Mechanistic insights into proton diffusion in Σ3 BaZrO₃ (210)[001] tilt grain boundary

The acceptor-doped BaZrO₃ (BZY) shows considerable bulk proton conductivity. However, the higher grain boundary (GB) resistances significantly limit the overall transport of protons in polycrystalline samples. The fundamental questions about the proton diffusion mechanism at the defects are still unclear. By performing detailed density functional theory (DFT) calculations, we have studied the diffusion properties of protons in GB, where the presence of large free space and hydrogen bond leads to a significant proton trapping in GB structure. By comparing the characteristics of proton diffusion in BZY and BZY-GB, we found that in both structures, the local lattice deformation is important in both the proton rotation and transfer steps involved in proton diffusion. By calculating the change in bond strength, the decrease of A ions during the proton rotation process in GB leads to a significant change in the bond strength of the remaining O and A ions, and for the increase in the number of oxygen donor and B ion interaction bonds during the proton transfer process in GB, both of which increase the energy barrier for local lattice deformation such as O–B rotational motion, thus inhibiting the proton diffusion in GB. Our results give mechanical insights into the high resistance to proton diffusion in GB, clarifying the sources of energy barriers for proton diffusion in GB, and lay the foundation for industrial applications of proton conductors.