Fabrication of CdS/NiFe LDH heterostructure for improved photocatalytic hydrogen evolution from aqueous methanol solution

2D CdS/NiFe LDH (short for layered double hydroxide) heterostructures were designed and fabricated by following a facile in-situ growth method. The CdS nanoparticles are well dispersed on the surface of NiFe LDH to form nanoscale heterojunctions, as suggested from the TEM and elemental mapping images. The composites with optimum CdS amount (15 wt%) take on notably higher hydrogen evolution activity (469 μmol h^?1 g^?1) than the independent CdS and NiFe LDH from aqueous methanol solution under xenon lamp irradiation. The nano-heterojunction notably promotes the H₂ evolution kinetics and greatly suppresses the recombination of photo-induced electrons and holes, which is responsible for the enhanced photocatalytic activity of the composites, as demonstrated by the reducing onset potential and increasing photocurrent of the composites in the photoelectrochemical experiments. The possible photocatalytic mechanism is proposed on the basis of the defined position of energy band edges.     

AuNi alloy nanoparticles supported on reduced graphene oxide as highly efficient electrocatalysts for hydrogen evolution and oxygen reduction reactions

Bimetallic nanoparticles of Au and Ni in the form of alloy nanostructures with varying Ni content are synthesized on reduced graphene oxide (rGO) sheets via a simple solution chemistry route and tested as electrocatalysts towards the hydrogen evolution (HE) and oxygen reduction (OR) reactions using polarization and impedance studies. The AuNi alloy NPs/rGO nanocomposites display excellent electrocatalytic activity which is found to improve with increasing Ni content in the AuNi/rGO alloy nanocomposites. For HER, the best AuNi alloy NPs/rGO electrocatalyst, the one with the highest Ni content, exhibits high activity with an onset overpotential approaching zero versus the reversible hydrogen electrode and an overpotential of only 37 mV at 10 mA cm^?2. Additionally, a low Tafel slope of 33 mV dec^?1 and a high exchange current density of 0.6 mA cm^?2 are measured which are very close to those of commercial Pt/C catalyst. Also, in the ORR tests, this electrocatalyst displays comparable activity to Pt/C. The Koutecky–Levich plots referred to a 4-electron mechanism for the reduction of dissolved O₂ on the AuNi alloy NPs/rGO catalyst. The electrocatalyst thus demonstrates excellent activity towards HER and ORR. Additionally, it exhibits outstanding operational durability and activation after 10,000^th cycles assuring its practical applicability.     

Mesocrystalline Ta2O5 nanosheets supported PdPt nanoparticles for efficient photocatalytic hydrogen production

We successfully synthesized mesocrystalline Ta₂O₅ nanosheets supported bimetallic PdPt nanoparticles by the photo-reduction method. The as-prepared mesocrystalline Ta₂O₅ nanosheets in this work showed amazing visible-light absorption, mainly because of the formation of oxygen vacancy defects. And the as-prepared bimetallic PdPt/mesocrystalline Ta₂O₅ nanaosheets also showed highly enhanced UV–Vis light absorption and highly improved photocatalytic activity for hydrogen production in comparison to that of commercial Ta₂O₅, mesocrystalline Ta₂O₅ nanosheets, Pd/mesocrystalline Ta₂O₅ nanosheets and Pt/mesocrystalline Ta₂O₅ nanosheets. The highest photocatalytic hydrogen production rate of PdPt/mesocrystalline Ta₂O₅ nanaosheets was 21529.52 g^?1 h^?1, which was about 21.2 times of commercial Ta₂O₅, and the apparent quantum efficiency of PdPt/mesocrystalline Ta₂O₅ nanaosheets for hydrogen production was about 16.5% at 254 nm. The highly enhanced photocatalytic activity was mainly because of the significant roles of PdPt nanoparticles for accelerating the charge separation and transport upon illumination. The as-prepared PdPt/mesocrystalline Ta₂O₅ nanaosheets in this work could serve as an efficient photocatalyst for green energy production.     

