Stabilized Li-decoration and enhanced hydrogen storage on reduced graphene oxides

Hydrogen storage properties of Li-decorated graphene oxides containing epoxy and hydroxyl groups are studied by using density functional theory. The Li atoms form Li₄O/Li₃OH clusters and are anchored strongly on the graphene surface with binding energies of −3.20 and −2.84 eV. The clusters transfer electrons to the graphene substrate, and the Li atoms exist as Li⁺ cations with strong adsorption ability for H₂ molecules. Each Li atom can adsorb at least 2H₂ molecules with adsorption energies greater than −0.20 eV/H₂. The hydrogen storage properties of Li-decorated graphene at different oxidation degrees are studied. The computations show that the adsorption energy of H₂ is −0.22 eV/H₂ and the hydrogen storage capacity is 6.04 wt% at the oxidation ratio O/C = 1/16. When the O/C ratio is 1:8, the storage capacity reaches 10.26 wt% and the adsorption energy is −0.15 eV/H₂. These results suggest that reversible hydrogen storage with high recycling capacities at ambient temperature can be realized through light-metal decoration on reduced graphene oxides.     

Assessment of the effect of electrophoretic deposition parameters on hydrogen storage performance of graphene oxide layer applied on nickel foam

In this study, the hydrogen storage capacity of the graphene oxide layer was studied electrochemically. The graphene oxide was synthesized by modified Hummers' method and applied on the nickel foam by electrophoretic deposition (EPD) method at different potentials (20 and 60 V) and times (20 and 60 min) to determine the effect of applied potential and time of deposition on the hydrogen adsorption performance. The hydrogen adsorption tests including charge-discharge test, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were conducted in 6 M KOH solution and at room temperature. Based on the achieved CV curves, the graphene oxide (GO) layer achieved at 60 V within 20 min has a higher electrochemical hydrogen adsorption capability compared to other obtained samples. The calculated hydrogen storage capacity is obtained 50.9 mA. h. g^?1. The rosette flower like morphology of the obtained GO layers at optimum condition, has an impressive effect on the improving electrochemical hydrogen adsorption based on morphology study by field emission scanning electron microscopy.     

Study on the hydrogen storage properties of the dual active metals Ni and Al doped graphene composites

The graphene nanosheets are synthesized by modified Hummer's method, based on which the dual active metals Ni and Al doped graphene composites are prepared through in-suit reaction and self-assembly with high-temperature reduction process. The molecular structure, morphology and specific surface area of graphene nanosheets are characterized systematically. The phase composition, surface morphology and hydrogen storage properties of dual active metals Ni and Al doped graphene composites are further investigated by X-ray diffraction, scanning electron microscopy and gas reaction controller. Results show that the graphene nanosheets have typical graphene feature, whose transparent graphene edges can be observed clearly, and the specific surface area is as high as 604.2 m² g⁻¹. The Ni and Al doped graphene composites are composed with Ni, Al and C phases, which have high hydrogen storage capacity and excellent hydriding/dehydriding stabilities. The maximum hydrogen storage uptake of such composites is up to 5.7 wt% at 473 K, and the dehydriding efficiency is high as 96%∼97% at the dehydriding temperature of 380 K. The hydrogen adsorption and desorption rate control step of the Ni and Al doped graphene composites is complied to the nucleation and two-dimensional growth mechanism.     

Boron-doping effect on the enhanced hydrogen storage of titanium-decorated porous graphene: A first-principles study

The exploitation of solid hydrogen storage materials is an important part of the large-scale application of hydrogen energy. However, Metal agglomeration is one of the main reasons that restrict the hydrogen storage performance of carbon-based hydrogen storage materials. Herein, we develop Ti-decorated boron doped porous graphene as a novel hydrogen storage material based on first-principles calculations. The geometry and electronic structure of Ti-decorated porous graphene with and without boron doped are calculated. Doping boron in porous graphene (PG) can significantly increase the metal-substrate interaction and prevents the formation of Ti-metal clusters. The Ti atom-decorated boron-doped porous graphene (Ti–B/PG) system can stably adsorb sixteen hydrogen molecules with a gravimetric hydrogen uptake of 8.58 wt%. The thermodynamic calculations prove a high usable capacity of the material, at the adsorbing and desorbing conditions of 25 °C, 30 atm and 100 °C, 3 atm. The excellent hydrogen capacity, good recyclability, and desirable desorption capacity of Ti–B/PG make it a very prospective material for hydrogen storage.     

