Kuang-Hsu Wu

A microporous–mesoporous carbon with graphitic structure for a high-rate stable sulfur cathode in carbonate solvent-based Li–S batteries

A microporous-mesoporous carbon with graphitic structure was developed as a matrix for the sulfur cathode of a Li-S cell using a mixed carbonate electrolyte. Sulfur was selectively introduced into the carbon micropores by a melt adsorption-solvent extraction strategy. The micropores act as solvent-restricted reactors for sulfur lithiation that promise long cycle stability. The mesopores remain unfilled and provide an ion migration pathway, while the graphitic structure contributes significantly to low-resistance electron transfer. The selective distribution of sulfur in micropores was characterized by X-ray photoelectron spectroscopy (XPS), nitrogen cryosorption analysis, transmission electron microscopy (TEM), X-ray powder diffraction and Raman spectroscopy. The high-rate stable lithiation-delithiation of the carbon-sulfur cathode was evaluated using galvanostatic charge-discharge tests, cyclic voltammetry and electrochemical impedance spectroscopy. The cathode is able to operate reversibly over 800 cycles with a 1.8 C discharge-recharge rate. This integration of a micropore reactor, a mesopore ion reservoir, and a graphitic electron conductor represents a generalized strategy to be adopted in research on advanced sulfur cathodes.

A Discussion on the Activity Origin in Metal-Free Nitrogen-Doped Carbons For Oxygen Reduction Reaction and their Mechanisms.

The origin of oxygen reduction reaction activity in metal-free N-doped carbons has been a stimulating, yet unsolved issue for the rational design of cost-effective electrocatalysts for fuel cells and metal-air batteries. At present, there are several inconsistent opinions on the materials chemistry and the mechanism of the oxygen reduction reaction (ORR) performed on this type of materials. This article provides a brief review of the current understanding of ORR processes and the history of electrocatalyst development. With special attention, the focus of the discussion is on the major contentions of the current opinions towards metal-free N-doped carbon chemistry and the arguments for the probable ORR mechanisms. By clarifying the fundamental aspects of each opinion, a converging consensus on N-doped carbon electrocatalysts can be established and thus facilitate the substantial development of large-capacity energy devices.

Revealing the Origin of Activity in Nitrogen-Doped Nanocarbons towards Electrocatalytic Reduction of Carbon Dioxide.

Carbon nanotubes (CNTs) are functionalized with nitrogen atoms for reduction of carbon dioxide (CO2 ). The investigation explores the origin of the catalysts activity and the role of nitrogen chemical states therein. The catalysts show excellent performances, with about 90 % current efficiency for CO formation and stability over 60 hours. The Tafel analyses and density functional theory calculations suggest that the reduction of CO2 proceeds through an initial rate-determining transfer of one electron to CO2 , which leads to the formation of carbon dioxide radical anion (CO2 (.-) ). The initial reduction barrier is too high on pristine CNTs, resulting in a very high overpotentials at which the hydrogen evolution reaction dominates over CO2 reduction. The doped nitrogen atoms stabilize the radical anion, thereby lowering the initial reduction barrier and improving the intrinsic activity. The most efficient nitrogen chemical state for this reaction is quaternary nitrogen, followed by pyridinic and pyrrolic nitrogen.

Structural Origin of the Activity in Mn3O4–Graphene Oxide Hybrid Electrocatalysts for the Oxygen Reduction Reaction

Non-precious metal oxide/carbon hybrid electrocatalysts are of increasing importance for the oxygen reduction reaction (ORR). A synergistic effect is commonly used to explain the superior ORR activity exerted by metal oxide/nanocarbon hybrids, and this effect is attributed to covalently coupled interfaces between the two materials. However, the origin of the high activity, the structure, and the electrocatalytic nature of the interface remain unclear. By combining X-ray photoelectron spectroscopy with synchrotron far-infrared spectroscopy, we resolved the interface structure between spinel manganese oxide nanocrystals and graphene oxide nanoribbons, and the role of this interface in the promoted ORR. Moreover, we demonstrated the excellent ORR activity by a functional synergism of the hybrid constituents through a series of comparative electrochemical experiments.

Reduction-induced surface amorphization enhances the oxygen evolution activity in Co3O4

Modifying crystalline Co3O4 by thermal H2 annealing and air exposure produced an amorphous surface layer consisting of mixed hydrated cobalt hydroxide/carbonate species and remarkably enhanced the oxygen evolution activity.

