Experimental and first principle studies on hydrogen desorption behavior of graphene nanofibre catalyzed MgH2

With the combination of experiment and first-principles theory, we have evaluated and explored the catalytic effects of graphitic nanofibres for hydrogen desorption behaviour in magnesium hydride. Helical form of graphene nanofibres (HGNF) have larger surface area, curved configuration and high density of graphene layers resulting in large quantity of exposed carbon sheet edges. Therefore they are found to considerably improve hydrogen desorption from MgH₂ at lower temperatures compared to graphene (onset desorption temperature of MgH₂ catalyzed by HGNF is 45 °C lower as compared to MgH₂ catalyzed by graphene). Using density functional theory, we find that graphene sheet edges, both the zigzag and armchair type, can weaken MgH bonds in magnesium hydride. When the MgH₂ is catalyzed with higher electronegative and reactive graphene edge of graphene, the electron transfer occurs from Mg to carbon, due to which MgH₂ is dissociated into hydrogen and MgH component. The Mg gets bonded with the graphene edge carbon atoms in the form of CMgH and CH bonds. In the as formed CMgH, the graphene edges “grab” more electronic charge as compared to the normal charge donation of Mg to H. This leads to the weakening of the MgH bond, causing hydrogen to desorbs at lower temperatures.     

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.     

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.     

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.     

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.     

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).     

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%.     

Dehydrogenation steps and factors controlling desorption kinetics of a MgCe hydrogen storage alloy

Experimentally systematical comparisons are carried out in this work to clarify dehydrogenation steps of Mg-based hydrogen storage alloys during the overall desorption process. Different forms of MgH₂CeH_2.73 composite powders are prepared by high energy ball milling, partial dehydrogenation and annealing. For partially dehydrogenated samples, the desorption temperature and desorption activation energy decrease significantly considering the fact that primary-precipitated metal Mg phase on the surface of MgH₂ can act as nucleate precursors. No significant difference in isothermal desorption kinetics is observed for MgH₂CeH_2.73 powders with different grain sizes. However, particle size reduction facilitates desorption at temperatures below 300 °C. As minor Ni is distributed on the surface, both onset and peak temperatures in thermal desorption decrease for MgH₂CeH_2.73 composite. The reduced activation energy by Ni addition is comparable to the value caused by partial dehydrogenation. Recombination of hydrogen atoms plays an important role during dehydrogenation. The obtains in this work can be expected to provide guidelines to improve desorption kinetics of Mg-based alloys.     

The catalytic effect of an inert additive (SrTiO3) on the hydrogen storage properties of 4MgH2Na3AlH6

Investigations on the catalytic effects of a non-reactive and stable additive, SrTiO₃, on the hydrogen storage properties of the 4MgH₂Na₃AlH₆ destabilized system were carried out for the first time. The Na₃AlH₆ compound and the destabilized systems used in the investigations are prepared using ball milling method. The doped system, 4MgH₂Na₃AlH₆SrTiO₃, had an initial dehydrogenation temperature of 145 °C, which 25 °C lower as compared to the un-doped system. The isothermal absorption and desorption capacity at 320 °C has increased by 1.2 wt% and 1.6 wt% with the addition of SrTiO₃ as compared to the 4MgH₂Na₃AlH₆ destabilized system. The decomposition activation energy of the doped system is estimated to be 117.1 kJ/mol. As for the XRD analyses at different decomposition stages, SrTiO₃ is found to be stable and inert. In addition to SrTiO₃, similar phases are found in the doped and the un-doped system during the decomposition and dehydrogenation processes. Therefore, the catalytic effect of the SrTiO₃ is speculated owing to its ability to modify the physical structure of the 4MgH₂Na₃AlH₆ particles through pulverization effect.     

Effect of combining Al,Mg, Ce or La oxides to extracted rice husk nanosilica on the catalytic performance of NiO during COx-free hydrogen production via methane decomposition

Amorphous nanosilica powder was extracted from rice husk and used as a catalyst support as well as a starting material for the preparation of different binary oxides, i.e., SiO₂Al₂O₃, SiO₂MgO, SiO₂CeO₂ and SiO₂La₂O₃. A series of supported nickel catalysts with the metal loading of 50 wt % were prepared by wet impregnation method and evaluated in methane decomposition to “CO_x-free” hydrogen production. The fresh and spent catalysts were extensively characterized by different techniques. Among the evaluated catalysts, both Ni/SiO₂Al₂O₃ and Ni/SiO₂La₂O₃ catalysts were the most active with an over-all H₂ yield of ca. 80% at the initial period of the reaction. This distinguishable higher catalytic activity is mainly referred to the presence of free mobile surface NiO and/or that NiO fraction weakly interacted with the support easily reducible at low temperatures. The Ni/SiO₂CeO₂ catalyst has proven a great potential for application in the hydrogen production in terms of its catalytic stability. The formation of Mg_xNi_(1?x)O solid solution caused the Ni/SiO₂MgO catalyst to lose its activity and stability at a long reaction time. Various types of carbon materials were formed on the catalyst surface depending on the type of support used. TEM images of as-deposited carbon showed that multi-walled carbon nanotubes (MWCNTs) and graphene platelets were formed on Ni/SiO₂, while only MWCNTs were deposited on all binary oxide supported Ni catalysts.     

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.     

