Improved hydrogen storage properties of MgH2 with Ni-based compounds

The nanoscaled Ni-based compounds (Ni₃C, Ni₃N, NiO and Ni₂P) are synthesized by chemical methods. The MgH₂-X (X = Ni₃C, Ni₃N, NiO and Ni₂P) composites are prepared by mechanical ball-milling. The dehydrogenation properties of Mg-based composites are systematically studied using isothermal dehydrogenation apparatus, temperature-programmed desorption system and differential scanning calorimetry. It is experimentally confirmed that the dehydrogenation performance of the Mg-based materials ranks as following: MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P. The onset dehydrogenation temperatures of MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P are 160 °C, 180 °C, 205 °C and 248 °C, respectively. The four Mg-based composites respectively release 6.2, 4.9, 4.1 and 3.5 wt% H₂ within 20 min at 300 °C. The activation energies of MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P are 97.8, 100.0, 119.7 and 132.5 kJ mol^?1, respectively. It' found that the MgH₂Ni₃C composites exhibit the best hydrogen storage properties. Moreover, the catalytic mechanism of the Ni-based compounds is also discussed. It is found that Ni binding with low electron-negativity element is favorable for the dehydrogenation of the Mg-based composites.     

Structural dependence of photocatalytic hydrogen production over La/Cr co-doped perovskite compound ATiO3 (A = Ca,Sr and Ba)

The crystal structure of a photocatalyst generally plays a pivotal role in its electronic structure and catalytic properties. In this work, we synthesized a series of La/Cr co-doped perovskite compounds ATiO₃ (M = Ca, Sr and Ba) via a hydrothermal method. Their optical properties and photocatalytic activities were systematically explored from the viewpoint of their dependence on structural variations, i.e. impact of bond length and bond angles. Our results show that although La/Cr co-doping helps to improve the visible light absorption and photocatalytic activity of these wide band gap semiconductors, their light absorbance and catalytic performance are strongly governed by the TiO bond length and TiOTi bond angle. A long TiO bond and deviation of TiOTi bond angle away from 180° deteriorate the visible light absorption and photocatalytic activity. The best photocatalytic activity belongs to Sr_0.9La_0.1Ti_0.9Cr_0.1O₃ with an average hydrogen production rate ～2.88 μmol/h under visible light illumination (λ ≥ 400 nm), corresponding to apparent quantum efficiency ～ 0.07%. This study highlights an effective way in tailoring the light absorption and photocatalytic properties of perovskite compounds by modifying cations in the A site.     

The effects of duty cycles on pulsed current electrodeposition of ZnNiAl2O3 composite on steel substrate: Microstructures,hardness and corrosion resistance

In this study, we examine the effect of duty cycles (33%, 50% and 67%) under square-wave galvanostatic pulses on the electrodeposition of zinc-nickel-alumina (ZnNiAl₂O₃) composites from a sulfate bath. XRD results showed that the dominant phases of the ZnNiAl₂O₃ electrodeposits were mixtures of Zn₂₁Ni₅ and Zn₂₂Ni₃ phases together with as Al₂O₃. The Ni content measured in the electrodeposits using EDS varied from 9.73 to 13.47 wt%. SEM results showed that finer and smoother surface electrodeposits were obtained by pulsed current electrodeposition at a low (33%) duty cycle. In addition, the corrosion properties of the electrodeposits were characterized by Tafel plots and electrochemical impedance spectroscopy (EIS), while the microhardness of the electrodeposits was measured by a Vickers hardness tester. In summary, this study revealed that pulsed current electrodeposition at a 33% duty cycle led to a finer and smoother surface morphology, an enhanced strength, a greater corrosion resistance, and a higher Ni content in the ZnNiAl₂O₃ composite coatings compared to plating at higher duty cycles or plating through DC electrodeposition.     

