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.     

Synthesis of nickel hydroxide/reduced graphene oxide composite thin films for water splitting application

Facile synthesis of highly efficient and low-cost electrocatalyst for oxygen evolution reaction (OER) is important for large-scale hydrogen production. Herein, nickel hydroxide/reduced graphene oxide (Ni(OH)₂/rGO) composite thin film was fabricated using dip-coating followed by electrodeposition method on Ni foam substrate at room temperature. The deposited composite film shows amorphous nature with ultra-thin Ni(OH)₂ nanosheets vertically coated on rGO surface, which provides large electrochemical surface area and abundant catalytically active sites. It exhibits a low overpotential of 260 mV @10 mA cm⁻² as compared to the pristine electrodes and excellent long-term stability up to 20 hours in 1 M KOH solution. The electrochemical active surface area and Tafel slope of the composite electrode are 20.2 mF cm⁻² and 35 mV dec⁻¹, respectively. The superior water oxidation performance is a result of high catalytically active sites and improved conductivity of the composite electrode.     

Simulation and characterization of hydrogen evolution reaction on porous NiCu electrode using surface response methodology

In this study, porous NiCu coating was applied on Ni foam and electrocatalytic activity for hydrogen evolution reaction was studied using the simulation method. In order to optimize the electrocatalytic activity of the fabricated coating on the Ni foam, the experiment was designed using the response surface methodology (RSM). The results demonstrated that in the optimal condition, the porous NiCu electrode requires only 203 and 310 mV vs. RHE overpotentials at the current densities of 10 and 100 mA cm⁻², respectively. In addition, the Tafel slope was 88.2 mV dec⁻¹ and the electrochemically active surface area was about 1790 cm². Moreover, the porous NiCu catalyst depicted favorable electrocatalytic durability and affords long-term electrolysis without activity degradation. This high electrocatalytic activity and stability of coating can be attributed to the active surface area due to porous structure growth, rapid separation of hydrogen gas bubbles and the synergistic effect between Ni and Cu. This study offers an effective fabrication method for three-dimensional electrodes for renewable energy resources.     

Electrochemical performance of metal-organic framework MOF(Ni) doped graphene

In the paper, MOF(Ni) and MOF(Ni) complex with graphene MOF(Ni)-GR(w%) are synthesized by solvothermal method. The structure, morphology and elemental composition of samples are analyzed by physical characterization such as SEM, XRD, EDS and XPS. The results show that MOF(Ni) and MOF(Ni)-GR(w%) with high crystallinity and less impurities are successfully synthesized. Different samples are loaded on the electrodes and their electrochemical performances are tested by electrochemical workstation. The results show that doping appropriate graphene in MOF(Ni) can effectively improve the electrochemical catalytic activity of the composites. When the content of graphene is 4%, MOF(Ni)-GR(4%) is of the highest electrochemical catalytic activity, its overpotential is 268 mV, the Tafel slope is 108 mV·dec^?1, and it has better electrochemical stability compared with that of Pt/C electrode. The ECSA and EIS of MOF(Ni)-GR(4%) are superior to that of MOF(Ni) in exploring the reasons for enhancing catalytic hydrogen evolution. The reason is that the addition of graphene enhances the conductivity of the composites and reduces the resistance of catalyst transportation. Moreover, the doping of graphene improves the specific surface area of the composites, and provides enough active sites for hydrogen evolution reaction (HER).     

Controlled galvanic decoration boosting catalysis: Enhanced glycerol electro-oxidation at Cu/Ni modified macroporous films

We report on glycerol electro-oxidation in alkaline medium at macroporous Ni electrodes decorated with Cu particles. Macroporous Ni film is electrodeposited, using hydrogen bubbles as dynamic templates, atop of a Cu substrate. This film shows good electrocatalytic activity towards glycerol oxidation reaction (GOR). The Ni film is further decorated with Cu via spontaneous deposition from CuSO₄ solution. This is done to enhance the catalytic activity of the film towards GOR. The morphology of the Cu-decorated Ni film is controlled using various additives such as KCl and (NH₄)₂SO₄ which are added to the Cu deposition bath. The as-prepared Cu-decorated Ni films are characterized by electrochemical measurements, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). It is found that these additives have tremendous effects on the morphology and the electrocatalytic activity of the decorating Cu particles.The decorated Ni foam showed superior electrocatalytic activity towards the GOR, as confirmed by the negative shift in the onset oxidation potential (ca. 100 mV) together with an increase in oxidation current that is up to 1.5-fold during the cyclic voltammetry (CV) measurements, compared to the undecorated Ni foam.     