Temperature-hydrogen pressure phase boundaries and corresponding thermodynamics for ZrCoH system

The thermodynamically and kinetically stable regions of the temperature–H₂ pressure phase boundaries for the ZrCoH system were established using the Temperature-Concentration-Isobar (TCI) method. Based on this, the enthalpy change and entropy change values of dehydrogenation and disproportionation reactions were successfully obtained. The average enthalpy change (ΔH) and entropy change (ΔS) estimated from the phase boundaries for dehydrogenation of ZrCoH₃ to ZrCo are respectively 103.07 kJ mol^?1H₂ and 148.85 J mol^?1 H₂ K^?1, which are well agreement with the data reported in literature. The average ΔH and ΔS were estimated to be ?120.91 kJ mol^?1H₂ and -149.32 J mol^?1 H₂ K^?1 for the disproportionation of ZrCoH₃, whereas the ΔH and ΔS were calculated to be ?84.6 kJ mol^?1H₂ and -92.29 J mol^?1 H₂ K^?1 for disproportionation of ZrCo. In addition, it was found from the established phase boundaries that the anti-disproportionation property of ZrCo alloy can be enhanced if the phase boundaries of hydrogenation/dehydrogenation are far away from the phase boundaries of disproportionation by adjusting the thermodynamics. Meanwhile, it is possible to keep ZrCo away from disproportionation even at high temperature of 650 °C under hydrogen atmosphere, if the temperature-H₂ pressure trajectory is carefully controlled without crossing the phase boundaries of disproportionation. Therefore, the established phase boundaries can be used as a guide to the eye avoiding disproportionation and improving the anti-disproportionation property of ZrCo alloy.     

Energy and exergy analyses of hybrid photocatalytic hydrogen production reactor for CuCl cycle

The present paper concerns electrochemical, energy, exergy and exergoeconomic analyses of a hybrid photocatalytic-based hydrogen production reactor which is capable of replacing the electrolysis sub-system of the CuCl thermochemical cycle. Several operating parameters, such as current density, reactor temperature, ambient temperature and electrode distance, are varied to study their effects on the hydrogen production rate, the cost of hydrogen production and energy and exergy efficiencies. The present results show that the voltage drops across the anolyte solution (sol 1), catholyte solution (sol 2), an anode, cathode, and cation exchange membrane vary from 0.005 to 0.016 V, 0.004–0.013 V, 1.67–2.168 V, 0.18–0.22 V and 0.06–0.19 V, respectively with an increase in current density from 0.5 to 1.5 A/cm². The energy and exergy efficiencies of the hybrid photocatalytic hydrogen production reactor decrease from 5.74 to 4.54% and 5.11 to 4.04%, respectively with an increase in current density.     

In situ fabrication of porous graphene electrodes for high-performance lithiumoxygen batteries

These years, LiO₂ batteries attract wide interest because of its high theoretical energy density. However, the catalytic activity and porous structure of cathode remains a great challenge. In this work, we developed a hierarchical porous graphene foam to serve as a battery cathode, which has much richer active sites for cathodic reaction and channels for Li⁺ transfer and O₂ diffusion. The cathode exhibits a superior specific capacity as high as 9559 mAh g^?1 at 57 mA g^?1 and remains a high-rate capability of 3988 mAh g^?1 at an increased current density of 285 mA g^?1. Benefiting from the well-designed cathode structure, the battery can be stably operated for 150 cycles with a stable voltage profile and voltage efficiency up to 65%. The well-designed graphene has a potential to be a superior free-standing cathode to other carbon-based materials due to its good combination of its hierarchical and porous structure, large surface area, abundant defects and excellent mechanical stability.     

