Stable photocatalytic hydrogen evolution from water over ZnO–CdS core–shell nanorods

Stability and efficiency are important to realize the practical applications of photocatalysts for photocatalytic hydrogen evolution from water splitting. ZnO–CdS core–shell nanorods with a wide absorption range were designed and synthesized by a two-step route. The ZnO–CdS core–shell nanorods exhibit stable and high photocatalytic activity for water splitting into hydrogen in the presence of S²⁻ and SO₃²⁻ as sacrificial reagents. Furthermore, the photocatalytic activity and stability of ZnO–CdS core–shell nanorods/RuO₂ co-catalyst is superior to that of ZnO–CdS core–shell nanorods/Pt co-catalyst. The merits of stable ZnO and CdS, core–shell and nanorod structures employed are considered to contribute to the favorable photocatalytic hydrogen evolution of ZnO–CdS core–shell nanorods.     

Formation of flammable hydrogen–air clouds from hydrogen leakage

The possibilities of the formation of a flammable cloud over the ground in an open atmosphere from the leakage of hydrogen stored at different temperatures are studied. The dispersion of hydrogen in the stable and unstable atmospheric conditions is determined using the Gaussian dispersion model. The efflux of hydrogen from the storage vessel is considered at velocities between 1 m/s and 1500 m/s, the latter corresponding to the upper limit of velocities arising from the choked flow. The dispersion analysis shows that flammable hydrogen–air clouds would not be formed over the ground under unstable atmospheric conditions for all efflux velocities and leakage areas and for the different temperatures of the hydrogen leak. However, under strongly stable atmospheric conditions, such as those associated with clear sky winter nights with low winds and temperature inversion in the planetary boundary layer, a flammable cloud is seen to be formed. This is particularly true for low temperature hydrogen efflux and very low velocities of the efflux.     

Hydrogen generation from catalytic hydrolysis of sodium borohydride solution using Cobalt–Copper–Boride (Co–Cu–B) catalysts

Co–Cu–B, as a catalyst toward hydrolysis of sodium borohydride solution, has been prepared through chemical reduction of metal salts, CoCl₂·6H₂O and CuCl₂, by an alkaline solution composed of 7.5wt% NaBH₄ and 7.5wt% NaOH. The effects of Co/Cu molar ratio, calcination temperature, NaOH and NaBH₄ concentration and reaction temperature on catalytic activity of Co–Cu–B for hydrogen generation from alkaline NaBH₄ solution have been studied. X-ray diffraction (XRD), scanning electron microscope (SEM) and Nitrogen adsorption–desorption isotherm have been employed to understand the results. The Co–Cu–B catalyst with a Co/Cu molar ratio of 3:1 and calcinated at 400 °C showed the best catalytic activity at ambient temperature. The activation energy of this catalytic reaction is calculated to be 49.6 kJ mol⁻¹.     

Synthesis and characterization of metastable Li–Mn–O spinels from Mn(V)

Li₃MnO₄ Mn(V) was synthesized from LiOH and LiMnO₄, and characterized by gravimetric, X-ray, and XPS analysis. This precursor was used to improve the calcination time required for the low temperature solid state synthesis of highly oxidized spinels, Li₂Mn₄O_8+x. TG calculations were used to show the effect of calcination conditions on phase purity. The effect of a second phase on cycling performance was evaluated.     

Ni catalysts derived from Mg–Al layered double hydroxides for hydrogen production from landfill gas conversion

A layered double hydroxide (LDH) precursor with a hydrotalcite-like structure containing Ni/Mg/Al cations was prepared. A series of Ni catalysts containing mixed-oxides and spinel phases were then obtained through thermal treatment of the LDH precursor. X-ray diffraction (XRD), transmission electron microscopy (TEM), and temperature-programmed reduction (TPR) revealed that the LDH derived Ni catalysts have well-dispersed nickel phases upon reduction. The thermal treatment temperatures have noticeable effects on the specific surface area, pore volume, phase transformation, particle size, and reducibility of the catalysts. Thermal treatment temperatures up to 700 °C promote the generation of mesopores which facilitate an increase in specific area and pore volume. Beyond 700 °C sintering occurs, mesopores collapse, and specific area and pore volume decrease. High thermal treatment temperatures favor the phase transformation to spinel solid solutions and the particle size growth. Metal-support interaction is enhanced but reducibility is hindered due to the formation of spinel solid solution phases. The LDH derived Ni catalysts were tested for landfill gas conversion at 750 °C and have shown excellent activity and stability in terms of methane conversion. At gas hourly space velocity (GHSV) of 240,000 h⁻¹ and pressure of 1 atm, 81% methane conversion was achieved during a 48 h test period without apparent catalyst deactivation.     

