Jianhuang Zeng

Hybrid PdAg alloy-Au nanorods: Controlled growth, optical properties and electrochemical catalysis

Controllable growth of high-quality hybrid nanostructures is highly desirable for the fabrication of hierarchical, complex and multifunctional devices. Here, PdAg alloys have been controllably grown at different locations on gold nanorods, producing dumbbell-like nanostructures with PdAg at the ends of the gold nanorods or branched nanostructures with PdAg grown almost perpendicular to the gold nanorods. The nucleation sites of PdAg alloys on the gold nanorods can be effectively tuned by varying the concentrations of H2PdCl4, AgNO3 and cetyltrimethylammonium bromide (CTAB). The dumbbell-like and branched nanostructures were characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), line-scanning energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and UV-Vis absorption spectroscopy. Their electrocatalytic performance was evaluated using ethanol oxidation as a probe reaction. The dumbbell-like nanostructures show a better anti-poisoning performance, but a worse electrochemical activity than the branched ones. The results provide guidelines for the controlled growth of complicated nanostructures for either fundamental studies or potential applications.

Electrostatic interaction based hollow Pt and Ru assemblies toward methanol oxidation

Mastery over the structure and/or composition of metal nanoparticles is an effective way to improve their catalytic activity on a mass basis. Herein, we report a facile solution route for the assembly of hollow Pt nanospheres (hPt) and ultrafine Ru nanoparticles based on electrostatic interactions. In this approach, negatively charged hollow Pt nanospheres with an average size of 12 nm and positively charged ultrafine Ru nanoparticles with an average size of 0.9 nm are first prepared, followed by the successful fabrication of hPt–Ru assemblies upon mixing the particles with opposite charges. The hPt–Ru assemblies at a Pt/Ru molar ratio of 2u2006:u20061 exhibit superior catalytic activity toward methanol oxidation in direct methanol fuel cells for the presence of a mixed-phase containing Pt and an effective oxophilic metal, and a smaller dilution effect on the Pt surface induced by Ru in the assemblies. This study offers a vivid example to demonstrate the integration of a second oxophilic metal into the active Pt catalyst capable of enhancing its catalytic properties by means of a physical construction.

A high-performance composite ORR catalyst based on the synergy between binary transition metal nitride and nitrogen-doped reduced graphene oxide

We report a composite catalyst in which binary transition metal nitride nanoparticles (NPs) were mounted on nitrogen-doped reduced graphene oxide (TiCoNx/N-rGO). The catalyst exhibited outstanding oxygen reduction activity in an alkaline medium. In its optimal form, our catalyst yielded a half-wave potential of 0.902 V (vs. RHE), ∼30 mV more positive than that of the commercial Pt/C catalyst, and its current density at 0.9 V (vs. RHE) reached 2.51 mA cm−2. The ORR activity of our transition metal nitride-mounted N-rGO was much higher than the activities of transition metal nitride alone or N-rGO alone, revealing a strong synergistic effect between the two materials. Further, the catalyst mounted with Ti and Co binary NPs exhibited higher ORR activity than the catalyst mounted solely with Ti nitride NPs, indicating the significant improvement gained by the addition of cobalt. XPS analysis results showed that the mounting of transition metal nitride clearly changed the amount and distribution of N species in the catalyst, causing the percentage of active pyridinic-N species to increase significantly. Moreover, changes in the binding energies of C and Ti atoms proved the synergy between TiCoNx NPs and N-rGO. We therefore ascribe the superior electrochemical activity of our TiCoNx/N-rGO catalyst to this synergy and to the improvement resulting from the addition of Co. In addition to its outstanding ORR activity, this catalyst also showed excellent stability and methanol tolerance, making it a promising Pt-free ORR catalyst for alkaline H2/O2 fuel cells and direct methanol fuel cells.

