The effect of Sn content on the electrocatalytic properties of Pt–Sn nanoparticles dispersed on graphene nanosheets for the methanol oxidation reaction

Direct methanol fuel cell (DMFC) electrode catalysts with improved electrochemical properties have been prepared by dispersing platinum–tin (Pt–Sn) nanoparticles onto graphene sheets. During the deposition, a majority of the oxygenated functional groups on the graphene oxide nanosheets were removed, resulting in the formation of graphene. Microstructural characterization shows that metallic Pt, Pt–Sn alloy and tin dioxide (SnO₂) nanoparticles were distributed on the graphene sheets, representing different lattice planes during the synthetic process. In terms of the electrocatalytic properties, graphene-supported Pt–Sn and graphene-supported Pt catalysts exhibited much higher current densities compared with that of commercial carbon black-supported Pt catalysts. Graphene-supported Pt–Sn increased the electrocatalytic activity, which is strongly influenced by the addition of Sn in its alloyed and oxidized forms, boosting the reaction more readily because of the lower oxidation potential.     

Temperature dependence of morphology and oxygen reduction reaction activity for carbon-supported Pd–Co electrocatalysts

In this study, we investigated the heat treatment temperature effect on the morphology and oxygen reduction reaction activity of carbon-supported Pd–Co alloy electrocatalysts. As prepared Pd–Co bimetallic nanoparticles showed a single-phase face-centered cubic disordered structure, and the mean particle size decreased with a Co content. In order to improve activity and stability, the catalysts were heat-treated in a temperature range of 300 to 700 °C. From the results of oxygen reduction reaction activity tests, the optimal heat treatment temperature was found to be 700 °C for the low Co content samples, while 300 °C was the best condition for the high Co content samples.     

Characterization of methanol-tolerant Pd–WO3 and Pd–SnO2 electrocatalysts for the oxygen reduction reaction in direct methanol fuel cells

Palladium (Pd) catalysts containing nanosized metal oxides, tungsten oxide (WO₃) and tin oxide (SnO₂), supported on carbon black (Pd–MO_x/C) were synthesized, and the effect of the metal oxide on the oxygen reduction reaction (ORR) in a direct methanol fuel cell (DMFC) was investigated. The SEM images showed that the Pd nanoparticles were highly dispersed on the carbon black, and the metal oxide particles were also distributed well. Pd/C and Pd–WO₃/C catalysts as cathode materials for the ORR in DMFCs showed activity similar to or better than that of Pt/C, whereas Pd–SnO₂/C showed no improvement in catalytic activity.     

IrO2–SnO2 mixtures as electrocatalysts for the oxygen reduction reaction in alkaline media

In this work, SnO₂ + IrO₂ mixed oxides are studied as electrocatalysts for the oxygen reduction reaction (ORR) in alkaline media by means of voltammetric techniques under controlled mass transfer conditions thanks to the use of rotating (ring) disk electrodes (RDE/RRDE). The oxides, prepared by sol–gel methodology, are supported on the disk electrodes using a thin layer of anionic exchange polymer as gluing agent. The amount of deposited polymer was optimized to avoid any limitation due to the diffusion of reactant/products across the film thickness. The mixed oxides were prepared at the following mole fractions of IrO₂: $ x_{{{\text{IrO}}_{ 2} }} $  = 0.15, 0.31, 0.55, 0.73, and 1. The role of composition was studied in terms of the reaction pathways and the relevant fraction of H₂O₂ production, together with the potentials of the onset of ORR. The fraction of sites able to give proton/hydroxyl and electron transfers is also determined and discussed. The results point to the best performance of low-Ir containing mixtures and to their low sensitivity to the presence of methanol, a key feature in the case of crossover in alkaline direct alcohol fuel cells.     

Carbon nano-tube supported Pt–Pd as methanol-resistant oxygen reduction electrocatalyts for enhancing catalytic activity in DMFCs

This work tries to study the problem of methanol crossover through the polymer electrolyte in direct methanol fuel cells (DMFCs) by developing new cathode electrocatalysts. For this purpose, a series of gas diffusion electrodes (GDEs) were prepared by using single-walled carbon nanotubes (SWCNTs) supported Pt–Pd (Pt–Pd/SWCNT) with different Pd contents at the fixed metal loading of 50 wt%, as bimetallic electrocatalysts, in the catalyst layer. Pt–Pd/SWCNT was prepared by depositing the Pt and Pd nanoparticles on a SWCNTs support. The elemental compositions of bimetallic catalysts were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES) system. The performances of the GDEs in the methanol oxidation reaction (MOR) and in the oxygen reduction reaction with/without the effect of methanol oxidation reaction were investigated by means of electrochemical techniques: cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS). The results indicated that GDEs with Pt–Pd/SWCNT possess excellent electrocatalytic properties for oxygen reduction reaction in the presence of methanol, which can originate from the presence of Pd atoms and from the composition effect.     

