A fast procedure for the estimation of the hydrogen storage capacity by cryoadsorption of metal-organic framework materials from their available porous properties

In order to identify the best porous materials for the cryogenic physisorption of hydrogen, high-throughput calculations are performed starting, i.e., from the collected information in crystallographic databases. However, these calculations, like molecular simulations, require specific training and significant computational cost. Herein, a relatively simple procedure is proposed to estimate and compare hydrogen uptakes at 77 K and pressure values from 40 bar starting from the porous properties of MOF materials, without involving simulation tools. This procedure uses definitions for adsorption and considers the adsorbed phase as an incompressible fluid whose pressure-density change is that for the liquid phase at 19 K. For the 7000 structures from the CoRE MOF database, the average error of the predictions is only of 1% from reference values at 100 bar, with an SD of ±8%. This accuracy is lower than that from simulation tools, but involving lower computational cost and training.     

Ab initio study of hydrogen adsorption on Zn2(NDC)2(diPyTz) metal-organic framework decorated with alkali and alkaline earth metal cations

We have studied effect of alkali and alkaline earth metal cations (Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺) decoration on hydrogen adsorption of the organic linker of Zn₂(NDC)₂(diPyTz) by employing three cluster models: diPyTz:mLi⁺ (m = 1–4), diPyTz:mLi⁺:nH₂ (m = 0,1,2 and n = 1–5) and diPyTz:1M⁺:1H₂ (M⁺ = Na⁺, K⁺, Be²⁺, Mg²⁺) complexes, using density functional theory (DFT) and second-order Moller–Plesset perturbation theory (MP2). The calculated binding energies show that decoration of the organic linker with alkali and alkaline earth metal cations enhanced H₂ interaction with diPyTz when compared with the pristine diPyTz. The atomic charges were derived by Mulliken, ChelpG and ESP methods. Finally, the atoms in molecules theory (AIM) were also applied to get more insight into the nature of the interaction of diPyTz and Li⁺. Results of AIM analysis show that N–Li⁺ bond in diPyTz organic linker's complex appears as shared electron interaction.     

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.     

Enhancement of hydrogen adsorption by alkali-metal cation doping of metal-organic framework-5

Over the past decade, metal-organic frameworks (MOFs) have been extensively studied as a novel approach to store hydrogen. The large surface area and volume of micropores that are intrinsic to MOFs make them ideal for gas adsorption. In addition, we chemically reduced MOF-5 by doping it with alkali metals (Li, Na, and K). We found that the H₂ uptake capacity of MOF-5 materials doped with Li, Na, and K exceeded that of a neutral framework by 24%, 68%, and 70%, respectively. Notably, at the same levels of doping, the Li⁺-doped framework exhibited the strongest H₂ binding, and the binding strength decreased sequentially in the order Li⁺ > Na⁺ > K⁺.     

Pd (II) decorated conductive two-dimensional chromium-pyrazine metal-organic framework for rapid detection of hydrogen

The utilization of H₂ for versatile application has demanded highly selective, low cost and rapid hydrogen sensors that are proficient in sensing H₂ near flammability limit. In this report, Cr^IIICl₂(pyrazine)₂ MOF with negatively charged pyrazine linkers in its structure is used for the stabilization of Pd (II) via charge transfer interactions. This material design turned an innocent MOF into selective hydrogen sensor that can respond (through decrease in resistance under dynamic sensing setup) to H₂ in 5–7 s with a detection range of 0.25%–1% H₂ concentration. A correlation of H₂ sensing characteristics and the structure-property relationship is established using density functional theory (DFT) calculations. The calculations suggested that near fermi level in Pd^II@CrPy, the bandwidth increases upon interaction with H₂ thereby the phase space for electron delocalization increases leading to better carrier mobility. This new approach not only yields novel sensing properties but also enables limited usage of precious metal to develop cost-effective sensors.     

