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 H 2 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 H 2 ), while satisfying ISO 26142 requirements for gas sensing. A machine‐learning regression model integrated into a sensor prototype enables robust, quantitative determination of H 2 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 H 2 O 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.