Hydride Research Articles

Hydrogen Absorption in Palladium‐Based Nanocrystals for Electrocatalysis Investigation

AbstractHydrogen absorption in transition metals has received enormous attention for applications such as the hydrogen storage, sensing and corrosion. In particular, the Pd‐H system is a simple and classical metal hydride system that serves as a platform for a deeper understanding of hydrogen behavior. In the past few years, Pd‐H has been reported as electrocatalyst in multiple reactions, whereas the impact of hydrogen absorption/desorption in electrocatalytic applications remains unclear. Therefore, in this concept, factors influencing the hydrogen adsorption/desorption of palladium in the gas phase, such as facet engineering, ligand effects, and alloying effects, are summarized. Subsequently, in the context of electrocatalysis, factors affecting electrochemical hydrogen adsorption/desorption are outlined. Particularly, prospects for the further investigation of electrochemical hydrogen adsorption/desorption in catalysis are discussed.

Time-Dependent Multi-Particle Model Describing the Hydrogen Absorption of Nanocrystalline Magnesium Powders: A Case Study

Classical kinetic models describing the hydrogen absorption of nanocrystalline metallic hydrides generally do not involve any parameter related to the change in the crystallite size during the hydrogenation at constant temperature. In the present investigation, ball-milled nanocrystalline Mg powders exhibiting lognormal crystallite size distribution have been subjected to hydrogen absorption in a Sievert-type apparatus. Partially absorbed states were achieved by interrupting the hydrogenation cycle at different hydrogen content, i.e., when 15%, 50%, and 90% of Mg powder transformed to MgH2. The evolution of the characteristic size of the nucleating MgH2 phase was determined from X-ray diffraction analysis. Considering the crystallite size distribution of the as-milled powder agglomerate as well as the growth during the isothermal hydrogenation process, a time-dependent multi-particle reaction function ∝CV¯t;R(t) was developed. It was shown unambiguously for this case study that the measured hydrogen absorption curve of the ball-milled Mg powder shows the best correlation with this model when it is compared to classical kinetic functions or the previously developed multi-particle reaction function excluding the change in the average crystallite size during hydrogenation.

Advances in CALPHAD Methodology for Modeling Hydrides: A Comprehensive Review

AbstractHydrides enable handling hydrogen at low pressure and near room temperature, offering higher volumetric densities than compressed or liquid hydrogen and enhancing safety. The CALPHAD method, rooted in the principles of thermodynamics, offers a systematic approach for predicting phase equilibria and thermodynamic properties in multicomponent materials. This comprehensive review paper aims to provide a detailed overview of the application of the CALPHAD method in the realm of metallic and complex hydrides. After an introduction to the fundamental thermodynamic aspects of hydrides, key elements of applying the CALPHAD method to model metal-hydrogen systems and complex hydrides are discussed. Subsequently, recent publications are reviewed, highlighting key findings and recent progresses in the field. Finally, the challenges that must be overcome to achieve further progress in this area are explored.

Event driven high speed data acquisition with IEEE 1588 synchronization for long pulse operations of Indian Test Facility for ITER DNB

The Indian Test Facility (INTF) is a negative Hydrogen ion based 100 kV, 60A, 5 Hz modulated NBI system having duty cycle of 3 s ON time to 20 s OFF time. Prime objective of the facility is to characterize ITER Diagnostic Neutral Beam (DNB) with full specifications, prior to shipment and installation in ITER. In order to fully characterize ITER DNB, a matured Data Acquisition and Control system (DACS) is needed which will be responsible for supervisory control of the facility and for acquiring and monitoring signals at run time. Apart from conventional DACS, high speed data acquisition is needed for diagnostics and other signals with sampling rate requirements of 100 MS/s. For acquiring data from such systems it is imperative that the data be acquired only for certain duration on the occurrence of the event. Such acquisition systems are classified as event driven data acquisition system and are widely used in fusion engineering community.Development and implementation of high speed event driven data acquisition systems pose challenges in terms of data buffering, data streaming and synchronization. The precise timing of the event occurrence with respect to the main pulse should also be known for which a timing and synchronization system is needed. To fulfill this purpose, IEEE 1588 protocol offers a network based method of synchronizing multiple nodes based on Ethernet with sub microsecond accuracy. The present work describes the development of an event driven high speed data acquisition in synchronization with IEEE 1588 based timing network. The work proposes the time stamping of the event from synchronized network integrated with the event acquisition platform. The developed system has been tested for pulse durations longer than 3600 s. The performance of event data acquisition system has been thoroughly characterized and improved by applying various software optimizations in LabVIEW. The present work although intended to be used for INTF can be used for any such facility in fusion or other areas.The paper provides results of IEEE 1588 based timing network in various configurations and event data acquisition system developed on high speed FPGA controlled digitizer board with prototype results.

