Grain-Boundary-Facilitated Palladium Hydride Formation Enables Humidity-Tolerant Catalytic Hydrogen Sensing

Reliable hydrogen sensing under high absolute humidity remains a significant challenge owing to the competitive adsorption of water molecules on the sensing material surface. Here, we report a humidity-tolerant catalytic H₂ sensor based on a grain-boundary-rich palladium nanostructure (Pd_GB) that operates without any external heating. The Pd_GB sensor functions reliably at 22°C-80°C and 0%-90% relative humidity, achieving a lower limit of detection (LLOD) of 100 ppm even at 80°C and 90% relative humidity (261 g m^-3 absolute humidity, corresponding to a > 3000-fold higher molar concentration of water vapor relative to the LLOD of H₂), while satisfying ISO 26142 requirements for gas sensing. A machine-learning regression model integrated into a sensor prototype enables robust, quantitative determination of H₂ concentration across a wide range of temperature and humidity conditions. Mechanistic investigations combining in situ XRD, temperature-programmed desorption, molecular dynamics simulations, and density functional theory calculations reveal that the exceptional humidity tolerance originates from the Pd grain boundaries, which accelerate PdH_x formation for weakening H₂O adsorption and suppress humidity-induced interference. These findings reveal a previously unexplored synergistic mechanism and establish a general materials-design strategy for next-generation humidity-tolerant catalytic gas sensors.