Hydrogen adsorption of O/N-rich hierarchical carbon scaffold decorated with Ni nanoparticles: Experimental and computational studies

Hierarchical carbon scaffold (HCS) with multi-porous structures, favoring hydrogen diffusion and physisorption is doped with 2–10 wt % Ni for storing hydrogen at ambient temperature. Due to N- and O-rich structure of melamine-formaldehyde resin used as carbon precursor, homogeneous distribution of heteroatoms (N and O) in HCS is achieved. 2 wt % Ni-doped HCS shows the highest hydrogen capacity up to 2.40 wt % H₂ (T = 298 K and p (H₂) = 100 bar) as well as excellent reversibility of 18.25 g H₂/L and 1.25 wt % H₂ (T = 298 K and p (H₂) = 50 bar) and electrical production from PEMFC stack up to 0.7 Wh upon eight cycles. Computations and experiments confirm strong interactions between Ni and heteroatoms, leading to uniform distribution small particles of Ni. This results in enhancing reactive surface area for hydrogen adsorption and preventing agglomeration of Ni nanoparticles upon cycling. Ni K-edge XANES spectra simulated from the optimized structure of Ni-doped N/O-rich carbon using DFT calculations are consistent with the experimental spectra and suggest electron transfer from Ni to hydrogen to form Ni–H bond upon adsorption. Considering low desorption temperature (323 K), not only chemisorbed hydrogen is involved in adsorption mechanisms but also physisorption and spillover of hydrogen.     

Strain and defect engineering of graphene for hydrogen storage via atomistic modelling

The adsorption of hydrogen molecules on monolayer graphene is investigated using molecular dynamics simulations (MDS). Interatomic interactions of the graphene layer are described using the well-known AIREBO potential, while the interactions between graphene and hydrogen molecule are described using Lennard-Jones potential. In particular, the effect of strain and different point defects on the hydrogen storage capability of graphene is studied. The strained graphene layer is found to be more active for hydrogen and show 6.28 wt% of H₂ storage at 0.1 strain at 77 K temperature and 10 bar pressure. We also studied the effect of temperature and pressure on the adsorption energy and gravimetric density of H₂ on graphene. We considered different point defects in the graphene layer like monovacancy (MV), Stone Wales (SW), 5-8-5 double vacancy (DV), 555–777 DV, and 5555-6-7777 DV which usually occur during the synthesis of graphene. At 100 bar pressure, graphene with 1% concentration of MV defects leads to 9.3 wt% and 2.208 wt% of H₂ storage at 77 K and 300 K, respectively, which is about 42% higher than the adsorption capacity of pristine graphene. Impact of defects on the critical stress and strain of defected graphene layers is also studied.     

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.     

Enhanced hydrogen adsorption in ordered mesoporous carbon through clathrate formation

Ordered mesoporous carbons were synthesized with a soft-template approach and modified with a water and tetrahydrofuran mixture having a H₂O/THF molar ratio of 17:1 as potential adsorbent media for hydrogen storage. Hydrogen adsorption equilibrium on the carbon adsorbents was measured gravimetrically at 270 K and hydrogen pressures up to 163 bar. Enhanced hydrogen adsorption was observed on the carbon adsorbents doped with 0.5 wt.% and 0.75 wt.% of H₂O/THF due to the combined effects of hydrogen adsorption on the carbon surface and formation of a binary H₂–H₂O–THF clathrate. Hydrogen adsorption capacities on the carbon adsorbents doped with 0.5 wt.%, 0.75 wt.% of H₂O/THF, and the pure carbon at 270 K and 163 bar are 0.747 wt.%, 0.646 wt.% and 0.585 wt.%, respectively. The hydrogen adsorption isotherms on all the doped carbon adsorbents are of typical Type III and can be well correlated by the Freundlich equation. A desorption hysteresis loop was observed on the carbon adsorbents doped with 0.5 wt.% and 0.75 wt.% of H₂O/THF, which was probably caused by the pore size difference during the adsorption and desorption steps.     

