Yanwen Ma

Nitrogen‐Doped Carbon Nanocages as Efficient Metal‐Free Electrocatalysts for Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is one of the most crucial factors limiting the performance of proton exchange membrane fuel cells due to its slow kinetics. [ 1 , 2 ] The development of effi cient ORR electrocatalysts is thus of great signifi cance. Today, platinum is usually used as the electrocatalyst for ORR; however, the large-scale application of fuel cells is hampered by the its scarcity and high cost. [ 3 ] In addition, Pt-based catalyst is sensitive to deactivation in the presence of CO, methanol and also susceptible to time-dependent drift. [ 1 ] Hence, great efforts have been devoted to exploring the advanced ORR catalysts to rival the commercial Pt/C catalyst in activity and durability with reduced Pt loading, [ 1 , 4–6 ] or nonprecious metals, [ 7–10 ] or even metalfree species. [ 11–16 ] Compared to Pt-based catalysts, metal-free catalysts have several noteable advantages, in that they do not suffer CO poisoning or crossover effects, have long-term operational stability, and are relatively cost-effective. Following the breakthrough in discovering the metal-free catalyst of PEDOT for ORR, [ 11 ] heteroatom-doped carbon nanotubes (CNTs) were demonstrated to be a new kind of promising metal-free electrocatalyst. Doping with either electron-rich nitrogen [ 12 ] or electrodefi cient boron [ 13 ] can transform CNTs into superb metal-free ORR catalysts, and the co-doping can provide further space for performance optimization, [ 14 ] which forms a scientifi cally interesting and technologically important subject today. However, there are still some key issues to address. First, enhancing the catalytic activity by increasing the specifi c surface area is an important challenge. This has proven to be diffi cult for doped CNTs, [ 17 , 18 ] although undoped CNTs have been prepared with high specifi c surface areas ( > 1200 m 2 g − 1 ) in the case of singlewalled CNTs. [ 19 ] In addition, the origin of the ORR activity for doped CNTs is still a matter of controversy and need to be further clarifi ed. [ 10 , 12 , 20–22 ] Since the metal impurities with ORR activity, i.e., the residue from the catalyst used in the growth of

Superhydrophobic and superoleophilic hybrid foam of graphene and carbon nanotube for selective removal of oils or organic solvents from the surface of water

A monolithic 3D hybrid of graphene and carbon nanotube was synthesized by two-step chemical vapor deposition. Owing to its superhydrophobic and superoleophilic properties, it can selectively remove oils and organic solvents from water with high absorption capacity and good recyclability.

Hydrophilic Hierarchical Nitrogen-Doped Carbon Nanocages for Ultrahigh Supercapacitive Performance.

The synergism of large surface area, multiscale porous structure, and good conductivity endows hierarchical carbon nanocages with high-level supercapacitive performances. Further nitrogen doping greatly improves the hydrophilicity, which boosts the supercapacitive performances to an ultrahigh specific capacitance of up to 313 F g(-1) at 1 A g(-1).

Synthesis of graphene–carbon nanotube hybrid foam and its use as a novel three-dimensional electrode for electrochemical sensing

Three-dimensional (3D) graphene–carbon nanotube (CNT) hybrids are synthesized by two-step chemical vapor deposition (CVD) under atmospheric pressure. As revealed by scanning electron microscopy (SEM), the hybrid is a monolithic graphene foam with conformal coverage of a dense CNT mesh. We further demonstrate that the obtained graphene–CNT hybrid foams can be used as novel 3D electrochemical electrodes for sensing applications. Specifically, the 3D graphene–CNT electrodes exhibit a high sensitivity (∼470.7 mA M−1 cm−2) and low detection limit (∼20 nM with S/N ≈ 9.2) for dopamine detection. Modified with horseradish peroxidase and Nafion, the 3D hybrid electrodes are also used to detect H2O2 with a high sensitivity (137.9 mA M−1 cm−2), low detection limit (∼1 μM with S/N ≈ 17.4), and wide linear detection range (10 μM–1 mM).

The synthesis of shape-controlled MnO2/graphene composites via a facile one-step hydrothermal method and their application in supercapacitors

Novel MnO2 petal nanosheet and nanorod/graphene composites are successfully fabricated by a facile one-step hydrothermal method through changing the content of the Mn source. The formation mechanism of different morphologies of MnO2/graphene composites have been studied. The structure of the MnO2/graphene is “sandwich”-like, with MnO2 petal nanosheets and nanorods homogeneously anchored on each side of the graphene. Furthermore, the MnO2/graphene composites with different shapes can be used for supercapacitor electrode materials. The experimental results show that the MnO2 petal nanosheet/graphene composite has better capacitance performance than that of the MnO2 nanorod/graphene composite. The MnO2 petal nanosheet/graphene composite shows excellent specific capacitance as high as 516.8 F g−1 at a scan rate of 1 mV s−1 in 1 M Na2SO4 electrolyte and good long-term cycle stability, indicating its potential application to act as a promising electrode material for high-performance supercapacitors. This study provides a facile and in situ method to prepare metal oxide/graphene composite materials and a novel scaffold to construct other metal oxides with graphene for energy storage.

