Hydrogen storage using physisorption – materials demands

A survey is presented of the storage capacities of a large number of different adsorbents for hydrogen at 77 K and 1 bar. Results are evaluated to examine the feasibility and perspectives of transportable and reversible storage systems based on physisorption of hydrogen on adsorbents. It is concluded that microporousadsorbents, e.g. zeolites and activated carbons, display appreciable sorption capacities. Based on their micropore volume (∼1 ml/g) carbon-based sorbents display the largest adsorption, viz. 238 ml (STP)/g, at the prevailing conditions. Optimization of sorbent and adsorption conditions is expected to lead to adsorption of ∼560 ml (STP)/g, close to targets set for mobile applications. Received: 9 January 2001 / Accepted: 27 January 2001 / Published online: 23 March 2001     

Hydrogen storage in carbon nanostructures – still a long road from science to commerce?

Hydrogen storage in carbon nanostructures is still at a research level and not yet mature for industrial application. For the time being it is unfair to compare carbon nanostructures for hydrogen storage at the same level as metal hydrides or other established storage technologies, as not yet enough research has been carried out. Nevertheless we compare carbon nanostructures with well-established hydrogen-storage technologies to develop a feeling of the needs and to identify where bottlenecks might exist. We try to sketch the long route for carbon nanostructures to become a commercial product for hydrogen storage, with a focus on mobile applications. Received: 4 August 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Storage of hydrogen on single-walled carbon nanotubes and other carbon structures

The sorption of hydrogen on carbon structures and nanostructures offers a way to reduce the storage pressure of hydrogen with respect to compression storage while achieving interesting gravimetric storage densities. The most readily available carbon structures, activated carbons, can achieve reproducible, high gravimetric storage densities under cryogenic operating conditions: 5–6% at 35 bar and 77 K, in excess of the normal density that would be present in the pore volume under compression at the same temperature and pressure. We discuss and compare the adsorption of hydrogen on high specific surface activated carbons, nanofibres and nanotubes from experimental and theoretical considerations. In particular, we present gravimetric and volumetric hydrogen sorption measurements on single-walled carbon nanotubes (SWNTs) at (1 bar, 77 K) and (1 bar, 295 K) within the context of our ongoing work on the storage of hydrogen on activated carbon and carbon nanostructures. BET surface area and XRD characterization results on SWNTs are also presented. The experiments were performed on as received, chemically treated and metal-incorporated SWNT samples. Hydrogen sorption capacities measured on treated samples ranged from 0 to about 1 wt.% at 1 bar and 295 K and reached about 4 wt.% at 1 bar and 77 K. Our results show that under certain conditions, SWNTs have better hydrogen uptake performance than large surface area activated carbons. PACS 81.07.de; 81.05.Uw; 68.43.h     

Preparation and hydrogen storage of activated rayon-based carbon fibers with high specific surface area

Activated carbon fibers were prepared from rayon-based carbon fibers by two step activations with steam and KOH treatments. Hydrogen storage properties of the activated rayon-based carbon fibers with high specific surface area and micropore volume have been investigated. SEM, XRD and Brunauer-Emmett-Teller (BET) were used to characterize the samples. The adsorption performance and porous structure were investigated by nitrogen adsorption isotherm at 77 K on the base of BET and density functional theory (DFT). The BET specific surface area and micropore volume of the activated rayon-based carbon fibers were 3144 m²/g and 0.744 m³/g, respectively. Hydrogen storage properties of the samples were measured at 77 and 298 K with pressure-composition isotherm (PCT) measuring system based on the volumetric method. The capacities of hydrogen storage of the activated rayon-based carbon fibers were 7.01 and 1.46 wt% at 77 and 298 K at 4 MPa, respectively. Possible mechanisms for hydrogen storage in the activated rayon-based carbon fibers are discussed.     

Hydrogen storage in sonicated carbon materials

The hydrogen storage in purified single-wall carbon nanotubes (SWNTs), graphite and diamond powder was investigated at room temperature and ambient pressure. The samples were sonicated in 5 M HNO₃ for various periods of time using an ultrasonic probe of the alloy Ti-6Al-4V. The goal of this treatment was to open the carbon nanotubes. The maximum value of overall hydrogen storage was found to be 1.5 wt %, as determined by thermal desorption spectroscopy. The storage capacity increases with sonication time. The sonication treatment introduces particles of the Ti alloy into the samples, as shown by X-ray diffraction, transmission electron microscopy, and chemical analysis. All of the hydrogen uptake can be explained by the assumption that the hydrogen is only stored in the Ti-alloy particles. The presence of Ti-alloy particles does not allow the determination of whether a small amount of hydrogen possibly is stored in the SWNTs themselves, and the fraction of nanotubes opened by the sonication treatment is unknown. Received: 18 December 2000 / Accepted: 18 December 2000 / Published online: 9 February 2001     

