Layered Materials for Energy Storage and Conversion / Edition 1

by Dongsheng Geng, Yuan Cheng, Gang Zhang

Overview

The considerable interest in graphene and 2D materials is sparking intense research on layered materials due to their unexpected physical, electronic, chemical, and optical properties. This book will provide a comprehensive overview of the recent and state-of-the-art research progress on layered materials for energy storage and other applications. With a brief introduction to layered materials, the chapters of this book gather various fascinating topics such as electrocatalysis for fuel cells, lithium-ion batteries, sodium-ion batteries, photovoltaic devices, thermoelectric devices, supercapacitors and water splitting. Unique aspects of layered materials in these fields, including novel synthesis and functionalization methods, particular physicochemical properties and consequently enhanced performance are addressed. Challenges and perspectives for layered materials in these fields will also be presented. With contributions from key researchers, Layered Materials for Energy Storage and Conversion will be of interest to students, researchers and engineers worldwide who want a basic overview of the latest progress and future directions.

Product Details

ISBN-13:	9781788014267
Publisher:	RSC
Publication date:	02/04/2019
Series:	Smart Materials Series , #34
Pages:	328
Product dimensions:	6.15(w) x 9.20(h) x (d)

About the Author

Dr. Zhang joined Peking Universityas a full professor in 2010. He received his B. Sci and PhD in physics from Tsinghua Universityin 1998 and 2002, respectively. From 2002 to 2004 he was a SMF research fellow at National University of Singapore (NUS) and from 2005 to 2006 at Stanford University. He joined the Institute of Microelectronics, A*STAR, Singapore, as a senior research engineer in 2006 and Department of Electronics, Peking Universityin 2010 as a full professor. His research is focused on the energy transfer and harvesting in nanostructured materials. He is a world recognized expert in the electrical and thermal properties simulation of nano materials. He developed several novel approaches for molecular dynamic, and quantum chemistry simulations. He has authored or co-authored more than 70 publications in the peer-reviewed international journals and conferences, including 5 in Nano letters and Nano Today, and 1 invited review article in NanoScale. He has delivered several invited talks in the international conferences. His research has gained him a number of international recognition and media highlight. He was awarded with an Outstanding Ph.D. thesis Award in Tsinghua University(2002), Singapore Millennium Foundation Fellowship (2002-2004), and IME Excellence Award (2008). Dr. Zhang's research achievements are also matched with his competency in teaching. In 2010, his PhD student received the Chinese Government Award for Outstanding Students Studying Abroad.

Read an Excerpt

CHAPTER 1

MOF-derived Materials for Extremely Efficient Electrocatalysis

ZHONGXIN SONG, LEI ZHANG, MATTHEW ZHENG AND XUELIANG SUN

1.1 Introduction

Nanostructured materials such as porous carbon, metal/metal oxides nanoparticles (NPs), and their composites have been widely investigated in the field of electrocatalysis. Metal–organic frameworks (MOFs) as precursors and/or templates for the design of functional porous materials have become a rapidly expanding research area in recent years. As their name suggests, MOFs are constructed by periodic coordination of metal ions and organic ligands to form one to three-dimensional (3D) networks. The organic linkers are enormously diverse and have a variety of configurations. The vast numbers of metal ions and organic linkers as well as their diverse assemblies thus have led to the reporting of more than 20 000 MOFs. In addition to their adjustable compositions, another distinctive advantage of MOFs is their ordered pore structure, uniform pore size, and high specific surface area in contrast to traditional microporous and mesoporous materials. The pore size of MOFs can be adjusted from a few angstroms to nanometres, which enables the access of reactants and removal of products within a specific size, ensuring rapid mass diffusion and transport.

The remarkable advantages of MOF-based materials in catalysis mainly arise from the inherent properties of MOF precursors: MOFs possess highly dense and uniformly dispersed active sites; the high surface area, porous structure, and open channels facilitate rapid mass transport and diffusion. These advantages enable MOF-based nanomaterials to be promising solid catalysts, especially in electrochemical catalysis. However, the coordinatively unsaturated metal sites and nonconductive organic ligands limit MOFs to certain electrocatalytic reactions. Fortunately, this challenge can be alleviated by two approaches: (i) functional modification: it is possible to convert the metal ions into metal/metal compounds while carbonizing the organic linker into a conductive carbon support by a precise post-treatment modification. (ii) Pore encapsulation: MOF structures can incorporate various catalytic active species into their pore space and behave as nanoreactors to host catalytic reactions. Moreover, the obtained nanocomposites derived from MOF precursors display high surface area, porous structure, and uniformly dispersed active sites, which were found to be important properties in electrocatalysis. In this chapter, we describe several unique structures and compositions of MOF-derived materials, and then highlight the recent progress of MOF-derived nanocomposites for electrocatalysis. Finally, the major challenges of MOF-based materials and their research opportunities for further development in electrocatalysis are discussed.

