This paper provides a retrospective summary of Kale and co-workers’ systematic studies on solid-state mixed-potential NO2 and CO sensors for high-temperature emission monitoring. The effects of solid electrolytes, sensing electrodes, and electrode thickness on the gas-sensing performance of the sensors are analyzed. The paper compares the differences in ionic conductivity, microstructure, and interfacial compatibility among electrolytes such as YSZ, ScSZ, α-Al2O3-doped ScSZ (16AlSZ), La0.85Sr0.15Ga0.8Mg0.2O(3−0.35/2), and (BaxLa1−x)2In2O5+x. The results indicate that the crystalline structure and densification characteristics of the electrolytes can significantly influence the development of mixed potentials, stability of response signals, and response kinetics. For CO sensing, nanoporous ITO coupled with ScSZ delivers a sensitivity of 87.1 mV/decade at 615 °C with a t90 of 4 s and shows clear microstructure dependence: ImageJ analysis gives an apparent surface porosity of 33.75% for spherical-ITO electrodes vs 26.24% for rod/spherical-ITO electrodes, consistent with faster reversible responses for the more open pore network. Thickness studies on ITO/ScSZ identify an optimal printed sensing-layer thickness window of ∼7–15 μm, balancing gas diffusion, effective TPB utilization, and polarization losses. For NO2 sensing, mixed-oxide/spinel electrodes achieve fast responses at intermediate temperatures, exemplified by 52.45 mV/decade with t90 = 6 s (CuO + CuCr2O4/ScSZ) and 36.69 mV/decade with t90 = 6 s (Pt/16AlSZ) over 500–650 °C. These quantitative outcomes provide practical guidelines for electrolyte/electrode pairing and thickness/microstructure design in mixed-potential NO2/CO sensors.
Meng et al. (Sun,) studied this question.