Observation of giant 3D graphenic vesicles encapsulating hydrogen

Abstract

Graphene is one of the few materials that is impermeable to hydrogen. Computational studies suggest that giant fullerenes such as C₇₂₀ may be candidates for practical hydrogen storage. The current state-of-the-art in hydrogen confinement by aromatic carbon structures are C₇₀ fullerenes which confine up to 2 molecules of H₂. No experimental demonstration of the encapsulation of bulk H₂ in 3D grahphenes has prevously been reported. Here we describe meso-scale, graphenic vesicles with diameters up to 90 nm, more than 110 times larger than C₇₀ and 35 times larger than C₇₂₀. Electron energy-loss spectrum imaging and core-loss spectroscopy indicate that vesicles may contain H₂ gas. ²H nuclear magnetic resonance studies confirm that these vesicles contain encapsulated H₂/D₂. The encapsulation has been found to persist for over 5 years at room temperature and ambient pressure, demonstrating the high stability and impermeability of the vesicle shells. The synthesis of these novel meso-graphenic structures and the demonstration of long-term, multi-year hydrogen encapsulation may open the door to 3D meso-graphenic materials as a new approach for practical hydrogen storage.

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1. Introduction

Since the discovery of graphene in 2004, the use of graphenic materials have permeated the hydrogen research space.¹ Graphene has proven to be a promising material for gas confinement, as not even hydrogen can penetrate a defect-free graphene sheet.² However, graphenic materials currently suffer from low gravimetric and volumetric hydrogen storage densities; are hampered by surface functionalities; and are currently not amenable to large scale synthesis.³ These drawbacks are shared by other 2D materials, such as hexagonal boron nitride that is further limited by functionalization difficulties due to strong B–N bonds.⁴

Recently, the 3D confinement of H₂ by a carbon material was accomplished by implanting one or two H₂ molecules into C₇₀ through “molecular surgery”.^5,6 Unlike the interaction of H₂ with 2D graphenic carbons, which requires high pressure or low temperature, the encapsulation of H₂ in these 3D carbon structures is permanent under ambient conditions. Computational studies have shown giant fullerenes, C_n where n > 700, to be promising methods of physical hydrogen storage, while subsequent computational studies have shown that it is feasible to produce C_n fullerenes where N = 10 000 (∼9 nm diameter).^7–9 Extension of 3D H₂ confinement by graphenic carbon to the meso scale (10 to hundreds of nanometers in diameter), would represent even greater promise for practical hydrogen storage. However, to our knowledge the largest fullerene that has been synthesized to date is reported to be C₅₀₀ (∼2 nm diameter).¹⁰

Regardless of substrate, the field of hydrogen confinement has been largely reliant on nanoscale interactions and mechanisms, with little work between the nano-scale in the literature body. We recently reported the observation of kinetically enhanced hydrogenation of MgB₂ upon modification with additives.^11,12 In the course of studying the morphological and chemical effects of hydrogenation on treated MgB₂ based materials, an electron energy-loss spectroscopy (EELS) study was conducted that revealed the presence of mesoscale vesicles. High resolution EELS and nuclear magnetic resonance (NMR) studies of these vesicles indicate that they contain confined hydrogen. Herein, we report the first instance, to our knowledge, of long-term confinement of hydrogen gas in a meso-sized 3D graphenic carbon material. While the fullerene studies laid the foundation for 3D encapsulated hydrogen, our observation of much larger 3D graphenic structures and confinement of bulk hydrogen represents a significant leap towards the development of practical hydrogen storage materials and raises new questions for future mechanistic studies. These findings may also represent new avenues of exploration for the synthesis and application of novel graphenic structures.

2. Methods

Mechanochemically treated MgB₂ samples were prepared by milling 1.2 grams of MgB₂ under argon at 700 rpm in a Fritsch Pulverisette 7 premium planetary mill. 10 mol% of Rieke Mg was added at the onset of milling, and 10 mol% of graphene was added during the last hour, totalling 5 hours of mill time under argon. The ball to powder ratio was maintained at 20 : 1, utilizing tungsten carbide mills and grinding media. After mechanochemical treatment, the samples were stored in an Ar glovebox until analysis.

