flexible graphene composite paper

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Fabrication of self-binding noble metal/flexible graphene composite paper Yang Dai a,b, Sendan Cai a,b, Weijing Yang Jingying Xie a,b,*, Jia Zhi a, Xuemei Ju a a b

a,b

, Lei Gao

a,b

, Weiping Tang

a,b

,

Center for Electrochemistry Research and Development, Shanghai Institute of Space Power Sources, Shanghai 200245, China Shanghai Engineering Center for Power and Energy Storage Systems, Shanghai 200245, China

A R T I C L E I N F O

A B S T R A C T

Article history:

Self-binding noble metal (Pt, Au, and Ag)/graphene composite papers as large as 13 cm in

Received 1 April 2012

diameter were fabricated using a flow-directed method where in situ reduced graphene

Accepted 23 May 2012

served as a ‘‘binder’’. The papers were characterized by X-ray diffraction, scanning and

Available online 31 May 2012

transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. This approach yielded well dispersed metals with various nanostructures both on and between the graphene layers to form papers with good conductivity and flexiblility. The 300 C-annealed Ag/graphene papers were evaluated as binder-free anodes for lithium ion batteries, delivering a reversible charge capacity of 689 mAh/g at a current density of 20 mA/g.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

Graphene, a well-defined two-dimensional structure of carbon atoms, has attracted tremendous attention because of its unique electronic, thermal and mechanical properties [1–4]. Owing to its unique nanostructure and extraordinary properties, graphene layers can play a unique role as potential building blocks as well as effective ‘‘binders’’ to composite metallic, semiconducting, and polymer particles [4–10]. Flexible papers with graphene layers as building blocks have been already fabricated by flow-directed assembly method [11–15]. However, there are very few reports about free standing composite papers with three-dimensional (3D) macroscale achieved by self-binding of graphene layers and metal particles. Here, we report the flow directed fabrication of self-binding large size (U = 13 cm) and flexible noble metal/graphene paper. The method is convenient and more importantly it is

environmentally friendly. In fact, using the flow directed method to prepare self-binding noble metal/graphene papers has various advantages. Besides its simplicity, the method, on one hand, allows the in situ reduced graphene layers acting as templates and ‘‘binders’’ to confine metal particles during the process, on the other hand, allows the metal particles adhering to the surface of the graphene layers to prevent the layers from restacking. The prepared paper is flexible, with a tunable structure and metal loading. The Ag/graphene paper annealed at 300 C serves as the binder free anodes for the lithium ion battery, showing a good cyclic capability with a reversible charge capacity of 689 mAh/g. This demonstrates the potential applications of noble metal/graphene papers for energy storage, especially as binder free anodes for flexible lithium thin film batteries, and the catalyst-current collector integrated electrodes for fuel cells.

* Corresponding author: Fax: 607 255 4742. E-mail address: [email protected] (J. Xie). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.05.053

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2.

Experimental section

2.1.

Materials

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Natural graphite powder was obtained from J&K Reagents Company. L-Ascorbic acid (VC), NaOH, P2O5, KMnO4, K2S2O8, concentrated H2SO4 (98%), concentrated HCl (36.5%), AgCH3COO, HAuCl4, HPtCl4 and 30% H2O2 aqueous solutions, all of analytical grade, were purchased from Guoyao group Chemical Reagents Company.

2.2.

Synthesis of graphite oxide

Graphite oxide (GO) was prepared using a modified Hummers method [16,17]. The as-synthesized GO was suspended in water to give a brown dispersion, followed by 3 times of dialysis to completely remove the residual salts and acids. The purified GO was then dispersed in distilled water and exfoliated by ultrasonication (Branson-3500H, 350 W) to form a 1 mg/mL suspension.

2.3.

