Hydrogen storage in absorbents as activated carbons has been rarely investigated; however, about 25 years ago, the development of new nanomaterials, initiated by Iijima’s discovery of carbon ...microtubules, started new hopes. Unfortunately, initial results on high hydrogen uptake in carbon nanotubes at ambient conditions could not be independently reproduced; however, at cryogenic conditions, these novel nanomaterials just behaved as activated carbons with an uptake proportional to the surface area. Shortly after, the development of coordination polymers with permanent porosity opened a new route to nanoporous materials with ultra-high internal surfaces. Mainly metal–organic frameworks (MOFs) have been attracting a great deal of attention in recent years, as very high gravimetric hydrogen capacities can be achieved at 77 K. Cryogenic storage by physisorption of hydrogen molecules will safely operate at low pressures, is fully reversible, and possesses fast kinetics. This mini-review shows the rapid development in this field over the past 25 years. Exemplarily, the main focus is on results obtained in the hydrogen storage laboratory in Stuttgart and their connection to Applied Physics A.
One of the greatest challenges of modern separation technology is separating isotope mixtures in high purity. The separation of hydrogen isotopes can create immense economic value by producing ...valuable deuterium (D) and tritium (T), which are irreplaceable for various industrial and scientific applications. However, current separation methods suffer from low separation efficiency owing to the similar chemical properties of isotopes; thus, high‐purity isotopes are not easily achieved. Recently, nanoporous materials have been proposed as promising candidates and are supported by a newly proposed separation mechanism, i.e., quantum effects. Herein, the fundamentals of the quantum sieving effect of hydrogen isotopes in nanoporous materials are discussed, which are mainly kinetic quantum sieving and chemical‐affinity quantum sieving, including the recent advances in the analytical techniques. As examples of nanoporous materials, carbons, zeolites, metal–organic frameworks, and covalent organic frameworks are addressed from computational and experimental standpoints. Understanding the quantum sieving effect in nanospaces and the tailoring of porous materials based on it will open up new opportunities to develop a highly efficient and advanced isotope separation systems.
The separation of a physicochemically almost identical isotopic mixture is the grandest challenge in modern separation technology. Recent advances of hydrogen‐isotope separation in a confined nanospace based on separation mechanisms of quantum effects, as well as computational and experimental studies on carbons, zeolites, metal–organic frameworks, and covalent organic frameworks, are discussed. Future perspectives are also suggested for realistic isotope separation platforms.
Spreading rumors on the Internet has become increasingly pervasive due to the proliferation of online social media. This paper investigates how rumors are amplified by a group of users who share ...similar interests or views, dubbed as an echo chamber. To this end, we identify and analyze 'rumor' echo chambers, each of which is a group of users who have participated in propagating common rumors. By collecting and analyzing 125 recent rumors from six popular fact-checking sites, and their associated 289,202 tweets/retweets generated by 176,362 users, we find that the rumors that are spread by rumor echo chamber members tend to be more viral and quickly propagated than those that are not spread by echo chamber members. We propose the notion of an echo chamber network that represents relations among rumor echo chambers. By identifying the hub rumor echo chambers (in terms of connectivity to other rumor echo chambers) in the echo chamber network, we show that the top 10% of hub rumor echo chambers contribute to propagation of 24% rumors by eliciting more than 36% of retweets, implying that core rumor echo chambers significantly contribute to rumor spreads.
Deuterium plays a pivotal role in industrial and scientific research, and is irreplaceable for various applications such as isotope tracing, neutron moderation, and neutron scattering. In addition, ...deuterium is a key energy source for fusion reactions. Thus, the isolation of deuterium from a physico-chemically almost identical isotopic mixture is a seminal challenge in modern separation technology. However, current commercial approaches suffer from extremely low separation efficiency (i.e., cryogenic distillation, selectivity of 1.5 at 24 K), requiring a cost-effective and large-scale separation technique. Herein, we report a highly effective hydrogen isotope separation system based on metal–organic frameworks (MOFs) having the highest reported separation factor as high as ∼26 at 77 K by maximizing synergistic effects of the chemical affinity quantum sieving (CAQS) and kinetic quantum sieving (KQS). For this purpose, the MOF-74 system having high hydrogen adsorption enthalpies due to strong open metal sites is chosen for CAQS functionality, and imidazole molecules (IM) are employed to the system for enhancing the KQS effect. To the best of our knowledge, this work is not only the first attempt to implement two quantum sieving effects, KQS and CAQS, in one system, but also provides experimental validation of the utility of this system for practical industrial usage by isolating high-purity D2 through direct selective separation studies using 1:1 D2/H2 mixtures.
