Potentiometric carbon dioxide sensor based on thin Li3PO4 electrolyte and Li2CO3 sensing electrode

Potentiometric CO2 gas sensors with thin-film lithium phosphate (Li3PO4) electrolytes were developed by using radio frequency (RF) magnetron sputtering. Li2CO3 and a mixture of Li2TiO3 and TiO2 were used as sensing and reference electrodes, respectively. By using the RF sputtering deposition process, we obtained a dense, crystalline, thin-film Li3PO4 electrolyte with good adhesion on the Al2O3 substrate. The thin-film Li3PO4 electrolyte had good ionic conductivity, i.e., 2.15 × 10−6 S cm−1 at 500 °C, and its activation energy was 0.97 eV. The thin-film Li3PO4 electrolyte was suitable for the miniaturization of potentiometric CO2 sensors. The thin-film potentiometric CO2 sensor provided relatively good sensing response for overall CO2 concentrations (500 to 3,000 ppm and 5 to 20 %) at 500 °C. The Nernstian slope of 78.2 mV/decade obtained for CO2 concentrations from 5 to 20 % at 500 °C was close to the theoretical value (76.6 mV/decade). Although the sensor’s reading deviated from the theoretical value at low CO2 concentrations (500 to 3,000 ppm), the sensor provided better sensing performance than a potentiometric CO2 sensor with a thick electrolyte. As a result, it was assumed that the thin-film sensor could be used to monitor the overall concentration of CO2 in the environment.