Hydrogen storage in complex hydrides: past activities and new trends

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Hydrogen storage in complex hydrides: Past activities and new trends

Intense literature and research efforts have focussed on the exploration of complex hydrides for energy storage applications over the past decades. A focus was dedicated to the determination of their thermodynamic and hydrogen storage properties, due to their high gravimetric and volumetric hydrogen storage capacities, but their application has been limited because of harsh working conditions for reversible hydrogen release and uptake. The present review aims at appraising the recent advances on different complex hydride systems, coming from the proficient collaborative activities in the past years from the research groups led by the experts of the Task 40 'Energy Storage and Conversion Based on Hydrogen' of the Hydrogen Technology Collaboration Programme of the International Energy Agency. An overview of materials design, synthesis, tailoring and modelling approaches, hydrogen release and uptake mechanisms and thermodynamic aspects are reviewed to define new trends and suggest new possible applications for these highly tuneable materials

Detecting Reactive Products in Carbon Capture Polymers with Chemical Shift Anisotropy and Machine Learning

Aminopolymers are attractive sorbents for CO2 direct air capture applications as their amines readily react with atmospheric levels of CO2 to form chemisorbed species. The identity of the chemisorbed species varies upon experimental conditions like amine chemistry, support material, CO2 loading, and humidity, forming a variety of carbonyl-type sites. 13C solid-state nuclear magnetic resonance (NMR) is often used to help elucidate the identity of the chemisorbed species however the chemical shift range for carbonyl sites is small and comparable to observed chemisorbed 13C peak widths. Herein, application of a 2D chemical shift anisotropy (CSA) recoupling pulse sequence (ROCSA) is used to obtain CSA tensor values at each isotropic chemical shift, overcoming the isotropic peak resolution limitation. CSA tensor values describe the local chemical environment and can readily differentiate between chemisorbed products. To aid this experimental technique, we also developed a k-nearest-neighbor (KNN) classification model to distinguish chemisorbed compounds via their CSA tensor parameters. The combination of 2D CSA measurements coupled with a KNN classification model enhances the ability to accurately identify chemisorbed products especially in the case of mixtures. This methodology is demonstrated on poly(ethylenimine) in a solid-support γ-Al2O3 exposed to CO2 followed by incomplete regeneration at 100 °C and shows a mixture of strongly bound chemisorbed products, ammonium carbamate and urea

Deep Neural Network Potential Demonstrates the Impact of Proton Transfer in CO2 Capture by Liquid Ammonia

The direct air capture of CO2 using aminopolymers can reduce the environmental impact caused by the still growing anthropogenic emissions of CO2 to the atmosphere. Despite the adsorption efficiency of aminopolymers even in ultradilute conditions, the mechanism of CO2 binding in condensed phase amines is still poorly understood. This work combines machine learning potentials, enhanced sampling and Grand Canonical Monte Carlo to directly compute experimentally-relevant quantities, such as the free energy and enthalpy of CO2 adsorption. Our free energy calculations elucidate the important role of solvent-mediated proton transfer on the formation of the most stable CO2-bound species: carbamate and carbamic acid. Liquid ammonia is used as a model system to study CO2 adsorption, but the methodology can be extended to amines with more complex chemical structure. The study of CO2 adsorption using machine learning brings computer simulations closer to the thermodynamic conditions of interest to experiments, thus paving the way to a more detailed study between the chemical composition of amines and their CO2 binding affinity

Contributions of CO2, O2 and H2O to the Oxidative Stability of Solid Amine Direct Air Capture Sorbents at Intermediate Temperature

Aminopolymer-based sorbents are preferred materials for extraction of CO2 from ambient air (direct air capture of CO2, or DAC) owing to their high CO2 adsorption capacity and selectivity at ultra dilute conditions. While those adsorptive properties are important, the stability of a sorbent is a key element in developing high-performing, cost-effective, and long-lasting sorbents that can be deployed at scale. Along with process upsets, environmental components such as CO2, O2, and H2O may contribute to long-term sorbent instability. As such, unraveling the complex effects of such atmospheric components on sorbent lifetime as they appear in the environment is a critical step to understanding sorbent deactivation mechanisms and designing more effective sorbents and processes. Here, PEI/Al2O3 sorbent is assessed over continuous and cyclic dry and humid conditions to determine the effect of the co-presence of CO2 and O2 on stability at an intermediate temperature of 70 °C. Thermogravimetric and elemental analysis in combination with in situ HATR-IR spectroscopy are performed to measure the extent of deactivation, elemental content, and molecular level changes in the sorbent due to deactivation. The thermal/thermogravimetric analysis results reveal that incorporating CO2 with O2 accelerates sorbent deactivation using these sorbents in dry and humid conditions compared to CO2-free air in similar conditions. In situ HATR-IR spectroscopy results of PEI deactivation under a CO2-air environment show the formation of primary amine species in higher quantity (compared to conditions without O2 or CO2), which arise due to C-N bond cleavage at the primary and secondary amine due to oxidative degradation. We hypothesize the formation of bound CO2 species such as carbamic acids catalyze C-N cleavage reactions in the oxidative degradation pathway by shuttling protons, resulting in a lower activation energy barrier for degradation, as probed by metadynamics simulations. In the cyclic experiment after 30 cycles, results show a gradual loss in stability (dry: 29%, humid: 52%) under CO¬2 containing air (0.04% CO2/21% O2 balance N2). However, the loss in capacity during cyclic studies is significantly less than continuous deactivation as expected