Preparation of noble metal/graphene composite paper

A mixed aqueous solution (200 mL), which contains GO (1 mg/ mL), ascorbic acid (10 mg/mL) and AgCH3COO (0.25 mg/L) or HAuCl4 (0.25 mg/L), or HPtCl4 (0.25 mg/L) was first treated by ultrasonication (40 kHz, 600 W) for 0.5 h. Unless specially mentioned, the noble metal/graphene(Au, Pt, Ag) paper refers to the one fabricated from the 0.25 mg/L metal precursor solution. Then the mixture was heated and maintained at 90 C for 45 min, during which a large amount of stably dispersed noble metal particle/graphene aqueous dispersion could be easily obtained. The aqueous dispersion was then filtered with a membrane (15 cm in diameter and 0.5 lm in pore size) to form noble metal/graphene composite paper. The resulted paper was peled and washed with enough deionized water to remove the excess VC and then put in the vacuum oven at 60 C overnight. The annealed pure graphene paper and Ag/graphene paper were obtained by putting the corresponding papers in a quartz tube at 300 C in the atmosphere of Ar 3% H2 for 3 h.

2.4.

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(k = 532.5 nm). The surface morphologies of the papers were visualized by scanning electron microscopy (SEM) using a Hitachi S4800 microscope. Transmission electron microscopy (TEM) was carried out on a JEOL JEM-2100F microscope to confirm the uniform despersion of the noble metal particles on the graphene sheets. The crystallographic structure of the prepared papers was examined by powder X-ray diffraction (XRD) on a Rigaku D/max-2600PC diffractometer with nick˚ ). X-ray photoelectron el-filtered Cu K a radiation (k = 1.5418 A spectroscopy (XPS) was performed using a RBD upgraded PHI5000C ESCA system (Perkin Elmer) with Mg Ka radiation (hm = 1253.6 eV). A Shirley background was removed from the spectra before deconvolution. The electrical conductivity of the composite papers was determined through resistance measurement with a SX1944 four point probe meter (Suzhou Telecommunication Instrument).

3.

Results and discussion

All the prepared noble metal/graphene composite papers are quite flexible. For example, Fig. 1 shows the as-prepared Ag/ graphene paper with a diameter of 13 cm and a thickness about 20 lm, which can be folded into a ‘‘paper plane’’, revealing the excellent flexibility and mechanical durability (as is shown in the inset of Fig. 1). The conductivities of the as-prepared Au/graphene, Pt/ graphene, Ag/graphene are 1457, 302, 2880 S/m, respectively, which are more than 3 orders of magnitude higher than the ever reported noble-metal promoted self-assembled 3D structures (2.5 · 10 1 S/m) [4].The conductivity depends on the type of the noble metals, contents, and annealing temperatures. Interestingly, the 300 C-annealed Ag/graphene paper, composed with 54 wt.% Ag, shows quite a high conductivity of 1 · 104 S/m. Fig. 2 shows the XRD spectra of the as-prepared graphene paper and noble metal/graphene papers. All the XRD spectra display a broad diffraction peak at around 22.5, revealing the partially exfoliated graphene. The allocated peaks of Pt, Au, Ag correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystalline planes of the FCC noble metals (JCPDS No. of Pt, Au and Ag are 04-0208, 04-0784, and 04-0783, respectively),

Electrochemical measurements

Electrodes were prepared by simply punching the 300 C-annealed Ag/graphene paper into 1.4 cm U disks. All the samples were heated at 60 C in vacuum for 10 h to remove residual moisture before being transferred to an Ar-filled glove box. Charge–discharge curves and cycle performance data for Ag/graphene paper electrodes (20 lm in thickness) assembled in a CR 2016-type coin cell were collected by a Land BT2100C battery cycler. Lithium metal foil served as the counter electrode and the electrolyte consisted of a 3:7 w/w mixture of ethylene carbonate and ethyl methyl carbonate, with lithium ions presented in the form of LiPF6 (1.5 M).

2.5.

Physical characterizations

The size of graphitic regions in the graphene paper was evaluated using a Renishaw Reflex with laser excitation

Fig. 1 – Digital camera image of the as-prepared Ag/ graphene paper with a diameter of 13 cm and the folded ‘‘paper plane’’ (insert).

