Fabrication of High-Surface-Area Graphene/Polyaniline Nanocomposites and Their Application in Supercapacitors

Graphene/polyaniline (PANI) nanocomposites were prepared by reducing graphene oxide with hydrazine in the presence of different amounts of polyaniline nanoparticles. In situ cryo-transmission electron microscope (TEM) images of a graphene oxide (GO)/PANI solution revealed that the PANI nanoparticles were anchored on the surface of the GO sheets. During the reduction, the as-adsorbed PANI nanoparticles were sandwiched between layers of graphene sheets. These PANI nanoparticles acted as spacers to create gaps between neighboring graphene sheets, resulting in a higher surface area compared to pure graphene. Graphene/PANI nanocomposites exhibited the high specific surface area of 891 m²/g. Utilizing this composite material, a supercapacitor with a specific capacitance of 257 F/g at a current density of 0.1 A/g has been achieved

Novel Pyrolyzed Polyaniline-Grafted Silicon Nanoparticles Encapsulated in Graphene Sheets As Li-Ion Battery Anodes

A simple method to fabricate graphene-encapsulated pyrolyzed polyaniline-grafted Si nanoparticles has been developed. Instead of using Si nanoparticles with a native oxide layer, HF-treated Si nanoparticles were employed in this work. The uniqueness of this method is that, first, a PANI layer over the Si nanoparticles was formed via the surface-initiated polymerization of aniline on the surface of aniline-functionalized Si nanoparticles; then, the PANI-grafted Si nanoparticles were wrapped by the GO sheets via π–π interaction and electrostatic attraction between the GO and the PANI. Finally, the GO and PANI were pyrolyzed, and this pyrolyzed PANI layer tightly binds the graphene sheets and the Si nanoparticles together in the composite. The composite materials exhibit better cycling stability and Coulombic efficiency as anodes in lithium ion batteries, as compared to pure Si nanoparticles and physically mixed graphene/Si composites. After 300 cycles at a current density of 2 A/g, the composite electrodes can still deliver a specific capacity of about 900 mAh/g, which corresponds to ∼76% capacity retention. The enhanced performance can be attributed to the absence of surface oxides, the better electronic conductivity, faster ion diffusion rate, and the strong interaction between the graphene sheets and the tightly bound carbon-coated Si nanoparticles

Hierarchical Nanocomposites of Vanadium Oxide Thin Film Anchored on Graphene as High-Performance Cathodes in Li-Ion Batteries

Hierarchical nanocomposites of V₂O₅ thin film anchored on graphene sheets were prepared by slow hydrolysis of vanadyl triisobutoxide on graphene oxide followed by thermal treatment. The nanocomposite possessed a hierarchical structure of thin V₂O₅ film uniformly grown on graphene, leading to a high specific surface area and a good electronic/ionic conducting path. When used as the cathode material, the graphene/V₂O₅ nanosheet nanocomposites exhibit higher specific capacity, better rate performance, and longer cycle life, as compared to the pure V₂O₅. The nanocomposite cathode was able to deliver a specific capacity of 243 mAh/g, 191 mAh/g, and 86 mAh/g at a current density of 50 mA/g, 500 mA/g, and 15 A/g, respectively. Even after 300 cycles at 500 mA/g, the composite electrode still exhibited a specific capacity of ∼122 mAh/g, which corresponds to ∼64% of its initial capacity. This enhanced electrochemical performance can be attributed to facile electron transport between graphene and V₂O₅, fast Li-ion diffusion within the electrode, the high surface area of the composites, and a pore structure that can accommodate the volume change during lithiation/delithiation, which results from the unique hierarchical nanostructure of the V₂O₅ anchored on graphene

Structural Modification of Graphene Sheets to Create a Dense Network of Defect Sites