Dispersion and catalytic ignition of hydrogen leaks within enclosed spaces

An experimental investigation is conducted into the nature of catalytic ignition of leaked hydrogen gas within an enclosure, and the nature of hydrogen dispersion under varied venting conditions. Using a 1/16th linear scale two-car garage as a model, and a platinum foil as a catalytic surface, it is found that for all conditions tested, catalytic ignition is observed after the leaked hydrogen comes in contact with the catalytic surface, which is initially at or near room temperature. After ignition, these surface reactions lead to steady-state surface temperatures in the range of 600–800 K, dependent on inlet conditions in terms of mixture composition and flow rate. In addition, varying the venting opportunities from the garage walls suggests that not only total area, but also the number and position of vents may impact the nature of hydrogen accumulation within an enclosed structure.     

The HER in alkaline media on Pt-modified three-dimensional Ni cathodes

Electrodeposited porous Ni layers and commercial Ni foams were submitted to spontaneous deposition of Pt, achieved by immersing the Ni substrates in H₂PtCl₆ solutions, at open circuit, to produce Pt-modified 3D Ni electrodes. When using Ni foams, the immersion was prolonged until the whole amount of H₂PtCl₆ in the solution had reacted. Such an approach, which granted an easy control of the Pt loading, could not be used for Ni electrodeposits, since they underwent significant corrosion. The true Pt surface area was determined by measuring, for each electrode, the hydrogen desorption charge according to methods described in the literature. The ratios between Pt surface area and Pt loading were higher for Ni foam electrodes than for porous Ni electrodeposits. Both kinds of Pt-modified Ni electrodes were used as cathodes for hydrogen evolution in 1 M KOH. Cathodes with Pt loading below 0.5 mg cm⁻² (referred to geometric surface area) evolved hydrogen at −100 mA cm⁻² with a −75 mV overpotential. The better activity of foam electrodes as compared to electrodeposits, especially at low Pt loading, was mainly due to their higher Pt surface area per unit Pt mass.     

Development of new type Ni-Mg texture-like structure hydrogen absorber

This study demonstrated the feasibility of a novel Mg vapor deposition treatment on Ni foam to synthesize a Ni-Mg texture-like structure as a new type of hydrogen absorber. Energy dispersive spectrometry (EDS) yielded an estimative value of the weight percent ratio of Ni and Mg of 71.8 and 20.5 in as-prepared Ni-Mg texture-like structure. The microstructural changes were also characterized by X-ray diffraction (XRD) and the formed hydride tetragonal-MgH₂ was confirmed. The unique combination of large surface area of catalyst (Ni) and hydrogen acceptor (Mg) reduced the hydrogenation and dehydrogenation temperatures and performed the capability of reversible hydrogen storage capacity up to 0.72 wt.% H₂ at 25 °C. Ni-Mg texture-like structure achieved significant hydriding-dehydriding performances at lower temperature than traditional Mg-based hydrogen storage alloys. A possible hydrogen storage mechanism was also discussed where the catalytic Ni foam with large surface area was shown to be a vital factor in improving hydriding-dehydriding kinetics.     

3D nickel foams with controlled morphologies for hydrogen evolution reaction in highly alkaline media

Water electrolysis is the cleanest method for hydrogen production, and can be 100% green when renewable energy is used as electricity source. When the hydrogen evolution reaction (HER) is carried out in alkaline media, nickel (Ni) is a low cost catalyst and an interesting alternative to platinum. Still, its performance has to be enhanced to meet the high efficiency of the nobler metals, an objective that requires further tailoring of the surface area and morphology of Ni-based electrode materials. Unlike commercially available porous Ni, these features can be easily controlled via electrodeposition, a one-step process, taking advantage of the dynamic hydrogen bubble template (DHBT). Generally, changes in surface porosity and morphology have been mainly achieved by altering the main parameters, such as the current density or the deposition time. However, very scarce work has been done on the role of supporting electrolyte (i.e., its concentration and composition) in tailoring the foam features and consequently their catalytic activity. Hence, this approach paves the way to optimum design of metallic foam structures that can be obtained only with modifications in the electrolytic bath. In this work, 3D Ni foams are obtained from different composition baths by galvanostatic electrodeposition in the hydrogen evolution regime on stainless steel current collectors. Their porosity and morphology are analysed by optical microscopy and SEM. The electrochemical performance is evaluated by cyclic voltammetry, while catalytic activity towards HER and materials’ stability in 8 M KOH are tested using polarisation curves and chronoamperometry measurements, respectively. The recorded high currents and extended stability of the Ni foams with dendritic morphology demonstrate its outstanding performance, making it an attractive cathode material for HER in highly alkaline media.     

