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.

Thin film metal hydride optical sensors, especially those made from tantalum, offer a large, hysteresis-free hydrogen sensing range, fast response times and great stability. However, due to the shift in tantalum’s hydrogen sensing ranges with rising temperatures, tantalum becomes inadequate for the detection of low hydrogen concentrations (<10+3 ppm) above 200 °C, making it unsuitable for high-temperature applications. We show that the properties of tantalum can be tailored by alloying tantalum with hafnium. Optical transmission measurements, ex situ and in situ X-ray diffraction and X-ray and neutron reflectometry are used to show that the introduction of Hf in Ta results in a solid solution with a stable structure with up to 21% Hf. Alloying Ta with Hf expands the unit cell, which alters the enthalpy of hydrogenation and shifts the sensing range to lower concentrations. Moreover, alloying Ta with Hf improves the sensitivity at low hydrogen concentrations (<10+3 ppm) and for temperatures exceeding 200 °C by about two times compared to pure Ta while preserving its large, hysteresis-free sensing range and excellent stability.

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.

Using in situ scanning transmission electron microscopy (STEM) and low-loss plasmon electron energy-loss spectroscopy (EELS), we reveal asymmetric transformation mechanisms during the hydrogenation and dehydrogenation of Mg thin films. Remarkably, during hydrogenation, the MgH2 phase can nucleate from either the bottom or top interface of a Mg thin film while symmetrically sandwiched between two Ti layers. This unexpected behavior, occurring under identical external conditions, highlights the critical role of nucleation barriers in the phase transformation process, challenging conventional diffusion-driven paradigms. In contrast, dehydrogenation proceeds exclusively via an H2 diffusion-controlled frontal growth originating from the top interface. These insights underscore the importance of understanding metal-to-metal hydride phase transformations for advancing hydrogen storage technologies and applications such as hydrogen sensing.

In Fig. 4(e) on page 6733 of this article, the legends in the graph for faradaic efficiency of CO and C2+ were misplaced. The original figure should be replaced with an updated one. Note that this correction does not have any impact on the main idea and conclusion of this article. The updated Fig. 4 is as follows. The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers. (Figure presented).

Metal hydrides are potential candidates for applications in hydrogen-related technologies, such as energy storage, hydrogen compression, and hydrogen sensing, to name just a few. However, understanding the electronic structure and chemical environment of hydrogen within them remains a key challenge. This work presents a new analytical pathway to explore these aspects in technologically relevant systems using hard x-ray photoelectron spectroscopy (HAXPES) on thin films of two prototypical metal dihydrides: YH2-δ and TiH2-δ. By taking advantage of the tunability of synchrotron radiation, a nondestructive depth profile of the chemical states is obtained using core-level spectra. Combining experimental valence-band (VB) spectra collected at varying photon energies with theoretical insights from density functional theory (DFT) calculations, a description of the bonding nature and the role of d versus sp contributions to states near the Fermi energy are provided. Moreover, a reliable determination of the enthalpy of formation is proposed by using experimental values of the energy position of metal s-band features close to the Fermi energy in the HAXPES VB spectra.

Carbon dioxide (CO2) electrolysis on copper (Cu) catalysts has attracted interest due to its direct production of C2+ feedstocks. Using the knowledge that CO2 reduction on copper is primarily a tandem reaction of CO2 to CO and CO to C2+ products, we show that modulating CO concentrations within the liquid catalyst layer allows for a C2+ selectivity of >80% at 200 mA cm−2 under broad conversion conditions. The importance of CO pooling is demonstrated through residence time distribution curves, varying flow fields (serpentine/parallel/interdigitated), and flow rates. While serpentine flow fields require high conversions to limit CO selectivity and maximize C2+ selectivity, the longer CO residence times of parallel flow fields achieve similar selectivity over broad flow rates. Critically, we show that parts of the catalyst area predominantly reduce CO instead of CO2 as supported by CO reduction experiments, transport modelling, and achieving a CO2 utilization efficiency greater than the theoretical limit of 25% for C2+ products.

