Surfactant-free Pd–Fe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation

Herein, a novel surfactant-free nanocatalyst of Pd–Fe bimetallic nanoparticles (NPs) supported on the reduced graphene oxide (Pd–Fe/RGO) were synthesized using a two-step reduction in aqueous phase. Electrochemical studies demonstrate that the nanocatalyst exhibits superior catalytic activity towards the formic acid oxidation with high stability due to the synergic effect of Pd–Fe and RGO. The optimized Pd–Fe/RGO (Pd:Fe = 1:5) nanocatalyst possess an specific activity of 2.72 mA cm^?2 and an mass activity of 1.0 A mg^?1_(Pd), which are significantly higher than those of Pd/RGO and commercial Pd/C catalysts.     

Hydrothermal synthesis of mixed crystal phases TiO2–reduced graphene oxide nanocomposites with small particle size for lithium ion batteries

A rutile and anatase mixed crystal phase of nano-rod TiO₂ and TiO₂–reduced graphene oxide (TiO₂–RGO) nanocomposites with small particle size were prepared via a facile hydrothermal approach with titanium tetrabutoxide and graphene oxide as the precursor. Hydrolysis of titanium tetrabutoxide and mild reduction of graphene oxide were simultaneously carried out. Compared with traditional multistep methods, a novel green synthetic route to produce TiO₂–RGO without toxic solvents or reducing agents was employed. TiO₂–RGO as anode of lithium ion batteries was characterized by extensive measurements. The nanocomposites exhibited notable improvement in lithium ion insertion/extraction behavior compared with TiO₂, indicating an initial irreversible capacity and a reversible capacity of 295.4 and 112.3 mA h g⁻¹ for TiO₂–RGO after 100 cycles at a high charge rate of 10 C. The enhanced electrochemical performance is attributed to increased conductivity in presence of reduced graphene oxide in TiO₂–RGO, a rutile and anatase mixed crystal phase of nano-rod TiO₂/GNS composites, small size of TiO₂ particles in nanocomposites, and enlarged electrode–electrolyte contact area, leading to more electroactive sites in TiO₂–RGO.     

Platinum–Iron nanoparticles supported on reduced graphene oxide as an improved catalyst for methanol electro oxidation

Platinum–Iron nanoparticles supported on reduced graphene oxide powder are synthesized by chemical reduction method as an anode catalyst for the methanol electro oxidation. The characterization of the catalyst has been investigated using physical and electrochemical methods. Prepared catalyst was characterized by scanning electron microscopy (SEM), TEM (Transmission electron microscopy), FT-IR (Fourier-transform infrared spectroscopy), Raman spectroscopy and, X-ray diffraction (XRD) and energy dispersive analysis of X-ray (EDX). Pt and Pt-Fe nanoparticles are uniformly dispersed on the surface of reduced graphene oxide (rGO) powder nanocomposite support. The catalytic properties of the catalyst for methanol electro-oxidation were thoroughly studied by electrochemical methods that involved in the cyclic voltammetry, linear sweep voltammetry (LSV), chronoamperometry and electrochemical impedance spectroscopy (EIS). The Pt-Fe/rGo exhibits high electrocatalytic activity, catalyst tolerance for the CO poisoning and catalyst durability for electro-oxidation of methanol compared to the Pt/rGo and commercial Pt/C catalyst. Therefore, the Pt-Fe/rGo catalyst is a good choice for application in direct methanol fuel cells.     

Hierarchical reduced graphene oxide supported dealloyed platinum–copper nanoparticles for highly efficient methanol electrooxidation

Exploiting highly efficient electrocatalysts through simple methods is very critical to the development of energy conversion technologies. Herein, we develop a hierarchical reduced graphene oxide supported dealloyed platinum–copper nanoparticle catalyst (Pt–Cu/RGO) by a facile one-step electrodeposition of graphene oxide in the presence of H₂PtCl₆ and copper ethylenediamine tetraacetate. The nanostructure and composition were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. Meanwhile, the electrocatalytic performance was investigated by cyclic voltammetry and chronoamperometry, showing that the Pt–Cu/RGO catalyst not only equips with an outstanding electrocatalytic activity for the methanol oxidation reaction (2.3 times that of commercial Pt/C catalyst), but also shows a robust durability and superior tolerance to CO poisoning. The excellent electrocatalytic performance could be attributed to the three-dimensional hierarchical structure, porous dealloyed nanoparticles and synergistic effect between each component.     

