Controlling grain boundary segregation to tune the conductivity of ceramic proton conductors

Acceptor-doped barium zirconates are of major interest as proton-conducting ceramics for electrochemical applications at intermediate operating temperatures. The proton transport through polycrystalline microstructures of yttrium doped barium zirconates is hindered by the presence of a positive space charge potential at grain boundaries. During high temperature sintering, the positive charge acts as a driving force for acceptor dopant segregation to the grain boundary. Acceptor segregation to grain boundaries has been observed in sintered ceramics, but the fundamental relationship between the segregation kinetics and the protonic conductivity is poorly understood. Here, we present a comprehensive study of the influence of acceptor dopant segregation on the electrochemical properties of grain boundaries in barium zirconate based protonic ceramics. To facilitate this study, we designed an out-of-equilibrium model material that is not in a state of thermodynamic equilibrium and displays no detectable Y segregation at its grain boundaries. This model material served as a starting point to measure the kinetics of segregation and the induced changes in grain boundary conductivity upon varying thermal histories. Furthermore, we correlated the electrochemical results from impedance spectroscopy to atomic resolution transmission electron microscopy and atom probe tomography. We discovered that acceptor dopant segregation drastically increases the proton conductivity in both our model system and several other application-relevant compositions. In all cases, high-temperature thermal treatments were necessary to equilibrate the space charge zones, allowing the segregation of cationic point defects to grain boundaries, compensating the core charge and resulting in high performance protonic ceramics.