Highly Water Durable Solid Electrolyte Type Ammonia Gas Sensor with Ammonium Lanthanum Niobate As an Auxiliary Sensing Electrode
Authors: Tamura Shinji , Yamashita Ryo , Shibata Makiko , Imanaka Nobuhito 
Published: May 2020

Ammonia (NH3) gas is one of the toxic gases causing a serious health damage, while NH3 is a useful chemical in industrial field. Therefore, for preventing unexpected accidents caused by inhalation, rapid, continuous, and reliable NH3 detection is greatly required by applying smart gas sensor which can put every NH3 emitting site. Among the various kinds of NH3 gas detection tool, the solid electrolyte type sensor is regarded to be a promising candidate for on time monitoring tool due to its selective and quantitative gas detection, thermal stability, and compactness in size, if both the low-temperature sensor operation and satisfactory water durability are realized.

In 2011, we have reported a solid electrolyte type NH3 gas sensor showing high water durability at 230°C by using Al3+ ion conducting (Al0.2Zr0.8)20/19Nb(PO4)3 solid electrolyte and water durable NH4
+-β-gallate auxiliary sensing electrode. However, the operation temperature of this sensor (230°C) at which the high sensitivity without electrical noise was obtained are still high for expecting a long-term sensor operation, and it is required to lower the operation temperature. Because a main reason for preventing the low-temperature operation is high electrical resistance of NH4
+-β-gallate, we can expect to lower the operation temperature by newly designing an auxiliary sensing electrode.

In this study, we focused on NH4LaNb2O7 in which NH4
+ ions existed stably in the layered structure, similar to the NH4
+-β-gallate solid, and fabricated the highly water durable NH3 gas sensor which can operate even at 180°C, by combining with (Al0.2Zr0.8)20/19Nb(PO4)3 solid electrolyte.

The NH4LaNb2O7 solid was obtained by the ion-exchange of RbLaNb2O7, which was synthesized by conventional solid-state reaction, in NH4NO3 eutectic solution. Similar to our previous sensor, the sensor element was devised by the combination of (Al0.2Zr0.8)20/19Nb(PO4)3 solid electrolyte, NH4LaNb2O7 auxiliary sensing electrode, and Al metal reference electrode. The NH3 gas sensing performances were investigated at 180°C in atmospheres whose water content was varied from 0 to 4.2 vol.%.

From the thermal analysis of the synthesized NH4LaNb2O7 solid, it was found that NH4
+ ions stably existed in (NH4)LaNb2O7 solid up to 200°C. Therefore, we decided the sensor operation temperature at 180°C which is slightly lower than the upper temperature limit of NH4LaNb2O7 solid for holding NH4
+ ions in its structure. The present sensor exhibited a stable and continuous sensor response to NH3 gas concentration change with a 90% response time of 6 minutes (Response time includes substitution time of test gas in the gas flow line) at 180°C even under humid atmosphere containing 4.2 vol% H2O. Furthermore, the sensor EMF outputs for each NH3 gas concentration were almost same even for varying humidity from 0 to 4.2 vol%, and the EMF changes by changing NH3 gas concentration obeyed a theoretical Nernst relationship over three months, indicating that the present sensor with NH4LaNb2O7 solid as an auxiliary sensing electrode showed the theoretical response for NH3 gas at 180°C with long term stability even in humid atmosphere.