Hydrogen storage on chemically activated carbons and carbon nanomaterials at high pressures

Hydrogen adsorption measurements have been carried out at different temperatures (298 K and 77 K) and high pressure on a series of chemically activated carbons with a wide range of porosities and also on other types of carbon materials, such as activated carbon fibers, carbon nanotubes and carbon nanofibers. This paper provides a useful interpretation of hydrogen adsorption data according to the porosity of the materials and to the adsorption conditions, using the fundamentals of adsorption. At 298 K, the hydrogen adsorption capacity depends on both the micropore volume and the micropore size distribution. Values of hydrogen adsorption capacities at 298 K of 1.2 wt.% and 2.7 wt.% have been obtained at 20 MPa and 50 MPa, respectively, for a chemically activated carbon. At 77 K, hydrogen adsorption depends on the surface area and the total micropore volume of the activated carbon. Hydrogen adsorption capacity of 5.6 wt.% at 4 MPa and 77 K have been reached by a chemically activated carbon. The total hydrogen storage on the best activated carbon at 298 K is 16.7 g H₂/l and 37.2 g H₂/l at 20 MPa and 50 MPa, respectively (which correspond to 3.2 wt.% and 6.8 wt.%, excluding the tank weight) and 38.8 g H₂/l at 77 K and 4 MPa (8 wt.% excluding the tank weight).     

Synthesis of graphene-like nanosheets and their hydrogen adsorption capacity

Graphene-like nanosheets have been synthesized by the reduction of a colloidal suspension of exfoliated graphite oxide. The morphology and structure of the graphene powder sample was studied using scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Raman spectroscopy. The graphene sheets are found to be in a highly agglomerated state, with many wrinkles. The sample has a BET surface area of 640 m²/g as measured by nitrogen adsorption at 77 K. Hydrogen adsorption-desorption isotherms were measured in the temperature range 77-298 K and at pressures of up to 10 bar. This gives hydrogen adsorption capacities of about 1.2 wt.% and 0.1 wt.% at 77 K and 298 K, respectively. The isosteric heat of adsorption is in the range of 5.9-4 kJ/mol, indicating a favourable interaction between hydrogen and surface of the graphene sheets. The estimated room temperature H₂ uptake capacity of 0.72 wt.% at 100 bar and the isosteric heat of adsorption of our sample are comparable to those of high surface area activated carbons, however significantly better than the recently reported values for graphene and a range of other carbon and nanoporous materials; single and multi walled carbon nanotubes, nanofibers, graphites and zeolites.     

Influence of sample cell physisorption on measurements of hydrogen storage of carbon materials using a Sieverts apparatus

In recent years, large fluctuations have been reported for measurements of the hydrogen storage of carbon materials using a Sieverts apparatus. To investigate this problem, helium gas adsorption was selected for comparison with the adsorption of hydrogen, and the results show that hydrogen but not helium was adsorbed onto the wall of the sample cell at ambient temperature. The adsorption capacity of the sample cell at 77 K is higher than that at ambient temperature. A series of adsorption tests was conducted with a LaNi₅ alloy to prove the influence of the physisorption, and the results show that an increase in the hydrogen storage capacity was resulted in when sample loading decreases. After correction for this hydrogen physisorption, the capacity was restricted between 1.38 and 1.41 wt.%. Multi-walled carbon nanotubes (MWCNTs), activated carbon (AC), single-walled carbon nanotubes (SWCNTs), graphite nanofibers (GNFs), and graphite oxide (GO) were also measured and corrected through this method.     

Activation, characterization and hydrogen storage properties of the mesoporous carbon CMK-3

Activation of mesoporous carbon CMK-3 with CO₂ for hydrogen storage was studied. Huge structure and texture changes emerged for the activated CMK-3 based on the characterization by using XRD, TEM and nitrogen adsorption at 77 K. The ordered mesoporous structure of CMK-3 gradually became disorder and its specific surface area and volume of pores especially micropores were enhanced remarkably. Hydrogen sorption measurement showed that the activation led to an obvious increase of the H₂ sorption capacity of CMK-3. The maximum H₂ uptake of 2.27 wt% at 77 K and 1 bar was obtained for the sample activated at 1223 K for 8 h. The small pores with the diameter smaller than 1 nm contributed greatly to the H₂ uptake, and were confirmed more effective than other pores for hydrogen storage.     

