Novel metal graphene framework (MGF) structures for hydrogen storage

Hierarchical novel metal graphene framework (MGF) structures designed, constructed and used for hydrogen storage by adsorption. Three different amounts of pillars used for connecting carboxylate functionalized graphene layers. The three-dimensional atomistic models optimized and then the aqua molecules removed which are coordinated to metal between graphene layers as bridge. Hydrogen storage abilities of aqua free models also investigated. The aqua existence made the structures more effective to store hydrogen according to simulation results. The highest storage capacity calculated for 181C.Aq named structure that includes aqua molecules inside could store 7.745 wt percentage hydrogen at 77 K and 100 bars pressure conditions. Hydrogen storage capacity decreases almost to half by removing aqua molecules for same structure. It is clear the coordinated aqua existence around the bridging atoms provides the spaces between graphene layers. In other word, the spaces between the graphene layers collapse by aqua removal so the hydrogen storage capacities decrease dramatically. Finally, spacing between graphene layers by bridges makes them very effective for hydrogen storage.     

Hydrogen adsorption around lithium atoms anchored on graphene vacancies

Density functional theory and molecular dynamics were used to study the interaction of a lithium atom with a vacancy inside a graphene layer. It was found that the lithium atom is adsorbed on this vacancy, with a binding energy much larger than the lithium cohesive energy. Then, the adsorption of hydrogen molecules around lithium atoms was studied. We found that at 300 K and atmospheric pressure, this system could store up to 6.2 wt.% hydrogen, with average adsorption energy of 0.19 eV per molecule. Thus, this material satisfies the gravimetric capacity requirements for technological applications. A complete desorption of hydrogen occurs at 750 K. However, a multilayer of this system would be required for practical reasons. Under atmospheric pressure and at 300 K, we found that a system made of multiple layers of this material is stable. The storage capacity remained at 6.2 wt.%, but all adsorbed molecules were dissociated. The average adsorption energy becomes 0.875 eV/H.     

Hydrogen storage on lithium modified silica based CHAbazite type zeolite,A computational study

Lithium atoms were dispersed in CHAbazite-type zeolite to investigate the effects of differing amount of dispersed atoms on hydrogen storage. Each structures were relaxed after lithium modification using Density Functional Theory calculations. The changes in the surface area and energetic properties, such as electrostatic potentials and charge transfers, were investigated for the final structures. Then, the partial charges which were calculated according to the Hirshfeld method, were assigned to the structures. Finally, charged structures were used for hydrogen adsorption calculations. It was found that increased amount of lithium atoms does not affect the hydrogen storage capacity continuously. So, after a certain number of dispersed atoms is reached, hydrogen storage capacity begins to decrease. The limit values were 12 and 3 for the low and high pressure ranges when the adsorbed amounts of hydrogen were 0.157 and 1.545% by weight for the same structures at 298 K, respectively. 12 and 3 lithium atoms dispersion enhance the storage capacity 6 and 3 times as much as pure silica based CHAbazite type zeolite.     

Hydrogen storage ability of porous carbon material fabricated from coffee bean wastes

The hydrogen storage ability at 298 and 77 K of porous carbon materials with microporous structures fabricated from coffee bean wastes through KOH activation was investigated regarding pore structure. The dependence of hydrogen storage ability on the pore structure of porous carbon materials was investigated at 298 and 77 K to clarify the storage mechanism of carbon materials. Hydrogen storage ability at 298 K was increased linearly with increasing of specific surface area increasing. The maximum amount of stored hydrogen was 0.6 wt.% on porous carbon material with 2070 m²/g specific surface area. The hydrogen storage ability at 77 K was 4.0 wt.% on the same sample. The hydrogen storage ability showed a linear relationship with the micro-pore volume size. These changes in the dependence of the hydrogen storage ability on pore size suggested that the storage configuration changed from two- to three-dimensional. The stored hydrogen densities in porous carbon materials calculated from these values were 5.7 and 69.6 mg/cm³ at 298 and 77 K, respectively. The change in density indicated that the state of stored hydrogen in porous carbon materials was filled up aggregational state, which is extremely close to the liquid state, and suggested the realizing of high hydrogen storage ability on carbon materials fabricated from agricultural waste.     

