Humidity effects on calcium zirconate ceramics in electrochemical hydrogen cells

In the eighties, Iwahara discovered that doped Sr and Ba cerates show significant proton conduction, at high temperature, and in hydrogen- or water-containing atmosphere [1]. Begin ‘90 it was proved that also doped zirconates (Sr, Ba, Ca) join this family of proton conducting perovskites [2]. Within this group, indium doped CaZrO3 is one of the more interesting materials for applications, being mechanically and chemically particularly stable, and producible in very dense form. Specifically, it was already succesfully incorporated in a hydrogen sensor for liquid aluminium [3]. In principle, this sensor works as a simple electrochemical hydrogen concentration cell described by Nernst. However, as already remarked phenomenologically from the beginning [1], a hydrogen concentration cell using these perovskites shows the theoretically predicted emf, only in the presence of humidity. In this paper we want to go deeper into the reasons for this phenomenon.