The effect of metal content on nickel-based catalysts obtained from hydrotalcites for WGSR in one step

Environmental problems related to the use of fossil fuels and the development of new technologies for energy production from hydrogen have renewed the interest for the water gas shift reaction (WGSR). For several years, the WGSR has been used to eliminate carbon monoxide from hydrogen-rich streams produced during natural gas reforming, as well as to increase hydrogen production. For reasons of economy, in industrial processes the reaction is carried out in two steps at different temperatures. There is, however, interest to develop catalysts able to operate at one step at intermediate temperatures. In a previous work, we had found that nickel-based catalysts prepared from hydrotalcite-like precursors were promising for WGSR in a single step. In the present work, the influence of the nickel amount on the properties of the catalysts was investigated, in order to find the best formulation for these catalysts. The samples were prepared by precipitation techniques and characterized by chemical analysis, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, specific surface area and porosity measurements, temperature programmed reduction, X-ray photoelectron spectroscopy and transmission electron microscopy. The catalysts were evaluated in WGSR at 1 atm and at different temperatures. During the heating of hydrotalcite-like precursors, aluminum cations were incorporated into the nickel oxide lattice, hindering the reduction of this oxide; however, the addition of zinc decreased this effect. The existing interaction between nickel oxide and alumina (γ-Al₂O₃) is comforted by the observation of Moiré fringes caused by the interference of crystallographic planes of both phases. All catalysts were active in the WGSR but the nickel-richest ones also produced methane, this effect being diminished or prevented by the addition of zinc to the catalysts. No simple relationship was found between nickel amount and the activity or selectivity of the catalysts probably due to the different interactions among the metals at different reaction temperatures. The most efficient catalyst was obtained by heating the N_0.37Z_0.37A_0.25 hydrotalcite, which produces a solid able to work in WGSR in one step at an intermediate temperature (300 °C) leading to 100% of carbon monoxide conversion without producing methane.     

Catalytic hydrolysis of sodium borohydride solution for hydrogen production using thermal plasma synthesized nickel nanoparticles

In the present study nickel nanoparticles were synthesized by thermal plasma route. In this method we obtained highly crystalline almost spherical nanoparticles with maximum number of particles having size around 30–50 nm. These nanoparticles were thoroughly characterized and employed as a catalyst for hydrogen production using hydrolysis of sodium borohydride (NaBH₄). The effect of initial concentration of NaBH₄, pH and temperature of solution on the rate of hydrogen production was investigated. Nickel nanoparticles exhibits first order reaction with respect to NaBH₄ concentration at elevated temperatures. After hydrolysis, the nickel nanoparticles showed presences of B–O and B–OH species on the nickel surface. The catalyst was found to be stable during 5 sequential cycles of test.     

Thermophysical and optical properties of NiCo2O4@ZrO2: A potential composite for thermochemical processes

The use of solar energy through thermochemical processes is an important approach to drive endothermic reactions to produce solar fuels such as hydrogen or syngas. This work reports the preparation and the thermophysical characterization of a porous composite based on zirconium dioxide (ZrO₂) and nickel cobaltite (NiCo₂O₄) nanoparticles for applications in thermochemical processes at high temperatures. The ZrO₂ supports were modified with NiCo₂O₄ nanoparticles by a low-cost and straightforward impregnation process following by thermal treatment at 773 K. The impregnated NiCo₂O₄ obtained is formed by nanoparticles with an average size of 50 nm favoring a complete and homogenous covering of ZrO₂ supports. The thermal properties of ZrO₂ supports and NiCo₂O₄@ZrO₂ composites were evaluated in the temperature range from 300 to 1250 K. Besides, the solar absorbance and thermal emittance values were measured. After depositing the nickel cobaltite nanoparticles in the supports, it has been observed that the thermal properties have changed slightly so that the added nanoparticles do not significantly change the thermal performance of the materials. The nickel cobaltite nanoparticles deposited on the surface of the ZrO₂ supports causes a strong increase in solar absorbance. This improves the efficiency of solar thermal conversion. Our results have shown that NiCo₂O₄@ZrO₂ has excellent characteristics to be used in solar thermochemical processes.     

