36 Search Results

Liquid organic hydrogen carriers (LOHCs) are compounds that store and release hydrogen in stable forms at high density. While one-way carriers such as methanol and ammonia have gained attention, their economic advantages are often realized from their use as an export product and direct use as a fuel. Liquid carrier materials that can instead be cycled for energy storage have promise for stationary power applications. In particular, the reversible LOHC system 1,4-butanediol (BDO, H₂-rich) and gamma-butyrolactone (GBL, H₂-lean) has a lower enthalpy of dehydrogenation compared to conventional cyclic hydrocarbons and can utilize non-precious metal copper-based catalysts. Here, in this study,more » BDO/GBL system capital and operating expenses are characterized for vapor phase versus liquid phase hydrogenation and dehydrogenation in a 10 MW backup power application corresponding to sizing of Tier 2 datacenters as well as other critical infrastructure such as hospitals. Costs are benchmarked against two incumbent technologies: a well-established methylcyclohexane/toluene carrier system and compressed gas storage. BDO/GBL storage costs are found to differ substantially between operating modes, with liquid phase hydrogenation coupled with liquid phase dehydrogenation leading to the lowest LCOS of $$\$$$$4.58/kg H₂ in the absence of byproduct formation. In this bounding case, LCOS for the BDO/GBL system is lower than for MCH/TOL ($$\$$$$6.97/kg H₂) and compressed gas ($$\$$$$8.48/kg H₂ at 170 bara and $$\$$$$12.05/kg H₂ at 350 bara). However, escalating costs of carrier replacement due to byproduct formation (ranging from an added $$\$$$$6–11/kg H₂) illustrate the need for highly selective catalysts to ensure BDO/GBL carrier viability.« less

Density functional theory simulations (DFT) have been used in combination with the climbing image nudged elastic band method (CI-NEB) to determine the energy barriers required to dissociate H₂ at four types of h-BN defects and evaluate the potential for reacted H species to hop to adjacent available sites. The defects were selected based on previous work in which ab initio thermodynamics was used to identify specific defects with optimal thermodynamics for reversible H₂ storage, that is, the defects would dissociate molecular H₂ while not binding H species too strongly so that hydrogen release is favorable. The four defects investigated aremore » the N and B monovacancies, which only offer either B or N defective sites respectively, as well as larger cluster vacancy defects named 3V(1B2N) and hexagonal 6V(3B3N), which present a mixture of B and N defect sites available for H binding. While homogeneous H₂ dissociation by B and N monovacancies have energy barriers larger than 0.4 eV, we found that mixed BN terminated defects have energy barrier lower than 0.4 eV for H₂ dissociation. In particular, H₂ dissociation by the hexagonal 6V(3B3N) defect, exclusively made of Frustrated Lewis Pairs (FLPs), is almost barrierless. The energy barrier for subsequent H hopping to available adjacent B or N sites of the defect were also calculated. It was found that the hopping of H species generally involves high energy barriers ranging from 0.26 eV to 2.57 eV. In conclusion, this work completes previous thermodynamic theoretical investigations and provides a more comprehensive picture of H₂ reaction with defective h-BN for potential H₂ storage and recovery.« less

Metal hydrides are known for their outstanding performance as materials for hydrogen storage and processing. These materials find applications for short- and long-term energy storage, compression and supply of hydrogen gas, thermal energy storage, as electrodes and electrolytes in rechargeable batteries, for the microstructural optimisation of functional materials, in thin film technologies, as catalysts, getters and in many other uses. After the discovery of the first binary metal hydrides back in the 19th century, their studies covered all possible binary M-H systems and expanded rapidly into the field of ternary hydrides following the recognition of the excellent hydrogen storage performancemore » of LaNi5- and TiFe-based materials, which operate efficiently at room temperature and at near-ambient H2 pressures. This review aims to provide an overview of the early works, as well as selected recent results on various classes of metal hydrides. It also covers the recent activities from the major contributing countries and continents, including USA, Europe, Japan, China and Australia. These studies relate to achieving the hydrogen storage systems goals set by the Department of Energy in the United States which inspired the research activities at the national and international level, through execution of the tasks on hydrogen-based energy storage managed by the International Energy Agency. The review is prepared by international experts in the field and covers the most important past developments and also presents the recent achievements in the field.« less

