atoms has good catalytic activity.In this paper,the density functional theory is used to study the advantages of PdHx as a catalyst.The adsorption energy of O on the catalyst is in a"volcano curve"relationship with the activity of the catalyst.The activity of the catalyst is judged by calculating the adsorption energy of O on the surface of the Pd-based catalyst,that is,the adsorption energy near the apex of the volcano corresponds to the maximum activity.It is found by calculation that PdHx undergoes cell expansion?stretching?due to the introduction of H atoms compared to Pd crystals.By comparing the Pd?1 1 1?faces of different stretching or compression,we obtain the cell stretching The conclusion of increasing the adsorption energy.At the same time,the effect of H on the adsorption energy was studied by controlling the H/Pd ratio,and it was found that the introduction of H atoms reduced the adsorption energy.Therefore,PdHx is affected by both cell expansion and H atom.The optimal PdHx structure can be found by controlling the cell size and the ratio of H.The calculations show that the Pd?2 2 1?crystal plane with H/Pd=1:3 and 7.6%expansion of the unit cell has good catalytic activity.