Hydrogen diffusion in the protonic conductor BaCe_1-xGd _xO_3-x2 from density functional theory

The diffusion barriers of protonic defects in Gd-doped BaCeO₃, a compound candidate as electrolyte for protonic ceramic fuel cells, have been investigated by density functional theory calculations, starting from a previously computed energy landscape consisting of 16 kinds of stable sites (eight close to dopants and eight far from them). The simplified string method has been used to determine accurately the minimum energy paths between those sites that might imply either proton reorientations, intraoctahedral, or interoctahedral hopping mechanisms. At contrast with simple cubic perovskites such as barium stannate or barium zirconate, very different values for energy barriers (from 0.02 to 0.58 eV) are found in this highly distorted orthorhombic perovskite, and no specific process appears to be clearly rate limiting. Some interoctahedral hoppings (when possible) are found to be more favorable than the intraoctahedral ones, while reorientations exhibit a wide range of energy barriers.