High-pressure hydrogenation and deuteration experiments were conducted using a custom-built high-pressure reactor equipped with a Newport Scientific compressor and constructed from 316 L stainless steel components. Samples were loaded inside the reaction vessel within an argon glovebox (maintaining levels of <0.1 ppm H₂O and <0.1 ppm O₂). Cylindrical stainless-steel slugs were positioned at the bottom of the pressure vessel to allow the loaded samples to stay within the primary heating zone. The reaction vessel was pressurized with hydrogen or deuterium (Matheson Tri-Gas) to achieve the target pressure of 400 bar gas at 300 °C, and kept under these conditions for 72 hours.

Powder samples for EELS measurements were transported, sealed under inert atmosphere, to the National Center for Electron Microscopy (NCEM) in the Molecular Foundry at Lawrence Berkeley Lab. Specimens were prepared in an argon glove box (<0.1 ppm H₂O and <0.1 ppm O₂) by dispersion onto a copper mesh transmission electron microscopy (TEM) grid with a ∼15 nm thick amorphous carbon substrate. Within the argon glove box, the TEM grid was mounted directly into an air-free sample holder equipped with an O-ring seal to prevent air exposure during transfer into the transmission electron microscope.

Annular dark field (ADF) STEM imaging and EELS measurements were carried out using the TEAM I at NCEM, a modified high-base 60–300 keV FEI Titan Scanning Transmission Electron Microscope (STEM) with a monochromator, chromatic and spherical aberration correctors, a continuum electron energy-loss spectrometer/energy filter, and a K3 CMOS camera (Gatan-Ameritek Inc.). The camera was operated in counting mode, enabling highest signal-to-noise and counting statistics at lowest electron dose. Spectrum images were collected with a 17 mrad. convergence semi-angle and 54 mrad collection semi-angle into the spectrometer. Probe currents of 100–150 pA were used for the measurements with pixel spacing of 1 or 2 nm and dwell times of 50 ms per probe position. H, C, O, B and Mg core-loss spectra and spectrum images were acquired at 300 keV in DualEELS mode, which acquires simultaneous spectra over the zero loss peak (ZLP) and selected higher energy loss regions. In this case, energy windows of 0–300 eV and 300–800 eV were selected. Because the focused probe (0.1 nm diameter) was smaller than the pixel sampling interval (1–2 nm) for the EELS spectrum images, the electron probe was rastered on a 16 × 16 nm grid during the 50 ms EELS acquisition at each spectrum image pixel, both to spread the electron dose over a larger sample area and to integrate the full volume of each 1 or 2 nm region. Spectrum images were compensated for energy shifts in the position of the ZLP during the scan. The core loss spectrum images were also deconvolved pixelwise using the simultaneously acquired ZLP at every probe location.

²H NMR experiments were performed on an Agilent 600 DD2, utilizing a 600 MHz 5 mm PFG OneNMR Probe with an acquisition time and relaxation delay of 0.1 s and a pulse width of 100 microseconds. ²H Spin echo experiments were performed utilizing the following pulse sequence:

Prepulse delay (100 ms) – 90° pulse (135 µs) – τ delay (155 µs) – 180° pulse (270 µs) – τ delay (155 µs) – acquisition (10 ms).

3. Results and discussion

3.1 Vesicle observation

In order to investigate the effect of additives on the MgB₂ hydrogenation cycle, EELS analyses were performed on a powder sample of MgB₂ that was ball milled with Rieke magnesium and graphene and exposed to 400 bar of H₂ gas. (see Methods section for details) EELS elemental maps revealed high concentrations of hydrogen, in regions that also contained high concentrations of carbon and boron (Fig. 1). These regions have the appearance of “bubbles”, or vesicles, observable as dark (more electron transparent) regions in annular darkfield (ADF) images (Fig. 2).

Fig. 1 EELS spectrum images (elemental maps) of a MgB₂ grain displaying the distributions of H, B, C and Mg. Arrows 1 & 2 indicate H₂ vesicles.

Fig. 2 Annular Dark Field (ADF) scanning transmission electron microscopy (STEM) image of a treated MgB₂ grain containing a “hydrogen confining vesicle” (outlined in red) with an apparent internal diameter of ∼90 nm.

All observed vesicles have an internal diameter at least one or two orders of magnitude larger than the single nanometer scale, up to ∼90 nm (Fig. 2).

