Multiscale Simulation Of Proton Conduction And Mechanical Behavior In Perovskite Barium Zirconate Electrolyte With Defects

Due to the negative impact of fossil fuels on the environment,it is necessary to develop technologies that can reduce or eliminate the demand for oil and coal.In this context,fuel cells have received a lot of attention because they provide an efficient way of converting chemical energy into electrical energy.It has been found that Barium zirconate(BZO),a solid oxide proton conductor,has the necessary characteristics of a good electrolyte material in a wide temperature range.However,the limitation of hydration and the slow conductivity in grain boundaries and dislocations reduce the feasibility of wide application of these materials,so it is still a challenge to improve the conductivity to the required level in practical application.Under this background,this paper uses Density functional theory(DFT),Molecular dynamics(MD),Finite element method(FEM)and their combined multi-scale methods to investigate the diffusion mechanism of protons in point defects,one-dimensional defects(dislocations)and two-dimensional defects(grain boundaries)and the effect of microstructure on the mechanical behavior of defects(cracks),so as to determine the strategy to improve the overall conductivity and mechanical properties of electrolyte materials,and provide further guidance for the practical application of perovskite oxides.Solid oxide materials conduct protons as soon as they are hydrated,however the mechanism of hydration at the defects is unknown.Through detailed DFT calculation,the hydration of BZO under the joint action of point defects(BZY)and grain boundaries(BZY-GB)was studied.Four key steps in the hydration line on the grain boundary structure(GB)of perovskite oxides were discovered: adsorption of water molecules on grain boundary surface,proton migration from grain boundary surface to grain boundary,proton migration in grain boundary structure,and oxide ion vacancy migration on grain boundary structure.Compared with proton jump mechanism,hydroxide ion migration has a lower energy barrier,which is proposed for proton conduction between the surface and subsurface of perovskite oxide.This study not only provides a mechanism insight for the hydration of BZO grain boundary structure,but also provides the importance of structural rearrangement when protons are incorporated into solid oxide materials.The higher grain boundary resistance greatly limits the overall transport of protons in polycrystalline samples.The diffusion characteristics of protons in GB were studied by DFT calculation.The large free space and the competing mechanism of hydrogen bonds led to the significant capture of protons in GB structure.By analyzing the lattice deformation between the initial,saddle and final structures,the proton transfer and hydroxide ion rotation are decomposed into three processes respectively.The decrease of A ion during the hydroxyl ion rotation of GB leads to the obvious change of the bond strength between the remaining O ion and A ion.For the increase of the number of interaction bonds between oxygen donor and B ion during the proton transfer of GB,both of these situations increase the energy barrier of local lattice deformation(such as O-B-O rotation movement),thus inhibiting the proton diffusion in GB.The results reveal the high resistance phenomenon of proton diffusion in GB from the perspective of dynamics,and quantify the source of energy barrier.For large structural defects such as dislocations,it is necessary to obtain accurate diffusion mechanism from the electronic structure of the dislocation core and avoid the influence of adjacent structures caused by periodic boundary conditions.Based on the multi-scale simulation theory of Quantum mechanics / Molecular mechanics(QM / MM),combined with LAMMPS and VASP package,the multi-scale calculation program was written to achieve the requirements of both accuracy and efficiency in calculation.The diffusion characteristics of protons on edge dislocations(BZY-D)in perovskite oxide are revealed by this program.No evidence of pipeline diffusion is found from the calculated migration energy barrier,which not only does not accelerate the diffusion of ions,but reduces the conductivity of ions.At the same time,the proton diffusion characteristics of BZY,BZY-D and BZY-GB are compared,and it is found that the local lattice deformation plays an important role in the hydroxyl ion rotation and proton transfer involved in proton diffusion.This study clarifies the diffusion mechanism of protons in dislocation cores and the source of related energy barriers,and provides an effective tool for the simulation calculation of irregular defects in materials.The Atom-to-Continuum(AtC)multi-scale simulation method combining molecular dynamics and finite element calculation was used to study the mechanical properties of electrolyte material BZO under the action of defects.At the macro level,FEM is used for loading and numerical solution;At the microscopic level,MD is used to simulate the atomic structure of BZO.In this method,the anisotropy of BZO atomic structure is taken into account,and the inherent natural law is determined by detailed modeling of microstructure.The accuracy of the multi-scale calculation method is verified by comparing the stress-strain curves of BZO with central crack under AtC method and MD simulation.Then,combined with finite element software,the multi-scale calculation under larger and more complex loading and boundary conditions is realized,and the mechanical response of cracks under macro and micro calculation is observed and discussed.Under the finite element simulation,the crack extends forward by splitting,while in MD simulation,the results show obvious direction dependence:(100)plane fracture involves in-plane slip,and the crack propagates forward perpendicular to the plane between Zr and O;On the(110)plane,the crack is unstable during its propagation,which leads to the zigzag propagation path,and the direction of the final crack is consistent with the initial direction.This study not only reveals the fracture mechanism of perovskite oxide BZO,but also puts forward a multi-scale calculation method,which provides a new way for the design and application of materials.