Preheated self-aligned graphene oxide for enhanced room temperature hydrogen storage

This study explored the hydrogen adsorption capacity of self-assembled aligned graphene oxide at room temperature. The characteristics of as-prepared graphene oxide were determined by scanning electron microscopy, Raman spectroscopy, and X-ray diffractometry techniques. Three different temperatures were taken for preheating, i.e., 25, 250, and 400 °C. The maximum adsorption pressure was given to 20 bar, and we evaluated the hydrogen adsorption competency at room temperature (25 ± 2 °C). The maximum hydrogen storage capacity was achieved ~2.5 wt%, which was found for the graphene oxide sample preheated at 400 °C. This hydrogen storage capacity was 67% and 40% more than the graphene oxide samples preheated at 25 and 250 °C, respectively. Such an enhancement of hydrogen storage capacity in the self-aligned graphene oxide samples at room temperature is attributed to reduced interlayer spacing and increased topological defects in preheated graphene oxide samples at 400 °C.     

Li-decorated double vacancy graphene for hydrogen storage application: A first principles study

Lithium decoration is an effective strategy for improving the hydrogen adsorption binding energy and the storage capacity in carbon nanostructures. Here, it is shown that Li-decorated double carbon vacancy graphene (DVG) can be used as an efficient hydrogen storage medium by means of Density Functional Theory (DFT) based calculations. The Li binding energy in DVG is 4.04 eV, which is much higher than that of pristine graphene. A maximum of four hydrogen molecules adsorb on Li decorated on one side of DVG and this leads to a gravimetric storage capacity of 3.89 wt% with an average adsorption binding energy of 0.23 eV/H₂. When Li is decorated on both sides of DVG, the gravimetric storage capacity reaches 7.26 wt% with a binding energy of 0.26 eV/H₂ which shows that desorption would take place at ambient conditions.     

Energy storage of thermally reduced graphene oxide

The energy-storage capacity of reduced graphene oxide (rGO) is investigated in this study. The rGO used here was prepared by thermal annealing under a nitrogen atmosphere at various temperatures (300, 400, 500 and 600 °C). We measured high-pressure H₂ isotherms at 77 K and the electrochemical performance of four rGO samples as anode materials in Li-ion batteries (LIBs). A maximum H₂ storage capacity of ∼5.0 wt% and a reversible charge/discharge capacity of 1220 mAh/g at a current density of 30 mA/g were achieved with rGO annealed at 400 °C with a pore size of approximately 6.7 Å. Thus, an optimal pore size exists for hydrogen and lithium storage, which is similar to the optimum interlayer distance (6.5 Å) of graphene oxide for hydrogen storage applications.     

Si-doping effect on the enhanced hydrogen storage of single walled carbon nanotubes and graphene

We identified several parameters that correlate with the hydrogen physisorption energy and physicochemical properties of heteronuclear bonding in single-walled carbon nanotubes (SWCNT) and graphene. These parameters were used to find the most promising heteronuclear doping agents for SWCNTs and graphene for enhanced hydrogen storage capacity. Si-doping was showed to increase the amount of physisorbed hydrogen on such surfaces. Grand Canonical Ensemble Monte Carlo (GCMC) simulations showed that the hydrogen storage capacity of 10 at% Si-doped SWCNT (Si-CNT10) could reach a maximum of 2.5 wt%, almost twice the storage capacity of undoped SWCNTs, which were showed to reach a maximum capacity of 1.4 wt% at room temperature. To achieve this capacity, debundling effects of the uneven surfaces of Si-doped SWCNTs were found to be necessary. Similarly, 10 at% Si-doping on graphene (Si-GR10) was showed to increase the hydrogen storage capacity from 0.8 to 2.4 wt%.     

Diatom frustule-graphene based nanomaterial for room temperature hydrogen storage

Development of a room temperature hydrogen storage material is vital for the realization of a hydrogen economy. Towards achieving this goal, we present the use of naturally occurring diatom frustule for the synthesis of a novel nanomaterial that can achieve a hydrogen storage capacity of ~4.83 wt% at 25 ^°C and ~20 bar H₂ equilibrium pressure. We have effectively combined the large surface area of few layer graphene(G), the unique physical and chemical properties of diatom frustules(D) such as chemical inertness and good porosity and the ability of transition metals and their alloys (Pd₃Co) to adsorb large amounts of hydrogen. The resulting nanomaterial (Pd₃Co-D(100)-G) has a surface area of 163.25 m²/g and pore volume of 0.84 cm³/g. The observations in the present study suggest that increased surface area and porosity play a key role in achieving high hydrogen storage capacity at relatively low H₂ equilibrium pressures and room temperature conditions.     