The use of an electron energy-loss spectrometer with CMOS technology and single-electron detection sensitivity, in conjunction with low incident beam currents (∼100 pA), enabled rapid acquisition of high signal-to-noise spectra without alteration of the specimens under the electron irradiation (see Methods). However, because TEM images provide projected 2D information, accurate volume information cannot be obtained from the observed vesicles, especially considering the morphological variation between vesicles on different grains. These vesicles have only been observable at (thin) grain edges, but it is assumed that bulk grains also contain vesicles in their interiors, which would not be visible via TEM imaging or EELS analysis due to greater thickness there. Due to the wide distribution of observed vesicle size and morphology, it can also be assumed that there is likely a higher distribution of smaller vesicles throughout these samples, and that there may also be interior grain vesicles that are larger than the observed ∼90 nm diameter vesicle. Further studies detailed below reveal that these vesicles confine gaseous hydrogen.

3.2 Characterization of meso-graphenic structures

EELS analysis of the vesicle wall shows a carbon K-edge that contains a strong feature at 285 eV and a broad feature at 292 eV, that correspond to peaks assigned as graphene 1s → π* and 1s → σ* excitations, respectively (Fig. 3b).¹³ The π* promotion is directly associated with sp² hybridization. Examination of the low loss region of the EELS spectra reveals π-plasmons at 4.8 and 5.8 eV (Fig. 4) at locations also exhibiting a strong carbon K-edge. The plasmon at 4.8 eV agrees with previous reports of single sheets of graphene, while the 1.0 eV shift to 5.8 eV correlates to the presence of a graphene bilayer, suggesting the presence of graphenic meso-structures.¹⁴ (The amorphous carbon substrate of the TEM grid shows no graphenic features). Additionally, the low loss region contains a prominent feature at 13 eV (Fig. 3c). This feature is the hydrogen-K core scattering edge, indicating that the regions of study correspond to a graphenic material that confines a hydrogen-containing species, subsequently confirmed via NMR (next section) to be H₂ gas.^15,16 These findings were unanticipated because the EELS analysis was carried out on a sample that was hydrogenated ∼5 years prior and stored in a glovebox at ambient temperature and pressure.

Fig. 3 Selected region in a grain with the strongest hydrogen signal. (a) ADF STEM image and (b and c) expanded EELS spectra from the selected region, displaying the hydrogen K-edge at 13 eV, the boron K-edge feature at 193 eV, as well as carbon contributions at 285 eV and 292 eV. The H₂-bearing vesicle is located in the center of the circled region.

Fig. 4 Low loss spectra comparison on and off vesicle wall, in a hydrogenated grain. On-vesicle low loss features contain π-plasmon resonances seen in graphene that are spatially correlated with a H₂ K-edge feature at 13 eV. These features are not observed off the vesicle.

Analysis of the boron K-edge reveals that the observed boron K-edge is missing the signature MgB₂ feature at 186 eV, known to be planar hole promotions seen in 6 membered rings of boron.^17–19 The boron K-edge contains only a strong feature at 193.6 eV, known to be associated with boron oxides/borates (Fig. 3b). The formation of B–C bonds in a material like graphitic BC₃ contains a signature feature at 190–192 eV, and it is therefore likely that the vesicle shells contain a thin coating of borates rather than domains of a B–C polymorph.^20,21

3.3 Verification of hydrogen confinement

To independently verify the presence of confined H₂, we undertook an examination of the materials by NMR spectroscopy. A variety of hydrogen containing contaminants, most notably water, could quite plausibly be misidentified as confined hydrogen by ¹H NMR spectroscopy. To avoid ambiguity, we carried out a study on samples that were exposed to 400 bar of D₂ gas instead of H₂. New samples were prepared utilizing the same procedure that was used in our previous hydrogenation study, then examined by ²H NMR spectroscopy. Remarkably, static ²H NMR spectra of these deuterated samples consistently display evidence of confined D₂. In consideration of the broadness of the observed signal, solid-state spin echo NMR pulse sequencing was utilized to measure the very broad resonance without distortion. The resulting spectrum yields a single broad peak centered at −29 ppm (Fig. 5). Despite the broad signal width, the observation of a solid state D₂ signal without the assistance of magic angle spinning indicates that the confined D₂ must be unbound, somewhat mobile, and dynamically similar to hydrogen previously observed to be confined in fullerene.²²

Fig. 5 ²H NMR of the broad resonance observed at −29 ppm.