Three-dimensional Ni(OH)2 nanoflakes/graphene/nickel foam electrode with high rate capability for supercapacitor applications

Supercapacitor, known as an important energy storage device, is also a critical component for next generation of hydrogen fuel cell vehicles. In this study, we report a novel route for synthesis of three-dimensional Ni(OH)₂/graphene/nickel foam electrode by electrochemical depositing Ni(OH)₂ nanoflakes on graphene network grown on nickel foam current collector and explore its applications in supercapacitors. The resulting binder-free Ni(OH)₂/graphene/nickel foam electrode exhibits excellent supercapacitor performance with a specific capacitance of 2161 F/g at a current density of 3 A/g. Even as the current density reaches up to 60 A/g, it still remains a high capacitance of 1520 F/g, which is much higher than that of Ni(OH)₂/nickel foam electrode. The enhanced rate capability performance of Ni(OH)₂/graphene/nickel foam electrode is closely related to the presence of highly conductive graphene layer on nickel foam, which can remarkably boost the charge-transfer process at electrolyte–electrode interface. The three-dimensional graphene/nickel foam substrate also significantly improves the electrochemical cycling stability of the electrodeposited Ni(OH)₂ film because of the strong adhesion between graphene film and electrodeposited Ni(OH)₂ nanoflakes. Results of this study provide an alternative pathway to improve the rate capability and cycling stability of Ni(OH)₂ nanostructure electrode and offer a great promise for its applications in supercapacitors.     

Three-dimensional structure of WS2/graphene/Ni as a binder-free electrocatalytic electrode for highly effective and stable hydrogen evolution reaction

Tungsten disulfide (WS₂) has attracted much attention as the promising electrocatalyst for hydrogen evolution reaction (HER). Herein, the three-dimensional (3D) structure electrode composed of WS₂ and graphene/Ni foam has been demonstrated as the binder-free electrode for highly effective and stable HER. The overpotential of 3D WS₂/graphene/Ni is 87 mV at 10 mA cm^?2, and the current density is 119.1 mA cm^?2 at 250 mV overpotential, indicating very high HER activity. Moreover, the current density of 3D WS₂/graphene/Ni at 250 mV only decreases from 119.1 to 110.1 mA cm^?2 even after 3000 cycles, indicating a good stability. The high HER performance of 3D WS₂/graphene/Ni binder-free electrode is superior than mostly previously reported WS₂-based catalysts, which is attributed to the unique graphene-based porous and conductive 3D structure, the high loading of WS₂ catalysts and the robust contact between WS₂ and 3D graphene/Ni backbones. This work is expected to be beneficial to the fundamental understanding of both the electrocatalytic mechanisms and, more significantly, the potential applications in hydrogen economy for WS₂.     

Improved H2 production of ZnO@ZnS nanorod-decorated Ni foam immobilized photocatalysts

ZnO@ZnS nanorod-decorated Ni foam was prepared as a self-supported photocatalyst for hydrogen generation through a two-step method, including the formation of the ZnO nanorod core by a hydrothermal method, and the fabrication of the ZnS shell by a sulfidation method. The impact of the ZnS shell thickness was studied, including the influence on the optical properties, surface wettability, separation of photoexcited charge carriers, and photocatalytic hydrogen generation performance. Formation of the core-shell ZnO@ZnS structure and the incorporation of the conductive Ni foam substrate can enhance the separation of photoexcited carriers of the immobilized photocatalyst. The formation of ZnO@ZnS nanorods on the Ni foam resulted in a change in the surface from hydrophobic to superhydrophilic. The porous texture of the Ni foam facilitates the effective contact between the sacrificial agent and the immobilized photocatalyst. The ZnO@ZnS/Ni foam photocatalyst that was synthesized using a sulfidation time of 4 h, (namely, NZS4), exhibited H₂ generation activity of 5860 μmol g^?1 h^?1, which is approximately three-fold that of the ZnO/Ni foam photocatalyst (named NZ). After being reused for three cycles, with a simple washing between cycles, the NZS4 photocatalyst retained 90% of its hydrogen generation activity.     