Reduced graphene oxide: a metal-free catalyst for aerobic oxidative desulfurization

Nanocarbons have been extensively used as metal-free alternatives to metal catalysts in many oxidative processes owing to their functional groups or defects, which have the activation ability toward oxygen molecules. Herein, reduced graphene oxide (rGO) could be used as catalysts in oxidative desulfurization reactions for the first time to remove sulfur-containing compounds from fuels. Superior catalytic activity and stability are obtained in the aerobic oxidative desulfurization process catalyzed by rGO. A broad range of sulfur-containing aromatic substrates could be removed effectively in this catalytic system. Carbonyl groups have proved to play crucial roles during the oxidation process based on an XPS analysis, a chemical titration method and a series of comparative experiments. The chemically active defects are also beneficial to the catalytic performance because carbonyl groups could be generated in situ on these defects under the reaction conditions. Based on the EPR results, a radical pathway is proposed. Oxygen molecules could interact with the carbon atoms adjacent to carbonyl groups to form adsorbed super oxygen anion radicals (rGO-OO˙−), which then attack the adjacent sulfur-centered cation radicals and generate the final product sulfone.

Synergy of nanoconfinement and surface oxygen in recrystallization of sulfur melt in carbon nanocapsules and the related Li–S cathode properties

We studied the recrystallization behaviours of sulfur melt in oxygen-containing carbon nanocapsules (CNCs). The effects of the oxidizing degree and the nanoconfinement of CNCs on sulfur recrystallization were investigated. We performed weak oxidation on CNCs by firstly grafting >C-Cl3 groups via a Friedel-Craft reaction and successive hydrolysis of >C-Cl3; and the strong oxidation was conducted in nitric acid. It is found that the weak oxidation preserved the CNC structure while the strong oxidation damaged the CNC morphology. Electron microscopy, X-ray diffraction, Raman spectroscopy and X-ray absorption spectroscopy were combined to characterize the sulfur crystallites in pristine and oxidized CNCs. The results revealed that the smaller sulfur crystallites preferentially formed in integrated CNCs (preserved nanoscale hollow structure) regardless of oxygen content; while the stronger oxidation and destruction of hollow structures fostered the growth of larger sulfur crystals. These results suggest a possible approach to control the growth of sulfur in carbon by combining oxygen and nanoconfinement effects, and hopefully to tune the related electrochemical properties in Li–S battery cathodes.

Direct Insight into Ethane Oxidative Dehydrogenation over Boron Nitrides

The ultimate objective of chemical conversion is to achieve 100 % selectivity from catalysis, and this is also a prodigious challenge for the conversion of light paraffins into olefins, because it involves controlled activation of highly stable aliphatic C−H bonds. Herein, we show that metal‐free boron nitride (BN) nanosheets not only enable the oxidative dehydrogenation of ethane to ethylene exclusively at near 10 % conversion, but they also deliver a remarkable 60 % selectivity at an ethane conversion of 78 % and remain stable over 400 h at 575 °C. Our operando infrared spectroscopy and 18O isotope tracer study explicitly demonstrates that B−O(H) active sites are formed at the edges of BN through the aid of ethane, and the dehydrogenation circle is completed over these B−O sites by the assistance of O2.

Solution phase synthesis of halogenated graphene and the electrocatalytic activity for oxygen reduction reaction

Metal-free carbon electrocatalyts for the oxygen reduction reaction (ORR) are attractive for their high activity and economic advantages. However, the origin of the activity has never been clearly elucidated in a systematic manner. Halogen group elements are good candidates for elucidating the effect, although it has been a difficult task due to safety issues. In this report, we demonstrate the synthesis of Cl-, Br- and I-doped reduced graphene oxide through two solution phase syntheses. We have evaluated the effectiveness of doping and performed electrochemical measurements of the ORR activity on these halogenated graphene materials. Our results suggest that the high electronegativity of the dopant is not the key factor for high ORR activity; both Br- and I-doped graphene promoted ORR more efficiently than Cl-doped graphene. Furthermore, an unexpected sulfur-doping in acidic conditions suggests that a high level of sulfide can degrade the ORR activity of the graphene material.

Nanodiamond core reinforced graphene shell immobilized Pt nanoparticles as a highly active catalyst for low temperature dehydrogenation of n-butane

Pt nanoparticles (NPs) immobilized on a core–shell hybrid carbon support allow the efficient direct dehydrogenation of n‐butane at low temperatures. The hybrid carbon support is composed of a nanodiamond core and a reinforced ultrathin graphene shell (ND@G), which improves the stability of the anchored Pt NPs against sintering under the reaction conditions. The as‐prepared Pt/ND@G catalyst demonstrates excellent catalytic performance (≈25 % conversion with >95 % selectivity toward olefins) at 450 °C for over 10 h as a result of strong metal–support interactions. The catalyst can also be fully regenerated by postoxidative thermal treatment.