Synergistic effects of des tabilization,catalysis and nanoconfinement on dehydrogenation of LiBH4

In this report, Ni and TiO₂ are successfully embedded into porous carbon aerogel (CA) (donated as NiTiO₂@CA). Meanwhile, the synergistic effect of Ni, TiO₂ and CA on the dehydrogenation properties of LiBH₄ is systematically studied. Ni@CA, TiO₂@CA and CA are also investigated for comparisons. Compared to other three materials, NiTiO₂@CA exhibits better performance when used as a carrier to support LiBH₄. More than 6.75 wt% H₂ is released from LiBH₄NiTiO₂@CA system in nearly 120 min at 350 °C, exhibiting a higher dehydrogenation capacity than that of LiBH₄Ni@CA (3.15 wt %), LiBH₄TiO₂@CA (5.15 wt%) and LiBH₄-CA (2.05 wt %), respectively. Furthermore, the apparent energy (Ea) calculated with Kissinger method is 118.8 kJ/mol, much lower than that of pure LiBH₄. Dehydrogenation performance of LiBH₄NiTiO₂@CA may be due to the synergetic effect of destabilization of TiO₂, catalysis of Ni, as well as the nanoconfinement of CA.     

Different modified multi-walled carbon nanotube–based anodes to improve the performance of microbial fuel cells

To clearly illustrate the activity effect of multi-walled carbon nanotubes (MWCNTs) and their functionality on anodic exoelectrogen in microbial fuel cells (MFCs), the growth of E. coli and anode biofilm on MWCNT-, MWCNTCOOH and MWCNTNH₂ modified anodes were compared with a bare carbon cloth anode. The activity effect was characterized by the amount of colony-forming units (CFUs), activity biomass, morphology of biofilms and cyclic voltammetric (CV). The results showed that MWCNTs, MWCNT-COOH and MWCNT-NH₂ exhibited good biocompatibility on exoelectrogenic bacteria. The performance of MFCs were improved through the introduction of MWCNT-modified anodes, especially in the presence of COOH/NH₂ groups. The MFCs with the MWCNTCOOHmodified anode achieved a maximum power density of 560.40 mW/m², which was 49% higher than that obtained with pure carbon cloth. In conclusion, the positive effects of MWCNTs and their functionality were evaluated for promoting biofilm formation, biodegradation and electron transfer on anodes. Specifically, the MWCNTCOOHmodified anode demonstrated the largest application potential for the development of MFCs.     

Study on steam reforming of coal tar over NiCo/ceramic foam catalyst for hydrogen production: Effect of Ni/Co ratio

The effect of Ni/Co ratio on the catalytic performance of NiCo/ceramic foam catalyst for hydrogen production by steam reforming of real coal tar was studied. The NiCo/ceramic foam catalyst was synthesized by deposition-precipitation (DP) method and characterized with different methods. The experiments were conducted in a two-stage fixed-bed reactor. The results showed that the reducibility of the metallic oxides in bimetallic NiCo/ceramic foam catalysts was influenced obviously by the Ni/Co ratio.Both gas and hydrogen yield increased first and then decreased with the decline of Ni/Co ratio, and the highest hydrogen yield of 31.46 mmol g^?1 was obtained when the Ni/Co ratio was 5/5. The lowest coke deposition of 0.34 wt% was generated at the same Ni/Co ratio. The lifetime test showed the catalyst maintained catalytic activity after 14 cycles (28 h), indicating the coal tar steam reforming on NiCo/ceramic foam catalyst is a promising method for hydrogen production.     

Trimetallic NiFeCo selenides nanoparticles supported on carbon fiber cloth as efficient electrocatalyst for oxygen evolution reaction

Trimetallic NiFeCo selenides (NiFeCoSe_x) anchored on carbon fiber cloth (CFC) as efficient electrocatalyst for oxygen evolution reaction (OER) in alkaline medium have been synthesized via a facile two-step method. Firstly, trimetallic NiFeCo (oxy) hydroxides have been electrodeposited on CFC support (NiFeCo/CFC). Secondly, a solvothermal selenization process has been used to convert NiFeCo/CFC into NiFeCoSe_x/CFC using N, N-dimethylformamide (DMF) as solvent. The composition and homogeneous distribution of NiFeCoSe_x/CFC nanoparticles are determined by XRD, XPS, SEM elemental mapping and EDX images. Furthermore, SEM images reveal that NiFeCoSe_x/CFC has volcano-shaped morphology with rough surface and homogenously distributed on the surface of CFC, which may provide more active sites for OER. The electrochemical measurements show that trimetallic NiFeCoSe_x/CFC possesses the better electrocatalytic activity with the lower overpotential (150 mV at 10 mA cm^?2), Tafel slope (85 mV dec^?1), larger double-layer capacitance (200 mF cm^?2) and long-term stability than unary or binary metal selenides. The enhanced activity of NiFeCoSe_x/CFC may be attributed to the trimetallic NiFeCo selenides and selenides-CFC synergistic interaction. It may offer a promising way to design transition multimetallic selenides supported on conductive support as electrocatalysts for OER.     