1.2 MOF-derived Materials: Structures and Compositions

Recently, MOFs have gained increasing attention due to their ordered framework and porous structures. Enabled by their diverse structures, high surface area, and permanent porosity, MOFs are considered as novel precursors to construct functional materials such as nanoporous carbon, nano-metal compounds (e.g. metal oxides/sulphides/carbides), and their composites. Generally, MOF crystals with controllable size, shape, and compositions can be obtained by incorporating the desired metal ions and organic ligands during the MOFs' synthesis. Subsequently, post-synthetic modification can be used to treat MOF precursors and transfer the precursors into MOF-derived materials with diverse nanostructures and compositions. This section describes the multiple nanostructures (e.g. porous, core–shell, hollow structures, and 3D composites structure, as shown in Figure 1.1) and compositions designed from novel MOF precursors, with an emphasis on their attractive properties and unique functions for boosting electrocatalysis.

1.2.1 Structures of MOF-derived Materials

1.2.1.1 MOF-derived Porous Carbon

MOFs can be used as ideal sacrificial templates to construct diverse nanostructured materials such as porous carbon, metal compounds, and their composites. These MOF-derived materials can inherit the advantages of MOF precursors, especially their high surface area and tailorable porosity. As an example, Yamauchi et al. used Zn-based MOFs (zeolitic imidazolate framework, ZIF-8) as self-sacrificing templates for the preparation of nitrogen-doped porous carbons (NPCs). The resulting NPCs not only maintained the polyhedral morphology of the ZIF-8 precursor, but also exhibited a large surface area (up to 1110 m2 g-1) and hierarchical porosity. The NPCs maintained an average pore size with a diameter of 1.06 nm, which was much closer to the pore size of the parent ZIF-8. By delicate design of the MOF precursor, for example, integrating the properties of ZIF-8 and ZIF-67 nanocrystals, the core–shell structured ZIF-8@ZIF-67 polyhedrons with ZIF-8 as the core and ZIF-67 as the shell can be well defined. By choosing ZIF-8 seeds with different sizes, the core size of ZIF-8 can be tuned. Simultaneously, varying the feeding molar ratio of Co2+ : Zn2+, the shell thicknesses of ZIF-67 can be adjusted. After thermal treatment of a ZIF-8@ZIF-67 precursor and then Co removal by acid etching, the functionalized nanoporous carbon hybrid with a core structure of nitrogen-doped porous carbon and shell of highly graphitic carbon (GC) was obtained. In particular, the resultant NPC@GC core–shell materials possessed an interconnected hierarchically micro/mesoporous structure due to the carbonization of organic linkers and removal of metal atoms from the parent ZIF-8@ZIF-67. These results suggest the possibility of designing MOF-derived nanocarbon with a porous character partially inherited from the parent MOFs.

1.2.1.2 MOF-derived Hollow Structures

Hollow structures with a high specific surface area (external and internal surface), enhanced material utilization and efficiency, as well as high stability, have been considered as one of the most attractive structures for energy storage and conversion application. Construction of hollow structures with tuneable architectures can be enabled by using MOF strategies. Recently, Guan et al. reported the ZIF-7 strategy for the fabrication of hollow structured cobalt/nitrogen-doped carbon (Co/NC) materials. In their work, ZIF-67 crystals were synthesized and self-assembled on the surface of polystyrene spheres (PS) to construct yolk–shell PS/ZIF-67 composite spheres. Subsequently, by a controlled pyrolysis, the PS/ZIF-67 composite was transformed into hollow particles with a single hole on the surface of the shell. During high temperature PS/ZIF-67 pyrolysis, the ZIF-67 micro-shells were transformed to Co/NC, while isolated Co nanocrystals were covered by a porous carbon layer. The PS sphere, a thermally degradable template, was decomposed and generated a strong gas outflux of hydrocarbon, leading to the formation of a single large-through hole on the shell of the resultant Co/NC spheres. Importantly, the authors found that the open size of the hole has a relationship with the heating rate of the pyrolysis. A higher heating rate decomposes PS more rapidly and results in a large-sized hole on the final hollow Co/NC shells.