The use of stainless steel and nickel alloys as low-cost cathodes in microbial electrolysis cells

Microbial electrolysis cells (MECs) are used to produce hydrogen gas from the current generated by bacteria, but low-cost alternatives are needed to typical cathode materials (carbon cloth, platinum and Nafion™). Stainless steel A286 was superior to platinum sheet metal in terms of cathodic hydrogen recovery (61% vs. 47%), overall energy recovery (46% vs. 35%), and maximum volumetric hydrogen production rate (1.5 m³ m⁻³ day⁻¹ vs. 0.68 m³ m⁻³ day⁻¹) at an applied voltage of 0.9 V. Nickel 625 was better than other nickel alloys, but it did not perform as well as SS A625. The relative ranking of these materials in MEC tests was in agreement with cyclic voltammetry studies. Performance of the stainless steel and nickel cathodes was further increased, even at a lower applied voltage (0.6 V), by electrodepositing a nickel oxide layer onto the sheet metal (cathodic hydrogen recovery, 52%, overall energy recovery, 48%; maximum volumetric hydrogen production rate, 0.76 m³ m⁻³ day⁻¹). However, performance of the nickel oxide cathodes decreased over time due to a reduction in mechanical stability of the oxides (based on SEM–EDS analysis). These results demonstrate that non-precious metal cathodes can be used in MECs to achieve hydrogen gas production rates better than those obtained with platinum.     

Electrochemical synthesis of hexagonal closed pack nickel: A hydrogen storage material

An in situ hydrogen generation and storage technique is demonstrated during the electrodeposition of hexagonal closed pack (HCP) nickel from NiCl₂-1-ethyl-3-methylimidazolium chloride (NiCl₂-EmimCl) and NiCl₂·6H₂O-1-ethyl-3-methylimidazolium chloride (NiCl₂·6H₂O-EmimCl) melts. During electrolysis, the dissolution of hydrogen in nickel takes place due to the electrolysis of water. This results in the production of HCP nickel. The hydrogen content of the electrodeposited nickel from NiCl₂-EmimCl was found to be 1.2 wt.%. Thermal analysis showed that the phase transformation from HCP nickel to FCC occurred at 462 °C, releasing hydrogen in the process.     

Enhancement of anaerobic hydrogen production by iron and nickel

The effects of iron and nickel on hydrogen (H₂) production were investigated in a glucose-fed anaerobic Continuous Flow Stirred Tank Reactor (ACSTR). Both iron and nickel improved the reactor performance and H₂ production was enhanced by 71% with the sole iron or nickel supplementation. In all cases, H₂ production yield was increased by lowering both ethanol and total metabolites production and increasing butyrate production. Furthermore, iron and nickel slightly increased biomass production while glucose degradation decreased with the supplementation of nickel. Dynamic changes in bacterial composition as analyzed by 16S rRNA gene-targeted denaturing gradient gel electrophoresis (DGGE) revealed that hydrogen was produced mainly by Clostridium butyricum strains and that nickel addition decreased the microbial diversity.     

Size and strain dependent activity of Ni nano and micro particles for hydrogen evolution reaction

In the context of constant research for the improvement of alkaline water electrolysis process using advanced electrocatalytic materials for the hydrogen evolution reaction (HER), various nickel particle based electrode materials were prepared and characterized. The synthesis of nickel hydroxide nanoparticles was performed in water in presence of three different stabilizers (CTAB, PVP and KBr). A thermal treatment at 400 °C under 5% H₂/Ar atmosphere led to nickel nanoparticles. Mechanically milled commercial micrometric particles and nanoparticles synthesised by a polyol route completed a series of Ni powders showing broad ranges of size (5 nm–73 μm) and strain (6 ppm–0.7%). The electrocatalytic activity of the resulting electrode materials was evaluated versus powder morphology. Their apparent and intrinsic activity and the mechanism of the HER were studied by electrochemical impedance spectroscopy (EIS) and steady-state polarisation. A change in the HER mechanism is observed depending on particle size. This first systematic study demonstrates that the smaller the size and the more defective the particles, the greater the electrocatalytic activity. As a matter of fact, appreciable cathodic current densities of 100 mA cm⁻² at ∼ −300 mV of overpotential were obtained for nickel nanoparticles with 5 nm size and 0.7% strain.     