High hydrogen uptake capacity of mesoporous nitrogen-doped carbons activated using potassium hydroxide

Mesoporous nitrogen-doped carbons activated using potassium hydroxide have large surface areas and high pore volumes, are studied for their hydrogen storage properties. The activated material can store 6.84 wt.% of hydrogen at 77 K under a pressure of 20 bar, which is estimated to correspond to a maximum capacity of 8.24 wt.% based on the Langmuir simulation. The hydrogen adsorption values show linear relationships with the volumes of micropores and small mesopores under those conditions, which indicates that the small mesopores between the sizes 2 and 3 nm also make contributions to the hydrogen adsorption at high pressure.     

High performance of nanoporous carbon in cryogenic hydrogen storage and electrochemical capacitance

Nanoporous carbon materials were synthesized by a two-step casting process using zeolite 13X as template. The nanoporous structures were characterized by X-ray diffraction, high resolution transmission electron microcopy and nitrogen sorption at 77 K, and the results show that pore filling in the zeolite channels could play an important role in the replication of zeolite-like structural order. Better pore filling led to a more ordered structure as well as higher surface area and pore volume. Further potassium hydroxide (KOH) activation improved the microporous texture to the carbon framework and resulted in higher surface area and pore volume. A large hydrogen uptake capacity of 6.30 wt.% has been achieved at 77 K and 20 bar. Besides, a high gravimetric capacitance of up to 160 F g⁻¹ and an energy density of 30 W h kg⁻¹ have been obtained when tested as an electrode for supercapacitors. The high performance in cryogenic hydrogen storage and electrochemical capacitance were closely correlated with the pore structures of the carbon materials.     

Experimental measurements and computer simulation of methane adsorption on activated carbon fibers

Three activated carbon fibers (ACFs) with different BET specific surface areas (SSAs) were prepared. Experimental characterization and methane adsorption on the ACFs were measured by the intelligent gravimetric analyzer (IGA-003, Hiden) at 258 and 298 K. Correlations proposed between the methane adsorption capacity and SSA indicate that the SSA plays an important role on storage amount at a given temperature. A detailed experimental investigation was focused on the sample ACF3 of the highest SSF of 1511 m²/g at five temperatures, from 258 to 298 K. The temperature dependence for methane adsorption amount on ACF3 at 1.8 MPa is proposed. It shows that temperature is vital to methane storage capacity for ACF3, and adsorption storage at the temperatures below 280 K is recommended for favorite uptakes. To model ACF3, the pores are described as slit-shaped with a pore size distribution that was determined by molecular simulation and the statistics integral equation. Predictions of methane adsorption, carried out at 258 and 298 K and high pressures by molecular simulation, indicate that our sample ACF3 can reach the uptake of 14.99 wt% at 4.0 MPa and 298 K, which is comparable with the best result in the literature.     

Synthesis, characterization and energy-related applications of carbide-derived carbons obtained by the chlorination of boron carbide

A nanoporous carbide-derived carbon (CDC) was synthesized by chlorination of boron carbide powder using hydrogen chloride as the reactive gas. The structure and texture of the CDCs were characterized by X-ray diffraction, high-resolution transmission electron microcopy and nitrogen adsorption at 77 K, which confirmed a structural and textural dependence on chlorination temperature and reaction time. The CDC technique to produce porous carbons is very attractive because it can obtain carbons with desired structure and porosity and the CDCs produced here show great potential for energy-related applications. Used as hydrogen storage materials, the hydrogen uptake capacity could reach 1.06 wt.% at 77K and 1 bar. When tested as electrodes for supercapacitors, specific surface capacitance value up to 0.403 F m⁻² and a capacitance retention ratio up to 86% (at a voltage scan rate of 50 mV s⁻¹) could be obtained.     