Noble metal-free FeN-CNFs as an efficient electrocatalyst for oxygen reduction reaction

Carbonaceous materials containing non-precious metal atoms and doped with nitrogen have enthralled stunning attention in the field of electrochemical energy conversion systems. Herein, we demonstrated a facile method to fabricate iron and nitrogen doped carbon nanofiber (FeN-CNFs) catalyst material from ferric chloride and interfacial synthesized polyaniline (PANI) nanofibers, by carbonization process in an inert atmosphere at 800 °C. Further, synthesized material was characterized by elemental analysis and X-ray photoelectron spectroscopy (XPS) that confirms the presence of FeN bonds. The structural and morphological features are studied using various microscopy and spectroscopy techniques. The oxygen reduction reaction (ORR) activity of synthesized catalyst materials was examined by rotating disk electrode experiments in 0.1 M KOH. Among all these synthesized materials FeN-CNFs material showed enhanced ORR activity regarding current density and onset potential. Also, FeN-CNFs catalyst exhibited tolerance to methanol and durability in comparison to commercial Pt/C catalyst. The superior performance of FeN-CNFs may be attributed due to the introduction of Fe and formation of FeN bond in catalyst material.     

NiPt sinter as a promising electrode for methanol electrocatalytic oxidation

Methanol is one of the chemical compounds utilized in fuel cells. The direct methanol fuel cell (DMFC) can be applied in many devices such as light electric vehicles and field equipment. Such a fuel cell is characterized by its high fuel energy density and low pollution. Despite many advantages of DMFCs, they are not commercially available, as the most efficient catalyst, which can be used in this process, has not been developed yet. Traditionally, it was platinum that was used in these fuel cells which is expensive and susceptible to CO poisoning. The solution to this is the use of bimetallic catalysts such as a NiPt system. In this study, we used a sintered NiPt electrode as the anode for the electrocatalytic oxidation of methanol. Based on our results, the sintered NiPt electrodes exhibited much higher activity in the oxidation of methanol, when compared with some conventional anodes.     

Characteristic of a multi-pass membrane separator for hydrogen separation through self-supported PdAg membranes

Dense PdAg membranes have shown immense potential to achieve high hydrogen purity required for proton exchange membrane (PEM) fuel cell. However, high hydrogen recovery and flux at lower transmembrane partial pressure is still a concern. In current study self-supported dense PdAg membranes were used to study the hydrogen recovery in a multi-pass membrane separator. Performance of a single and four collective membranes are tested in a single (without baffle) and multi-pass (with longitudinal baffles) membrane separator. Further, array of membrane configurations were tested experimentally by using longitudinal baffles and placing membranes at different locations. The hydrogen recovery for each configuration was measured experimentally. Experiments were performed using binary gas mixture 50H₂:50N₂ (v/v) at 3 bar pressure, 673 K temperature and gas-hourly space velocity (GHSV) 43 h^?1. The best assembly was further tested with typical methanol reformate gas composition by using simulated gas mixture of 50H₂:30N₂:18CO₂:2CO (v/v) at same operating condition. Numerical simulations were performed by using commercial software ANSYS 14.5 to understand the flow dynamics inside the separator with and without baffle. The results demonstrate that a multi-pass membrane separator enables to control hydrogen partial pressure radially along the length of reactor. This resulted in 33% enhancement in hydrogen recovery with multi-pass in comparison to single pass membrane separator.     

Development of hydrogen production by liquid phase plasma process of water with NiTiO2/carbon nanotube photocatalysts

Hydrogen evolution by water photocatalysis using liquid phase plasma system was disserted over metal-loaded TiO₂ photocatalysts. Carbon nanotube was applied as a support for the metal-loaded TiO₂ nanocrystallites. Photocatalytic activities of the photocatalysts were estimated for hydrogen production from water. Hydrogen was produced from the photodecomposition of water by liquid phase plasma irradiation. The rate of hydrogen evolution was improved by the metal loading on the TiO₂ surface. TiO₂ nanocrystallites were incorporated above 40 wt% onto the carbon nanotube support. The carbon nanotubes could be applied as a useful photocatalytic support for the fixation of TiO₂. Hydrogen evolution was enhanced by the Ni loading on the TiO₂ nanocrystallites supported on the carbon nanotube. Hydrogen evolution was increased apparently with addition of the alcohols which contributes as a kind of sacrificial reagent promoting the photocatalysis.     