Pt/graphene composites were synthesized by loading platinum nanoparticles onto graphene and etched at 1000 °C in a hydrogen atmosphere. This results in the formation of a dense array of nanostructured defect sites in the graphene, including trenches, nanoribbons, islands, and holes. These defect sites result in an increase in the number of unsaturated carbon atoms and, consequently, enhance the interaction of the CO₂ molecules with the etched graphene. This leads to a high capacity for storing CO₂; 1 g of the etched samples can store up to 76.3 cm³ of CO₂ at 273 K under ambient pressure

Facile Preparation of Graphene/SnO₂ Xerogel Hybrids as the Anode Material in Li-Ion Batteries

SnO₂ has been considered as one of the most promising anode materials for Li-ion batteries due to its theoretical ability to store up to 8.4 Li⁺. However, it suffers from poor rate performance and short cycle life due to the low intrinsic electrical conductivity and particle pulverization caused by the large volume change upon lithiation/delithiation. Here, we report a facile synthesis of graphene/SnO₂ xerogel hybrids as anode materials using epoxide-initiated gelation method. The synthesized hybrid materials (19% graphene/SnO₂ xerogel) exhibit excellent electrochemical performance: high specific capacity, stable cyclability, and good rate capability. Even cycled at a high current density of 1 A/g for 300 cycles, the hybrid electrode can still deliver a specific capacity of about 380 mAh/g, corresponding to more than 60% capacity retention. The incorporation of graphene sheets provides fast electron transfer between the interfaces of the graphene nanosheets and the SnO₂ and a short lithium ion diffusion path. The porous structure of graphene/xerogel and the strong interaction between SnO₂ and graphene can effectively accommodate the volume change and tightly confine the formed Li₂O and Sn nanoparticles, thus preventing the irreversible capacity degradation

Rate-Dependent, Li-Ion Insertion/Deinsertion Behavior of LiFePO₄ Cathodes in Commercial 18650 LiFePO₄ Cells

We have performed operando synchrotron high-energy X-ray diffraction (XRD) to obtain nonintrusive, real-time monitoring of the dynamic chemical and structural changes in commercial 18650 LiFePO₄/C cells under realistic cycling conditions. The results indicate a nonequilibrium lithium insertion and extraction in the LiFePO₄ cathode, with neither the LiFePO₄ phase nor the FePO₄ phase maintaining a static composition during lithium insertion/extraction. On the basis of our observations, we propose that the LiFePO₄ cathode simultaneously experiences both a two-phase reaction mechanism and a dual-phase solid-solution reaction mechanism over the entire range of the flat voltage plateau, with this dual-phase solid-solution behavior being strongly dependent on charge/discharge rates. The proposed dual-phase solid-solution mechanism may explain the remarkable rate capability of LiFePO₄ in commercial cells

<i>In Situ</i> X‑ray Near-Edge Absorption Spectroscopy Investigation of the State of Charge of All-Vanadium Redox Flow Batteries

Synchrotron-based <i>in situ</i> X-ray near-edge absorption spectroscopy (XANES) has been used to study the valence state evolution of the vanadium ion for both the catholyte and anolyte in all-vanadium redox flow batteries (VRB) under realistic cycling conditions. The results indicate that, when using the widely used charge–discharge profile during the first charge process (charging the VRB cell to 1.65 V under a constant current mode), the vanadium ion valence did not reach V­(V) in the catholyte and did not reach V­(II) in the anolyte. Consequently, the state of charge (SOC) for the VRB cell was only 82%, far below the desired 100% SOC. Thus, such incompletely charged mix electrolytes results in not only wasting the electrolytes but also decreasing the cell performance in the following cycles. On the basis of our study, we proposed a new charge–discharge profile (first charged at a constant current mode up to 1.65 V and then continuously charged at a constant voltage mode until the capacity was close to the theoretical value) for the first charge process that achieved 100% SOC after the initial charge process. Utilizing this new charge–discharge profile, the theoretical charge capacity and the full utilization of electrolytes has been achieved, thus having a significant impact on the cost reduction of the electrolytes in VRB