Optical hydrogen sensors have the power to reliably detect hydrogen in an inherently safe way, which is crucial to ensure safe operation and prevent emissions of hydrogen as an indirect greenhouse gas. These sensors rely on metal hydride material that can reversibly absorb hydrogen when it is present in the environment, and as a result, change their optical properties. To apply this technology along hydrogen infrastructure, in hydrogen-powered planes and other vehicles, it is crucial that these sensors can operate down to −60 °C, a challenge so far unaddressed. Here, it is showed that metal hydride hydrogen sensing materials can be used to detect hydrogen optically down to −60 °C in just a couple of seconds and across a hydrogen concentration range of 0.02–100% with a 1% change in transmission per order of magnitude change in hydrogen concentration. The in-situ X-ray diffraction and optical transmission measurements show that Ta, Ta₈₈Pd₁₂, Ta₈₈Ru₁₂, and Pd₆₀Au₄₀ can gradually, reversibly and hysteresis-free absorb hydrogen while providing sufficient optical contrast. Specifically, Ta₈₈Ru₁₂ possesses the largest optical contrast and the swiftest response down to 6 s at −60 °C. These results confirm the operational viability and foretell new applications of metal hydride hydrogen sensing in cold conditions.

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Optical hydrogen sensors based on metal hydrides have distinct advantages over other types of hydrogen sensors as they can be made small, do not require the presence of oxygen, and have a large sensing range. The working principle is based on the fact that when exposed to an atmosphere containing hydrogen, a metal hydride absorbs hydrogen, which in turn changes the optical properties. In a micro-mirror configuration, this change in optical properties can be measured by measuring the reflectivity of light. Tantalum alloys have been identified as suitable sensing material owing to their large sensing range, hysteresis-free response and fast response times. Here, we rationally develop a micro-mirror hydrogen sensor based on a tantalum-alloy as sensing layer. We first study the optical contrast of Ta_0.88Pd_0.12 thin films with a Pd_0.6Au_0.4 catalyst layer deposited on substrates with various catalyst and sensing layer thicknesses in reflection. Modeling the experimental results shows that the total optical contrast, that is the change of the reflectivity with a changing hydrogen concentration, is a strong interplay of wavelength and hydrogen-concentration dependent reflection, attenuation and amplification coefficients of both the Ta_0.88Pd_0.12 thin films with a Pd_0.6Au_0.4 catalyst layer. These effects may either constructively or destructively contribute to the overall signal, making carefully choosing the wavelength and layer thicknesses essential. Using optimal values of the wavelength and layer thicknesses, we successfully construct and test a micro-mirror sensor that can detect hydrogen over at least 5 orders of magnitude in hydrogen concentration without any hysteresis.

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Grasping (electronic) structure changes during photochromic processes is crucial for fully understanding the photochromic effect in rare-earth oxyhydride films. In this study, we employ in situ UV illumination positron annihilation lifetime spectroscopy (PALS) to investigate the time evolution of open-volume defects and metallic domains during photodarkening and bleaching in yttrium oxyhydride films. The PALS depth profiles before and after a photodarkening-bleaching cycle reveal a light-induced increase in open-volume defects, that occurs homogeneously throughout the oxyhydride layer. The time-dependent PALS measurements show that upon photodarkening, a fast initial formation of metallic domains occurs, as well as a fast release of loosely bounded hydrogen from vacancy clusters and nanopores. During further photodarkening, the concentration of divacancy-like defects gradually increases due to the aggregation of light-induced hydrogen vacancies with preexisting yttrium monovacancies. After the UV illumination is stopped, two subsequent bleaching phases are observed. During the first bleaching phase, a strong correlation between the shortest positron lifetime 𝜏1 and the photochromic contrast is seen in both samples, suggesting that metallic domains disappear and, correspondingly, positron trapping at yttrium monovacancies and divacancy-like defects increases. During the second bleaching phase, a subsequent correlation between 𝜏1 and the photochromic contrast is observed in the more H-rich sample, which is related to the disappearance of larger metallic domains. After bleaching, most of the metallic domains and the photoexcited electrons in the matrix have disappeared, while the formed small vacancy complexes and larger vacancies remain stable. Our PALS study suggests that the formation of metallic domains is the cause of photodarkening, and the formed vacancy defects are important for understanding the memory effect.@en