Evaluation of electrochemical hydrogen storage capability of graphene oxide multi-layer coating

Due to its unique physical and chemical properties, graphene oxide (GO) has excellent potential in energy-saving applications, especially in hydrogen storage materials such as bulk and layer coatings. A three-layer GO/Ni/GO coating was applied successfully on the Ni-foam through the hybrid coating process. GO was firstly synthesized by modified Hummers' method and then deposited on Ni-Foam using an electrophoretic deposition (EPD) method at 40, 60, and 80 V at various times (1 and 2 h). Furthermore, Ni-layer was applied on the first applied to GO layer using a Ni-electroplating bath at 3 V for 3 min. For evaluation of the electrochemical hydrogen storage performance, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were carried out in 1 M NaOH solution at room temperature. Obtained results showed a significant improvement in the amount of adsorbed hydrogen. The hydrogen storage capacity was increased from 88 to 741 F. g^?1, and load transfer resistance decreased from 260 to 35 Ω for uncoated and coated porous substrates, respectively, which is due to the achieved high specific area. Based on the hydrogen storage capacity, the optimum EPD method is 60 V and 2 h.     

Cerium hydride generated during ball milling and enhanced by graphene for tailoring hydrogen sorption properties of sodium alanate

In this study, NaAlH₄?based hydrogen storage materials with dopants were prepared by a two-steps in-situ ball milling method. The dopants adopted included Ce, few layer graphene (FLG), Ce + FLG, and CeH_2.51. The hydrogen storage materials were studied by non-isothermal and isothermal hydrogen desorption measurements, X-ray diffractions analysis, cycling sorption tests, and morphology analysis. The hydrogen storage performance of the as-prepared NaAlH₄ with Ce addition is much better than that with CeH_2.51 addition. This is due to that the impact of Ce occurs from the body to the surface of the materials. The addition of FLG further enhances the impact of Ce on the hydrogen storage performance of the materials. The hydrogen storage capacity, hydrogen sorption kinetics, and cycle performance of NaAlH₄ with Ce + FLG additions are all better than NaAlH₄ materials with the addition of either Ce or FLG alone. The NaAlH₄ with Ce and FLG addition starts to release hydrogen at 85 °C and achieves a capacity of 5.06 wt% after heated to 200 °C. The capacity maintains at 4.91 wt% (94.7% of the theoretical value) for up to 8 cycles. At 110 °C, the material can release isothermally a hydrogen capacity of 2.8 wt% within 2 h. The activation energies for the two hydrogen desorption steps of NaAlH₄ with Ce and FLG addition are estimated to be 106.99 and 125.91 kJ mol^?1 H₂, respectively. The related mechanisms were studied with first-principle and experimental methods.     

Improved hydrogen storage properties of MgH2 by the addition of KOH and graphene

Mg hydride is a competitive candidates for hydrogen storage based on its high gravimetric hydrogen capacity and accessibility. In this study, a small amount of KOH and graphene were added into MgH₂ by high energy ball milling. MgH₂ doped with both KOH and graphene has a greatly improved hydrogen storage performance. The existence of graphene and the in-situ formed KMgH₃ and MgO decreased activation energy of MgH₂ to 109.89 ± 6.03 kJ/mol. The both KOH and graphene doped sample has a reversibly capacity of 5.43 wt % H₂ and can released H₂ as much as 6.36 times and 1.84 times faster than those of undoped sample and only KOH doped sample at 300 °C, respectively. The addition of graphene not only can provide more “H diffusion channels”, but also can disperse the catalyst.     

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.     

Storage of atomic hydrogen in multilayer graphene

The world is moving rapidly to embrace renewable energy sources. One energy carrier, hydrogen, is a growing player in this field. This study was motivated by a desire to find alternative hydrogen storage mechanisms using processes that are cost effective with low environmental impact and good energy storage efficiency. This paper presents initial research findings on the novel approach of employing the patented RMIT Proton Battery to store atomic hydrogen in a multilayer graphene electrode using an acid electrolyte. This is a very different approach to conventional hydrogen energy storage systems. The paper reveals that one supplier's product achieves a 0.35 wt% reversible hydrogen storage in a multilayer graphene material with 0.35 nm layer separation and a specific surface area of 720 m²/g.     