Effect of single atomic layer graphene film on the thermal stability and hydrogen permeation of Pd-coated Nb composite membrane

Pd coated Nb-base composite membranes are preferable in the fields of hydrogen permeation. However, the rapid reduction of hydrogen permeability caused by high-temperature interfacial diffusion of Pd and Nb atoms hinders their large-scale application. In this paper, a single atomic layer graphene film was used for improving the thermal stability of a hydrogen-permeable composite membrane comprising a Pd coating on the Nb substrate. First, the graphene film was transferred onto the surface of the “clean” niobium substrate. Then a thin palladium coating was deposited on it by magnetron sputtering to form the niobium/graphene/palladium (Nb/Gr/Pd) composite membrane. The interfacial stability was evaluated in the temperature range of 673–973 K under vacuum, and the hydrogen permeation behavior was studied by gas-driven permeation method at 573–823 K. The results show that the single atomic layer graphene film can effectively compress the interdiffusion of Pd coating and Nb substrate and achieve a good hydrogen permeability below 823 K. However, it would be broken due to the micro-deformation of Nb substrate, the high mobility of Pd atoms, and the grain growth at a higher temperature. Therefore, it is concluded that the single atomic layer graphene film is unsuitable as an intermediate hindering layer for Nb-based hydrogen-permeable membranes.     

Fabrication of three-dimensional nanoporous nickel films with tunable nanoporosity and their excellent electrocatalytic activities for hydrogen evolution reaction

Three-dimensional (3D) nanoporous nickel films were fabricated by a novel and facile method. The fabrication process involved the heat treatment of the electrodeposited zinc layer on nickel substrate and the subsequent electrochemical dealloying. The mutual diffusion of Ni and Zn during the heat treatment resulted in the formation of the Ni₂Zn₁₁ alloy surface film. The 3D nanoporous nickel films with open pores and interconnected ligaments were obtained by the electrochemical dealloying of relatively active zinc from the alloy surface film. As the electrodeposited zinc amount increased, the thickness, pore diameter and pore density of the nanoporous nickel films became larger. In our experimental range, the thickest nanoporous nickel film presented a thickness of 8 μm and an average pore diameter of 700 nm. The as-prepared 3D nanoporous nickel films exhibited much higher electrocatalytic activity for hydrogen evolution reaction (HER) than smooth nickel foil, and their electrocatalytic activities for HER enhanced with increase in the porosity and thickness. It was concluded that the enhanced electrocatalytic activity and excellent electrochemical stability for HER of the as-prepared 3D nanoporous nickel films can be ascribed to their unique nanostructured characteristics.     

Electrochemical fabrication of Ni–Mo nanostars with Pt-like catalytic activity for both electrochemical hydrogen and oxygen evolution reactions

Synthesis of electrocatalysts with excellent performance for hydrogen and oxygen evolution are the main challenges for production of hydrogen by electrochemical water splitting method. Here, Ni–Mo nanostars were created by electrochemical deposition process at different morphologies and their electrocatalytic behavior was studied for hydrogen and oxygen evolution reactions in 1.0 M KOH solution. Increased electrochemically active surface area due to the nanostars formation, improved intrinsic electrocatalytic activity, increased surface wettability, as well as being binder-free during electrode production, resulted in excellent electrocatalytic behavior. For optimized condition, 60 mV and 225 mV overpotential are needed for generating the current density of 10 mA.cm-² in HER and OER process respectively in the alkaline medium. The lower slope of the electrode compared to the other electrodes also indicated that the kinetics of HER on the surface of the electrode was better. Also, there was very little change in the potential during the stability test, indicating the excellent electrocatalytic stability of the synthesized electrode. The present study introduces a rational, cost-effective and binder-free method for the synthesis of high performance electrocatalysts.     

One-step potentiostatic electrodeposition of Ni–Se–Mo film on Ni foam for alkaline hydrogen evolution reaction

Exploring efficient, abundant, low-cost and stable materials for hydrogen evolution reaction (HER) is highly desired but still a challenging task. Herein, Ni–Se–Mo electrocatalysts supported on nickel foam (NF) substrate were synthesized by a facile one-step electrodeposition method. The Ni–Se–Mo film presents high electrocatalytic activity and stability toward HER, with a low overpotential of 101 mV to afford a current density of 10 mA cm⁻² in 1.0 M KOH medium. Such excellent HER performance of Ni–Se–Mo film induced by the synergistic effects from Mo-doped Ni–Se film leads to the fast electron transfer. This work provides the validity of interface engineering strategy in preparing highly efficient transition metal chalcogenides based HER electrocatalysts.     

