A Highly Sensitive and Selective NH3 Sensor Based on a Spherical In2O3-Sensing Electrode Prepared Using the Hydrothermal Method

In2O3 material with special morphology was synthesized using the hydrothermal method and was used as the sensing electrode for a mixed potential ammonia sensor based on yttria-stabilized zirconia electrolyte. The In2O3 sensing electrode (In2O3-SE) was sintered at different temperatures and was characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (SEM). The influence of the In2O3-SE sintering temperature on the performance of the sensor was investigated by testing the NH3 response at 500 °C. The results showed that the sensor attached with In2O3-SE sintered at 900 °C exhibited the largest response value (–102.1 mV for 250 ppm NH3) at 500 °C. Moreover, the response value of the sensor varied almost linearly with the logarithm of NH3 concentration in the range of 20–400 ppm, which was consistent with the mixed potential mechanism, as verified by polarization and electrochemical impedance spectroscopy (EIS) tests. Furthermore, the sensor also displayed high sensitivity to NH3, but weak cross-sensitivities to various interfering gases (CHs, NOx, CO, CO2 and H2O) at 500 °C. It exhibited slight signal drifts both in the continuous 26-h test and in the interval test over a period of one month, which showed that the sensor attached with 900 °C-sintered In2O3-SE has acceptable short- and long-term stability.