Microstructure and activation characteristics of Mg–Ni alloy modified by multi-walled carbon nanotubes

An Mg–6 wt% Ni alloy was fabricated by a casting technique and the drilled chips ball-milled by high energy ball milling to be examined for their hydrogenation modified with multi-walled carbon nanotubes (MWCNTs). The activation characteristics of ball-milled alloy are compared with those of the materials obtained by ball milling with 5 wt% MWCNTs for 0.5, 1, 2, 5 and 10 h. MWCNTs enhanced the absorption kinetics considerably in all cases. The hydrogen content of the modified powder with MWCNTs reached maximum hydrogen capacity within 2 min of exposure to hydrogen at 370 °C and 2 MPa pressure. X-ray diffraction analysis provided evidence that no carbon-containing phase was formed during milling. However, milling with MWCNTs reduced the crystallite size, even if the milling was carried out for only an hour. The rate-controlling steps of the hydriding reactions at different milling times were determined by fitting the respective kinetic equations. Evidence is provided that nucleation and growth of hydrides are accelerated drastically by a homogenous distribution of MWCNTs on the surface of the ball-milled powders. We show that MWCNTs are very effective at promoting the hydriding/dehydriding kinetics, as well as in increasing the hydrogen capacity of the magnesium alloy.     

Electrochemical characterization of activated carbon–ruthenium oxide nanoparticles composites for supercapacitors

The high specific capacitance of ruthenium oxide (denoted as RuO_x) nanoparticles prepared by a modified sol–gel method with annealing in air for supercapacitors was demonstrated in this work. The specific capacitance of activated carbon (denoted as AC) measured at 5 mA/cm² is significantly increased from 26.8 to 38.7 F/g by the adsorption of RuO_x nanoparticles with ultrasonic weltering in 1 M NaOH for 30 min. This method is a promising tool in improving the performance of carbon-based double-layer capacitors. The total specific capacitance of a composite composed of 90 wt.% AC and 10 wt.% RuO_x measured at 25 mV/s is about 62.8 F/g, which is increased up to ca. 111.7 F/g when RuO_x has been previously annealed in air at 200 °C for 2 h. The specific capacitance of RuO_x nanoparticles was promoted from 470 to 980 F/g by annealing in air at 200 °C for 2 h. The nanostructure of RuO_x was examined from the transmission electron microscopic (TEM) morphology.     

Synthesis and electrocatalytic performance of MWCNT-supported Ag@Pt core–shell nanoparticles for ORR

Ag@Pt core–shell nanoparticles with different Ag/Pt ratios were supported on multi walled carbon nanotubes (MWCNTs) and used as electrocatalysts for PEMFC. The morphology of the electrocatalyst samples was characterized by XRD and HRTEM. It was found that the Ag@Pt/MWCNTs catalyst exhibited a core–shell nanostructure. And the CV and LSV results demonstrated that such core–shell materials exhibited attractive electrocatalytic activity. Moreover, the specific electrochemically active area (EAS) of the Ag@Pt/MWCNTs catalyst is 70.63 m² g⁻¹, which is higher than the values reported in the literature.     

Cu-MOF@Co-MOF derived Co–Cu alloy nanoparticles and N atoms co-doped carbon matrix as efficient catalyst for enhanced oxygen reduction

We prepare a series of N-containing Cu-MOF@Co-MOF core-shell templates by employing two-step feed-in pathway and then step by step calcining these materials to obtain the N-doped carbon layers encapsulated Co–Cu alloy nanoparticles as efficient catalysts for enhanced oxygen reduction. The purpose of Cu-MOF designed as core materials is to increase the number of openly active sites when the templates are calcined at high temperature. During calcining, the Cu atoms facilitate the dispersion of Co on the surface of Co–Cu alloy nanoparticles, which efficiently avoid the agglomeration of Co during carbonization. When being used as cathode oxygen reduction catalysts, the optimal Co–Cu(3:1)@CN-900 material exhibits outstanding performance that can be comparable with the 20 wt% Pt/C catalysts, and the onset potential and half-wave potential of Co–Cu(3:1)@CN-900 is 60 mV and 37 mV higher than that of monometallic Co@CN-900 catalysts, respectively. Moreover, the active sites of Co–Cu(3:1)@CN-900 catalysts are also carefully investigated.     

