Nanoconfined water exhibits unique properties compared to bulk water due to limited quantities, frustrated hydrogen bonding, and surface interactions, which are fundamental for energy storage and transport applications. We integrate machine learning–accelerated ab initio molecular dynamics with x-ray diffraction (XRD) and inelastic neutron scattering (INS) to systematically analyze the thermodynamic and dynamic behavior of water confined between functionalized (-F, -O, and -OH) two-dimensional (2D) Ti₃C₂T_x MXene layers. As water intercalates between layers, the interlayer spacing exhibits layer-dependent staging characteristics. The water polarization can be flipped by the count and morphology of intercalated molecules interacting with MXene surface groups, resulting in varying electrostatic potential profiles. On the basis of interfacial electrostatic potential, hydrogen bond lifetime, and molecular orientation, we establish a linear combination of exponential model describing water diffusivity. These computational insights align well with experimental x-ray and neutron measurements, suggesting strategies for tuning water morphology and transport by tailoring MXene surface chemistry and water content for electrochemical energy storage and nanofluidic applications.

All-solid-state batteries have great potential to outperform conventional lithium-ion batteries in both safety and energy density, as the solid electrolyte can potentially accommodate high-energy-density anodes such as metallic lithium or silicon more safely. However, the high-valence cations present in most highly conductive solid electrolytes facilitate reductive decomposition at low potentials, leading to significant irreversible lithium inventory loss. Preventing this requires the development of solid electrolytes that are thermodynamically stable at low operating potentials while providing high ionic conductivity and sufficient oxidative stability. To realize this, we explored a new family of Li-rich antifluorite irreducible solid electrolytes, Li2.65S0.35NxP0.65–x, the first reported nitrido-phosphido-sulfide, and investigated their application in all-solid-state batteries. The optimized composition Li2.65S0.35N0.15P0.5 possesses a remarkably high ionic conductivity of 1.05 mS cm–1, as well as a relatively high oxidative stability of 1.15 V vs Li+/Li for this class of materials. Ab initio molecular dynamics and density functional theory simulations reveal that enhanced Li diffusion is the result of enlarged diffusion bottleneck sizes. These are a consequence of (i) substitution with smaller anions or (ii) increased electrostatic repulsion from the substitution with high-valence anions. Importantly, the oxidative stability makes Li2.65S0.35N0.15P0.5 exhibit good compatibility with Si anodes, and in conjunction with the high ionic conductivity, this enables a high initial Coulombic efficiency of 94.2% as well as a stable cycle life of a full cell with a micron silicon–Li2.65S0.35N0.15P0.5 anode and a LiCoO2–Li3InCl6 cathode. This work highlights the potential of irreducible solid electrolytes for the design of all-solid-state batteries with low-potential and high-energy-density anodes.

Sulfide corrosion of Cu is rapid, and hydrogen atoms produced by its cathodic half-reaction could adsorb on the Cu surface and diffuse into the Cu, potentially leading to hydrogen embrittlement. However, in solutions with low concentrations of SH⁻, absorption of hydrogen into Cu is not observed by ex situ hydrogen analysis, although it is unclear whether this is due to the lack of absorption, the outgassing of hydrogen from the Cu before it can be measured, or another mitigation mechanism. Herein, hydrogen uptake into Cu and the development of Cu2S layers during corrosion by SH⁻ were studied by in situ neutron reflectometry and electrochemical impedance spectroscopy. The method relies on a 4-nm Ti layer beneath 50 nm of Cu to trap hydrogen that may penetrate the Cu. Additionally, elastic recoil detection analysis and Rutherford backscattering spectrometry were used to measure hydrogen. While no increase in hydrogen was detected in either the Ti or Cu layers, a higher concentration of hydrogen was observed in the outer Cu2S layer (2560 ppm) than in the underlying Cu (244 ppm), demonstrating that bisulfide-driven corrosion does not lead to hydrogen absorption into the Cu. These results have implications for deep geological repositories utilizing Cu corrosion barriers.

