Recent advanced in energy harvesting and storage ...home.skku.edu/~nesel/paper files/194.pdf · Transition metal dichacogenide Hexagonal boron nitride Energy applications abstract

FlatChem xxx (2017) xxx–xxx

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FlatChem

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Recent advanced in energy harvesting and storage applications withtwo-dimensional layered materials

http://dx.doi.org/10.1016/j.flatc.2017.07.0062452-2627/� 2017 Published by Elsevier B.V.

⇑ Corresponding author.E-mail address: [email protected] (S.-W. Kim).

1 These authors contributed equally to this work.

Please cite this article in press as: S.A. Han et al., Recent advanced in energy harvesting and storage applications with two-dimensional layered maFlatChem (2017), http://dx.doi.org/10.1016/j.flatc.2017.07.006

Sang A Han 1, Ahrum Sohn 1, Sang-Woo Kim ⇑School of Advanced Materials Science & Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea

a r t i c l e i n f o

Article history:Received 30 April 2017Revised 17 July 2017Accepted 24 July 2017Available online xxxx

Keywords:Two-dimensional materialGrapheneTransition metal dichacogenideHexagonal boron nitrideEnergy applications

a b s t r a c t

Because of the depletion of existing fossil fuels and environmental pollution issues, securing sustainablegreen energy is globally becoming an important issue. To solve this problem, various complementarymeasures, such as solar cells, fuel cells and thermal power generation are being studied. Also, everythingbecomes user-centered, society is increasingly dependent on larger amount of data. In order to analyzesuch large amount of data and provide customized services to users, a small, semi-permanent powersource that is continuously driven is required. Because current technologies have limitations on life-time, size, and mechanical properties, it is very important to develop next-generation ultra-compact,light-weight energy generating devices. Two-dimensional (2D) layered materials, such as graphene,hexagonal boron nitride, and transition metal dichalcogenides have shown potential as peculiar energymaterials due to their unique properties. In this paper, we will give an overall review about recent pro-gress in energy applications of 2D-based layered structure materials. First, a brief introduction of synthe-sis method and characterization of 2D layered materials are presented. Then, the energy application of 2Dlayered structure materials will be discussed in the field of batteries, solar cells, hydrogen storage, super-capacitors, and nanogenerators.

� 2017 Published by Elsevier B.V.

Contents

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Top-down method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Bottom-up method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Structure and properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Energy applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Batteries. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Batteries based on graphene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Batteries based on TMDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Solar cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Solar cell based on graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Solar cell based on TMDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Solar cells based on h-BN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

H2 production and storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

H2 storage based on graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00H2 production based on TMDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00H2 storage based on h-BN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Supercapacitors based on graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

terials,

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2 S.A Han et al. / FlatChem xxx (2017) xxx–xxx

Supercapacitors base on TMDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Supercapacitors based on h-BN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

PleaseFlatCh

Nanogenerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Nanogenerators based on graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Nanogenerators based on TMDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Nanogenerators based on h-BN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Introduction

There are many different kinds of materials. If we classify thosematerials as zero-dimensional (0D) to 2D, carbon-based materialscan be classified as fullerene (0D), carbon nanotubes (CNT, 1D),and graphene (2D). In the last few years, because of its high appli-cability in various fields, there are many research gas been done on2D materials, especially graphene [1–3]. Graphene is consists ofcarbon atoms with sp2-bonded into a single-layer honeycomb lat-tice and has a zero-band gap semiconductor or semimetalproperties.

If we expand to compound materials, similar to graphene, theycan also be classified 0D to 2D: quantum dot (0D), nanorods, wire(1D), and nanosheets (2D). Currently the research on othergraphene-like 2D materials, such as hexagonal boron nitride (h-BN) and transition metal dichacogenides (TMDs), has made greatprogress. For example, h-BN has an atomic structure like that ofgraphene, but has very different electrical and optical properties.Molybdenum disulfide (MoS2) among TMDs have a Van der Waalsforce layered structure and show semiconducting properties. Thegeneral advantages of 2D materials are transparency, mechanicalstability, and new properties in the monolayer. We can also applythem to new types of devices, such as high-performance, flexible,transparent devices.

The 2D materials have been widely employed in various appli-cations, such as sensing, electrochemical energy storage and con-version, catalysis, composites, and transistors. In this review, wefocus on the typical applications of 2D materials in the energy con-version and storage field. To begin with, we describe the synthesis,electronic structures, and basic properties of 2D materials. Sequen-tially and emphatically, we discuss the recent advances that havemade in both the energy harvesting and the storage field, includingbatteries, solar cells, hydrogen production and storage, superca-pacitors, and nanogenerators using the 2D layered materials.

Synthesis

The fabrication methods of 2D materials can be classified asbottom-up and to p-down, including mainly three approaches,for example, mechanical exfoliation (top-down) [1,3], electro-chemical/liquid isolation (top-down) [4–8], and chemical vapordeposition (bottom-up) [9,10]. Various synthesis methods for mak-ing high performance devices using 2D materials are being studied.