Adsorption dynamics of per- and polyfluoroalkyl substances (PFAS) on activated carbon: Interplay of surface chemistry and PFAS structural properties

Using activated carbon (AC) to adsorb per- and polyfluoroalkyl substances (PFAS) is highly practical for environmental remediation. However, examining the surface chemistry of carbonaceous material with regard to the removal of various PFAS has been limited, as the focus has often solely been on perfluorooctanoic acid and perfluorooctane sulfonic acid. Consequently, this study systematically explores the role of AC surface chemistry in the adsorption of PFAS with various structural properties using ACs that have been modified for different degrees of acidity, basicity, and hydrophobicity. Batch adsorption studies have demonstrated that the affinity of PFAS for the AC surface is negatively correlated with surface acidity. The removal performance at an adsorbent dose of 2 mg/L and an initial PFAS concentration of 600 ng/L is 27.4–91.4 %, 57.2–95.6 %, 92.4–99.2 %, and 89.1–99.6 % for an AC with an acidic surface, unmodified AC, defunctionalized AC, and AC with basic surface, respectively. Analyses of the adsorption isotherms, kinetics, and removal performance of the four types of ACs and 14 PFAS in water at different pH and ionic strengths reveal that hydrophobic, electron donor–acceptor, and electrostatic interactions, as well as negative charge-assisted hydrogen bond formation are necessary to describe the key adsorption characteristics observed in the experiments. The relative significance of the adsorption mechanisms depends on the PFAS structural properties, AC surface chemistry, and water composition. This extensive discourse contributes substantially to the design of an efficient carbonaceous adsorbent with specific surface moieties for removing PFAS from water.

Analytical and experimental evaluation of LaNi5-xMx, where M = Fe, Mn, and Al hydrides for hydrogen compression applications

Metal Hydride Hydrogen Compressors (MHHC) will play a vital role in the hydrogen economy in the coming times. Developing a more efficient and optimized MHHC requires revisiting the existing materials and selecting the best-suited material. The current work compares LaNi5-xMx, where M = Fe, Mn, and Al hydrides, through a thermodynamic evaluation of the compressor's performance. The operating conditions have been chosen as 20 to 40 °C for absorption and 80 to 140 °C for desorption to provide a baseline for comparing the materials. The performance parameters include thermal efficiency, pressure ratio, and throughput (i.e. mass of hydrogen delivered per 100 g of hydride (wt.%)). The Pressure Concentration Isotherms (PCIs) are taken from the published literature, and the equilibrium pressures at the desired temperature conditions are calculated from the van't Hoff equation. LaNi4.8Al0.2 gives good performance throughout the studied temperature and pressure range. The throughput of LaNi4.8Al0.2 drops from 1.4 wt.% to around 1 wt.% at high absorption temperatures. A separate experimental study of LaNi4.8Al0.2 is documented where it is used in a prototype of MHHC, and its performance parameters are compared with the thermodynamic analysis. The reaction kinetics, throughput, pressure ratio, and efficiency are reported. A pressure ratio of 4.2 is achieved with an efficiency of 5.2 % at 3 bar supply pressure, and absorption and desorption temperatures of 30 and 140 °C, respectively.

Impact of fins and cooling fluid on the hydrogenation process in a LaNi5-Based metal hydride reactor for hydrogen storage

This study investigates the influence of different reactors incorporating copper circular fins within the hydride, along with variations in the reactor coolant—specifically water, oil, and nitrogen. Employing user-defined functions in the commercial software Ansys Fluent 15, a 2D numerical model was developed to assess thermal characteristics (temperature, heat flux, etc.) and kinetics of MH-hydrogenation, with a focus on the cooling fluid type. The reactor experiences diverse pressures (5–15 bar), an initial temperature of 298 K, and a coolant speed of 1 m/s. Validation of numerical results was performed against experimental data. The study delves into absorption properties of the storage reactor, examining crucial parameters such as hydrogen concentration, heat transfer coefficient of forced convection, and average temperature within the reactor. The findings reveal that coupling internal fins with water or oil as a coolant yields optimal performance, especially on an industrial scale. Nitrogen, on the other hand, exhibited the least favorable thermal performance, while the addition of fins significantly improved the cooling performance of the storage unit. Results were quantitatively analyzed across various scenarios to enhance understanding of the synergistic role of fins and cooling fluids in the storage performance of H2 within LaNi5 alloy.