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Fig. 2 – XRD patterns of the noble metal/graphene composite papers.

showing the formation of the metal particles of Au, Pt, Ag during the self-binding process. The reduction process takes only less than an hour, suggesting that Vitamin C is an effective reductant [18,19]. Broad peaks for the noble metal/graphene papers correspond to the inter-sheet spacings of 3.90, 3.71, ˚ , respectively, comparable to that of graphene paper and 3.91 A ˚ ). The variation of the d-spacing values may be attrib(3.70 A uted to the interactions between the various noble nano-particles and the graphene layers. However, no obvious peaks for graphite or GO are observed in the XRD spectra, suggesting the destruction of the regular stacking of the graphite or GO [4,12]. That can be attributed to the corrugations of reduced graphene layers or metal particles, which can prevent the

strong p–p attraction and thus reduce the restacking of graphene layers [15]. Fig. 3 exhibits the plane-view and the cross-sectional (inset) SEM images of the as-prepared pure and self-binding noble metal/graphene papers. Fig. 3(a) exhibits the crinkled surfaces of the pure graphene paper, and the loose orderly stacking of graphene layers in cross section (inset) compared to the hydrazine-reduced GO paper with a relatively more compact structure [15]. From Fig. 3(b)–(d), nano-particles are embedded uniformly in between the graphene layers in the plane view. And it can be seen from the cross-sectional view that the loose layers act as the ‘‘super conducting binder’’ to self-bind the metal particles. However, the shape of the particles and the configuration of the layers are quite different. It’s worthwhile to note that as high as 54 wt.% Ag can be loaded on the paper, suggesting the excellent composite capability of the in situ reduced graphene layers (see Fig. S1). TEM images of the heterostructure of the noble metal/ graphene dispersion in water before filtering are shown in Fig. 4. The nano-particles are scattered randomly upon the transparent carbon sheets without obvious aggregation (Fig. 4(a) and (b)) except the Pt/graphene composite (Fig. 4(c) and (d)). Few particles are spreading out off the support, indicating the majority of the particles have been bound to the in situ reduced graphene layers before filtering. And as shown in Fig. 4, the shapes and particle sizes of the metals are quite different, which can be ascribed to the different interactions between metals and graphene layers. It is interesting to see that primary Pt particles of about 2 nm pile together to form ‘‘blackberry’’ shaped secondary particles. To get a deeper insight into the particle shaping, some surfactants could be added in the future work.

Fig. 3 – SEM images of the (a) pure graphene paper, (b) Au/graphene paper (c) Pt/graphene paper (d) Ag/graphene paper (the main pictures from the plane view, and the inset pictures from the correspondent cross-sectional view).

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Fig. 4 – TEM image of the noble metal/graphene composite before filtering, (a) Au/graphene, (b) Ag/graphene, (c) Pt/graphene, (d) Pt/graphene at high magnification.

Fig. 5 shows the Raman spectra of the composite papers. The two bands at about 1349 and 1583 cm 1 correspond to the disorder-induced D band and the in-plane vibration of graphene lattice (G band), respectively. The intensity ratios of the D and G bands (ID/IG) for Au, Pt and Ag composite papers are 1.3, 1.1 and 0.9, respectively. The ID/IG ratios determined by the Raman spectra of the noble metal/graphene

Fig. 5 – Raman spectra of the as-prepared noble metal/ graphene composite papers.

papers verify the existence of significant graphitic regions and indicate the disordered graphene structures with smaller sp2 domains in the as-prepared papers [12]. The atomic ratio of carbon and oxygen (C/O) in the prepared papers is obtained by taking the ratio of C1s to O1s peak areas in XPS spectra [15]. The C/O ratios are 5.6, 4.5, 4.0 and 10.4 for Au/graphene, Pt/graphene, Ag/graphene and 300 Cannealed Ag/graphene papers respectively. As shown in Fig. 6 of C1s, four different peaks at 284.5, 286.6, 287.8, and 289 eV are observed, corresponding to sp2 C, C–O, C@O, and –O–C@O groups respectively. The difference in C/O ratios of oxygen functional groups of different noble metal/graphene papers may result from the diversified catalytic activity of the attached metal particles. The calculation of the C 1s spectra of Ag/graphene paper shows an increase in the area of sp2 C even though that of the C–O group decreases sharply after annealing at 300 C with Ar 3% H2. Note that, after the 300 C annealing, the C/O ratio increases from 4 to 10.4, suggesting that the annealing is most likely to remove the C–O group. Many metals, including noble metals such as gold and silver can be alloyed with lithium [21]. Especially, silver could form alloys with a high lithium content (up to AgLi12) and keep electrochemical activity toward lithium at very negative potentials. However, the Ag electrode suffers from mechanical strains during lithium intercalation/de-intercalation, leading to the poor cycling performance. Our 300 C-annealed Ag/