Synthesis of carbon-supported binary FeCo–N non-noble metal electrocatalysts for the oxygen reduction reaction

In this paper, a carbon-supported binary FeCo–N/C catalyst using tripyridyl triazine (TPTZ) as the complex ligand was successfully synthesized. The FeCo–TPTZ complex was then heat-treated at 600 °C, 700 °C, 800 °C, and 900 °C to optimize its oxygen reduction reaction (ORR) activity. It was found that the 700 °C heat-treatment yielded the most active FeCo–N/C catalyst for the ORR. XRD, EDX, TEM, XPS, and cyclic voltammetry techniques were used to characterize the structural changes in these catalysts after heat-treatment, including the total metal loading and the mole ratio of Fe to Co in the catalyst, the possible structures of the surface active sites, and the electrochemical activity. XPS analysis revealed that Co–N_x, Fe–N_x, and C–N were present on the catalyst particle surface. To assess catalyst ORR activity, quantitative evaluations using both RDE and RRDE techniques were carried out, and several kinetic parameters were obtained, including overall ORR electron transfer number, electron transfer coefficient in the rate-determining step (RDS), electron transfer rate constant in the RDS, exchange current density, and mole percentage of H₂O₂ produced in the catalyzed ORR. The overall electron transfer number for the catalyzed ORR was ～3.88, with H₂O₂ production under 10%, suggesting that the ORR catalyzed by FeCo–N/C catalyst is dominated by a 4-electron transfer pathway that produces H₂O. The stability of the binary FeCo–N/C catalyst was also tested using single Fe–N/C and Co–N/C catalysts as baselines. The experimental results clearly indicated that the binary FeCo–N/C catalyst had enhanced activity and stability towards the ORR. Based on the experimental results, a possible mechanism for ORR performance enhancement using a binary FeCo–N/C catalyst is proposed and discussed.     

Effect of MEA fabrication techniques on the cell performance of Pt–Pd/C electrocatalyst for oxygen reduction in PEM fuel cell

The effect of three different membrane electrode assembly (MEA) fabrication techniques, catalyst-coated substrate by direct spray (CCS) and catalyst-coated membrane by direct spray (CCM-DS) or decal transfer (CCM-DT), on the performance of oxygen reduction in a proton exchange membrane (PEM) fuel cell was carried out under identical conditions of Pt–Pd/C electrocatalyst loading. The results indicated that the fabrication technique had only a very slight effect on the ohmic resistance of the PEM fuel cell but it significantly affected the charge transfer resistance and open circuit voltage (OCV). The cells prepared by the CCM method, and particularly by decal transfer, exhibited a significantly higher OCV but a lower ohmic and charge transfer resistance compared with the other investigated fabrication techniques. By using cyclic voltammetry with H₂ adsorption, it was found that the electrochemical active area of the electrocatalyst prepared by CCM-DT was higher than those prepared by CCS and CCM-DS by around 1.76- and 1.05-fold, respectively. Under a H₂/O₂ system at 0.6 V, the cells with MEA made by CCM-DT provided the highest cell performance of around 350 mA/cm², significantly greater than those prepared by the CCS and CCM-DS (149 and 42 mA/cm², respectively).     

One-pot synthesis of graphene–BiOBr nanosheets composite for enhanced photocatalytic generation of reactive oxygen species

Graphene oxide (GO), BiOBr and graphene–BiOBr nanosheets composites (BiOBr–RG) were synthesized and characterized. It can be found that except for BiOBr nanosheets with pure tetragonal phase were grown uniformly on the graphene surface, little graphene layer also can load on the surface of BiOBr evenly. And we found that the graphene can change the conduction band (CB) and valence band (VB) of BiOBr toward enhanced photocatalytic activity for reactive oxygen species (ROS) generation than that of BiOBr under visible-light irradiation.     

Efficient chemoselective reduction of nitro compounds and olefins using Pd–Pt bimetallic nanoparticles on functionalized multi-wall-carbon nanotubes

We report the synthesis of novel Pd–Pt bimetallic nanoparticle catalysts using functionalized multi-wall carbon-nanotubes and utilization of them to reductions. The carbon nanotube-supported bimetallic nanoparticle catalysts showed improved activity in reduction reactions, compared with that of mono metal-supported catalysts. Under the optimized reaction conditions, various nitro compounds and alkenes were cleanly reduced at ambient temperature. Furthermore, this catalytic system exhibits excellent activity and high chemoselectivity for nitro compounds in the presence of other functional groups labile to hydrogenation. After the reaction, the catalysts could be collected through filtration, and reused for 10 times without any loss of catalytic activity.     