Thermodynamic analysis of a membrane-assisted chemical looping reforming reactor concept for combined H2 production and CO2 capture

There is great consensus that hydrogen will become an important energy carrier in the future. Currently, hydrogen is mainly produced by steam reforming of natural gas/methane on large industrial scale or by electrolysis of water when high-purity hydrogen is needed for small-scale hydrogen plants. Although the conventional steam reforming process is currently the most economical process for hydrogen production, the global energy and carbon efficiency of this process is still relatively low and an improvement of the process is key for further implementation of hydrogen as a fuel source. Different approaches for more efficient hydrogen production with integrated CO₂ capture have been discussed in literature: Chemical Looping Combustion (CLC) or Chemical Looping Reforming (CLR) and membrane reactors have been proposed as more efficient alternative reactor concepts relative to the conventional steam reforming process. However, these systems still present some drawbacks. In the present work a novel hybrid reactor concept that combines the CLR technology with a membrane reactor system is presented, discussed and compared with several other novel technologies. Thermodynamic studies for the new reactor concept, referred to as Membrane-Assisted Chemical Looping Reforming (MA-CLR), have been carried out to determine the hydrogen recovery, methane conversion as well as global efficiency under different operating conditions, which is shown to compare quite favorably to other novel technologies for H₂ production with CO₂ capture.     

Impacts of nano-metal oxides on hydrogen production in anaerobic digestion of palm oil mill effluent – A novel approach

In the present study, hydrogen production from palm oil mill effluent (POME) was investigated with the incorporation of nanoparticles (NPs) comprising of nickel (NiO) and cobalt oxides (CoO). The NPs of NiO and CoO were prepared using hydrothermal method and were further applied to analyse, their effect on hydrogen production. The results demonstrated that, a maxima volumetric hydrogen production rate of 21 ml H₂/L-POME/h with the hydrogen yield of 0.563 L H₂/g-COD_removed was obtained with 1.5 mg/L concentration of NiO NPs. On the other hand, the addition of CoO NPs produced maximum volumetric hydrogen production rate of 18 ml H₂/L-POME/h with a hydrogen yield of 0.487 L H₂/g-COD_removed with 1.0 mg/L of CoO NPs. Results showed that addition of optimal concentration of NiO and CoO NPs to the POME enhances the hydrogen yield by 1.51 and 1.67 fold respectively. Besides, this addition of NiO and CoO enhanced the COD removal efficiency by 15 and 10% respectively as compared to an un-additive NPs POME. The toxicity of NPs was also tested using bacterial viability test, which revealed that application of 3.0 mg/L of NiO and CoO NPs to modified Luria-Bertani (LB) medium had 63% and 83% reduction in bacterial cell growth. The results concluded that supplementation of NiO and CoO NPs under an optimal range to the wastewater can improve the hydrogen productivity.     

Electrospun carbon nanofiber-encapsulated NiS nanoparticles as an efficient catalyst for hydrogen production from hydrolysis of sodium borohydride

Carbon nanofibers (CNFs) incorporating NiS nanoparticles (NPs), namely NiS@CNFs were prepared by one-step electrospinning and successfully employed as a catalyst for hydrogen production from hydrolytic dehydrogenation of sodium borohydride (SBH). As-prepared NiS@CNFs, composed of polyacrylonitrile (PAN), nickel acetate, and ammonium sulfide, was calcined at 900 °C in argon atmosphere, and characterized using standard surface science techniques. The combined results revealed the growth of NiS NPs inside the CNFs, hence confirmed the presence of elemental Ni, S, and C. The as-prepared NiS@CNFs catalyst has a significantly higher surface area (650.92 m²/g) than the reported value of 376 m²/g. Importantly, this catalyst exhibited a much higher catalytic performance, for H₂ production from SBH, than that of Ni@CNFs, as evidenced by its low activation energy (∼25.11576 kJ/mol) and their R_max values of 2962 vs. 1770 mL/g·min. Recyclability tests on using NiS@CNFs catalyst showed quantitatively production (∼100% conversion) of H₂ from SBH and retained up to 70% of its initial catalytic activity after five successive cycles. The low cost and high catalytic performance of the designed NiS@CNFs catalyst enable facile H₂ production from readily available hydrogen storage materials.     