Top-down method

A mechanical exfoliation method is a top-down method.Mechanical exfoliation using adhesive tapes offer to be an efficientapproach to creating high-quality monolayer samples (Fig. 1a)[11]. The advantage of this method is that it can produce high-quality samples, but they are very small, so the method can beapplied only in fundamental research and prototypical demos.Liquid-phase exfoliation method is one of the top-down methods

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to get single or multi-layer 2D nanosheets. However, these requirelengthy high-temperature treatment, and cannot produce on alarge scale. Nevertheless, the samples prepared by liquid-exfoliation are covered by the intercalation agent.

Bottom-up method

The bottom-up method is a major breakthrough in producinglarge scale 2D materials for practical applications. This method isdivided into oxide thin-film replacement and chemical vapor depo-sition (CVD). The replacement of oxide thin-film is mainly used inthe synthesis of TMDs (Fig. 1d). Mo or tungsten (W) thin films weredeposited by evaporator and then replaced with sulphur (S) orselenium (Se). In this method, the thickness of the oxide thin filmdetermines the thickness of the TMDs materials [12,13]. Howeverit is difficult to deposit an oxide thin film with an atomic-scalelayer, such as mono- or bi-layer thickness. The CVD method suc-ceeded in fabricating a large area TMDs with controllable layernumbers (Fig. 1e) [14–16]. For graphene and h-BN, there are manystudies of the large-area growth by the CVD method using copper(Cu), iron (Fe), gold (Au), ruthenium (Rh), etc., as a substrate. Fur-thermore, thin nanosheets grown by CVD method can be easilytransferred to arbitrary substrates, enabling stacking with other2D materials to fabricate heterostructures [17].

Structure and properties

2D materials have a layered structure with van der Waals forcebetween layers. They are classified into graphene with metallicproperties, h-BN with insulating properties, and TMDs with semi-conducting properties. Graphene is an allotrope of carbon and itsstrong covalent bonds provide in-plane stability of 2D crystals, asshown in Fig. 2a. Graphene is a zero-gap semiconductor or semi-metal materials, and the electronic-band structure is includes bothmetallic and semiconducting characteristics [18]. Graphene hasmany unusual properties. It has a very high intrinsic electronmobility of 2 � 106 cm2/V�s [19], a high thermal conductivity of5000W/m�K [20], a Young’s modulus of �1.0 TPa [21] and its the-oretical specific surface area is also very large. In addition, since itis composed of one layer, it absorbs very little visible light, and itstransmittance to light with a wavelength of 500 nm is 97.7% [22].Graphene is 100 times more electric than Cu, and can move elec-trons more than 100 times faster than single-crystalline silicon(Si), which is mainly used as a semiconductor.

TMDs materials are consist of a combination of two atoms; atransition metal (M, groups 4–10) and a chalcogen (X), such as sul-fur (S), selenium (Se), or tellurium (Te). Depending on the group oftransition metal, TMDs materials shows layered and non-layeredstructure. Group 4–7 transition elements has a layered structure,while the group 8–10 transition metals has non-layered structures[23]. Fig. 2b shows the general layered structure of TMDs materi-als. The thickness of TMDs are 6–7 Å, which consists of a stackedlayer combined with weak van der Waals forces [24]. The size of

rvesting and storage applications with two-dimensional layered materials,

http://dx.doi.org/10.1016/j.flatc.2017.07.006

Fig. 1. (a)–(c) The top-down methods of mass-production of these 2Dmaterials, including mechanical exfoliation, liquid isolation, and electrochemical exfoliation. (d) and (e)Bottom-up methods to produce large-area 2D materials with high quality from Refs. [12–17].

Fig. 2. Atomic structure of (a) graphene, (b) TMDs, and (c) h-BN. (d) Periodic table showing possible combinations of TMDs from Ref. [31]. (e) Electronic character of differentlayered TMDs from Ref. [23].

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the chalcogen and metal atom affect to the bonding length of theM-X atom and is shown between 3.15 Å and 4.03 Å. The uniqueelectronic properties of TMDs induced by the filling of the non-bonding d bands from the group 4 to group 10 species [23]. Whenthe orbitals are partially occupied, the TMDs exhibit metallic prop-erties, whereas when they are fully occupied, they shows semicon-ducting properties. The chalcogen atoms have little effect onelectronic structure compared to metal atoms; however, expansionof the d band reduces the band gap by increasing the number of thechalcogen atoms [24]. Fig. 2e summarizes the electronic characterof different layered TMDs [23]. The bulk TMDs material has anindirect band gap and the monolayer TMDs material has a directband gap according to both the theoretical calculations and theexperimental results [25,26].