A core–shell Pd1Ru1Ni2@Pt/C catalyst with a ternary alloy core and Pt monolayer: enhanced activity and stability towards the oxygen reduction reaction by the addition of Ni

A core–shell structured catalyst, Pd1Ru1Ni2@Pt/C, with a ternary alloy as its core and a Pt monolayer shell was prepared using a two-stage strategy, in which Pd1Ru1Ni2 alloy nanoparticles were prepared by a chemical reduction method, and then the Pt monolayer shell was generated via an underpotential deposition method. It was found that the addition of Ni to the core played an important role in enhancing the catalysts oxygen reduction activity and stability. The optimal molar ratio of Pdu2006:u2006Ruu2006:u2006Ni was about 1u2006:u20061u2006:u20062; the catalyst with this optimal ratio had a half-wave potential approximately 65 mV higher than that of a PdRu@Pt/C catalyst, and its mass activity was up to 1.06 A mg−1 Pt, which was more than five times that of a commercial Pt/C catalyst. The catalysts structure and composition were characterized using X-ray powder diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectrometry. The core–shell structure of the catalyst was demonstrated by the EDS mapping results and supported by the XPS results. We also performed a stability test that confirmed the catalysts superior stability in comparison to that of commercial JM Pt/C (20 wt% Pt).

A Co-doped porous niobium nitride nanogrid as an effective oxygen reduction catalyst

Transition metal nitrides have recently attracted significant interest as electrocatalysts for the oxygen reduction reaction (ORR) owing to their low electrical resistance and good corrosion resistance. In this paper, we describe the preparation of a Nb-based binary nitride material with a porous nanogrid morphology/structure. The catalyst exhibited good catalytic activity and high stability towards oxygen reduction. We also intensively investigated the effect that doping with a second transition metal had on the performance of the catalyst. We found that the ORR activity of NbN could be enhanced significantly by enriching the d electrons of Nb through doping with a second transition metal, and that doping with cobalt resulted in the best improvement. Our optimal catalyst, Nb0.95Co0.05N, had an ORR activity ∼4.6 times that of NbN (current density @ 0.6 V vs. the RHE). XPS results revealed that Co doping increased the proportion of Nb in a low-valence state, which may be one of the most important reasons for the enhanced performance. Another important reason is the high surface area resulting from the porous nanogrid morphology. As transition metal doping is an attractive way to enhance the activity of nitride catalysts, our work may provide an effective pathway to achieve this.

Nitrogen self-doped carbon nanoparticles derived from spiral seaweeds for oxygen reduction reaction

In this work, nitrogen self-doped porous nanoparticles were synthesized through a low cost and simple method with spiral seaweed as a source of nitrogen and carbon. Transmission electron microscopy (TEM), nitrogen adsorption–desorption, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analysis showed that nitrogen was successfully doped into the framework of porous nanostructures. The nitrogen self-doped porous nanomaterial featured a high surface area and micro/mesoporous structures. The fabricated nanomaterial was then used as a metal-free catalyst for oxygen reduction reaction (ORR). This catalyst exhibited improved electrocatalytic activity, long-term operation stability, and high CH3OH tolerance for ORR in alkaline fuel cells compared with commercial Pt/C catalysts. The influence of different nitrogen species formed in different atmospheres on ORR activity was further investigated. This study shows that spirulina is a suitable nitrogen and carbon source for various carbon-based materials for the development of metal-free efficient catalysts for applications beyond fuel cells.

Randomly oriented Ni–P/nanofiber/nanotube composite prepared by electrolessly plated nickel–phosphorus alloys for fuel cell applications

In this work, we have synthesized a new type of hybridized composites, Ni–P/CNT–CNFs, which refers to carbon nanotube (CNT)—hybridized carbon nanofibers (CNFs) through electrolessly plated nickel–phosphorus (Ni–P) alloys. The composites combine the merits of CNTs with high electronic conductivity and CNFs with abundant defect sites via the junction of electrolessly deposited Ni–P. The materials have been extensively characterized by scanning electron microscope, transmission electron microscopy, X-ray diffraction, Brunauer–Emmett–Teller (BET), thermogravimetric analysis and hydrophilicity analysis. Electrochemical evaluations for oxygen reduction reaction (ORR) are carried out in 0.1xa0M KOH with and without 1xa0M methanol. In addition to serving as desirable candidates as electrocatalyst supports, the randomly oriented hybridized composites show satisfactory activities to ORR and excellent methanol tolerance in alkaline solutions. This generalized method is applicable for the preparation of a broad range of composite materials with different active components simply by variations in electroless plating bath (for example, coppercobalt or bimetallic plating).