MWCNTs and Cu2O sensitized TiFe2O3 photoanode for improved water splitting performance

Fe₂O₃ and Cu₂O, both earth abundant materials are used in functionalizing Ti doped Fe₂O₃ photoanodes with Cu₂O and MWCNTs for improving photoelectrochemical performance for hydrogen generation. Pristine Ti doped Fe₂O₃ are fabricated by spray pyrolysis deposition method on the conducting ITO coated glass substrate. Two different modifications are adopted to improve the photoelectrochemical performance of pristine sample by subsequent deposition of multi walled carbon nano tubes (MWCNTs) alone and also in combination with Cu₂O. Better photoresponse in modified samples is attributed to increase in conductivity and promotion of electron transport to Fe₂O₃ layer due to presence of MWCNTs while formation of heterojunction also promotes charge transfer kinetics by effective separation of charge carriers. Offering high photocurrent density of 5.17 mA cm^?2 at 1 V vs SCE, high open circuit voltage (V_oc), least resistance, higher negative flat band potential (V_fb), TiFe₂O₃/(MWCNTs + Cu₂O), emerges as the most photoactive sample. High applied bias photon to current conversion efficiency (ABPE) value of 4.6% is obtained for the modified sample against 0.07% ABPE for TiFe₂O₃ photoanodes.     

Hydrogen production by a PdAg membrane reactor during glycerol steam reforming: ANN modeling study

In the present work, an artificial neural networks (ANNs) model has been developed for investigation of glycerol steam reforming (GSR) process with PdAg membrane reactor (MR) in the presence of Co/Al₂O₃ catalyst. Reaction pressure and sweep factor as independent variables (Inputs) and glycerol conversion, hydrogen recovery, hydrogen yield, H₂ selectivity, CO selectivity and CO₂ selectivity as dependent variables (outputs) are chosen for ANN modeling of GSR. The ANN model was developed by feed-forward back propagation network with trainlm algorithm and topology (2: 10: 6) and Sigmoid transfer function for hidden and output layers. A good agreement between predicted values using ANN with experimental results was observed (R² and MSE values were 0.9998 and 3.48 × 10^?6 (based on normalized data), respectively). Modeling results indicated that all selected factors (reaction pressure and sweep factor) were effective on output variables. It was found that the reaction pressure with a relative importance of 59% was the most effective parameter in the GSR process with PdAg MR in the presence of Co/Al₂O₃ catalyst.     

Theoretical evaluation of PdAg membrane reactor performance during biomass steam gasification for hydrogen production using CFD method

In recent years, biomass has been introduced as a promising solution for environmental crisis. Biomass steam gasification is a valuable process for hydrogen production. Main problem of this process is low conversion and low partial pressure of hydrogen in product stream. PdAg membrane reactor (MR) can be used in biomass steam gasification to improve the process efficiency. Hence, Computational fluid dynamic (CFD) method was used in this study for a detail modeling and analyzing the biomass steam gasification in a two-dimensional PdAg MR. After good agreement of CFD model results with literature experimental data, simulation results was indicated that the PdAg MR has better efficiency compared with traditional reactor (TR). Biomass conversion of near 100%, CO selectivity in the range 0–14 and H₂ recovery of 70% in the best condition were achieved. In addition, different flow patterns (cocurrent and counter-current modules) were compared for MR and overall efficiency (biomass conversion) of counter-current model was obtained higher than co-current model. In summary, for all operating conditions and modules, PdAg MR was showed better efficiency compared with TR.     

Effective excitons separation on graphene supported ZrO2TiO2 heterojunction for enhanced H2 production under solar light

A novel photocatalyst comprises of ZrO₂TiO₂ immobilized on reduced graphene oxide (rGO) – a ternary heterojunction (ZrO₂TiO₂/rGO) was synthesized by using facile chemical method. The nanocomposite was prepared with a strategy to achieve better utilization of excitons for catalytic reactions by channelizing from metal oxide surfaces to rGO support. TEM and XRD analysis results revealed the heterojunction formed between ZrO₂ and single crystalline anatase TiO₂. The mesoporous structure of ZrO₂TiO₂ was confirmed using BET analysis. The red shift in absorption edge position of ZrO₂TiO₂/rGO photocatalyst was characterized by using diffuse reflectance UV–Visible spectra. ZrO₂TiO₂/rGO showed greater interfacial charge transfer efficiency than ZrO₂TiO₂, which was evidenced by well suppressed PL intensity and high photocurrent of ZrO₂TiO₂/rGO. The suitable band gap of 1.0 wt% ZrO₂TiO₂/rGO facilitated the utilization of solar light in a wide range by responding to the light of energy equal to as well as greater than 2.95 eV by the additional formation of excited high-energy electrons (HEEs). ZrO₂TiO₂/rGO showed the enhanced H₂ production than TiO₂/rGO, which revealed the role of ZrO₂ for the effective charge separation at the heterojunction and the solar light response. The optimum loading of 1.0 wt% of ZrO₂ and rGO on TiO₂ showed the highest photocatalytic performance (7773 μmolh^?1$g_{cat}^{?1}$) for hydrogen (H₂) production under direct solar light irradiation.     