Physisorbed hydrogen has generally been reported to have chemical shifts in the range of 1–6 ppm, that is, upfield from the experimental isotropic chemical shift of H₂ gas, δ = 7.40 ppm.^23,24 This contrasts the resonance for external hydrogen gas intercalated into the fcc lattice of fullerenes, observed at 4.8 ppm, and endohedral hydrogen inserted into a fullerene, which was observed at −24 ppm.^22,25,26 Mamone et al. (2013) postulated that the dominant influence on the chemical shift of the confined hydrogen arises from contributions of both isotropic and anisotropic effects of the C₇₀ cage itself.²² In the case of the graphenic meso-structures observed in our material, the further shift to −29 ppm, the largest upfield shift observed to date for H₂/D₂ gas, can be explained to first order by considerations of aromaticity and vesicle morphology. It has been established that an increase in the size of an aromatic system results in larger shielding effects of nuclei that are orthogonal to the aromatic surface and aligned with the NMR applied external magnetic field, B₀, due to stronger induced ring currents.²⁷ Molecules of a mobile gas confined within an impermeable shell would undoubtedly experience these alignments and orientations, as confinement disallows orientations parallel to the confining surface. Therefore, the shift indicates that the vesicle walls likely contain large aromatic graphenic domains. Furthermore, the formation of asymmetrical vesicles that appear to possess curved walls suggests the observed NMR resonance is the result of compounding effects where aromatic ring currents combine with anisotropic effects from the geometry of the vesicle shell, imparting higher shielding effects, in agreement with literature reports on strained and curved graphene systems.^28,29 The broad width of the observed resonance is most likely due to the broad distribution of inter-vesicle morphology, as interactions between confined H₂/D₂ and the vesicle walls differ depending on the size and shape of the confining vesicle, in contrast to that of a uniform fullerene cage for a fixed fullerene size. Additionally, the possibility of smaller vesicles/fullerene type structures, below the limit of spectroscopic detection for this type of feature, and their contributions to the observed NMR resonance cannot be ignored and are likely included in the observed broad resonance.

3.4 Impermeability

We have found hydrogen confinement to persist for over 5 years. Thus, we conclude the graphenic shell of the vesicle is highly impermeable to hydrogen. This observation is in agreement with Bunch et al. (2008) and subsequent literature reports that find single sheets of graphene to be impermeable to all gasses.^2,30–32 However, confinement of hydrogen by graphene has only been reported to occur at graphene-substrate interfaces.³³ To our knowledge, this is the first instance of the full encapsulation and confinement of bulk hydrogen by a 3D graphenic material in volumes extending beyond the nanoscale of fullerenes.

3.5 Meso-confinement

Assuming fullerene-like dimensionality, the vesicle seen in Fig. 2 would possess an interior diameter of approximately 90 nm. This would correspond to a spherical volume of 380 000 cubic nanometers was recently reported that the largest possible single wall fullerene is C₁₀₀₀₀ and that expansion past this size results in the formation of carbon onions.⁹ A C₁₀₀₀₀ fullerene would possess a diameter of ∼9 nm, whereas the observed vesicle of ∼90 nm diameter would correlate to a C_n fullerene with an N = ∼1 000 000.⁸ This suggests that these observed vesicles comprise a non-fullerene 3D graphene system, in agreement with the EELS observations of only graphenic single and bilayers. The above size estimation is derived by assuming a spherical structure. However, it is likely that the vesicles have an ellipsoidal shape, such that the unobserved third dimension may be substantially smaller than the two observed dimensions. Regardless, even considering the third dimension equivalent to the width of a single C₆₀ fullerene, (the smallest known precedent), the volume potential of these vesicles are exponentially higher than anything currently reported. These vesicles are at least one order of magnitude larger than previously reported dimensions in C₇₀ fullerenes. The “meso” scale of these vesicles lies between well-established nano-confinement and micro-confinement seen in hollow glassy spheres.³⁴

It is difficult to make quantitative claims as to the population of hydrogen confined within the vesicles without extensive additional analytical study. In addition to the uncertainty about the range of vesicle size, the hydrogen pressure in the vesicles is an open question, as the pressure in structural voids can vary widely.³⁵ However, a simple rough estimate utilizing the van der Waal radius of hydrogen implies that a spherical vesicle with a 90 nm diameter could contain on the order of 10⁵–10⁶ molecules of H₂. While this value would be difficult to verify, it can be taken as an upper limit of the amount of H₂ that could be confined within the vesicle that was observed. This parameter has serious implications for the hydrogen storage potential associated with these features and future exploration of the hydrogen pressures associated with the meso-scale confinement that we have observed is clearly warranted.

