Shuangyin Wang

BCN Graphene as Efficient Metal‐Free Electrocatalyst for the Oxygen Reduction Reaction

Abstract : The cathodic oxygen reduction reaction (ORR) is an important process in fuel cells and metal air batteries.[1 3] Although Pt-based electrocatalysts have been commonly used in commercial fuel cells owing to their relatively low overpotential and high current density, they still suffer from serious intermediate tolerance, anode crossover, sluggish kinetics, and poor stability in an electrochemical environment. This, together with the high cost of Pt and its limited nature reserves, has prompted the extensive search for alternative low-cost and high-performance ORR electrocatalysts. In this context, carbon-based metal-free ORR electrocatalysts have generated a great deal of interest owing to their low-cost, high electrocatalytic activity and selectivity, and excellent durability.[4 9] Of particular interest, we have previously prepared vertically aligned nitrogendoped carbon nanotubes (VA-NCNTs) as ORR electrocatalysts, which are free from anode crossover and CO poisoning and show a threefold higher catalytic activity and better durability than the commercial Pt/C catalyst.[4]

Polyelectrolyte Functionalized Carbon Nanotubes as Efficient Metal-free Electrocatalysts for Oxygen Reduction

Having a strong electron-withdrawing ability, poly(diallyldimethylammonium chloride) (PDDA) was used to create net positive charge for carbon atoms in the nanotube carbon plane via intermolecular charge transfer. The resultant PDDA functionalized/adsorbed carbon nanotubes (CNTs), either in an aligned or nonaligned form, were demonstrated to act as metal-free catalysts for oxygen reduction reaction (ORR) in fuel cells with similar performance as Pt catalysts. The adsorption-induced intermolecular charge-transfer should provide a general approach to various carbon-based efficient metal-free ORR catalysts for oxygen reduction in fuel cells, and even new catalytic materials for applications beyond fuel cells.

Vertically Aligned BCN Nanotubes as Efficient Metal-Free Electrocatalysts for the Oxygen Reduction Reaction: A Synergetic Effect by Co-Doping with Boron and Nitrogen

Abstract : The oxygen reduction reaction (ORR) is an important process in many fields, including energy conversion (fuel cells, metal air batteries), corrosion, and biosensing. For fuel cells, the cathodic oxygen reduction is a major factor limiting their performance. The ORR can proceed either through a four-electron process to directly combine oxygen with electrons and protons into water as the end product, or a less efficient two-step, two-electron pathway involving the formation of hydroperoxide ions as intermediate. Oxygen reduction also occurs, albeit too slowly to be of any practical significance, in the absence of an ORR catalyst on the cathode. Platinum nanoparticles have long been regarded as the best catalyst for the ORR and are still commonly used in fuel cells due to their relatively low overpotential and high current density with respect to other commercial catalysts. However, the ORR kinetics on the Pt-based electrode is sluggish, and the Pt electrocatalyst still suffers from multiple drawbacks, such as susceptibility to fuel crossover from the anode, deactivation by CO, and poor stability under electrochemical conditions. In addition, the high cost of Pt and its limited natural reserves are the major barriers to mass-market fuel cells for commercial applications.

Polyelectrolyte-Functionalized Graphene as Metal-Free Electrocatalysts for Oxygen Reduction

Poly(diallyldimethylammonium chloride), PDDA, was used as an electron acceptor for functionalizing graphene to impart electrocatalytic activity for the oxygen reduction reaction (ORR) in fuel cells. Raman and X-ray photoelectron spectroscopic measurements indicate the charge transfer from graphene to PDDA. The resultant graphene positively charged via intermolecular charge-transfer with PDDA was demonstrated to show remarkable electrocatalytic activity toward ORR with better fuel selectivity, tolerance to CO posing, and long-term stability than that of the commercially available Pt/C electrode. The observed ORR electrocatalytic activity induced by the intermolecular charge-transfer provides a general approach to various carbon-based metal-free ORR catalysts for oxygen reduction.

PtRu nanoparticles supported on 1-aminopyrene-functionalized multiwalled carbon nanotubes and their electrocatalytic activity for methanol oxidation.