Enhanced thermal diffusivity and dehydrogenation of 2LiNH2MgH2 by doping with super activated carbon

Doping with the additives in metal-N–H system has been regarded as one of the most effective approaches to improve its hydrogen storage properties. Herein, we prepared super activated carbon (SuperC) through the activation of commercial activated carbon by KOH and evaluated its effect on dehydrogenation properties of 2LiNH₂MgH₂. Our studies show that doping with SuperC could effectively lower its dehydrogenation temperatures. For instance, 2LiNH₂MgH₂–10 wt% SuperC can release 4.86 wt% of hydrogen upon heating up to 300 °C with the onset and peak dehydrogenation temperatures of 71 °C and 168 °C, respectively. Moreover, the release of byproduct NH₃ was successfully suppressed. Measurement of thermal diffusivity suggests that the enhanced dehydrogenation properties may be ascribed to the improved heat transfer for solid-solid reaction resulting from doping with SuperC.     

CNF-grafted carbon fibers as a binder-free cathode for LithiumOxygen batteries with a superior performance

In this work, we report the significant enhancement of the electrochemical performance and flexibility of a lithium–oxygen battery by introducing a free-standing, binder-free carbon nano-fibers (CNF) grafted carbon paper cathode with a bimodal pore architecture. The small pore structures (～100 nm) accommodated Li₂O₂, and the large pore structures (～10 μm) enabled effective oxygen diffusion without clogging the pores. This kind of cathode overcame some troubles of the cathode prepared by spraying coating method, such as the low utilization of substrate surface, the unreasonable aperture structure and the aggregation of active carbon material. As a result, this electrode structure imparted stability to active sites during the recovery of discharge products to the initial state, providing long-term cyclability of more than 800 cycles in a 1 M LiTFSI/TEGDME electrolyte system. In addition, the battery output a discharge capacity as high as 20000 mAh g^?1 at 468 mA g^?1 and exhibited a charge/discharge rate as high as 1136 mA g^?1 (0.57 mA cm^?2). The test results suggest that these CNF-grafted carbon papers have the potential to be used for oxygen/air electrodes for next-generation lithium-oxygen batteries, though the present results need to be improved to achieve performance of practical significance, namely with regard to (i) cathode mass loading to get higher areal capacity, and (ii) cycling performance at higher current density.     

CuOCr2O3 core-shell structured co-catalysts on TiO2 for efficient photocatalytic water splitting using direct solar light

Synthesis of core-shell structured CuOCr₂O₃ nanoparticles as co-catalyst to improve the photocatalytic hydrogen evolution performance of TiO₂ was demonstrated. The effect of co-catalyst loading on TiO₂ and the nature of the reactor was found to be more significant for H₂ production under direct solar light. The formation of 9.3 nm Cr₂O₃ shell over CuO core in the CuOCr₂O₃ nanostructured co-catalyst was confirmed using transmission electron microscopy. A very high H₂ production rate of 82.39 and 70.4 mmol h^?1 g^?1_cat was observed with quartz and pyrex reactors under direct solar light of irradiation 96–100 mW/cm², respectively. This is almost three times higher than that of bare TiO₂ under similar experimental conditions. The core-shell co-catalyst loaded on TiO₂ by simple mechanical mixing method which is useful for bulk scale synthesis in practical applications. The observed high H₂ production was explained with plausible mechanism where the synergic effect of CuOCr₂O₃ co-catalyst loaded TiO₂ surface that reduces the effective charge carriers recombination and impeded backward reaction by the Cr₂O₃ thin layer. The presence of Cu²⁺ and absence of Cu⁺ and metallic Cu was confirmed using XPS analysis. The effect of co-catalyst loading and sacrificial agent concentration on the photocatalytic hydrogen production was also reported. The stability of the CuOCr₂O₃ core-shell NPs loaded TiO₂ photocatalyst under the direct solar light was examined by continuous cycling for three days and it was found to be 81 and 70% of photocatalyst activity is retained after 3 days in the quartz and pyrex reactor systems, respectively.     

