Influences of graphene oxide support on the electrochemical performances of graphene oxide-MnO2 nanocomposites

MnO2 supported on graphene oxide (GO) made from different graphite materials has been synthesized and further investigated as electrode materials for supercapacitors. The structure and morphology of MnO2-GO nanocomposites are characterized by X-ray diffraction, X-ray photoemission spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and Nitrogen adsorption-desorption. As demonstrated, the GO fabricated from commercial expanded graphite (denoted as GO(1)) possesses more functional groups and larger interplane gap compared to the GO from commercial graphite powder (denoted as GO(2)). The surface area and functionalities of GO have significant effects on the morphology and electrochemical activity of MnO2, which lead to the fact that the loading amount of MnO2 on GO(1) is much higher than that on GO(2). Elemental analysis performed via inductively coupled plasma optical emission spectroscopy confirmed higher amounts of MnO2 loading on GO(1). As the electrode of supercapacitor, MnO2-GO(1) nanocomposites show larger capacitance (307.7 F g-1) and better electrochemical activity than MnO2-GO(2) possibly due to the high loading, good uniformity, and homogeneous distribution of MnO2 on GO(1) support

2020 roadmap on two-dimensional materials for energy storage and conversion

Energy storage and conversion have attained significant interest owing to its important applications that reduce CO2 emission through employing green energy. Some promising technologies are included metal-air batteries, metal-sulfur batteries, metal-ion batteries, electrochemical capacitors, etc. Here, metal elements are involved with lithium, sodium, and magnesium. For these devices, electrode materials are of importance to obtain high performance. Two-dimensional (2D) materials are a large kind of layered structured materials with promising future as energy storage materials, which include graphene, black phosporus, MXenes, covalent organic frameworks (COFs), 2D oxides, 2D chalcogenides, and others. Great progress has been achieved to go ahead for 2D materials in energy storage and conversion. More researchers will join in this research field. Under the background, it has motivated us to contribute with a roadmap on ‘two-dimensional materials for energy storage and conversion

Energy storage applications of graphene-based materials

This thesis focuses on the investigation of chemically prepared graphene/ graphene composites and their application as new energy storage materials. The surface chemistry of graphene was found to be important for the growth of conducting polymer and further influences the magnitude of increase of specific capacitance. Nitrogen-doped graphene (N-graphene) and non-precious metal nitride catalyst (FeCo-N complex) are successfully prepared and demonstrate its use as a noble metal-free catalyst to study the catalytic active center for the oxygen reduction reaction (ORR). The bonding state of the N atoms was found to have a significant effect on the selectivity and catalytic activity for ORR. The ORR mechanism and the nature of the active centers of graphene based non-precious metal catalyst were rationalized based on UPS, XPS, XRD, and linear sweep voltammetry data. The data suggest that non-precious metal catalysts can be effective for ORR.DOCTOR OF PHILOSOPHY (SPMS

Durable freestanding hierarchical porous electrode for rechargeable zinc-air batteries

The development of freestanding bifunctional air cathodes for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desirable for the next generation of flexible rechargeable metal-air batteries. It remains challenging to achieve efficient OER and ORR bifunctionality on a single lightweight and inexpensive electrode. In this article, a metal-free, and freestanding air cathode based on vertically aligned carbon nanotubes (VACNTs) functionalized with N, P heteroatoms doped carbon is first reported. In addition to the high catalytic activity caused by N, P heteroatoms doping, the importance of efficient gas diffusion and electron transfer provided by the VACNT-GF hierarchical structure is highlighted. The carbonization temperature has been identified to have pronounced effect on catalytic activity, and the samples with P-N bonds have smaller ORR and OER overpotentials, while the quantitative atomic ratio of either P or N has little effect on catalytic activity. The resulting air electrode achieved a high peak power density of 56 mW cm-2 at a current density of 120 mA cm-2, outperforming Pt/C- and IrO2-based rechargeable Zn-air batteries. The zinc-air battery assembled with the air electrode also showed good cyclability, which exceeded that of cells with the Pt/C//IrO2 catalyst. The increase of voltage difference between the charge and discharge platform was 0.2 V for the cell assembled with N,P-doped VACNT-based freestanding air cathode after 75 h of operation at 10 mA cm-2, which was less than half of that of cells with Pt/C//IrO2 catalyst. Impedance analysis further reveals the good performance results from the favorable mass transfer of the electrode.Ministry of Education (MOE)We gratefully acknowledge the National Natural Science Foundation of China for supporting this research through Grant No. 51502135 and the Singapore Ministry of Education for supporting this research through Grant AcRF Tier 1 (Reference No. RG103/16)