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.

Rare-Earth oxyhydrides (REH3-2xOx) are characterized by photodarkening when illuminated by photons having an energy exceeding that of the band gap. We propose that the film is segregated in hydrogen rich and hydrogen poor areas. Upon illumination, the excited electrons reduce the three-valent cations inducing an insulator to metal transition in the hydrogen rich entities. These small metallic oxyhydride clusters are responsible for the enhanced optical absorption. In the surrounding semiconductor matrix the photoexcitation induces a transition from p to n-type conductivity. This persistent photoconductivity is due to trapping of the holes by hydride ions. As a result, the Fermi level rises above the conduction band inducing a Burstein-Moss effect and a large increase in the conductivity.

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.

Rare earth oxyhydrides REO_xH_(3-2x), with RE = Y, Sc, or Gd and a cationic FCC lattice, are reversibly photochromic in nature. It is known that structural details and anion (O^2-:H^-) composition dictate the efficiency of the photochromic behavior. The mechanism behind the photochromism is, however, not yet understood. In this study, we use ¹H, ²H, ¹⁷O, and ⁸⁹Y solid-state NMR spectroscopy and density functional theory (DFT) calculations to study the various yttrium, hydrogen, and oxygen local environments, anion oxidation states, and hydride ion dynamics. DFT models of YO_xH_(3-2x) with both anion-ordered and anion-disordered sublattices are constructed for a range of compositions and show a good correlation with the experimental NMR parameters. Two-dimensional ¹⁷O-¹H and ⁸⁹Y-¹H NMR correlation experiments reveal heterogeneities in the samples, which appear to consist of hydride-rich (x ≈ 0.25) and hydride-poor domains (x ≈ 1) rather than a single composition with homogeneous anion mixing. The compositional variation (as indicated by the different x values in YO_xH_(3-2x)) is determined by comparing static ¹H NMR line widths with calculated ¹H-¹H dipolar couplings of yttrium oxyhydride models. The 1D ¹⁷O MAS spectrum demonstrates the presence of a small percentage of hydroxide (OH^-) ions. DFT modeling indicates a reaction between the protons of hydroxides and hydrides to form molecular hydrogen (H⁺ + H^- → H₂). ¹H MAS NMR indicates the presence of a mobile component that, based on this finding, is attributed to trapped molecular H₂ in the lattice.

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.

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.

Rare-earth (RE) oxyhydride thin films show a color-neutral, reversible photochromic effect at ambient conditions. The origin of the photochromism is the topic of current investigations. Here, we investigated the lattice defects, electronic structure, and crystal structure of photochromic YHxOy and GdHxOy thin films deposited by magnetron sputtering using positron annihilation techniques and X-ray diffraction, in comparison with Y, YH∼1.9, Y2O3, Gd, GdH∼1.8, and Gd2O3 films. Positron annihilation lifetime spectroscopy (PALS) reveals the presence of cation monovacancies in the as-deposited Y and YH∼1.9 films at concentrations of ∼10-5 per cation. In addition, vacancy clusters and nanopores are found in the as-prepared YHxOy and Y2O3 films. Doppler broadening positron annihilation spectroscopy (DB-PAS) of the Y- A nd Gd-based films reflects the transition from a metallic to an insulating nature of the RE metal, metal hydride, semiconducting oxyhydride and insulating oxide films. In-situ illumination DB-PAS shows the irreversible formation predominantly of di-vacancies, as PALS showed that cation mono-vacancies are already abundantly present in the as-prepared films. The formation of di-vacancies supports conjectures that H-(and/or O2-) ions become mobile upon illumination, as these will leave anion vacancies behind, some of which may subsequently cluster with cation vacancies present. In addition, in RE oxyhydride films, partially reversible shifts in the Doppler parameters are observed that correlate with the photochromic effect and point to the formation of metallic domains in the semiconducting films. Two processes are discussed that may explain the formation of these metallic domains and the changes in optical properties associated with the photochromic effect. The first process considers the reversible formation of metallic nanodomains with reduced O: H composition by transport of light-induced mobile hydrogen and local oxygen displacements. The second process considers metallic nanodomains resulting from the trapping of photoexcited electrons in an eg orbital at the yttrium ions surrounding positively charged hydrogen vacancies that are formed by light-induced removal of hydrogen atoms from octahedral sites. When a sufficiently large concentration, on the order of ∼10%, is reached in a certain domain of the film, band formation of the eg electrons may occur, leading to an Anderson-Mott insulator-metal transition like the case of yttrium trihydride in these domains.