By appropriate design of the composition of MOF precursors and by carefully controlling their subsequent post-treatment, hollow materials with unique architectures can be achieved. For instance, yolk–shell hollow structures and multi-shelled hollow polyhedrons have been fabricated by innovative MOFs' synthesis and post-synthetic modification. In 2012, Kuo and co-workers coated pre-synthesized palladium (Pd) nanocrystals with a Cu2O layer, then in situ synthesized an outer shell of polycrystalline ZIF-8. The clean Cu2O layer with a capping-agent-free surface contributed to the growth of a ZIF-8 shell, which was simultaneously dissolved by the protons' environment during the ZIF-8 synthesis. The trace amount of Cu2O residue can be removed by treatment with a solution of 3% NH4OH in methanol, thus forming the obvious void between the metal Pd cores and the ZIF-8 shells. The morphology and property of the metal nanocrystals were well preserved during the coating of the ZIF-8 shell. Through this strategy, a series of metal nanocrystals@ZIF-8 yolk–shell composites can be constructed. The yolk–shell structures suitably incorporate the functions of metal cores, porous shells, and the cavity between the core and shell, which provides a typical example in the rational design MOF-derived materials with hollow architectures. Recently, Lou et al. presented a two-step method to synthesize box-in-box double-shelled nanocages with different shell compositions. In their experiment, the uniform Co-based ZIF-67 NPs were synthesized in the first step. The ZIF-67 NPs were then dispersed in an ethanol solution of Ni(NO3)2 to generate ZIF-67/Ni-Co layered double hydroxides (LDH) yolk–shelled structures. After thermal calcination in air, the ZIF-67 cores and Ni-Co LDH shells can be further transformed into Co3O4 and NiCo2O4 nanocages, respectively. Both the Co3O4 inner shell and the NiCo2O4 outer shell can remain intact after thermal treatment, resulting in Co3O4-NiCo2O4 double-shelled nanocages. This two-step strategy can be applied to ZIF-67 NPs with different particle sizes, thus giving rise to the size-controlled synthesis of Co3O4-NiCo2O4 double-shelled nanocages. Several kinds of double-shelled nanocages with inner Co3O4 and outer metal oxide (e.g. Fe/Mg/CoOx) nanocages can be developed via this highly versatile strategy.

1.2.2 Compositions of MOF-derived Materials

1.2.2.1 Metal-free Nanocarbon

Due to the presence of carbon-containing organic linkers in MOF crystals, nanocarbon materials are easily constructed through carbonization of MOF precursors and removal of metal species. Thus far, several Zn-based MOFs, such as ZIF-8, MOF-5, and MOF-74 have been demonstrated to be promising self-sacrificial precursors to produce metal-free nanocarbon. Generally, The zeolitic imidazolate frameworks (ZIFs) have been adopted as both templates and precursors to develop nitrogen-doped porous nanocarbons, due to their highly ordered porosity and organic ligands with rich nitrogen. In 2014, Zhang et al. demonstrated the in situ synthesis of NPCs by high temperature pyrolysis of a ZIF-8 precursor. This approach enables the simultaneous incorporation of carbon and nitrogen species, resulting in the formation of NPCs after the carbonization of a ZIF-8 precursor. Additionally, the remarkable characteristics of ZIF-8 such as being rich in nitrogen, having hierarchical porosity, as well as ordered 3D networks, could be passed to the derivative material of NPCs (Figure 1.2a). Moreover, it was revealed that the total nitrogen content in NPCs could be controlled from 23.9 to 5.82 at% when the carbonization temperature was increased from 700 to 1000 °C. Considering that organic ligands are also composed of various heteroatoms (N, P, S, etc.) other than carbon, heteroatoms-doped nanocarbon with highly dispersed doping sites could also be designed from MOF precursors. To further tune the electronic properties and conductivity of nanocarbon, incorporating multi-heteroatoms into the MOF-derived nanocarbon has been increasingly investigated in recent years. For example, Sun et al. reported the facile synthesis of MOF-derived nanoporous carbon couple-doped with nitrogen and sulphur atoms. Thiourea was used as a sulphur precursor, which was encapsulated into the pore structures of a ZIF-8 template (as a carbon and nitrogen source). Under a high-temperature carbonization process, nitrogen and sulphur couple-doped nanoporous carbon (N/S-NPC) was produced (Figure 1.2b). The physical characterizations indicated that N/S-NPC demonstrated a high specific surface area, graphitic and porous structure, and was rich in nitrogen and sulphur doping sites. The X-ray photoelectron spectroscopy indicated that 5.4 at% nitrogen and 0.3 at% sulphur were doped in the as-prepared N/S-NPC. Significantly, it was revealed that the MOF-derived N/S-co-doped nanocarbon exhibited excellent catalytic activity for the oxygen reduction reaction (ORR).

