Direct Enrichment of Metallic Single-Walled Carbon Nanotubes by Using NO₂ as Oxidant to Selectively Etch Semiconducting Counterparts

We report an efficient method for enriching high-purity metallic single-walled carbon nanotubes (m-SWCNTs) by using NO₂ as oxidant to remove semiconducting components at 220 °C. After etching, m-SWCNTs with purity higher than 90% were obtained. The surviving m-SWCNTs retain an intact structure without any extra defects on their surface

Pt Nanoparticles Densely Coated on SnO₂‑Covered Multiwalled Carbon Nanotubes with Excellent Electrocatalytic Activity and Stability for Methanol Oxidation

A new electrocatalyst exhibiting enhanced activity and stability is designed from SnO₂-covered multiwalled carbon nanotubes coated with 85 wt % ratio Pt nanoparticles (NPs). This catalyst showed a mass activity 6.2 times as active as that of the commercial Pt/C for methanol oxidation, owing to the unique one-dimensional structure. Moreover, the durability and antipoisoning ability were also improved greatly. The enhanced intrinsic performance was ascribed to the densely connected networks of Pt NPs on the SnO₂ NPs

Sulfur- and Nitrogen-Doped, Ferrocene-Derived Mesoporous Carbons with Efficient Electrochemical Reduction of Oxygen

Development of inexpensive and sustainable cathode catalysts that can efficiently catalyze the oxygen reduction reaction (ORR) is of significance in practical application of fuel cells. Herein we report the synthesis of sulfur and nitrogen dual-doped, ordered mesoporous carbon (SN-OMCs), which shows outstanding ORR electrocatalytic properties. The material was synthesized from a surface-templating process of ferrocene within the channel walls of SBA-15 mesoporous silica by carbonization, followed by in situ heteroatomic doping with sulfur- and nitrogen-containing vapors. After etching away the metal and silica template, the resulting material features distinctive bimodal mesoporous carbon frameworks with high nitrogen Brunauer–Emmett–Teller specific surface area (of up to ∼1100 m²/g) and uniform distribution of sulfur and nitrogen dopants. When employed as a noble-metal-free electrocatalyst for the ORR, such SN-OMC shows a remarkable electrocatalytic activity; improved durability and better resistance toward methanol crossover in oxygen reduction can be observed. More importantly, it performs a low onset voltage and an efficient nearly complete four-electron ORR process very similar to the observations in commercial 20 wt % Pt/C catalyst. In addition, we also found that the textural mesostructure of the catalyst has superseded the chemically bonded dopants to be the key factor in controlling the ORR performance

MnO₂ Nanofilms on Nitrogen-Doped Hollow Graphene Spheres as a High-Performance Electrocatalyst for Oxygen Reduction Reaction

Platinum is commonly chosen as an electrocatalyst used for oxygen reduction reaction (ORR). In this study, we report an active catalyst composed of MnO₂ nanofilms grown directly on nitrogen-doped hollow graphene spheres, which exhibits high activity toward ORR with positive onset potential (0.94 V vs RHE), large current density (5.2 mA cm^–2), and perfect stability. Significantly, when it was used as catalyst for air electrode, a zinc–air battery exhibited a high power density (82 mW cm^–2) and specific capacities (744 mA h g^–1) comparable to that with Pt/C (20 wt %) as air cathode. The enhanced activity is ascribed to the synergistic interaction between MnO₂ and the doped hollow carbon nanomaterials. This easy and cheap method paves a way of synthesizing high-performance electrocatalysts for ORR

Shell Structure Control of PPy-Modified CuO Composite Nanoleaves for Lithium Batteries with Improved Cyclic Performance