In general, SiO2 and aluminum oxide (Al2O3), which is an oxide,are used as an insulating substrate for the research on 2D materi-als. The surface of such an oxide changes the intrinsic properties of2D materials because of charge trap effect and scattering of elec-trons caused by dangling bonds and rough surface [27]. Hence,h-BN has been attracting attention as a new substrate that com-pensates for the disadvantages of existing substrates, in order tostudy the intrinsic properties of 2D materials. The h-BN is an insu-lating material that consists of an equal amount of boron and nitro-gen with strong covalent bonding (sp2-hybridized) as shown inFig. 2c; it does not have unsaturated bonds on its surface and hasa flat structure at the atomic level. Also, it is transparent and flex-ible like graphene and has excellent mechanical properties, whichmight be comparable to those of graphene. Optical property isanother important parameter for h-BN nanosheets, especially inoptoelectronic devices. The h-BN nanosheets show very high trans-parency (transmittance over 99%) in the wavelength range of 250–900 nm, and evince a sharp absorption peak below 250 nm. Thetheoretical calculation of the optical band gap of monolayer h-BNis 6.07 eV [28]. The optical band gap of a few layer h-BNnanosheets were reported to be of 5.92 eV, and for bulk h-BN, itis about 5.2 eV [29]. It means that the optical bandgap of h-BNnanosheets reduced with an increasing number of layers, whichis associated with layer–layer interactions leading to the disper-sion of electronic bands and decline of the band gap [30].

Energy applications

Batteries

A battery consists of electrochemical energy storage devicesthat can reversibly convert chemical energy to electrical power.Among these batteries, the Lithium-Ion Battery (LIB) is one of themost widely used and is the typical battery in modern applications.The way these batteries perform depends on the properties of theelectrode materials. In a decade, many reports about the batterybased on graphene and TMDs for high-performance have beenpublished. Recently, 2D materials based on the Sodium-Ion Battery(SIB) also have been studied to overcome the limits of LIBs such ascost and energy density.

Batteries based on grapheneSince the Lithium Ion Battery (LIB) has been investigated, gra-

phite is widely used as the anode material for LIBs, because ithas several advantages, e.g., low cost, natural abundance, and highCoulombic efficiency. Han et al. calculated that graphene has amaximum theoretical capacity of 740 mAh/g on the basis of itsdouble-layer adsorption configuration, whereas graphite anodeshave a theoretical capacity of 372 mAh/g [31]. Besides, graphenecan also store lithium (Li)-ions on edges and defects, and the syn-thesized LiC2 has a capacity of up to 1116 mAh/g [32]. Experimen-

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tally, Yoo et al. revealed the capacity of graphene to be 540 mAh/g,which is much larger than that of graphite. They also increased thecapacity up to 730 mAh/g and 784 mAh/g, respectively, by theincorporating of macromolecules of carbon nanotube and buck-minsterfullerene to graphene [33]. Several researchers alsoattempted to improve capacity by adding other dopants to gra-phene. Reddy et al. reported double enhancement of reversible dis-charge capacity using Li-ion intercalation in a pristine grapheneelectrode [34]. Wu et al. also showed the possibility of a promisinganode for high power and energy LIBs as using N- or B-doped gra-phene under high-rate charge and discharge conditions (Fig. 3a).The doped graphene had a high reversible capacity of>1040 mAh/g at a low rate of 50 mA/g, as shown in Fig. 3b and c[35]. Increasing the demand for Li-ions also increases the cost ofLi-ions. Recently, 2D materials based on the SIB have been studiedto overcome the limits of LIBs, such as cost and energy density.However, there are bottlenecks to future development such asthe low capacity and poor rate. Denis et al. used graphene as anodematerial for SIBs with uniform coated antimony sulphide (stibnite).As a result, their sample had a high capacity of 730 mAh/g at50mA/g, and excellent rate capability up to 6C [36].

Batteries based on TMDsOne layer of TMDs consists of three atom layers; i.e., one layer

of metal atoms exist between two chalcogen layers, and each layeris stacked weakly by van der Waals forces. Such a structure makesLi-ions and Na-ions easily intercalated and deintercalated fromTMDs. As a result, most TMDs have higher theoretical capacitiesthan those of the graphite anode of 372 mAh/g [31]. For example,calculated MoS2 had a high theoretical capacity of 670 mAh/g[37]. Thus TMDs became promising candidates to be the electrodematerials for batteries. As shown for graphene, many results aboutthe use of TMDs in LIBs can be found in the literature. Wang et al.reported both experimental and theoretical study of MoSe2nanocrystals as the anode materials for LIBs. As the anode, thenanocrystalline MoSe2 yielded initial discharge of 782 mAh/g andcharge capacities of 600 mAh/g at a current of 0.1 C [38]. Liuet al. achieved also a high Li storage capacity of 805 mAh/g at a cur-rent of 0.1 A/g using ordered mesoporous WS2 with a large surfacearea and a narrow pore [39]. Hwang et al. reported an excellentrate capability of 53.1 A/g at 50 �C and a reversible capacity of700 mAh/g (Fig. 3d–f) using MoS2 [40]. Recently, SIBs are promis-ing alternatives of LIBs due to the abundant reserves and low costof sodium (Na)-ions. Ko et al. reported that yolk–shell-structuredMoSe2 and MoO3 microspheres. Their unique structure had initialdischarge capacities of 527 and 465 mAh/g in the voltage rangeof 0.001–3 V and discharge capacities after 50 cycles were 433and 141 mAh/g, respectively [41]. Also, Share et al. achieved highreversible capacity above 200 mAh/g using WSe2 for electrode ofthe SIB [42].