Crown-like Zn0.5Cd0.5S/M-TiO2 composites derived from metal-organic frameworks for photocatalytic hydrogen production

Photocatalytic hydrogen production has been recognized as one of the most desirable approaches to overcome the worldwide energy and environmental issues. Here, novel sea urchin-like Zn_0.5Cd_0.5S and mesoporous TiO₂ (M-TiO₂) are designed, and a series of crown-like Zn_0.5Cd_0.5S/M-TiO₂ composites with different contents of M-TiO₂ are synthesized by hydrothermal method. The optimum hydrogen production rate of composites reaches 180.4 mmolh^?1g^?1 with the AQE up to 48.9% at 420 nm, which is 3.5 and 216 times that of pure Zn_0.5Cd_0.5S and the M-TiO₂, respectively. The outstanding performance of optimized Zn_0.5Cd_0.5S/M-TiO₂ composite prepared in this work exceeds most reported Cd-S-based catalysts. The improvement on the photocatalytic performance of composites is mainly due to the enlarged specific surface area, the exposure of more active sites, and the enhancement of the electron-hole separation efficiency.     

Structural changes of a NiFe-based metal-organic framework during the oxygen-evolution reaction under alkaline conditions

The design of new supramolecular complexes and metal-organic frameworks (MOFs) as water-oxidizing catalysts requires the stability studies because many metal-organic compounds are decomposed under the harsh conditions of water oxidation. Metal-organic frameworks have been extensively reported as catalysts for water splitting toward hydrogen production. Recently, a NiFe MOF has been claimed as an excellent catalyst for oxygen-evolution reaction (OER) under alkaline conditions (KOH (1.0 M)). Herein, using electrochemical methods, X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, transmission electron microscopy, and Raman spectroscopy, the stability of this NiFe MOF was investigated during OER. In the Raman spectrum, the peaks related to C–C and C–O in 1100–1550 cm^?1 become weak after OER, indicating the decrease of the carboxylate functional group. Comparing energy-dispersive spectra for the MOF before and after OER shows a decrease in carbon, but an increase in oxygen contents. Similar to the results of linear sweep voltammetry, energy-dispersive spectroscopy spectra suggest that the surface of the MOF converts into an oxide-based structure after OER. Transmission electron microscopy also shows some new crystalline areas after OER. The crystalline areas indicate a crystal lattice spacing of 0.21–0.23 nm, corresponding to (012) plane of the NiFe layered double hydroxide. Taken together, these experiments show that during OER the MOF converts to NiFe oxide with significant defects and imperfections in the regular geometrical arrangement of the ions, and the NiFe oxide is a candidate for catalyzing OER. This finding could be a roadmap for progress in the field of sustainable catalysis.     

On the application of standard isotherms to hydrogen adsorption in microporous materials

A mathematical framework for simulating equilibrium hydrogen adsorption isotherms in porous materials and estimating the values of key parameters associated with the adsorption process is developed. Explicit expressions for the excess, adsorbed, compressed and absolute masses, for any model isotherm, are derived. The modelling framework is used in combination with five standard equilibrium isotherm models to simulateexperimental data for Prussian blue analogues, nitropussides and metal-organic frameworks via nonlinear regression. The surface areas, the affinity and heterogeneity factors, and the pressure-dependent adsorption volumes are calculatedand compared to values available in the literature and the sensitivity of the results to the number of data points is quantified. The consistency of the results using different isotherm models is evaluated.     

Improved design of metal-organic frameworks for efficient hydrogen storage at ambient temperature: A multiscale theoretical investigation

A multiscale theoretical technique is used to examine the combination of different approaches for hydrogen storage enhancement in metal-organic frameworks at room temperature and high pressure by implementation lithium atoms in linkers. Accurate MP2 calculations are performed to obtain the hydrogen binding sites and parameters for the following grand canonical Monte Carlo (GCMC) simulations. GCMC calculations are employed to obtain the hydrogen uptake at different thermodynamic conditions. The results obtained demonstrate that the combination of different approaches can improve the hydrogen uptake significantly. The hydrogen content reaches 6.6 wt% at 300 K and 100 bar satisfying DOE storage targets (5.5 wt%).     

Understanding hydrogen adsorption performance of lithium-doped MIL-101(Cr) by molecular simulations: Effects of lithium distribution