Hydrogen generation from hydrolysis of sodium borohydride by cubic Co–La–Zr–B nano particles as novel catalyst

Cubic Co–La–Zr–B nano particles were prepared in situ for the first time from the reduction of Co(II), La(III) and Zr(IV) chloride by sodium borohydride in methanol under reflux condition. Poly N-vinyl-2-pyrrolidone (PVP) as stabilizing agent was used for preparation of Co–La–Zr–B nano particles. Obtained powders were characterized by XRD, BET, ICP, SEM, TEM and UV–vis techniques. XRD patterns declare that under argon atmosphere only metalboride phase has been crystallized and it was not seen any oxide phase of metals. TEM image depicts that PVP stabilized nano particles are square shaped particles that containing many nanoclusters. Cubic Co–La–Zr–B nano particles were also confirmed by SEM image. Co–La–Zr–B is highly active catalysts for hydrogen generation from the hydrolysis of sodium borohydride. The reported work also includes the full experimental details for the collection of a wealth of kinetic data to determine the activation energy (E_a = 53 kJ mol⁻¹) and effects of the catalyst dosage, amount of NaBH₄, and temperature on the rate of the catalytic hydrolysis of sodium borohydride. Catalytic hydrolysis of NaBH₄ is first order with respect to the catalyst concentration and also first order to the NaBH₄ concentration in the case of cubic Co–La–Zr–B nano particles.     

Novel Ni–Co–B hollow nanospheres promote hydrogen generation from the hydrolysis of sodium borohydride

Ni–Co–B hollow nanospheres were synthesized by the galvanic replacement reaction using a Co–B amorphous alloy and a NiCl₂ solution as the template and additional reagent, respectively. The Ni–Co–B hollow nanospheres that were synthesized in 60 min (Ni–Co–B-60) showed the best catalytic activity at 303 K, with a hydrogen production rate of 6400 mL_hydrogenmin^?1g_catalyst^?1 and activation energy of 33.1 kJ/mol for the NaBH₄ hydrolysis reaction. The high catalytic activity was attributed to the high surface area of the hollow structure and the electronic effect. The transfer of an electron from B to Co resulted in higher electron density at Co sites. It was also found that Ni was dispersed on the Co–B alloy surface as result of the galvanic replacement reaction. This, in turn, facilitated an efficient hydrolysis reaction to enhance the hydrogen production rate. The parameters that influenced the hydrolysis of NaBH₄ over Ni–Co–B hollow nanospheres (e.g., NaOH concentration, reaction temperature, and catalyst loading) were investigated. The reusability test results show that the catalyst is active, even after the fifth run. Thus, the Ni–Co–B hollow nanospheres are a practical material for the generation of hydrogen from chemical hydrides.     

Recovery of metal values from spent nickel–metal hydride rechargeable batteries

A hydrometallurgical process is developed for the separation and recovery of metal values such as nickel, cobalt and rare earths from spent nickel–metal hydride (Ni–MH) rechargeable batteries. After removal of the external case, the electrode materials are dissolved in 2 M sulfuric acid solution at 95°C. The resulting liquor contains typically (g l⁻¹), 10.6 Ni, 0.85 Co, 1.70 Fe, 0.36 Zn, 0.21 Al, 0.54 Mn, 1.73 La, 0.10 Ce, 0.33 Pr, 1.10 Nd and 0.032 Sm. The pH is around 0.4. The rare earth values are recovered from the liquor by means of a solvent extraction circuit with 25% bis(2-ethylhexyl) phosphoric acid (D2EHPA) in kerosene, followed by precipitation with oxalic acid. A mixed rare earth oxide of about 99.8% purity is obtained after calcination of the precipitate. The total yield of rare earths approaches 93.6%. The cobalt and nickel in the raffinate are effectively separated by solvent extraction with 20% bis(2,4,4-tri-methylpenthyl) phosphinic acid (Cyanex 272) in kerosene. The individual cobalt and nickel are then recovered as oxalates by the addition of oxalic acid. Cobalt and nickel oxalates with purities close to 99.6% and 99.8%, respectively, are obtained. The overall recoveries are over 96% for both cobalt and nickel. A total flowsheet of the process for recovery of rare earths, cobalt and nickel from spent Ni–MH batteries is proposed.     