Solar cell

For several decades, Si has been the most commonly used basicmaterial for solar cells. Si has a band gap well matched to the solarspectrum, as well as a tunable work function using variousdopants, leading to easier fabrication of the PN junction. However,decreasing the device size to micron size, devices based on Si havereached their limits, for example, it is difficult to control thenanometer thickness, there is no short channel effect, and thereis extremely low light absorption for a nanometer sized height.To overcome these problems, graphene, TMDs, and h-BN areemerging as a substitute for Si, because of their unique optoelec-tronic properties and attractive application potentials, as seen inthe last ten years.



Fig. 3. (a) Schematic structure of the binding conditions of N (left side) and B (right side) in a graphene lattice from Ref. [35]. (b) Cycle performance and coulombic efficiencyas a function of cycle number of the N-doped graphene electrode from Ref. [35]. (c) Cycle performance and coulombic efficiency as a function of cycle number of the B-dopedgraphene electrode from Ref. [35]. (d) Schematic of preparation of MoS2 nanoplates from Ref. [40]. (e) Scanning electron microscope (SEM) image of MoS2 nanoplates fromRef. [40]. (f) Charge capacity as a function of cycle number from Ref. [40].

Fig. 4. (a) J-V characteristics using functionalized graphene sheets with oxygen-containing sites (red) and Pt (black) from Ref. [49]. (b) Energy diagram of the forward-biasedgraphene sheet/n-Si Schottky junction. UG (4.8 � 5.0 eV), Un-Si(4.25 eV) is the work function of graphene sheet and n-Si, respectively. V0 is the built-in potential. Ub is thebarrier height. v is the electron affinity of Si (4.05 eV). Eg is the band gap of Si (1.12 eV) and EF is the energy of the Fermi level. Vbias is the applied voltage from Ref. [50]. (c)Light J–V curves of the cells illuminated with simulated AM 1.5 G from Ref. [51].


Solar cell based on grapheneGraphene has several unique properties useful for solar cells;

e.g., it has no band gap, and has extremely high carrier mobility(�20,000 cm2 v�1 s�1) at room temperature [43], high opticaltransparence [44], and mechanical flexibility. These propertiesmake graphene a promising candidate as the electrode [45], holetransport layer [46] and the active layer for solar cells. Arco et al.reported graphene films obtained by CVD can be used to transpar-ent and conductive electrodes in organic photovoltaic cells. Solarcell with CVD-grown graphene as the transparent conductiveanode showed a power conversion efficiency of 1.8%. This PCE ishigher than that of the device commonly used ITO electrode(1.27%) [47]. Park et al. fabricated ZnO nanowire arrays on gra-phene and achieved global power conversion efficiency of 4.2%approaching the performance of ITO-based devices with similar


architectures [48]. These two results clearly showed that graphenecan replace ITO and then open up a new field of solar cells. In addi-tion, graphene can be considered as a substitute material for Pt asthe electrode of dye-sensitized solar cells (DSSCs) due to its lowcost, high optical transparence, and excellent electrocatalytic prop-erties. Roy-Mayhew et al. used graphene sheets with oxygen-containing sites as the counter electrode of a DSSC, as shownFig. 4a. Their results revealed that the DSSCs based on the graphenehad a PCE of 5.0% which was comparable to the 5.5% of Pt-basedsolar cells [49]. Xue et al. also achieved a PCE of 7.07% using theN-doped graphene as counter electrodes in DSSCs, which was com-parable to the Pt-based counter electrode with a PCE of 7.44% [50].Besides, graphene without a band gap can form the Schottky junc-tion with semiconductors and are used as the active layer for solarcells, as shown in Fig. 4b. Li et al. reported the high PCE up to 1.5%



Fig. 6. (a) Digital photographs of MoS2/GaAs and MoS2/h-BN/GaAs from Ref. [62].(b) J-V curves in the dark and under AM 1.5 G illumination of undoped and dopedMoS2/h-BN/GaAs solar cells from Ref. [62]. (c) Performance stability of the MoS2/h-BN/GaAs solar cell under AM 1.5 G illumination from Ref. [62].


as combining graphene and n-type Si wafer (Fig. 4c) [51]. And Miaoet al. achieved a PCE of 8.6% using single layer graphene with bis(trifluoromethanesulfonyl)amide/n-Si Schottky junction [52].

Solar cell based on TMDsMost TMDs have a tunable band gap. Although the bulk mate-

rial has an indirect band gap and absorbs near infrared light,single-layers TMDs are considered to be promising materials forthe solar cells. Decreasing the thickness of TMDs, TMDs have thedirect band gap due to the quantum confinement effect and thisband gap well matches the range of visible light [53]. Bernardiet al. demonstrated that the TMD materials could absorb up to5–10% of incident sunlight in a thickness of less than 1 nm andhave been shown to absorb sunlight at an order of magnitudehigher than GaAs and Si, the most commonly used solar absorbers[54]. They also calculated that a Schottky-junction solar cell con-sisting of a graphene/MoS2 stack with a PCE of �1%, whereas thatof WS2/MoS2 is 1.5%. Fontana et al. reported that a multi-layerMoS2 channel can be hole-doped by palladium contacts, yieldingMoS2 p-type transistors. Using this, they manufactured workingphotovoltaic devices with �1% PCE [55]. Pospischil et al. fabricateda p-n junction diode based on an electrostatically doped WSe2monolayer, and obtained PCE and electroluminescence efficienciesof �0.5% and �0.1%, respectively [56]. However, since the PCE ofthe solar cells that use only TMDs is lower than that of GaAs orSi, there have been attempts to improve solar cell efficiency bycombining TMDs with other materials. Tsai et al. fabricatedlarge-scale MoS2 monolayers with p-Si as photovoltaic operationwith a power conversion efficiency of 5.23%. [57] Besides, whenTMDs combine with the polymer, they have potential applicationsas solar cells. Yu et al. reported MoS2 nanomembrane–Au Schottkybarrier achieved a PCE of 1.8% [58]. Sun et al. fabricated the 2DMoS2 nanosheets as hole-extraction layers for organic solar cells.Their devices, based on P3HT:PC61BM, and PTB7:PC71BM, bothwith MoS2 nanosheets as the hole extraction layers, achieved a

