Until now, the NDIR typed physical sensor has been used which is precise but expensive, large and difficult to maintain, which requires development of electrochemically potentiometric typed $CO_2$ sensors with good selectivities, technological simplicity, quantitative characteristics, compactness, and low price.
The first potentiometric $CO_2$ sensor based on carbonates was proposed by Gauthier et al. This typed sensor was $K_2CO_3-electrolyte$ based symmetrical cell using a reference gas, which had the gas sealing problem. A few years later, Maruyama et al. developed the tip-shaped $CO_2$ sensor based on NASICON electrolyte, working with the sensing electrode made of sodium carbonate and the open reference electrode. However, thermodynamically undefined status of the reference electrode caused the sensor to show the high drift effect. To solve this problem, Maier et al. developed $CO_2$ sensor with the reference electrode using the two phase mixture, which was not easily controlled and had poor reproducibility of synthesizing the reference material. So the key factor is that the reference electrode should make the potential fixed in the sensing performance. In this study, YSZ was adopted as a material of reference electrode and NASICON was chosen as a sodium ion conductor which led the bi-electrolyte typed $CO_2$ sensor to be created. Difficulty of sintering the pelletized NASICON and perfectly bonding between NASICON and YSZ was solved by thermal diffusion. Calcined and cold isostatic pressed NASICON pellet was successfully coupled with YSZ by thermal diffusion. $CO_2$ sensors fabricated by the simple and new technique showed good sensitivity up to theoretical values and good short term durability in various $CO_2$ concentrations. Furthermore, in terms of activity of sodium oxide, the small deviation from the value obtained by T. Maruyama et. al. and G.M. Kale et al. was shown.
When fabricating bi-electrolyte $CO_2$ sensors, $Na_2CO_3$, $Na_2CO_3-BaCO_3$ mixture (1:1.7 molar ratio), and $Na_2CO_3-BaCO_3-SiO_2$ (nano-silica, 5.9 weight percent) mixture were used as auxiliary phases and YSZ was used as a reference material. The structure of this new typed sensor was as followed
상용화된 광방식의 이산화탄소 센서는 비싸고 계속적인 유지 보수가 요구되므로 값싸고 기술적으로 간단한 전기화학식 센서의 개발이 요구되고 있다. 이제껏 개발된 전기화학식 센서는 기준전극에서 열역학적으로 정의가 잘 되지 않아서 기준전극을 대체가능한 음이온 전해질을 이용해서 양이온 전해질과 접합한 이산화탄소 센서를 개발하였다.