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

Abstract

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.