High NH₃ selectivity of NiFe₂O₄ sensing electrode for potentiometric sensor at elevated temperature

NiFe₂O₄ was synthesized using sol-gel method for sensing electrode material of YSZ based ammonia sensor. NiFe₂O₄-SEs sintered at 1100 °C, 1150 °C and 1200 °C were characterized by XRD, the BET method and ESEM. By testing the NH₃ response of different sensors at 650 °C, it was observed that the 1150 °C sintered sensor had the largest response value (-104.3 mV for 320 ppm NH₃) and the highest sensitivity (-77.56 mV/decade), which were related to the most TPB sites and the moderate gas phase catalytic reaction. The response values of the sensor varied almost linearly with the logarithm of 20-320 ppm NH₃ at 600-750 °C, which was consistent with mixed-potential mechanism testified by polarization and EIS tests. When the oxygen concentration was at 7-10 vol %, its effect on the response value was within 3%. When the water vapor concentration was 3, 6 and 9 vol %, the ammonia response value was 95.1%, 92.9% and 88.7% of the values when there was no water vapor, respectively. The sensor showed very weak cross sensitivities to NO_x, but non-negligible SO₂ cross sensitivity. It also displayed slight signal drifts in weekly tests in eight weeks, which showed that the sensor attached with NiFe₂O₄-SE has a good long-term stability.