Improvement of Al3+ ion conductivity by F doping of (Al0.2Zr0.8)4/3.8NbP3O12 solid electrolyte for mixed potential NH3 sensors

New Al3+ ion conducting solid electrolytes (Al0.2Zr0.8)4/3.8NbP3O12-xF2x(0 ≤x ≤ 0.4) with Nasicon-structure are successfully prepared by solid state reaction method. The influences of the doped F- content on the properties of the (Al0.2Zr0.8)4/3.8NbP3O12-xF2x samples are investigated using X-ray powder diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The results show that F- doping can effectively improve the sinterability and the total conductivity of the (Al0.2Zr0.8)4/3.8NbP3O12-xF2x samples. Among the solids series, (Al0.2Zr0.8)4/3.8NbP3O11.7F0.6 shows the highest conductivity of 1.53 × 10−3 S cm−1at 500 °C, which is approximately 7.9 times higher than that of the undoped (Al0.2Zr0.8)4/3.8NbP3O12. The ion transference number of the samples is higher than 0.99 at 300-700 °C. On the basis of the promising properties, a mixed-potential type NH3 sensor based on (Al0.2Zr0.8)4/3.8NbP3O11.7F0.6 electrolyte and In2O3 sensing electrode has been developed. The sensing performance of the sensor is evaluated. The mixed-potential type sensor can work at relatively low temperatures of 200-350 °C and an excellent sensitivity of 99.71 mV/decade at 250 °C is obtained. The sensor also displays excellent stability and reproducibility, accompanied by low cross-sensitivities to CO2, CH4 and H2.