We have designed and realized a temperature and pressure controlled cell for Neutron Reflectometry (NR) and Small Angle Neutron Scattering (SANS) that is compatible with simultaneous optical transmission and resistivity measurements. The cell can accommodate samples up to 102 mm (4 inch) in diameter, can be pressurized from vacuum up to 10 bar gas pressure and the sample temperature can be controlled up to 350°C. The four single crystal quartz windows ensure both a good neutron and optical transmission and hence can be used in combination with in-situ optical transmission measurements. We present the cell and illustrate its performance with a series of neutron reflectometry experiments performed on Ta based thin films under a hydrogen containing atmosphere.

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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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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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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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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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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

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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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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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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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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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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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 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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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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Thin films of rare earth metal oxyhydrides show a photochromic effect, the precise mechanism of which is yet unknown. Here, we made thin films of NdH3-2xOx and show that we can change the band gap, crystal structure, and photochromic contrast by tuning the composition (O2-:H-) via the sputtering deposition pressure. To protect these films from rapid oxidation, we add a thin ALD coating of Al2O3, which increases the lifetime of the films from 1 day to several months. Encapsulation of the films also influences photochromic bleaching, changing the time dependency from first-order kinetics. As well, the partial annealing which occurs during the ALD process results in a dramatically slower bleaching speed, revealing the importance of defects for the reversibility (bleaching speed) of photochromism.

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Catalyzing capping layers on metal hydrides are employed to enhance the hydrogenation kinetics of metal hydride-based systems such as hydrogen sensors. Here, we use a novel experimental method to study the hydrogenation kinetics of catalyzing capping layers composed of several alloys of Pd and Au as well as Pt, Ni, and Ru, all with and without an additional PTFE polymer protection layer and under the same set of experimental conditions. In particular, we employ a thin Ta film as an optical indicator to study the kinetics of the catalytic layers deposited on top of it and which allows one to determine the absolute hydrogenation rates. Our results demonstrate that doping Pd with Au results in significantly faster hydrogenation kinetics, with response times up to five times shorter than Pd through enhanced diffusion and a reduction in the activation energy. On the other hand, the kinetics of non-Pd-based materials turn out to be significantly slower and mainly limited by the diffusion through the capping layer itself. Surprisingly, the additional PTFE layer was only found to improve the kinetics of Pd-based capping materials and has no significant effect on the kinetics of Pt, Ni, and Ru. Taken together, the experimental results aid in rationally choosing a suitable capping material for the application of metal hydrides and other materials in a hydrogen economy. In addition, the used method can be applied to simultaneously study the hydrogenation kinetics in thin-film materials for a wide set of experimental conditions.@en

Direct electrochemical nitrogen reduction holds the promise of enabling the production of carbon emission-free ammonia, which is an important intermediate in the fertilizer industry and a potential green energy carrier. Here we show a strategy for ambient condition ammonia synthesis using a hydrogen permeable nickel membrane/electrode that spatially separates the electrolyte and hydrogen reduction side from the dinitrogen activation and hydrogenation sites. Gaseous ammonia is produced catalytically in the absence of electrolyte via hydrogenation of adsorbed nitrogen by electrochemically permeating atomic hydrogen from water reduction. Dinitrogen activation at the polycrystalline nickel surface is confirmed with 15N2 isotope labeling experiments, and it is attributed to a Mars-van Krevelen mechanism enabled by the formation of N-vacancies upon hydrogenation of surface nitrides. We further show that gaseous hydrogen does not hydrogenate the adsorbed nitrogen, strengthening the benefit of having an atomic hydrogen permeable electrode. The proposed approach opens new directions toward green ammonia.

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