Tailored design of architecturally controlled Pt nanoparticles with huge surface areas toward superior unsupported Pt electrocatalysts

Catalyst Development for High-Temperature Polymer Electrolyte Membrane Fuel Cell (HT-PEMFC) Applications

A constant increase in global emission standard is causing fuel cell (FC) technology to gain importance. Over the last two decades, a great deal of research has been focused on developing more active catalysts to boost the performance of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFC), as well as their durability. Due to material degradation at high-temperature conditions, catalyst design becomes challenging. Two main approaches are suggested: (i) alloying platinum (Pt) with low-cost transition metals to reduce Pt usage, and (ii) developing novel catalyst support that anchor metal particles more efficiently while inhibiting corrosion phenomena. In this comprehensive review, the most recent platinum group metal (PGM) and platinum group metal free (PGM-free) catalyst development is detailed, as well as the development of alternative carbon (C) supports for HT-PEMFCs.

Noble metal nanodendrites: growth mechanisms, synthesis strategies and applications

Inorganic nanodendrites (NDs) have become a kind of advanced nanomaterials with broad application prospects because of their unique branched architecture. The structural characteristics of nanodendrites include highly branched morphology, abundant tips/edges and high-index crystal planes, and a high atomic utilization rate, which give them great potential for usage in the fields of electrocatalysis, sensing, and therapeutics. Therefore, the rational design and controlled synthesis of inorganic (especially noble metals) nanodendrites have attracted widespread attention nowadays. The development of synthesis strategies and characterization methodology provides unprecedented opportunities for the preparation of abundant nanodendrites with interesting crystallographic structures, morphologies, and application performances. In this review, we systematically summarize the formation mechanisms of noble metal nanodendrites reported in recent years, with a special focus on surfactant-mediated mechanisms. Some typical examples obtained by innovative synthetic methods are then highlighted and recent advances in the application of noble metal nanodendrites are carefully discussed. Finally, we conclude and present the prospects for the future development of nanodendrites. This review helps to deeply understand the synthesis and application of noble metal nanodendrites and may provide some inspiration to develop novel functional nanomaterials (especially electrocatalysts) with enhanced performance.

Surfactant-Assisted Synthesis of Pt Nanocubes Using Poly(N-isopropylacrylamide) Nanogels

Poly(N-isopropylacrylamide) (PNIPAM) nanogels were prepared by emulsion polymerization using sodium dodecyl sulfate (SDS) and employed as a capping agent in platinum nanoparticle (Pt NP) synthesis by liquid-phase reduction with hydrogen gas. When the PNIPAM nanogels were used without removing SDS, that is, a slight amount of SDS was included in the reaction solution, Pt nanocubes (NCs) were predominantly produced (>80%). The proportion of the resultant Pt NCs was much higher than that obtained using the PNIPAM linear polymer (∼60%). To clarify the effects of the three-dimensional polymer network and SDS, we synthesized Pt NPs using the PNIPAM nanogel without SDS (SDS-free PNIPAM nanogel) and found that Pt NCs are rarely formed, and most NPs obtained have an irregular shape. When only SDS was used as a capping agent, NCs were hardly obtained, but other polyhedral NPs were formed. Furthermore, the use of SDS together with the PNIPAM polymer led to the decrease in the proportion of the Pt NCs compared with that obtained using only the linear polymer. These results indicate that the enhancement of the Pt NC proportion using the PNIPAM nanogel with SDS is attributable to not only the three-dimensional polymer network of the PNIPAM nanogel but also the assist of SDS as a capping agent.

Facile Room-Temperature Synthesis of a Highly Active and Robust Single-Crystal Pt Multipod Catalyst for Oxygen Reduction Reaction