Base-metal catalysts based on porous layered double hydroxides for alkaline-free sodium borohydride hydrolysis

Catalyzed hydrolysis of sodium borohydride (SBH) has demonstrated promise for generation of a pure hydrogen stream for use with fuel cells. In designing an improved continuous hydrogen generator that uses the substantial heat released in the hydrolysis reaction to more effectively separate the sodium borate by-product, we sought a robust base-metal catalyst that could tolerate the exothermic reaction under flow conditions. Working under base-free conditions in ethanol solvent we identified reduced nickel and iron-containing particles supported on layered double hydroxides (LDHs) as robust catalysts. Catalytic activity was enhanced further using high surface area hierarchical supports prepared using the ‘inverse opal’ method. In particular, macroporous NiMgAl and FeMgAl LDHs produced 0.4 and 1.0 mol of hydrogen per minute per mole of active metal of the supported catalyst in aqueous ethanol solvent.     

First-principles study of hydrogen behavior in vanadium-based binary alloy membranes for hydrogen separation

Vanadium-based alloys are considered to be one of the most promising hydrogen separation membranes due to their high hydrogen permeability. In this study, we investigate the dissolution and diffusion behaviors of hydrogen in vanadium-based binary alloys, V₁₅M (where M = Al, Ti, Cr, Fe, Ni and Nb) alloys, using first-principles method based on density functional theory. The dissolution of hydrogen in V₁₅M alloys is affected by both the elastic and electronic properties, but the elastic effect is the main factor. The H solution energies in the alloys follow the sequence: VTi < VNb < VAl < VCr < VNi < VFe, and a smaller atom size increase the H solution energy. Therefore, the addition of alloying elements with smaller atomic sizes can reduce the solubility of hydrogen in vanadium and inhibit hydrogen embrittlement. For hydrogen diffusion, alloying elements Al, Ti and Nb can be good candidates because they have a higher diffusion coefficient. The VTi alloy has the highest hydrogen permeability, but will have serious hydrogen embrittlement due to the increased H solubility.     

Dimensionally stable NiFe@Co/Ti nanoporous electrodes by reactive deposition for water electrolysis

The development of efficient and dimensionally stable electrode plates is of a significant challenge for the oxygen evolution reaction in the industrial water electrolysis process. In this work, structurally stable electrode plates are developed based on the nanostructured NiFe catalysts on highly porous and dimensionally stable Co reactive deposited on Ti substrates, NiFe@Co/Ti. SEM analysis shows the hierarchically structured micro- and nano-porous structure of the Co electrode on Ti substrates by reactive deposition route. The surface area of the reactive deposited Co is 3 times larger than that of the conventional electrodeposited Co electrode, providing highly porous and stable base for the subsequent deposition of NiFe electrocatalysts. The as-prepared NiFe@Co/Ti electrode exhibits high catalytic activity towards oxygen evolution in alkaline solutions, achieving an onset potential of as low as 1.44 V (η = 210 mV) and delivering a current of 10 mA cm^?2 at an overpotential of 0.26 V. Most importantly, the electrode shows excellent stability with negligible degradation under the discharge current density at 100 mA cm^?2 for 100 h, demonstrating the practical applicability of the NiFe@Co/Ti nanostructured electrodes for industrial scale water electrolysis.     

