Methodology for the study of mixed transport properties of a Zn-doped SrZr0.9Y0.1O3−δ electrolyte under reducing conditions

Abstract

The mixed ionic–electronic transport properties of the protonic ceramic electrolyser material SrZr_0.9Y_0.1O_3−δ, with the addition of 4 mol% ZnO as a sintering additive, are analysed under reducing conditions. The study is performed by means of an active-load modification of the classical electromotive-force method to account for the non-negligible effect of the electrodes on the obtained electrical-transport numbers. The methodology is developed in detail in order to link the electrochemical criteria to simulated equivalent circuits. The observed electromotive force of the system is considerably affected by the introduction of the polarisation resistance of the electrodes in the corresponding analysis, resulting in a high deviation between the present results and those obtained by a classical analysis without attending to electrode effects. Under wet reducing conditions (pH₂ ≈ 0.05 atm, pH₂O ≈ 3 × 10⁻³–10⁻² atm), the oxide-ionic transport number is negligible in the range of 600–900 °C, whereas pure protonic conductivity is observed for temperatures ≤700 °C and pH₂O ≥ 5.6 × 10⁻³ atm. For higher temperatures and/or lower pH₂O, mixed protonic–electronic conduction is exhibited. The electronic contribution under reducing conditions is consistent with n-type electronic behaviour.