Synergetic effects of K,Ti and F on the hydrogen storage properties of the LiNH system

In this paper, the hydrogen storage properties of the LiNH₂LiH system doped with K₂TiF₆ were investigated and discussed. Interestingly, the hydrogen storage properties are significantly enhanced by introducing K₂TiF₆ into the LiNH₂LiH system. By doping 5 mol% K₂TiF₆ in the LiNH₂LiH system, we obtain the hydrogen desorption peak temperature (233 °C) at a heating rate of 10 °C min^?1, which is approximately 66 °C lower than that of the pristine LiNH₂LiH system. Moreover, the system begins to desorb H₂ at 75 °C, which is approximately 124 °C lower than in the pristine LiNH₂LiH system. The isothermal desorption kinetics at 250 °C and 300 °C clearly reflects the dramatically improved kinetic properties. Additionally, the reversibility of the LiNH₂LiH system can be drastically enhanced by adding K₂TiF₆. We propose that the dehydrogenation property of the K₂TiF₆-doped LiNH₂LiH sample is improved by the synergetic effects of K, Ti and F.     

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.     

One-step synthesis of CuCo alloy/Mn2O3Al2O3 composites and their application in higher alcohol synthesis from syngas

CuCo alloy/Mn₂O₃Al₂O₃ composites were synthesized by a facile one-step sol–gel method using citric acid as a chelating agent. The catalytic performance of the as-prepared catalysts was investigated in reaction of CO hydrogenation to higher alcohols. According to the characterization data obtained by TG-DSC, XRD, TPR, BET, ICP, SEM, TEM and XPS, a stronger interaction between the Cu and Co ions in the CuCo₂O₄ particles led to the formation of CuCo alloy in the reduced catalysts, and the Mn/Al molar ratio significantly influenced the performance of the catalysts. Mn₂O₃Al₂O₃ composites reduce the unwanted CoAl₂O₄ spinel phase, offer tunable pore sizes and surface areas, and also appear to act as barriers to hinder the CuCo alloy particles sintering. The results suggest that the metal nanoparticles of CuCo alloy together with Mn₂O₃Al₂O₃ contributed to the high selectivity of higher alcohols as well as the good stability. A Mn/Al molar ratio of 5/3 was found to be most suitable for the catalyst properties in terms of activity and product distribution.     

Platinum-decorated palladium-nanoflowers as high efficient low platinum catalyst towards oxygen reduction

A three-dimensional, low platinum (Pt) catalyst was prepared by decorating platinum on the palladium nanoflowers (PdNF) by an underpotential deposition (UPD) method. The PdNF was synthesized by a solvothermal approach, using oleic acid as the template and benzyl alcohol as the solvent-reducing agent. The obtained Pd with a morphology of uniform nanoflowers is composed of plentiful nanosheets. After decorating with platinum, the catalyst PdNF@Pt exhibits much higher activity for the oxygen reduction reaction (ORR) compared to commercial Pt/C (Pt 20 wt%). The interaction between deposited Pt and PdNF was revealed by XPS analysis, and the high performance of the PdNF@Pt catalyst was attributed to following two aspects: the increased of dispersion of platinum based on PdNF substrate, and the increased intrinsic activity of the active sites caused by the interaction of Pt and Pd NF.     

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.     

Non-precious co-catalysts boost the performance of TiO2 hierarchical hollow mesoporous spheres in solar fuel cells

A main challenge hindering the development of efficient solar fuel cell systems is the identification of robust, cost-effective, and abundant catalysts. Herein, we demonstrate the ability to synthesize photoactive, relatively cheap and abundant catalyst for the solar-assisted water splitting. The proposed catalyst is a composite of CoCu/graphene immobilized on hierarchical hollow mesoporous Titania. Diffuse Reflectance analysis showed visible light absorption for (CoCu) _2%TiO₂/RGO with an estimated band gap of 2.41 eV, as compared to 3.13 eV for Titania. Photoluminescence spectra showed a significant decreasing in recombination rate of photogenerated electron-hole pair for (CoCu) _2%TiO₂/RGO nanocomposites. Upon their use as photoanodes in solar fuel cells, the fabricated nanocomposites show 14-fold increase in photocurrent density compared to Titania. This enhancement was confirmed via the measurement of electron and phonon life times. The results attained in this study demonstrate a step toward using non-precious co-catalysts to boost the performance of photocatalysts in solar fuel cells.     

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.     

Effect of heat treatment on AlMgGaInSn alloy for hydrogen generation through hydrolysis reaction

The composition, microstructure and corrosion behavior of AlMgGaInSn alloy in cast and heat-treatment were investigated by XRD (X-ray diffraction), SEM (scanning electron microscope) and EDS (energy dispersive spectrometer). The hydrogen evolution parameters, and the electrochemical properties based on different heat-treatment parameters conditions and immersion temperature were also tested. As the heat-treated temperature increased, the second phases were found to be spheroidizing or ellipsoidal shape due to the diffusion and solid solution. The reaction can be divided into three stages: i) the amalgamation initial stage; ii) the micro-galvanic reaction for propagation corrosion; iii) the dissolution-precipitation reaction for uniform corrosion. The hydrolysis rate reached the maximum value when the sample was annealed at temperature of 500 °C for 9 h. The hydrogen generation rate and the open electrochemical potential of the activated aluminum under water are both depending on heat treatment time, heat treatment temperature, and reaction temperature. A corrosion mechanism was also proposed in which Mg₂Sn and eutectic phase acted as the induction reaction stage during hydrolysis reaction.     

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.