Effect of in-situ boron doping on hydrogen adsorption properties of carbon nanotubes

Floating catalyst chemical vapor deposition method was used for the synthesis of boron doped carbon nanotubes (BCNTs) using ethanol, triethyl borate and ferrocene as carbon source, boron source and catalyst precursor, respectively. The synthesized BCNTs were characterized by transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy (XPS). The hydrogen adsorption activity was studied for BCNTs along with undoped single walled and multi walled carbon nanotubes. Significant enhancement in the hydrogen storage value was found in doped CNTs as compared to the other undoped CNTs. Hydrogen storage for BCNTs was found to be 2.5 wt% at 10 bar and 77 K. In-situ doped BCNTs gives higher hydrogen adsorption as compared to ex-situ doped BCNTs. The Langmuir adsorption isotherm was found to be suitable for describing the adsorption isotherm as compared with Freundlich isotherm. Maximum adsorption capacity was about 9.8 wt% at 77 K. Pseudo second order kinetics was followed by BCNTs for hydrogen adsorption.     

Temperature dependence of hydrogen adsorption properties of nickel-doped mesoporous silica

The temperature dependence of the hydrogen adsorption properties of nickel-doped mesoporous silica (MCM-41) synthesized by a direct hydrothermal method was investigated by measuring the amount of hydrogen adsorbed at pressures up to 100 kPa at 298, 373, and 473 K. Nickel-doped MCM-41 adsorbed more hydrogen than undoped MCM-41 and metallic nickel at ≈298/0, 373/0, and 473 K/0 kPa due to chemical adsorption enhanced by the highly dispersed nickel particles. Chemical adsorption increased with increasing nickel content and adsorption temperature, suggesting the presence of adsorption sites. The nickel doping also brought the spillover effect, which enhances the physical adsorption of hydrogen. The spillover effect was enhanced at high nickel contents and adsorption temperatures.     

Interpreting the hydrogen adsorption on organic groups functionalized MOF-5s by statistical physics model

The hydrogen adsorption isotherms at equilibrium on four adsorbents (MOF-5 and three modified MOF-5s named, CH₃-MOF-5, Br-MOF-5 and Cl-MOF-5) were studied using a monolayer model with four adsorption sites energies. The analytical expression of this model was developed using the grand canonical ensemble in statistical physics by taking some working hypotheses involving some physicochemical parameters which can describe the adsorption process. These parameters are: four numbers of hydrogen adsorbed molecules per site (n₁, n₂, n₃ and n₄), four receptor site densities (N_M1, N_M2, N_M3 and N_M4), four saturation adsorbed quantities (Q₁, Q₂, Q₃ and Q₄) and four adsorption energies (??₁, ??₂, ??₃ and ??₄). The evolutions of these parameters in relation with temperature were discussed to understand and interpret the adsorption process at different temperatures. Fitting results revealed that the adsorption of hydrogen on MOF-5 is an exothermic physisorption process. The adsorption surface is inhomogeneous with many site energies. The fitting of the adsorption site is achieved by an aggregate of hydrogen molecules. The adopted model expression is used to derive the thermodynamic potential functions which govern the sorption mechanism such as entropy S_a, free enthalpy of Gibbs G and internal energy E_int.     

Revealing the interfacial phenomenon of metal-hydride formation and spillover effect on C48H16 sheet by platinum metal catalyst (Pt (n=1-4)) for hydrogen storage enhancement

