Investigation of porous silicon thin films for electrochemical hydrogen storage

In this study, nanoporous silicon (PS) layers have been elaborated and used for hydrogen storage. The effect of the thickness, porosity and specific surface area of porous silicon on the amount of hydrogen chemically bound to the nanoporous silicon structures is studied by Infrared spectroscopy (FTIR), cyclic voltammetry (CV), contact angle and capacitance –voltage measurements. The electrochemical characterization and hydrogen storage were carried out in a three-electrode cell, using sulfuric acid 3 M H₂SO₄ as electrolyte by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge. The results indicate the presence of two oxidation peaks at 0.2 V and 0.4 V on the anodic side corresponding to hydrogen desorption and a reduction peak at −0.2 V on the cathodic side corresponding to the sorption of hydrogen. Moreover, the EIS studies performed on PS electrode in 3 M H₂SO₄ show that the hodograph contains a semicircle at high frequency region and a line in the lower frequency zone. An equivalent circuit has been proposed; the values of the equivalent circuit elements corresponding to the experimental impedance spectra have been determined and discussed. Finally, the highest hydrogen storage in PS was 86 mAh/g. This storage capacity decreases by only 7% of the initial capacity value, after 40 cycles.     

Tin carbide monolayers decorated with alkali metal atoms for hydrogen storage

In this work, a density-functional study of hydrogen storage in tin carbide monolayers (2DSnC) decorated with alkali metals atoms (AM) such as Li, Na, and K, is reported. The most stable adsorption site for these alkali metal atoms on the 2DSnC is above a tin atom. The results indicate that the alkali metal atoms are chemisorbed on the 2DSnC and that electronic charge is transferred from the decorating atom to the 2DSnC. In all the studied cases, the hydrogen molecules are physisorbed on the AM-2DSnC (AM = Li, Na, and K) complexes and then these systems could be used for hydrogen storage. In particular, it is found that the K-2DSnC monolayer has the highest hydrogen-storage capacity, where a single potassium atom can adsorb up to 6 hydrogen molecules, followed by Na-2DSnC with 5 hydrogen molecules and Li-2DSnC with 3 hydrogen molecules. Finally, it can be estimated that when the K, Na and Li adatom-coverings respectively attain 40%, 44% and 70%, the hydrogen-storage gravimetric capacities of AM-2DSnC could overcome the US-DOE recommended target of 5.5 wt% for onboard automotive systems.     

First-principles study of hydrogen storage on Si atoms decorated C60

Hydrogen storage capacity of Si_nC₆₀ is studied via first-principles theory based on DFT and Canonical Monte Carlo Simulation (CMCS). It is shown that Si atoms strongly prefer D-site rather than other sites and in these structures maximum number of hydrogen molecule onto any Si atom is one. Each Si atom adsorbs one hydrogen molecule in molecular form and with proper binding energies when Si atom is placed in any D-site of C₆₀. Si atoms enhance remarkably hydrogen storage capability in fullerene.     

Theoretical study of hydrogen storage by spillover on porous carbon materials

Hydrogen storage by spillover in porous carbon material (PCM) has achieved great success in experiments. During the past 20 years, a large number of theoretical works have been performed to explore the hydrogen spillover mechanism, look for high-performance hydrogen storage materials and high-efficiency catalysts. In this paper, we summarize and analyze the results of the past researches, and draw the following conclusions: (1) In PCM surface, the stability of chemisorbed H can be reached through phase nucleation process, which can be initiated in the vicinity of surface impurities or defects. (2) To achieve the 2020 U.S. Department of Energy (DOE) target, the PCM material used for hydrogen storage by spillover should have a sp² carbon ratio greater than 0.43 and a surface area less than 3500 m²/g, which gives us an inspiration for exploring hydrogen spillover materials. (3) Due to a high barrier, the hydrogen spillover almost can not be initiated on pure PCM substrate at room temperature. By introducing the defects or impurities (e.g. holes, carbon bridges, oxygen functional groups, boron atoms and fluorine atoms), the spillover barriers can be reduced to a reasonable range. In addition, hydrogen atoms may also migrate in a gas phase. (4) According to our previous results of kinetic Monte Carlo simulations, there is a linear relationship between the reaction temperature and the migration barrier. The optimal barrier for the hydrogen spillover should be in the range of 0.60–0.88 eV. (5) Once the hydrogen atoms are chemically adsorbed on the carbon substrate, it is difficult to diffuse again due to the strong strength of C–H bond. Several theoretical diffusion mechanisms have been proposed. For example, the H atoms in physisorption state can diffuse freely on carbon surfaces with high mobility, using the shuttle gases (e.g. BH⁻₄, H₂O, HF and NH₃) to make the migration thermodynamically possible and decrease the migration barrier, the H atoms diffuse inside the interlayer space of the bi- and tetralayer graphene, and introducing the impurities on the surface to facilitate the hydrogen diffusion. (6) The H desorption through the directly recombination or the reverse spillover is unlikely to occur at normal temperature. The Eley-Rideal reaction may be the only possible mechanism for desorption of the adsorbed H atoms in carbon substrate. Finally, we have made a prospect for further research works on hydrogen storage by spillover.     