Exploring Nanomaterials for Hydrogen Storage: Advances, Challenges, and Perspectives.

Hydrogen energy heralded for its environmentally friendly, renewable, efficient, and cost-effective attributes, stands poised as the primary alternative to fossil fuels in the future. Despite its great potential, the low volumetric density presents a formidable challenge in hydrogen storage. Addressing this challenge necessitates exploring effective storage techniques for a sustainable hydrogen economy. Solid-state hydrogen storage in nanomaterials (physically or chemically) holds promise for achieving large-scale hydrogen storage applications. Such approaches offer benefits, including safety, compactness, lightness, reversibility, and efficient generation of pure hydrogen fuel under mild conditions. This article presents solid-state nanomaterials, specifically nanoporous carbons (activated carbon, carbon fibers), metal-organic frameworks, covalently connected frameworks, nanoporous organic polymers, and nanoscale metal hydrides. Furthermore, new developments in hydrogen fuel cell technology for stationary and mobile applications have been demonstrated. The review outlines significant advancements thus far, identifies key barriers to practical implementation, and presents a perspective for future sustainable energy research. It concludes with recommendations to enhance hydrogen storage performance for cost-effective and long-lasting utilization.

CFD simulation of heat and mass transfer processes in a metal hydride hydrogen storage system, taking into account changes in the bed structure

CFD modelling of heat-and-mass transfer in metal hydride (MH) beds is an important step in the development and optimisation of MH reactors for hydrogen storage, compression and separation/purification. In the existing models, the mass conservation equation includes the density of solid in the non-hydrogenated (ρs) and hydrogen-saturated (ρss) states. To provide constant porosity of the MH bed during H2 absorption/desorption, assumption ρs<ρss is typically taken. However, this assumption contradicts to well-known fact that hydrogenation of hydride-forming alloys is accompanied by the expansion of crystal lattice of the metallic matrix and, therefore, by the decrease of the solid density by 15–25%.This work presents results of the modelling which takes this effect into account. The model assumes that the density of the MH changes linearly as the starting hydride-forming alloy is saturated with hydrogen, while the volume of the MH bed remains constant. The last assumption is a limiting estimation because it provides the maximum possible reduction in the porosity during H2 absorption process.The model was validated using published experimental data on a cylindrical MH reactor filled with 422 g of LaNi5. It was shown that accounting the realistic changes in the MH density can significantly affect the simulation results.

Experimental and numerical investigations on synergistic coupling of metal hydride hydrogen storage systems with low-temperature proton exchange membrane fuel-cell

Hydrogen has emerged as a promising alternative for various energy needs, and metal hydride hydrogen systems offer significant potential due to their near-ambient working conditions and safe storage. This study presents experimental and numerical investigations on a lab-scale metal hydride fuel cell system to showcase its viability in addressing future energy demands. The experimental setup includes a 41-tube Metal Hydride (MH) reactor with an outer cooling jacket, containing 3.75 kg of La0.7Ce0.1Ca0.3Ni5. The output of the reactor is coupled to a 1 kW fuel cell, with controlled hydrogen discharge at temperatures ranging from 30 to 50 °C through numerical modelling. The study found that maintaining the reactor at 40 °C was sufficient to sustain the required bed pressure for a 1 kW fuel cell. Numerical simulations revealed that a 1 kW fuel cell operating at a current density of 0.8 A/cm2 and 50 °C rejected approximately 1.3 kW of waste heat. Moreover, 3.75 kg MH reactor could discharge hydrogen at a constant rate of 13 l/min for 36.3 min with just 125–225 W of heat input, highlighting the thermal coupling potential between these systems. The energy efficiency of the coupled MH-PEMFC system increased by 1.8 times to reach 49 % at 50 °C. Additionally, the waste heat generated proved sufficient to provide the required endothermic heat to multiple MH reactors, with a single reactor utilizing only 16.3 % of the total waste heat. This integrated system demonstrates promising efficiency and sustainability for future energy applications.