Thin film metal hydrides have traditionally been studied (as a model system) for hydrogen storage applications. Meanwhile, other applications including optical switchable mirrors, optical hydrogen sensors, and the use of thin film metal hydrides to study diffusion have emerged. From a more fundamental point of view, surface effects and the fact that thin film metal hydrides have to obey constraints on their volumetric expansion which may lead to high internal stresses. This ensures that thin film metal hydrides may have a significantly different response to hydrogen as their bulk counter parts. This can lead to fascinating phenomena such as suppression of phase transitions, altering the nature of phase transitions and shifting of plateau pressures. This chapter discusses the thermodynamics of thin film metal hydrides, common experimental techniques, and several thin film metal hydrides grouped by application. First, we discuss thin film metal hydrides for hydrogen storage applications focussing on magnesium and magnesium alloys. Next, we consider switchable mirrors based on rare-earth metal hydrides. Third, we discuss metal hydrides including palladium-based and tantalum-based materials for hydrogen sensing applications. In addition, we discuss experimental techniques including hydrogenography and neutron reflectometry that are specifically suited to studying metal hydride thin films as well as how thin film metal hydrides can be applied to study hydrogen diffusion.

Palladium hydride is a model system for studying metal-hydrogen interactions. Yet, its bulk electronic structure has proven difficult to directly probe, with most studies to date limited to surface-sensitive photoelectron spectroscopy approaches. This work reports the first in situ ambient-pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) study of hydrogen incorporation in Pd thin films, providing direct access to bulk chemical and electronic information at elevated hydrogen pressures. Structural characterization by in situ X-ray diffraction and neutron reflectometry under comparable conditions establishes a direct correlation between hydrogen loading, lattice expansion, and electronic modifications. Comparison with density functional theory (DFT) reveals how hydrogen stoichiometry and site occupancy govern the density of occupied states near the Fermi level. These results resolve long-standing questions regarding PdH and establish AP-HAXPES as a powerful tool for probing the bulk electronic structure of metal hydrides under realistic conditions.

Anode-free aqueous zinc metal batteries (AZMBs) offer significant potential for energy storage due to their low cost and environmental benefits. Ti₃C₂T_x MXene provides several advantages over traditional metallic current collectors like Cu and Ti, including better Zn plating affinity, lightweight, and flexibility. However, self-freestanding MXene current collectors in AZMBs remain underexplored, likely due to challenges with Zn deposition reversibility. This study investigates the combination of a Ti₃C₂T_x self-freestanding film with advanced electrolyte engineering, specifically examining the effects of Li-salt and propylene carbonate (PC) as additives on Zn plating reversibility. While using Li⁺ ions as an additive alone facilitates uniform Zn deposition on bulk metals through the electrostatic shielding effect, the addition of Li-salt negatively impacts Zn plating uniformity on Ti₃C₂T_x. Meanwhile, using PC additive alone forms an organic SEI layer on Ti₃C₂T_x and causes Zn agglomeration. The use of both additives together results in a ZnF₂-containing hybrid SEI layer with improved interfacial kinetics, promoting more uniform Zn deposition. This approach achieves an average Coulombic efficiency (CE) of 96.8% over 150 cycles (a maximum CE of 97.8%). The study highlights the strategic difference in electrolyte design, emphasizing the need for tailored approaches to optimize Zn deposition on MXenes, contrasting with traditional metallic current collectors.

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.

One of the major challenges in advancing polymer-inorganic hybrid solid electrolytes (HSEs) lies in comprehending and controlling their internal structure. In addition, the intricate interplay between multiple phases further complicates efforts to establish the structure-property relationships. In this study, by introducing a multifunctional LiI additive to an HSE compromising of polyethylene oxide (PEO) polymeric electrolyte and the fast lithium-ion conductor Li₆PS₅Cl, the relationship between the bulk and interface structure and ascertaining their impact on lithium-ion dynamics within the HSE is disentangled. Using multidimensional solid-state nuclear magnetic resonance, we find that the addition of LiI stabilizes the internal interfaces and enhances lithium-ion mobility. A kinetically stable solid-electrolyte interphase is formed at the lithium-metal anode, increasing the critical current density to 1.3 mA cm⁻², and enabling long-term stable cycling of lithium symmetric cells (>1200 h). This work sheds light on tailoring the structure of HSEs to improve their conductivity and stability for enabling all-solid-state lithium-metal batteries.

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.

Hydrogen, a key component of a net-carbon free society, requires precise sensing solutions. This research focuses on the development of metal hydride-coated Fibre Bragg Grating (FBG) based hydrogen sensors, marking a significant step towards the realisation of multipoint hydrogen sensing systems - a growing demand in the industry. The performance of three FBG sensors coated with nanometre-thick tantalum, palladium, and palladium-gold hydrogen sensing metal thin films, deposited via magnetron sputtering, is presented. Among these, the novel tantalum sensor exhibited the best performance, achieving a minimum detection limit of 50 ppm and and an enhanced sensitivity below 0.1% H₂ levels at room temperature.

