Electrical conduction in polycrystalline semiconducting ZnO electrodes. Importance of the grain boundaries

The electrical properties of pure and bismuth-doped polycrystalline sintered zinc oxide pellets have been compared with those of electrodes of similar composition but operating in contact with an aqueous electrolyte. In good accordance with the model proposed for varistors, the electrical properties of the bismuth-doped dry pellets are dominated by the intergranular barriers; the zero-field resistivity is high and an applied polarization is divided into individual potential drops, each corresponding to one grain boundary barrier. These properties are altered when the pellets operate in contact with an aqueous electrolyte, the resistivity of the bismuth-doped pellets decreasing strongly. The behavior of the polycrystalline electrodes is similar to that of single crystal electrodes : an applied anodic polarisation is entirely sustained in a single space charge layer at the electrode-electrolyte interface. The results are a consequence of the wetting of the grain boundaries by the electrolyte at the surface of the pellets allowing a direct electron injection in the grain boundaries.