An accurate volumetric differential pressure method for the determination of hydrogen storage capacity at high pressures in carbon materials

Widely different hydrogen adsorption capacities have been reported for a variety of carbon materials which have attracted attention for hydrogen storage. This has led to doubts as to the validity of some of the claims and it has been suggested that one possible reason for the disparate hydrogen sorption capacities may lie in the inaccurate measurement of the hydrogen adsorbed. The aim of the work described in this paper was to make a contribution to this debate by developing a means and method of producing repeatable, accurate measurements of hydrogen sorption capacity in carbon materials. The apparatus developed is a volumetric differential pressure set-up operating at up to 10 MPa and the method has a conservative limit of detection of 0.1 wt% and an accuracy of ±0.05 wt%, using 1.0-2.5 g samples of the carbon materials studied. These included a carbon nanofiber sample and a series of activated carbons, the latter displaying a direct correlation between the BET effective surface area and the hydrogen sorption capacity of the materials. The amount of hydrogen adsorbed was less than 1 wt% for all the carbons examined.     

Experimental evidence of an upper limit for hydrogen storage at 77 K on activated carbons

An upper limit for hydrogen storage at 77 K on activated carbons was clearly observed in the present experimental work. Such a limit is around 6.4 wt.%, i.e., close to the theoretical limit of 6.8 wt.%. Results of hydrogen storage were obtained in three independent laboratories using volumetric and gravimetric devices. Lab-made activated carbons (ACs) were found to have higher capacities than those of the commercial material AX-21. A maximum excess hydrogen storage capacity of 6.0 wt.% at 77 K and 4 MPa was obtained. This maximum was reduced to 0.6 wt.% at 298 K and 5 MPa. ACs with surface areas (S_BET) as high as 3220 m² g⁻¹ were prepared from chemical activation of anthracites with alkali (Na and K) hydroxides. At 77 K and 4 MPa, excess hydrogen storage capacity was directly correlated with S_BET for ACs having S_BET values lower than 2630 m²/g. Hydrogen uptake at 77 K also correlated with micropore volume and strongly depended on average pore diameter.     

The optimum average nanopore size for hydrogen storage in carbon nanoporous materials

A thermodynamical model of hydrogen storage in slitpores is presented and applied to carbon and BN nanoporous materials. The model accounts for the quantum effects of the molecules in the confining potential of the slitpores. A feature of the model is a new equation of state (EOS) of hydrogen, valid over a range of pressures wider than any other known EOS, obtained using experimental data in the range 77-300 K and 0-1000 MPa, including data in the region of solid hydrogen. The model reproduces the experimental hydrogen storage properties of different samples of activated carbons and carbide-derived carbons at 77 and 298 K and at pressures between 0 and 20 MPa, for an average nanopore width of about 5 Å. The model predicts that in order to reach the US Department of Energy hydrogen storage targets for 2010, the nanopore widths should be equal to or larger than 5.6 Å for applications at low temperatures, 77 K, and any pressure, and about 6 Å for applications at 300 K and at least 10 MPa.     

Numerical estimation of hydrogen storage limits in carbon-based nanospaces

Theoretical limits of the hydrogen adsorption in carbon nanospaces are modeled using Monte Carlo simulations. A detailed analysis of storage capacity of slit pores has been performed as a function of the pore size, gas pressure (up to 100 bars) and temperature of adsorption (77 and 298 K). The H₂-slit wall interaction has been modeled assuming energies of adsorption ranging from 4.5 kJ/mol (pure graphene surface) to 15 kJ/mol (hypothetical chemically modified graphene). The quantum nature of H₂ has been incorporated in the calculations using the Feynman-Hibbs approach. It has been shown that in a hypothetical chemically modified porous carbon, with energy of adsorption of 15 kJ/mol or higher and pore size between 0.8 and 1.1 nm, the gravimetric and volumetric storage capacity can achieve targets required for practical applications. The relation between the energy of adsorption and the effective delivery has been discussed.     

Hydrogen storage properties of Pd nanoparticle/carbon template composites

Theoretical studies predict improved hydrogenation properties for hybrid carbon/metal composites. The hydrogen storage capacity of ordered porous carbon containing Pd clusters was measured. The C/Pd composite was obtained by chemical impregnation of an ordered porous carbon template (CT) with a H₂PdCl₄ solution followed by a reduction treatment. 10 wt.% of palladium clusters were introduced in the carbon porosity; the Pd clusters (2 nm in size) being homogeneously distributed. Thermodynamic hydrogenation properties of both Pd-free CT and the Pd-10 wt.% CT composite have been determined by hydrogen isotherm sorption measurements and thermal desorption spectroscopy (TDS) analysis. The introduction of the palladium into the carbon matrix does not increase the hydrogen storage capacity at 77 K and 1.6 MPa, since here the hydrogen uptake is being attributed to physisorption on the carbon. However, at room temperature and moderate pressure (0.5 MPa), the filling of the CT with 10 wt.% nanocrystalline Pd results in an hydrogen uptake eight times larger than that of the Pd-free CT. After the second cycle, a good reversibility is observed. TDS measurements confirm that the sharp increase of the hydrogen uptake is due to the presence of the Pd clusters in the carbon porosity.     