Fig. 5. (a) Schematic of the graphene/MoS2/n-Si solar cell from Ref. [60]. (b) J-Vcurves of a trilayer-graphene/MoS2/n-Si solar cell with a 9 nm-thick MoS2 layer; thecurves were measured under AM 1.5 G illumination conditions from Ref. [60].Photovoltaic parameters, including the open-circuit voltage VOC, short-circuitcurrent density JSC, fill factor FF, and photovoltaic efficiency g, determined from thiscurve are indicated in the figure. Schematics of band diagrams for the solar cells.The photovoltaic processes (a) in the graphene/n-Si and (b) in the graphene/MoS2/n-Si solar cells are shown from Ref. [60].


PCE of 4.03% and 8.11%, respectively [59]. Recently, severalresearchers reported a PCE of above 10% using TMDs. Tsuboiet al. achieved a high PCE of 11.1% with the optimized trilayer-graphene/MoS2/n-Si solar cell (Fig. 5) [60].

Solar cells based on h-BNThe h-BN displays a wide band gap of 6.2 eV and is a ceramic

material with exceptional chemical and thermal stability [61].Therefore, h-BN layers have been used as interfacial layers for solarcells. Miyata et al. reported that incorporating h-BN into a MoS2/GaAs heterostructure can suppress the static charge transfer, andthe obtained MoS2/h-BN/GaAs solar cell exhibited an improvedPCE of 5.42%. Also, when they employed chemical doping and elec-trical gating into their solar cell device, a PCE of 9.03% wasachieved, as shown Fig. 6 [62]. Meng et al. introduced a few-layer h-BN to engineer the graphene/Si interface for improving aPCE for the solar cell. The h-BN acted as an effective electron-blocking/hole-transporting layer and appropriated band alignment withSi, thus the interface recombination was suppressed, and the open-circuit voltage, VOC, was remarkably increased. As a result, a max-imum PCE of 10.93% was achieved by combining an h-BN inter-layer [63]. Also, h-BN is demonstrated to be a new materials assurface passivation in DSSCs to reduce interfacial carrier recombi-nation Shanmugam et al. observed a 57% improvement the PCE ofthe DSSC using h-BN coated semiconductor TiO2 over that of thedevice without surface passivation. This passivation helped tonot only minimize recombination of electron-hole pair at theTiO2/dye/electrolyte interfaces but also significantly lower darksaturation current in the low forward bias region [64].

H2 production and storage

Hydrogen (H2) is the most promising clean and renewableenergy source as a future energy carrier because of its environmen-tal friendliness and high energy density and. To use H2 as fuel, twomain issues should be solved: production and storage. Most widelyused method for production of H2 is water splitting method. Alsothe production of H2 is affected to the hydrogen evolution reaction(HER) catalysts [65]. It is known to Pt are most used as HER cata-lysts because of their excellent catalytic activity [66]. However,their low abundance and high cost make Pt catalysts difficult for



Fig. 7. (a) Hydrogen absorption/desorption (at 200 �C and 15bar H2/300 �C and 0 bar) for the prepared rGO-Mg multilaminates from Ref. [71]. (b) Hydrogen absorption/desorption cycling of rGO-Mg multilaminates that were first exposed to air overnight. The first 5 cycles were performed at 250 �C and 15bar H2/350 �C and 0 bar, and theadditional 20 cycles at 200 �C and 15bar H2/300 �C and 0 bar from Ref. [71]. (c) XRD spectra of rGO-Mg after absorption/desorption (the bottom bars represent the XRDpatterns of Mg (red), MgH2 (pink), Mg(OH)2 (green) and MgO (blue)) from Ref. [71].


commercial applications. Storage of H2 is also an important issue.Solid-state storage is thus being investigated as an alternative. Inthis part, we introduce graphene and BN as H2 storage materialsand TMDs as H2 generation materials.

H2 storage based on grapheneGraphene has been highlighted as the best potential material

for H2 storage. By Birch reduction, few-layer graphene could storeof �5 wt% of hydrogen [67]. Besides, graphene oxide could achievethe maximum storage capacity of 4.8 wt% at 77 K and 9.0 MPapressure [68]. Lee et al. reported the gravimetric capacity of�5 wt% hydrogen using combination with Ca atoms andgraphene-based nanostructures [69]. Also, Zhou et al. revealedthe hydrogen storage capacity could be increased to 16 wt% whenLi atoms covered on both sides [70]. Recently, Cho et al. con-structed Mg nanocrystals on atomically thin and gas- selectivereduced GO (rGO) sheets for exceptionally dense hydrogen storage(6.5 wt% and 0.105kg H2 per liter in the total composite) as shownFig. 7 [71].

