Accurate, cost-efficient, and safe hydrogen sensors will play a key role in the future hydrogen economy. Optical hydrogen sensors based on metal hydrides are attractive owing to their small size and costs and the fact that they are intrinsically safe. These sensors rely on suitable sensing materials, of which the optical properties change when they absorb hydrogen if they are in contact with a hydrogen-containing environment. Here, we illustrate how we can use alloying to tune the properties of hydrogen-sensing materials by considering thin films consisting of tantalum doped with ruthenium. Using a combination of optical transmission measurements, ex situ and in situ X-ray diffraction, and neutron and X-ray reflectometry, we show that introducing Ru in Ta results in a solid solution of Ta and Ru up to at least 30% Ru. The alloying has two major effects: the compression of the unit cell with increasing Ru doping modifies the enthalpy of hydrogenation and thereby shifts the pressure window in which the material absorbs hydrogen to higher hydrogen concentrations, and it reduces the amount of hydrogen absorbed by the material. This allows one to tune the pressure/concentration window of the sensor and its sensitivity and makes Ta_1-yRu_y an ideal hysteresis-free tunable hydrogen-sensing material with a sensing range of >7 orders of magnitude in pressure. In a more general perspective, these results demonstrate that one can rationally tune the properties of metal hydride optical hydrogen-sensing layers by appropriate alloying. ... Read more

As hydrogen can make an important contribution in the energy transition, its detection is necessary for safe application. Optical hydrogen sensors are a promising alternative to conventional hydrogen sensors as they are smaller, less expensive and safer. These sensors show a change in optical transmission upon exposure to hydrogen, allowing the partial hydrogen pressure to be probed. The optical response and the structural properties of Pd capped TaRu thin films are studied. A sensing range of 7 orders of magnitude in partial hydrogen pressure is observed, with a significant change in optical transmission upon hydrogenation. Exposure to hydrogen results in a phase transition of the Pd capping layer, leading to the hysteresis in the optical response of the sensor. This phase transition can be suppressed by alloying the capping layer. A sub-second response time is observed for large hydrogen pressures, which can be improved significantly using an additional PTFE layer. Phase transitions of the TaRu layer are suppressed by the
nanoconfinement of the sensing layer, and a solid solution is formed. The sensing range of TaRu can tuned by alloying Ta with Ru, resulting in a shift of the sensing range to higher hydrogen pressures. Implementing the suggested improvements would result in a hydrogen sensor with a sub-second response time and a tuneable sensing range of 7 orders of magnitude free of any hysteresis, proving TaRu to be an effective material for hydrogen sensing. ... Read more