Uniformly dispersed carbon-supported bimetallic ruthenium–platinum electrocatalysts for the methanol oxidation reaction

Reducing the Pt-based electrocatalysts to sub-nanometer sizes is an effective way to achieve high utilization of noble metals. Herein, we report a successive route to synthesize carbon-supported bimetallic ruthenium–platinum electrocatalysts (Ru–Pt/C) with uniform dispersion and fine sizes. In this strategy, carbon-supported Ru nanoparticles (Ru/C) with a mean size of 1.4xa0nm are firstly prepared in a mixture of ethylene glycol and water, and the Pt precursors are then reduced in the presence of pre-formed Ru/C. The average diameter of the bimetallic Ru–Pt particles on carbon supports is 1.9xa0nm, which corresponds to one to two Pt layers deposited on the surface of Ru seeds. The as-prepared bimetallic Ru–Pt/C electrocatalysts are analyzed by the CO stripping voltammetry, a diagnostic electrochemical tool. Compared with the commercial PtRu/C catalyst and the control PtRu/C prepared by a conventional co-reduction method, the bimetallic Ru–Pt/C has higher electrochemical surface area (92.5xa0m2 g−1) and mass activity (483 A g−1) for methanol oxidation reaction. The strategy reported in this study is effective to produce fine bimetallic Ru–Pt particles (less than 2.0xa0nm) with uniform dispersion and high activity.

Controlled synthesis of carbon nanofibers over electrolessly plated metal foam catalysts on polyurethane for fuel cell applications

AbstractnSynthesis of carbon nanofibers (CNFs) with different morphologies has attracted great attentions due to their broad energy applications. Chemical vapor deposition (CVD) method is often employed to produce CNFs, and the morphology of the resulting CNFs is largely dependent on the catalysts used in this process. In this work, 3-D ordered polyurethane foam has been electrolessly plated with a layer of nickel catalysts, which are then used to catalytically decompose acetylene to yield CNFs via CVD. Catalysts with different compositions (nickel, cobalt, copper or their alloys) can be made through changes in the electroless plating baths and then used to synthesize CNFs with different morphologies. The as-prepared CNFs synthesized in this work can be used directly as supports for fuel cell electrocatalysts. The method reported in this work offers a general protocol to synthesize a wide variety of carbon filaments with tailored compositions and properties.n

A Facile and Environmentally Friendly One-Pot Synthesis of Pt Surface-Enriched Pt-Pd(x)/C Catalyst for Oxygen Reduction

AbstractPlatinum surface-enriched bimetallic PtPd nanoparticles are synthesized via a facile and eco-friendly approach. Significantly different formation processes of single Pt, Pd, and bimetallic PtPd nanoparticles under various conditions have been monitored by UV-visible spectra. The size of the bimetallic Pt-Pd nanoparticles has been found to be significantly affected by the pH of the starting precursor solutions. Uniform bimetallic Pt-Pd nanoparticles with an average diameter of ~3xa0nm are formed in the basic solutions; however, the particle size can be as large as 11xa0nm when synthesized in acidic media. The detailed morphology, composition, and structure of the carbon-supported bimetallic Pt-Pd electrocatalysts have been extensively characterized and correlated with their electrochemical properties as evaluated using cyclic voltammetry and single-cell test. The formation of Pt surface-enriched Pt-Pd bimetallic nanoparticles has been confirmed by X-ray photoelectron spectroscopy and has been interpreted by the exclusively reduced Pt around the newly formed Pd nuclei due to the catalytic action of Pd, which in turn curbs the unfavorable growth of the bimetallic nanoparticles. The electrochemical tests indicate that the optimized Pt-Pd/C with reduced cost exhibits competitive catalytic performance toward oxygen reduction reaction and superior tolerance to methanol over the state-of-the-art Pt/C.n Graphical abstractA facile and environmentally friendly one-pot synthesis of Pt surface-enriched Pt-Pd(x)/C catalyst for oxygen reduction.