Preparation of Pt nanoparticle-loaded three-dimensional Fe3O4/carbon with high electro-oxidation activity

Program for New Century Excellent Talents in Fujian Province [X12103]; Natural Science Foundation of China [20701031]; Natural Science Foundation of Fujian Province of China [C0710045]A three-dimensional (3D) Fe3O4/carbon material functionalized with amino and hydroxyl groups was synthesized by decomposing 2,4,5-trichlorophenol/ferrocene mixture in the presence of ammonia and polyethylene glycol in solvothermal conditions at 250 degrees C for 30 h by a one-step process. The 3D Fe3O4/carbon materials can be loaded with Pt nanoparticles without adding any reducing agent; Pt-loaded 3D Fe3O4/carbon hybrid materials have superior electrochemical catalytic activity toward methanol oxidation and the oxidation current density on them is nearly triple that on a commercial Pt/C catalyst. (C) 2010 Elsevier Ltd. All rights reserved

Solvothermal syntheses of hollow carbon microspheres modified with -NH2 and -OH groups in one-step process

Program for New Century Excellent Talents in Fujian Province [X12103]; Natural Science Foundation of China [20701031]; Natural Science Foundation of Fujian Province of China [C0710045]Homogeneous and monodisperse hollow carbon microspheres (HCMS) functionalized with amino and hydroxyl groups are synthesized by decomposing 2,4,6-tribromopheno/ferrocene mixture in the presence of ammonia via a one-step solvothermal process at 250 degrees C for 24 h. The effect of experimental conditions on the morphology of carbon microspheres has been investigated systematically. The surface of the HCMS is modified with amino and hydroxyl groups through the synthesis process as confirmed by infra-red spectroscopy and X-ray photoelectron spectroscopy. The pore-size distribution and the specific area of macropores are measured by nitrogen adsorption-desorption and mercury intrusion porosimetry. The as-synthesized HCMS has the high Brunauer-Emmett-Teller surface area of 289 m(2)/g and macropores whose diameter is mostly larger than 100 nm. To investigate the chemical reactivity of functionalized groups on the surface of HCMS, Au and Ag nanoparticles are successfully loaded onto HCMS by direct reduction of HAuCl4 or AgNO3 without adding any reducing agent. (C) 2010 Elsevier Ltd. All rights reserved

A cathode for Li-ion batteries made of vanadium oxide on vertically aligned carbon nanotube arrays/graphene foam

A hierarchically structured free-standing V2O5/vertically aligned carbon nanotube/graphene foam (V2O5-VA-CNTs/GF) coated by PEDOT was designed. Instead of forming a thick coating layer around, the V2O5 nanobelts disperse uniformly among the CNTs forest without severe aggregations. The PEDOT-V2O5-VA-CNTs/GF delivered a reversible capacity of 296.8 mAh g−1 at 1C, and has capacity retention of 113.3 mAh g−1 at 5C after 1000 cycles. First principles calculations indicate the addition of VA-CNTs to V2O5 electrode could improve the electronic conductivity and facilitate Li-ion adsorption, which lead to the outstanding Li-ion storage and conversion behaviour

High-performance asymmetric pseudocapacitor cell based on cobalt hydroxide/graphene and polypyrrole/graphene electrodes