Extensive study on laminar free film condensation from vapor–gas mixture

The dimensionless velocity component method was successfully applied in a depth investigation of laminar free film condensation from a vapor–gas mixture, and the complete similarity transformation of its system of governing partial differential equations was conducted. The set of dimensionless variables of the transformed mathematical model greatly facilitates the analysis and calculation of the velocity, temperature and concentration fields, and heat and mass transfer of the film condensation from the vapor–gas mixture. Meanwhile, three difficult points of analysis related to the reliable analysis and calculation of heat and mass transfer for the film condensation from the vapor–gas mixture were overcome. They include: (i) correct determination of the interfacial vapor condensate saturated temperature; (ii) reliable treatment of the concentration-dependent densities of vapor–gas mixture, and (iii) rigorously satisfying the whole set of physical matching conditions at the liquid–vapor interface. Furthermore, the critical bulk vapor mass fraction for condensation was proposed, and evaluated for the film condensation from the water vapor–air mixture, and the useful methods in treatment of temperature-dependent physical properties of liquids and gases were applied. With these elements in place, the reliable results on analysis and calculation of heat and mass transfer of the film condensation from the vapor–gas mixture were achieved.The laminar free film condensation of water vapor in the presence of air was taken as an example for the numerical calculation. It was confirmed that the presence of the non-condensable gas is a decisive factor in decreasing the heat and mass transfer of the film condensation. It was demonstrated that an increase of the bulk gas mass fraction has the following impacts: an expedited decline in the interfacial vapor condensate saturation temperature; an expedited decrease in the condensate liquid film thickness, the condensate liquid velocity, and the condensate heat and mass transfer. It was found that an increase of the wall temperature will increase the negative effect of the non-condensable gas on heat and mass transfer of the film condensation from the vapor–gas mixture.     

Ultrafine cobalt–molybdenum–boron nanocatalyst for enhanced hydrogen generation property from the hydrolysis of ammonia borane

The hydrolysis of ammonia borane (NH₃BH₃) is recognized as an efficient way of hydrogen generation if it can be effectively catalyzed. In this work, a series of cobalt–molybdenum–boron (Co–Mo–B) nanoparticles (NPs) on copper (Cu) foil are introduced as catalysts for NH₃BH₃ hydrolysis by electroless deposition method. The influence of the depositing pH value on the catalytic property is investigated by adjusting the pH value ranged from 10.5 to 12.0. By optimizing the value to 11, the ultrafine Co–Mo–B NPs with the grain size around 4.3 nm show the best catalytic property for NH₃BH₃ hydrolysis. The hydrogen generation rate reaches 5818.0 mL·min⁻¹·g⁻¹ when the hydrolysis temperature is 298 K. The thermodynamic tests show that the lower activation energy (E_a) is estimated to be 59.3 kJ·mol⁻¹. It can be found that the catalytic property in this work overtakes that of partial non-precious metal NPs, and is even better than some precious metal NPs previously reported. The hydrolysis reaction of NH₃BH₃ catalyzed by ultrafine Co–Mo–B NPs is a non-spontaneous process. In addition, the cycling ability of the ultrafine Co–Mo–B NPs is also studied and the results demonstrate that the catalyst is a recyclable one toward the hydrolysis of NH₃BH₃ under mild reaction conditions.     

Hydrogen production from water-splitting reaction based on RE-doped ceria–zirconia solid-solutions

The effect of rare earth (RE = Tb, Pr and La) dopant on the catalytic performance of RE-doped ceria–zirconia (CZRE) solid-solutions for oxygen storage capacity and hydrogen production activity has been successfully investigated. The sustainability of the solid-solutions even after the reduction was confirmed by XRD. Raman analysis showed that the addition of RE element in CZ system significantly decreased the intensity of the characteristic fluorite peak (462–474 cm⁻¹) indicating a highly deformed structure than CZ system which can enhance the oxygen mobility and redox property of these materials and the order of the intensity decrease was Pr > Tb > La. The XPS measurements revealed that the CZPr sample has a homogeneous distribution of Ce/Zr and also showed a high enrichment of Pr on the particle surface than the others. Among the CZRE solid-solution catalysts tested, CZPr catalyst showed the best catalytic performance for high OSC and hydrogen production from water-splitting reaction.     

