Oxygen Reduction Electrocatalyst Based on Strongly Coupled Cobalt Oxide Nanocrystals and Carbon Nanotubes

Surface structure dependent electrocatalytic activity of Co₃O₄ anchored on graphene sheets toward oxygen reduction reaction

Catalytic activity is primarily a surface phenomenon, however, little is known about Co₃O₄ nanocrystals in terms of the relationship between the oxygen reduction reaction (ORR) catalytic activity and surface structure, especially when dispersed on a highly conducting support to improve the electrical conductivity and so to enhance the catalytic activity. Herein, we report a controllable synthesis of Co₃O₄ nanorods (NR), nanocubes (NC) and nano-octahedrons (OC) with the different exposed nanocrystalline surfaces ({110}, {100}, and {111}), uniformly anchored on graphene sheets, which has allowed us to investigate the effects of the surface structure on the ORR activity. Results show that the catalytically active sites for ORR should be the surface Co²⁺ ions, whereas the surface Co³⁺ ions catalyze CO oxidation, and the catalytic ability is closely related to the density of the catalytically active sites. These results underscore the importance of morphological control in the design of highly efficient ORR catalysts.

Hybrid material design for energy applications: impact of graphene and carbon nanotubes

This article reviews the origin and progress of inorganic/nanocarbon hybrid material research in my Ph.D. career. Building chemical bonds between inorganic active materials and nanocarbon substrates is the key to controlled hybrid material synthesis that allows for controlling the size and morphology of the materials and enhancing electron transport within the electrodes. Our inorganic/nanocarbon hybrid materials manifest superior electrochemical performance for asymmetrical supercapacitors, ultrafast nickel-iron batteries, lithium ion batteries, lithium-sulfur batteries, and electrocatalysis of oxygen reduction and evolution reactions. X-ray absorption near edge structure (XANES) spectroscopy has been utilized to characterize the chemical bonding and charge transfer at the interface of inorganic nanocrystals and nanocarbon substrates. Probing the physical and chemical states of the hybrid materials under electrochemical reaction conditions is an important future direction in this area.

Metal (metal = Fe, Co), N codoped nanoporous carbon for efficient electrochemical oxygen reduction

Fe, N codoped nanoporous carbon (N–Fe–nC) exhibits superior oxygen reduction activity with lower overpotential and comparable electron transfer number compared with Pt/C.

Ni0.33Mn0.33Co0.33Fe2O4 nanoparticles anchored on oxidized carbon nanotubes as advanced anode materials in Li-ion batteries

Ni_0.33Mn_0.33Co_0.33Fe₂O₄ nanoparticles anchored on oxidized carbon nanotubes as anode materials exhibit a significantly improved electrochemical performance in Li-ion batteries.

Two-step synthesis of boron and nitrogen co-doped graphene as a synergistically enhanced catalyst for the oxygen reduction reaction

Boron and nitrogen co-doped graphene was synthesized as synergistically enhanced catalyst for oxygen reduction reaction via a two-step doping strategy.

Co2Nx/nitrogen-doped reduced graphene oxide for enzymeless glucose detection

Co₂N_x/nitrogen-doped reduced graphene oxide (Co₂N_x/NG) is synthesized by electrostatic co-precipitation of Co and rGO followed by high-temperature nitridation, which can serve as an efficient catalyst for sensitive glucose detection due to the unique electrocatalytic property of Co₂N_xand synergistic effect between Co₂N_xand N-doped rGO.

Chemical bonding in amorphous Si-coated carbon nanotubes as anodes for Li ion batteries: a XANES study

The Si–O–C bonding and its evolution upon electrochemical cycling in a Si-coated carbon nanotube anode are unveiled by X-ray absorption spectroscopy studies.

Residual metallic impurities within carbon nanotubes play a dominant role in supposedly “metal-free” oxygen reduction reactions

ORR electrocatalysis on the supposedly metal-free carbon nanotubes is in fact due to the presence of residual metallic impurities within.

P-modified and carbon shell coated Co nanoparticles for efficient alkaline oxygen reduction catalysis

The present carbon shell coated Co nanoparticles of which the surface composites are modified by phosphorus incorporation, exhibit efficient and durable oxygen reduction activities in alkaline medium.

Advanced zinc-air batteries based on high-performance hybrid electrocatalysts

Primary and rechargeable Zn-air batteries could be ideal energy storage devices with high energy and power density, high safety and economic viability. Active and durable electrocatalysts on the cathode side are required to catalyse oxygen reduction reaction during discharge and oxygen evolution reaction during charge for rechargeable batteries. Here we developed advanced primary and rechargeable Zn-air batteries with novel CoO/carbon nanotube hybrid oxygen reduction catalyst and Ni-Fe-layered double hydroxide oxygen evolution catalyst for the cathode. These catalysts exhibited higher catalytic activity and durability in concentrated alkaline electrolytes than precious metal Pt and Ir catalysts. The resulting primary Zn-air battery showed high discharge peak power density ~265 mW cm(-2), current density ~200 mA cm(-2) at 1 V and energy density >700 Wh kg(-1). Rechargeable Zn-air batteries in a tri-electrode configuration exhibited an unprecedented small charge-discharge voltage polarization of ~0.70 V at 20 mA cm(-2), high reversibility and stability over long charge and discharge cycles.