Here, we study the structural and optical properties of tetragonal β-tantalum-sputtered thin films both ex situ and when exposed to hydrogen, with a focus on optical hydrogen sensing applications. Using optical transmission measurements, out-of-plane and in-plane X-ray diffraction, and X-ray and neutron reflectometry, we show that thin film β-tantalum gradually, reversibly, and hysteresis-freely absorbs hydrogen with an increasing hydrogen pressure/concentration. The gradual absorption of hydrogen with increasing hydrogen concentrations induces a change in the optical transmission and reflection. These quantities change reversibly and are hysteresis-free over at least 5 orders of magnitude in hydrogen pressure/concentration, making β-tantalum a suitable hydrogen sensing material. At all partial hydrogen pressures studied, we observe that the volumetric expansion, hydrogen-to-metal ratio, and lattice expansion are substantially smaller than for body-centered cubic α-tantalum.

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Palladium thin films have been studied as hydrogen sensing materials and applied to variety of optical hydrogen sensors. Recently, tantalum has emerged as an attractive option for hydrogen sensing materials due to its broad sensing range and flexibility in tuning the sensing range by modifying the alloying composition or elements. Following the demand for optical hydrogen sensors for aerospace applications, testing the performance of hydrogen sensing materials is of interest. This work examines the optical response in respect to changing hydrogen concentrations and thermal expansion of palladium-gold (Pd_0.65Au_0.35) and tantalum-ruthenium (Ta_0.97Ru_0.03 and Ta_0.91Ru_0.09) thin films at temperatures similar to a hydrogen combustion engine. Our results suggest that tantalum-ruthenium alloys are suitable for sensing hydrogen from ambient temperatures up to 270^◦C because its low detection limit (0.01% of hydrogen in the atmosphere) is well below the explosive limit of hydrogen (4% of hydrogen in the atmosphere).

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Hydrogen, which serves as a major driver of sustainable aviation, requires precise sensing methods. This study focuses on the development of a metal hydride-coated tilted fibre Bragg grating (TFBG) based sensor for hydrogen detection, with tantalum as a novel sensing material for fibre optic hydrogen sensing. Magnetron sputtering has been employed to deposit nanometer-scale metal films onto the optical fibre surface of the TFBG structure. In this proof of concept work, changes in both amplitude and the centre wavelength of cladding resonances of the TFBG transmission spectrum were observed in the hydrogen concentration range from 0.01% to 4% at room temperature.

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Cycling stability of the photochromic effect in rare-earth oxyhydride thin films is of great importance for long-term applications such as smart windows. However, an increasingly slower bleaching rate upon photochromic cycling was found in yttrium oxyhydride thin films; the origin of this memory effect is yet unclear. In this work, the microstructural changes under six photodarkening-bleaching cycles in YHxOy and GdHxOy thin films are investigated by in situ illumination Doppler broadening positron annihilation spectroscopy, complemented by positron annihilation lifetime spectroscopy (PALS) investigations on YHxOy films before and after one cycle. For the first three cycles, the Doppler broadening S parameter after bleaching increases systematically with photodarkening-bleaching cycle, and correlates with the bleaching time constant extracted from optical transmittance measurements. This suggests that the microstructural evolution that leads to progressively slower bleaching involves vacancy creation and agglomeration. PALS suggests that during a photodarkening-bleaching cycle, divacancies are formed that are possibly composed of illumination-induced hydrogen vacancies and preexisting yttrium monovacancies, and vacancy clusters grow, which might be due to local removal of hydrogen. If bleaching is a diffusion-related process, the formed vacancy defects induced by illumination might affect the diffusion time by reducing the diffusion coefficient. Hydrogen loss could also be a key factor in the reduced bleaching kinetics. Other microstructural origins including domain growth, or formation of OH- hydroxide groups, are also discussed with respect to the slower bleaching kinetics. During the fourth to sixth photodarkening-bleaching cycle, reversible shifts in the Doppler S and W parameters are seen that are consistent with the reversible formation of metallic-like domains, previously proposed as a key factor in the mechanism for the photochromic effect.