Cobalt oxide preparation from waste LiCoO2 by electrochemical–hydrothermal method

Cobalt ions, extracted from waste LiCoO₂ by using a nitric acid leaching solution, are potentiostatically transformed into cobalt hydroxide on a titanium electrode and cobalt oxide is then obtained via a dehydration procedure. In linear sweep voltammetry, distinct cathodic current peak is observed and indicates that hydroxide ions are formed near the electrode via the electroreduction of dissolved oxygen and nitrate ions give rise to an increase in the local surface pH of the titanium. Under appropriate pH conditions, island-shaped cobalt hydroxide is precipitated on the titanium substrate and heat treatment of the cobalt hydroxide results in the formation of cobalt oxide.     

Epoxy-activated acrylic particulate polymer-supported Co–Fe–Ru–B catalyst to produce H2 from hydrolysis of NH3BH3

Epoxy-activated acrylic particulate polymer, namely Eupergit CM, supported Co–Fe–Ru–B catalyst (EP/Co–Fe–Ru–B) for the first time was used to produce H₂ from hydrolysis of NH₃BH₃. The EP/Co–Fe–Ru–B showed very effective performance in the production of H₂ from the hydrolysis of NH₃BH₃. Various techniques such as XRD, SEM-EDS, ICP-OES, and TEM have been used to characterize these catalysts. The parameters on the hydrolysis reaction of NH₃BH₃ such as the effect of metal amount, the effect of Ru percentage, the effect of NH₃BH₃ concentration, the effect of NaOH concentration, the amount of catalyst, temperature, and catalyst durability were investigated in detail. Eupergit CM based polymer support and Ru particles have been found to be highly effective in H₂ production reactions. The hydrogen production rate (HGR) of the EP/Co–Fe–Ru–B catalyst was found to be 36,978 mL/min/g_cat, which was quite good compared to the values reported in the literature. In addition, the activation energy (Ea) of the polymer-supported Co–Fe–Ru–B catalyst was determined as 24.91 kJ/mol.     

Nickel versus cobalt catalysts for hydrogen production by ethanol steam reforming: Ni–Co–Zn–Al catalysts from hydrotalcite-like precursors

The urea hydrolysis method allowed to prepare well-crystallized Ni–Co–Zn–Al Layered Double Hydroxides to be used as precursors of mixed oxide catalysts for the Ethanol Steam Reforming (ESR) reaction. The calcination of the layered precursors gives rise to high surface area mixed oxides, being actually a mixture of a rock salt phase (NiO), a wurtzite phase (ZnO) and a spinel phase.     

High-performance cobalt–tungsten–boron catalyst supported on Ni foam for hydrogen generation from alkaline sodium borohydride solution

Low cost and catalytically effective transition metal catalysts are highly wanted in developing on-demand hydrogen generation system for practical onboard application. By using a modified electroless plating method, we have prepared a robust Co–W–B amorphous catalyst supported on Ni foam (Co–W–B/Ni foam catalyst) that is highly effective for catalyzing hydrogen generation from alkaline NaBH₄ solution. It was found that the plating times, calcination temperature, NaBH₄ and NaOH concentrations all exert considerable influence on the catalytic effectiveness of Co–W–B/Ni foam catalyst towards the hydrolysis reaction of NaBH₄. Via optimizing these preparation and reaction conditions, a hydrogen generation rate of 15 L/min g (Co–W–B) has been achieved, which is comparable to the highest level of noble metal catalyst. In consistent with the observed pronounced catalytic activity, the activation energy of the hydrolysis reaction using Co–W–B/Ni foam catalyst was determined to be only 29 kJ/mol. Based on the phase analysis and structural characterization results, the mechanism underlying the observed dependence of catalytic effectiveness on the calcination temperature was discussed.     