In situ X-ray absorption spectroscopic studies of anodically deposited binary Mn–Fe mixed oxides with relevance to pseudocapacitance

Binary Mn–Fe oxides with different Mn/Fe content ratios were prepared by anodic deposition. The deposited oxides were studied by in situ X-ray absorption spectroscopy (XAS) in 2 M KCl solution during the charging–discharging process. The experimental results clearly confirmed that the oxidation states of both Mn and Fe changed back and forth with adjusting the applied potential, contributing to the pseudocapacitive characteristics of the binary oxides. It was also found that, within a potential range of 1 V, because of Fe oxide addition the variation in the Mn oxidation state was increased from 0.70 (+3.25 to +3.95 for plain Mn oxide) to 0.81 (+3.12 to +3.93 for Mn90Fe10 oxide or +3.10 to +3.91 for Mn75Fe25 oxide), while the Fe oxide itself demonstrated an oxidation state shift of only 0.55. Accordingly, the optimum pseudocapacitance of the binary Mn–Fe oxide could be only achieved when the amount of Fe oxide was properly controlled. The highest specific capacitance of 255 F g⁻¹ was obtained with a Mn/Fe atomic ratio of 90/10, while plain Mn oxide revealed a capacitance of only 205 F g⁻¹.     

Monodisperse nickel–palladium alloy nanoparticles supported on reduced graphene oxide as highly efficient catalysts for the hydrolytic dehydrogenation of ammonia borane

Addressed herein is the catalysis of reduced graphene oxide-supported monodisperse NiPd alloy nanoparticles (NPs) (rGO-NiPd) in the hydrolytic dehydrogenation of ammonia borane (AB). This is the first example of the use of NiPd alloy NPs as catalyst in the hydrolytic dehydrogenation of AB. Monodisperse NiPd alloy NPs (3.5 nm) were synthesized by co-reduction of nickel(II) acetate and palladium(II) acetylacetonate in oleylamine (OAm) and borane-tert-butylamine complex (BTB) at 100 °C. The current recipe allowed to control the composition of NiPd alloy NPs and to study the composition-controlled catalysis of rGO-NiPd in the hydrolytic dehydrogenation of AB. Among the all compositions tested, the Ni₃₀Pd₇₀ was the most active one with the turnover frequency of 28.7 min⁻¹. The rGO-Ni₃₀Pd₇₀ were also durable catalysts in the hydrolytic dehydrogenation of AB providing 3650 total turnovers in 35 h and reused at six times without deactivation. The detailed reaction kinetics of hydrolytic dehydrogenation of AB revealed that the reaction proceeds first order with respect to the NiPd concentration and zeroth order with respect to the AB concentration. The apparent activation energy of the catalytic dehydrogenation of AB was also calculated to be E_a^app = 45 ± 2 kJ*mol⁻¹.     

Enhanced hydrogen storage performance of three-dimensional hierarchical porous graphene with nickel nanoparticles

Three-dimensional hierarchical porous graphene with nickel nanoparticles (3DHPG-Ni) was synthesized through electrostatic assembly method with the assistance of poly (methyl methacrylate) (PMMA) template and subsequent removal of PMMA template by calcination. The morphology, microstructure and hydrogen adsorption properties of 3DHPG-Ni nanocomposites were examined in detail. The obtained 3DHPG-Ni nanocomposite exhibited hierarchical porous structure composed of macro-, meso- and micropores, high specific surface area (925 m² g^?1), large pore volume (0.58 cm³ g^?1) and excellent hydrogen storage capacity. Under the pressure of 5 bar, 3DHPG-Ni nanocomposite showed a maximum hydrogen capacity of 4.22 wt% and 1.95 wt% at 77 K and 298 K, respectively, demonstrating that the as-prepared 3DHPG-Ni nanocomposite was supposed to be a promising material with outstanding properties for practical applications in the field of hydrogen storage. The three-dimensional hierarchical porous structure, evenly distributed Ni nanoparticles and hydrogen spillover effect were responsible for the enhanced hydrogen storage capacities.     