Pulsed electrodeposition of reduced graphene oxide on NiNiO foam electrode for high-performance supercapacitor

Through electrodeposition, controlling hydrogen evolution reaction and selective electrochemical dealloying of copper from NiCu porous foam, highly nanoporous nickel and nickel oxide is fabricated on the copper surface. Electrochemically reduced graphene oxide (ERGO) is loaded on the NiNiO foam as high-performance electrodes for supercapacitors through pulsed galvanostatic reduction of drop casted graphene oxide nanosheets at different duty cycles and frequencies. Surface morphology and composition of fabricated ERGO/NiNiO foam composite electrodes are characterized using scanning electron microscopy (SEM), powder X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Raman Spectroscopy. Electrochemical impedance spectroscopy (EIS) measurements, galvanostatic charge/discharge (GCD) and cyclic voltammetry (CV) are carried out to study the electrochemical behavior of ERGO/NiNiO foam electrodes. From structural and electrochemical characterizations, optimized parameters for pulse duty cycle and frequency were found to be 10% and 1000 Hz, respectively. As a result, the ERGO/NiNiO foam film (i_c = ?10 mA/cm², f = 1000 Hz and DC = 10%) provides a specific capacitance of 2298 F/g in 1 M KOH at a current density of 1 A/g. Stability study of fabricated film represents a long cycling life up to 4000 cycles with 0.7% decay in specific capacitance at the high current density of 20 A/g in the potential range of 0–0.6 V vs. saturated calomel electrode (SCE).     

Electrodeposited PCo nanoparticles in deep eutectic solvents and their performance in water splitting

A facile one-step route has been developed to electrodeposite PCo nanoparticles on a nickel foam in deep eutectic solvents. The as-prepared catalyst exhibits excellent performance towards both hydrogen evolution reaction and oxygen evolution reaction. Only 62 mV and 320 mV overpotentials were required to reach a current density of 10 mA cm^?2 for hydrogen evolution reaction and oxygen evolution reaction, respectively. That current density is measured at the voltage of 1.59 V for an overall water splitting when used as both anode and cathode. The scanning electron microscopy images indicate a high dispersion of the PCo sample on the Ni foam. The prepared material possesses a relative high ECSA and a low charge transfer resistance, indicating a large number of active sites for water splitting.     

PtPd nanoparticles supported on sulfonated nitrogen sulfur co-doped graphene for methanol electro-oxidation

In this paper, sulfonated nitrogen sulfur co-doped graphene (S-NS-GR) nanocomposite, i.e., nitrogen sulfur co-doped graphene functionalized with SO₃H group as a novel catalyst support material was prepared. PtPd nanoparticles (PtPd NPs) were deposited on the surface of S-NS-GR by a facile electrochemical approach. The morphology and structure of Pd-PtNPs/S-NS-GR were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and electrochemical impedance spectroscopy (EIS), respectively. In addition, the electrocatalytic performance of catalyst for methanol oxidation reaction (MOR) was systematically studied by cyclic voltammetry and chronoamperometry in alkaline media. Compared with PtPd NPs supported on nitrogen sulfur co-doped graphene (Pt-PdNPs/NS-GR), the excellent performance of Pd-PtNPs/S-NS-GR is mainly ascribed to the embedding of abundant functional groups (SO₃H) into the NS-GR layers, which not only facilitate the homogeneous distribution of metal NPs, but also strengthen the interaction between metals and support material, thus improve the stability of catalyst in MOR.