The Impact of Nanoscale Percolation in Yttrium‐Doped BaZrO₃ on the Oxygen Ion and Proton Conductivities: A Density Functional Theory and Kinetic Monte Carlo Study

Abstract

Acceptor-doped BaZrO 3 is known for its high proton conductivity, making it an advanced energy material for various applications, for example, electrolyzers, fuel cells, or methane-conversion cells. [1][2][3][4][5] For this reason, many groups are investigating proton migration in acceptor-doped BaZrO 3 from the past to the present day. [2,[6][7][8] Recently, we could show that proton motion in yttrium-doped BaZrO 3 is determined by two phenomena: proton trapping by yttrium ions and the formation of nanoscale percolation pathways of yttrium ions. [9] A single yttrium dopant traps protons on firstand second-nearest neighbor sites, and at low dopant fractions, where the dopants are isolated from each other, trapping results in a decrease in average proton mobility. With increasing dopant fractions there are, however, local structures containing several Y ions in firstand second-nearest neighborhood. Within these nanoscale structures, the trapping zones of all Y ions overlap, and due to the low proton migration energies inside of these local structures, protons can move very fast within them. This phenomenon leads to a strong increase in proton mobility and proton conductivity with increasing dopant fraction, and thus, we named it in our previous publication [9] nanoscale percolation. We emphasize that these nanoscale structures are not necessarily connected, and they do not necessarily cross the whole sample, as in classical percolation, but they can be isolated local structures with fast proton transport, depending on the dopant fraction.In this article, we will investigate the impact of nanoscale percolation in yttrium-doped BaZrO 3 on its oxygen ion conductivity. Although it is known that at low temperatures the oxygen ion conductivity is much smaller than the proton conductivity, there are three reasons for this theoretical study. 1) Oxygen ions are mobile by means of oxygen vacancies, that will be trapped by Y ions, similar to protons. However, there seems to be no detailed theoretical study on the influence of defect interactions on the mobility of oxygen vacancies in Y-doped BaZrO 3 , although BaZrO 3 can be regarded as a prototype perovskite-structured oxide; 2) It is a priori unclear whether the phenomenon of

Incorporation of protons and hydroxide species in BaZrO3 and BaCeO3

“…64,65 We calculate a barrier of 0.72 eV in BZO, similar to the value of 0.66 eV recently calculated in another study. 66 For BCO, we calculate barriers ranging from 0.45-0.50 eV. We are unaware of previous NEB calculations on V 2+ O migration in BCO, and experimental activation energies are higher (0.69-0.90 eV 1,67,68 ).…”

Revised Centrality Measures Tell a Robust Story of Ion Conduction in Solids

“…Molecular dynamics (MD) simulations have been used to probe ion conduction in many systems, including ionic solids. − In systems where moves can be cataloged, kinetic Monte Carlo (KMC) allows different time scale moves by choosing a move based on its probability and advancing the simulation clock based on the move’s rate constant. − To overcome the challenges of both the distinct time and length scale important to ion conduction, a variety of multiscale methods have been developed and applied successfully to shed light on critical ion conduction technologies. , Many of these methods generate trajectories that can be probed to understand the system dynamics. Trajectories of ion conduction in solids may include many conducting ion diffusion steps near traps and a few escapes to fast conduction regions intermingled with smaller movements of other ions.…”