A study on the hydrogen storage capacity of Ni-plated porous carbon nanofibers

In this study, we prepared highly porous carbon-nanofiber-supported nickel nanoparticles as a promising material for hydrogen storage. The porous carbons were activated at 1050 °C, and the nickel nanoparticles were loaded by an electroless metal-plating method. The textural properties of the porous carbon nanofibers were analyzed using N₂/77 K adsorption isotherms. The hydrogen storage capacity of the carbons was evaluated at 298 K and 100 bar. It was found that the amount of hydrogen stored was enhanced by increasing nickel content, showing 2.2 wt.% in the PCNF-Ni-40 sample (5.1 wt.% and 6.4% of nickel content and dispersion rate, respectively) owing to the effects of the spill-over of hydrogen molecules onto the metal–carbon interfaces. This result clearly indicates that the presence of highly dispersed nickel particles can enhance high-capacity hydrogen storage.     

Hydrogen physisorption in high SSA microporous materials - A comparison between AX-21_33 and MOF-177 at cryogenic conditions

The two most promising materials for a hydrogen cryo-adsorption tank, activated carbon AX-21_33 and metal-organic framework MOF-177, have been investigated in the pressure range up to 2 MPa and at temperatures from 77 K to 125 K and at room temperature. The total hydrogen storage, including adsorbed hydrogen and gaseous hydrogen, has been determined for both samples. The results were evaluated with respect to the operating conditions of a tank system at cryogenic conditions, assuming a maximum tank pressure of 2 MPa and a minimum back pressure for the hydrogen consumer of 0.2 MPa. AX-21_33 shows a usable capacity of 3.5 wt.% in the case of isothermal operation at 77 K and 5.6 wt.%, if the tank is loaded at 77 K and the temperature is increased by 40 K during unloading. Under the same conditions, MOF-177 has a usable capacity of 6.1 wt.% and 7.4 wt.%, respectively. The results show that the heat of adsorption has a high impact on the amount of hydrogen remaining in a tank after unloading and that the heat management plays a crucial role for the design of a cryogenic tank system.     

Optimization of the pore structure of nickel/graphite hybrid materials for hydrogen storage

Nickel/graphite hybrid materials were prepared by mixed acid treatment of graphite flakes, following metal nanoparticle deposition. The textural properties were studied by BET surface area measurement and t-plot methods with N₂/77 K adsorption isotherms. The hydrogen storage characteristics of the nickel/graphite at 298 K and 10 MPa were studied using a pressure-composition-temperature apparatus. The pore structure of the materials was studied as a function of processing conditions. In the optimum material, the hydrogen storage capacity was as high as 4.48 wt.%. The total amount of storage was not proportional to the specific surface area or metal content of the adsorbate. A dipole-induced model on nickel/carbon surfaces is proposed for the hydrogen storage mechanism.     

Potassium decorated γ-graphyne as hydrogen storage medium: Structural and electronic properties

Different sites for K adsorption in γ-graphyne were investigated using density functional theory (DFT) calculations and optical and structural properties of the structures were examined. For the most stable structures, we put one H₂ molecule in different directions on the various sites to evaluate the hydrogen adsorption capability of them. Then, one to nine H₂ molecules in sequence were added to the best structure. Results show that clustering of the K atoms is hindered on the graphyne surface and the most desirable adsorption site for K atom is the hollow site of 12-membered ring with adsorption energy of 5.86 eV. Also, this site is the best site for H₂ adsorption onto K-decorated graphyne with E_das of −0.212 eV. Adding of number of H₂ molecule on this site shows that K atom can bind nine H₂ molecules at one side of the graphyne with the average adsorption energy of 0.204 eV/H₂. Therefore, for one side ca. 8.95 wt % and for both sides of the graphyne with a K atom in each side ca. 13.95 wt % of the hydrogen storage capacity can be achieved. This study shows that K-decorated graphyne can be a promising candidate for the hydrogen storage applications.     

Hydrogen storage in Ni-B nanoalloy-doped 2D graphene

Two-dimensional graphene material is doped with Ni-B nanoalloys via a chemical reduction method, and shows that the optimal graphene doped with Ni (0.14 wt.%) and B (0.63 wt.%) has a hydrogen capacity of 2.81 wt.% at 77 K and 106 kPa, which is more than twice of that of the pristine graphene. The measured adsorption isotherms of hydrogen and nitrogen suggest that the Ni-B nanoalloys function as catalytic centers to induce the dissociative adsorption of hydrogen (spillover) on the graphene. The Ni-B nanoalloys without using any noble metal may be a promising catalyst for hydrogen storage application.     

Optimization of activated carbons for hydrogen storage

Activated carbons (ACs) having hydrogen storage performances among the highest reported so far (i.e. 6.6 wt. % at 77 K and 4 MPa) are presented. These materials were prepared by chemical activation of anthracite with KOH. The effects of two experimental parameters: KOH/anthracite weight ratio (W) and activation temperature (T), on the hydrogen storage capacity were studied by application of central composite design and response surface methodology. A quadratic model was developed for correlating W and T to the hydrogen storage capacity. The analysis of variance showed that W the only significant parameter in the range of the experimental conditions tested. Our optimized AC showed higher hydrogen capacity in terms of absolute and excess storage properties than the well-known MAXSORB-3.     

