Characterization of a NASICON based potentiometric CO2 sensor

Open CO2 sensors work in a differential mode and the response must only depend on the CO2 pressure. To obtain a more stable response, the O2 reference electrode consists of a material which can exchange oxygen with gas and sodium ions (Na+) with the NASICON membrane. Such an electrode is composed of a mixture of sodium titanates Na2Ti3O7–Na2Ti6O13 and La1−xSrxMnO3 (LSM). This latter compound is added to improve the oxygen electrode reaction. The sensing electrode is composed of gold and a mixture of sodium and barium carbonates (Na2CO3 and BaCO3). Different compositions of the two electrodes are investigated in a large range of CO2 pressure. Sensors with a weight composition of carbonates about 3:1 in sodium and barium for the sensing electrode show a nernstian response in the 385–668 °C temperature range. In this range, the experimental slopes are about 90% of the theoretical value. Below 385 °C, a logarithmic function of the CO2 pressure is still observed but deviations from the Nernst behaviour occur. A thermodynamic approach based on the standard potential E° point of view reveals a good agreement between theory and experiments.