Fuel cells powered by hydrogen from secure and renewable sources are the ideal solution for non-polluting vehicles, and extensive research and development on all aspects of this technology over the past fifteen years has delivered prototype cars with impressive performances. But taking the step towards successful commercialization requires oxygen reduction electrocatalysts--crucial components at the heart of fuel cells--that meet exacting performance targets. In addition, these catalyst systems will need to be highly durable, fault-tolerant and amenable to high-volume production with high yields and exceptional quality. Not all the catalyst approaches currently being pursued will meet those demands.

Electrocatalyst approaches and challenges for automotive fuel cells

Lithium (Li) metal is an ideal anode material for rechargeable batteries due to its extremely high theoretical specific capacity (3860 mA h g−1), low density (0.59 g cm−3) and the lowest negative electrochemical potential (−3.040 V vs. the standard hydrogen electrode). Unfortunately, uncontrollable dendritic Li growth and limited Coulombic efficiency during Li deposition/stripping inherent in these batteries have prevented their practical applications over the past 40 years. With the emergence of post-Li-ion batteries, safe and efficient operation of Li metal anodes has become an enabling technology which may determine the fate of several promising candidates for the next generation energy storage systems, including rechargeable Li–air batteries, Li–S batteries, and Li metal batteries which utilize intercalation compounds as cathodes. In this paper, various factors that affect the morphology and Coulombic efficiency of Li metal anodes have been analyzed. Technologies utilized to characterize the morphology of Li deposition and the results obtained by modelling of Li dendrite growth have also been reviewed. Finally, recent development and urgent need in this field are discussed.

Lithium metal anodes for rechargeable batteries

The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core–shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

http://repository.ust.hk/ir/bitstream/1783.1-77094/1/acs.chemrev.5b00462_withlink.pdf

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

This review analyses the literature from the early 1990s until the beginning of 2003 and covers the use of carbon nanotubes (CNT) and nanofibers as catalysts and catalysts supports. The article is composed of three sections, the first one explains why these materials can be suitable for these applications, the second describes the different preparation methods for supporting metallic catalysts on these supports, and the last one details the catalytic results obtained with nanotubes or nanofibers based catalysts. When possible, the results were compared to those obtained on classical carbonaceous supports and explanations are proposed to clarify the different behaviors observed.

Carbon nanotubes and nanofibers in catalysis

Graphene sheets, which possess unique nanostructure and a variety of fascinating properties, can be considered as promising nanoscale building blocks of new composites, for example, a support material for the dispersion of nanoparticles. Here, we present a general approach for the preparation of graphene−metal particle nanocomposites in a water−ethylene glycol system using graphene oxide as a precursor and metal nanoparticles (Au, Pt and Pd) as building blocks. These metal nanoparticles are adsorbed on graphene oxide sheets and play a pivotal role in catalytic reduction of graphene oxide with ethylene glycol, leading to the formation of graphene−metal particle nanocomposites. The typical methanol oxidation of graphene−Pt composites in cyclic voltammograms analyses indicated its potential application in direct methanol fuel cells, bringing graphene−particle nanocomposites close to real technological applications.

Graphene−Metal Particle Nanocomposites

A sonochemical process was developed to treat carbon nanotubes in nitric and sulfuric acids to create surface functional groups for metal nanoparticle deposition. Carbon nanotubes treated in the sonochemical process are shown to lead to the deposition of uniformly dispersed high loading Pt nanoparticles, which have not been achieved with carbon nanotubes treated in reflux processes. Pt nanoparticles of a size less than 5 nm and loading up to 30 wt % with little aggregation were synthesized on the sonochemically treated carbon nanotubes. Cyclic voltammetry measurements in 1.0 M H2SO4 showed that the Pt nanoparticles on carbon nanotubes are more than 100% active in the electrochemical adsorption and desorption of hydrogen than the Pt nanoparticles supported on carbon black. This enhancement of electrochemical activity is attributed to the unique structures of carbon nanotubes and the interactions between the Pt nanoparticles and the carbon nanotube support. The ability to synthesize high loading Pt on carbo...