Bandgap control of p-n heterojunction of Cu–Cu2O @ ZnO with modified reduced graphene oxide nanocomposites for photocatalytic hydrogen evolution

Here, we describe the in-situ synthesis of multicomponent ZnO-based photocatalysts for hydrogen production. We fabricated ZnO coupled with Cu–Cu₂O nanoparticles and modified reduced graphene oxide (mRGO) to ameliorate hydrogen production. The simultaneous introduction of mRGO and Cu–Cu₂O enhanced the generation rate of photocatalytic hydrogen to 3085.02 μmol g^?1 h^?1 due to significant alteration of the electronic structure. The bandgap energy of the prepared catalysts decreased from 3.2 eV for pristine ZnO to 2.64 eV for a composite containing 15% Cu–Cu₂O. The optimal designed heterostructure efficiently separates photo charge carriers and prevents charge carriers’ recombination by accelerating charge transfer with the help of mRGO and metallic Cu and as a result leading to efficient hydrogen yields.     

Ni–P and Ni–Cu–P modified carbon catalysts for methanol electro-oxidation in KOH solution

The electrocatalytic oxidation of methanol was studied on Ni–P and Ni–Cu–P supported over commercial carbon electrodes in 0.1 M KOH solution. Cyclic voltammetry and chronoamperometry techniques were employed. Electroless deposition technique was adopted for the preparation of these catalysts. The effect of the electroless deposition parameters on the catalytic activity of the formed samples was examined. They involve the variation of the deposition time, pH and temperature. The scanning electron micrography showed a compact Ni–P surface with a smooth and low porous structure. A decreased amount of nickel and phosphorus was detected by EDX analysis in the formed catalyst after adding copper to the deposition solution. However, an improvement in the catalytic performance of Ni–Cu–P/C samples was noticed. This is attributed to the presence of copper hydroxide/nickel oxyhydroxide species. It suppresses the formation of γ-NiOOH phase and stabilizes β-NiOOH form. Linear dependence of the oxidation current density on the square root of the scan rate reveals the diffusion controlled behaviour.     

Nanocrystalline structure and electrochemical hydrogen storage properties of the as-milled Mg–V–Ni–Fe–Zn-based materials

Among the electrode materials for Ni-MH batteries, the Mg alloy electrodes such as MgNi, Mg₂Ni, REMg₁₂, La₂Mg₁₇ are considered the most suitable anode materials due to their high discharge capacity and low cost. However, the poor electrochemical cycling stability prevents its practical application. In this paper, Mg_50-xV_xNi₄₅Fe₃Zn₂ (x = 0, 1, 2, 3, 4) + 50 wt% Ni alloys were prepared by partially replacing Mg with V and using mechanical ball milling techniques with amorphous and nanocrystalline structures. Electrochemical tests showed that the ball-milled alloy had good electrochemical uptake and release performance. The maximum release performance is achieved in the first cycle. After that, the discharge level and cycle stability increased significantly with increasing ball grinding time and V content.     

Structural and electrochemical characterization of YBa(Fe,Co,Cu)2O5+δ layered perovskites as cathode materials for solid oxide fuel cells

Single- and double-doped YBa(Fe,Co,Cu)₂O_5+δ layered perovskites are prepared by solid state reaction method and their structural characteristics, thermal expansion coefficient, oxygen nonstoichiometry, electrical conductivity, and electrochemical performance are comparatively studied. The substitution of Co by Fe or/and Cu significantly improves thermal expansion properties as compared to undoped YBaCo₂O_5+δ. Electrochemical tests demonstrate the promising performance of synthesized materials as cathode materials at intermediate temperatures. Single doped YBaCuCoO_5+δ cathodes reveal the lowest polarization resistance equal to 0.24 and 0.78 Ω cm² at P(O₂) P^?1 = 0.2 at temperature of 800 and 700 °C, respectively.     