Palladium nanoparticles on carbon supports (Pd/C) are effective for catalyzing hydrogen release from aqueous formate solutions but typically suffer from a gradual decrease of activity. This study finds two primary factors influencing activity: (i) the number of available surface Pd sites, and (ii) metal-support interactions which depend on the nature of the C support. We propose that the Pd/C catalyst is structure insensitive and undergoes Ostwald ripening to yield an active ‘conditioned’ catalyst with dispersion plateauing between ca. 15–20 %. Contrary to earlier studies, in-situ XANES experiments show that PdO is not an active catalyst for formate dehydrogenation. Calcination ofmore » Pd/C before dehydrogenation increases the catalytic activity which suggests a beneficial, albeit temporary, modification of the metal support interaction. N-containing supports minimize nanoparticle growth and also increase activity through a metal-support interaction. In conclusion, these findings advance our understanding of catalyst evolution and stability in formate dehydrogenation systems.« less

Here, we used density functional theory simulations in combination with ab initio thermodynamics to determine the H₂ partial pressure (p_H2)-dependent energetics associated with H₂ activation and recovery at various defect sites in hexagonal boron nitride (h-BN). We found that some defects are very reactive with hydrogen, thereby definitely trapping hydrogen in defective h-BN. However, depending on hydrogen partial pressure, less reactive defect sites can be populated. Because of the lower binding capability of these sites, they would allow hydrogen to be recycled and recovered. For small defect sizes, we found that hydrogen preferentially binds to nitrogen sites by forming N–Hmore » bonds, and if no N sites are available then boron sites would be the next to bind hydrogen. Hydrogen dissociation via frustrated Lewis pair is found to be more favorable than forming only N–H bonds but only if the defect size is large enough to accommodate steric effects. For specific conditions such as T = 400 K, p_H2 = 1 bar, and only considering one molecular H₂ per defect, three defects, namely, the N monovacancy, 3V(1B2N), and hexagonal 6V(3B3N) could play a role in both the activation and recycling of H₂ as they would be reacting enough to allow a favorable splitting of H₂ while not binding too strongly to allow its recovery. More broadly, a range of p_H2 and hydrogen loading conditions were investigated for different types of defects and the finding suggests that p_H2 could be used to fine-tune the Gibbs free energy of hydrogenation, thereby allowing several types of defects at different hydrogen loading contents to play a role in the activation/recovery process of H₂ in defective h-BN.« less

Carbon-based materials, such as graphite and its functionalized/doped derivatives, are promising lightweight layered materials for hydrogen activation and storage. Their propensity to control the thermodynamics of hydrogen binding and the kinetics of hydrogen mobility strongly depends on the speciation and the arrangement of dopants. In this study, we demonstrate precise control over dopant speciation and clustering in nitrogen-containing layered carbon materials during hydrothermal synthesis. Through extensive spectroscopic characterization and first principles simulations, we demonstrate that the formation of N-motifs can be controlled by the choice of precursor and synthesis temperature. The distinct three-dimensional architecture and porosity in graphene oxide andmore » carbon nitride-derived materials furnish a synthetic pathway for precise control over the local and global structure of nitrogen-doped carbon materials and their activity toward the activation of molecular hydrogen.« less

