Exploring Al doped ZnO for the oxygen electrode of reversible solid oxide electrochemical cells

ZnO is a semiconductor with a reactive surface due to the presence of oxygen vacancies, but its application in solid oxide electrochemical cells (SOCs) was limited by its low electronic conductivity. Aluminum-doped zinc oxide (AZO), an n -type transparent conductive oxide, is explored for the reversible SOCs. Interestingly, 2 at. % of alumina doping in zinc oxide (AZO-2) increased the electric conductivity of ZnO to 1 Scm −1 at 800 °C. The K chem (5.98 x 10 −8  cm s −1 ) of AZO-2 from electric conductivity relaxation is 91% higher than that of ZnO. AZO-2 electrode on Y 0.16 Zr 0.84 O 1.92 (8YSZ) electrolyte exhibits a polarization resistance ( R p ) of 0.73 Ω cm 2 at open-circuit voltage and 800 °C, and the reaction is limited by the surface exchange of oxygen species. The surface exchange current density of AZO-2 cathode on Y 0.16 Zr 0.84 O 1.92 is 0.0736 A cm -2 at 800 °C. AZO-2 electrode can be activated after 20 cycles of switching bias, and the R p is further decreases to 0.26 Ω cm 2 . This study confirms that AZO with an enhanced electric conductivity can be used for the oxygen electrode of a reversible SOC due to the fast surface oxygen exchange over the defective surface with couple Zn 2+ and O 2− vacancies.