Development of Ceramic Li-Electrolyte Based CO₂ Sensors for Temperatures Ranging From Ambient to High Temperature

Solid-state electrochemical CO₂ gas sensors composed of an electrolyte and two porous electrodes have been used extensively in the automobile and bio-chemical industry. Based on the field of application, the working temperature of the sensor ranges from room temperature to 600 °C. Two potentiometric CO₂ sensors that work at different temperature ranges were developed in this work. A potentiometric CO₂ gas sensor with Li₃PO₄ electrolyte and BaCO₃ coated Li₂CO₃ sensing electrode was developed and the sensing electrode was characterized in order to understand its sensing mechanism under humid conditions. This potentiometric CO₂ sensor showed humidity-interference-free sensing response for high CO₂ concentrations (5~25%) at high temperatures (T > 400 °C). In addition, the sensor showed good reproducibility and long-term stability under humid conditions. In the sensing electrode, the BaCO₃ layer improved the resistance against humidity as a chemical barrier, while the inner Li₂CO₃ layer was responsible for the CO₂ sensing. However, the sensor in which the eutectic layer covered the entire sensing electrode showed good sensing behavior under dry and humid conditions.

Lately, low-temperature CO₂ sensors have been attracting attention due to their low power consumption and easy sensor miniaturization, since a heater is unnecessary. We have developed a low-temperature CO₂ sensor based on lithium lanthanum titanate (LLT) electrolyte in dry conditions that requires further improvement. Lithium lanthanum titanate (LLT) electrolytes were prepared by a conventional solid-state method. The impedance of the LLT electrolyte was measured over the temperature range of 300 to 473 K and the frequency range of 5 Hz and 13 MHz. Activation energies for the Li ionic conduction for grain boundary and grain were estimated to be 0.47 and 0.31 eV, respectively. It was found that LLT is a good ionic conductor at low temperatures and a good candidate as an electrolyte for low-temperature electrochemical cells. A La_2/3-xLi_xTiO₃(LLT)-based CO₂ sensor with a mixture of CeO₂, Au, and Li₂CO₃ as the sensing electrode has been developed and shown to have relatively stable sensing behaviors at 200 °C under dry conditions. However, this sensor showed non-Nernstian behavior because electrochemical reactions were not fast enough on the sensing electrode and the solid electrolyte may have some electronic conduction. In addition, the observed sensitivity was less than the theoretical prediction. By adding K₂CO₃ on the sensing electrode, the sensitivity of the low-temperature CO₂ sensor was slightly improved. However, the sensing signals of the sensors were degraded by water vapor under humid conditions due to the formation of KHCO₃ or K₂CO₃•mH₂O (m= 2, 3, or 6). To optimize the low-temperature CO₂ sensor, a more active sensing electrode is needed, which may be achieved by controlling the size of particles and their distribution on the electrode. In addition, a thinner electrolyte with pure ionic conduction is also required.