Jianyi Lin

Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction

We present two different ways to fabricate nitrogen-doped graphene (N-graphene) and demonstrate its use as a metal-free catalyst to study the catalytic active center for the oxygen reduction reaction (ORR). N-graphene was produced by annealing of graphene oxide (G-O) under ammonia or by annealing of a N-containing polymer/reduced graphene oxide (RG-O) composite (polyaniline/RG-O or polypyrrole/RG-O). The effects of the N precursors and annealing temperature on the performance of the catalyst were investigated. The bonding state of the N atom was found to have a significant effect on the selectivity and catalytic activity for ORR. Annealing of G-O with ammonia preferentially formed graphitic N and pyridinic N centers, while annealing of polyaniline/RG-O and polypyrrole/RG-O tended to generate pyridinic and pyrrolic N moieties, respectively. Most importantly, the electrocatalytic activity of the catalyst was found to be dependent on the graphitic N content which determined the limiting current density, while the pyridinic N content improved the onset potential for ORR. However, the total N content in the graphene-based non-precious metal catalyst does not play an important role in the ORR process.

High-performance flexible asymmetric supercapacitors based on a new graphene foam/carbon nanotube hybrid film

In this work, we report the fabrication of a new 3D graphene foam (GF)/carbon nanotube (CNT) hybrid film with high flexibility and robustness as the ideal support for deposition of large amounts of electrochemically active materials per unit area. To demonstrate the concept, we have deposited MnO2 and polypyrrole (Ppy) on the GF/CNT films and successfully fabricated lightweight and flexible asymmetric supercapacitors (ASCs). These ASCs assembled from GF/CNT/MnO2 and GF/CNT/Ppy hybrid films with high loading of electroactive materials in an aqueous electrolyte are able to function with an output voltage of 1.6 V, and deliver high energy/power density (22.8 W h kg−1 at 860 W kg−1 and 2.7 kW kg−1 at 6.2 W h kg−1). The rate performance can be further improved with less loading of electroactive materials (10.3 kW kg−1 at 10.9 W h kg−1). The ASCs demonstrate remarkable cycling stability (capacitance retention of 90.2–83.5% after 10u2006000 cycles), which is among the best reported for ASCs with both electrodes made of non-carbon electroactive materials. Also the ASCs are able to perfectly retain their electrochemical performance at different bending angles. These ASCs demonstrate great potential as power sources for flexible and lightweight electronic devices.

A V2O5/Conductive‐Polymer Core/Shell Nanobelt Array on Three‐Dimensional Graphite Foam: A High‐Rate, Ultrastable, and Freestanding Cathode for Lithium‐Ion Batteries

A thin polymer shell helps V2O5 a lot. Short V2O5 nanobelts are grown directly on 3D graphite foam as a lithium-ion battery (LIB) cathode material. A further coating of a poly(3,4-ethylenedioxythiophene) (PEDOT) thin shell is the key to the high performance. An excellent high-rate capability and ultrastable cycling up to 1000 cycles are demonstrated.

Self‐Assembly of Honeycomb‐like MoS2 Nanoarchitectures Anchored into Graphene Foam for Enhanced Lithium‐Ion Storage

Honeycomb-like MoS2 nanoarchitectures anchored into 3D graphene foam are successfully fabricated as a high-performance positive electrode for enhanced Li-ion storage. The unique 3D interpenetrating honeycomb-like structure is the key to the high performance. High reversible capacity, superior high-rate capability, and excellent cycling stability are demonstrated.

VO2 nanoflake arrays for supercapacitor and Li-ion battery electrodes: performance enhancement by hydrogen molybdenum bronze as an efficient shell material

Hydrogen molybdenum bronze (HMB) is electrochemically deposited as a homogeneous shell on VO2 nanoflakes grown on graphene foam (GF), forming a GF + VO2/HMB integrated electrode structure. Asymmetric supercapacitors based on the GF + VO2/HMB cathode and neutral electrolyte are assembled and show enhanced performance with weaker polarization, higher specific capacitance and better cycling life than the unmodified GF + VO2 electrode. Capacitances of 485 F g−1 (2 A g−1) and 306 F g−1 (32 A g−1) are obtained because of the exceptional 3D porous architecture and conductive network. In addition, the GF + VO2/HMB electrodes are also characterized as the cathode of lithium ion batteries. Very stable capacities at rates up to 30 C are demonstrated for 500 cycles. This new type of shell material is expected to have its generic function in other metal oxide based nanostructures.

