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Graphene Ribbon-based Nanomaterials for Electrochemical Energy Conversion and Storage

Final Report Summary - GRAPHENE (Graphene Ribbon-based Nanomaterials for Electrochemical Energy Conversion and Storage)

In this project, we prepared a series of graphene and graphene ribbon-based materials for electrochemical energy storage and conversion.

1. According to the project plan proposed, we successfully prepared highly porous graphene materials for flexible supercapacitor device. We successfully prepared a highly porous graphene on carbon cloth (PG/CC) via an electrophoretic deposition process. With this process, in order to obtain high porous graphene on carbon cloth, fine-size graphene should be used. When large-size graphene was used, carbon cloth was only covered by graphene (G/CC) without porous surface. PG/CC was used as electrodes for Flexible All-Solid-State Supercapacitors (FASSSs). The porous structure of PG/CC electrode significantly increased the surface area of graphene and thus the specific capacitance (79.19 F g-1 for PG/CC and 32.35 F g-1 for G/CC). The macroscopic porous morphology of carbon cloth as the electrode matrix enhanced the integration between electrode and electrolyte, which is favorable for the ion diffusion and electron transport. The excellent mechanical stability and flexibility of PG/CC ensures the device with good flexibility. The resultant PG/CC based FASSSs showed high specific capacitance, good cycling stability, and enhanced energy density and power density.

2. The development of flexible, environmentally friendly, low cost, and safe energy storage devices has attracted tremendous interest for the applications of flexible electronic devices. In the second work, for the first time, we fabricated a flexible supercapacitor using the hybrid graphene/carbon nanotube (G-CNT) composites on carbon cloth (CC) as advanced binder-free electrodes. We successfully developed a graphene oxide-assisted electrophoretic deposition (EPD) method to prepare porous hybrid G-CNT layer on the carbon fiber surface of carbon cloth. In the EPD process, graphene oxide could act as charging additives to ensure the successful co-deposition of graphene and CNT. On the other hand, the existence of CNTs in the hybrid G-CNT layer on CC could prevent the reduced graphene from re-stacking and thus increase the surface area of the hybrid G-CNT/CC. And also CNTs may enhance the electrical conductivity of G-CNT. The as-fabricated flexible supercapacitor based on G-CNT/CC electrodes shows significantly enhanced supercapacitor performance in terms of specific capacitance, rate capability, and energy and power density. Besides, compared with the traditional paste/press fabrication technique of supercapacitors, G-CNT/CC obtained by the EPD process shows higher specific capacitance, higher maximum power density, and lower resistance.

Therefore, the strategy used in this work has a significant impact on the development of the electrode materials for flexible supercapacitors. It is believed that the as-prepared G-CNT/CC may also find potential applications in other fields such as dye-sensitized solar cells, fuel cells, lithium-air batteries, and lithium sulfur batteries etc.

3. Even though graphene and graphene-based nanomaterials have been extensively studied as electrode materials for various electrochemical energy conversion and storage devices, to the best of our knowledge, there is no report on the use of graphene ribbon as electrode materials for electrochemical energy devices, especially in fuel cell, either there is even no experimental reports on the electrochemical properties of graphene ribbons. Therefore, the investigation of the fundamental electrochemical properties of graphene ribbons and their applications in electrochemical energy conversion and storage devices is in significant demand. In this work, we doped graphene ribbons with nitrogen for ORR at the cathode in alkaline fuel cells. As we shall see later, the N-graphene ribbons (N-GR) showed a much better electrocatalytic activity than undoped counterparts for oxygen reduction.

4. In the fourth work, we developed a facile one-pot reaction for simultaneous reduction of graphene oxide to graphene, nitrogen doping of graphene with urea, and deposition of Pt nanoparticles on doped graphene to prepare N-doped grapheme supported Pt nanoelectrocatalysts (Pt/N-Graphene) for methanol oxidation.. The obtain Pt/N-graphene are characterized by X-ray powder diffraction, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy while the electrocatalytic activity were evaluated by cyclic voltammetry and chronoamperometry techniques. Compared with the updoped Pt/graphene catalyst, Pt/N-graphene presented excellent activity and durability towards methanol oxidation reaction contributed by the modified Pt electronic properties and enhanced interaction with support by nitrogen doping. This work provides a facile, green and economic single-step synthetic approach for the synthesis of Pt/N-graphene electrocatalyst which show significantly improved electrocatalytic activity for methanol oxidation.

5. In the fifth work, Fe2O3 supported on nitrogen-doped graphene (Fe2O3/N-rGO) hydrogel was prepared by a facial one-pot hydrothermal method. The efficient Fe2O3 loading and nitrogen doping of graphene was realized with this method. The morphology and structure of the samples were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectra, X-ray diffraction, and nitrogen isothermal adsorptiondesorption.

The chemical environment of the surface composition of the samples was recorded by X-ray photoelectron spectroscopy. The electrochemical performance was tested with a three-electrode system in the aqueous electrolyte of 1 M KOH. The electrochemical measurement demonstrated that Fe2O3/N-rGO shows a specific capacitance as high as 618 F g-1 at a discharge current density of 0.5 A g-1. Even at the current density of 10 A g-1, the specific capacitance is still as high as 350 F g-1. After 1000 cycles, the capacity retention is still maintained at 85%.

The methodologies developed through this project are very useful for the technical development of the fabrication of graphene and graphene ribbon materials and their application in the electrochemical energy storage and conversion devices.

The research results based on the above study completed this year have been published in two international journals: Journal of Materials Chemistry A and Nano Energy, and presented on one international conference.