The effect of Nb deficiency and YSZ composite on the response characteristics of MnNb2O6 sensing electrodes in mixed potential SO2 sensors

Accurate and real-time detection of SO 2 at industrial emission sources is important for emission control, yet reliable sensing under high-temperature and relatively high-concentration conditions remains challenging. In this study, a YSZ-based mixed-potential SO 2 sensor was developed by regulating Nb nonstoichiometry in MnNb 2-x O 6 sensing electrodes and YSZ compositing. Nb deficiency altered particle coarsening, the apparent pore architecture of the electrode layer, and the defect chemistry. Among the single-phase sensing electrodes, MnNb 1.9 O 6 exhibited larger pore sizes and the highest oxygen-vacancy concentration, achieving a sensitivity of −14.76 mV/decade over 50-500 ppm SO 2 at 400 °C. Further compositing with YSZ produced more uniformed pore structure and increased the electrochemically active triple-phase-boundary (TPB) region, with MnNb 1.8 O 6 /20 wt% YSZ achieved the highest sensitivity of −18.74 mV/decade. A comparison of the composition series and YSZ-composite electrodes indicates that electrode microstructure and accessible TPB sites are the primary determinants of the electrochemical response, which enhance SO 2 diffusion and increase the number of active sites. Additionally, the increased oxygen vacancies also provide a synergistic contribution by promoting oxygen ion transport, further enhancing the overall reaction rate. The optimized sensor also showed good repeatability and 15-day stability, highlighting its potential for high-concentration SO 2 monitoring under industrially relevant dynamic-flow conditions.