Computational insights of alkali metal (Li / Na / K) atom decorated buckled bismuthene for hydrogen storage

The mechanism of hydrogen molecule adsorption on 2D buckled bismuthene (b-Bi) monolayer decorated with alkali metal atoms was studied using density functional theory based first principles calculations. The decorated atoms Li, Na and K exhibited distribution on surface of b-Bi monolayer with increasing binding energy of 2.6 eV, 2.9 eV and 3.6 eV respectively. The adsorption of H₂ molecule on the slabs appeared stable which was further improved upon inclusion of van der Waals interactions. The adsorption behaviour of H₂ molecules on the decorated slabs is physisorption whereas the slabs were able to bind up to five H₂ molecules. The average adsorption energy per H₂ molecules are in range of 0.1–0.2 eV which is good for practical applications. The molecular dynamics simulation also confirmed the thermodynamic stabilities of five H₂ molecules adsorbed on the decorated slabs. The storage capacity values are found 2.24 wt %, 2.1 wt %, and 2 wt %, for respective cases of Li, Na and K atoms decorated b-Bi. The analysis of the adsorbed cases pointed to electrostatic interaction of Li and H₂ molecule. The adsorption energies, binding energies, charge analysis, structural stability, density of states, and hydrogen adsorption percentage specifies that the decorated b-Bi may serve as an efficient hydrogen storage material and could be an effective medium to interact with hydrogen molecules at room temperature.     

Rice husk derived graphene-like material: Activation with phosphoric acid in the absence of inert gas for hydrogen gas storage

In this study, the effect of concentration of phosphoric acid (H₃PO₄) towards the physicochemical properties of rice husk derived graphene (GRHA) in the absence of inert gas was investigated. From TGA analysis, it was found that GRHA 1:3 possessed the highest weight loss (24.66%) due to the highest reactivity towards H₃PO₄. The FTIR shows that graphene-like material was obtained as the –OH groups were vanished in GRHA structure after activation. Raman spectroscopy and XRD analysis indicated that the produced GRHA is in amorphous state and has few layers of graphene. GRHA 1:3 showed the greatest improvement in their porous structure including the highest surface area (315.07 m²/g) with the largest pore volume (0.2069 cm³/g) as compared to other samples. From the static adsorption test, it was confirmed that GRHA 1:3 stored the highest amount of hydrogen compared to other samples with 1.95 wt % contributed by its excellent porosity and surface area. To further understand the kinetics of hydrogen adsorption on GRHA, pseudo-first model and pseudo-second model was plotted. Pseudo second model was the best fitted model which indicated that the gas molecule adsorbed in the GRHA material via chemisorption. Additionally, from the kinetic study it was found that the adsorption process of GRHA 1:3 was controlled by multi-step adsorption process.     

Hierarchical porous graphene-based carbons prepared by carbon dioxide activation and their gas adsorption properties

Hierarchical porous graphene-based carbons (HPGCs) have been prepared by a simple carbon dioxide activation of graphite oxide. The effects of activation temperature on the structural and textural properties as well as gas adsorption capacities of the resultant carbons have been investigated. The HPGCs showed hierarchically micro-meso-macroporous structures, high specific surface areas of up to 532 m² g⁻¹, and large pore volumes of up to 1.67 cm³ g⁻¹. Moreover, the HPGC materials were demonstrated to be efficient for CO₂ and H₂ adsorption. The HPGC-850, which was obtained after two hours of activation at 850 °C, exhibited the highest adsorption capacities of 7.74 wt% (1.76 mmol g⁻¹) at 273 K and 1 bar for CO₂ and of 0.75 wt% (3.76 mmol g⁻¹) at 77 K and 1 bar for H₂.     