A Green Approach to the Synthesis of High-Quality Graphene Oxide Flakes via Electrochemical Exfoliation of Pencil Core

A simple, green and cost-effective approach has been reported to synthesize high-quality graphene oxide (GO) flakes via electrochemical exfoliation of pencil cores in aqueous electrolytes. The exfoliated GO flakes exhibit excellent electrocatalytic activity and toxicity tolerance for oxygen reduction reactions in alkaline solution. Our present results are promising for scaled-up preparation and further commercialization of graphene oxide in a low-cost and environmentally friendly way.

Large area patterned arrays of aligned carbon nanotubes via laser trimming

We develop a simple to implement, inherently parallel and high throughput technique for the fabrication of large areas of patterned aligned multi-wall carbon nanotube (CNT) arrays deposited on silicon or quartz substrate. This technique makes use of a parallel or converging laser beam from a high power pulsed laser for the destruction of aligned CNTs with a copper grid as lithography mask to define patterned aligned CNT arrays. The wavelength of the laser beam used is 248 nm and the average energy per pulse is 500 mJ. Using this technique, an extensive area of patterned CNT arrays as large as 3 × 5 mm2 can be fabricated without the use of any pre-patterned substrate. In addition, we were able to control the size of the features created by (1) using different copper grids and (2) using a converging beam. Exposing the sample to different numbers of laser pulses allows us to generate families of CNTs with different uniform lengths. Furthermore, using two overlapping grids as a lithography mask, we managed to create a regular array of features with sizes as small as 2.5 μm.

MoS2 architectures supported on graphene foam/carbon nanotube hybrid films: highly integrated frameworks with ideal contact for superior lithium storage

Three-dimensional (3D) nanoworm-like MoS2 architectures have been successfully synthesized and directly supported on graphene foam/carbon nanotubes (GF/CNT) hybrid films. The sp2-hybridized GF/CNT films provide robust frameworks with an ideal contact for the nucleation and subsequent massive growth of the MoS2 architectures, while acting as an efficient current collector with a conductive contact for binder-free electrodes. The as-prepared hierarchical MoS2@GF/CNT electrode shows capacities of 1368 mA h g−1 and 823 mA h g−1 at current densities of 200 mA g−1 and 5000 mA g−1, and can retain 81.3% of the initial reversible capacity up to 120 cycles.

Advanced Energy Storage Devices: Basic Principles, Analytical Methods, and Rational Materials Design

Abstract Tremendous efforts have been dedicated into the development of high‐performance energy storage devices with nanoscale design and hybrid approaches. The boundary between the electrochemical capacitors and batteries becomes less distinctive. The same material may display capacitive or battery‐like behavior depending on the electrode design and the charge storage guest ions. Therefore, the underlying mechanisms and the electrochemical processes occurring upon charge storage may be confusing for researchers who are new to the field as well as some of the chemists and material scientists already in the field. This review provides fundamentals of the similarities and differences between electrochemical capacitors and batteries from kinetic and material point of view. Basic techniques and analysis methods to distinguish the capacitive and battery‐like behavior are discussed. Furthermore, guidelines for material selection, the state‐of‐the‐art materials, and the electrode design rules to advanced electrode are proposed.

Nanostructured trimetallic Pt/FeRuC, Pt/NiRuC, and Pt/CoRuC catalysts for methanol electrooxidation

We report the preparation and characterization of metal–carbon nanocomposites (NiRuC, FeRuC, and CoRuC) with bimetallic Ni–Ru, Fe–Ru, and Co–Ru nanoparticles incorporated into the pore walls of ordered mesoporous carbon, which were synthesized via a template strategy. Pt nanoparticles were deposited on the nanocomposites separately, which is different from the traditional alloying method. Nitrogen adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and thermogravimetric analysis techniques were used to characterize the materials. It was found that bimetallic nanoparticles (Ni–Ru, Fe–Ru, and Ru–Co) are homogenously dispersed in the carbon matrix and Pt nanoparticles with a size of less than 5 nm are widely distributed within the nanocomposites. The Pt/CoRuC catalyst shows better catalytic activity for the methanol oxidation reaction (MOR) than Pt on FeRuC or NiRuC, and its performance is also closer to that of the commercial PtRu catalyst with a slightly higher metal loading. A kinetic-based impedance model was used to simulate the electrochemical properties of the catalysts and matches well with the MOR performances of the catalysts. The promotional effect of the bimetallic/carbon nanocomposites on the catalytic activity of Pt was evidenced, and more importantly, the ligand effect was demonstrated by our results to be the major factor in the enhancement. Our investigation not only provides further insight into the roles of Fe, Ni and Co in MOR, but also assists in the design and synthesis of the new types of nanostructured electrocatalyst supports.