Nano Today
Volume 9, Issue 5, October 2014, Pages 668-683
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Review
Graphene and its composites with nanoparticles for electrochemical energy applications

https://doi.org/10.1016/j.nantod.2014.09.002Get rights and content

Highlights

  • The new chemistry used to synthesize high quality graphene, especially liquid phase exfoliation (LPE), is highlighted.

  • The methodologies developed to dope graphene with heteroatoms to modify and control graphene properties are outlined.

  • The general approaches to prepare graphene–nanoparticle composites are presented.

  • The applications of graphene-based nanocomposites in electrochemical energy storage (lithium based batteries and supercapacitors) and conversion devices (fuel cells and electrolyzers) are summarized.

Abstract

Graphene is a two dimensional (2D) planar and hexagonal array of carbon atoms and has been studied extensively as advanced nanomaterials for important technological applications. This review summarizes the recent developments in chemistry, materials and energy applications of graphene, doped graphene and their composites with nanoparticles (NPs). It first highlights the new chemistry used to synthesize high quality graphene. It then outlines the methodologies developed to dope graphene with heteroatoms to modify and control graphene properties. It further describes the general approaches to graphene–NP composites via either direct NP growth onto graphene or self-assembly of the pre-formed NPs on graphene surface. These graphene–NP composites provide some ideal systems for studying synergistic effects between graphene and NPs on catalysis. The review focuses on applications of graphene–NP composites in increasing electrochemical energy storage density and in catalyzing chemical reactions with much desired electrochemical efficiencies.

Introduction

Graphene is a two dimensional (2D) planar and hexagonal array of carbon atoms. Each of these carbons is sp2-hybridized and is linked together by three strong Csingle bondCsingle bondσ bonds of 120° apart. The unhybridized p-orbital is perpendicular to the sp2-hybridization plane, conjugating with the same p-orbitals on other carbon atoms via π interaction across the entire 2D plane surface [1]. With σ bond of 1.46 Å and lattice parameter of 2.46 Å [2], graphene possesses very unique electronic and mechanical properties [3]: its valence band mirrors its conduction one with two bands intersecting at its Fermi level [4], [5], giving graphene the unique zero-gap semiconductor properties with charge carrier mobility >2 × 105 cm2/Vs at an electron density of 2 × 1011 cm−2 [6], [7], [8] and thermal conductivity >3000 W/mK [9]; the strong σsingle bondπ bonds make graphene mechanically robust with its Young's modulus >0.5–1 TPa [10]. The complete planar exposure of the carbon atoms renders graphene a theoretical surface area >2500 m2/g [11], [12]. The delocalized π electrons are also responsible for various interactions between any two graphene layers and between graphene and a substrate [4]. Thus, graphene has been coupled with metals [13], [14], alloys [15], oxides [16], [17] and other polymeric compounds to form composites [18], [19], [20], [21], [22]. The properties of graphene can be further modified by the introduction of heteroatoms in the graphitic plane [23]. The replacement of a carbon atom with another atom breaks the electro-neutrality of the σ–π network within graphene, making the 2D network more susceptible to chemical reactions [24]. The new properties evolved from graphene and its hybrid structures have been explored extensively for advanced technological applications in electronics, optics, catalysis and energy storage/conversion [2], [25], [26], [27], [28], [29], [30], [31], [32], [33].

This review intends to summarize the recent developments in chemistry, materials and energy applications of graphene, doped graphene and their composites with nanoparticles (NPs). It includes both the work from the author's groups and the representative examples from others to highlight the important aspects on graphene chemistry and applications. It will first summarize new chemistry used to synthesize high quality graphene. It will also outline the methodologies developed to dope graphene with heteroatoms to modify and control graphene properties. The more controlled synthesis leading to high quality graphene and its solutions in various solvents allows for rational assembly of NPs on graphene surface, providing an ideal system to study synergistic effect between graphene and NPs on the enhanced catalytic properties. The review will focus on applications of graphene-based nanomaterials in electrochemical energy storage (lithium based batteries and supercapacitors) and conversion devices (fuel cells and electrolyzers).

Section snippets

Synthesis of graphene via liquid phase exfoliation

Synthesis of high quality graphene is key to understanding and controlling its physical and chemical properties [34], [35]. Figure 1 summarizes the methods (as well as the costs associated with the methods) used to prepare graphene. Graphite is the natural choice as a starting precursor to produce graphene as the graphitic layers are already present within the graphite structure and graphene can be obtained by simply “peeling off” these graphitic layers via mechanical and/or chemical means [36]

Doped graphene

Chemical modification of graphene with other elements has been explored extensively to improve intrinsic properties, especially catalytic properties, of graphene derivatives. It is observed that doping heteroatoms (e.g., N, B, P or S) into the graphene lattice can tune the electronic and geometric features of the resultant graphene by providing more active sites for stronger molecular adsorption [60], [61], [62]. Such doping is important to further tune graphene properties for electrocatalytic