Ni–Fe nanocubes embedded with Pt nanoparticles for hydrogen and oxygen evolution reactions

Electrochemical water-splitting is widely regarded as one of the essential strategies to produce hydrogen energy, while Metal-organic frameworks (MOFs) materials are used to prepare electrochemical catalysts because of its controllable morphology and low cost. Herein, a series of trimetallic porous Pt-inlaid Ni–Fe nanocubes (NCs) are developed with bifunctions of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In the process of prepare the electrochemical catalysts, Pt nanoparticles are uniformly embedded in the Fe–Ni PBA cube structure, and ascorbic acid is employed as a reducing agent to reduce Pt²⁺ to Pt nanoparticles. In this work, the cubic structure of Fe–Ni PBA is maintained and the noble metal Pt nanoparticles are embedded. Remarkably, the formation of PBA cubes, Pt inlay and reduction are completed in one step, and Pt nanoparticles are embedded by a simple method for the first time. By employing acid etching method, a porous structure is formed on the PBA cube, which increases the exposed area of the catalyst and provides more active sites for HER and OER. Due to the porous structure, highly electrochemical active surface area and the embedded of highly dispersed Pt nanoparticles, the porous 0.6 Ni–Fe–Pt nanocubes (NCs) exhibits excellently electrocatalytic performance and durable stability to HER and OER. In this work, for HER and OER, the Tafel slopes are 81 and 65 mV dec⁻¹, the overpotential η at the current density of 10 mA cm⁻² are 463 and 333 mV, and the onset potential are 0.444 and 1.548 V, respectively. And after a 12-h i-t test and 1000 cycles of cyclic voltammetry (CV), it maintained high stability and durability. This work opens up a new preparation method for noble metal embedded MOF materials and provided a new idea for the preparation of carbon nanocomposites based on MOF.     

Monodisperse palladium–nickel alloy nanoparticles assembled on graphene oxide with the high catalytic activity and reusability in the dehydrogenation of dimethylamine–borane

Herein, a highly efficient and stable palladium nickel nanoparticles (PdNi NPs) supported on graphene oxide (GO) was synthesized, characterized and applied for the dehydrogenation of dimethyl ammonia borane (DMAB). The monodisperse PdNi NPs has been synthesized via the ultrasonic double solvent reduction method in the presence of oleylamine and GO as support matrices. The structure morphology and properties of PdNi@GO NPs were characterized by using different techniques such as UV–VIS, XPS, TEM, HRTEM and XRD methods. The PdNi@GO NPs was found to be highly effective and stable in the dehydrogenation of DMAB. This catalyst with the turnover frequency of 271.9 h^?1 shows one of the best results among the all prepared catalysts in literature for the dehydrogenation of DMAB. The apparent activation parameters of the catalytic dehydrogenation reaction were also calculated; apparent activation energy (E_a,app) = 38 ± 2 kJ mol^?1, activation enthalpy (ΔH^#_,app) = 35 ± 1 kJ mol^?1 and activation entropy (ΔS^#_,app) = ?102 ± 1 J K^?1 mol^?1.     