Synthesis and Electrochemical Characterization of Uniformly-Dispersed High Loading Pt Nanoparticles on Sonochemically-Treated Carbon Nanotubes

Functionalization of carbon nanotubes (CNTs) is important for enhancing deposition of metal nanoparticles in the fabrication of supported catalysts. A facile approach for oxidizing CNTs is presented using a sonochemical method to promote the density of surface functional groups. This was successfully employed in a previous study [J. Phys. Chem. B 2004, 108, 19255] to prepare highly dispersed, high-loading Pt nanoparticles on CNTs as fuel cell catalysts. X-ray photoelectron spectroscopy (XPS), transmission electron microscopy, cyclic voltammetry, and settling speeds were used to characterize the degree of surface functionalization and coverage. The sonochemical method effectively functionalized the CNTs. A mixture of CO/CO and COO was observed along with evidence for weakly bound CO at longer treatment times. The integrated XPS C 1s core level peak area ratios of the oxidized-to-graphitic C oxidation states, as well as the atom % oxygen from the O 1s level, showed an increase in peak intensity (attributed ...

Sonochemical Oxidation of Multiwalled Carbon Nanotubes

To create new catalyst materials for fuel cell applications, multiwalled carbon nanotubes (CNTs) were functionalized with CO, COC, COO, and COH groups using a sonochemical treatment method under ac...

Pt Nanoparticle Binding on Functionalized Multiwalled Carbon Nanotubes

Experimental studies of the fractal morphology of flame-generated aggregates are described here, considering not only the fractal dimension, D f , but also the fractal prefactor (lacunarity), k g , both of which are shown to be needed to fully characterize aggregates. Measurements were made using angular light scattering (ALS) and thermophoretic sampling followed by transmission electron microscopy (TEM) for soot aerosols found in laminar and turbulent flame environments. D f and the prefactor k g were simultaneously inferred from ALS measurements using the optical properties of aggregates composed of small primary particles. TEM-based inferences of these fractal properties involve analysis of aggregate-projected images from which the actual morphologies are obtained by correlating the radius of gyration to the outer radius (half of the maximum length) of an aggregate. Both of our procedures for determining the detailed morphology of aggregates yield D f = 1.7 ± 0.15 and k g = 2.4 ± 0.4 for carbonaceous soot, in good agreement with earlier TEM measurements involving multiple angle images. Furthermore, we show that these values also seem to be valid for other materials such as alumina aggregates, suggesting that not only the fractal dimension but also the fractal prefactor are universal properties of aggregates found in combustion environments due to similar mechanisms of aggregation. This universality for D f and k g is observed at various positions in four different flame types with nine various gaseous and liquid fuels for aggregates' with mean primary particle radii and number of primary particles in aggregates in the range 12-26 nm and 2-10 4 , respectively.

Fractal Morphology Analysis of Combustion-Generated Aggregates Using Angular Light Scattering and Electron Microscope Images

Bimetallic Pt−Ru alloy catalysts have been demonstrated to be more active than pure Pt catalysts in the electrooxidation of methanol. We report here a study on Pt−Ru nanoparticle catalysts supported on sonochemically functionalized carbon nanotubes. The catalysts were prepared by directly reducing the corresponding salts, K2PtCl4 and K2RuCl5, in an ethylene glycol aqueous solution containing dispersed carbon nanotubes. Three catalysts of different Pt to Ru atomic ratios, namely, Pt53Ru47, Pt69Ru31, and Pt77Ru23, were prepared for investigation of the compositional effects. It was shown that highly dispersed bimetallic Pt−Ru alloy nanoparticles with no agglomeration can be synthesized on the carbon nanotubes with average particle sizes of less than 3.0 nm in diameter. The Pt−Ru nanoparticles are uniform and cover only the outside of the carbon nanotubes. It was found that the polyol process produced alloy compositions that are not consistent with the metal ratios in the precursors. It was also found that t...

Yangchuan Xing

Papers

Synthesis and Electrochemical Characterization of Uniformly-Dispersed High Loading Pt Nanoparticles on Sonochemically-Treated Carbon Nanotubes

Sonochemical Oxidation of Multiwalled Carbon Nanotubes

Pt Nanoparticle Binding on Functionalized Multiwalled Carbon Nanotubes

Fractal Morphology Analysis of Combustion-Generated Aggregates Using Angular Light Scattering and Electron Microscope Images

Pt−Ru Nanoparticles Supported on Carbon Nanotubes as Methanol Fuel Cell Catalysts