Metal-organic frameworks (MOFs) have been recognized as one of the most compelling physical adsorption hydrogen storage materials owing to their ultrahigh surface area and excellent hydrogen adsorption performance. In order to further improve their hydrogen adsorption performance, lithium doping is an effective approach to increase the number of hydrogen adsorption sites as well as enhance the interaction strength towards hydrogen molecules according to grand canonical Monte Carlo(GCMC) simulations. However, in previous simulation studies, lithium ions were commonly assumed to be randomly distributed in MOF frameworks. In fact, the lithium-doped MOFs were prepared by immersing MOFs in a lithium salt solution and then drying them under high temperatures, in which the distribution of Li⁺ in MOF frameworks is elusive. In this work, the lithium-doped MIL-101 models (i.e., Immersion model) with varying lithium contents were constructed according to experimental operation and their hydrogen adsorption performance from GCMC simulations was also investigated in comparison with the equivalent models with randomly distributed lithium ions (i.e., Random model). It is found that in contrast to the uniform distribution of lithium ions in Random model, the accumulation of lithium ions was inspected in Immersion models especially at high loadings, leading to the reduced pore size. On the contrary, the hydrogen adsorption capacities of Immersion models are significantly improved owing to the enhanced interaction strength with hydrogen molecules resulting from the reduced pore size and the strengthened charged-induced dipole interaction.     

Simulation of hydrogen storage tank packed with metal-organic framework

Hydrogen adsorption in high surface metal-organic framework (MOF) has generated significant interest over the past decade. We studied hydrogen storage processes of MOF-5 hydrogen storage systems with adsorbents of both the MOF-5 powder (0.13 g/cm³) and its compacted tablet (0.30 g/cm³). The charge–discharge cycles of the two MOF-5 adsorbents were simulated and compared with activated carbon. The physical model is based on mass, momentum and energy conservation equations of the adsorbent-adsorbate system composed of gaseous and adsorbed hydrogen, adsorbent bed and tank wall. The adsorption process was modeled using a modified Dubinin–Astakov (D–A) adsorption isotherm and its associated variational heat of adsorption. The model was implemented by means of finite element analysis software Comsol Multiphysics™, and the system simulation platform Matlab/Simulink™. The thermal average temperature from Comsol simulation is used to fill the gap between the system model and the multi-dimensional models. The heat and mass transfer feature of the model was validated by the experiments of activated carbon, the simulated pressure and temperatures are in good agreement with the experimental results. The model was further validated by the metal-organic framework of Cu-BTC and is being extended its application to MOF-5 in this study. The maximum pressure in the powder MOF-5 tank is much higher than that in the activated carbon tank due to the lower adsorbent density of MOF-5 and resulting lower hydrogen adsorption. The maximum pressure in the compacted MOF-5 tank is a little bit lower than that in the activated carbon tank due to the higher adsorbent density and resulting higher hydrogen adsorption. The temperature swings during the charge–discharge cycle of both MOF-5 tanks are higher than that of the activated carbon tank. These are caused mainly by pressure work in the powder MOF-5 tank and by adsorption heat in the compacted MOF-5 tank. For both MOF-5 hydrogen storage systems, the lumped parameter models implemented by Simulink agree well with experimental pressures and with pressures and thermal average temperatures from Comsol simulation.     

An investigation of structural and hydrogen adsorption properties of microporous metal organic framework (MMOF) materials

Microporous metal organic frameworks (MMOFs) have garnered great attraction as adsorbent materials for on-board hydrogen storage. To enhance hydrogen adsorption, we have carried out a systematic study to synthesize, characterize, and modify crystal structures of a number of MMOFs and to investigate their pore characteristics. In addition, their hydrogen adsorption properties at cryogenic and ambient temperature over a range of pressures are analyzed.     

Density functional study of hydrogen spillover on direct Pd-doped metal-organic frameworks IRMOF-1

The direct doping of small Pd cluster on IRMOF-1 and its influence on H₂ adsorption were investigated using periodic density functional methods. Our calculations indicate that the Pd cluster prefers to be deposited on the linker. The doped Pd cluster can not only play the important role for dissociating H₂ molecules, but also enhance the H interaction with IRMOF-1 by altering its electronic structures. As a result, hydrogen spillover in IRMOF-1 by directly doping Pd catalysts is thermodynamically feasible and the odd numbered H bottlenecks on undoped IRMOF-1 are eliminated. In addition, we discuss a working mechanism for releasing H₂ from the chemisorbed states.     

Ambient temperature hydrogen storage in porous materials with exposed metal sites

Cross-linked porous polymeric complexes with exposed metal sites are synthesized for room temperature hydrogen storage via physisorption. At 298 K and 100 atm, PTF-Cr exhibits high excess hydrogen storage capacity up to 1.5 wt% with Qst of 11.5 kJ mol^?1 while PTF-Mg exhibits 0.5 wt% with Qst of 8 kJ mol^?1. The result provides insight for development of future storage materials with exposed transition metals.     