Highly efficient oxygen reduction electrocatalysts based on winged carbon nanotubes

Developing electrocatalysts with both high selectivity and efficiency for the oxygen reduction reaction (ORR) is critical for several applications including fuel cells and metal-air batteries. In this work we developed high performance electrocatalysts based on unique winged carbon nanotubes. We found that the outer-walls of a special type of carbon nanotubes/nanofibers, when selectively oxidized, unzipped and exfoliated, form graphene wings strongly attached to the inner tubes. After doping with nitrogen, the winged nanotubes exhibited outstanding activity toward catalyzing the ORR through the four-electron pathway with excellent stability and methanol/carbon monoxide tolerance. While the doped graphene wings with high active site density bring remarkable catalytic activity, the inner tubes remain intact and conductive to facilitate electron transport during electrocatalysis.

Design of meso-TiO2@MnO(x)-CeO(x)/CNTs with a core-shell structure as DeNO(x) catalysts: promotion of activity, stability and SO2-tolerance

Developing low-temperature deNOx catalysts with high catalytic activity, SO2-tolerance and stability is highly desirable but remains challenging. Herein, by coating the mesoporous TiO2 layers on carbon nanotubes (CNTs)-supported MnOx and CeOx nanoparticles (NPs), we obtained a core-shell structural deNOx catalyst with high catalytic activity, good SO2-tolerance and enhanced stability. Transmission electron microscopy, X-ray diffraction, N2 sorption, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction and NH3 temperature-programmed desorption have been used to elucidate the structure and surface properties of the obtained catalysts. Both the specific surface area and chemisorbed oxygen species are enhanced by the coating of meso-TiO2 sheaths. The meso-TiO2 sheaths not only enhance the acid strength but also raise acid amounts. Moreover, there is a strong interaction among the manganese oxide, cerium oxide and meso-TiO2 sheaths. Based on these favorable properties, the meso-TiO2 coated catalyst exhibits a higher activity and more extensive operating-temperature window, compared to the uncoated catalyst. In addition, the meso-TiO2 sheaths can serve as an effective barrier to prevent the aggregation of metal oxide NPs during stability testing. As a result, the meso-TiO2 overcoated catalyst exhibits a much better stability than the uncoated one. More importantly, the meso-TiO2 sheaths can not only prevent the generation of ammonium sulfate species from blocking the active sites but also inhibit the formation of manganese sulfate, resulting in a higher SO2-tolerance. These results indicate that the design of a core-shell structure is effective to promote the performance of deNOx catalysts.

Metal-organic frameworks as a tunable platform for designing functional molecular materials

Metal-organic frameworks (MOFs), also known as coordination polymers, represent an interesting class of crystalline molecular materials that are synthesized by combining metal-connecting points and bridging ligands. The modular nature of and mild conditions for MOF synthesis have permitted the rational structural design of numerous MOFs and the incorporation of various functionalities via constituent building blocks. The resulting designer MOFs have shown promise for applications in a number of areas, including gas storage/separation, nonlinear optics/ferroelectricity, catalysis, energy conversion/storage, chemical sensing, biomedical imaging, and drug delivery. The structure-property relationships of MOFs can also be readily established by taking advantage of the knowledge of their detailed atomic structures, which enables fine-tuning of their functionalities for desired applications. Through the combination of molecular synthesis and crystal engineering, MOFs thus present an unprecedented opportunity for the rational and precise design of functional materials.

A nitrogen-doped graphene/carbon nanotube nanocomposite with synergistically enhanced electrochemical activity

A new kind of nitrogen-doped graphene/carbon nanotube nanocomposite can be synthesized by a facile hydrothermal process under mild conditions, which exhibits synergistically enhanced electrochemical activity for the oxygen reduction reaction. This research provides a new route to access a metal-free electrocatalyst with high activity under mild conditions.

Monodisperse M(x)Fe(3-x)O4 (M = Fe, Cu, Co, Mn) nanoparticles and their electrocatalysis for oxygen reduction reaction

Sub-10 nm nanoparticles (NPs) of M(II)-substituted magnetite MxFe3-xO4 (MxFe1-xO•Fe2O3) (M = Mn, Fe, Co, Cu) were synthesized and studied as electrocatalysts for oxygen reduction reaction (ORR) in 0.1 M KOH solution. Loaded on commercial carbon support, these MxFe3-xO4 NPs showed the M(II)-dependent ORR catalytic activities with MnxFe3-xO4 being the most active followed by CoxFe3-xO4, CuxFe3-xO4, and Fe3O4. The ORR activity of the MnxFe3-xO4 was further tuned by controlling x and MnFe2O4 NPs were found to be as efficient as the commercial Pt in catalyzing ORR. The MnFe2O4 NPs represent a new class of highly efficient non-Pt catalyst for ORR in alkaline media.