Hollow nanoporous NiPd catalysts with enhanced performance for ethanol electro-oxidation

In this work, bimetallic NiPd hollow nanoporous (HNiPd) catalysts are prepared by in-situ deposition of Pd nanoparticles (Pd NPs) on hollow Ni (HNi) microspheres. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) reveal the hollow nanoporous essence of HNiPd catalysts. Meanwhile, using high-angle annular dark-field scanning TEM (HAADF-STEM) and elemental mapping, it is found that tiny dendritic-like NiPd nanocomposites attach on the exterior of microspheres. The content of Pd is easily tailored to constitute HNiPd catalysts with different Ni/Pd atomic ratios. Further electrochemical evaluation vindicates that the as-prepared HNiPd catalysts have a good catalytic activity and stability toward ethanol oxidation reaction (EOR) in alkaline medium. Notably, the peak current density of HNi_3.1Pd catalyst and the chronoamperometric current density of HNi_4.6Pd catalyst are 4 and 2 times of Pd/C (JM) catalyst, respectively, which show that HNiPd catalysts hold great potential in application of alkaline direct ethanol fuel cells (DEFCs).     

Characterization and performance evaluation of PtRu/CTiO2 anode electrocatalyst for DMFC applications

In this study, the effect of introduction of titania (TiO₂) material into PtRu/C anode electrocatalyst on the performance of direct methanol fuel cells (DMFCs) was investigated. TiO₂ materials were first synthesized applying a sol–gel method and then incorporated directly into commercial PtRu/C anode electrocatalyst with different TiO₂ weight ratios (5, 15, and 25 wt.%) to improve the performance of the DMFC. For comparison, the anode electrocatalysts with the same TiO₂ weight ratios were also prepared using commercial TiO₂ materials. The performance tests of the DMFCs based on these composite anode electrocatalysts were conducted and their performances were also compared to that of a DMFC based on a traditional anode electrocatalyst (PtRu/C) under various operating conditions. In addition, 4 h short-term stability tests were conducted for all the manufactured DMFCs. The highest power densities were found as 705.12 W/m² and 709.32 W/m² at 80 °C and 1 M for the DMFCs based on PtRu/CTiO₂ anode electrocatalysts containing 5 wt.% of commercial and in-house TiO₂, respectively. The results of the short-term stability tests showed that introduction of 5 wt.% of commercial TiO₂ into commercial PtRu/C anode electrocatalyst improved its stability characteristics significantly.     

Enhancement of hydrogen storage properties in 4MgH2 Na3AlH6 composite catalyzed by TiF3

In this work, we have investigated the hydrogen release and uptake pathways storage properties of the MgH₂Na₃AlH₆ with a molar ratio of 4:1 and doped with 10 wt% of TiF₃ using a mechanical alloying method. The doped composite was found to have a significant reduction on the hydrogen release temperature compared to the un-doped composite based on the temperature-programme-desorption result. The first stage of the onset desorption temperature of MgH₂Na₃AlH₆ was reduced from 170 °C to 140 °C with the addition of the TiF₃ additive. Three dehydrogenation steps with a total of 5.3 wt% of released hydrogen were observed for the 4MgH₂Na₃AlH₆-10 wt% TiF₃ composite. The re/dehydrogenation kinetics of 4MgH₂Na₃AlH₆ system were significantly improved with the addition of TiF₃. Kissinger analyses showed that the apparent activation energy, E_A, of the 4MgH₂Na₃AlH₆ doped composite was 124 kJ/mol, 16 kJ/mol and 34 kJ/mol lower for un-doped composite and the as-milled MgH₂, respectively. It was believed that the enhancements of the MgH₂Na₃AlH₆ hydrogen storage properties with the addition of TiF₃ were due to formation of the NaF, the AlF₃ and the Al₃Ti species. These species may played a synergetic catalytic role in improving the hydrogenation properties of the MgH₂Na₃AlH₆ system.     

Improved performance of 3CSiC photocathodes by using a pn junction

To improve the performance of 3CSiC photocathodes, we formed a pn junction at the 3CSiC surface. Using current–voltage measurements for Schottky contacts on 3CSiC, the Schottky barrier height and depletion layer width of 3CSiC having a pn junction was observed to be larger than those of 3CSiC without the junction. By measuring photocurrent and spectral responses, the 3CSiC photocathodes with the pn junction exhibited larger photocurrent and higher quantum efficiencies compared with 3CSiC without the pn junction. Using a Pt cocatalyst on the 3CSiC photocathode with the pn junction, the solar-to-hydrogen energy conversion efficiency was measured at 0.72%.     