An isotope-selective responsive system based on molecular recognition in porous materials has potential for the storage and purification of isotopic mixtures but is considered unachievable because of ...the almost identical physicochemical properties of the isotopes. Herein, a unique isotope-responsive breathing transition of the flexible metal–organic framework (MOF), MIL-53(Al), which can selectively recognize and respond to only D2 molecules through a secondary breathing transition, is reported. This novel phenomenon is examined using in situ neutron diffraction experiments under the same conditions for H2 and D2 sorption experiments. This work can guide the development of a novel isotope-selective recognition system and provide opportunities to fabricate flexible MOF systems for energy-efficient purification of the isotopic mixture.
The commercialization of hydrogen as a clean source of energy is a vital requirement for overcoming the anticipated energy crisis. In addition, the capture of CO2 and commercialization of methane as ...an efficient and clean alternative to polluting gasoline are important goals. To this end, we have developed a nanoporous activated carbon material prepared from renewable resources that has a high storage capacity for various gases. Sugar beet leaves were converted to graphite flakes and decorated with polymer nanoparticles, giving rise to a highly porous activated carbon through chemical activation. The developed porous carbon has a high surface area (2800 m2 g−1) and specific pore volume (1.86 cm3 g−1), as well as high nitrogen and oxygen contents. The combination of high surface area, pore volume, and nitrogen and oxygen contents provided superior storage capacity for various gases. The total hydrogen storage capacities at 20 bar were 5.9 and 0.15 wt% at 77 and 298 K, respectively. In addition, the physical upper limit of hydrogen storage capacity was also evaluated using Brunauer–Emmett–Teller isotherms at the liquefaction temperature of hydrogen (20 K). A value of 14.1 wt% was obtained, which is the highest reported value for a porous carbon. The CO2 capture and CH4 storage capacities at room temperature and 20 bar were 19.65 and 7.6 mmol g−1, respectively, which are also among the highest values reported for porous carbon materials. Furthermore, the separation selectivity for CO2/CH4 binary mixtures was evaluated based on the ideal adsorbed solution theory (IAST) model and found to be 4.6.
•Different WO3 nanostructures were synthesized using hydrothermal method.•WO3 nanostructures were employed for electrochromic and supercapacitor application.•WO3 nanoplates showed prominent ...electrochromic activity.•WO3 nanoplates exhibited an aerial capacitance of 72.6 mF cm−2.
We report the direct synthesis of various WO3 nanostructures (nanoplates, nanobricks, and stacked nanosheets) on fluorine-doped tin oxide conducting substrates for electrochromic and pseudocapacitive energy storage applications. These nanostructures were formed by varying the pH of the hydrothermal solution, which led to monoclinic and triclinic crystal structures. Among these structures, vertically aligned WO3 nanoplates showed good electrochromic properties, with rapid and reversible response of the colored and bleached states in 0.5 M H2SO4 electrolyte. Moreover, the vertically aligned WO3 nanoplates exhibited promising energy storage behavior as a negative electrode material with a higher areal capacitance of 72.6 mF cm−2 in 0.5 M Na2SO4 electrolyte and better electrochemical performance than the nanobricks and stacked nanosheets. The two-dimensional WO3 nanoplates exhibit strong potential for use in smart windows and negative-electrode pseudocapacitors.