Porous 3D Few‐Layer Graphene‐like Carbon for Ultrahigh‐Power Supercapacitors with Well‐Defined Structure–Performance Relationship

3D few-layer graphene-like carbon with hierarchical open porous architecture is obtained by a new in situ Cu template method, leading to top-level supercapacitive performance, especially state-of-the-art power density. An effective new approach is demonstrated, which can extend the understanding of structure-performance relationships for many electrochemical energy-storage systems and form a significant complement to classical electrochemical impedance spectroscopy.

Conductive graphene fibers for wire-shaped supercapacitors strengthened by unfunctionalized few-walled carbon nanotubes.

Graphene fibers are a promising electrode material for wire-shaped supercapacitors (WSSs) that can be woven into textiles for future wearable electronics. However, the main concern is their high linear resistance, which could be effectively decreased by the addition of highly conductive carbon nanotubes (CNTs). During the incorporation process, CNTs are typically preoxidized by acids or dispersed by surfactants, which deteriorates their electrical and mechanical properties. Herein, unfunctionalized few-walled carbon nanotubes (FWNTs) were directly dispersed in graphene oxide (GO) without preoxidation or surfactants, allowing them to maintain their high conductivity and perfect structure, and then used to prepare CNT-reduced GO (RGO) composite fibers by wet-spinning followed by reduction. The pristine FWNTs increased the stress strength of the parent RGO fibers from 193.3 to 385.7 MPa and conductivity from 53.3 to 210.7 S cm(-1). The wire-shaped supercapacitors (WSSs) assembled based on these CNT-RGO fibers presented a high volumetric capacitance of 38.8 F cm(-3) and energy density of 3.4 mWh cm(-3). More importantly, the performance of WSSs was revealed to decrease with increasing length due to increased resistance, revealing a key issue for graphene-based electrodes in WSSs.

The self-assembly of shape controlled functionalized graphene–MnO2 composites for application as supercapacitors

Graphene–MnO2 nanocomposites with different morphologies were obtained by a facile self-assembly method. The formation mechanism of graphene–MnO2 composites with different shapes of MnO2 is discussed in detail. Nanostructured MnO2 with different morphologies was distributed on the surface of graphene uniformly. The prepared graphene–MnO2 composites could be used as electrode materials for supercapacitors. The graphene–MnO2 (flowerlike nanospheres) composite (405 F g−1) exhibited better capacitive performance than that of the graphene–MnO2 (nanowires) composite (318 F g−1) at a current density of 1.0 A g−1. The synergistic effect of graphene and MnO2 endowed the composite with high electrochemical capacitance. Moreover, the graphene–MnO2 (flowerlike nanospheres) composite showed a fast charge–discharge process and high cyclic stability.

Facile synthesis of shape-controlled graphene–polyaniline composites for high performance supercapacitor electrode materials

Graphene–polyaniline (PANI) nanocomposites with different morphologies were successfully fabricated by an effective one-step hydrothermal method. The morphologies of PANI could be controlled from nanowires to nanocones by adjusting the amount of aniline with the assistance of an ultrasonication process. By taking the advantages of the high conductivity of graphene and the pseudocapacitance of PANI, graphene–PANI composites were used as an example for the application to the supercapacitor electrode materials. The cyclic voltammograms (CV) and galvanostatic charge–discharge measurements demonstrate that the graphene–PANI shows excellent electrochemical properties. The graphene–PANI nanowire composite (724.6 F g−1) exhibited higher specific capacitance than that of the graphene–PANI nanocone composite (602.5 F g−1) at a current density of 1.0 A g−1. Furthermore, the graphene–PANI nanowire composite exhibited outstanding capacitive performance with a high specific capacitance of 957.1 F g−1 at 2 mV s−1 and a high cycle reversibility of 90% after charge–discharge 1000 cycles. The improved electrochemical properties of the graphene–PANI nanocomposites suggest their promising applications to high-performance supercapacitors.

Synthesis of a graphene/polyaniline/MCM-41 nanocomposite and its application as a supercapacitor

A ternary nanocomposite of graphene/polyaniline (PANI)/porous silica MCM-41 (MCM-41) was prepared by a hydrothermal method. The amount of graphene oxide (GO) in the graphene/PANI/MCM-41 nanocomposite had a strong effect on the supercapacitor performance. The experimental results showed that the specific capacitance of the graphene/PANI/MCM-41 nanocomposite could reach the highest value when the GO content was 50%. The specific capacitance of the nanocomposite was 405 F g−1 at a current density of 0.8 A g−1. Furthermore, over 91.4% of the original capacitance was retained after repeating the galvanostatic charge–discharge for 1000 cycles. The performance of the prepared supercapacitor containing different amounts of GO were studied in detail.