Economical production of highly active and robust Pt catalysts on a large scale is vital to the broad commercialization of polymer electrolyte membrane fuel cells. Here, we report a low-cost, one-pot process for large-scale synthesis of single-crystal Pt multipods with abundant high-index facets, in an aqueous solution without any template or surfactant. A composite consisting of the Pt multipods (40 wt %) and carbon displays a specific activity of 0.242 mA/cm² and a mass activity of 0.109 A/mg at 0.9 V (versus a reversible hydrogen electrode) for oxygen reduction reaction, corresponding to ∼124% and ∼100% enhancement compared with those of the state-of-the-art commercial Pt/C catalyst (0.108 mA/cm² and 0.054 A/mg). The single-crystal Pt multipods also show excellent stability when tested for 4500 cycles in a potential range of 0.6-1.1 V and another 2000 cycles in 0-1.2 V. More importantly, the superior performance of the Pt multipods/C catalyst is also demonstrated in a membrane electrode assembly (MEA), achieving a power density of 774 mW/cm² (1.29 A/cm²) at 0.6 V and a peak power density of ∼1 W/cm², representing 34% and 20% enhancement compared with those of a MEA based on the state-of-the-art commercial Pt/C catalyst (576 and 834 mW/cm²).

Ascorbic Acid-Assisted Polyol Synthesis of Iron and Fe/GO, Fe/h-BN Composites for Pb2+ Removal from Wastewaters

Iron powders and Fe/graphene oxide and Fe/boron nitride composites were synthesized by means of a polyol synthesis method. The effect of NaOH/Fe and ascorbic acid/Fe ratios on the characteristics of synthesized products were evaluated. The samples were characterized by X-ray diffraction, scanning and transmission electron microscopy, low-temperature nitrogen adsorption and Raman-spectroscopy. Ascorbic acid-assisted polyol synthesis resulted in the 10-fold decrease of the iron particles' size and almost 2-fold increase of lead removal efficiency. The deposition of iron on the surface of graphene oxide lead to the formation of small 20-30 nm sized particles as well as bigger 200-300 nm sized particles, while the reduction in presence of boron nitride resulted in the 100-200 nm sized particles. The difference is attributed to the surface state of graphene oxide and boron nitride. Adsorption properties of the obtained materials were studied in the process of Pb²⁺ ion removal from wastewater.

Highly Catalytic Pt Nanoparticles Grown in Two-Dimensional Conducting Polymers at the Air-Water Interface

We report a new approach to the synthesis of uniform, high areal density Pt nanocrystals supported by conducting polymers. The key strategy is the use of ice-templated, two-dimensional polyaniline nanosheets at the air-water interface as a platform for expediting Pt nucleation. Highly crystalline Pt nanoparticles with a narrow size distribution of 2.7 ± 0.3 nm and a high electrochemically active surface area of 94.57 m² g^-1 were obtained. Pt NPs were strongly anchored to the polyaniline nanosheets, and demonstrated high current densities, good durability for the methanol oxidation reaction, and excellent carbon monoxide tolerance, all of which are unprecedented. The idea established in this study could be applied to the production of a wide range of other catalysts with enhanced activities.

Self-Assembled Dendritic Pt Nanostructure with High-Index Facets as Highly Active and Durable Electrocatalyst for Oxygen Reduction

The durability issues of Pt catalyst should be resolved for the commercialization of proton exchange membrane fuel cells. Nanocrystal structures with high-index facets have been recently explored to solve the critical durability problem of fuel cell catalysts as Pt catalysts with high-index facets can preserve the ordered surfaces without change of the original structures. However, it is very difficult to develop effective and practical synthetic methods for Pt-based nanostructures with high-index facets. The current study describes a simple one-pot synthesis of self-assembled dendritic Pt nanostructures with electrochemically active and stable high-index facets. Pt nanodendrites exhibited 2 times higher ORR activity and superior durability (only 3.0 % activity loss after 10 000 potential cycles) than a commercial Pt/C. The enhanced catalytic performance was elucidated by the formation of well-organized dendritic structures with plenty of reactive interfaces among 5 nm-sized Pt particles and the coexistence of low- and high-index facets on the particles.

Ultrathin Rhodium Oxide Nanosheet Nanoassemblies: Synthesis, Morphological Stability, and Electrocatalytic Application

Inspired by graphene, ultrathin two-dimensional nanomaterials with atomic thickness have attracted more and more attention because of their unique physicochemical properties and electronic structure. In this work, the atomically thick ultrathin Rh₂O₃ nanosheet nanoassemblies (Rh₂O₃-NSNSs) were obtained by oxidizing the atomically thick ultrathin Rh nanosheet nanoassemblies with HClO. For the first time, Rh-based nanostructures were used as the oxygen evolution reaction (OER) electrocatalyst in an alkaline medium. Surprisingly, the as-prepared Rh₂O₃-NSNSs displayed extremely improved catalytic activity and durability for the OER compared with those of the commercial Ir/C catalyst and most recently reported Ir-based electrocatalysts. The result indicated Rh-based nanostructures that have great promise to become a potential candidate for efficient OER electrocatalyst because of the similarity of Rh and Ir prices. These experimental results demonstrated the reasonable morphological control of Rh₂O₃ nanostructures could significantly improve their catalytic activity and durability during heterogeneous catalysis.

