Unraveling vacancy-modulated proton transport beyond trapping in partially hydrated Y-doped BaZrO3 using a charge-aware foundation potential

Proton transport in yttrium-doped BaZrO 3 is widely interpreted through dopant-associated trapping models derived from fully hydrated lattices. However, protonic ceramic electrochemical cells operate under partial hydration, where oxygen vacancies remain intrinsic to the defect chemistry and may fundamentally alter the local transport landscape. The effect of vacancy-dopant configuration on proton trapping under operating-relevant conditions remains unresolved. In the present work, proton migration in partially hydrated Ba(Zr 0.8 Y 0.2 )O 3-δ is examined by explicitly resolving the placement of vacancies within Y-Y-coordinated environments. Charge-aware foundation-potential atomistics, combined with climbing-image nudged-elastic-band calculations, quantify migration barriers and thermodynamic asymmetry for all symmetry-allowed nearest-neighbor proton hops. While fully hydrated Y-Y coordination exhibits pronounced trapping characterized by asymmetric transition states and facile back-hopping, partial hydration qualitatively alters the transport regime. A vacancy positioned between two Y dopants is thermodynamically favored and remains unoccupied by protons due to unfavorable hydroxyl migration. Instead, cooperative electrostatic stabilization and lattice relaxation enable low-barrier, nearly symmetric interoctahedral proton migration across all symmetry-allowed pathways. These findings demonstrate that proton mobility under operating partial hydration is governed by vacancy configuration rather than dopant proximity alone, providing mechanistic guidance for designing high-conductivity protonic ceramics.