Catalyzed hydrogen spillover for hydrogen storage on microporous organic polymers

Catalyzed hydrogen spillover for hydrogen storage on microporous organic materials has been studied in this work. The method, i.e. “preparation of Pt nanoparticle first and then in situ formation of microporous materials” has been developed for the synthesis of microporous hypercrosslinked polymers with highly dispersed Pt nanoparticles. Hydrogen adsorption isotherms are measured at 77.3 K and up to 1.13 bar, and 298.15 K and up to 19 bar. By containing 2 wt % Pt nanoparticles, the hydrogen storage capacity of hypercrosslinked polymers is enhanced to 0.21 wt % at 298.15 K and 19 bar. Compared to the similar materials without Pt nanoparticles, the H₂ adsorption amount has been enhanced by a factor of 1.75.     

Synthesis and hydrogen storage properties of ultrafine Mg-Zn particles

Mg-6.9 at.% Zn ultrafine particles (UFPs) were prepared by hydrogen plasma-metal reaction (HPMR) method. The electron microscopy study revealed that they were spherical in shape with particle size in the range 100-700 nm. Each fine particle was composed of single crystal structure of α-Mg(Zn) solid solution and amorphous structure of Mg-Zn alloy. After one absorption and desorption cycle, these UFPs transformed from the single crystal into the nanocrystalline structure and the mean particle size changed from 400 to 250 nm. It was found that the Mg-Zn UFPs could absorb 5.0 wt.% hydrogen in 20 min at 573 K and accomplish a high hydrogen storage capacity of 6.1 wt.% at 573 K. The fine particle size, nanocrystalline structure and the low oxide content of the obtained sample promoted the hydrogen sorption process with low hydrogen absorption activation energy of 56.3 kJ/mol. The enhanced hydrogen sorption properties of high absorbing rate and high storage capacity were due to the improved kinetics rather than the change in enthalpy.     

Optimizing the conditions of multi-walled carbon nanotubes surface activation and loading metal nanoparticles for enhanced hydrogen storage

In this study, the effect of surface activation of multi-walled carbon nanotubes (MWCNTs) by KOH along with loading of cobalt and lithium nanoparticles on the surface of MWCNTs are investigated. In the first step, surface activation parameters, i.e. MWCNT/KOH weight ratio, activation temperature, and activation time are optimized to give the highest hydrogen uptake. According to obtained results, the optimum synthesis conditions are MWCNT/KOH weight ratio of 1:5, 800 °C, and 1 h of activation duration. Afterward, cobalt and lithium metal nanoparticles are doped discretely on the surface of activated nanotubes. It is demonstrated that amounts of loaded cobalt and lithium metals are 5.5 and 1.9% wt, respectively. In addition, it is revealed that the amount of hydrogen storage capacity for cobalt-loaded and lithium-loaded MWCNTs are 1.06% wt. and 1.33% wt., respectively (at 278 K) which are higher than the capacity of pristine and activated MWCNT samples.     

Effect of temperature on activated carbon nanotubes for hydrogen storage behaviors

In this work, activated multi-walled carbon nanotubes (Acti-MWNTs) with well-developed pore structures, a highly specific surface area, and higher hydrogen adsorption capacities due to CO₂ activation were prepared. The activation was performed at activation temperatures in the range of 500–1100 °C. The microstructure and crystallinity of the Acti-MWNTs were evaluated with a transmission electron microscope (TEM) and an FT-Raman spectrometer, respectively. The textural properties of the Acti-MWNTs were investigated by using a nitrogen gas sorption analyzer at 77 K. The hydrogen storage capacities of the Acti-MWNTs were investigated by BEL-HP at 298 K/100 bar. The hydrogen storage capacities of the Acti-MWNTs were enhanced to 0.78 wt.% by increasing activation temperatures to 900 °C, which resulted in the formation of a defective structure in the Acti-MWNTs. This result indicated that the CO₂ activation was one of the most effective methods to develop the textural properties, as well as to enhance the hydrogen storage capacities of MWNTs.     