A study on the hydrogen storage performance of graphene–Pd(T)–graphene structure

The 1–6 H₂ molecule adsorption energy and electronic properties of sandwich graphene–Pd(T)–Graphene (G–Pd(T)–G) structure were studied by the first-principle analysis. The binding energies, adsorption energies, and adsorption distances of Pd atoms-modified single-layer graphene and bilayer graphene with H₂ molecules at B, H, T adsorption sites were calculated. In bilayer graphene, the adsorption properties at T sites were found to be more stable than those at B and H sites. The binding energy of Pd atoms (4.16 eV) on bilayer graphene was higher than the experimental cohesion energy of Pd atoms (3.89 eV), and this phenomenon eliminated the impact of metal clusters on adsorption properties. It was found that three H₂ molecules were stably adsorbed on the G–Pd(T)–G structure with an average adsorption energy of 0.22 eV. Therefore, it can be speculated that G–Pd(T)–G is an excellent hydrogen storage material.     

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.     

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.     

Spectroscopic characterization of Mn–Co–Ce mixed oxides,active catalysts for COPROX reaction

Mn–Co–Ce mixed oxides are active and selective catalysts for the CO preferential oxidation (COPROX), which is a promising process for the purification of hydrogen streams. In this work, we report a careful spectroscopic characterization of the said system, with the aim of relating its physical chemistry properties to the catalytic behavior. In all the Co–Mn–Ce samples, we detected the formation of partially developed (Mn,Co)₃O₄ mixed spinels. The presence of these species, which can be reduced during the TPR experiments at an intermediate temperature range (300–600 °C), was also suggested by XRD and LRS. XPS results show that in all cases the catalytic surface is enriched in Mn, while the opposite occurs for Co. As regards the catalytic activity, we observed that the best formulations were those containing intermediate Mn/Co ratios (1/4 and 1/1), which can be ascribed to the promoting effect of Mn in improving the redox properties of Co active sites and provoking an increase in surface area. The best catalyst, which has a Mn/Co ratio of 1/4, was very stable after 75 h of time-on-stream with CO₂ included in the feed.     

Study on the hydrogen storage performance of graphene(N)–Sc–graphene(N) structure

The electronic properties of a sandwich graphene(N)–Sc–graphene(N) structure and its average adsorption energies after the adsorption of 1, 3, 5, 7, 10, and 14H₂ molecules were investigated by first principles. The average binding energies and adsorption distances of Sc atoms at different adsorption sites in N-doped bilayer graphene (N–BLG) were calculated. It was found that Sc atoms located at different adsorption sites of BLG generated metal clusters. The binding energy of the Sc atom located at the TT position of N–BLG (5.19 eV) was higher than the experimental cohesion energy (3.90 eV), and eliminated the impact of metal clusters on adsorption properties. It was found that the G(N)–Sc–G(N) system could stably adsorb 10 hydrogen molecules with an average adsorption energy of 0.24 eV. Therefore, it can be speculated that G(N)–Sc–G(N) is an excellent hydrogen storage material.     

Post-mortem evaluation of oxidized atmospheric plasma sprayed Mn–Co–Fe oxide spinel coatings on SOFC interconnectors

Interconnects employed in solid oxide fuel cells require electrically conductive protective coatings such as those based on manganese cobalt oxide spinels in order to prevent evaporation of volatile Cr(VI)-compounds and to minimize high temperature corrosion. MnCo_2−xFe_xO₄ based (where x = 0.1 and 0.3) oxide spinel protective coatings were manufactured by the atmospheric plasma spraying process on Crofer 22 APU substrates. The coated substrates were oxidized at 700 °C in air for 1000 h and post-mortem analyses were conducted to study the performance of the thermal sprayed coatings. During the high temperature oxidation, a four-point on-line measurement technique was used for area specific resistance studies. The MnCo_1.7Fe_0.3O₄ coating was tested together with the La_0.85Sr_0.15Mn_1.1O₃-spacer.     