Polypyrrole (PPy)-modified CuO nanocomposites (NCs) with various shell structures have been synthesized by controlling the polymerization time of pyrrole in the presence of leaf-like CuO nanobelts (NBs) as wire templates. The synthesized CuO/PPy NCs and CuO NBs are characterized by XRD, FT–IR, TGA, SEM, TEM, STEM, and EDX line analysis/elemental mapping. The formation mechanism of CuO/PPy core–shell NCs is also illustrated. Electrochemical lithium-storage properties of all the products are evaluated by using them as anode materials for Li-ion batteries (LIBs). It is found that the polymerization time of pyrrole plays a significant role in affecting the shell structures and subsequent lithium-storage properties of the hybrid CuO/PPy NCs. With the extension of polymerization time, CuO/PPy NCs gradually form typical core–shell structures, where the doped PPy with increasing content is steadily and uniformly coated on the CuO surface. Correspondingly, the discharge/charge capacity and cyclic durability of CuO/PPy NCs are significantly enhanced. For the core–shell NCs made by the 3 h polymerization, a greatly improved initial capacity of 1114 mAh g^–1 and a high reversible capacity of 760 mAh g^–1 are achieved, which are much better than those of the bare CuO NBs and the NCs without core–shell structures. The improved performance of core–shell CuO/PPy NCs can be attributed to their advantageous structure features that buffer volume variations of the rigid CuO, allow short Li-ion diffusion length, form good interface interaction between PPy and CuO for charge transfer, and avoid direct contacts between CuO and electrolytes during charge–discharge processes. This study indicates that the structural tuning of polymer/metal oxide composites by controlling the polymerization time is a simple and promising way to improve the electrode performance of NCs for energy storage

Fully Reversible Conversion between SnO₂ and Sn in SWNTs@SnO₂@PPy Coaxial Nanocable As High Performance Anode Material for Lithium Ion Batteries

In this report, we designed a novel SWNTs@SnO₂@PPy coaxial nanocable as superior anode material for the first time. The nanosized SnO₂ particles (2–3 nm) were uniformly distributed between one dimension SWNTs core and PPy shell, as confirmed by XRD, SEM, and TEM characterizations. As an anode material for lithium ion batteries, this composite delivered a high capacity of 823 mAh g^–1 at 150 mA g^–1 after 100 cycles. Even at a high rate of 3000 mA g^–1, a reversible capacity of 480 mAh g^–1 still remained. Furthermore, the SnO₂ in this composite exhibited a large capacity of 1486 mAh g^–1 as well as good capacity retention of 95% over 100 cycles. This result indicated the completely reversible reaction between Li_4.4Sn and SnO₂, greatly improving the theoretical capacity of SnO₂ from 782 to 1493 mAh g^–1

An Effective Integrated Design for Enhanced Cathodes of Ni Foam-Supported Pt/Carbon Nanotubes for Li‑O₂ Batteries

Designing an effective microstructural cathode combined with a highly efficient catalyst is essential for improving the electrochemical performance of Li-O₂ batteries (LOBs), especially for long-term cycling. We present a nickel foam-supported composite of Pt nanoparticles (NPs) coated on self-standing carbon nanotubes (CNTs) as a binder-free cathode for LOBs. The assembled LOBs can afford excellent electrochemical performance with a reversible capacity of 4050 mAh/g tested at 20 mA/g and superior cyclability for 80 cycles with a limited capacity of 1500 mAh/g achieved at a high current density of 400 mA/g. The capacity corresponds to a high energy density of ∼3000 Wh/kg. The improved performance should be attributed to the excellent catalytic activity of highly dispersed Pt NPs, facile electron transport via loose CNTs connected to the nickel foam current collector, and fast O₂ diffusion through the porous Pt/CNTs networks. In addition, some new insights from impedance analysis have been proposed to explain the enhanced mechanism of LOBs

Ammonia Defective Etching and Nitrogen-Doping of Porous Carbon toward High Exposure of Heme-Derived Fe–N_<i>x</i> Site for Efficient Oxygen Reduction

The utilization of metal and nitrogen doped carbon as a Pt-free oxygen reduction electrocatalyst depends largely on the homogeneous composition of the metal–nitrogen sites with limited content. Herein a simple and feasible ammonia defective activation strategy is explored on ordered mesoporous carbon (APC) to confine hematin precursor and suppress the formation of inorganic Fe-based derivatives during pyrolysis. Thus, a hierarchically nanoporous Fe/N/APC catalyst with high numbers of exposed iron–nitrogen sites exhibits an impressive performance for oxygen reduction reaction in alkaline media, with large diffusion-limited current density and positive half-wave potential with respect to commercial Pt/C catalyst. The enhanced ORR properties can be majorly ascribed to synergistic contributions of high numbers of exposed catalytic sites completion from high contents of Fe–N and pyridinic N along with the fast mass-transport properties arising from the etched high permeable porous structure. When applied as cathodic catalyst in Zn-air battery, it demonstrates a power density of 200 mW cm^–2 and a specific capacity of 605 mA h g^–1_Zn higher than those of Pt/C catalyst