Enhancement of electrocatalytic activity towards hydrogen evolution reaction by boron-modified nickel nanoparticles

Nanosized nickel particles have been synthesized by three different routes: polyol, microemulsion and precipitation/reduction methods. Nickel nanoparticles have been evaluated as electrocatalysts for the hydrogen evolution reaction (HER). The electrocatalysts have been characterized by using X-ray diffraction (XRD), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). Their electrocatalytic performance in the hydrogen evolution reaction has been evaluated by means of the Tafel curves recorded in alkaline medium. The activity for the hydrogen evolution reaction increases with the increasing amount of reduced Ni in the electrocatalysts. Remarkably, the formation of a nickel-boride alloyed phase (Ni₃B) is responsible for the higher activity of the sample prepared by the precipitation/reduction method for the HER. The crystalline phase Ni₃B appears to be responsible for the very high activity in hydrogen production.     

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

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

Nickel oxide/Au porous nanobelts: Synthesis and visible light-driven photocatalytic hydrogen production

Nickel oxide/Au porous nanobelts were synthesized by hydrolysis of nickel (II) sulfate in the presence of Au nanoparticles under hydrothermal condition and subsequent calcination of Ni(OH)₂/Au precursor in air. XPS measurement indicated the existence of both NiO and Ni₂O₃ in nickel oxide/Au porous nanobelts. Heterojunctions between NiO and Ni₂O₃, as well as between NiO and Au, were observed in HRTEM images. Under visible light irradiation (λ > 420 nm), nickel oxide/Au porous nanobelts effectively produced hydrogen from water in the presence of Na₂S and Na₂SO₃ as sacrificial electron donor. The highest rate of hydrogen production over nickel oxide/Au porous nanobelt was 38.5 times higher than that of commercial NiO loaded with the same amount of Au. The photoluminescence spectra and transient photocurrent responses of nickel oxide/Au nanobelts reveal that the improved photocatalytic activity is due to the increased electron–hole separation at NiONi₂O₃ and NiOAu interfaces. This work shows that low-cost nickel oxide could be used as visible light-active photocatalyst for hydrogen production.     

2D materials-based membranes for hydrogen purification: Current status and future prospects

Hydrogen (H₂) is the most viable energy carrier to replace fossil fuels and achieve zero net emission. To realize hydrogen-based society, the development of energy-efficient and high-purity H₂ production techniques is one of the key issues as currently mature H₂ purification technologies are energy-intensive. Membrane separation is an attractive option to obtain high-purity H₂ gas with low energy consumption. However, membrane separation processes are often limited by the low performance of commercial polymeric membranes despite the progress made over the last few decades. As an alternative, two-dimensional nanomaterials (2DNMs) have recently drawn tremendous attention as membrane materials thanks to their unique physical and chemical properties. Herein, we seek to offer a comprehensive review of 2DNM-based membranes for H₂ gas separation. Also, we discuss the current technological challenges and provide our perspectives for future research.     

An efficient ternary photocatalyst via anchoring nickel complex and nickel oxides onto carbon nitride for visible light driven H2 evolution

Developing earth abundant, active and stable photocatalysts for water splitting is a critical but challenging procedure for efficient conversion and storage of sustainable energy. Here, a ternary photocatalyst was rationally prepared for efficient H₂ production by covalently anchoring a nickel molecule cocatalyst (NiL) onto graphitic carbon nitride nanosheets (CN) and introducing nickel oxides (NiO_x) as hole-transport materials. The lower H₂ overpotential by NiL and the faster separation of photoinduced carriers by NiO_x nanoparticles account for the efficient H₂ generation of CN without the help of noble metals. Eventually, the prepared NiL/NiO_x/CN catalyst exhibited excellent performance for H₂ evolution (289 μmol g^?1 h^?1) in TEOA solution under visible light irradiation, which is superior to 3NiL/CN (161 μmol g^?1 h^?1) and CN (Null). Furthermore, a possible mechanism of photocatalytic H₂ production for NiL/NiO_x/CN is proposed based on a series of electrochemical measurements. The noble-metal-free photocatalyst developed in this work will pave a new way to synthesize low-cost multicomponent photocatalysts for solar conversion.     