Melaleuca bark based porous carbons for hydrogen storage

Typical porous carbons were obtained from waster biomass, melaleuca bark activated by potassium hydroxide (KOH), and characterized by XRD, SEM, TEM, FTIR, XPS and N₂-sorption. The different samples with tunable morphologies and texture were prepared by controlling synthesis reaction parameters. The resulting samples demonstrate both high surface area (up to 3170 m² g⁻¹) and large hydrogen storage capacity (4.08 wt% at 77 K and 10 bar), implying their great potential as hydrogen storage materials.     

Lithium decoration characteristics for hydrogen storage enhancement in novel periodic porous graphene frameworks

Hydrogen storage in porous materials by physical adsorption is being discussed to provide widespread usage of hydrogen energy systems. One of the recent hydrogen storage media that store hydrogen physically is Porous Graphene Frameworks (PGFs). In the study, three different PGFs were constructed by using Benzene-1,3,5-tricarboxylic acid (BTC), 4,40,400-Benzene-1,3,5-triyltribenzoate (BTB) and 4,40,400-(benzene-1,3,5triyl-tris (benzene-4,1-diyl))tribenzoate (BBC) organic linkers. The geometries of the structures were optimized and lithium atoms were dispersed inside. Then, thirty-three different structures were derived. Finally, hydrogen storage capacities and surface areas of each structure were computed. It was found out that 160 lithium dispersed Graphene-BBC structure has the highest hydrogen storage capacity with 4.26 wt % at 298K and 100 bars while 70 lithium dispersed graphene-BTB structure store 9.81 wt % hydrogen at 77K and 4 bars, and lithium free graphene-BBC structure store 20,68 wt % hydrogen at 77K and 100 bars. Lithium dispersion enabled extra surfaces for Graphene-BTB and Graphene-BBC structures to the limits. But surface area of Graphene-BTC structure decreased with lithium dispersion. The number of limits for Graphene-BTB and Graphene-BBC named structures were 70 and 200 lithium atoms, respectively. At the final it is pointed out that constructed novel PGFs could store comparable and relatively high hydrogen in various conditions. The existence of lithium atoms played a minor role to enhance hydrogen storage capacity but the limits are critically important to reach maximum capacity.     

Polyacrylonitrile-based highly porous carbon materials for exceptional hydrogen storage

In this paper, a common low-cost chemical material-polyacrylonitrile (PAN) is transformed into porous carbon with excellent specific surface area (2564.6–3048.8 m² g⁻¹) and highly concentrated micropore size distribution (0.7–2.0 nm). Benefit to the unique structure, the as-prepared materials show appealing hydrogen adsorption capacity (4.70–5.94 wt % at 20 bar, 7.15–10.14 wt % at 50 bar), demonstrating a promising prospect of practical application. This work also confirmed that the narrow and deep ultramicropore (<0.7 nm) could facilitate adsorption of hydrogen molecules significantly at atmospheric pressure, and the volume increase of supermicropore (0.7–2.0 nm) could lead to hydrogen capacity promotion at relative high pressure (>20 bar), which provides valuable guidance for the construction of ideal porous adsorbent for efficiency hydrogen storage.     

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.     

Novel porous carbons synthesized from polymeric precursors for hydrogen storage

In this work, porous carbons were prepared from polymeric ion-exchangeable resin by a chemical activation method in order to obtain novel hydrogen storage materials, and the adsorption characteristics of the porous carbons were investigated. The textural properties were studied by BET and D–R methods with adsorption isotherms. The hydrogen storage behaviors of the porous carbons at 298 K and 100 bar were studied using a PCT apparatus. In the observed post-activation result, the hydrogen storage capacity was markedly improved. However, it was also found that the total amount of adsorption was not proportional to the specific surface area of the adsorbates. This indicates that hydrogen storage could be a function not only of specific surface area or total pore volume but also of microporevolume fraction or the average pore size of adsorbents.     

Photocatalytic generation of hydrogen coupled with in-situ hydrogen storage

To date, hydrogen generation and storage are two separated processes. We report on a new concept where photocatalytically generated hydrogen is simultaneously stored in-situ within the material photo-generating hydrogen. To this aim, we successfully synthesised a “forest” of vertically aligned TiO₂ nanotubes decorated with Pd nanoparticles acting as the hydrogen store. Upon illumination of TiO₂, hydrogen was effectively generated and full storage of hydrogen within the Pd nanostructures was achieved within 100 min. This result demonstrates new avenues on the possibility of designing hybrid nanostructures for the effective use of hydrogen as an energy vector.     