Abstract Graphene‐like materials are of interest for large‐scale hydrogen storage applications due to their lightweight, durable, and scalable properties. Nitrogen‐doping minimizes kinetic limitations in diffusion and recombination on surfaces, however, the role of graphitic nitrogen (GN) and pyridinic nitrogen (PN) is not well understood. Nitrogen‐doped graphene is synthesized on Ru(0001) using chemical vapor deposition (CVD) of pyridine and ion irradiation. Scanning tunneling microscopy (STM), x‐ray photoelectron spectroscopy (XPS), and density functional theory (DFT) are used to identify the structure, location, and thermodynamic stability of nitrogen species within the graphene moiré. CVD of pyridine results in a low nitrogen concentration (<0.1at%),more » while the post‐growth nitrogen ion irradiation allows us to increase the concentration further. The concentration of GN and PN is controlled by varying the ion dose and annealing temperature. Comparison of measured and simulated STM images of GN and PN yield an excellent agreement, allowing us to confidently establish that GN is preferentially located near the center of the Atop region, while PN is located in the valley region of the graphene moiré. This report explicitly confirms the site assignments and provides a foundation for the site synthesis and analysis of structural and electronic properties that drive the reactivity of N‐doped graphene.« less

A detailed analysis of the N(1s) and C(1s) X-Ray Photoelectron Spectroscopy (XPS) is made, where the measured XPS is compared with theoretical Sudden Approximation (SA) intensities and theoretical XPS Binding Energies (BEs). There is remarkably good agreement between the theoretical predictions and the measured XPS; in particular, the different full width at half maximum values for the C(1s) and N(1s) BEs are explained in terms of unresolved C(1s) BEs for the different C atoms in pyridine. Here, this work demonstrates that the combination of theory and XPS measurements can extract analysis of the XPS relevant to the molecular electronic structure.more » The theory used is based on fully relativistic self-consistent field solutions of the Dirac–Coulomb Hamiltonian, and the SA is used to determine relative XPS intensities.« less

A Liquid Organic Hydrogen Carrier (LOHC) enables the storage and transport of hydrogen at ambient pressures and temperatures in a safe and convenient form using current infrastructure. However, it is challenging to directly compare reactivity and selectivity for hydrogen release, especially when comparing the catalytic efficiencies of neat LOHCs to highly diluted LOHCs in different solvents, reaction conditions, and catalysts. This work evaluates the role of solvents in catalysis and quantifies the energy efficiency of the overall process. The presence of solvent dilutes the volumetric density of available hydrogen, but may be necessary to achieve optimal catalysts stability, reactivity, andmore » product selectivity. With respect to the reaction conditions as determined by thermodynamics, solvents with higher vapor pressures than that of the carrier can cause the erroneous impression of a more favorable reaction equilibrium. Concerning energy efficiency, solvents can result in increased energy demand for hydrogen release as the inert solvent must be heated to reaction temperatures required for release of H₂ from the LOHC. Further, this work recommends that investigations of catalyst reactivity should be carried out at different ratios of solvent to LOHC to understand how the reactivity changes and what the implications are for maximizing energy density and catalyst stability and reactivity. Investigations should also consider how these implications will affect the technical needs of applications intended for the LOHC system. Based on the results of this study, it is advised to focus research activities on LOHC systems with a gravimetric solvent content below about 50% as the thermodynamic disadvantages become very pronounced beyond this threshold.« less

Carbon sorbent materials have shown great promise for solid-state hydrogen (H₂) storage. Modification of these materials with nitrogen (N) dopants has been undertaken to develop materials that can store H₂ at ambient temperatures. In this work density functional theory (DFT) calculations are used to systematically probe the influence of curvature on the stability and activity of undoped and N-doped carbon materials toward H binding. Specifically, four models of carbon materials are used: graphene, [5,5] carbon nanotube, [5,5] D_5d-C₁₂₀, and C₆₀, to extract and correlate the thermodynamic properties of active sites with varying degrees of sp² hybridization (curvature). From the calculationsmore » and analysis, it is found that graphitic N-doping is thermodynamically favored on more pyramidal sites with increased curvature. In contrast, it is found that the hydrogen binding energy is weakly affected by curvature and is dominated by electronic effects induced by N-doping. These findings highlight the importance of modulating the heteroatom doping configuration and the lattice topology when developing materials for H₂ storage.« less