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This paper studies the structural and optical properties of tantalum–iron-, tantalum–cobalt-, and tantalum–nickel-sputtered thin films both ex situ and while being exposed to various hydrogen pressures/concentrations, with a focus on optical hydrogen sensing applications. Optical hydrogen sensors require sensing materials that absorb hydrogen when exposed to a hydrogen-containing environment. In turn, the absorption of hydrogen causes a change in the optical properties that can be used to create a sensor. Here, we take tantalum as a starting material and alloy it with Fe, Co, or Ni with the aim to tune the optical hydrogen sensing properties. The rationale is that alloying with a smaller element would compress the unit cell, reduce the amount of hydrogen absorbed, and shift the pressure composition isotherm to higher pressures. X-ray diffraction shows that no lattice compression is realized for the crystalline Ta body-centered cubic phase when Ta is alloyed with Fe, Co, or Ni, but that phase segregation occurs where the crystalline body-centered cubic phase coexists with another phase, as for example an X-ray amorphous one or fine-grained intermetallic compounds. The fraction of this phase increases with increasing alloyant concentration up until the point that no more body-centered cubic phase is observed for 20% alloyant concentration. Neutron reflectometry indicates only a limited reduction of the hydrogen content with alloying. As such, the ability to tune the sensing performance of these materials by alloying with Fe, Co, and/or Ni is relatively small and less effective than substitution with previously studied Pd or Ru, which do allow for a tuning of the size of the unit cell, and consequently tunable hydrogen sensing properties. Despite this, optical transmission measurements show that a reversible, stable, and hysteresis-free optical response to hydrogen is achieved over a wide range of hydrogen pressures/concentrations for Ta–Fe, Ta–Co, or Ta–Ni alloys which would allow them to be used in optical hydrogen sensors@en

The thermal stability of an equilibrium phase may be tuned due to lattice strain and distortion induced by nanosizing. We apply these effects to destabilize magnesium hydride, a promising hydrogen storage material owing to its high gravimetric hydrogen density but with a too high operating temperature/low supply pressure of hydrogen for most practical applications. The destabilization is attempted with MgH₂ in contact with high entropy alloy (HEA), in which multiple metal atoms lead to lattice strain and distortion. Here, two HEAs, CrMnFeCoNi with a face centered cubic (fcc) structure and TiVZrNbHf with a body centered cubic (bcc) structure, were prepared. Subsequently, they were cosputtered with Mg to synthesize Mg−HEA thin films, respectively. Although, in the Mg−CrMnFeCoNi thin films, miscible metals with Mg as Co and Ni may hamper the formation of independent Mg domains, a small proportion of Mg atoms form destabilized MgH₂. In contrast, Mg and TiVZrNbHf domains are chemically segregated at the nanoscale in the Mg−TiVZrNbHf thin films. The formation of nanometer-sized Mg domains is promoted by atomic rearrangement following the structural change of TiVZrNbHf from a bcc to an fcc structure upon hydrogenation, resulting in distorted and destabilized MgH₂. Our strategy to use HEAs and the structural change upon hydrogenation for the formation of destabilized MgH₂ is effective and opens up the possibility for the development of advanced and low-cost hydrogen storage and supply systems.

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Metal hydrides have been widely studied as hydrogen sensing materials and applied to various optical sensor configurations. With the increasing interest in using hydrogen as an energy source across sectors involving combustion processes, there is a growing demand for reliable hydrogen sensors operating at temperatures above 100 °C. Therefore, it is necessary to evaluate the performance of potential hydrogen sensing materials at elevated temperatures. We conducted experiments to observe the optical response and structural characteristics of palladium, palladium–gold, tantalum, and tantalum-alloy thin films with respect to varying hydrogen concentrations from 28 °C to 270 °C. Our results demonstrate that the optical response of palladium and palladium–gold diminish at 270 °C. However, tantalum provides a remarkable optical response to hydrogen concentrations below 1% for all the observed temperatures and a stable response at 270 °C for 350 cycles. Our measurement results show that tantalum is the most suitable material for detecting hydrogen within the range of 0.01% to 100% at temperatures ranging from 28 °C to 270 °C.