SANS characterization of porous magnesium for hydrogen storage

Porous magnesium was produced through the thermal decomposition of various additives in an effort to increase hydrogen storage capacity. Samples were characterized using SANS and different theoretical models were applied to the results and discussed. The polydisperse self-assembled (PSA) model was found to best represent the scattering from these materials as this model incorporates the polydispersity of the pores and allows for variations in structure factor. Pure magnesium produced using the same thermal method absorbed a negligible amount of hydrogen, and hydrogen uptake was found to increase with increasing porosity as determined using the PSA model. Maximum hydrogen uptake (1.3%) was found when 0.3% Cs₂CO₃ and 0.5% Ni were combined as an additive during thermal treatment. In addition, the development of porosity was found to promote hydrogen desorption at lower temperatures. SANS represents an indispensible method by which to characterize materials and the PSA model described in this work has the potential to be extremely useful in the characterisation of porous metallic systems.     

Degradation of biomass components to prepare porous carbon for exceptional hydrogen storage capacity

Porous carbon has been constructed in various strategies for hydrogen storage. In this work, a simple-effective strategy was proposed to transform sustainable biomass into porous carbon by degrade partial lignin and hemicellulose with Na₂SO₃ and NaOH aqueous mixture. This method collapses the biomass structure to provide more active sites, and also avoid the generation and accumulation of non-porous carbon nanosheets. As a result, the as-prepared sample possesses high specific surface area (2849 m² g^?1) and large pore volume (1.08 cm³ g^?1) concentrating almost completely on micropore. Benefit to these characteristics, the as-prepared sample exhibits appealing hydrogen storage capacity of 3.01 wt% at 77 K, 1 bar and 0.85 wt% at 298 K, 50 bar. The isosteric heat of hydrogen adsorption is as high as 8.0 kJ mol^?1, which is superior to the most biochars. This strategy is of great significance to the conversion of biomass and the preparation of high-performance hydrogen storage materials.     

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.     

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.     

Modeling hydrogen storage in boron‐substituted graphene decorated with potassium metal atoms

Boron‐substituted graphene decorated with potassium metal atoms was considered as a novel material for hydrogen storage. Density functional theory calculations were used to model key properties of the material, such as geometry, hydrogen packing, and hydrogen adsorption energy. We found that the new material has extremely high hydrogen storage capacity: 22.5 wt%. It is explained by high‐density packing of hydrogen molecules into hydrogen layers with specific geometry. In turn, such geometry is determined by the composition and topology of the material. Copyright © 2014 John Wiley & Sons, Ltd.     

Release of hydrogen during transformation from porous silicon to silicon oxide at normal temperature

The use of porous silicon as an energy carrier is investigated. NaOH and solid Mg alloy are used to introduce OH⁻ in water to react with the porous silicon and the porous silicon treated with Mg alloy in water is converted to transparent silicon oxide hydride. The amount and release rate of hydrogen from the reaction between porous silicon and water are determined and the efficiency is also studied. The total amount of released hydrogen does not vary much with the pH value but the release rate is sensitive to the pH value. The average amount of hydrogen produced form porous silicon can reach 63.2 mmol per gram of porous silicon. A moderate rate of about 1.77 mol of H₂ per mol of porous silicon can be obtained per day with the aid of the Mg alloy. This technique is potential useful in supplying hydrogen to fuel cells at normal temperature.     

High hydrogen storage capacity of rice hull based porous carbon

A kind of porous carbon with high specific surface area (approximately 4000 m²/g) was prepared from rice hull through carbonization and sodium hydroxide activation. The effects of preparation parameters on the characteristics of the porous carbon were studied. The properties of these porous carbon samples were investigated by X-ray diffraction and scanning electron microscope (SEM) and Fourier transform infrared spectroscopy. The rice hull based porous carbon exhibits high hydrogen storage capacity of 7.7 wt% at 77 K and 1.2 MPa.     

Theoretical discovery of high capacity hydrogen storage metal tetrahydrides

Hydrogen energy is attractive energy carrier due to its high energy density, abundant, environmentally friendly and renewable etc. However, the search for the high capacity hydrogen storage material is still a great challenge. In addition, the hydrogen storage materials should have excellent catalytic activity and superior mechanical properties to meet dehydrogenation and transportation. Here, we report on a novel metal tetrahydride that can effectively improve the hydrogen storage capacity. We obtain two novel metal tetrahydrides: TiH₄ and VH₄ based on the phonon dispersion and thermodynamically, respectively. In particular, those metal tetrahydrides not only exhibit good dehydrogenation behavior but also show superior mechanical properties. We demonstrate that the high hydrogen storage capacity of those tetrahydrides derives from the alternative stacking of metal layer and hydrogen layer. However, the excellent dehydrogenation process is attributed to the van der Waals interaction between hydrogen layers. Finally, the thermodynamic properties of TiH₄ and VH₄ are discussed.