One-pot synthesis of PtRu nanodendrites as efficient catalysts for methanol oxidation reaction

Bimetallic Pt-based nanodendrites are of particular interest in various catalytic applications due to their high surface areas and low densities. Herein, we provide a facile method for one-pot synthesis of PtRu nanodendrites via the co-reduction of Pt and Ru precursors in oleylamine by H₂. The as-fabricated PtRu nanodendrites exhibit superior catalytic activity and durability compared with PtRu nanocrystals (NCs), synthesized under the same reaction conditions, and the commercial Pt/C catalyst towards the methanol oxidation reaction (MOR).

Facile synthesis of hollow dendritic Ag/Pt alloy nanoparticles for enhanced methanol oxidation efficiency

Hollow dendritic Ag/Pt alloy nanoparticles were synthesized by a double template method.

Further Insight into the Definite Morphology and Formation Mechanism of Mesoporous Silica KCC-1

The unique three-dimensional pore structure of KCC-1 has attracted significant attention and has proven to be different compared to other conventional mesoporous silica such as the MCM-41 family, SBA-15, or even MSN nanoparticles. In this research, we carefully examine the morphology of KCC-1 to define more appropriate nomenclature. We also propose a formation mechanism of KCC-1 based on our experimental evidence. Herein, the KCC-1 morphology was interpreted mainly on the basis of compiling all observation and information taken from SEM and TEM images. Further analysis on TEM images was carried out. The gray value intensity profile was derived from TEM images in order to determine the specific pattern of this unique morphology that is found to be clearly different from that of other types of porous spherical-like morphologies. On the basis of these results, the KCC-1 morphology would be more appropriately reclassified as bicontinuous concentric lamellar morphology. Some physical characteristics such as the origin of emulsion, electrical conductivity, and the local structure of water molecules in the KCC-1 emulsion were disclosed to reveal the formation mechanism of KCC-1. The origin of the KCC-1 emulsion was characterized by the observation of the Tyndall effect, conductometry to determine the critical micelle concentration, and Raman spectroscopy. In addition, the morphological evolution study during KCC-1 synthesis completes the portrait of the formation of mesoporous silica KCC-1.

Highly branched PtCu bimetallic alloy nanodendrites with superior electrocatalytic activities for oxygen reduction reactions

Morphology control is a promising strategy to improve the catalytic performance of Pt-based catalysts. In this work, we reported a facile synthesis of PtCu bimetallic alloy nanodendrites using Brij 58 as a template. The highly branched structures and porous features offer relatively large surface areas, which is beneficial to the enhancement of the catalytic activity for oxygen reduction reactions in fuel cells. In addition, the elimination of carbon supports showed an important effect on the stability of the catalysts. By tuning the ratio of Pt and Cu precursors, PtCu nanodendrites were almost four times more active on the basis of an equivalent Pt mass for oxygen reduction reactions than the commercial Pt/C catalyst.

Preparation of a platinum electrocatalyst by coaxial pulse arc plasma deposition

We have developed a new method of preparing Pt electrocatalysts through a dry process. By coaxial pulse arc plasma deposition (CAPD), highly ionized metal plasma can be generated from a target rod without any discharged gases, and Pt nanoparticles can be deposited on a carbon support. The small-sized Pt nanoparticles are distributed over the entire carbon surface. From transmission electron microscopy (TEM), the average size of the deposited Pt nanoparticles is estimated to be 2.5 nm, and their size distribution is narrow. Our electrocatalyst shows considerably improved catalytic activity and stability toward methanol oxidation reaction (MOR) compared with commercially available Pt catalysts such as Pt black and Pt/carbon (PtC). Inspired by its very high efficiency toward MOR, we also measured the catalytic performance for oxygen reduction reaction (ORR). Our PtC catalyst shows a better performance with half-wave potential of 0.87 V, which is higher than those of commercially available Pt catalysts. The higher performance is also supported by a right-shifted onset potential. Our preparation is simple and could be applied to other metallic nanocrystals as a novel platform in catalysis, fuel cells and biosensors.