Hydrogen is a worldwide green energy carrier, however due its low storage capacity, it has yet to be widely used as an energy carrier. Therefore, the quantum chemical method is being employed in this investigation for better understand the hydrogen storage behaviour on Pt _(n = 1-4) cluster decorated C₄₈H₁₆ sheet. The Pt_(n = 1-4) clusters are strongly bonded on the surface of C₄₈H₁₆ sheet with binding energies of ?3.06, ?4.56, ?3.37, and ?4.03 eV respectively, while the charge transfer from Pt_(n = 1-4) to C₄₈H₁₆ leaves an empty orbital in Pt atom, which will be crucial for H₂ adsorption. Initially, the molecular hydrogen is adsorbed on Pt_(n = 1-4) decorated C₄₈H₁₆ sheet through the Kubas interaction with adsorption energies of ?0.85, ?0.66, ?0.72, and ?0.57 eV respectively, while H–H bond is elongated due to the transfer of electron from σ (HH) orbital to unfilled d orbital of the Pt atom, resulting in a Kubas metal-dihydrogen complexes. Furthermore, the dissociative hydrogen atoms adsorbed on Pt_(n = 1-4) decorated C₄₈H₁₆ sheet have adsorption energies of ?1.14 eV, ?1.02 eV, ?0.95 eV, and ?1.08 eV, which are greater than the molecular hydrogen adsorption on Pt_(n = 1-4) cluster supported C₄₈H₁₆ sheet with lower activation energy of 0.007, 0.109, 0.046, and 0.081 eV respectively. To enhance the dissociative hydrogen adsorption energy, positive and negative external electric fields are applied in the charge transfer direction. Increasing the positive electric field makes H–H bond elongation and good adsorption, whereas increasing the negative electric field results H–H bond contraction and poor adsorption. Thus, by applying a sufficient electric field, the H₂ adsorption and desorption processes are can be easily tailored.     

Synthesis,characterization and hydrogen adsorption in mixed crystals of MOF-5 and MOF-177

Mixed MOF crystals with morphology similar to that of pure MOF-5 and pure MOF-177 were synthesized using two organic solvents: dimethylformamide (DMF) and diethylformamide (DEF). The mixed crystals were characterized with XRD, SEM and TGA for their physical properties and also evaluated for their hydrogen adsorption properties. The XRD and SEM results suggest that the mixed crystals are different from pure MOF-5 and pure MOF-177. The DMF-derived mixed MOF crystals have a slightly higher specific surface area, smaller pore diameter and greater pore volume than those of the DEF-derived crystals, and seem to be a better adsorbent than the DEF-derived crystals, which was confirmed by the higher hydrogen and nitrogen adsorption capacities on the DMF-derived crystals. The hydrogen adsorption capacities on the mixed MOF crystals are lower than those of pure MOF-5 and MOF-177. It was also observed that the hydrogen diffusion time constant increases with hydrogen pressure, and the heat of hydrogen adsorption decreases with adsorbed hydrogen amount on both mixed crystals.     

Equilibrium,kinetics and enthalpy of hydrogen adsorption in MOF-177

Metal–organic framework (MOF-177) was synthesized, characterized and evaluated for hydrogen adsorption as a potential adsorbent for hydrogen storage. The hydrogen adsorption equilibrium and kinetic data were measured in a volumetric unit at low pressure and in a magnetic suspension balance at hydrogen pressure up to 100 bar. The MOF-177 adsorbent was characterized with nitrogen adsorption for pore textural properties, scanning electron microscopy for morphology and particle size, and X-ray powder diffraction for phase structure. The MOF-177 synthesized in this work was found to have a uniform pore size distribution with median pore size of 12.7 Å, a higher specific surface area (Langmuir: 5994 m²/g; BET: 3275 m²/g), and a higher hydrogen adsorption capacity (11.0 wt.% excess adsorption, 19.67 wt.% absolute adsorption) than previously reported values on MOF-177. Freundlich equation fits well the hydrogen adsorption isotherms at low and high pressures. Diffusivity and isosteric heat of hydrogen adsorption were estimated from the hydrogen adsorption kinetics and equilibrium data measured in this work.     