Room-temperature hydrogen storage performance of metal-organic framework/graphene oxide composites by molecular simulations

Metal-organic framework/graphene oxide (MOF/GO) composites have been regarded as potential room-temperature hydrogen storage materials recently. In this work, the influence of MOF structural properties, GO functional group contents and different amounts of doped lithium (Li⁺) on hydrogen storage performance of different MOF/GO composites were investigated by grand canonical Monte Carlo (GCMC) simulations. It is found that MOF/GO composites based on small-pore MOFs exhibit enhanced hydrogen storage capacity, whereas MOF/GO based on large-pore MOFs show decreased hydrogen storage capacity, which can be ascribed to the novel pores at MOF/GO interface that favors the enhanced hydrogen storage performance due to the increased pore volume/surface area. By integrating the small-pore MOF-1 with GO, the hydrogen storage capacity was enhanced from 9.88 mg/go up to 11.48 mg/g. However, the interfacial pores are smaller compared with those in large-pore MOFs, resulting in significantly reduced pore volume/surface area as well as hydrogen storage capacities of large-pore MOF/GO composite. Moreover, with the increased contents of hydroxyl, epoxy groups as well as carboxyl group modification, the pore volumes and specific surface areas of MOF/GO are decreased, resulting in reduced hydrogen storage performance. Furthermore, the room-temperature hydrogen storage capacities of Li⁺ doped MOF/GO was improved with increased Li⁺ at low loading and decrease with the increased Li⁺ amounts at high loading. This is due to that the introduced Li⁺ effectively increases the accessible hydrogen adsorption sites at low Li⁺ loading, which eventually favors the hydrogen adsorption capacity. However, high Li⁺ loading causes ion aggregation that reduces the accessible hydrogen adsorption sites, leading to decreased hydrogen storage capacities. MOF-5/GO composites with moderate Li⁺ doping achieved the optimum hydrogen storage capacities of approximately 29 mg/g.     

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.     

Superalkali NLi4 decorated graphene: A promising hydrogen storage material with high reversible capacity at ambient temperature

This paper investigates the decoration of superalkali NLi₄ on graphene and the hydrogen storage properties by using first principles calculations. The results show that the NLi₄ units can be stably anchored on graphene while the Li atoms are strongly bound together in the superalkali clusters. Decoration using the superalkali clusters not only solve the aggregation of metal atoms, it also provide more adsorption sites for hydrogen. Each NLi₄ unit can adsorb up to 10 H₂ molecules, and the NLi₄ decorated graphene can reach a hydrogen storage capacity 10.75 wt% with an average adsorption energy ?0.21 eV/H₂. We also compute the zero-point energies and the entropy change upon adsorption based on the harmonic frequencies. After considering the entropy effect, the adsorption strengths fall in the ideal window for reversible hydrogen storage at ambient temperatures. So NLi₄ decorated graphene can be promising hydrogen storage material with high reversible storage capacities.     

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.     

Improved room-temperature hydrogen storage performance of lithium-doped MIL-100(Fe)/graphene oxide (GO) composite

Li⁺ doping is regarded as an effective strategy to enhance the room-temperature hydrogen storage of metal-organic frameworks (MOFs). In this work, Li⁺ is doped into both MIL-100(Fe) and MIL-100(Fe)/graphene oxide (GO) composite, and it is demonstrated that the hydrogen uptake of Li⁺ doped MIL-100(Fe)/GO (2.02 wt%) is improved by 135% compared with Li⁺ doped MIL-100(Fe) (0.86 wt%) at 298 K and 50 bar, which is ascribed to its higher isosteric heat of adsorption (7.33 kJ/mol) resulting from its more accessible adsorption sites provided by doped Li⁺ ions and ultramicropores. Grand canonical Monte Carlo (GCMC) simulation reveals that Li⁺ ions distributing in the interface between MIL-100(Fe) and GO within MIL-100(Fe)/GO composite is favorable for hydrogen adsorption owing to the increased number of adsorption sites, thus contributing to the enhanced hydrogen storage capacity. These findings demonstrate that MIL-100(Fe)/GO is a more promising Li⁺ doping substrate than MIL-100(Fe).     

Enhanced reversible hydrogen storage in nickel nano hollow spheres

Nickel is a good catalyst for the dissociation of molecular hydrogen to atomic form, but suffers from a negligible hydrogen storage capacity (∼10⁻⁴ wt% at 25 °C and 1 atm). The current investigation pertains to the enhancement in the reversible hydrogen storage capacity of Ni via storage in molecular form; thus utilizing a recently developed multi-mode storage philosophy. Ni nano hollow spheres (NiHS) have been synthesized using hydrothermal method (outer diameter of ∼300 nm and shell thickness ∼30 nm). Pressure-composition-isotherms and temperature programmed desorption curves have been used to characterize the hydrogen storage capacity and to establish the reversibility of the process. An enhancement in the reversible storage capacity by a factor of 7 × 10³ (7,00,000%) is obtained at 25 °C and 150 bar pressure. The capacity is further enhanced to 0.91 wt% hydrogen by utilizing a pressure of 300 bar. Ni plays a dual catalytic role in the absorption and desorption process.