Hydrogen storage properties of Mg–Ce–Ni nanocomposite induced from amorphous precursor with the highest Mg content

The effect of Ce and Ni contents on the glass-forming ability (GFA) of Mg–Ce–Ni system in the Mg-rich corner of Mg–Ce–Ni system is revealed. Ce is more advantageous for the GFA of Mg-rich Mg–Ce–Ni system than Ni, and the lowest Ce content is ∼5 at.% to obtain the fully amorphous alloy. Amorphous alloy with the highest Mg content, Mg₉₀Ce₅Ni₅, was obtained by melt-spinning. With the amorphous alloy as precursor, nanostructure multi-phases compositae was prepared by crystallizing it in hydrogenation process. The compositae with reversible hydrogen storage capacity of 5.3 wt.% shows much faster kinetics and lower MgH₂ desorption activation energy than those of induction-melt Mg₉₀Ce₅Ni₅ alloy. Both in situ formed nanosized Mg₂Ni and CeH_2.73 act as effective catalysts and significantly improve the hydrogen storage properties of MgH₂.     

Extracting equivalent circuit parameters of lead–acid cells from sparse impedance measurements

A method for extracting the equivalent circuit parameters of a lead–acid battery from sparse (only three) impedance spectroscopy observations at three different frequencies is outlined. The method is ideal for finding the parameters in an equivalent circuit consisting of bulk resistance, a reaction resistance and a constant phase element (CPE). Electrochemical impedance spectroscopy (EIS) measurements were made on spirally wound 2.5 Ah lead–acid cells. The equivalent circuit parameters found using our method closely compare with those extracted from the EIS measurements. The complete EIS spectrum calculated using the estimated circuit parameters at all other frequencies also closely matched the actual measurements.     

Performance and stability of SOFC anode fabricated from NiO–YSZ composite particles

Ni–YSZ cermet anodes for solid oxide fuel cells (SOFCs) were fabricated at various sintering temperatures from NiO–YSZ composite particles made by spray pyrolysis (SP) technique. NiO particles covered with fine YSZ (Y₂O₃ stabilized ZrO₂) particles were used as the composite particles, and the initial ratio of Ni and YSZ was set at 75:25 (mol%). As a result, the cermet anode sintered at 1350 °C showed the morphology in which fine YSZ grains were uniformly dispersed on the surface of Ni grain network. Electrical performance such as electrochemical activity and internal resistance of a Ni–YSZ cermet anode changed with sintering temperature. The anode fabricated at 1350 °C showed the highest electrical performance. Especially, a single cell voltage with the Ni–YSZ cermet anode kept very stable for 8000 h at 1000 °C in the SOFC operation condition of H₂—3% H₂O and air. The cermet anode after a long-term test had its initial morphology. It indicates that the Ni–YSZ cermet anode fabricated from NiO–YSZ composite particles is a very promising material for its practical use as SOFCs.     

CdO–CdS nanocomposites with enhanced photocatalytic activity for hydrogen generation from water

Nanocomposites of CdO–CdS have been prepared in ethylene glycol water mixture followed by heating at 300 °C. TEM and XRD studies confirmed the atomic scale mixing of CdO and CdS nanoparticles, leading to the formation of CdSO₃ phase at the interfacial region between CdO and CdS. Photocatalytic studies for hydrogen generation from water show an enhanced activity for CdO–CdS composites compared to individual components namely CdO or CdS nanoparticles. Based on optical absorption, surface area measurements, steady state and time resolved fluorescence studies, it is established that, enhanced absorption in the visible region, higher surface area and increase in lifetime of the charge carriers are responsible for the observed increase in hydrogen yield from water when composite sample was used as the photocatalyst compared to individual components. The composite sample when combined with Pt as co-catalyst exhibit a large increase in the photocatalytic activity.     

Hydrogen generation from alkaline NaBH4 solution using a dandelion-like Co–Mo–B catalyst supported on carbon cloth

In this study, a dandelion-like Co–Mo–B catalyst was prepared on carbon cloth (CC) by two-step electrodeposition method for the first time. The composition and microscopy are characterized by XRD and SEM technology. The results revealed that the as-synthesized Co–Mo–B catalyst exhibited high hydrogen generation rate (1280.80 mL min^?1 g^?1) and low activation energy (51.0 kJ mol^?1) for the hydrolysis of alkaline NaBH₄ solution. The results reveal that the reason might be due to high specific surface of the novel dandelion-like nanostructure and the synergistic effect of Co, Mo and B. Moreover, the catalytic activity was closely related to NaOH concentration, and OH^– anions were competitive with BH₄^– anions in alkaline NaBH₄ solution to transfer to the catalyst surface.