Selective and tunable H2 adsorption/sensing performance of W-doped graphene under external electric fields: A DFT study

W-doped graphene and its selective gas adsorption/sensing performance are studied through first-principles density functional theory (DFT) calculations. A single W atom is stably anchored into the graphene plane with a high binding energy of ?9.325 eV. The W-doped graphene interacts more strongly with H₂ compared to NH₃, CH₄, CO, SO₂ or H₂S. The H₂ adsorption system also has a higher adsorption energy of ?1.035 eV. Furthermore, the W-doped graphene exhibits the highest sensor response to H₂ with the largest number of transferred charges and the biggest change in the band gap. A negative electric field improves the interaction between the H₂ and the W-doped graphene by increasing the adsorption energy and promoting charge transfer. However, the adsorption of the H₂ is significantly weakened upon the application of a positive electric field; the adsorbed H₂ is easily desorbed from the W-doped graphene with a modulated recovery time as short as ～4.099 s at room temperature (300 K) upon a +0.4 V Å^?1 increase in the electric field. These results reveal that the W-doped graphene has promising selective and tunable H₂ adsorption/sensing performance upon the application of external electric fields.     

Improved hydrogen storage properties of Mg-based nanocomposite by addition of LaNi5 nanoparticles

Mg (200 nm) and LaNi₅ (25 nm) nanoparticles were produced by the hydrogen plasma-metal reaction (HPMR) method, respectively. Mg–5 wt.% LaNi₅ nanocomposite was prepared by mixing these nanoparticles ultrasonically. During the hydrogenation/dehydrogenation cycle, Mg–LaNi₅ transformed into Mg–Mg₂Ni–LaH₃ nanocomposite. Mg particles broke into smaller particles of about 80 nm due to the formation of Mg₂Ni. The nanocomposite showed superior hydrogen sorption kinetics. It could absorb 3.5 wt.% H₂ in less than 5 min at 473 K, and the storage capacity was as high as 6.7 wt.% at 673 K. The nanocomposite could release 5.8 wt.% H₂ in less than 10 min at 623 K and 3.0 wt.% H₂ in 16 min at 573 K. The apparent activation energy for hydrogenation was calculated to be 26.3 kJ mol⁻¹. The high sorption kinetics was explained by the nanostructure, catalysis of Mg₂Ni and LaH₃ nanoparticles, and the size reduction effect of Mg₂Ni formation.     

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.     

Synthesis and characterization of porous HKUST-1 metal organic frameworks for hydrogen storage

Hydrogen storage capacity has been investigated on a copper-based metal organic framework named HKUST-1 with fine structural analyses. The crystalline structure of HKUST-1 MOF has been confirmed from the powder X-ray diffraction and the average particle diameter has been found about 15–20 μm identified by FE-SEM. Nitrogen adsorption isotherms show that HKUST-1 MOF has approximately type-I isotherm with a BET specific surface area of 1055 m²g⁻¹. Hydrogen adsorption study shows that this material can store 0.47 wt.% of H₂ at 303 K and 35 bar. The existence of Cu (II) in crystalline framework of HKUST-1 MOF has been confirmed by pre-edge XANES spectra. The sharp feature at 8985.8 eV in XANES spectra represents the dipole-allowed electron transition from 1s to 4p_xy. In addition, EXAFS spectra indicate that HKUST-1 MOF structure has the Cu–O bond distance of 1.95 Å with a coordination number of 4.2.     

Tailoring morphological and chemical properties of covalent triazine frameworks for dual CO2 and H2 adsorption

The development of functional porous samples suitable as gas-adsorption materials is a key challenge of modern materials chemistry to face with global warming or issues related to renewable energy-storage solutions. Herein, a set of five Covalent Triazine Frameworks (CTFs) featured by high specific surface area (SSA, up to 3201 m² g^?1) and N content as high as 12.2 wt% have been prepared through a rational synthetic strategy and exploited with respect to their gas uptake properties. Among CTFs from this series, CTF-pDCB/DCI^HT (4) combines ideal morphological and chemico-physical properties for CO₂ and H₂ adsorption. Noteworthy, besides ranking among CTFs with the highest CO₂ adsorption capacity reported so far (up to 5.38 mmol g^?1 at 273 K and 1 bar), 4 displays a H₂ excess uptake at 77 K of 2.84 and 5.0 wt% at 1 and 20 bar, respectively, outperforming all CTF materials and 2D Porous Organic Polymers of the state-of-the-art.     