Heterogeneity of activated carbons in adsorption of aniline from aqueous solutions

The heterogeneity of activated carbons (ACs) prepared from different precursors is investigated on the basis of adsorption isotherms of aniline from dilute aqueous solutions at various pH values. The APET carbon prepared from polyethyleneterephthalate (PET), as well as, commercial ACP carbon prepared from peat were used. Besides, to investigate the influence of carbon surface chemistry, the adsorption was studied on modified carbons based on ACP carbon. Its various oxygen surface groups were changed by both nitric acid and thermal treatments. The Dubinin-Astakhov (DA) equation and Langmuir-Freundlich (LF) one have been used to model the phenomenon of aniline adsorption from aqueous solutions on heterogeneous carbon surfaces. Adsorption-energy distribution (AED) functions have been calculated by using an algorithm based on a regularization method. Analysis of these functions for activated carbons studied provides important comparative information about their surface heterogeneity.     

Challenges in hydrogen adsorptions: from physisorption to chemisorption

In this short review, we will briefly discuss the story of hydrogen storage, its impact on clean energy application, especially the challenges of using hydrogen adsorption for onboard application. After a short comparison of the main methods of hydrogen storage (high pressure tank, metal hydride and adsorption), we will focus our discussion on adsorption of hydrogen in graphitic carbon based large surface area adsorbents including carbon nanotubes, graphene and metal organic frameworks. The mechanisms, advantages, disadvantages and recent progresses will be discussed and reviewed for physisorption, metal-assisted storage and chemisorption. In the last section, we will discuss hydrogen spillover chemisorption in detail for the mechanism, status, challenges and perspectives. We hope to present a clear picture of the present technologies, challenges and the perspectives of hydrogen storage for the future studies.     

Heterogeneity of activated carbons with different surface chemistry in adsorption of phenol from aqueous solutions

The heterogeneity of activated carbons is investigated on the basis of adsorption isotherms of phenol from dilute aqueous solutions at different values of pH in the solution. The original carbon studied was prepared from polyethyleneterephtalate (PET). Its various oxygen surface functionalities were systematically changed by additional nitric acid and heat treatments. The Dubinin-Astakhov adsorption-isotherm equation was used to evaluate the parameters characterizing the adsorption of phenol from dilute water solutions on activated carbon surfaces. Adsorption energy distribution functions were calculated by the INTEG algorithm, based on a regularization method. Analysis of distribution functions for activated carbons provides significant comparative information about their energetic heterogeneity. Moreover, a comparison of the resulting energies obtained from the distributions can be made with enthalpic data.     

Synthesis and properties of new nitrogen-doped nanostructured carbon materials obtained by templating of mesoporous silicas with aminosugars

The negative templating synthesis process has been applied to prepare nanostructured carbon materials with a high nitrogen content. SBA-15 silica template was impregnated with the following carbon precursors: sucrose, glucose and amino-glucose. The structure of the materials was investigated by SAXS, WAXS and TEM. Nitrogen functions were analyzed by XPS and the textural parameters of the carbons were studied by nitrogen and CO₂ adsorption. X-ray and TEM studies confirmed that a pore nanostructure is inherited from the silica templates. XPS analysis showed that the nitrogen content of the materials can be controlled between 2 and 5 wt% and that N atoms are strongly bonded in the carbon structure in heterocycles or nitrile functions. An important result is that these nanostructured carbon materials exhibits interesting textural properties with BET surface areas ranging between 1000 and . Moreover, the study of the influence of nitrogen on the textural and structural parameters of the resulting carbon materials shows that nitrogen plays an active role during the synthesis process. This observation is also supported by the speciation of nitrogen in the nanostructured carbon materials.     

Interaction between nanobuds and hydrogen molecules: A first-principles study

Hydrogen storage materials are crucial for the wide application of hydrogen in fuel cells. In this Letter, the interaction between hydrogen molecules and nanobuds has been studied using the Dmol3 package. The results show that the adsorption energies of hydrogen molecules onto nanobuds range from 0.069 eV to 0.115 eV, and that the adsorption energies are not sensitive to the nanobuds' structures but closely related to the number of carbon atoms around H₂ molecules. The energy barrier of a hydrogen molecule entering C176 is 2.38 eV. Each C176 nanobud can accommodate four H₂ molecules. The stress existing in nanobuds induces alterative charge distribution, which can improve the hydrogen storage performance of nanobuds to a certain extent.     