H2 production based on TMDsRecent studies have proven that newly emerging TMDs are a

promising, noble-metal-free electrocatalyst for HER because oftheir suitable characteristics as HER such as high chemical stabil-ity, low cost and excellent electrocatalyticactivity. Huang et al. fab-ricated the unique structure which includes vertical few-layeredMoSe2 nanosheets on SnO2 nanotubes. Their samples leaded toexcellent HER catalytic activity with a low onset potential of�0.11 V and a small Tafel slope of 51 mV per decade [72]. AndZhou et al. reported hierarchical ultrathin MoSe2�x nanosheetsexhibited excellent HER activity with a small overpotential of�170 mV, large cathodic currents, and a Tafel slope of 98 mV perdecade [73]. Besides, Zhou et al. demonstrated the constructionof MoSe2�NiSe nanohybrids with a low onset potential of�150 mV, and a small Tafel slope of 56 mV per decade[74].

H2 storage based on h-BNh-BN also is promising material for storing H2 because of higher

H2 chemisorption. Weng et al. developed a novel BN material withporous microbelts. Their special BN exhibited high and reversibleH2 uptake from 1.6 to 2.3 wt% at 77 K [75]. One year later, the samegroup reported that highly porous and sponge-like BN with ultra-high surface area up to 1900 m2/g. Their BN enables to high andreversible H2 sorption capacities from 1.65 to 2.57 wt% at 1 MPaand �196 �C [76]. Also, Lei et al. synthesized oxygen-doped atomiclayered BN nanosheets with a storage capacity of 5.7 wt% under5 MPa at room temperature [77].


Supercapacitors

Supercapacitors are a promising alternative storage because oftheir intrinsic performance advantages, such as low cost of mainte-nance, safe operation, and ultrahigh power density. Supercapaci-tors can be divided into two types; electrochemical double layercapacitors (EDLCs) and surface pseudocapacitors on electrodes.EDLCs store energy by charge accumulation at the electrode–elec-trolyte interface via polarization, and pseudocapacitors storeenergy via fast and reversible surface redox reactions [78].

Supercapacitors based on grapheneGraphene is promising as a next-generation energy storage

material because of its high volumetric specific capacitance of300 F/cm3 [79]. In 2008, Stoller et al. created a novel supercapac-itor using graphene, and their supercapacitor showed excellentperformance with specific capacitances of 99 and 135 F/g inorganic and aqueous electrolytes, respectively [80]. Yana et al.demonstrated a graphene–MnO2 composite had specific capaci-tance as high as 310 F/g at 2 mV/s (even 228 F/g at 500 mV/s)[81]. Also, Li et al. fabricated a monolithic ultra-thick and densecarbon electrode with graphene assembly for delivering highcapacitance of 150 F/cm3 in an ionic liquid electrolyte [82].Recently, Chini et al. reported improving the performance ofsupercapacitor using porous graphene with an increased numberof edges. This material had the specific capacitance with 185 F/g,especially the case of porous graphene with nano composites has357 F/g [83].

Supercapacitors base on TMDsThe study of TMD in energy storage is in primary stages com-

pared to that of the grahene. However, TMDs have been widelyused as electrode of pseudocapacitors because of their highercapacitance. Cao et al. reported finger-like MoS2 microsupercapac-itors exhibited a high area capacitance of 8 mF/cm2 and excellentcyclability, superior to that reported for graphene-based microsu-percapacitors [84]. Huang et al. achieved a high specific capaci-tance of about 576 F/g at 5 mV/s scan rate, and a good long-termcycling stability of 82% over 3000 cycles using MoS2 [85]. Wanget al. reported the synthesis of novel hybrid core/shell metal sul-fides with a conductive Ni3S2 core. When they were tested assupercapacitor electrodes, the Ni3S2@MoS2 heterostructure exhi-bits about twice the capacitance (848 F/g) of a pristine Ni3S2 sam-ple (425 F/g), excellent rate capability (46.6% capacity retention at20 A/g), and outstanding cycling stability (91% retention after 2000cycles) [86].



Fig. 8. (a) SEM images of bulk BN (inset, high resolution SEM) from Ref. [88]. (b)Transmission electron microscope image of boron nitride/reduced graphene oxide(BN/rGO) from Ref. [88]. (c) Capacitance as function of current density of RGO(black) and BN/rGO (red) from Ref. [88] (d) Cyclic performances of BN/rGO from Ref.[88].

Fig. 9. (a) Schematic diagrams of device fabrication and output voltage and current destacked 2 L-, 3 L-, and 4 L graphene triboelectric nanogenerator under a vertical compreelectric power generator and current output from the device according to the forward andelectric field applied perpendicularly to the graphene sheet induces an equibiaxial strainpiezoresponse amplitude at the resonance as a function of applied a.c. voltage from Ref



Supercapacitors based on h-BNGuo et al. demonstrated that h-BN capacitors also could be a

good specific capacitor of 6.8 mF/cm2 [87]. Gao et al. reported syn-thesized BN/rGO nanocomposites had a high specific capacitance(140 F/g at 2 A/g), good rate performance (71.5 F/g at 50 A/g), andexcellent cyclic stability (105.5% capacitance retention after 1000cycles) as shown in Fig. 8 [88]. Saha et al. fabricated BN/rGO witha high specific capacitance of �824 F/g at a current density of4 A/g, and these composite materials showed better electrical con-ductivity than the bulk h-BN did [89].

