Abstract
The development of air electrodes with superior surface oxygen exchange properties at intermediate temperatures is crucial for improving the efficiency of protonic ceramic fuel cells. This study evaluated the surface exchange properties of Co(3)O(4) dispersed protonic conductors, BaZr(0.9)Sc(0.1)O(2.95). Although Co(3)O(4) is widely acknowledged as superior dissociative adsorption catalysts, there is still ambiguity regarding the enhancement mechanisms of their surface exchange properties by Co(3)O(4), as well as their optimal composition to achieve high catalytic activity. To overcome these difficulties, this study elucidated the effect of the chemical states and composition of composites on their surface exchange properties by evaluating their chemical states and surface exchange reaction rates with several compositions prepared at different temperature conditions using a vibrating-sample magnetometer and the pulse isotope exchange technique. For samples annealed at a high temperature, it became evident that the surface exchange activity became the most active by adding only 1 vol % Co(3)O(4) and indicated an abrupt decline above this composition despite an increase in the volume of the catalysts. This was attributed to the combined effect of the high dissociative adsorption activity of the Co-containing solid solutions formed at a high temperature and a decrease in oxygen vacancies due to hole compensation. For samples annealed at intermediate temperature, their chemical states remained unchanged from those of the original milled powders, and their surface exchange properties monotonically improved with an increase in the volume of Co(3)O(4). Based on the results, different chemical states of composites derived from different preparation conditions lead to completely different activation behavior of the surface exchange reaction.