Biomass waste-derived activated carbon for the removal of arsenic and manganese ions from aqueous solutions

The goal of this study is to investigate the preparation of low-cost activated carbon from bean pods waste and to explore their potential application for the removal of heavy metals from aqueous solutions. Conventional physical (water vapor) activation was used for synthesizing the adsorbent. The obtained carbon was employed for the removal of As (III) and Mn (II) from aqueous solutions at different initial concentrations and pH values. Adsorption for both ions follows Langmuir-type isotherm, the maximum loading capacities for arsenic (III) and Mn (II) ions being 1.01 and 23.4 mg g⁻¹, respectively. According to the experimental data, it can be inferred that the basic character of the surface, i.e. the high content of basic groups, favors adsorption of ions. Arsenic adsorption capacity on the carbon obtained from agricultural waste was found to be similar to this of more expensive commercial carbons showing high adsorption capability. Regarding manganese adsorption, herein obtained carbon presented higher uptake adsorption than that of activated carbons reported in the literature.     

Hydrogen adsorption in microporous alkali-doped carbons (activated carbon and single wall nanotubes)

Adsorption of hydrogen gas was tested in microporous doped carbons: activated carbon (1600 m²/g) and single wall carbon nanotubes (SWNTs). The isotherms of adsorption of LiC₁₈ and KC₂₄ doped microporous activated carbons were determined in the range [0–30 bar] at room temperature and 77 K. The chemisorption ratio observed at room temperature increases with increasing the alkali/carbon rate. The isotherm profiles of doped activated carbon at 77 K show no clear enhancement of the sorption ratio compared to the raw activated carbon.The adsorption sites of potassium doped SWNTs with closed end were determined by neutron diffraction experiment using deuterium gas. The K-doped SWNTs were found only slightly intercalated by K ions so that empty cavities are preserved in between the tubes. At room temperature, the chemisorption of deuterium was not observed in doped SWNTs bundles, but only in the KC₈ graphite intercalation compound impurities. At low temperature, the isotherms analysis and neutron diffraction experiments have shown that D₂ molecules are physisorbed in the free interstitial voids in between the tubes within the bundles.     

氢分子在Na2Al6团簇上的吸附和解离性能

氢的物理和化学吸附是氢存储的基本形式，而H₂分子的解离能垒是决定可逆储氢动力学性能的重要因素.纳米团簇是研究材料储氢性能的重要物质层次，研究氢与Na-Al团簇的相互作用性质能够了解纳米尺度的Na-Al氢化物的储氢性能.本文利用密度泛函理论，计算研究了H₂分子在较小的合金团簇Na₂Al₆上的吸附与解离性能.结果表明H₂分子在Na₂Al₆团簇上是弱的物理吸附，但很容易发生解离.氢分子的解离能垒很低，解离可以在环境温度下发生，纳米结构的Na₂Al₆团簇具有良好的化学储氢性能.     

Preparation of activated carbons previously treated with sulfuric acid: A study of their adsorption capacity in solution

In the present work, cedar wood has been used as raw material for the preparation of activated carbons. The influence of a previous treatment with sulfuric acid on the textural properties of the carbonized and activated samples has been investigated. Finally, the adsorption capacity of para-nitrophenol in aqueous solution has been studied and the corresponding adsorption isotherms have been fitted to Langmuir's equation. The experimental results indicate that the previous dehydration of the raw material with sulfuric acid gives rise to an improvement in the porous texture and adsorption capacity of the activated carbons.     

Influence of activated carbon surface acidity on adsorption of heavy metal ions and aromatics from aqueous solution

Adsorption of toxic heavy metal ions and aromatic compounds onto activated carbons of various amount of surface C-O complexes were examined to study the optimum surface conditions for adsorption in aqueous phase. Cadmium(II) and zinc(II) were used as heavy metal ions, and phenol and nitrobenzene as aromatic compounds, respectively. Activated carbon was de-ashed followed by oxidation with nitric acid, and then it was stepwise out-gassed in helium flow up to 1273 K to gradually remove C-O complexes introduced by the oxidation. The oxidized activated carbon exhibited superior adsorption for heavy metal ions but poor performance for aromatic compounds. Both heavy metal ions and aromatics can be removed to much extent by the out-gassed activated carbon at 1273 K. Removing C-O complexes, the adsorption mechanisms would be switched from ion exchange to Cπ-cation interaction for the heavy metals adsorption, and from some kind of oxygen-aromatics interaction to π-π dispersion for the aromatics.     