A dually spontaneous reduction and assembly strategy for hybrid capsules of graphene quantum dots with platinum–copper nanoparticles for enhanced oxygen reduction reaction

The unique hybrid capsules of graphene quantum dots (GQDs) with platinum (Pt)–copper (Cu) nanoparticles have been prepared by a dually spontaneous reduction and assembly process of oxidized graphene quantum dots (ox-GQDs) and Pt precursor by galvanic displacement reaction on Cu microspheres. Cu@PtCu core–shell microspheres were first prepared by reaction of Cu with platinum (IV), which subsequently induced the reduction and assembly of ox-GQDs on them at room temperature. After removal of the Cu cores, the resultant capsules of PtCu@GQDs were collected. Based on the unique hybrid structure, PtCu@GQDs capsules showed remarkably enhanced catalytic activity for oxygen reduction reaction (ORR) with a 2 times higher mass activity and a 40 mV more positive onset potential than that of commercial Pt black, indicating the promise of the newly-prepared PtCu@GQDs capsules as catalysts for fuel cell applications.     

An update on the mechanism of the graphene–NO reaction

Cognizant of the key experimental facts from studies of carbonaceous solids ranging from soot to graphite, we performed a quantum chemistry study of the interaction of NO monomer or dimer with one or more zigzag sites. Thermodynamic and kinetic results were used to examine two alternative mechanisms proposed in the literature, and to compare them with the graphene–O₂ reaction mechanism. The chemisorption stoichiometry similarities are striking; but the differences, especially regarding the intermediate role of N₂O, have important practical implications. Monomer chemisorption on an isolated site is a dead-end and temporarily inhibiting process, similar to that of formation of a stable C–O surface complex in the graphene–O₂ reaction. When two sites are available, successive monomer adsorption eventually leads to N₂O formation subsequent to parallel reorientation of the first NO molecule. If three contiguous sites are available, N₂ and CO are the principal products. Chemisorption of the dimer provides a straightforward path to N₂ and CO₂ when one site is available and to N₂ and CO when two sites are available. The formation of N₂O is also feasible in this case, both during adsorption and desorption; in the adsorption phase it is very sensitive to the details of the electron pairing processes.     

RRDE study of oxygen reduction on Pt nanoparticles inside Nafion®: H2O2 production in PEMFC cathode conditions

Dihydrogen peroxide production on platinum particles supported on carbon inside a proton exchange membrane (Nafion^®), that is, under the working conditions of PEMFC cathodes, is rather small at the usual oxygen reduction potentials. As on bulk platinum, a four-electron mechanism appears to be the main pathway, with particle size and carbon substrate effects on the H₂O₂ production. A large increase in the H₂O₂ contribution takes place at low potentials, that is, at the working potentials of PEMFC anodes.     

Pd–Ru/C as the electrocatalyst for hydrogen peroxide reduction

Pd–Ru, Pd and Ru nanoparticles supported on Vulcan XC-72 carbon were prepared by chemical reduction of PdCl₂ and/or RuCl₃ in aqueous solution using NaBH₄ as the reducing agent. Transmission electron microscopy measurements showed that Pd–Ru particles were uniformly dispersed on carbon. The particle size of Pd–Ru is around 5–9 nm. X-ray diffraction analysis indicated that Ru formed alloy with Pd in Pd–Ru/C catalyst. The electroreduction of hydrogen peroxide on Pd–Ru/C, Pd/C and Ru/C in H₂SO₄ solution was examined by linear sweep voltammetry and chronoamperometry measurements. Results revealed that Pd–Ru/C catalyst exhibited higher electrocatalytic activity for hydrogen peroxide reduction than Pd/C and Ru/C. All the catalysts showed good stability for hydrogen peroxide electroreduction in H₂SO₄ electrolyte.     