Graphene/Ni–Fe layered double hydroxide nano composites as advanced electrode materials for glucose electro oxidation

NickelIron Layered Double Hydroxide nano composites were electrochemically synthesized on graphene/glassy carbon electrode at a constant potential. The surface morphology and the structure of the electrodes were characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), elemental mapping analysis, X-ray diffraction (XRD) and Atomic force microscopy (AFM). This electrode was studied for glucose electro-oxidation reaction using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy (EIS) techniques. Results confirmed high catalytic activity, stability of the graphene/NiFe LDH electrode and glucose electro oxidation reaction on this electrode is under the effect of diffusion process. Also in comparison of some previous reported methods for the glucose electro oxidation, graphene/NiFe LDH shows a high diffusion coefficient as an electro catalyst for glucose electro oxidation. Electrical equivalent circuits for electrodes is obtained by using the Zview software. The low electrochemical charge transfer resistance (R_ct) was obtained on the graphene/NiFe LDH due to the presence of NiFe LDH nano composite.     

Tailoring the hydrogen absorption desorption's dynamics of MgMgH2 system by titanium suboxide doping

Titanium suboxide (TiO) is one of the best catalysts which improved the hydrogen absorption-desorption property of MgH₂ Mg system. The TiO catalyzed Mg MgH₂ have shown a remarkably reduced apparent activation energy and enhanced the hydrogen absorption-desorption kinetics. The X-ray photoelectron spectroscopy (XPS) analysis has indicated that the oxidation state of Ti in TiO remains unchanged during ball milling and hydrogen absorption-desorption of TiO-doped-MgH₂. The X-ray diffraction (XRD) analysis further confirms the XPS result. The TiO has shown the excellent catalytic effect on the MgMgH₂ system which remarkably reduced the hydrogen absorption-desorption temperatures.     

Synergic effects of VLi and Ti doping on hydrogen desorption on LiBH4 (010) surface: A first-principles investigation

Ti-doping and Li-vacancy (V_Li) crucially affect the dehydrogenation properties of LiBH₄ surface. However, theoretical investigations on individual Ti or V_Li could not completely explain experimental observations. In this article, we investigated the synergistic effects of co-existing Ti and V_Li on the dehydrogenation properties of LiBH₄ (010) surface. Our result shows mutual stabilization between Ti-dopant and Li-vacancy, implying expectable co-existence of Ti and V_Li. Thermodynamic destabilization from composite Ti + V_Li defect agrees with experiments better than that from single Ti or V_Li. The kinetic barrier on Ti + V_Li decorated surface also becomes closer to experimental result. Therefore, the co-existing Ti and V_Li synergistically and crucially affect the dehydrogenation thermodynamics and kinetics on LiBH₄ surface. The electronic structure further reveals strong HH, BB, and TiB bonds as well as weakened BH bond in transition states on Ti + V_Li co-existed surface, which is the main factor of low kinetic barrier.     

Sol-gel synthesis of Er3+:Y3Al5O12/TiO2Ta2O5/MoO2 composite membrane for visible-light driving photocatalytic hydrogen generation