Salicylaldimine-Ni complex supported on Al2O3: Highly efficient catalyst for hydrogen production from hydrolysis of sodium borohydride

In this study, the Ni-based complex catalyst containing nickel of 1% supported on Al₂O₃ is used as for the hydrogen production from NaBH₄ hydrolysis. The maximum hydrogen production rate from hydrolysis of NaBH₄ with Ni-based complex catalyst supported on Al₂O₃ containing nickel of 1% is 62535 ml min^?1 g^?1 (complex catalyst containing 1 wt% Ni). The resulting complex catalyst is characterised by XRD, XPS, SEM, FT-IR, UV, and BET surface area analyses. The Arrhenius activation energy is found to be 27.29 kJ mol^?1 for the nickel-based complex catalyst supported on Al₂O₃. The reusability of the catalyst used in this study has also been investigated. The Ni-based complex catalyst supported on Al₂O₃ containing nickel of 1% is maintained the activity of 100% after the fifth use, compared to the first catalytic use. The n value for the reaction rate order of NaBH₄ is found to be about 0.33.     

Assessment of four different cathode materials at different initial pHs using unbuffered catholytes in microbial electrolysis cells

Nickel foam (NF), stainless steel wool (SSW), platinum coated stainless steel mesh (Pt), and molybdenum disulfide coated stainless steel mesh (MoS₂) electrodes have been proposed as catalysts for hydrogen gas production, but previous tests have primarily examined their performance in well buffered solutions. These materials were compared using two-chamber microbial electrolysis cells (MECs), and linear sweep voltammetry (LSV) in unbuffered saline solutions at two different initial pHs (7 and 12). There was generally no appreciable effect of initial pH on production rates or total gas production. NF produced hydrogen gas at a rate of 1.1 m³ H₂/m³·d, which was only slightly less than that using Pt (1.4 m³ H₂/m³·d), but larger than that obtained with SSW (0.52 m³ H₂/m³·d) or MoS₂ (0.67 m³ H₂/m³·d). Overall hydrogen gas recoveries with SSW (29.7 ± 0.5 mL), MoS₂ (28.6 ± 1.3 mL) and NF (32.4 ± 2 mL) were only slightly less than that of Pt (37.9 ± 0.5 mL). Total energy recoveries, based on the gas produced versus electrical energy input, ranged from 0.75 ± 0.02 for Pt, to 0.55 ± 0.02 for SSW. An LSV analysis showed no effect of pH for NF and Pt, but overpotentials were reduced for MoS₂ and SSW by using an initial lower pH. At cathode potentials more negative than −0.85 V (vs Ag/AgCl), NF had lower overpotentials than the MoS₂. These results provide the first assessment of these materials under practical conditions of high pH in unbuffered saline catholytes, and position NF as the most promising inexpensive alternative to Pt.     

Catalytic effect of bis (cyclopentadienyl) nickel II on the improvement of the hydrogenation-dehydrogenation of Mg-MgH2 system

Bis(cyclopentadienyl) nickel II is one of the best precursors of nickel catalyst which remarkably improved the hydrogen absorption-desorption of Mg–MgH₂ system. The X-ray photoelectron spectroscopy (XPS) and Furrier Transformed Infrared Spectroscopy (FTIR) analyses revealed that bis (cyclopentadienyl) nickel II decomposed into metallic nickel during ball milling with MgH₂. The nickel thus formed has homogeneously doped over the Mg - MgH₂ surface. The Ni-doped Mg-MgH₂ have shown the excellent catalytic effect on hydrogen absorption-desorption. The catalyzed MgH₂ could desorb hydrogen below 225 °C (T_onset) under Ar flow, and absorb hydrogen at 50 °C under 1.5 MPa H₂ pressure. The hydrogen absorption-desorption temperatures are remarkably decreased as compared to the uncatalyzed Mg-MgH₂ system under the identical experimental conditions.