Static and dynamic studies of hydrogen adsorption on nanoporous carbon gels

Although hydrogen is considered to be one of the most promising green fuels, its efficient and safe storage and use still raise several technological challenges. Physisorption in porous materials may offer an attractive means of $H_2$ storage, but the state-of-the-art capacity of these kinds of systems is still limited. To overcome the present drawbacks a deeper understanding of the adsorption and surface diffusion mechanism is required along with new types of adsorbents developed and/or optimised for this purpose. In the present study we compare the hydrogen adsorption behaviour of three carbon gels exhibiting different porosity and/or surface chemistry. In addition to standard adsorption characterisation techniques, neutron spin-echo spectroscopy (NSE) has been also applied to explore the surface mobility of the adsorbed hydrogen. Our results reveal that both the porosity and surface chemistry of the adsorbent play a significant role in the adsorption of $H_2$ in these systems.     

Ab initio and periodic DFT investigation of hydrogen storage on light metal-decorated MOF-5

The effect of light metal (M = Li, Be, Mg, and Al) decoration on the stability of metal organic framework MOF-5 and its hydrogen adsorption is investigated by ab initio and periodic density functional theory (DFT) calculations by employing models of the form BDC:M₂:nH₂ and MOF-5:M₂:nH₂, where BDC stands for the benzenedicarboxylate organic linker and MOF-5 represents the primitive unit cell. The suitability of the periodic DFT method employing the GGA-PBE functional is tested against MP2/6-311 + G* and MP2/cc-pVTZ molecular calculations. A correlation between the charge transfer and interaction energies is revealed. The metal-MOF-5 interactions are analyzed using the frontier molecular orbital approach. Difference charge density plots show that H₂ molecules get polarized due to the charge generated on the metal atom adsorbed over the BDC linker, resulting in electrostatic guest-host interactions.Our solid state results show that amongst the four metal atoms, Mg and Be decoration does not stabilize the MOF-5 to any significant extent. Li and Al decoration strengthened the H₂-MOF-5 interactions relative to the pure MOF-5 exhibited by the enhanced binding energies. The hydrogen binding energies for the Li- and Al-decorated MOF-5 were found to be sensible for allowing reversible hydrogen storage at ambient temperatures. A high hydrogen uptake of 4.3 wt.% and 3.9 wt.% is also predicted for the Li- and Al-decorated MOF-5, respectively.     

Performances comparison of adsorption hydrogen storage tanks at a wide temperature and pressure zone

Large-scale application of hydrogen requires safe, reliable and efficient storage technologies. Among the existing hydrogen storage technologies, cryo-compressed hydrogen (CcH₂) storage has the advantages of high hydrogen storage density, low energy consumption and no ortho-para hydrogen conversion. But it still needs higher hydrogen storage pressure when reaching higher hydrogen storage density. In order to reduce hydrogen storage pressure and improve storage density, solid adsorption technology is introduced in CcH₂. Activated carbon and metal-organic framework materials (MOFs) are employed as adsorbents in this paper. The gravimetric/volumetric hydrogen storage capacities of different adsorption tanks are studied and compared with the hydrogen storage conditions of 1–55 MPa at 77–298 K. The results show that the hydrogen storage density of CcH₂ combined with adsorption is higher than that of pure adsorption hydrogen storage, and the storage pressure is lower than that of pure CcH₂ under the same hydrogen storage capacity. And the combination of two hydrogen storage technologies can achieve a high hydrogen storage capacity equivalent to that of liquid hydrogen at a lower pressure.     

A model study of effect of M = Li,Na, Be,Mg, and Al ion decoration on hydrogen adsorption of metal-organic framework-5