By using TiO₂ and Ta₂O₅ colloids, a stable and efficient visible-light driven photocatalyst, Er³⁺:Y₃Al₅O₁₂/TiO₂Ta₂O₅/MoO₂ composite membrane, was successfully prepared via sol–gel dip coating method at room temperature. The XRD, FTIR, SEM, TEM and EDX results confirm that approximately spherical Er³⁺:Y₃Al₅O₁₂ nanoparticles were embedded in TiO₂Ta₂O₅ matrix. UV–vis absorption and PL spectra of Er³⁺:Y₃Al₅O₁₂ were also determined to confirm the visible absorption and ultraviolet emission. The photocatalytic hydrogen generation was carried out by using methanol as sacrificial reagent in aqueous solution under visible-light irradiation. Furthermore, some main influence factors such as heat-treated temperature, heat-treated time and molar ratio of TiO₂ and Ta₂O₅ on visible-light photocatalytic hydrogen generation activity of Er³⁺:Y₃Al₅O₁₂/TiO₂Ta₂O₅/MoO₂ composite membrane were studied in detail. The experimental results showed that the photocatalytic hydrogen generation activity of Er³⁺:Y₃Al₅O₁₂/TiO₂Ta₂O₅/MoO₂ composite membrane heat-treated at 550 °C for 3.0 h was highest when the molar ratio of TiO₂ and Ta₂O₅ was adopted as 1.00:0.50. And that a high level photocatalytic activity can be still maintained after four cycles. In addition, a possible mechanism for the visible-light photocatalytic hydrogen generation of the Er³⁺:Y₃Al₅O₁₂/TiO₂Ta₂O₅/MoO₂ membrane was proposed based on PL spectra.     

Crystalline-to-amorphous transformation of tantalum-containing oxides for a superior performance in unassisted photocatalytic water splitting

Crystalline tantalum-containing oxides are usually taken as the advanced photocatalysts for water splitting. How about the amorphous counterparts? In this work, a transformation of crystalline Na₂Ta₂O₆ (CNa₂Ta₂O₆) to amorphous TaO_x (Am-TaO_x) was achieved by a facial hydrothermal method. We proposed a transformation mechanism based on nucleation-dissolution -recrystallization and further intensified the influence of base concentration on the composition, crystallinity, and morphology (CCM) as confirmed by XRD, TEM, EDS. N₂-physisorption, Raman, IR, and XPS analysis. It is found that when comparing to the crystalline counterparts, amorphous samples possessed higher surface area, abundant surface hydration layers and H⁺ adsorption, showing an unassisted photocatalytic water splitting with a rate of 70 ± 7 μmol g^?1 h^?1, much larger than that of 15 ± 1 μmol g^?1h^?1 of CNa₂Ta₂O₆, 11 ± 1 μmol g^?1h^?1 of crystalline Ta₂O₅ (CTa₂O₅), 30±2 μmol g^?1h^?1 of mixture with crystalline Ta₂O₅ and amorphous Na_xTa_yO_z (CTa₂O₅/Am-Na_xTa_yO_z), and even 4.6 × 10^?4 μmol g^?1h^?1 for commercial TiO₂. This observation is beneficial from the short diffusion paths of amorphous state for charge carriers, amount of catalytic sites, and stronger reducing ability. These findings develop a novel and efficient pathway towards synthesizing the different CCM of tantalum-containing compounds under hydrothermal conditions and could open opportunities for further investigating the photocatalytic property of tantalum-containing materials.     

Experimental study on the coal tar hydrocracking process over different catalysts

In the paper, the results of the study on a laboratory process of hydrocracking of coal tar are presented. High-temperature coal tar was hydrocracked in an autoclave reactor, at 400–413 °C and 7,3–9,6 MPa over 5 different, hydrocracking catalysts to study the yields and characteristics of the products. Liquid products were obtained with a yield of 88–92 wt.% and gas products with a yield of 12–8 wt.%. After dehydration by azeotropic distillation, liquid products were separated on the oil fraction boiling below 360 °C, and pitch fractions boiling above this temperature. In the balance of distillation, it is clear that only the cracking activity of NiW/Al₂O₃ and Y zeolite catalysts is so high that the raw material containing 5 vol.% fraction boiling below 200 °C provides a product comprising approx. 12,5 vol.% of this fraction. The influence of five different catalysts on the yield of 18 poly aromatic hydrocarbons (PAHs) in the hydrocracking products have been investigated by GC/MS. The results indicate that NiW/Al₂O₃ and Y zeolite catalysts have the highest catalytic activity for light aromatic formation (the highest yield of fraction boiling below 200 °C) and aromatic condensed aromatic cracking (PAHs). After the hydrocracking NiW/Al₂O₃ catalyst, the total yields of PAHs decrease 52.7% as that of raw coal tar.