Thin film based sensors for a continuous monitoring of hydrogen concentrations

Abstract

The transition toward a more sustainable energy infrastructure is inextricably connected to the development of reliable hydrogen sensors. One of the most promising configurations for such a device is a thin film based fiber optic detector. We demonstrate that with such a device not only the detection of a specific threshold concentration is possible, but also a quantitative determination of the hydrogen concentration. A varying hydrogen concentration can be measured optically using Pd–Ta alloy sensing materials. The optical properties of a Pd–Ta sensing layer change in a one-to-one relation with the applied partial hydrogen pressure. Thus, in a thin film optic fiber configuration, the hydrogen concentration can be quantitatively determined up to 300mbar at room temperature in reflection mode. This Pd–Ta sensor remains optically and mechanically stable up to at least 60 hydrogenation cycles without any indication of degradation. The response time during absorption and desorption is fast (typical<50s) as compared to Pd based detectors. Furthermore, pressure composition isotherm measurements indicate that the hysteresis between hydrogen absorption and desorption becomes very small for Pd–Ta alloys containing more than 5at.% Ta, which results in an almost one-to-one relation between the applied hydrogen pressure and the optical response. However, due to the loss of optical response in reflection at higher Ta concentrations, the most optimal composition range between 5.0 and 6at.% Ta. Furthermore, we find that optical transmission measurements are more sensitive and allow to use higher Ta concentrations of up to 7at.% and enables the measurement of pressures up to 1000mbar H2. The presented research is a first step toward the development of a quantitative fiber optic hydrogen sensor for the measurements of hydrogen concentrations in processes involving the production, transportation, storage and use of hydrogen.