The effect of light metal ion decoration of the organic linker in metal-organic framework MOF-5 on its hydrogen adsorption with respect to its hydrogen binding energy (ΔB.E.) and gravimetric storage capacity is examined theoretically by employing models of the form MC₆H₆:nH₂ where M = Li⁺, Na⁺, Be²⁺, Mg²⁺, and Al³⁺. A systematic investigation of the suitability of DFT functionals for studying such systems is also carried out. Our results show that the interaction energy (ΔE) of the metal ion M with the benzene ring, ΔB.E., and charge transfer (Q_trans) from the metal to benzene ring exhibit the same increasing order: Na⁺ < Li⁺ < Mg²⁺ < Be²⁺ < Al³⁺. Organic linker decoration with the above metal ions strengthened H₂-MOF-5 interactions relative to its pure state. However, amongst these ions only Mg²⁺ ion resulted in ΔB.E. magnitudes that were optimal for allowing room temperature hydrogen storage applications of MOF-5. A much higher gravimetric storage capacity (6.15 wt.% H₂) is also predicted for Mg²⁺-decorated MOF-5 as compared to both pure MOF-5 and Li⁺-decorated MOF-5.     

Effects of structural modifications on the hydrogen storage capacity of MOF-5

Here, we describe the preparation of four structurally modified MOF-5s and carried out a systematic study of the effects of the structural modifications on the evolution of the crystal structure, pore characteristics, and H₂ capacities of MOF-5s. The structural modifications were found to significantly influence the pore characteristics, and the specific surface areas of the MOF-5s decreased with the evolution of an ultrafine porosity. These changes were correlated with an increase in the H₂ storage capacity of the MOF-5 (from 1.2 to 2.0 wt% at −196 °C and 1 bar). The structural modifications also enhanced the thermal stability of the MOF-5s (the decomposition temperature increased from 438 °C to 510 °C). These results are particularly useful for the design of favorable MOF-based adsorbents with a high H₂ uptake coupled with a high thermal stability.     

The adsorption of H2 on Fe-coated C5H5 and its application in hydrogen storage

Based on density functional theory, the capacities of FeC₅H₅, Fe₂C₅H₅ and one-dimensional (FeC₅H₅)_∞ nanowire as hydrogen storage media were investigated. The results show that FeC₅H₅ and Fe₂C₅H₅ can adsorb five and ten H₂ molecules, respectively, and form stable FeC₅H₅(H₂)₅ and Fe₂C₅H₅(H₂)₁₀ systems. The hydrogen storage capacities of the two systems are 7.63 wt% and 10.15 wt%, while the average adsorption energies are 0.49 and 0.73 eV/H², indicating that FeC₅H₅ and Fe₂C₅H₅ are excellent hydrogen storage media. In addition, (FeC₅H₅)_∞ nanowire can also adsorb H₂ molecules (1.62 wt%). Most importantly, the magnetic and electrical properties of the nanowire are sensitive to the additional H₂, thus (FeC₅H₅)_∞ can be used for selecting and detecting H₂ molecules.     

Determination of the equilibrium constant and standard free energy of the over-potentially deposited hydrogen for the cathodic H2 evolution reaction at the Pt–Rh alloy electrode interface using the phase-shift method

The Langmuir adsorption isotherm of the over-potentially deposited hydrogen (OPD H) for the cathodic H₂ evolution reaction (HER) at the Pt–Rh (Pt:Rh; 80:20 wt%) alloy/0.5 MH₂SO₄ aqueous electrolyte interface has been studied using cyclic voltammetric and ac impedance techniques. The behavior of the phase shift (0°−φ90°) for the optimum intermediate frequency can be linearly related to that of the fractional surface coverage (1θ0) of the OPD H for the cathodic HER at the interface. The phase-shift profile (−φ vs. E) for the optimum intermediate frequency, i.e., the phase-shift method, can be used as a new electrochemical method to determine the Langmuir adsorption isotherm (θ vs. E) of the OPD H for the cathodic HER at the interface. At the Pt–Rh alloy electrode interface, the equilibrium constant (K) and the standard free energy (ΔG_ads) of the OPD H are 2.2×10⁻⁴ and 20.9 kJ/mol, respectively. At the steady state, the behaviors of the cyclic voltammogram and the Langmuir adsorption isotherm of the OPD H for the cathodic HER at the Pt–Rh alloy electrode interface are similar to those of the pure Pt electrode interfaces. At the steady state, the effect of Rh on the OPD H for the cathodic HER can be neglected at the Pt–Rh (Pt:Rh; 80:20 wt%) alloy/0.5 MH₂SO₄ aqueous electrolyte interface.     