Hydrogen adsorption properties of in-situ synthesized Pt-decorated porous carbons templated from zeolite EMC-2

To increase the interaction between the adsorbed hydrogen and the adsorbent surface to improve the hydrogen storage capacity at ambient temperature, decorating the sorbents with metal nanoparticles, such as Pd, Ni, and Pt has attracted the most attention. In this work, Pt-decorated porous carbons were in-situ synthesized via CVD method using Pt-impregnated zeolite EMC-2 as template and their hydrogen uptake performance up to 20 bar at 77, 87, 298 and 308 K has been investigated with focus on the interaction between the adsorbed H₂ and the carbon matrix. It is found that the in-situ generated Pt-decorated porous carbons exhibit Pt nanoparticles with size of 2–4 nm homogenously dispersed in the porous carbon, accompanied with observable carbon nanowires on the surface. The calculated H₂ adsorption heats at/near 77 K are similar for both the plain carbon (7.8 kJ mol⁻¹) and the Pt-decorated carbon (8.3 kJ mol⁻¹) at H₂ coverage of 0.08 wt.%, suggesting physisorption is dominated in both cases. However, the calculated H₂ adsorption heat at/near 298 K of Pt-decorated carbon is 72 kJ mol⁻¹ at initial H₂ coverage (close to 0), which decreases dramatically to 20.8 kJ mol⁻¹ at H₂ coverage of 0.014 wt.%, levels to 17.9 at 0.073 wt.%, then gradually decreases to 2.6 kJ mol⁻¹ at 0.13 wt.% and closes to that of the plain carbon at H₂ coverage above 0.13 wt.%. These results suggest that the introduction of Pt particles significantly enhances the interaction between the adsorbed H₂ and the Pt-decorated carbon matrix at lower H₂ coverage, resulting in an adsorption process consisting of chemisorption stage, mixed nature of chemisorption and physisorption stage along with the increase of H₂ coverage (up to 0.13 wt.%). However, this enhancement in the interaction is outperformed by the added weight of the Pt and the blockage and/or occupation of some pores by the Pt nanoparticles, which results in lower H₂ uptake than that of the plain carbon.     

Transition metal atom embedded graphene for capturing CO: A first-principles study

The adsorption of CO and H₂ on single-metal-atom (Fe, Co, Ni and Cu) embedded graphene (M-G) has been studied using density functional theory calculations. Fe-G and Co-G can capture up to three CO molecules per metal atom strongly, but tend to weakly or not adsorb H₂ molecules. Under standard conditions (298.15 K and 1 bar), they show a high adsorption selectivity ratio for CO over H₂. The density of states analysis reveals that the strong adsorption between CO and Fe(Co)-G results from the hybridization between d states of Fe (Co) and sp states of CO. Our findings suggest that Fe-G and Co-G can be used as a filter membrane for removing CO efficiently in the feed gas of hydrogen fuel cells.     

Direct Z-scheme anatase/rutile bi-phase nanocomposite TiO2 nanofiber photocatalyst with enhanced photocatalytic H2-production activity

Design and preparation of direct Z-scheme anatase/rutile TiO₂ nanofiber photocatalyst to enhance photocatalytic H₂-production activity via water splitting is of great importance from both theoretical and practical viewpoints. Herein, we develop a facile method for preparing anatase and rutile bi-phase TiO₂ nanofibers with changing rutile content via a slow and rapid cooling of calcined electrospun TiO₂ nanofibers. The phase structure and composition, surface morphology, specific surface area, surface chemical composition and element chemical states of TiO₂ nanofibers were analyzed by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption and X-ray photoelectron spectroscopy (XPS). By a rapid cooling of 500 °C-calcined electrospun TiO₂ precursor, anatase/rutile bi-phase TiO₂ nanofibers with a roughly equal weight ratio of 55 wt.% anatase and 45 wt.% rutile were prepared. The enhanced H₂ production performance was observed in the above obtained anatase/rutile composite TiO₂ nanofibers. A Z-scheme photocatalytic mechanism is first proposed to explain the enhanced photocatalytic H₂-production activity of anatase/rutile bi-phase TiO₂ nanofibers, which is different from the traditional heterojunction electron–hole separation mechanism. This report highlights the importance of phase structure and composition on optimizing photocatalytic activity of TiO₂-based material.     