Feasibility of tailoring for high isosteric heat to improve effectiveness of hydrogen storage in carbons

The idea that increasing the enthalpy of adsorption increases the adsorptive capacity of carbon and makes it a better storage material for hydrogen is examined here considering the entire adsorption-desorption cycle. Structural modifications of carbon are examined to reveal the complex relationships between the enthalpy of adsorption, the pore volume, and the amount of hydrogen delivered over the course of a single cycle. The results provide an understanding of the connection between enthalpy and effective storage capacity in carbon materials and serve as a guide toward the search for an adsorbent which satisfies the DOE targets. Extensive GCMC simulations show that carbons having single graphene walls are optimal for hydrogen storage and that attempts to increase the enthalpy of adsorption either by increasing the wall thickness or by decreasing the pore size are detrimental to adsorptive capacity over a complete cycle from charging to exhaustion. It is found that carbon nanotubes display the same trend as slit pore carbons. The search for an adsorbent suitable for hydrogen storage should be aimed at the discovery of an entirely new high-capacity adsorbent with an enthalpy of adsorption of 15 kJ/mol, intermediate between that of carbon (4-6 kJ/mol) and metal hydrides (30-75 kJ/mol).     

Carbon aerogels from acetic acid catalysed resorcinol-furfural using supercritical drying for hydrogen storage

Organic aerogels were derived from acetic acid catalysed resorcinol and furfural and then dried directly in supercritical carbon dioxide without the use of a solvent exchange process. These aerogels were further carbonised in nitrogen and activated in CO₂ in order to obtain their corresponding carbon aerogels. The carbon aerogels prepared by this method had a greater proportion of micropores in addition to a much shorter preparation time (on the order of days) than those prepared by other studies. The effect of different drying techniques on the microstructure of the wet gels was investigated by nitrogen adsorption at cryogenic liquid nitrogen temperature. Nitrogen adsorption at 77 K allowed the determination of surface areas and pore volumes, further analysed by the Dubinin-Radushkevich model and density functional theory model. The surface area and micropore volume of carbon aerogels prepared by this method increased by 19% and 12%, and accordingly, hydrogen uptake capacity was increased by 10% from 4.9 ± 0.2 wt.% to 5.4 ± 0.3 wt.% at 4.6 MPa and 77 K.     

High surface area carbide-derived carbon fibers produced by electrospinning of polycarbosilane precursors

Highly porous carbide-derived carbon fibers have been synthesized by electrospinning of polycarbosilane with subsequent pyrolysis and chlorination. The resulting ultrathin fibers show specific surface areas up to 3116 m² g⁻¹ and very high storage capacities for hydrogen up to 3.86 wt.% at 17 bar and 77 K. Due to the outstanding adsorption performance and other properties such as high temperature stability and the unique CDC fiber shape, this new kind of fiber material offers promising possibilities for several applications like air or liquid filters or textiles for protective clothing. Application as a flexible electrode material for supercapacitors is conceivable.     

Influence of oxidation process on the adsorption capacity of activated carbons from lignocellulosic precursors

A set of activated carbon materials non-oxidised and oxidised, were successfully prepared from two different lignocellulosic precursors, almond shell and vine shoot, by physical activation with carbon dioxide and posterior oxidation with nitric acid. All samples were characterised in relation to their structural properties and chemical composition, by different techniques, namely nitrogen adsorption at 77 K, elemental analysis (C, H, N, O and S), point of zero charge (PZC) and FTIR. A judicious choice was made to obtain carbon materials with similar structural properties (apparent BET surface area ∼ 850-950 m²g⁻¹, micropore volume ∼ 0.4 cm³g⁻¹, mean pore width ∼ 1.2 nm and external surface area ∼ 14-26 m²g⁻¹). After their characterisation, these microporous activated carbons were also tested for the adsorption of phenolic compounds (p-nitrophenol and phenol) in the liquid phase at room temperature. The performance in liquid phase was correlated with their structural and chemical properties. The oxidation had a major impact at a chemical level but only a moderate modification of the porous structure of the samples. The Langmuir and Freundlich equations were applied to the experimental adsorption isotherms of phenolic compounds with good agreement for the different estimated parameters.     