Nanoscale cobalt doped carbon aerogel: microstructure and isosteric heat of hydrogen adsorption

The incorporation of nanoscale Co particles (with sizes from a few nanometres) into porous carbon aerogels (CAs) was investigated. Elemental maps of the nanoscale metal particles embedded within CA were obtained using energy filtered transmission electron microscopy. The microstructure of Co doped carbon aerogels was further investigated using small angle X-ray scattering and nitrogen adsorption at 77 K. The isosteric heat of adsorption (Qst) was investigated as a function of hydrogen uptake at temperatures from 77 K to 110 K over the pressure range of 0-0.25 MPa. The isosteric heat of adsorption at low H₂ concentration for Co doped CA (9.0 kJ mol⁻¹) was found to be higher than for pure CA (5.8 kJ mol⁻¹).     

Effect of Zn/Co ratio in MOF-74 type materials containing exposed metal sites on their hydrogen adsorption behaviour and on their band gap energy

Simultaneous incorporation of Zn and Co ions into MOF-74 during the crystallization process has been studied, covering the whole Zn/Co concentration range (0-100% Co). The characterization techniques used, including X-ray diffraction, DR-UV-visible spectroscopy, N₂ adsorption isotherms and thermogravimetric analysis, strongly evidence the successful incorporation of both cations into the material framework, producing the crystallization of MOF-74 with 100% Co and MOF-74 with 100% Zn starting from Zn-free and Co-free initial mixtures, respectively, under the same conditions. H₂, CH₄ and CO₂ uptakes of MOF-74 type materials generally increase with framework Co content at any pressure, suggesting a relevant role of cobalt in the adsorption process. As expected, the comparison between the gas adsorption behavior of Co-substituted MOF-5 and Co-containing MOF-74 materials shows that exposed metal sites play a key role in MOF performance in adsorption. On the other hand, the good agreement between the evolution of the experimental band gap values as a function of the Co content and the computational-based predictions found in the literature further supports the coexistence of Zn²⁺ and Co²⁺ ions forming the MOF-74 metal clusters. Finally, we found that variations of both isosteric heat of hydrogen adsorption and band gap energy with the metal cluster composition show a parallel trend, although it is not systematic in the whole range of Zn/Co ratio. Indeed, a minimum band gap energy value and a maximum isosteric heat of H₂ adsorption value were found to appear simultaneously for Co-rich samples still having some Zn rather than for all-Co samples.     

Characterization and hydrogen storage properties of SnO2 functionalized MWCNT nanocomposites

Structural, spectral, morphological, thermal and hydrogen storage properties of the multi-walled carbon nanotubes functionalized with SnO₂ particles (MWCNT/SnO₂) heat treated at 300, 350 and 400 °C in air were systematically investigated. X-ray diffraction from (110), (101) and (211) planes of SnO₂, (002) plane of MWCNT and shift towards higher angle confirmed the formation of composites. XPS, Raman, FTIR and TGA analyses revealed that C–O bond on the surface of MWCNT acts as the nucleation sites for SnO₂ which resulted in a strong interaction between MWCNT and SnO₂. Presence of C, Sn and O was confirmed by EDX and XPS analyses. Hydrogen adsorption was carried out using hydrogenation set-up and H₂ adsorption/desorption behavior of the composites were studied employing Raman and thermogravimetric analyses. Size, morphology and interaction between MWCNT and SnO₂ were impacted significantly by the heat treatment which resulted in high hydrogen storage capacity of 2.13 and 2.62 wt % for 15 and 30 min hydrogenation time for the nanocomposite heat treated at 400 °C.     

