STOICHIOMETRY RELAXATION IN OXIDES WITH MOBILE OXYGEN VACANCIES, PROTONS AND HOLES: TEMPERATURE DEPENDENCE AND TRAPPING EFFECTS

摘要

Materials with three mobile carriers (oxygen vacancies, protons, holes) are of interest as cathode materials for proton-conducting ceramic fuel cells. The incorporation of protons into such mixed-conducting oxides can occur by acid-base hydration of oxygen vacancies as well as by hydrogen uptake. [1,2] Depending on conditions, one-fold or two-fold conductivity relaxation after pH_2O steps is observed.[2-6] Exact analytical relations for these processes were derived in ref. [2]. For a complete description four diffusion coefficients are required, comprising direct as well as "indirect" terms. The complex nonmonotonic kinetic behavior is related to the fact that in a three carrier system the electroneutrality condition does not lead to a simple coupling between the carrier fluxes. Numerical simulations for a wide range of materials and conditions are presented.[2,6] They allow us to identify the conditions for the transition from one-fold to two-fold relaxation, and give a natural explanation for the "moving boundary phenomenon" observed in ref. [3].