Self-standing,hybrid three-dimensional-porous MoS2/Ni3S2 foam electrocatalyst for hydrogen evolution reaction in alkaline medium

Self-standing and hybrid MoS₂/Ni₃S₂ foam is fabricated as electrocatalyst for hydrogen evolution reaction (HER) in alkaline medium. The Ni₃S₂ foam with a unique surface morphology results from the sulfurization of Ni foam showing a truncated-hexagonal stacked sheets morphology. A simple dip coating of MoS₂ on the sulfurized Ni foam results in the formation of self-standing and hybrid electrocatalyst. The electrocatalytic HER performance was evaluated using the standard three-electrode setup in the de-aerated 1 M KOH solution. The electrocatalyst shows an overpotential of 190 mV at ?10 mA/cm² with a Tafel slope of 65.6 mV/dec. An increased surface roughness originated from the unique morphology enhances the HER performance of the electrocatalyst. A density functional approach shows that, the hybrid MoS₂/Ni₃S₂ heterostructure synergistically favors the hydrogen adsorption-desorption steps. The hybrid electrocatalyst shows an excellent stability under the HER condition for 12 h without any performance degradation.     

Cyclic voltammetry electrodeposition of self-standing Ni–Fe–Sn ternary alloys for accelerating alkaline hydrogen evolution

Designing cost-effective and high-efficiency non-noble electrocatalysts for the hydrogen evolution reaction (HER) remains a significant challenge for electrochemical water splitting to store clean and renewable energy. Herein, we have developed Ni–Fe–Sn electrocatalysts grown on Ni foam (Ni–Fe–Sn@NF) for the HER through a simple and facile synthetic route of cyclic voltammetry electrodeposition. The optimized Ni–Fe–Sn electrocatalysts possess excellent electrocatalysis toward hydrogen evolution with a low overpotential of 103 mV at a current density of 10 mA cm⁻², together with a small Tafel slope value of 97.4 mV·dec⁻¹ in 1 M KOH. The electrocatalyst also features excellent stability even after 2000 cycles and strong durability after 50 h under alkaline condition. The high HER performance can be attributed to the moderately optimized electronic structure of the Ni, Fe, and Sn center and bind-free nature of the electrode, which facilitates electron transfer and speeds up reaction kinetics. Moreover, the improved electrochemical surface area also enhances the performance on hydrogen production. This strategy presents a facile and simple tactic for the synthesis of noble-metal-free electrocatalysts with excellent HER performance.     

Kinetics of the hydrogen evolution reaction on NiMn graphite modified electrode

The mechanism and kinetics of the hydrogen evolution reaction (HER) on graphite modified with Ni and NiMn electrode (G/Ni and G/NiMn) in 0.1 M NaOH solution were studied using the methods of steady-state polarization, electrochemical impedance spectroscopy, cyclic voltammetry and open circuit potential transient. The addition of Mn to Ni significantly increases the catalytic activity in HER due to higher real surface area and higher intrinsic activity. The simulation of the data obtained from these methods, using nonlinear fitting procedure allowed us to determine the rate constants of Volmer, Heyrovsky and Tafel steps associated with the mentioned reaction. The kinetics results indicate that HER mechanism for G/NiMn electrode at low negative potentials is a serial combination of Volmer and parallel Tafel and Heyrovsky steps. At high negative potentials where the hydrogen coverage reaches its limiting value, a Tafel line with the slope of −125 mV dec⁻¹ is obtained. In this potential region the mechanism of the HER follows Volmer-Heyrovsky while the Tafel step has negligible contribution. Open circuit potential measurements for G/Ni and G/NiMn at different charging currents show that hydrogen absorption into the electrode material occurs.     

Hydrogen adsorption on graphene foam synthesized by combustion of sodium ethoxide

Hydrogen storage is a crucial technology for the realization of a carbon-neutral society. However, few materials have been able to approach useful hydrogen storage capacity at reasonable temperatures and pressures. Graphene has an extremely high surface-area-to-weight ratio, is strong, cheap, chemically inert, and environmentally benign. As such it may be an ideal substrate for hydrogen storage. Here we present synthesis of graphene foam by combustion of sodium ethoxide. This technique is low-cost, scalable, and results in a three-dimensional graphene network with a surface area of more than 1200 m²/g. It is applied as a hydrogen storage material at liquid nitrogen temperature, with a capacity of 2.1 wt%.