Performance improvement of metal hydride hydrogen compressors using electromagnetic induction heating

While there are some hydrogen refueling stations (HRS) functioning in different parts of the world, their use is not widespread enough, primarily due to their expensive cost. Hydrogen compression is a main contributor to the capital and operation costs of the HRS. By improving H2 compression technology, it is possible to optimize the infrastructure for refueling with hydrogen in terms of cost and performance. The use of metal hydride hydrogen compressors (MHHCs), which have the potential to be inexpensive and have the ability to use waste heat and renewable energy sources for the H2 compression, is a promising solution to overcome this issue. The duration of the H2 compression cycle is nevertheless a serious challenge due to the metal hydride (MH) bed's low heat conductivity. As a heating technique to improve the performance of MHHCs, electromagnetic induction (EMI) is examined numerically for the first time in this work. The dynamic behavior of a two-stage MHHC with each MH reactor having an external heat exchanger and being ringed by a copper coil traversed by an alternating current is described by a two-dimensional mathematical model, which was established and successfully verified by the reference data. Numerical simulations were performed with the help of this model, and the findings showed that the EMI heating method is faster than the heat transfer fluid (HTF) heating technique. For instance, at a delivery temperature of 373 K and a supply pressure of 20 bar, it is possible to produce 106 NL H2 per kilogram of HPMH at a pressure of 97 bar with a 74 % shorter compression time than with the HTF heating technique.

Negative net global warming potential hydrogen production through biomass gasification combined with chemical looping: Environmental and economic assessments

The integration of biomass gasification (BG) with chemical looping hydrogen production (CLHP) offers one prospective approach for green hydrogen production. The environmental and economic assessments are conducted to evaluated the sustainable performance of the BG-CLHP system during the hydrogen production cycle (HPC). The results showed that the environmental impacts of BG-CLHP are concentrated in freshwater aquatic ecotoxicity (FAETP), human toxicity (HTP), and acidification (AP). The BG-CLHP system can provide a more cost-competitive hydrogen supply solution for hydrogen refueling stations compared the offshore wind or photovoltaic (PV) powered electrolysis, given the same hydrogen production target. For global warming potential (GWP) and cost, the BG-CLHP demonstrates excellent integrated performance with GWP values of −15.13 and −17.00 kg CO2, eq/kg H2, for the Air case and Oxygen case, and the costs of 3.05 and 2.82 $/kg H2, respectively, and sensitivity analysis reveals that the CLHP stage is the primary contributor to GWP. Compared with existing hydrogen production routes, BG-CLHP can still be improved in terms of costs and the technical maturity, however, the excellent negative GWP properties of BG-CLHP make it will play a more important role in future large-scale hydrogen production.

Thermal management techniques for enhancing hydrogen adsorption and desorption in metal hydride reactors: a short recent review

This manuscript presents a short recent review of various techniques used for thermal management enhancement in metal hydride reactors, emphasizing their effect on hydrogen adsorption and desorption. Mainly, the article highlights 4 techniques: The incorporation of various fin types, the integration of heat exchangers within the metal hydride reactor, the addition of heat transfer enhancing material into the MH bed, and the addition of phase change material (PCM). The analysis presented provides brief insightful information about the state of Metal hydride reactors now, laying the groundwork for future studies and advancements in this area.

Atomistic-scale insights into hydrogen diffusion barrier in nickel hydride: Complex interplay between short- and long-range hydrogen arrangement and hydrogen concentration

Metal hydrides, such as those containing Pd, Ni and Mg, are being considered as potential candidates for hydrogen storage applications. The sluggish hydrogen uptake/release in such materials at ambient conditions presents a major issue. The timescales are dictated mainly by diffusion, as it is usually the rate-controlling step. In order to identify practical strategies for improving the kinetics, one must therefore gain an atomistic-scale understanding of the hydrogen diffusion, which is generally lacking from experiments. We address this gap by analyzing the hydrogen hopping mechanism and the associated barriers in two million different hydrogen configurations in NiHx (x = 0–1) using computational techniques. A distribution of hopping barriers is observed. The barriers lie between 0.3–0.7 eV. Three factors, namely, the short- and long-range hydrogen arrangement and the hydrogen concentration influence the barrier. We also show that depending on these factors, two different H hopping mechanisms may prevail. At higher H concentration, the lattice expands by as much as 19 % by volume, making it easier for the hydrogen atom to hop. Therefore, hydrogen diffusion is sluggish in the initial α phase, which may explain the experimentally-observed induction behavior. This study highlights the complexity associated with the metal hydride systems and provides the basis for further molecular scale kinetic studies of metal hydride systems.

Hydrogen Storage Capacity of Cobalt Cluster Ions.