Experimental and Computational Interrogation of Fast SCR Mechanism and Active Sites on H‑Form SSZ-13

Experiments and density functional theory (DFT) models are combined to develop a unified, quantitative model of the mechanism and kinetics of fast selective catalytic reduction (SCR) of NO/NO₂ mixtures over H-SSZ-13 zeolite. Rates, rate orders, and apparent activation energies collected under differential conditions reveal two distinct kinetic regimes. First-principles thermodynamics simulations are used to determine the relative coverages of free Brønsted sites, chemisorbed NH₄⁺, and physisorbed NH₃ as a function of reaction conditions. First-principles metadynamics calculations show that all three sites can contribute to the rate-limiting N–N bond forming step in fast SCR. The results are used to parametrize a kinetic model that encompasses the full range of reaction conditions and recovers observed rate orders and apparent activation energies. Observed kinetic regimes are related to changes in most-abundant surface intermediates

Thermal modulation of reaction equilibria controls mass transfer in CO2-binding organic liquids

CO2-Binding organic liquids (CO2BOLs) are non-aqueous solvents which may reduce the parasitic energy of carbon capture processes. These solvents exhibit surprising mass transfer behavior: at fixed pressure driving force, the flux of CO2 into CO2BOLs decreases exponentially with increased temperature, a phenomenon not observed in aqueous amines. Here, we demonstrate that this phenomenon is primarily driven by a shift in reaction equilibrium, which reduces the degree to which chemical reactions enhance the CO2 flux. First-principles surface renewal models quantitatively reproduce mass transfer data for CO2 absorption into 2-EEMPA, IPADM-2-BOL and DBU:Hexanol across a range of temperatures. Density functional theory calculations are used to identify structural modifications likely to improve the CO2 flux. These findings reveal a fundamental trade-off between CO2 flux and the energy required for solvent regeneration, and provide a theoretical foundation for rational solvent design and the development of physics-informed mass transfer models.</p

A Nanoscale Ternary Amide‐rGO Composite with Boosted Kinetics for Reversible H2 Storage

Metal amides are attractive candidates for hydrogen storage due to their high volumetric and gravimetric hydrogen densities. However, the sluggish kinetics and competing side reactions during hydrogen uptake and release limit their practical use. Here, a novel nanoconfined Li2Mg(NH)2@reduced graphene oxide (rGO) composite is presented, which is fabricated using a melt-infiltration method with a minimum weight penalty of only 2&nbsp;wt.%. The presence of rGO ensures close contact between the active phases, effectively preventing aggregation during cycling process. As a result, the reversible capacity of Li2Mg(NH)2@rGO reaches 4.42&nbsp;wt.%, with no capacity degradation observed after multiple cycling. Theoretical calculations show that rGO catalyzes the hydrogen bond cleavage at the Mg-amide/Li hydride interface, leading to local dehydrogenation hotspots and significantly improves kinetics of dehydrogenation compared to the bulk counterpart. This study provides a promising strategy for designing metal imide-based composites to overcome the kinetic limitations and improve their reversible hydrogen storage performance

Enhanced Hydrogen Bonding via Epoxide-functionalization Restricts Mobility in Poly(ethylenimine) for CO2 Capture

Epoxide-functionalization has emerged as an effective strategy for enhancing the oxidative stability of poly(ethylenimine)-based CO2 capture sorbents. However, the underlying mechanism remains largely unexplored. Here we combine first-principles modeling, material synthesis, and characterizations to investigate the impact of epoxide-functionalization on hydrogen bonding and mobility in poly(ethylenimine) (PEI). Blue-moon ensemble and deep potential molecular dynamics simulations reveal that epoxide-functionalization leads to stronger hydrogen bonding involving hydroxyl groups. Synthesized branched PEI samples with and without propylene-oxide (PO) functionalization are characterized using DSC, NMR relaxometry, and fluorescent probes, demonstrating that PO-functionalization significantly reduces BPEI mobility. These findings suggest that the enhanced oxidative stability of epoxide-functionalized PEI can be attributed to the formation of strong hydrogen bonds with hydroxyl groups, which restrict the mobility of PEI and decelerate mobility-dependent radical propagation reactions responsible for polymer degradation. Strategies for further tuning hydrogen bond environment are proposed based on these findings