Electrode-electrolyte BIMEVOX system for moderate temperature oxygen separation

Electrochemical separation of oxygen from air is a promising application fo r oxide conductor solid electrolytes. However, several important specificat ions are required in order to obtain an efficient separation device. First of all, the electrolyte material must exhibit a high conductivity at modera te temperature. From this point of view, a new family of materials called B IMEVOX ideally fulfils this condition. Secondly, a typical separation devic e must comport two electrodes an opposite faces of the electrolyte. These e lectrodes must act as electronic collectors but also, at the cathodic side, as an oxygen dissociation catalyst. BIMEVOX electrolytes exhibit ionic con ductivity values that can allow work at temperature below 500 degrees C. Th e classical electrode approach, like in solid oxide fuel cells, consists in using a specific mixed oxide, for instance strontium lanthanum manganite o r cobaltite. However, the lower the temperature, the lower the efficiency o f these electrodes which quickly appears as the limiting factor. In previou s work on bismuth lead oxide electrolytes, we proposed a new approach that consists of using the surface of the bismuth-based electrolyte itself as th e catalyst, the electron collection being then performed by a co-sintered m etallic grid. This 'in-situ' electrode system provides many advantages, par ticularly it eliminates the problem of the chemical compatibility between e lectrode and electrolyte materials. Taking into account the presence of bot h catalytic vanadium and bismuth cations in BIMEVOX, we checked under these conditions the separation of oxygen from air for different electrolytes (B ICOVOX, BICUVOX, BIZNVOX) at various temperatures in the range 430-600 degr ees C. For instance, using a BICOVOX pellet with a gold grid inserted on ea ch side makes it possible to separate oxygen with nearly 100% efficiency fo r current density values up to 1000 mA/cm(-2). For higher intensity values, the faradic efficiency progressively but reversibly decreases. Similar res ults were obtained with copper and zinc derivatives. A model, in which part of the electrolyte material converts reversibly into electrode material is proposed to interpret the behavior of the oxygen separation membrane under operating conditions. (C) 1998 Elsevier Science B.V. All rights reserved.