In this paper, we investigate by ab initio DFT how the O:H ratio influences the formation and lattice energy, metastability, and optical properties of Y and La anion-disordered ROxH3-2x oxyhydrides. To achieve this, a set of special quasirandom structures (SQS) is introduced to model anion-disorder along the whole RH3-R2O3 composition line. A comparison with an extensive set of anion-ordered polymorphs of the same composition shows the comparable energy of the anion-disordered phase, which, in particular, in the H-rich composition interval showed the lowest relative energy. In turn, the metastability of the anion-disordered phase depends on the cation size (Y versus La), which determines the maximum H content above which the CaF2-type structure itself becomes unstable. To overcome the accuracy limitations of classical DFT, the modified Becke-Johnson (mBJ) scheme is employed in the study of the electronic properties. We show that major differences occur between H-rich and O-rich R oxyhydrides, as the octahedral H- present for x<1 form electronic states at the top of the valence band, which reduce the energy band gap and dominate the electronic transitions at lower energies, thus increasing the refractive index of the material in the VIS-nIR spectral range. Comparing the DFT results to experimental data on photochromic Y oxyhydride films reinforces the hypothesis of anion-disorder in the H-rich films (x<1), while it hints towards some degree of anion ordering in the O-rich ones (x>1). Our paper exemplifies a strategy to calculate ab initio the electronic/optical properties of a wide range of materials with occupational disorder.

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).

Finding alternative ways to tailor the electronic properties of a catalyst to actively and selectively drive reactions of interest has been a growing research topic in the field of electrochemistry. In this Letter, we investigate the tuning of the surface electronic properties of electrocatalysts via polymer modification. We show that when a nickel oxide water oxidation catalyst is coated with polytetrafluoroethylene, stable Ni-CFx bonds are introduced at the nickel oxide/polymer interface, resulting in shifting of the reaction selectivity away from the oxygen evolution reaction and toward hydrogen peroxide formation. It is shown that the electron-withdrawing character of the surface fluorocarbon molecule leaves a slight positive charge on the water oxidation intermediates at the adjacent active nickel sites, making their bonds weaker. The concept of modifying the surface electronic properties of an electrocatalyst via stable polymer modification offers an additional route to tune multipathway reactions in polymer/electrocatalyst environments, like with ionomer-modified catalysts or with membrane electrode assemblies.

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.

Metal hydrides may play a paramount role in a future hydrogen economy. While most applications are based on nanostructured and confined materials, studies considering the structural response of these materials to hydrogen concentrate on bulk material. Here, using in situ in- and out-of-plane X-ray diffraction and reflectometry, we study the fcc ↔fct transition in Hf thin films, an optical hydrogen-sensing material. We show that the confinement of Hf affects this transition: compared to bulk Hf, the transition is pushed to a higher hydrogen-to-metal ratio, the tetragonality of the fct phase is reduced, and phase coexistence is suppressed. These nanoconfinement effects ensure the hysteresis- free response of hafnium to hydrogen, enabling its remarkable performance as a hydrogen-sensing material. In a wider perspective, the results highlight the profound influences of the nanostructuring and nanoconfinement of metal hydrides on their structural response to hydrogen with a significant impact on their applicability in future devices.