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

Hollow micro-/nanostructures are of great interest in many current and emerging areas of technology. Perhaps the best-known example of the former is the use of fly-ash hollow particles generated from coal power plants as partial replacement for Portland cement, to produce concrete with enhanced strength and durability. This review is devoted to the progress made in the last decade in synthesis and applications of hollow micro-/nanostructures. We present a comprehensive overview of synthetic strategies for hollow structures. These strategies are broadly categorized into four themes, which include well-established approaches, such as conventional hard-templating and soft-templating methods, as well as newly emerging methods based on sacrificial templating and template-free synthesis. Success in each has inspired multiple variations that continue to drive the rapid evolution of the field. The Review therefore focuses on the fundamentals of each process, pointing out advantages and disadvantages where appropriate. Strategies for generating more complex hollow structures, such as rattle-type and nonspherical hollow structures, are also discussed. Applications of hollow structures in lithium batteries, catalysis and sensing, and biomedical applications are reviewed.

Hollow Micro-/Nanostructures: Synthesis and Applications**

Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries

Developing highly efficient catalysts for the oxygen reduction reaction (ORR) is key to the fabrication of commercially viable fuel cell devices and metal–air batteries for future energy applications. Herein, we review the most recent advances in the development of Pt-based and Pt-free materials in the field of fuel cell ORR catalysis. This review covers catalyst material selection, design, synthesis, and characterization, as well as the theoretical understanding of the catalysis process and mechanisms. The integration of these catalysts into fuel cell operations and the resulting performance/durability are also discussed. Finally, we provide insights into the remaining challenges and directions for future perspectives and research.

Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction

With the approaching commercialization of PEM fuel cell technology, developing active, inexpensive non-precious metal ORR catalyst materials to replace currently used Pt-based catalysts is a necessary and essential requirement in order to reduce the overall system cost. This review paper highlights the progress made over the past 40 years with a detailed discussion of recent works in the area of non-precious metal electrocatalysts for oxygen reduction reaction, a necessary reaction at the PEM fuel cell cathode. Several important kinds of unsupported or carbon supported non-precious metal electrocatalysts for ORR are reviewed, including non-pyrolyzed and pyrolyzed transition metal nitrogen-containing complexes, conductive polymer-based catalysts, transition metal chalcogenides, metal oxides/carbides/nitrides/oxynitrides/carbonitrides, and enzymatic compounds. Among these candidates, pyrolyzed transition metal nitrogen-containing complexes supported on carbon materials (M–Nx/C) are considered the most promising ORR catalysts because they have demonstrated some ORR activity and stability close to that of commercially available Pt/C catalysts. Although great progress has been achieved in this area of research and development, there are still some challenges in both their ORR activity and stability. Regarding the ORR activity, the actual volumetric activity of the most active non-precious metal catalyst is still well below the DOE 2015 target. Regarding the ORR stability, stability tests are generally run at low current densities or low power levels, and the lifetime is far shorter than targets set by DOE. Therefore, improving both the ORR activity and stability are the major short and long term focuses of non-precious metal catalyst research and development. Based on the results achieved in this area, several future research directions are also proposed and discussed in this paper.

A review on non-precious metal electrocatalysts for PEM fuel cells

By using a facile one-pot synthetic route, we designed a one-dimensional hybrid nanostructure by introducing small amounts of Au species (0.9 wt %) dispersed over the amorphous tin oxide overlayers uniformly supported on coaxial carbon nanotubes as electrode materials for application in lithium ion battery. The as-synthesized CNT@SnO2−Au nanocables were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, elemental mapping, and X-ray photoelectron spectroscopy; such hybrid nanostructure markedly improves electrode performance, especially for the rate performance. The capacities of 467 and 392 mA h g−1 were obtained at rates of 3.6 A g−1 and 7.2 A g−1, respectively.

One-Pot Synthesis of Carbon Nanotube@SnO2−Au Coaxial Nanocable for Lithium-Ion Batteries with High Rate Capability

In this paper, successful one-step chemical synthesis of Co−Pt hollow spheres with adjustable composition was developed in an effort to explore new synthesis methods, simplify the synthesis procedure, and improve the synthetic efficiency. In this synthesis, a direct thermolytic reduction of platinum acetylacetonate [Pt(CH3COHCOCH3)2] and cobalt(II) acetate [Co(CH3COO)2] in refluxing ethyl glycol was explored. In the reacting mixture, an anionic surfactant sodium dodecyl sulfate (SDS) was used as the capping and structure-directing agent. These Co−Pt hollow spheres demonstrated enhanced electrocatalytic activity toward methanol oxidation in comparison with Pt nanoparticles, which is crucial for anode electrocatalysis in DMFCs.

Facile Synthesis of Co−Pt Hollow Sphere Electrocatalyst

Coating of multi-walled carbon nanotube with SnO2 films of controlled thickness and its application for Li-ion battery

Bimetallic PtPb for formic acid electro-oxidation

Methanol-tolerant MoN electrocatalyst synthesized through heat treatment of molybdenum tetraphenylporphyrin for four-electron oxygen reduction reaction

Zhenyao Wang

Papers

One-Pot Synthesis of Carbon Nanotube@SnO2−Au Coaxial Nanocable for Lithium-Ion Batteries with High Rate Capability

Facile Synthesis of Co−Pt Hollow Sphere Electrocatalyst

Coating of multi-walled carbon nanotube with SnO2 films of controlled thickness and its application for Li-ion battery

Bimetallic PtPb for formic acid electro-oxidation

Methanol-tolerant MoN electrocatalyst synthesized through heat treatment of molybdenum tetraphenylporphyrin for four-electron oxygen reduction reaction