Nanogenerators

Recently, research on nanogenerators has been continuingbecause of the fuel depletion and environmental problems, andthe need for sustainable environment-friendly energy. Nanogener-ators are a type of technology that converts mechanical/thermalenergy, as produced by small-scale physical change, into electric-ity. Nanogenerators have three typical approaches: piezoelectric,triboelectric, and pyroelectric. A 2D based nanogenerator has alower output power performance than conventional piezo andtribo nanogenerators, but they are well suited for applications inself-powered electronics fields that are increasingly smaller andintegrated.

nsity from a Cu foil-grown 1 L graphene triboelectric nanogenerator and randomlyssive force of 1 kgf from Ref. [90]. (b) Schematic diagrams of water-droplet-basedreverse motion of the water from Ref. [91]. (c) Unit cells of doped atom and externaland polarization change from Ref. [93]. (d) Topography of the graphene samples and. [94].




Nanogenerators based on grapheneKim et al. first demonstrated electrical energy harvesting using

graphene by mechanical stress (Fig. 9a) [90]. In this paper, CVD-grown graphene was used as a friction material to fabricate a tribo-electric generators. One layer (1 L), two layers (2 L), three layers(3 L), four layers (4 L), and few-layer graphene grown on Cu andNi foil-based nanogenerator were fabricated, and their output volt-age and output current density were measured under mechanicalstrains. The graphene triboelectric nanogenerator based on 1 L gra-phene exhibited a high output voltage and output current densityof 5 V and 500 nA/cm2, respectively. Additionally, the regularlystacked few-layer graphene based nanogenerator shows enhancedoutput voltage and output current density to 9 V and 1.2 mA/cm2,respectively [90], Kwak et al. demonstrate large electric powergeneration using a single moving water droplet on a monolayergraphene (Fig. 9b), generating an output of 1.9 mW [91], which isalmost 100 times larger than the power output achieved in previ-ous reports [92]. Oon the surface of the polytetrafluoroethylene(PTFE) substrate, a strong negative potential was generated by tri-boelectrification between PTFE and deionized water. The triboelec-tric potential lead to the accumulation of positive and negativecharges on the top and bottom surfaces of graphene, respectively.

In general, it was thought that there is no piezoelectric phe-nomenon in graphene. However, Mitchell et al. reported thatpiezoelectric characteristics of graphene can be engineered byselective surface adsorption of various atoms by calculation(Fig. 9c) [93]. In this paper, they showed that piezoelectricity ofgraphene is formed by doping atoms in a single sheet of grapheneto destroy the inverse symmetry. Recently, it has been confirmedthat a single layer graphene on a SiO2/Si substrate exhibits a piezo-electric phenomenon. Rodrigues et al. reported that the results ofan experimental study of piezoelectrcity of single layer grapheneon SiO2 calibration grating substrates by confocal Raman spec-troscopy and piezoresponse force microscopy (PFM) (Fig. 9d)

Fig. 10. (a) Trigonal prismatic molybdenum disulfide (2H-MoS2), where Mo (transition mion structural, elastic, and piezoelectric properties of 2H-MX2, where M = Mo or W, and XRef. [95]. (b) Optical image of the single-atomic layer MoS2 flake, flexible device, operati1GO external load and short-circuit current response of a single-layer MoS2 device unstructure of the monolayer MoS2 and schematic image of the measurement configuratiozigzag edge boron nitride nano ribbon and the variation in band gaps of monolayer h-Bunder the uniaxial tensile strain from Ref. [100].


[94]. Piezoelectric phenomenon was observed on the supportedgraphene part where van der Waals and/or chemical interactionbetween the SiO2 surface and the graphene layer can induce ananisotropic strain and detectable PFM signal. The piezoelectricphenomenon of graphene layers was due to the chemical interac-tion between graphene atoms and underlying oxygen from theSiO2 substrate. Piezoelectric effect is high (d33 � 1.4 nm V�1, thatis, more than twice that of the best piezoelectric ceramics, suchas modified lead zirconate titanate).

