(Invited) Solid State Electrochemical Gas Sensors: Fundamentals, Materials and Applications
Authors: Thangadurai Venkataraman , Mulmi Suresh 
Published: February 2020

Solid-state electrochemical gas sensor (SSEGS) typically functions on the principles based on galvanic cells i.e. the output signals (e.g., V, I etc.) provides the information on the gas of interest. Monitoring oxygen in automobiles is a good example of solid-state potentiometric gas sensors, where the potential difference (V) helps translate to determine air-to-fuel ratio.1 The generation of potential could be measured in three different configurations:2 (i) directly measuring mobile species (type I), (ii) indirectly measuring immobile components (type II), and (iii) analyzing other species by employing auxiliary solid-phases (type III). Fast Na+ and Li+ ion conducting Na-β”-alumina, NASICON-type and Li6BaLa2Ta2O12 garnet have been continuously investigated as potentiometric sensor.3,4 Although all these types cover wide range of gaseous concentration, the technique struggles on providing high resolution results at lower gas concentrations. On the other hand, amperometric (I) gas sensors are known to operate over a narrow range of gas concentrations with higher resolutions. However, both solid-state potentiometric and amperometric gas sensors face the challenges of not being able to accurately detect the industrially important gases (e.g., CO2, SOx, H2S, CH4) due to the cross-sensitivities and stability issues, particularly at high temperatures. In other words, these challenges demand new materials with better selectivity, sensitivity and stability under aggressive environments (e.g., high temperature, toxic gases). Unlike potentiometric and amperometric sensors, resistive-type (R) sensors cover large number of materials including ion, electron and mixed conducting semiconductor-based materials (e.g., SnO2, TiO2, CuO). The research on resistive-type sensors has been further extended to various p- and n-type semiconductor-based perovskites owing to their remarkable stability at elevated temperatures as well as excellent structural flexibility to accommodate desired dopants.5 Thus, this emphasizes the importance of fundamental research and improvements on materials’ characteristic properties to overcome the persisting challenges with SSEGS. Here, we attempt to discuss the transition from fast-ion conducting ceramics to semiconductor-based mixed conductors highlighting the necessity of SSEGSs for current industrial applications.

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