Three‐dimensional covalent organic frameworks (COFs) have been demonstrated as a new class of templates for nanoparticles. Photodecomposition of the Pd(η3‐C3H5)(η5‐C5H5)@COF‐102 inclusion compound ...(synthesized by a gas‐phase infiltration method) led to the formation of the Pd@COF‐102 hybrid material. Advanced electron microscopy techniques (including high‐angle annular dark‐field scanning transmission electron microscopy and electron tomography) along with other conventional characterization techniques unambiguously showed that highly monodisperse Pd nanoparticles ((2.4±0.5) nm) were evenly distributed inside the COF‐102 framework. The Pd@COF‐102 hybrid material is a rare example of a metal‐nanoparticle‐loaded porous crystalline material with a very narrow size distribution without any larger agglomerates even at high loadings (30 wt %). Two samples with moderate Pd content (3.5 and 9.5 wt %) were used to study the hydrogen storage properties of the metal‐decorated COF surface. The uptakes at room temperature from these samples were higher than those of similar systems such as Pd@metal–organic frameworks (MOFs). The studies show that the H2 capacities were enhanced by a factor of 2–3 through Pd impregnation on COF‐102 at room temperature and 20 bar. This remarkable enhancement is not just due to Pd hydride formation and can be mainly ascribed to hydrogenation of residual organic compounds, such as bicyclopentadiene. The significantly higher reversible hydrogen storage capacity that comes from decomposed products of the employed organometallic Pd precursor suggests that this discovery may be relevant to the discussion of the spillover phenomenon in metal/MOFs and related systems.
In the frame: Pd@COF‐102 hybrid material (COF=covalent organic framework; see figure) is a rare example of a metal‐nanoparticle‐loaded porous crystalline material with a very narrow size distribution, cavity size matching, and an absence of larger agglomerates even at high loadings (30 wt %). Hydrogen storage studies showed that the H2 capacities were enhanced by a factor of 2–3 as a result of Pd impregnation on COF‐102 at room temperature and 20 bar.
Hydrogen isotope mixtures can be separated either by confinement in small pores i.e., “kinetic quantum sieving” (KQS) or by strong adsorption sites i.e., “chemical affinity quantum sieving” (CAQS). ...MOFs are excellent candidates for study of these quantum effects, due to their well‐defined, tunable pore structures and the potential to introduce strong adsorption sites directly into the framework structure. In this microreview we summarize the recent status of hydrogen isotope separation using MOFs and future strategies relating to it. Furthermore, a state‐of‐the‐art technique for the direct measurement of selectivity with regard to isotope mixtures is introduced. Experimental results relating to separation factors with different pore apertures in the case of KQS and the role of open metal sites in that of CAQS as a function of temperature and gas pressure are given. Furthermore, technologically relevant parameters such as feasible operating pressure and temperature are discussed with respect to possible applications in a temperature swing process.
Two mechanisms for hydrogen isotope separation using MOFs are introduced: “kinetic quantum sieving (KQS) with confinement in small pores” and “chemical affinity quantum sieving (CAQS) by strong adsorption sites”. In addition, the recent status and future strategies for hydrogen isotope separation with the aid of MOFs is discussed.
A semitechnical route (optimized by BASF SE) to synthesize MOF-74/174-M (M = Mg2+, Ni2+) efficiently in ton-scale production is presented with the goal of mobile and stationary gas storage ...applications especially for hydrogen as future energy carrier. In addition, a new member of these series of materials, MOF-184-M (M = Mg2+, Ni2+) is introduced using ligand exchange strategy in order to produce a more porous analogue (possessing large aperture) without loss of crystallinity. This family comprising MOF-74/174/184 are characterized systematically for hydrogen adsorption properties by volumetric measurements with a Sieverts’ apparatus. Replacing the linker by a longer one results in an increase of the BET area from 984 to 3154 m2/g and an enhancement of the excess cryogenic (77 K) hydrogen storage capacity from 1.8 to 4.7 wt%. The heat of adsorption of linker exchanged MOF-174/184 (as a function of uptake) shows similar values to the parent MOF-74, indicating successful construction of expanded MOFs in large scale production. Finally, a usable capacity of these MOFs is investigated for mobile application, revealing that the increasing surface area without strong binding metal sites through longer linker exchange is one of important parameters for improving usable capacity.
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•A large-scale synthesis method for MOF-74/74(II) have been introduced.•A newly developed large pore MOF-184 is synthesized by a ligand exchange strategy.•Replacing a linker by longer one shows enhanced BET area and H2 storage capacity.•Usable capacity is improved by increasing BET area w/o strong binding sites.
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