Redox-Paired Oxide/Nitride Electrodes for Humidity-Tolerant Fuel-Cell NO₂ Sensing

Fuel-cell-type electrochemical gas sensors are attractive for low-power monitoring but often suffer from humidity-sensitive baselines and limited long-term stability in nitrogen dioxide (NO₂) detection. Here, we address this challenge by implementing redox pairing between chemically orthogonal electrodes in an asymmetric TiN/SnO₂ heteroelectrode, in which metallic TiN serves as a NO₂-reduction cathode and oxygen-vacancy-rich SnO₂ functions as an oxygen evolution anode, separated by a Nafion membrane. The resulting room-temperature NO₂ sensor exhibits a sensitivity of 1.572 μA ppm^-1, a theoretical detection limit of 18 ppb, and 8/9 s response/recovery times, while maintaining strong selectivity, minimal humidity-induced drift, and >98% signal retention over 180 days. These figures of merit surpass those of symmetric TiN-TiN and SnO₂-SnO₂ sensors and previously reported fuel-cell-type NO₂ sensors. Mechanistic analysis combining in situ infrared spectroscopy and density functional theory shows that SnO₂ drives water oxidation whereas TiN stabilizes NO₂-derived intermediates and mediates electron transfer. Machine-learning-guided screening further identifies TiN/SnO₂ as an optimal candidate within a broader oxide/nitride design space, highlighting redox-paired electrodes as a promising strategy for humidity-tolerant fuel-cell-type gas sensing.