A new synthesis method for the preparation of high-performance PtRu electrocatalysts on multiwalled carbon nanotubes (MWCNTs) is reported. In this method, bimetallic PtRu electrocatalysts are deposited onto 1-aminopyrene (1-AP)-functionalized MWCNTs by a microwave-assisted polyol process. The noncovalent functionalization of MWCNTs by 1-AP is simple and can be carried out at room temperature without the use of expensive chemicals or corrosive acids, thus preserving the integrity and the electronic structure of MWCNTs. PtRu electrocatalysts on 1-AP-functionalized MWCNTs show much better distribution with no formation of aggregates, higher electrochemically active surface area, and higher electrocatalytic activity for the electrooxidation of methanol in direct methanol fuel cells as compared to that on conventional acid-treated MWCNTs and carbon black supported PtRu electrocatalysts. PtRu electrocatalysts on 1-AP-functionalized MWCNTs also show significantly enhanced stability.

Vertically Aligned BCN Nanotubes with High Capacitance

Using a chemical vapor deposition method, we have synthesized vertically aligned BCN nanotubes (VA-BCNs) on a Ni-Fe-coated SiO(2)/Si substrate from a melamine diborate precursor. The effects of pyrolysis conditions on the morphology and thermal property of grown nanotubes, as well as the nanostructure and composition of an individual BCN nanotube, were systematically studied. It was found that nitrogen atoms are bonded to carbons in both graphitic and pyridinic forms and that the resultant VA-BCNs grown at 1000 °C show the highest specific capacitance (321.0 F/g) with an excellent rate capability and high durability with respect to nonaligned BCN (167.3 F/g) and undoped multiwalled carbon nanotubes (117.3 F/g) due to synergetic effects arising from the combined co-doping of B and N in CNTs and the well-aligned nanotube structure.

Nitrogen-Doped Carbon Nanotube/Graphite Felts as Advanced Electrode Materials for Vanadium Redox Flow Batteries

Nitrogen-doped carbon nanotubes have been grown, for the first time, on graphite felt (N-CNT/GF) by a chemical vapor deposition approach and examined as an advanced electrode for vanadium redox flow batteries (VRFBs). The unique porous structure and nitrogen doping of N-CNT/GF with increased surface area enhances the battery performance significantly. The enriched porous structure of N-CNTs on graphite felt could potentially facilitate the diffusion of electrolyte, while the N-doping could significantly contribute to the enhanced electrode performance. Specifically, the N-doping (i) modifies the electronic properties of CNT and thereby alters the chemisorption characteristics of the vanadium ions, (ii) generates defect sites that are electrochemically more active, (iii) increases the oxygen species on CNT surface, which is a key factor influencing the VRFB performance, and (iv) makes the N-CNT electrochemically more accessible than the CNT.

Controlled synthesis of dendritic Au@Pt core?shell nanomaterials for use as an effective fuel cell electrocatalyst

We report the controlled synthesis of dendritic Au@Pt core-shell nanomaterials. The size and morphology of the Au cores and the Pt shell thickness of the Au@Pt core-shell nanostructures could be easily tuned. It was found that the directing agent and the reducing agent play critical roles in the synthesis of dendritic Au@Pt core-shell nanomaterials. For comparison purposes, conventional Au@Pt core-shell nanoparticles and monometallic Pt nanoparticles were also synthesized by the successive reduction method. Transmission electron microscopy (TEM) observations demonstrated the dendritic surface of the products obtained. The UV-visible (UV-vis) spectroscopy results and a comparison of the average diameter between the dendritic Au@Pt and conventional Au@Pt confirmed the relatively loose Pt shells around Au cores for the dendritic Au@Pt. The as-prepared dendritic Au@Pt showed enhanced electrocatalytic activity for methanol oxidation in acid medium, compared to the conventional Au@Pt and monometallic Pt.

Graphene oxide-assisted deposition of carbon nanotubes on carbon cloth as advanced binder-free electrodes for flexible supercapacitors

We successfully developed a graphene oxide-assisted electrophoretic deposition (EPD) method to prepare the porous hybrid graphene–carbon nanotube (G–CNT) layer on the carbon fiber surface of carbon cloth (CC). The as-fabricated flexible supercapacitor based on the G–CNT/CC electrodes shows significantly enhanced supercapacitor performance.

Boron-doped carbon nanotube-supported Pt nanoparticles with improved CO tolerance for methanol electro-oxidation.

Boron doping into carbon nanotubes improves the CO tolerance and increases the utilization efficiency of the supported Pt nanoparticles. Boron doping leads to a uniform deposition of Pt nanoparticles and also facilitates the removal of CO from the Pt surface, resulting in an increase in the electrocatalytic activity for methanol oxidation.