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.     

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.     

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.     

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.     

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).     

Interface stability between bare,MnCo spinel coated AISI 441 stainless steel and a diopside-based glass-ceramic sealant

This study is focused on a diopside-based glass-ceramic sealant for solid oxide fuel cells and its compatibility with AISI 441 stainless steel interconnect. The morphological and chemical stability with both bare and MnCo spinel coated AISI 441 steel, after 3500 h exposure at 800 °C in air, is reviewed and discussed. Post-mortem samples are morphologically and chemically analysed by SEM-EDS. Reaction products at the glass-ceramic/bare AISI 441 interface, resulting from the reaction of Mg from the sealant and Cr and Mn from the steel, are detected, without affecting negatively the integrity of the joints. In the case of MnCo spinel coated AISI 441, interactions between the glass-ceramic and the outer part of the MnCo spinel coating, along with crystallization of oxides rich in Si and Mg, are detected, but still no corrosion phenomena are present. The glass-ceramic is found to be compatible with both bare and coated AISI 441.     

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.     

AuCu alloys deposited on titanium dioxide nanosheets for efficient photocatalytic hydrogen evolution

This work first reports AuCu alloys deposited on the surface of TiO₂ nanosheets (TiNs) to form heterojunction. A simple deposition-precipitation method was used to construct a new type of AuCu/TiNs heterostructures through gradually depositing Au and Cu nanoparticles on TiNs. Such structures served the dual advantage of constructing a heterostructure which can improve visible light absorption, and the formation of a Schottky barrier between AuCu alloys (lower Fermi level) and TiNs (higher Fermi level) which can suppress the recombination of photo-generated charge carriers to improve the overall photocatalytic activity. The mass ratio of Au and Cu in the AuCu/TiNs heterostructures and the sequence and method of their deposition are found to be the important factors which affect the photocatalytic performance. When the mass ratio of Au to Cu was determined to be 1: 1, the AuCu/TiNs heterostructure exhibited the best photocatalytic performance for hydrogen production from water splitting (over 9 times than TiNs, 1.47 times than Au/TiNs, and 1.75 times than Cu/TiNs).     

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.     

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.     

One-pot synthesis of ultrasmall PtAg nanoparticles decorated on graphene as a high-performance catalyst toward methanol oxidation

A one-pot synthesis method is utilized for the fabrication of ultrasmall platinum-silver nanoparticles decorated on graphene (PtAg/G) catalyst. This method has several advantages such as inexpensiveness, simplicity, low temperature, surfactant free, reductant free, being environmentally friendly and greenness. In this work, graphene and silver formate were dispersed in ultrapure water in an ultrasonic bath at 25 °C followed by through a galvanic displacement reaction; to prepare PtAg/G, PtCl₂ was added to the suspension under mild stirring condition. The morphology, crystal structure and chemical compositions of the as-fabricated PtAg/G and Pt/C catalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Energy dispersive X-ray spectroscopy (EDS) techniques. Electrochemical techniques, including cyclic voltammetry (CV) and chronoamperometry (CA) measurements were used to analyze the electrochemical activity of the PtAg/G and Pt/C catalysts. The TEM images illustrate the uniform distribution of ultrasmall PtAg nanoparticles with the average size of 2–3 nm on the graphene nanosheets. The PtAg/G promoted the current density 2.46 times as much as Pt/C with a negative shift in onset oxidation potential and peak potential for oxidation reaction of methanol. Besides, the novel PtAg/G catalyst shows large electrochemically active surface area, lower apparent activation energy, and higher levels of durability in comparison to the Pt/C catalyst for the oxidation of methanol. The PtAg/G catalyst depicts extraordinary catalytic performance and stability to those of the Pt/C catalyst toward methanol oxidation in alkaline media.     

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.     

Kinetics of iron ore pellets reduced by H2N2 under non-isothermal condition

Direct reduction experiments under non-isothermal conditions are induced to simulate reaction in the actual hydrogen shaft furnace. Morphology of metalized pellets is analyzed through optical microscope. Weight loss during reduction is recorded and the model of un-reacted core is adopted for dynamic analysis in sections. Compressive strengths of products are also detected. Results show that reduction rates under heating conditions are lower compared with the isothermal situation. The un-reacted core still exists in products. It is found through kinetics analysis that the reaction is firstly mix controlled by the interfacial chemical reaction and internal diffusion, and then controlled dominantly by the interfacial chemical reaction as the temperature and efficient reduction gas content increase gradually with reaction going on. The compressive strengths under heating condition are also lower than the value obtained at constant temperature of 900 °C.This phenomenon may be caused by the crystalline transformation and volume expansion during the dominating Fe₂O₃ to Fe₃O₄ reduction at lower temperature. This study can provide scientific guide for rational utilization of hydrogen energy in iron making.     

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.     

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.     

Highly active screen-printed IrTi4O7 anodes for proton exchange membrane electrolyzers

Electroceramic support materials can help reducing the noble-metal loading of iridium in the membrane electrodes assembly (MEA) of proton exchange membrane (PEM) electrolyzers. Highly active anodes containing Ir-black catalyst and submicronic Ti₄O₇ are manufactured through screen printing technique. Several vehicle solvents, including ethane-1,2-diol; propane-1,2-diol and cyclohexanol are investigated. Suitable functional anodic layer with iridium loading as low as 0.4 mg cm^?2 is obtained. Surface properties of the deposited layers are investigated by atomic force microscopy (AFM). The most homogeneous coating with the highest electronic conductivity is obtained using cyclohexanol. Tests in PEM electrolyzer operating at 1.7 V and 40 °C demonstrate that the CCM with anode coated with cyclohexanol presents a 1.5-fold higher Ir-mass activity than that of the commercial CCM.     

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.     

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.     

Hydrogen-iodide decomposition over PdCeO2 nanocatalyst for hydrogen production in sulfur-iodine thermochemical cycle

A highly active and stable catalyst for hydrogen-iodide decomposition reaction in sulfur-iodine (SI) cycle has been prepared in the form of PdCeO₂ nanocatalyst by sol-gel method with different calcination temperatures (300 °C, 500 °C, and 700 °C). XRD and TEM confirmed a size around 6–8 nm for PdCeO₂ particles calcined at 300 °C. Raman study revealed large number oxygen vacancies in PdCeO₂-300 when compared to PdCeO₂-500 and PdCeO₂-700. With increase in calcination temperature, the average particle size increased whereas the specific surface area and number of oxygen vacancies decreased. Hydrogen-iodide catalytic-decomposition was carried out in the temperature range of 400°C–550 °C in a quartz-tube, vertical, fixed-bed reactor with 55 wt % aqueous hydrogen-iodide feed over PdCeO₂ catalyst using nitrogen as a carrier gas. PdCeO₂-300 showed hydrogen-iodide conversion of 23.3%, which is close to the theoretical equilibrium conversion of 24%, at 550 °C. It also showed a reasonable stability with a time-on-stream of 5 h.