Here, we show that we can synthesize free-standing palladium nanoparticles with a size of about 5 nm embedded in a fluorinated polymer matrix using magnetron codeposition and a subsequent annealing step. Indeed, we deposit with magnetron sputtering at the same time PTFE and Pd, and a subsequent thermal annealing step under a hydrogen atmosphere ensures agglomeration of the Pd atoms into small nanoparticles. This scalable vapor-based method allows deposition on all kinds of surfaces, including substrates and optical fibers. Using a combination of transmission electron microscopy, grazing-incidence diffraction, neutron and X-ray reflectometry, and X-ray photoelectron spectroscopy, we characterize the nanocomposite films and the palladium particles inside. These palladium nanoparticles could have a variety of applications in catalysis, hydrogen compressors, and optical hydrogen sensors. For the later application, we show using optical transmission measurements that the nanoparticles can reversibly absorb hydrogen, having well-defined steps in optical transmission when the hydrogen pressure is changed. Owing to their small size, the polymer matrix, and high surface-to-volume ratio, the nanoparticles show subsecond response times to changes in hydrogen concentration.

The development of reliable hydrogen sensing materials for subzero environments is crucial for aviation, cryogenic storage, and hydrogen infrastructure applications. In this study, we investigate tetragonal β-tantalum (β-Ta) thin films at −60 °C to assess their potential for optical hydrogen sensing. In situ X-ray diffraction (XRD) measurements reveal a reversible lattice expansion upon hydrogen exposure, with β-Ta exhibiting a smaller volumetric expansion compared to α-Ta, indicating lower hydrogen solubility. Optical transmission measurements demonstrate a monotonic and fully reversible optical response across a range of hydrogen pressures, free of any hysteresis. However, β-Ta exhibits prolonged response times at low temperatures due to diffusion-limited kinetics, as confirmed by power-law response rate analysis and direct diffusion front measurements. Although β-Ta offers a temperature-independent resolution and structural robustness, its slower response time suggests the need for further microstructural optimizations to enhance hydrogen diffusion.

Hybrid solid electrolytes (HSEs) leverage the benefits of their organic and inorganic components, yet optimizing ion transport and component compatibility requires a deeper understanding of their intricate ion transport mechanisms. Here, macroscopic charge transport is correlated with local lithium (Li)-ion diffusivity in HSEs, using poly(ethylene oxide) (PEO) as matrix and Li6PS5Cl as filler. Solvent- and dry-processing methods were evaluated for their morphological impact on Li-ion transport. Through multiscale solid-state nuclear magnetic resonance analysis, we reveal that the filler enhances local Li-ion diffusivity within the slow polymer segmental dynamics. Phase transitions indicate inhibited crystallization in HSEs, with reduced Li-ion diffusion barriers attributed to enhanced segmental motion and conductive polymer conformations. Relaxometry measurements identify a mobile component unique to the hybrid system at low temperatures, indicating Li-ion transport along polymer-filler interfaces. Comparative analysis shows solvent-processed HSEs exhibit better morphological uniformity and enhanced compatibility with Li-metal anodes via an inorganic-rich solid electrolyte interphase.

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.

Many electronic and electrochemical devices rely on the exchange of light elements such as hydrogen and oxygen with the environment. Understanding and tailoring the device functionality requires accurate information about the concentration and chemical bonding of such species inside a solid, which is particularly difficult if several species are exchanged. In LaNiO3 thin films in situ transport experiments reveal a re-entrant metal–insulator transition upon hydrogen exposure. The origin of this unusual behavior can be understood by combining information about the stoichiometry and chemical bonding of hydrogen and oxygen as determined by neutron reflectometry and x-ray absorption spectroscopy, respectively. In addition to the metallic parent phase, an insulating phase with composition LaNiO2.65 and a re-entrant metallic phase with composition LaNiO2.15(OH)0.5 are identified. They can be inter-converted by redox reactions in different external environments. The methodology employed offers new insights into the mechanisms underlying the influence of hydrogen in functional devices.

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.

The Goos-Hänchen (GH) shift describes a phenomenon in which a specularly reflected beam is translated along the reflecting surface such that the incident and reflected rays no longer intersect at the surface. Using a neutron spin-echo technique and a specially designed magnetic multilayer mirror, we have measured the relative phase between the reflected up and down neutron spin states in total reflection. The relative GH shift calculated from this phase shows a strong resonant enhancement at a particular incident neutron wave vector, which is due to a waveguiding effect in one of the magnetic layers. Calculations based on the observed phase difference between the neutron states indicate a propagation distance along the waveguide layer of 0.65 mm for the spin-down state, which we identify with the magnitude of the giant GH shift. The existence of a physical GH shift is confirmed by the observation of neutron absorption in the waveguide layer. We propose ways in which our experimental method may be exploited for neutron quantum-enhanced sensing of thin magnetic layers.