Room temperature synthesis of Cu[Fe(CN)6]·XH2O cube derived ferric oxide@cupric oxide alloy ball on nitrogen-doped graphene as highly efficient electrochemical water splitting

Establishing non-precious metals with high efficiency for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysis are extremely essential for renewable energy technologies. Herein, we achieve the synthesis of metal-organic framework precursor of Cu[Fe(CN)₆]·XH₂O cube at room temperature, which further derived ferric oxide@cupric oxide alloy ball. On this foundation, we creatively synthesized ferric oxide@cupric oxide alloy ball on nitrogen-doped graphene (NG@Fe₂O₃/CuO alloy ball), where Fe₂O₃/CuO alloy ball is evenly anchored on nitrogen-doped graphene. It is worth noting that nitrogen doping, reduction of graphene oxide and conversion of Cu[Fe(CN)₆]·XH₂O cube precursor into Fe₂O₃/CuO alloy ball can be simultaneously realized by facile one-step calcination. Moreover, synergistic effect between the nitrogen-doped graphene and Fe₂O₃/CuO alloy ball can enhance the overall electrocatalytic performance of the catalyst by playing specific roles. The outstanding catalytic activity, long-term durability and stability make NG@Fe₂O₃/CuO alloy ball to become a promising non-precious electrocatalyst for electrochemical water oxidation.     

Pt and Pt–Ag nanoparticles supported on carbon nanotubes (CNT) for oxygen reduction reaction in alkaline medium

In this work, we investigated the effect of the carbon nanotubes (CNT) as alternative support of cathodes for oxygen reduction reaction (ORR) in alkaline medium. The Pt and Pt–Ag nanomaterials supported on CNT were synthesized by sonochemical method. The crystalline structure, morphology, particle size, dispersion, specific surface area, and composition were investigated by XRD, SEM-EDS, TEM, HR-TEM, N₂ adsorption-desorption and XPS characterization. The electrochemical activity for ORR was evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) in alkaline medium. The electrochemical stability was researched by an accelerated degradation test (ADT). Pt/CNT showed the better electrocatalytic activity towards ORR compared with Pt–Ag/CNT and Pt/C. Pt/CNT exhibited higher specific activity (1.12 mA cm^?2 _Pt) than Pt/C (0.25 mA cm^?2 _Pt) which can be attributed to smaller particle size, Pt-CNT interaction, and Pt load (5 wt%). The Pt monometallic samples supported on CNT and Vulcan showed higher electrochemical stability after ADT than Pt–Ag bimetallic. The ORR activity of all materials synthesized proceeded through a four-electron pathway. Furthermore, the EIS results showed that Pt/CNT exhibited the lower resistance to the transfer electron compared with conventional Pt/C and Pt–Ag/CNT.     

Performance and durability of Ni–Co alloy cermet anodes for solid oxide fuel cells

Ni alloys are examined as redox-resistant alternatives to pure Ni for solid oxide fuel cell (SOFC) anodes. Among the various candidate alloys, Ni–Co alloys are selected due to their thermochemical stability in the SOFC anode environment. Ni–Co alloy cermet anodes are prepared by ammonia co-precipitation, and their electrochemical performance and microstructure are evaluated. Ni–Co alloy anodes exhibit high durability against redox cycling, whilst the current-voltage characteristics are comparable to those of pure Ni cermet anodes. Microstructural observation reveals that cobalt-rich oxide layers on the outer surface of the Ni–Co alloy particles protect against further oxidation within the Ni alloy. In long-term durability tests using highly humidified hydrogen gas, the use of a Ni–Co cermet with Gd-doped CeO₂ suppresses degradation of the power generation performance. It is concluded that Ni–Co alloy cermet anodes are highly attractive for the development of robust SOFCs.     

Comparison study of electrocatalytic activity of reduced graphene oxide supported Pt–Cu bimetallic or Pt nanoparticles for the electrooxidation of methanol and ethanol

Pt–Cu bimetallic nanoparticles supported on reduced graphene oxide (Pt–Cu/RGO) were synthesized through the simple one-step reduction of H₂PtCl₆ and CuSO₄ in the presence of graphene oxide (GO) at room-temperature. The Pt–Cu/RGO was characterized with UV–vis spectrophotometer, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy and its catalytic behavior for the direct oxidation of methanol was investigated. Compared to Pt/RGO and Pt/C catalysts, Pt–Cu/RGO hybrids exhibited markedly superior catalytic activity for the electrocatalytic oxidation of methanol and ethanol. This improved catalytic activity can be attributed to the dendritic structure of the Pt–Cu bimetallic nanoparticles.     