1.2.2.2 Transition Metal/Metal Compound-decorated Nanocarbon

MOFs are one class of compounds consisting of metal ions strongly coordinated to organic ligands to form one-, two- or three-dimensional structures. Based on such composition, MOFs can be converted into metal-decorated nanocarbon materials (e.g. metal/carbon, metal/metal-compound/ carbon) by pyrolysis treatment at a range of temperatures between 400–1000 °C. Yamauchi's group synthesized cobalt-decorated N-doped porous carbon (Co/NPC) by pyrolysis of a ZIF-67 precursor at 600–800 °C under N2 atmosphere. It was found that the rise in annealing temperature from 600 to 800 °C generated an increase in the crystallization of the prepared Co NPs. It was found that higher temperatures contribute to better graphitization of the porous carbon matrix and results in enhanced conductivity and stability for the as-prepared Co/NPC. This approach allowed the combination of the catalytic active sites of Co NPs with the conductive and porous N-doped nanocarbon matrix, which was considered as a potential material for electrocatalysis. Known from the metal–organic composition, the MOF precursors can be used to develop (i) metal NPs-doped carbon hybrids, or (ii) porous metal oxide materials. Li et al. used a one-step air calcination strategy to directly convert a Co-MOF (ZIF-67) precursor into hierarchically porous Co3O4 with honeycomb-like architectures. The Co3O4 architectures displayed abundant porosity and oxygen vacancy, which exhibit distinct electrocatalytic activity for the oxygen evolution reaction (OER). As is commonly understood, the metal oxide NPs tend to agglomerate under electrochemical catalysis conditions, which is seriously detrimental to their activity and stability. Encapsulation of these metal oxide species into a porous stable substrate has been considered as a potential method to develop highly stable metal hybrids due to the pore entrapping effect. Hou et al. embedded Co3O4 NPs in N-doped porous carbon (Co3O4/NPC) by a two-step thermal decomposition of ZIF-67. Firstly, the ZIF-67 precursor was heat-treated in an inert atmosphere to obtain Co/NPC. Then, by oxidation of Co3O4/NPC in air at 350 °C, Co3O4/NPC was finally achieved. The developed Co3O4/NPC product maintained the geometry of the parent ZIF-67 framework and generated highly stable Co3O4 NPs. The novel integration of Co3O4 NPs into N-doped carbon networks demonstrated a strong synergistic effect between Co3O4 and NPC, which enhanced the intrinsic properties of each individual component.

Apart from the metal oxide derivatives, metal phosphides (MPs) have gained great attention because of their excellent activity in electrocatalysis applications. In comparison to traditional synthetic methods of MPs, MOF-oriented approaches provide the opportunity to tailor MP materials with a high surface area and well-defined porous structure. In 2015, You et al. reported a ZIF-67-derived synthetic route to prepare porous CoPx/NC polyhedrons composed of CoP and Co2P NPs embedded in a N-doped carbon matrix (as shown in Figure 1.3a). In their experimental synthesis, the ZIF-67 precursors first underwent pyrolysis in an Ar atmosphere to obtain Co–N–C polyhedrons, which were subjected to phosphating at 300 °C with an extra phosphorous source (sodium hypophosphite monohydrate) to achieve the resultant CoPx/NC polyhedrons. The X-ray diffraction (XRD) patterns indicated that metallic Co NPs with a crystallite size of B33 nm were transformed into CoPx NPs (composed of CoP and Co2P) after the phosphating reaction. Energy dispersive X-ray spectroscopy (EDS) mapping confirmed the presence of Co, P, N, and C components in the resultant Co-Px/NC, and showed evidence that the P element is highly localized within the region of Co, indicating the formation of CoPx NPs.

(Continues…)

---

Excerpted from "Layered Materials for Energy Storage and Conversion"
by .
Copyright © 2019 The Royal Society of Chemistry.
Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

---

Table of Contents

MOF-derived Materials for Extremely Efficient Electrocatalysis;
Two-dimensional Layered Materials for High-performance Lithium-ion Batteries;
Intercalation-based Layered Materials for Rechargeable Sodium-ion Batteries;
Ionic Liquid Electrolytes for Graphene-based Supercapacitors with an Ultrahigh Energy Density;
Properties and Applications of Layered Thermoelectric Materials;
Graphene–Carbon Nanotube Hybrid Films for Highperformance Photovoltaic Devices;
Metal–Organic Frameworks (MOFs) as Potential Hybrid Ferroelectric Materials;
Synthetic Techniques and Functionalization Approaches of 2D Transition Metal Dichalcogenides;
High Electrocatalytic Performance of Twodimensional Layered MoS2-based Materials for the Hydrogen Evolution Reaction