Nanocomposites of graphene and NPs

The large surface area, electrical conductivity and mechanical strength make graphene the most promising substrate to be coupled with NP catalysts to achieve high catalytic performance for energy applications [100], [101]. Depending on the Fermi level/work function of graphene and metal NPs, interfacial charge transfer can be controlled from graphene to NPs or from NPs to graphene [102], [103]. Therefore the graphene–NP junction area can become catalytically “hot” spots to catalyze the

Applications of graphene–NP composites

Among all promising energy storage and conversion devices studied for alternative energy applications, Li-based batteries, supercapacitors and fuel cells have attracted tremendous attention [121], [122], [123], [124], [125]. The basic structure of these devices includes two electrodes (cathode and anode), separator/electrolyte (permeable for ions but nonconductive to electrons). The performance of these devices mainly depends on the physical and chemical properties of their electrode materials.

Graphene-based composite catalysts for other reactions

Graphene-based composite catalysts have also been explored to catalyze many other types of reactions. Here we only highlight a few of them that are important for energy applications.

The graphene–NP composite catalysts have been used to convert solar energy into chemical energy by splitting water into hydrogen photocatalytically. For example, rGO–ZnxCd1−xS exhibits a fast H2 production rate of 1824 μmol/hg and excellent quantum efficiency of 23.4% under a 420 nm solar irradiation, superior to that

Conclusions

Thanks to its unique chemical structure and excellent conductivity, transparency, mechanical strength, porosity and electrochemical properties, graphene has been extensively studied for various applications, especially for electrochemical energy conversion and storage. To fulfill the increasing demands of graphene, several synthesis methods have been explored in terms of the quality, mass production and cost. Presently, research has been focused on the well-controlled synthesis of graphene with

Acknowledgments

Work at Brown was supported in part by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, the Fuel Cell Technologies Program, by the U.S. Army Research Laboratory and the U.S. Army Research Office under the Multi University Research Initiative MURI (W911NF-11-1-0353) on “Stress-Controlled Catalysis via Engineered Nanostructures”; work in Peking University was supported by the NSFC-RGC Joint Research Scheme (51361165201), NSFC (51125001, 51172005), Beijing Natural

Qing Li is a postdoctoral research associate at Brown University under the supervision of Prof. Shouheng Sun. He received his Ph.D. in Chemistry from Peking University in 2010 and worked as a postdoctoral research associate at Los Alamos National Laboratory from 2011 to 2013. His research interests include functional nanomaterials and their applications in PEM fuel cells, metal-air batteries and biosensors.

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    Qing Li is a postdoctoral research associate at Brown University under the supervision of Prof. Shouheng Sun. He received his Ph.D. in Chemistry from Peking University in 2010 and worked as a postdoctoral research associate at Los Alamos National Laboratory from 2011 to 2013. His research interests include functional nanomaterials and their applications in PEM fuel cells, metal-air batteries and biosensors.

    Nasir Mahmood obtained his BS degree in 2009 in Chemistry from Punjab University and MS degree in 2011 in Materials and Surface Engineering from National University of Science and Technology, Pakistan. He joined Peking University in 2011, where he is currently pursuing his Ph.D. in Materials Science and Engineering under the guidance of Prof. Yanglong Hou. His research involves the synthesis of graphene/graphene-based nanomaterials and their application in energy storage and conversion devices.

    Jinghan Zhu received her B.Sc. in Materials Science and Engineering from the University of Science and Technology Beijing (USTB, China) in 2011. She has been pursuing her Ph.D. under the supervision of Prof. Yanglong Hou in the Department of Materials Science and Engineering at Peking University since 2011. Her research interests are the chemical synthesis of graphene based nanomaterials and their magnetic and catalytic applications.

    Yanglong Hou received his Ph.D. in Materials Science from Harbin Institute of Technology (China) in 2000. After a short post-doctoral training at Peking University, he worked at the University of Tokyo from 2002 to 2005 as JSPS foreign special researcher and also at Brown University from 2005 to 2007 as postdoctoral researcher. He joined Peking University in 2007, and now is a Professor of Materials Science. His research interests include the design and chemical synthesis of functional nanoparticles and graphene, and their biomedical and energy related applications.

    Shouheng Sun received his Ph.D. in Chemistry from Brown University in 1996. He was a postdoctoral fellow from 1996 to 1998 and a research staff member from 1998 to 2004 at the IBM T.J. Watson Research Center. He joined the Chemistry Department of Brown University in 2005. He is now the Professor of Chemistry and Engineering at Brown. His research interests are in nanomaterials synthesis, self-assembly, and applications in catalysis, nanomedicine and energy storage.

    1

    These two authors contributed equally to this review article.

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