In-situ construction of Mn–Fe mixed oxides nanoparticles on reduced graphene oxide with superior lithium storage property

In this paper, porous Mn₃O₄–Fe₃O₄ nanoparticles with highly uniform composition are in-situ anchored on reduced graphene oxide (rGO) nanosheets by a simple cyanometallic framework template method. Thanks to the synergistic effects between the porous Mn₃O₄–Fe₃O₄ nanoparticles and the well-conductive rGO nanosheets, the Mn₃O₄–Fe₃O₄/rGO composites present superior electrochemical lithium storage performances with a great reversible capacity of 1013 mAh g^?1 after 100 cycles at 0.1 A g^?1, satisfactory rate capability of 510 mAh g^?1 at 3.0 A g^?1, and eminent long-term cycle stability of 804 mAh g^?1 after 500 cycles at 0.5 A g^?1. It is demonstrated that the rGO can not only act as a conducting matrix, but also buffer the volume expansion and avoid the aggregation of the Mn₃O₄–Fe₃O₄ nanoparticles during charging-discharging. The work provides a simple strategy for designing and fabricating advanced multi-component metal oxide-based anodes for high-performance lithium-ion battery.     

Flower-like MOF-derived Co–N-doped carbon composite with remarkable activity and durability for electrochemical hydrogen evolution reaction

Electrochemical hydrogen evolution reaction (HER) is one of the most economical, sustainable, and attractive methods to produce hydrogen. Contemporarily, it is still a challenge to develop low-cost catalysts with high activity and durability for HER. Herein, we report a simple strategy to develop a Co–N-doped carbon electrocatalyst derived from a new cobalt metal-organic framework (MOF). The new flower-like 2D→3D MOF {[Co(BIPA)(5-OH-bdc)](DMF)}_n (1) was constructed based on bis(4-(1H-imidazol-1-yl)phenyl)amine (BIPA) and 5-hydroxyisophthalic acid (5-OH-H₂bdc). After that, the Co–N-doped carbon composite Co-MOF-800 was prepared via calcination of MOF 1. Interestingly, Co-MOF-800 exhibited excellent electrocatalytic activity and durability for HER. The onset potential (0.12 V) and E_j=10 value (0.193 V) of the Co-MOF-800 electrode were comparable to that of the most active non-precious metal HER electrocatalysts derived from other MOFs. The HER performance of Co-MOF-800 was stable without degradation even after long-term cycling.     

Ni–Fe/Mg(Al)O alloy catalyst for carbon dioxide reforming of methane: Influence of reduction temperature and Ni–Fe alloying on coking

Various Ni–Fe/Mg(Al)O alloy catalysts were obtained by calcination of Ni–Fe–Mg–Al hydrotalcite-like compounds, followed by reduction at different temperatures (973–1173 K). The characterizations of XRD and STEM-EDX suggest that the resulting Ni–Fe alloy particles are composition-uniform and size-controllable. The alloy composition is little affected by the reduction temperature, whereas the particle size (5.8–8.2 nm) increases with the increase of reduction temperature. This property is ascribed to the homogeneous distribution of nickel and iron species during the catalyst preparation. All of the Ni–Fe/Mg(Al)O alloy catalysts show relatively high and stable activity for CH₄–CO₂ reforming during 25 h of investigation at 773–1073 K. Particularly, the 973 K-reduced catalyst exhibits higher coke-resistance due to its smaller particle size. E_a-CH₄ and CH₄-TPSR measurements indicate that Ni–Fe alloying inhibits CH₄ dissociation. It is considered that during DRM CH₄ is dissociated at the Ni sites and CO₂ may be activated at the metal-support interface as well as the Fe sites. Ni–Fe alloying may inhibit CH₄ dissociation and/or promote CO₂ activation, thus contributing to the suppression of coke deposition.