3.6 Formation mechanism

Our findings a priori suggest the transformation of graphene into meso-structured vesicles. Indeed, the formation of fullerenes from graphite and graphene has been directly observed and has been the subject of mechanistic consideration.³⁶ However, intact graphene sheets are rarely seen during transmission electron microscopy explorations after ball milling, and bulk carbon structures have never been observed in carbon doped MgB₂ before hydrogenation. Nevertheless, the formation of meso-structured vesicles from interactions between MgB₂ and graphene during high temperature/pressure hydrogenation raises interesting questions as to the reactivity between carbon and MgB₂. As vesicles are only observed in hydrogenated materials, the formation of Mg(BH₄)₂ may play a role in the assembly of the vesicle walls. It has been reported that under hydrogen pressure and at elevated temperature, Mg(BH₄)₂ experiences a molten phase, in agreement with experimental observations where free flowing MgB₂ powders coalesce into rocky aggregates after high-pressure hydrogenation.³⁷ This liquid phase may be responsible for graphenization, as carbon is freed during the transition between carbon doped MgB₂ to Mg(BH₄)₂. However, the modest temperatures involved in this formation mechanism suggests a yet unknown process, as graphenization usually requires either the use of a transition metal catalyst and/or temperatures ranging from 600 to >1000 °C.³⁸ The use of graphene nanoplatelets in the milling of these samples cannot be ignored. It is possible that these meso-structures are assembled from graphene platelets that are incorporated into bulk boride under high hydrogen pressure. Additionally, the boron oxides observed on the vesicle shell may also be contributing in a yet unknown manner. Further exploration of the mechanism(s) of formation of closed 3D structures and their encapsulation and permanent confinement of H₂ are clearly warranted and will be the subject of further studies.

4 Conclusions

We have, for the first time, observed the formation of hydrogen containing vesicles of “meso” dimensions. EELS analysis indicates that the walls of the vesicles are composed of a material containing aromatic carbon. The presence of molecular hydrogen within the graphenic vesicles was cross validated by EELS and NMR studies (with substitution of deuterium). The chemical shift observed for the hydrogen in these materials indicates that the chemical environment of the confined hydrogen is similar to that of hydrogen confined in fullerenes, although our observations indicate confinement in a significantly larger aromatic system. To date, this is the first report of bulk H₂/D₂ permanently confined in a graphenic material. Most notably, we have observed that the vesicles are indefinitely impermeable to hydrogen/deuterium at room temperature and ambient pressure.

The combination of hydrogen impermeability and the extension of meso-confinement to large graphenic domains opens doors to new opportunities in bulk hydrogen storage, and the development of new 3D graphenes.

Author contributions

C. S prepared samples and wrote the manuscript with input from C. M. J, H. A. I, J. P. B, V. S. C. M. J designed the hydrogenation and NMR experiments. H. A. I and J. P. B designed and performed STEM experiments and analysis. W. Y. Y. performed NMR experiments and developed solid state pulse sequences. V. S and B. C. D performed hydrogenation and deuteration experiments.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the Supplementary Information (SI). Supplementary information: 1. Boron K-edge of EELS spectrum. 2. Low energy loss spectrum of an alternative carbon “vesicle”. See DOI: https://doi.org/10.1039/d5ta06250k.

Acknowledgements

The authors gratefully acknowledge research support from the U.S. Department of Energy (DOE), Office of Science Established Program to Stimulate Competitive Research, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office through the Hydrogen Storage Materials Advanced Research Consortium (HyMARC). Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, (NTESS) LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration (DOE/NNSA) under contract DE-NA0003525. Any subjective views or opinions that might be expressed in the written work do not necessarily represent the views of the U.S. Government. The publisher acknowledges that the U.S. Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this written work or allow others to do so, for U.S. Government purposes. The DOE will provide public access to results of federally sponsored research in accordance with the DOE Public Access Plan. STEM imaging and EELS were carried out at the National Center for Electron Microscopy/Molecular Foundry (NCEM/MF) at Lawrence Berkeley National Laboratory supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Chengyu Song and James Ciston (NCEM/MF) for technical assistance.

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