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Hydrogen, crucial in industrial and environmental realms, demands precise sensing methods. This study focuses on the design of a metal hydride-coated tilted fibre Bragg grating (TFBG) based sensor for hydrogen detection, introducing tantalum as a novel sensing material for fibre optic hydrogen sensor development. To facilitate the ellipsometry inspection of optical constants, magnetron sputtering technique has been employed to deposit nanometer-scale metal films onto a glass substrate. Numerical modeling results are presented for mode analysis of the proposed sensor design, analyzing transverse mode behavior for sensor optimization. The study also provides insights into other TFBG sensor design considerations, suggesting potential hydrogen sensing applications, such as hydrogen-powered aviation and storage solutions.

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Nanostructured metal hydrides could play a key role in a hydrogen economy. The nanostructuring or confinement of these materials as, e.g., thin films significantly affects the structural and functional properties. For tantalum hydride, a versatile hydrogen sensing material, we show that the confinement of tantalum as a thin film extends the solubility limit by suppressing the phase transition observed in bulk upon hydrogenation. Different from bulk, the body centered cubic unit cell continuously deforms with unequal lattice constants and angles between lattice vectors. This deformation ensures that the volumetric expansion is realized in the out-of-plane direction, and surprisingly, completely elastic in nature. The first-order phase transition suppression combined with the continuous elastic deformation of the tantalum unit cell over an extraordinary wide solubility range ensures the superb performance of tantalum and its alloys as a hysteresis-free optical hydrogen sensing range over a hydrogen pressure/concentration range of over 7 orders of magnitude.

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At ambient conditions, rare-earth oxyhydride thin films show reversible photochromism and photoconductivity, while their mechanism and relation are still unclear. In this work, this question is explored with in situ time-resolved measurements of both optical and transport properties of Gd-based oxyhydride thin films. It is found that p-type large polaron conduction is the initial mechanism of charge transport; however, upon photo-darkening, a 10⁴-fold increase of conductivity occurs, and n-type carriers become dominant. Further, photochromism and photoconductivity are shown to originate from a single process, as indicated by the fact that the photoconductivity is exponentially proportional to the increase of optical absorption. This exponential relation, notably, cannot stem from any of the optically absorbing species thought responsible for photochromism and, therefore, suggests that their formation is accompanied by a concerted increase of negative charge carriers in the Gd oxyhydride films.

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Photochromism has been reported for several rare-earth (RE) metal oxyhydride thin films and is characterized by a reversible darkening of the sample when exposed to light with energy greater than its optical bandgap. Here, we extend the range of known photochromic RE-oxyhydrides to include samarium oxyhydrides. These SmH3−2xOx thin films are made by reactive magnetron sputtering of as-deposited SmH1.9+δ and post-oxidation in the air to the oxyhydride phase. The deposition pressure during sputtering is used to control the resultant properties of the Sm-oxyhydride film, such as the optical bandgap, cubic lattice constant, photochromic contrast, and photochromic bleaching speed. Using Sm as the RE-cation results in slower bleaching speeds compared to other lanthanides. We posit that this is due to the stability of the Sm2+ state and the difficulty to oxidizing it back to the original RE3+ state. This points to the key role of the RE-cation charge state for the optical properties of the material.@en

Ammonia is an indispensable commodity and a potential carbon free energy carrier. The use of H permeable electrodes to synthesize ammonia from N ₂, water and electricity, provides a promising alternative to the fossil fuel based Haber-Bosch process. Here, H permeable Ni electrodes are investigated in the operating temperature range 25–120 °C, and varying the rate of electrochemical atomic hydrogen permeation. At 120 °C, a steady reaction is achieved for over 12 h with 10 times higher cumulative NH ₃ production and almost 40-fold increase in faradaic efficiency compared to room temperature experiments. NH ₃ is formed with a cell potential of 1.4 V, corresponding to a minimum electrical energy investment of 6.6 kWh kg ⁻¹ (Figure presented.). The stable operation is attributed to a balanced control over the population of N, NH _x and H species at the catalyst surface. These findings extend the understanding on the mechanisms involved in the nitrogen reduction reaction and may facilitate the development of an efficient green ammonia synthesis process.