Attachment-based growth: building architecturally defined metal nanocolloids particle by particle

This review highlights the principles and recent mechanistic insight into the synthesis of metal nanostructures using nanoparticles as primary building blocks.

Self-assembly: an option to nanoporous metal nanocrystals

Nanoporous metal nanocrystals involving both nanoscale effects and nanopore properties hold enormous promising potential for various important applications due to their unique structures such as large surface area per unit volume and interconnected open framework. Self-assembly, as an excellent option, has been developed to control the desired structure and rational performance of nanoporous metal nanocrystals. After identifying features of their methodologies and structures, the control of synthesis based on the self-assembly technique is thoroughly discussed. The development of the self-assembly synthesis methodology is then presented in detail. The emerging application, mainly in catalysis, and corresponding design are finally summarized.

Autocatalysis and selective oxidative etching induced synthesis of platinum-copper bimetallic alloy nanodendrites electrocatalysts

The controllable synthesis of noble metal alloy nanostructures with highly branched morphology has attracted much attention because of their specific physical and chemical properties. This article reports the synthesis of platinum-copper bimetallic alloy nanodendrites (Pt-Cu BANDs) by a facile, one-pot, templateless, and seedless hydrothermal method in the presence of poly(allylamine hydrochloride) (PAH) and formaldehyde (HCHO). The morphology, composition, and structure of Pt-Cu BANDs are fully characterized by various physical techniques, demonstrating Pt-Cu BANDs are highly alloying, porous, and self-supported nanostructures. The formation/growth mechanism of Pt-Cu BANDs is explored and discussed based on the experimental observations. The autocatalytic growth and interdiffusion are responsible for the formation of Pt-Cu alloy whereas selective oxidative etching results in dendritic morphology of Pt-Cu alloy nanostructures. In addition, the electrocatalytic activity and stability of Pt-Cu BANDs for the methanol oxidation reaction (MOR) are investigated by various electrochemical techniques. The synthesized Pt-Cu BANDs show higher electrocatalytic activity and stability than commercially available Pt black.

Mesoporous nano/micro noble metal particles: synthesis and applications

The morphology, size and composition often govern the physical and chemical properties of noble metal units with a size in the nano or micro scale. Thus, the controlled growth of noble metal crystals would help to tailor their unique properties and this would be followed by their practical application. Mesoporous nano/micro noble metal units are types of nanostructured material that have fascinating properties that can generate great potential for various applications. This review presents a general view on the growth mechanisms of porous noble metal units and is focused on recent progresses in their synthetic approaches. Then, their potential applications in the field of drug delivery, cell imaging and SERS substrates, as well as fuel cell catalysts are overviewed.

Synthesis of chestnut-bur-like palladium nanostructures and their enhanced electrocatalytic activities for ethanol oxidation

We report a facile method for the synthesis of Pd nanostructures with highly open structure and huge surface area by reducing Na₂PdCl₄ with ascorbic acid and using cetylpyridinium chloride (CPC) as a surfactant in an aqueous solution. The prepared Pd nanostructures had an average overall size of 70 nm and were composed of dozens of needle-like thin arms, originating from the same core, with an average thickness of 2.3 nm; the arms looked like chestnut-burs. Time evolution of Pd nanostructures implied that small Pd particles generated at the early stage of the reaction by fast reduction grew via the particle attachment growth mechanism. The morphology and size of the Pd nanostructures could be readily controlled by varying the concentration of CPC; depending on the amount of CPC, the reduction rates varied the morphology of the Pd nanostructures. Because of the huge surface area and possible catalytically active sites, the prepared chestnut-bur-like Pd nanostructures exhibited greater electrocatalytic activity toward ethanol electrooxidation compared to other Pd nanocatalysts, including cubic and octahedral Pd nanocrystals, and even commercial Pd/C.