The adsorption of H2 on gas-phase Con± (n = 5-12) clusters at 300 K and the desorption of H2 from ConHm± upon heating were studied experimentally by combining thermal desorption spectrometry and mass spectrometry to elucidate the hydrogen storage property of the Co clusters. Hydrogen atoms adsorbed well on Con+ (n = 5, 10-12) and Con- (n = 5-12) at 300 K to form ConHm±. The atomic ratios, m/n, for ConHm- (0.9-1.4) were higher than those for ConHm+ (0.2-1.1). According to density functional theory (DFT) calculations, the first few H2 molecules had a tendency to dissociatively adsorb onto the Co clusters. Further, the bonding nature of the H atoms was ionic, similar to the H atoms in the metallic hydrides. In contrast, H2, adsorbed molecularly, was explained in terms of σ complex formation. H2 molecules were desorbed from the clusters upon heating. The temperature dependences showed that ConHm- released H2 at a higher temperature (700-800 K) than ConHm+ (600-700 K), suggesting that Con- should have a higher affinity to hydrogen than Con+. The desorption temperatures were lower than those of VnHm+, which was consistent with the fact that the adsorption energies of H2 were lower for the Co clusters than those for the V clusters. The low adsorption energies were ascribed to their large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps in the Co clusters.

Gadolinium Doping Modulates the Enzyme-like Activity and Radical-Scavenging Properties of CeO2 Nanoparticles.

Their unique physicochemical properties and multi-enzymatic activity make CeO2 nanoparticles (CeO2 NPs) the most promising active component of the next generation of theranostic drugs. When doped with gadolinium ions, CeO2 NPs constitute a new type of contrast agent for magnetic resonance imaging, possessing improved biocatalytic properties and a high level of biocompatibility. The present study is focused on an in-depth analysis of the enzyme-like properties of gadolinium-doped CeO2 NPs (CeO2:Gd NPs) and their antioxidant activity against superoxide anion radicals, hydrogen peroxide, and alkylperoxyl radicals. Using an anion-exchange method, CeO2:Gd NPs (~5 nm) with various Gd-doping levels (10 mol.% or 20 mol.%) were synthesized. The radical-scavenging properties and biomimetic activities (namely SOD- and peroxidase-like activities) of CeO2:Gd NPs were assessed using a chemiluminescent method with selective chemical probes: luminol, lucigenin, and L-012 (a highly sensitive luminol analogue). In particular, gadolinium doping has been shown to enhance the radical-scavenging properties of CeO2 NPs. Unexpectedly, both bare CeO2 NPs and CeO2:Gd NPs did not exhibit SOD-like activity, acting as pro-oxidants and contributing to the generation of reactive oxygen species. Gadolinium doping caused an increase in the pro-oxidant properties of nanoscale CeO2. At the same time, CeO2:Gd NPs did not significantly inhibit the intrinsic activity of the natural enzyme superoxide dismutase, and CeO2:Gd NPs conjugated with SOD demonstrated SOD-like activity. In contrast to SOD-like properties, peroxidase-like activity was observed for both bare CeO2 NPs and CeO2:Gd NPs. This type of enzyme-like activity was found to be pH-dependent. In a neutral medium (pH = 7.4), nanoscale CeO2 acted as a prooxidant enzyme (peroxidase), while in an alkaline medium (pH = 8.6), it lost its catalytic properties; thus, it cannot be regarded as a nanozyme. Both gadolinium doping and conjugation with a natural enzyme were shown to modulate the interaction of CeO2 NPs with the key components of redox homeostasis.

Optimal design and simulation investigation for high-density hydrogen storage tanks filled with rare earth-based (RE-Ca)(Ni-Co)5 optimized alloy

The slow kinetic rate due to strong thermal effect limits the practical application of metal hydride tanks in high-density hydrogen storage. In this work, we concentrate on a comprehensive experimental and numerical investigation of metal hydride beds with rare earth-based (RE-Ca)(Ni-Co)5 optimized alloy to explore faster reaction rates. Two different cylindrical tanks were manufactured to reflect de-/hydrogenation characteristics. A study on the temperature evolution in the loose powder bed was performed first, followed by five other different bed configurations: alternated powder layers with Al fiber layers, powder mixed with Al fibers, compacted discs (mixture of powder, resin and graphite), alternated compacts with Al fins and alternated Al foil-wrapped compacts with Al fins. Among them, the compacts provide good heat transfer capacity, with additional Al foils and fins for optimum thermal diffusion, reducing the reaction time by almost 50 %. Stress concentrations caused by powder accumulation can also be avoided thanks to the stable structure. Finally, the numerical model was adapted to simulate the de-/hydrogenation processes of the optimized bed and the experimental data matched well with the computational results, confirming the reliability of the model. It lays the foundation for the design and operation optimization of high-density hydrogen storage in the future.