Nanogenerators based on TMDsIt has been found that TMD materials theoretically have piezo-

electric properties. Duerloo et al. reported piezoelectric constantsof h-BN, MoS2, MoSe2, MoTe2, WS2, WSe2 and WTe2 by densityfunctional theory (Fig. 10a) [95]. After that, Wu et al. identifiedthe piezoelectric phenomenon and piezoelectric output perfor-mance of MoS2 experimentally, as shown in Fig. 10b [96]. In thispaper, they demonstrated that the array integration of single-layer MoS2 flakes with an odd number of atomic layers generateoscillating piezoelectric voltage and current outputs. A singlemonolayer MoS2 flake strained by 0.53% produce a peak outputof 15 mV and 20 pA, corresponding to a power density of2 mW�2 and a 5.08% mechanical-to-electrical energy conversionefficiency. Zhu et al. reported the observation of molecular piezo-electricity in free-standing monolayer MoS2 crystals [97]. They alsoprovided a mesoscopic method to investigate of the absolutepiezoelectric direction of the 2D crystals, which is crucial to thevalleytronic devices and edge engineering with the angular depen-dence of piezoelectricity of MoS2. Kim et al. reported directionaldependent piezoelectric effects in monolayer MoS2 grown byCVD for flexible piezoelectric nanogenerators [98]. It was foundthat the output power obtained from a nanogenerator with thearmchair direction of MoS2 is about twice that from a nanogener-ator with the zigzag direction of MoS2 under the same strain of

etal) atoms are silver, and S (chalcogenide) atoms are yellow, and trends in relaxed-= S, Se, or Te. The relaxed-ion d11 coefficient values are listed as an example from

on scheme of the single-layer MoS2 piezoelectric device, and voltage response withder periodic strain in two different principal directions from Ref. [96]. (c) Atomicn for the lateral PFM on the monolayer MoS2 from Ref. [98]. (d) Atomic structure ofN and zigzag edge boron nitride nano ribbon with different widths from 1 to 6 nm




0.48% and the strain velocity of 70 mm/s. The unique directionaldependent piezoelectric effect of the CVD-grown triangular-shaped single-crystalline monolayer MoS2 flake was qualitativelystudied by using lateral PFM (Fig. 10c).

Nanogenerators based on h-BNThe piezoelectric properties of h-BN have been theoretically

studied in a few papers [95,99,100]. Michel et al. give a theoreticallattice of 2D h-BN which is based on a crystal model with partiallycovalent and ionic bonding [93]. The calculated phonon spectra isvery close to the in-plane inelastic X-ray scattering results of pho-non dispersions in 3D h-BN [101], and to the calculation of ab initiosimulation. Qi et al. also reported that the band gaps of zigzagboron nitride nanoribbon can be changed by applying a uniaxialtensile strain within the elastic range (Fig. 10d) [100]. Furthermore,they found that the smaller the gap, the wider the nanoribbon,because there are localized edge states instead of quantumconfinement of the bulk states under the same strain [99].Unlike the previous paper [90], which used graphene as a frictionmaterial for fabricating triboelectric nanogenerators, there is notriboelectric generator research using h-BN itself as a frictionmaterial. However, Han et al. reported a simple approach for theformation of high-quality, damage-free, large-scale uniform forma-tion of Al2O3 with a balanced stoichiometry on a CVD-grown lay-ered h-BN/graphene [102]. Both the experimental and thesimulation results clearly shows the importance role of h-BN as abuffer layer for deposit dielectric layer in the fabrication ofgraphene-based electronic and energy devices. An output voltageand output current density of 1.2 V and 150 nA/cm2 were observed,respectively, under a vertical compressive force of 1 kgf.

Conclusions and perspectives

In this review, we mainly surveyed the recent progress in thefield of graphene, TMDs and h-BN. Various methods have beenused to synthesis 2D-layered materials, such as mechanical exfoli-ation, chemical exfoliation, and CVD methods. For the large areagrowth needed for real devices, the CVD method is spotlighted,but the growth mechanism is still unclear. However, because oftheir unique physical and chemical properties, 2D-layered materi-als, such as graphene, TMDs and h-BN showed enormous potentialfor new and promising energy applications, especially the 2Dmaterials used as the anode material of LIBs and SIBs, solar cells,H2 storage materials, H2 generation materials, supercapacitors,and nanogenerators. Such properties and applications of 2D mate-rials are a relatively new but exciting and rapidly expanding area ofresearch, but there are many new scientific issues which need to beovercome. Therefore, in this new field, 2D layered materials pro-vide new challenges and opportunities for researchers.

Acknowledgements

The authors acknowledge financial support from the Frame-work of International Cooperation Program managed by NationalResearch Foundation of Korea (NRF-2015K2A2A7056357) and‘‘Human Resources Program in Energy Technology” of the KoreaInstitute of Energy Technology Evaluation and Planning (KETEP),granted financial resource from the Ministry of Trade, Industry &Energy, Republic of Korea (No. 20154030200870).

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Recent advanced in energy harvesting and storage applications with two-dimensional layered materialsIntroductionSynthesisTop-down methodBottom-up method

Structure and propertiesEnergy applicationsBatteriesBatteries based on grapheneBatteries based on TMDs

Solar cellSolar cell based on grapheneSolar cell based on TMDsSolar cells based on h-BN

H2 production and storageH2 storage based on grapheneH2 production based on TMDsH2 storage based on h-BN

SupercapacitorsSupercapacitors based on grapheneSupercapacitors base on TMDsSupercapacitors based on h-BN

NanogeneratorsNanogenerators based on grapheneNanogenerators based on TMDsNanogenerators based on h-BN

Conclusions and perspectivesAcknowledgementsReferences

Recent advanced in energy harvesting and storage ...home.skku.edu/~nesel/paper files/194.pdf · Transition metal dichacogenide Hexagonal boron nitride Energy applications abstract

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