Oxidation and reduction effects of propane–oxygen on Pd–chlorine/alumina catalysts

Pd–chloride precursor salt was used to prepare Pd/Al₂O₃ catalysts. TPSR measurements showed three distinct reactions for the oxidation of propane on palladium surface under excess of hydrocarbon: complete oxidation, steam reforming and propane hydrogenolysis. Propane oxidation on palladium catalysts was related to the Pd²⁺ sites observed on Pd/Al₂O₃ through infrared of adsorbed carbon monoxide. In fresh catalysts reduced by H₂, the IR spectra showed the linear and bridge adsorbed CO species on the Pd⁰ surface. After propane reaction, a new band at 2130 cm^-1 related to CO adsorption on Pd²⁺ species was noted. Carbon monoxide species adsorbed on Pd⁰ were also observed in all samples after reaction. Our results suggest surface ratios of Pd⁰/PdO during the propane oxidation. On the other hand, time on stream conversions of the complete oxidation of propane were affected by either the water generated during the reaction or added as a reactant at 10 vol%. The water generated by the reaction helped to eliminate chlorine residues in the form of oxychloride species leading to an increasing of the activity. However, the presence of water into the reaction mixture caused a strong decreasing of the activity. The inhibition mechanism of propane oxidation in the presence of water consisted in the dissociative adsorption of water on palladium sites with the possible formation of palladium hydroxide (Pd–OH) at the surface, diminishing the number of active surface sites. Dynamic fluctuations into the reaction conditions supported the idea that a pseudo‐equilibrium adsorption–desorption of water was reached. After water removal or increasing in the reaction temperature the equilibrium was shifted to the direction of OH–Pd decomposition. This behavior suggests that the inhibitory effect of water is a reversible phenomenon, being a function of the amount of water and the reaction temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ordered mesoporous Fe (or Co)–N–graphitic carbons as excellent non-precious-metal electrocatalysts for oxygen reduction

Novel mesoporous Fe (or Co)–N_x–C non-precious-metal catalysts (NPMCs) have been fabricated by a simple nanocasting-pyrolysis method using 1,10-phenanthroline metal chelates as the precursors. Owing to the ordered hexagonal mesostructures, appropriate surface area, large-pore channels, and well-distributed metal–N_x moieties embedded within the graphitic carbon backbones, the prepared metal–N_x–C materials exhibit excellent catalytic activity for oxygen reduction reaction (ORR) in both alkaline and acidic media. The prepared Fe–N_x–C materials, when prepared with an optimized catalyst loading on the electrode, exhibit more positive ORR onset-potential and half-wave potential (E_1/2) than commercial Pt/C catalysts and the previously reported NPMCs in 0.1 M KOH electrolyte. They also have the comparable ORR onset-potential and current densities to Pt/C electrode in 0.1 M HClO₄ electrolyte. Moreover, ORR over mesoporous Fe–N_x–C was found to proceed by the direct four-electron mechanism with high selectivity in both electrolytes. The mesoporous Fe–N_x–C materials demonstrated higher ORR catalytic activity compared to the NPMCs made by alternative methods. Analysis of the catalytic behavior, structure and nature of surface species of N_x–C materials allows us to ascribe the origin of the excellent ORR catalytic activity of mesoporous Fe (or Co)–N_x–C in both electrolytes to Fe (or Co)–N_x moieties embedded within the graphitic carbon frameworks.     

Fishnet-like Ni–Fe–N co-modified graphene aerogel catalyst for highly efficient oxygen reduction reaction in an alkaline medium

Journal of Applied Electrochemistry - The bimetallic nanoparticles of Ni and Fe co-modified reduced graphene oxide (rGO) aerogel were prepared by a fishnet-like one-step hydrothermal method, and...     

Preparation of Pt–Co alloy catalysts by electrodeposition for oxygen reduction in PEMFC

Direct current (DC) and pulse current (PC) electrodeposition of Pt–Co alloy onto pretreated electrodes has been conducted to fabricate catalyst electrodes for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). The effect of plating mode and pulse plating parameters on the Pt–Co alloy catalyst structure, composition and electroactivity for the ORR in PEMFC has been investigated. The electrodeposited Pt–Co alloy catalyst indicates higher electrocatalytic activity towards the ORR than the electrodeposited Pt catalyst. The activity of the electrodeposited Pt–Co catalysts is further improved by applying the current in a pulse waveform pattern. The electrodeposition mode and the pulse plating parameters do not have the significant effect on the Pt:Co composition of deposited catalysts, but show the substantial effect on the deposit structures produced. The Pt–Co catalysts prepared by PC electrodeposition have finer structures and contain smaller Pt–Co catalyst particles compared to that produced by DC electrodeposition. By varying the Pt concentration in deposition solution, the Pt:Co composition of the electrodeposited catalyst that exhibits the highest activity is found. The Pt–Co alloy catalyst with the Pt:Co composition of 82:18 obtained at the charge density of 2 C cm⁻², the pulse current density of 200 mA cm⁻², 5% duty cycle and 1 Hz was found to yield the best electrocatalytic activity towards the ORR in PEMFC.