An Ambient Gasochromic Hydrogen Gas Sensor Based on a Pd/Mo Bimetallic Catalyst and Azo Dyes

As the global transition to a hydrogen economy accelerates, detecting hydrogen leaks under ambient conditions becomes increasingly important. Traditional semiconductor-based sensors are unsuitable for this purpose due to their high operating temperatures. Herein, we report a gasochromic hydrogen sensor that functions at room temperature and provides a colorimetric response upon exposure to hydrogen gas at a concentration as low as 0.11 v/v %. The sensor features a methyl red (an azo dye) impregnated into a paper strip functionalized with Pd nanoparticles and a Mo-diaminobipyridine (dabpy) complex. Upon exposure to continuous H2, the paper strip undergoes a rapid, irreversible color change from red to colorless within 100 s. This response is triggered by a cascade of events that begins with the dissociative adsorption of H2 on the surface of Pd nanoparticles to reduce the Mo(VI) center in the Mo-dabpy complex. The resulting Mo(V) species then reduces the azo group of methyl red, yielding the colorless form. Notably, removing the dabpy ligand or replacing Mo(VI) with other metal ions such as V(V) or W(VI) does not produce any visible color change. The sensor exhibits high selectivity against CO, CO2, N2, O2, and NH3. The sensor retained a clear H2 response even after 14 days of storage. The low-cost sensors are ideal for large-scale deployment as leak indicators for hydrogen infrastructure. Furthermore, their passive nature allows them to be integrated into personal protective equipment for enhanced safety.