Hydrogen storage by physisorption: beyond carbon

Hydrogen storage using physisorption requires higher desorption temperatures than those possible using conventional adsorbents such as carbon. Using computational design, we predict that several materials have extremely strong physisorption interactions with hydrogen, including 12 kJ/mol heat of adsorption for hydrogen on some sites. Experimental adsorption isotherms on one of the materials, boron oxide, confirm the calculations, and large coverage is observed at temperatures as high as the boiling point of methane, 115 K. Since these materials have sp²-like bonding, they should be amenable to the rich variety of chemical manipulations that have been used with carbon.     

Water vapor adsorption onto activated carbons prepared from cattle manure compost (CMC)

Activated carbons were prepared from cattle manure compost (CMC) using zinc chloride activation. The structural and surface chemical characteristics of CMC-based activated carbons were determined by N₂ adsorption-desorption and Boehm titration, respectively. The water vapor adsorption properties of the prepared activated carbons with various pore structure and surface nature were examined, and the mechanism of water adsorbed onto activated carbon was also discussed. The results show that the adsorption of water vapor on carbons begins at specific active sites at low relative humidity (RH), followed by micropore filling at medium RH through the formation of pentamer cluster of water molecules in the narrow micropores. The water vapor adsorption capacity of activated carbon is predominantly dependent on its pore volume and surface area. Although capillary condensation is not the mechanism for water adsorption onto activated carbon, water can adsorb on narrow mesopore to some extent.     

Guest-host interaction in energy storage systems

Charge transfer reactions are the most frequent processes met during the conversion of chemical energy into electrical one. Intercalation/insertion processes are the best examples of these phenomena. The type of interaction/binding between guest species and host material decides about the reversibility of the processes. The important drawback of the carbon anode in Li-ion cells is often its irreversible capacity. It correlates with the active surface area and it can be significantly diminished through pyrolytic carbon coating. In situ⁷Li-NMR measurement is a perfect method for monitoring the type of Li-C bonding during the insertion/deinsertion process. Supercapacitor is the second category of attractive energy storage system. The main operation of a capacitor is based on an electrostatic attraction; however, very often pseudocapacitance effects take place. Intensive research is devoted to electrochemical hydrogen storage where the type of C-hydrogen bonding is crucial for practical application of this process. The reversibility of hydrogen insertion into the carbon network is ensured by a weak chemical bonding. Carbon materials with electrosorbed hydrogen play a perfect role of negative electrode in supercapacitor. Attractive host-guest interactions take place during the performance of various supercapacitor electrode materials, e.g. nitrogen-doped carbons with modified electronic structure, layered double hydroxides, conducting polymers, etc.     

Preparation of activated carbons previously treated with hydrogen peroxide: Study of their porous texture

Cedar wood was used as raw material for the preparation of activated carbons by treatment with hydrogen peroxide of different concentrations. The samples were next carbonised and activated under CO₂ atmosphere. The activated carbons were characterised by means of the adsorption isotherms of N₂ at 77 K, as well as by applying the Density Functional Theory (DFT) method and mercury porosimetry. The experimental results corresponding to the activated samples indicate a more remarkable porous development as a consequence of the treatment with hydrogen peroxide, probably due to the elimination of surface complexes produced during the activation step. The DFT diagrams point out that the activating treatment favours the development of medium and narrow-size micropores whereas the carbonisation process leads to the development of wide micropores of size close to that corresponding to mesopores.     

Heterogeneity of activated carbons in adsorption of phenols from aqueous solutions—Comparison of experimental isotherm data and simulation predictions

Surface heterogeneity of activated carbons is usually characterized by adsorption energy distribution (AED) functions which can be estimated from the experimental adsorption isotherms by inverting integral equation. The experimental data of phenol adsorption from aqueous solution on activated carbons prepared from polyacrylonitrile (PAN) and polyethylene terephthalate (PET) have been taken from literature. AED functions for phenol adsorption, generated by application of regularization method have been verified. The Grand Canonical Monte Carlo (GCMC) simulation technique has been used as verification tool. The definitive stage of verification was comparison of experimental adsorption data and those obtained by utilization GCMC simulations. Necessary information for performing of simulations has been provided by parameters of AED functions calculated by regularization method.