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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.

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To develop an understanding of the photochromic effect in rare-earth metal oxyhydride thin films (REH3-2xOx, here RE = Y), we explore the aliovalent doping of the RE cation. We prepared Ca-doped yttrium oxyhydride thin films ((CazY1-z)HxOy) by reactive magnetron cosputtering with Ca doping concentrations between 0 and 36 at. %. All of the films are semiconductors with a constant optical band gap for Ca content below 15%, while the band gap expands for compositions above 15%. Ca doping affects the photochromic properties, resulting in (1) a lower photochromic contrast, likely due to a lower H- concentration, and (2) a faster bleaching speed, caused by a higher pre-exponential factor. Overall, these results point to the importance of the H- concentration for the formation of a "darkened"phase and the local rearrangement of these H- for the kinetics of the process.

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The energy axes of the RBS and ERD data (contained in Figures 2a,b,d,e, and S4) were originally underestimated, and the corrected figures appear below and in the Supporting Information. The change is in the conversion from raw data to the energy scale, which was initially converted incorrectly. The rescaled x-axis does not change the data conclusions since the assignment of peaks to atoms remains the same and the intensity of the peaks is unaffected. Hence, it has no influence on the calculations and conclusions in the original text. (Figure presented).

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The electrochemical nitrogen reduction reaction (NRR) is a promising alternative to the current greenhouse gas emission intensive process to produce ammonia (NH₃) from nitrogen (N₂). However, finding an electrocatalyst that promotes NRR over the competing hydrogen evolution reaction (HER) has proven to be difficult. This difficulty could potentially be addressed by accelerating the electrocatalyst development for NRR by orders of magnitude using high-throughput (HTP) workflows. In this work, we developed a HTP gas diffusion electrode (GDE) cell to screen up to 16 electrocatalysts in parallel. The key innovation of the cell is the use of expanded Polytetrafluoroethylene (ePTFE) gas diffusion layers (GDL) which simplifies the handling of catalyst arrays compared to carbon fabrics and enables sufficient N₂ mass transport. We demonstrate the robustness of the HTP workflow by screening 528 bimetallic catalysts of composition AB (A,B = Ag, Al, Au, Co, Cu, Fe, Mn, Mo, Ni, Pd, Re, Ru, W) for NRR activity. None of the materials produced ammonia significantly over background level which emphasizes the difficulty of finding active electrocatalysts for NRR and narrows down the search space for future studies.

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Thin films of rare-earth metal oxyhydrides, such as yttrium oxyhydrides (YH3-2xOx), show a photochromic effect where the transparency of the films decreases reversibly upon exposure to UV light. However, the exact mechanism behind this effect is unknown. In this paper, we describe the behavior of YH3-2xOx thin films, with different O2-:H- ratios, under dark and illuminated conditions using in situ muon spin relaxation (μ+SR), and compare that to an oxygen-free reference compound, yttrium dihydride (YH2-δ). The muon acts as a local magnetic probe in our compounds, giving information related to electronic, structural, and photochromic properties. Although YH2-δ is the parent compound to YH3-2xOx, the muon behavior in these two materials is different - the muon electrostatically interacts primarily with H- (dihydride) or O2- (oxyhydride) - leading to the use of different theoretical models. For YH2-δ, we observed the formation of an entangled H-μ complex and the onset of Mu+ diffusion and H- rearrangement above 150 K (EA,Γ=67±13meV). For the oxyhydrides, we adopted a transition state model, where Mu0 formation and gradual Mu+ recovery take place, accompanied by the formation of a Mu+-O2- complex and a polaron at the Y cation. The activation energy (EA,dia) associated with Mu+ recovery is dependent on lattice relaxation and is lower for thin films of higher H content (EA,dia=29-45meV). In situ illumination further reduces this energy barrier for all measured oxyhydrides, suggesting that the photochromic effect involves a reversible structural rearrangement during photodarkening.

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