Comparative analysis of single and paired metal hydrides based thermal energy storage system

This study aims to determine which thermal energy storage (TES) system is most suited for a concentrated solar power (CSP) plant. The question is which the best TES system, whether utilizing a single metal hydride or employing dual metal hydrides with or without reaction heat recovery via phase change material (PCM). The authors noted that no studies have been conducted to address this question thus far. Consequently, the primary aim of this paper is to compare three different TES systems in order to ascertain the viability, efficacy, and performance of the appropriate TES system. The examined TES systems included: (i) TES system 1, based on a single metal hydride tank coupled to a hydrogen tank through an H2 compressor; (ii) TES system 2, based on paired metal hydride tanks without recovering reaction heat; and (iii) TES system 3, based on paired metal hydride tanks incorporating a PCM for reaction heat recovery. To simulate and evaluate the performance of the three TES systems under various operating conditions, a mathematical model and computer program were created and successfully verified. By examining into their thermodynamic performance and practical viability, this study attempts to provide the strengths and limitations of each system. A comparison of the three TES systems' performance indicators indicates that the TES system 1 has the potential to provide a specific power output of 500.8 W/kg of alloy, which is 50.1 % and 76.4 % greater than the TES systems 2 and 3, respectively. With a volumetric storage capacity of 755 MJ/m3, the TES system 2 outperforms the other two TES systems by around 40 %. The TES system 3 can operate with 85.9 % energy storage efficiency. This efficiency is 30.3 % higher than that of the TES system 1 and 10.2 % higher than that of the system 2. Ultimately, the decision to choose either a TES system based on a single metal hydride or one involving paired metal hydrides is hinges on the specific requirements, limitations, and priorities of the TES system under consideration.

Planar Tetranuclear Uranium Hydride Cluster Supported by ansa‐Bis(cyclopentadienyl) Ligands

Polynuclear metal hydride clusters play important roles in various catalytic processes, with most of the reported polynuclear metal hydride clusters adopting a polyhedral three‐dimensional structure. Herein, we report the first example of a planar tetranuclear uranium hydride cluster [(CpCMe2CMe2Cp)U]4(μ2‐H)4(μ3‐H)4 (U4H8). It was synthesized by reacting an ansa‐bis(cyclopentadienyl) ligand‐supported uranium chloride precursor [(CpCMe2CMe2Cp)U]3(μ2‐Cl)3(μ3‐Cl)2 with NaHBEt3. The presence of hydrides in U4H8 was confirmed by NMR spectroscopy and its reactivity with phenol and carbon tetrachloride. DFT calculations also facilitated the determination of the hydrides' positions in U4H8, featuring four bridging μ2‐H ligands and four face‐capping μ3‐H ligands, with the four U centers arranged in a rhombic geometry. The U4H8 represents not only the first example of planar polynuclear actinide metal hydride cluster but also the uranium hydride cluster with the highest nuclearity reported to date.

Planar Tetranuclear Uranium Hydride Cluster Supported by ansa-Bis(cyclopentadienyl) Ligands.

Polynuclear metal hydride clusters play important roles in various catalytic processes, with most of the reported polynuclear metal hydride clusters adopting a polyhedral three-dimensional structure. Herein, we report the first example of a planar tetranuclear uranium hydride cluster [(CpCMe2CMe2Cp)U]4(μ2-H)4(μ3-H)4 (U4H8). It was synthesized by reacting an ansa-bis(cyclopentadienyl) ligand-supported uranium chloride precursor [(CpCMe2CMe2Cp)U]3(μ2-Cl)3(μ3-Cl)2 with NaHBEt3. The presence of hydrides in U4H8 was confirmed by NMR spectroscopy and its reactivity with phenol and carbon tetrachloride. DFT calculations also facilitated the determination of the hydrides' positions in U4H8, featuring four bridging μ2-H ligands and four face-capping μ3-H ligands, with the four U centers arranged in a rhombic geometry. The U4H8 represents not only the first example of planar polynuclear actinide metal hydride cluster but also the uranium hydride cluster with the highest nuclearity reported to date.