GCMC-surface area of carbonaceous materials with N2 and Ar adsorption as an alternative to the classical BET method

In this paper we apply a new method for the determination of surface area of carbonaceous materials, using the local surface excess isotherms obtained from the Grand Canonical Monte Carlo simulation and a concept of area distribution in terms of energy well-depth of solid-fluid interaction. The range of this well-depth considered in our GCMC simulation is from 10 to 100 K, which is wide enough to cover all carbon surfaces that we dealt with (for comparison, the well-depth for perfect graphite surface is about 58 K). Having the set of local surface excess isotherms and the differential area distribution, the overall adsorption isotherm can be obtained in an integral form. Thus, given the experimental data of nitrogen or argon adsorption on a carbon material, the differential area distribution can be obtained from the inversion process, using the regularization method. The total surface area is then obtained as the area of this distribution. We test this approach with a number of data in the literature, and compare our GCMC-surface area with that obtained from the classical BET method. In general, we find that the difference between these two surface areas is about 10%, indicating the need to reliably determine the surface area with a very consistent method. We, therefore, suggest the approach of this paper as an alternative to the BET method because of the long-recognized unrealistic assumptions used in the BET theory. Beside the surface area obtained by this method, it also provides information about the differential area distribution versus the well-depth. This information could be used as a microscopic finger-print of the carbon surface. It is expected that samples prepared from different precursors and different activation conditions will have distinct finger-prints. We illustrate this with Cabot BP120, 280 and 460 samples, and the differential area distributions obtained from the adsorption of argon at 77 K and nitrogen also at 77 K have exactly the same patterns, suggesting the characteristics of this carbon.     

Intrinsic linear scaling of hydrogen storage capacity of carbon nanotubes with the specific surface area

The linear scaling of the gravimetric hydrogen storage capacity of single- and multi-walled carbon nanotubes (SWNTs and MWNTs) with the specific surface area is investigated at ambient temperature (298 K) and technically relevant pressures (0.9–1.6 MPa). All samples are found to adsorb hydrogen reversibly and their adsorption exhibits type-II BET isotherms according to the IUPAC classification. While there is strong sample-dependency on their pressure–composition isotherms, all of them follow the Henry's Law in the pressure range under consideration. A comparison of the observed slope of specific surface area versus gravimetric storage capacity with that of a theoretically predicted one using a hypothetical condensation model and that of chemically modified carbon nanotubes revealed that the hydrogen storage capacity depends on the accessibility of internal surfaces of nanostructured carbon. The linear scaling of hydrogen storage capacity with the respective specific surface area suggests that the hydrogen adsorption in carbon nanotubes depends on the specific surface area and is irrespective of the type of the nanotubes that is used.     

Synthesis of carbon nanofibers and measurements of hydrogen storage

The synthesis of carbon nanofibers was carried out by catalytic decomposition of ethylene in presence of hydrogen. Bimetallic catalysts, e.g. Fe-Cu or Ni-Cu, were synthesized by coprecipitation, reduction-precipitation and reverse microemulsion techniques and were proven to have a strong influence on the morphology of the nanofibers. The best results in terms of synthesis homogeneity were obtained by supporting the bimetallic catalyst on a high surface area silica support by the “incipient wetness” method. The hydrogen storage capacity of carbon nanofibers was tested in a custom made Sievert apparatus operating up to 160 bar and 450 °C. Several “in situ” activation procedures were experimented, however according to our data carbon nanofibers do not seem a suitable candidate for hydrogen storage. With the purpose of promoting a “spillover” function, 2 wt.% Pd-doped nanofibers were prepared. After loading at 77 bar, a hydrogen storage of 1.38 ± 0.30 wt.% was measured at room temperature.