Multi-generation overgrowth induced synthesis of three-dimensional highly branched palladium tetrapods and their electrocatalytic activity for formic acid oxidation

Highly branched noble metal nanostructures are highly attractive for catalytic applications owing to their specific physical and chemical properties. In this work, three-dimensional highly branched palladium tetrapods (Pd-THBTs) have been constructed in the presence of polyvinylpyrrolidone (PVP) through one-step hydrothermal reduction of ethylenediamine-tetramethylene phosphonate-palladium(II) (EDTMP-Pd(II)) by formaldehyde. The morphology and structure of the Pd-THBTs were fully characterized and the growth mechanism was explored and discussed based on the experimental observation. The concave Pd tetrahedra grew into highly branched Pd tetrapods consisting of four nanothorn-like branches with tetrahedral dimensions through interesting multi-generation nanocrystal overgrowth. The electrocatalytic activities of the as-synthesized Pd-THBTs toward formic acid oxidation were also studied by cyclic voltammetry and chronoamperometry. The Pd-THBTs showed higher catalytic activity and stability for formic acid oxidation than the commercial Pd black.

A universal approach to the preparation of colloidal mesoporous platinum nanoparticles with controlled particle sizes in a wide range from 20 nm to 200 nm

We report a universal method for the preparation of mesoporous Pt nanoparticles with tunable particle sizes by changing the surfactant concentration.

Synthesis of highly branched gold nanodendrites with a narrow size distribution and tunable NIR and SERS using a multiamine surfactant

Gold nanodendrites with a long and densely branched morphology were fabricated by a seed-mediated method in a solution containing gold nanoparticles (AuNPs), bis(amidoethyl-carbamoylethyl)octadecylamine (C18N3), HAuCl4, and the reducing agent ascorbic acid (AA). The length and density of the branches could be mediated by changing the AuNP seed and AA concentrations. The amphiphilic C18N3 molecules function as a template and induce the unique morphology of the AuNPs/C18N3 structures. The localized surface plasmon resonance (LSPR) peaks of the gold nanodendrites can be modulated from the visible (∼530 nm) to the near-infrared region (∼1100 nm) of the electromagnetic spectrum. Surface-enhanced Raman scattering (SERS) signals using rhodamine can also be mediated by changing the seed and AA concentrations. These unique highly branched gold nanodendrites with a narrow size distribution and tunable NIR and SERS spectra should have great potential in sensing applications.

Dimethylformamide-mediated synthesis of water-soluble platinum nanodendrites for ethanol oxidation electrocatalysis

Herein we describe the synthesis of water-soluble platinum nanodendrites in dimethylformamide (DMF), in the presence of polyethyleneimine (PEI) as a stabilizing agent. The average size of the dendrites is in the range of 20-25 nm while their porosity can be tuned by modifying the concentration of the metal precursor. Electron tomography revealed different crystalline orientations of nanocrystallites in the nanodendrites and allowed a better understanding of their peculiar branching and porosity. The high surface area of the dendrites (up to 22 m(2) g(-1)) was confirmed by BET measurements, while X-ray diffraction confirmed the abundance of high-index facets in the face-centered-cubic crystal structure of Pt. The prepared nanodendrites exhibit excellent performance in the electrocatalytic oxidation of ethanol in alkaline solution. Sensing, selectivity, cycleability and great tolerance toward poisoning were demonstrated by cyclic voltammetry measurements.

Facile solution synthesis of Ag@Pt core-shell nanoparticles with dendritic Pt shells

Ag@Pt nanoparticles with various dendritic Pt shells were successfully synthesized by using nonionic surfactants at room temperature. Our recent study demonstrated that the addition of nonionic surfactant plays an important key role in the synthesis of dendritic Pt nanostructures (J. Am. Chem. Soc., 2010, 132, 13636). Here we extend this synthetic concept to prepare various Ag@Pt nanoarchitectures. The different nanostructured Pt shells on the Ag core were confirmed by ultraviolet-visible absorption spectroscopy and transmission electron microscopy. As a preliminary electrochemical application, the obtained Ag@Pt nanostructures were applied in the methanol oxidation reaction (MOR) in 0.5 M H(2)SO(4) solution containing 0.5 M methanol. The Ag@Pt nanoparticles with thin dendritic Pt shells show superior CO-tolerance performance with a I(f)/I(b) value reaching 3.71. Our Ag@Pt nanostructures with good CO tolerant activity will be prominent catalysts for MOR.