Metastable CeZrO4 polymorph for high-sensitivity nitrogen dioxide detection

Accurate, real-time detection of nitrogen dioxide (NO ₂ ) is essential for enforcing next-generation vehicle emission standards and ensuring environmental safety. Electrochemical devices offer a cost-effective solution; however, current sensing materials have limited sensitivity, stability, and selectivity. Here, we report a metastable polymorph of CeZrO ₄ , stabilized by liquid-nitrogen quenching, as a high-performance NO ₂ -sensing material. The metastable tetragonal structure permits high concentrations of Ce ³⁺ and oxygen vacancies, and has high-energy surfaces, which collectively facilitate NO ₂ adsorption and charge transport. The resulting sensor achieves an unprecedented 24,000% current response to 10 ppm NO ₂ , compared to 3,300% for the equilibrium cubic phase and 3,050% for the best reported literature material, while maintaining stable operation under variable humidity and temperature. These findings demonstrate that metastable phases offer an effective route to tune sensing materials, and the current example provides a scalable platform for precise, durable, and sensitive NO ₂ monitoring as required, e.g., in the Euro 7 emission framework.