Hydrogen is a cornerstone of the emerging net-zero carbon economy, and its widespread deployment demands sensitive, stable, and scalable detection technologies. In this study, we present a comparative performance analysis of Fibre Bragg Grating (FBG) sensors coated with nanometre-thick metal hydride-forming layers—tantalum (Ta), tantalum-palladium alloy (Ta_0.88 Pd_0.12), palladium (Pd), and palladium-gold alloy (Pd _0.6 Au_0.4)—for optical hydrogen sensing. The integration of Ta and Ta _0.88 Pd_0.12, two tantalum-based metal hydrides, with FBG sensors is introduced here for the first time, offering a promising alternative to conventional Pd-based materials. All coatings were deposited via magnetron sputtering and tested under controlled hydrogen exposure across concentrations ranging from 0.001% to 100% H₂. The Ta-based FBGs exhibited outstanding performance, showing a remarkably linear relative wavelength shift over the full tested range (0.001% to 100% H₂), with sensitivity detectable down to 10 ppm—the lowest concentration achievable in the current setup. Both Ta and Ta_0.88 Pd_0.12 sensors exhibited fully reversible and hysteresis-free response characteristics, with rapid response and recovery. Among them, the Ta_0.88 Pd_0.12 sensor with a 100 nm coating demonstrated the highest logarithmic sensitivity of ∼9 pm/decade(%H₂), corresponding to a 9 pm wavelength shift for every tenfold increase in hydrogen concentration between 0.001% and 100% H₂. In contrast, Pd and Pd _0.6 Au_0.4 sensors showed degraded performance at low concentrations and greater signal hysteresis. These results underscore the potential of Ta and Ta _0.88 Pd_0.12 coatings as robust and high-performance alternatives to conventional Pd-based materials for next-generation distributed fibre-optic hydrogen sensing systems.

All-solid-state batteries receive ample attention due to their promising safety characteristics and energy density. The latter holds true if they are compatible with next-generation high-capacity anodes, but most highly ion-conductive solid electrolytes decompose at low operating potentials, leading to lithium loss and increased cell resistances. Here the dynamic stability of solid electrolytes that can improve all-solid-state battery performance is demonstrated. Halide electrolytes Li₃YCl₃Br₃ and Li₂ZrCl₆, considered unstable at low potentials, are found to exhibit structurally reversible redox activity beyond their electrochemical stability windows, increasing compatibility with anodes and contributing to capacity without compromising ionic conductivity. The benefit of this dynamic stability window is demonstrated with cost-effective red phosphorus anodes, resulting in high reversible capacities (2,308 mAh g⁻¹), high rate capacity retention (1,024 mAh g⁻¹ at 7.75 mA cm⁻²) and extended cycle life (61% retention after 1,780 cycles). Furthermore, high areal capacity (7.65 mAh cm⁻²) and stability (70% retention after 1,000 cycles) are achieved for halide-based full cells with red phosphorous anodes. The beneficial redox activity of halide electrolytes greatly expands their application scenarios and suggests valuable battery design principles to enhance performance.

The growing demand for safe, cost-efficient, high-energy and high-power electrochemical energy storage devices has stimulated the development of aqueous-based supercapacitors with high capacitance, high rate capability, and high voltage. 2D titanium carbide MXene-based electrodes have shown excellent rate capability in various dilute aqueous electrolytes, yet their potential window is usually narrower than 1.2 V. In this study, we show that the potential window of Ti₃C₂T _x MXene can be efficiently widened to 1.5 V in a cost-effective and environmentally benign polyethylene glycol (PEG) containing molecular crowding electrolyte. Additionally, a pair of redox peaks at −0.25 V/−0.05 V vs. Ag (cathodic/anodic) emerged in cyclic voltammetry after the addition of PEG, yielding an additional 25% capacitance. Interestingly, we observed the co-insertion of the molecular crowding agent PEG-400 during the Li⁺ intercalation process based on in-situ x-ray diffraction analysis. As a result, Ti₃C₂T _x electrodes presented an interlayer space change of 4.7 Å during a complete charge/discharge cycle, which is the largest reversible interlayer space change reported so far for MXene-based electrodes. This work demonstrates the potential of adding molecular crowding agents to improve the performance of MXene electrodes in aqueous electrolytes and to enlarge the change of the interlayer spacing.