Chemical Diffusion Coefficient of H₂O in AB_(1−x)B^′_xO_(3−x/2)‐Type Perovskites

Abstract

Many oxygen‐deficient oxide perovskites are oxygen ion conductors in a dry environment. However, in a humid atmosphere, they absorb water to fill up oxygen vacancies, release quasi‐free protons into the structure, and thereby become proton conductors. Absorption of water requires the transport of both protons and oxygen ions. This effectively leads to the transport of H2O under its chemical potential gradient. The present manuscript develops relationships between the chemical diffusion coefficient of H2O and the diffusion coefficients of protons and oxygen vacancies with an emphasis on the role of solution thermodynamics.

Transport in Proton Conductors for Fuel-Cell Applications: Simulations, Elementary Reactions, and Phenomenology

“…For a given water partial pressure, D ˜H2 O increases as temperature increases, because of dehydration and the increasing self-diffusion coefficients for both involved species. Recently, there has been some controversy about the validity of the model; 294 however, it is indeed strictly valid in the entire hydration range under the given assumptions. 295 It should be mentioned that, apart from oxide ion vacancies existing at low water partial pressures, the appearance of electronic holes (h • ) must be considered at high oxygen activities.…”

“…For a given water partial pressure, D̃ H 2 O increases as temperature increases, because of dehydration and the increasing self-diffusion coefficients for both involved species. Recently, there has been some controversy about the validity of the model; however, it is indeed strictly valid in the entire hydration range under the given assumptions 16 Self-diffusion coefficient of oxide ion vacancies in different perovskite-type oxides, which equals the chemical water diffusion coefficient in the fully hydrated state (see text).…”

Modeling the Steady-State and Transient Response of Polarized and Non-Polarized Proton-Conducting Doped-Perovskite Membranes

“…25,26 In this case, the defect transport fluxes can be expressed approximately as functions of the defect concentration gradients alone. Considering two major defects (OH O and V •• O for the doped BaCeO 3 ), Kreuer, et al 27 derived an expression of H 2 O flux in terms of the H 2 O concentration gradient and chemical diffusion coefficient, which is an explicit function of hydration degree and the self-diffusion coefficients of OH 28 presented an alternative form of H 2 O chemical diffusion coefficient. Under the state-steady conditions, local thermodynamic equilibrium within the membrane is usually assumed, and the permeation rates of the defect and gaseous species through the membrane are obtained by averaging the local defect fluxes through the membrane, rather than directly solving defect conservation equations.…”

Transport in Proton Conductors for Fuel‐Cell Applications: Simulations, Elementary Reactions, and Phenomenology

“…For a given water partial pressure, D H 2 O increases as temperature increases, because of dehydration and the increasing self-diffusion coefficients for both involved species. Recently, there has been some controversy about the validity of the model; 294 however, it is indeed strictly valid in the entire hydration range under the given assumptions. 295 It should be mentioned that, apart from oxide ion vacancies existing at low water partial pressures, the appearance of electronic holes (h • ) must be considered at high oxygen activities.…”