Grand canonical Monte Carlo simulations of hydrogen adsorption in carbon aerogels

Hydrogen storage plays a fundamental role in the future hydrogen energy system, and carbon aerogel is one of the most potential hydrogen storage materials because of its high gravimetric and volumetric density on hydrogen adsorption. In this paper, the amorphous structure of carbon, obtained by a numerical simulation process by using the molecular dynamic and Monte Carlo methods, as well as the primary unit method, was intercepted as a sphere structure for numerical annealing to build a carbon nanosphere, which serves as the basic unit to reconstruct the carbon aerogel's skeleton by the Diffusion Limited Cluster Aggregation (DLCA) method. The hydrogen adsorption in carbon aerogel was simulated by using the self-coding parallel grand canonical Monte Carlo (GCMC) method. The influences of particle diameter, density, temperature, pressure, and specific surface area on the hydrogen adsorbing capacity in carbon aerogel were analyzed in detail. The results showed that the carbon aerogel's hydrogen storage capacity with a specific surface area of 2680 m²/g could reach 4.52 wt % at 77 K and 3.0 MPa.     

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.     

The performance of green carbon as a backbone for hydrogen storage materials

We investigate the use of carbonized bamboo, which has an organic porous structure, as a hydrogen storage material. Bamboo samples were thermally treated at 800, 900, 1000, and 1100 °C for 24 h. The pore size and hydrogen storage capacity of each sample were measured by N₂ and H₂ gas sorption up to 1.13 bar at 77 K. The maximum hydrogen storage was exhibited by the sample treated at 900 °C, which reached 1.35 wt% at 1.13 bar/77 K. The results showed that the bamboo, one of the green carbons, has the potential to be used as an environmental-friendly carbon backbone for hybrid hydrogen storage materials.     

Improvement of hydrogen storage capacity on the palladium-decorated N-doped graphene sheets as a novel adsorbent: A hybrid MD-GCMC simulation study

A hybrid molecular simulation, as a combination of molecular dynamics and Grand Canonical Monte Carlo simulation, is performed to investigate the storage capacity of hydrogen in carbon nanostructure adsorbents. Pure graphene sheet, nitrogen-doped graphene sheet, palladium-decorated graphene sheet and nitrogen-doped graphene sheet decorated with palladium atoms are selected for this purpose. Palladium is added to the structure in atomic and nanoparticle forms. Initially, all selected systems are optimized using density functional theory (DFT). The atomic charges of various structures are incorporated in the hybrid simulation. Then, hybrid simulations of hydrogen adsorption in different structures are performed at a temperature of 300 K in the pressure range of 1–40 bar. Simulation results show that among various structures, the simultaneous doping of graphene sheet with nitrogen atom and decoration of sheets by palladium atoms could increase the storage capacity by about 437% in comparison to pure graphene. In addition, the atomic form of palladium is more efficient than its nanoparticle form. Finally, comparing the adsorption capacity of the proposed structure with the target set by the US Department of Energy for 2020 indicates that proposed nanostructure can improve this target for hydrogen storage in comparison with previous carbon structure materials.     

Computational evaluation of superalkali-decorated graphene nanoribbon as advanced hydrogen storage materials

In this study, we proposed that homo superalkali NM₄ clusters with high tetrahedral geometry, can be applied to develop high-performance hydrogen storage materials. Moreover, their special bonding structures and chemical stability make them ideal units for decoration of different kinds of pristine monolayers. We made a trial to decorate the NLi₄ clusters onto the 1D graphene nanoribbon, and employed density functional theory (DFT) computational studies to solve its electronic structure, and further evaluate its applicability in hydrogen storage. We found that the electronic charges on Li atoms were successfully transferred to the pristine monolayer, thus a partial electronic field around each Li atom was formed. This subsequently leads to the polarization of the adsorbed hydrogen molecules, and further enhances the electrostatic interactions between the Li atoms and hydrogen. Each NLi₄ cluster can adsorb at most 16 hydrogen molecules. For this novel material, its total capacity of hydrogen storage can reach to 11.2 wt %, surpassing the target value of 5.5 wt %, set by the U.S department of energy (DOE) [1], making itself an ideal unit for advanced energy materials design.     

Investigation of hydrogen storage capacity of various carbon materials

Hydrogen storage capacity of various carbon materials, including activated carbon (AC), single-walled carbon nanohorn, single-walled carbon nanotubes, and graphitic carbon nanofibers, was investigated at 303 and 77 K, respectively. The results showed that hydrogen storage capacity of carbon materials was less than 1 wt% at 303 K, and a super activated carbon, Maxsorb, had the highest capacity (0.67 wt%). By lowering adsorption temperature to 77 K, hydrogen storage capacity of carbon materials increased significantly and Maxsorb could store a large amount of hydrogen (5.7 wt%) at a relatively low pressure of 3 MPa. Hydrogen storage capacity of carbon materials was proportional to their specific surface area and the volume of micropores, and the narrow micropores was preferred to adsorption of hydrogen, indicating that all carbon materials adsorbed hydrogen gas through physical adsorption on the surface.