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.     

Concepts for improving hydrogen storage in nanoporous materials

Hydrogen storage in nanoporous materials has been attracting a great deal of attention in recent years, as high gravimetric H₂ capacities, exceeding 10 wt% in some cases, can be achieved at 77 K using materials with particularly high surface areas. However, volumetric capacities at low temperatures, and both gravimetric and volumetric capacities at ambient temperature, need to be improved before such adsorbents become practically viable. This article therefore discusses approaches to increasing the gravimetric and volumetric hydrogen storage capacities of nanoporous materials, and maximizing the usable capacity of a material between the upper storage and delivery pressures. In addition, recent advances in machine learning and data science provide an opportunity to apply this technology to the search for new materials for hydrogen storage. The large number of possible component combinations and substitutions in various porous materials, including Metal-Organic Frameworks (MOFs), is ideally suited to a machine learning approach; so this is also discussed, together with some new material types that could prove useful in the future for hydrogen storage applications.     

2-Dimensional metal-organic framework derived N-doped carbon immobilized Pd nanoparticles for hydrogen release from formic acid

Hydrogen release from formic acid is a significant energy supply route. However, the current catalysts suffer from low catalytic efficiency and stability. Herein, a porous N-doped carbon material with high N content (16.87%) and a large surface area (1544 m²·g^?1) were designed using a 2-dimensional metal-organic framework and etching agent potassium chloride. Due to its high N content and large surface area, ultrafine Pd particles are uniformly distributed on the porous N-doped carbon support, which effectively enhances excellent reactivity and enables a TOF value of 2365 h^?1 under additive-free conditions. The research also revealed that the strong interaction between Pd particles and pyridinic-N species can significantly slow metal agglomeration. Hence, the Pd/NC_ZIF-L (KCl) displayed good activity even after 10 cycling experiments, and it is a particularly competitive catalyst for hydrogen release from formic acid.     

Metal-organic framework derived Ni/Mo2C/Mo2TiC2Tx@NC as an efficient electrocatalyst for enhanced hydrogen production

Metal-organic framework's (MOF) shortcomings, such as poor conductivity, poor stability, and easy aggregation, impede its development in various application fields. Ni/Mo₂C/Mo₂TiC₂T_x@NC, a high-performance electrocatalyst for hydrogen evolution reaction, was prepared by incorporating a Mo₂TiC₂T_x MXene conductive matrix into MOF (namely C–Y). The Ni/Mo₂C/Mo₂TiC₂T_x@NC electrocatalyst demonstrates a remarkable HER ability with an overpotential of 105 and 134 mV and Tafel slope of 58 and 75 mV dec⁻¹ at a current density of 10 mA cm⁻² in 0.5 M H₂SO₄ and 1.0 M KOH, respectively. The outperformed HER activity of Ni/Mo₂C/Mo₂TiC₂T_x@NC catalyst is ascribe to the introduction of conductive Mo₂TiC₂T_x MXene as a carrier to improve the poor conductivity of MOF, the synergistic effect of Ni and Mo₂C nanoparticles, and the protective effect of the carbon layer. The work not only provides an experimental approach to address the problem of poor conductivity of MOF, but also provides a high-performance electrocatalyst for HER reactions. By utilizing MOFs and MXene as the precursor and the conducting carrier, our work provides some experimental reference for fabrication of multi-component inexpensive electrocatalysts.     

Preparation of a porous carbon from ferrocene-loaded polyaniline and its use in hydrogen adsorption

A porous carbon made of polyaniline with different ferrocene loadings was prepared through carbonization and thermal chemistry activation with KOH. The ferrocene served as a pore-forming agent and a resource of iron nanoparticles. N₂ adsorption/desorption measurements showed that the specific surface area and pore volume ranged from 2681 to 3246 m² g⁻¹ and from 1.56 to 2.06 cm³ g⁻¹, respectively, with increasing ferrocene loadings. Similarly, hydrogen adsorption also increased from 5.3 to 6.2 wt% at 77 K/5 MPa and 0.6 wt% to 0.85 wt% at 293 K/8 MPa. Scanning electron microscopy, X-ray diffraction and energy dispersive X-ray analysis showed that iron nanoparticles were embedded in the carbon matrix or dispersed on the surface. The large specific surface area and big pore volume improved the original hydrogen adsorption heat up to 7.2 kJ mol⁻¹ for the best sample.