Cobalt hydroxide nanowires network grown on nitrogen modified microwave exfoliated graphite oxide (NMEG) with a specific capacitance of 610 F/g is successfully synthesized by a chemical precipitation method. Asymmetric-type pseudocapacitors are fabricated with Co(OH)2/NMEG and polypyrrole (PPy)/rG-O applied as positive and negative electrodes respectively. The electrochemical properties of the electrodes in three electrode and two electrode systems are systematically investigated in 1 M KOH electrolyte. Various supercapacitor devices, such as rG-O//Co(OH)2/NMEG, NG//NG, rG-O//rG-O, and PPy/rG-O//PPy/rG-O are assembled with electrochemical performance evaluated by cyclic voltammetry and galvanostatic charge/discharge measurements. The Co(OH)2/NMEG//PPy/rG-O asymmetric supercapacitor cells can achieve a high cell voltage of 1.6 V and an energy density up to 24.9 Wh/kg with an active materials loading of ∼5 mg/cm2, significantly higher than that of rG-O//Co(OH)2/NMEG (19.3 Wh/kg), NG//NG (16.4 Wh/kg), rG-O//rG-O (15.3 Wh/kg) and PPy/rG-O//PPy/rG-O (9.4 Wh/kg) supercapacitor devices under the same measurement environment. The PPy/rG-O is a superior negative electrode to match cobalt/nickel oxides/hydroxides based positive electrodes for supercapacitor devices

Selection of graphene dopants for Na3V2(PO4)3 graphene composite as high rate, ultra long-life sodium-ion battery cathodes

Na3V2(PO4)3 (NVP) is one of the most promising cathode materials for sodium-ion batteries because of its stability, safety, and high reversible capacity. However, the sluggish Na-ion diffusion and poor electronic conductivity of NVP often hinder electrochemical performance, thus requiring compositing with carbon materials, such as graphene to improve the material. In this work, the effect of doping species of graphene on the electrochemical performance of NVP/graphene composites was systematically investigated and vigorously compared. 3D porous NVP fabricated by sol-gel method with 3 nm of carbon coating layers was deposited on graphene sheets with different surface functionalities (GO, N-rGO and P-rGO). NVP/N-rGO composites have low charge transfer resistance and high Na+ diffusion coefficient than that of NVP powder, NVP/P-rGO, and NVP/GO, which delivered a specific capacity of 113.9 mAh g−1 at 0.5C with a capacity retention up to 88.42% after 5000 cycles at 20 C. The superior sodium storage performance derives from the pyridinic and pyrrolic N doping in graphene, which triggers defective and active site numbers but maintains moderate graphitization to accelerate the Na+ and electron transportation.This work was supported by the National Natural Science Foundation of China (Grant No. 51502135), and Primary Research & Developement Plan of Jiangsu Province (BE2016183). The authors are grateful to Prof. Xia Hui, Nanjing University of Science & Technology for access to equipment. The authors are grateful to Prof. Jianmin Ma and Dr Zengxi Wei for discussion and theoretical calculation of NVP capacity

A free-standing Li4Ti5O12/graphene foam composite as anode material for Li-ion hybrid supercapacitor

The demand of highly efficient energy storage system has significantly increased along with the rapid development of electric vehicles and hybrid electric vehicles. However, the energy and power density of standard energy storage devices, such as Li-ion batteries or supercapacitors still cannot meet the requirement of the state-of-the-art electric vehicles. Herein, free-standing Li4Ti5O12/graphene foam (LTO/GF) composite is synthesized via a hydrothermal method, and applied as anode material for Li-ion hybrid supercapacitor. The as-synthesized LTO/GF delivers specific capacities of 186, 179 and 175 mAh g−1 at 0.2, 0.5 and 1 C, respectively. Li-ion hybrid supercapacitors have been assembled with LTO/GF as anodes and activated carbon as cathodes, which have energy densities of 46 and 26 Wh kg−1 at power densities of 625 and 2500 W kg−1, respectively. Furthermore, the hybrid supercapacitor exhibits a superior cycle performance with capacity retention of 83% after 4000 cycles at 1 A g−1. GF sponge substrate can speed up ions and electrons transport with short diffusion lengths and large electrode/electrolyte contact area. Hierarchically structured LTO/GF electrode is lightweight, flexible, and is promising for energy storage applications