Three-dimensional reduced graphene oxide–Mn3O4 nanosheet hybrid decorated with palladium nanoparticles for highly efficient hydrogen evolution

A three-dimensional (3D) reduced graphene oxide–Mn₃O₄ nanosheet (Mn₃O₄@rGO) hybrid was achieved by simple electrodeposition technique. Small palladium nanoparticle were homogeneously anchored onto Mn₃O₄@rGO substrate through the reduction of palladium salt. The interpenetrating network architecture of Mn₃O₄@rGO greatly inhibited the aggregation of 2D sheets of Mn₃O₄ and rGO, and the open 3D orientation of the Mn₃O₄@rGO hybrid nanosheets on the electrode facilitated both mass transport and electron transfer as well as maximally exposed active sites. The introduction of Mn₃O₄ enhanced the structural and electrochemical stability of rGO. The as-synthesized Pd/Mn₃O₄@rGO hybrid was employed as an electrocatalyst for electrocatalytic hydrogen evolution reaction (HER). The electrocatalyst showed a low overpotential of 20 mV at 10 mA cm^?2, a small Tafel slope of 48.2 mV dec^?1, and a large exchange current density of 0.59 mA cm^?2. Importantly, the catalyst possessed superior durability with 85.87% of catalytic activity after a long-time test (10 h). This work presents a simple and efficient stratagy to construct high-performance electrocatalysts for energy and environmental applications.     

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.     

Ti–Fe mixed oxides as photocatalysts in the generation of hydrogen under UV-light irradiation

Ti–Fe mixed oxides were prepared by sol-gel and used as photocatalysts for the generation H₂ from water. The solids were characterized by SEM-EDS, N₂ physisorption, XRD, UV–Vis and XPS spectroscopy. The mixed oxides present larger specific surface areas (83–205 m²/g) than that of pure TiO₂ (64 m²/g). The XRD patterns of the Ti–Fe solids resemble that of anatase titania. The band gap energies of the solids vary from 3.0 to 2.5 eV and are smaller than that of TiO₂ (3.2 eV). The mixed oxides were tested as photocatalysts in the production of hydrogen from water using methanol as a sacrificial agent. In all cases, their catalytic activities were higher than that exhibited by TiO₂ after 10 h of reaction.     

Enhanced electrochemical and hydrogen storage properties of La–Mg–Ni-based alloy electrode using a Ni and N co-doped reduced graphene oxide nanocomposite as a catalyst

To enhance the electrochemical property of a La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy, a three-dimensional (3D) reduced graphene oxide (rGO)-supported nickel and nitrogen co-doped (Ni–N@rGO) nanocomposite is fabricated by an impregnation method and introduced into the La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy. The results show that the reversible hydrogen storage property and the comprehensive electrochemical performance of the La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy are enhanced effectively when it is modified by the Ni–N@rGO nanocomposite. The high-rate dischargeability values at a discharge current density of 1500 mA g⁻¹ for the La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy and Ni–N@rGO-modified samples are 0.0% and 70.5%, respectively. Additionally, the anodic peak currents for the unmodified alloy electrode is 892 mA g⁻¹. Under the catalytic action of the Ni–N@rGO nanocomposite, the value increases to 2307 mA g⁻¹, which is 2.59 times larger than that of unmodified samples. The results also indicate that the diffusion ability of the hydrogen atom in the alloy electrode body enhances significantly when modified by the Ni–N@rGO nanocomposite. The hydrogen diffusion coefficient for the La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy electrode increases from 3.93 × 10⁻¹⁰ cm² s⁻¹ to 6.15 × 10⁻¹⁰ cm² s⁻¹ when is modified by Ni–N@rGO nanocomposite. These improvements in the comprehensive electrochemical properties are mainly attributed to the excellent electrochemical activity and conductivity of the Ni–N@rGO nanocomposite.     