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.     

Melamine assisted preparation of nitrogen doped activated carbon from sustainable biomass for H2 and CO2 storage

Activated carbon (AC), as an effective solid adsorbent, is extensively employed in H₂ and CO₂ storage. To enhance its adsorption capability and selectivity, it is necessary to increase its surface area and dope heteroatoms by a simple and environment-friendly method. In this work, nitrogen doped activated carbon (NAC) has been synthesized from sustainable biomass by direct activation with the assistance of melamine. The obtained NAC with 2.1 wt% N dopants possesses a high surface area (2477.27 m²/g) and pore volume (1.93 cm³/g). The NAC displayed enhanced H₂ uptake capacity (2.29 wt% at 77 K, 1 bar and 0.83 wt% at 298 K, 100 bar) and adequate CO₂ uptake capacity (2.85 mmol/g at 298 K, 1 bar and 4.49 mmol/g at 273 K, 1 bar). Activation mechanism with the assistance of melamine was proposed in accordance with the experimental data. The facile method of preparing NAC is potential for large-scaled production.     

Synthesis and hydrogen adsorption properties of a new iron based porous metal-organic framework

A new metal-organic framework [Fe₃O(OOC-C₆H₄-COO)₃(H₂O)₃]Cl·(H₂O)_x was synthesized with a specific surface area of 2823 m²/g and a lattice parameter of 88.61 Å. Isostructural with MIL-101, this compound exhibits similar hydrogen adsorption properties, with maximum adsorption capacity of 5.1wt.% H at 77 K. The adsorption enthalpy of hydrogen for MIL-101 and ITIM-1 (MIL-101Fe) at zero coverage was calculated for a wide temperature range of 77 K ÷ 324 K, considering corrections for the variation of hydrogen gas entropy with the temperature. The resulted adsorption enthalpy is 9.4 kJ/mol for MIL-101, in excellent agreement with the value reported in literature from microcalorimetric measurements, and a value of 10.4 kJ/mol at zero coverage was obtained for ITIM-1 (MIL-101Fe).     

Efficient catalytic abatement of greenhouse gases: Methane reforming with CO2 using a novel and thermally stable Rh–CeO2 catalyst

2% Rh–CeO₂ catalyst was synthesized using the hard template method and characterized by means of N₂ adsorption/desorption, XRD and H₂-TPR methods. The prepared powdered catalyst exhibited high thermal stability and high surface area with negligible sintering during 24-h exposure to 973 K in an inert atmosphere. During the temperature programmed methane dry reforming reaction between 473 and 1073 K, an increase in the molar H₂/CO ratio from 0.3 at 623 K to as high as 0.96 at 1073 K was observed. Besides H₂ and CO, H₂O was identified among reaction products, originating from the simultaneously occurring reverse water-gas shift reaction. During the isothermal test performed at 923 K, the 2% Rh–CeO₂ catalyst exhibited stable performance and produced syngas with the average H₂/CO ratio equal to 0.62. A relative drop of catalyst activity equal to 11% was observed within 70-h time on stream at 1023 K, with the average H₂/CO ratio at the reactor outlet equal to 0.71.     

Bench-scale WGS membrane reactor for CO2 capture with co-production of H2

This study investigated the water-gas shift reaction in a bench-scale membrane reactor (M-WGS), where three supported Pd membranes of 44 cm in length and ca. 6 μm in thickness were used, reaching a total membrane surface area of 580.6 cm². The WGS reaction was studied with the syngas mixture: 4.0% CO, 19.2% CO₂, 15.4% H₂O, 1.2% CH₄ and 60.1% H₂, under high temperature/pressure conditions: T = 673 K, p_feed = 20–35 bar(a), p_perm = 15 bar(a), mimicking CO₂ capture with co-production of H₂ in a natural gas fired power plant. High reaction pressure and high permeation of Pd membranes allowed for near complete CO conversion and H₂ recovery. Both the membranes and the membrane reactor demonstrated a long-term stability under the investigated conditions, indicating the potential of M-WGS to substitute conventional systems.