The theoretical study of the bimetallic Ni/Pd,Ni/Pt and Pt/Pd catalysts for hydrogen spillover on penta-graphene

The catalytic property of the bimetallic Ni/Pd, Ni/Pt and Pt/Pd particles for hydrogen spillover on penta-graphene (PG) are studied by using the first-principles and kinetic Monte Carlo (KMC) calculations. The bimetallic Ni/Pd, Ni/Pt and Pt/Pd particles can be stably decorated on PG surface with binding energies in the range of 4.15–5.52 eV. The adsorption enthalpies of H₂ molecules on bimetallic particles are in the range of ?11.56–?15.35 kcal/mol. The H atom can migrate from the bimetallic particles to PG with the migration barriers range from 0.67 to 0.95 eV. The KMC simulations show that the hydrogen spillover reactions can occur at a suitable temperature (260–361 K), which meet DOE target for onboard hydrogen storage systems applied to light-duty vehicles. In the study, the highest occupied molecular orbital and electric field analysis shows that the bimetal mixing can reduce the hydrogen adsorption enthalpy, and thereby reduce the H migration barrier, which displays a synergistic effect for hydrogen spillover.     

Effect of platinum doping of activated carbon on hydrogen storage behaviors of metal-organic frameworks-5

In this work, we prepared platinum doped on activated carbons/metal-organic frameworks-5 hybrid composites (Pt-ACs-MOF-5) to obtain a high hydrogen storage capacity. The surface functional groups and surface charges were confirmed by Fourier transfer infrared spectroscopy (FT-IR) and zeta-potential measurement, respectively. The microstructures were characterized by X-ray diffraction (XRD). The sizes and morphological structures were also evaluated using a scanning electron microscopy (SEM). The pore structure and specific surface area were analyzed by N₂/77 K adsorption/desorption isotherms. The hydrogen storage capacity was studied by BEL-HP at 298 K and 100 bar. The results revealed that the hydrogen storage capacity of the Pt-ACs-MOF-5 was 2.3 wt.% at 298 K and 100 bar, which is remarkably enhanced by a factor of above five times and above three times compared with raw ACs and MOF-5, respectively. In conclusion, it was confirmed that Pt particles played a major role in improving the hydrogen storage capacity; MOF-5 would be a significantly encouraging material for a hydrogen storage medium as a receptor.     

Modeling and interpretations by the statistical physics formalism of hydrogen adsorption isotherm on LaNi4.75Fe0.25

Three theoretical expressions for the adsorption isotherms of hydrogen on LaNi_4.75Fe_0.25 alloy at 303 K and 313 K have been established. Our objective in this modeling is to select the adequate model that presents a high correlation with the experimental curves. The establishment of these new expressions is based on statistical physics formalism. This method has allowed the estimation of physicochemical parameters in the theoretical model. The parameters intervening in the adsorption process have been deduced directly from experimental adsorption isotherms by numerical simulation. We will mainly introduce four parameters affecting the adsorption process, namely; the density of hydrogen receptor sites N_M, the number of molecules per site and the hydrogen adsorption energy. Then we apply the model to calculate thermodynamics functions which govern the adsorption mechanism such as entropy, free enthalpy and internal energy.     

Modeling hydrogen adsorption in the metal organic framework (MOF-5, connector): Zn4O(C8H4O4)3

Density functional theory investigation is performed to understand the underlying mechanism of hydrogen adsorption in the MOF-5 by using for first time the connector structure. The analysis of chemical bonds of the connector's atoms shows a good agreement between experimental and theoretical results. In particular, we show that this material has a desorption temperature of 115 K and an initial hydrogen storage capacity around 1.57 wt% which are close to the experimental values. We consider the coupling-energy mechanism to explore the most stable configurations in multiple adsorption sites namely metallic, carboxylic and cyclic sites. Three orientations which are vertical, horizontal and sloping are taking into account. The results show that the metallic and cyclic sites are more stable for multiple hydrogen molecule storage and the system reaches 4.57 wt% as a gravimetric storage capacity which is located in the interval 4.50–5.20 wt% found experimentally. In addition, the desorption temperature is improved significantly.