In-situ assemblage of Fe–V–P/graphene aerogel based on prussian blue analogue as a high-performance electrocatalyst for oxygen evolution reaction

The carbon scaffold with high conductivity is suitable to enhance the catalytic activity of prussian blue analogues (PBAs) in oxygen evolution reaction (OER) of water splitting. Herein, a two-step strategy is developed to synthesize Fe–V–P/graphene aerogel based on prussian blue analogue (Fe–V–P/GA) electrocatalysts, which possess dense arrangement of anomalous octahedron structure. In the vanadium-modified PBAs (FeV–PBAs), Fe/V with a series of different molar ratios has been investigated and when the molar ratio of Fe/V is 1:1, the catalyst achieves a fairly high specific surface area expressed by the double-layer capacitance of 5.29 mF cm⁻² and requires overpotentials of only 234.0 and 314.3 mV to attain the benchmark current density of 10 and 50 mA cm⁻² during OER process. Besides, the catalyst owns satisfactory stability and the current density remains almost constant during stability tests lasting up to 70 h. As revealed via electrochemical kinetics analysis and the spectroscopic measurement, embellished FeV–PBAs not only lead to larger tangible area, more accessible sites and higher durable stability, but also provide formation of high valence state of iron and vanadium species, strong modification of electronic state and faster kinetics. This study provides a novel mode of thinking to consummate the design of 3D construction so as to boost the catalytic effect of transition metal catalysts for efficient oxygen evolution.     

Superior electrocatalytic activity of PtSrCoO3−δ nanoparticles supported on functionalized reduced graphene oxide-chitosan for ethanol oxidation

Here, SrCoO_3?δ was synthesized through a sol gel method and characterized with X-ray powder diffraction and scanning electron microscopy techniques. Graphene oxide was synthesized by a modified Hummers' method, then functionalized with 1, 1′-dimethyl-4, 4′-bipyridinium dichloride (methyl viologen) accompanied by Chitosan to prepare novel MV-RGO-CH. This was used as a support for nanoparticles to get PtSrCoO_3?δ/MV–RGO-CH nanocomposite. Transmission electron microscopy image was used to show the morphology and distribution of nanoparticles. The electrocatalytic activity of PtSrCoO_3?δ/MV–RGO-CH nanocomposite for ethanol electrooxidation was investigated by cyclic voltammetry, chronoamperometry, CO_ads stripping voltammetry and electrochemical impedance spectroscopy techniques. The effects of some experimental factors for ethanol oxidation on the prepared nanocomposite were investigated and the optimum conditions were suggested. PtSrCoO_3?δ/MV–RGO-CH nanocomposite showed higher catalytic activities than Pt/MV–RGO-CH for ethanol electrooxidation indicating that PtSrCoO_3?δ/MV–RGO-CH nanocomposite could be a promising catalyst for direct ethanol fuel cells applications.     

Improved de/re-hydrogenation properties of MgH2 catalyzed by graphene templated Ti–Ni–Fe nanoparticles

The present investigation deals with the excellent catalytic effect of graphene templated Ti–Ni–Fe nanoparticles (Ti–Ni–Fe@Gr) on de/re-hydrogenation characteristics of MgH₂. The catalytic effect of Ti–Ni–Fe@Gr on MgH2 has also been compared with Ti@Gr, Ni@Gr, and Fe@Gr. It has been found that Ti–Ni–Fe@Gr lowers the onset desorption temperature up to 252 °C with improved kinetics and cyclability for the hydrogen release and absorption from MgH₂. The presence of a multivalence environment around Mg/MgH₂ has been analyzed by XPS analysis which gives the evidence of possible electronic exchange between the catalyst and Mg/MgH₂ during de-/rehydrogenation. Since Mg/MgH₂ and Ti–Ni–Fe are both anchored on graphene template, agglomeration detrimental to cycling is not possible. Thus negligible degradation of 0.22 wt% has been observed even after 24 cycles of de/re-hydrogenation.