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Fig. 6 – XPS spectra of the noble metal/graphene composite papers (a) Au/graphene, (b) Pt/graphene, (c) Ag/graphene, (d) Ag/ graphene paper annealed at 300 C, with Ar: 3% H2, for 3 h.

graphene paper was evaluated as the binder-free anode for lithium ion batteries. Benefiting from the layer-by-layer structure of the self-binding electrode, the paper electrode can buffer the volume change effect of Ag during charge and discharge. Compared to the as-prepared Ag/graphene paper, the XRD spectra of the annealed Ag/graphene paper shows sharper peaks of Ag. That may be due to the well crystallization of the Ag after annealing (Fig. S2). And from Fig. S3, some small particles are visible with fog-drop shape, which may result from the recoagulation of partially melted Ag particles. However, the melting point of bulk Ag is above 960 C. The phenomena may be ascribed to the smaller size of Ag particles (<1 lm) and their interactions with graphene layers during annealing. The charge and discharge profiles of the annealed Ag/ graphene paper electrode at the current density of 20 and 50 mA/g are shown in Fig. 7. The mass loading of Ag is 35 wt.% based on the EDS analysis. The paper electrode delivers a specific discharge and charge capacity of 1195 and 689 mAh/g, with a columbic efficiency of 57.7% at the current density of 20 mA/g during the initial cycle. The initial capacity loss (42.3%) could be attributed to various causes such as the

formation of the solid electrolyte interface film and the reduction of oxygen functional groups on the surface of graphene. Similar to the previous reports of graphene paper electrodes or polymer-binded graphene electrodes [12,13], the pure graphene paper shows the hard-carbon behavior without clear plateaus during charge and discharge (Fig. S4), with initial reversible charge capacity of 273 and 210 mAh/g at the current density of 20 and 50 mA/g, respectively. The first discharge curve of the annealed Ag/graphene paper does not show clear plateaus above 0.2 V (vs. Li/Li +), and a large part of the capacity occurs below 0.5 V (vs Li/Li+) with a plateau profile. This could be associated with the lithium ion adsorption, lithium alloying with the Ag, and other irreversible reactions [12,20]. From the initial charge profile, two clear plateaus appear at 0.1 and 0.26 V, which could be assigned to the formation of AgLix (x = 2.7–3.5) and AgLix (x = 0.87–1.74), respectively [21]. Similar to the first charge curve, the second discharge curve also exhibits two corresponding plateaus at 0.05 and 0.12 V, delivering a discharge capacity of 705 mAh/g. It should be noted that, the thickness of the paper is about 20 lm, which is much thicker than the reported papers [12,13], and the electrochemical performance of the paper

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Fig. 8 – Capacity retention profile of the 300 C-annealed Ag/ graphene paper at 50 mA/g (0–3 V).

ter performance for further paper battery applications, some advanced work should be done, such as the optimization of the annealing temperature and Ag contents, or the dimension reduction and the shaping of Ag particles through adding some surfactants.

4.

Fig. 7 – Charge and discharge profiles of the 300 C-annealed Ag/graphene paper at (a) 20 and (b) 50 mA/g (0–3 V, based on the total mass of the self-binding electrode).

electrode is surely related to the thickness. However, compared to the graphene paper (Fig. S4) and previous reports [12,13], the reversible capacity has increased significantly. This could be originated from the lithium alloying with the Ag particles and the successful reduction of the restacking of graphene layers by self-binding Ag particles. Under a 50 mA/g current rate, the annealed Ag/graphene paper exhibits an initial discharge capacity of 996 mAh/g with a columbic efficiency of 46.3% and the reversible capacity is 461 mAh/g (Fig. 7b). Similar to the poor rate performance of the graphene paper electrode, the Ag/graphene paper also suffers from the kinetic barrier for the diffusion of Li+ ions out of the anode during delithiation [13]. Fig. 8 exhibits the capacity retention curve of the annealed Ag/graphene paper electrode at the current density of 50 mA/ g. The capacity levels out at about the 20th cycle, with a charge capacity retention of 339 mAh/g at the 100th cycle. The cyclic performance is comparatively better than Ag and Ag-based anodes for lithium ion batteries. This owes to the layer-by-layer Ag/graphene self-binding structure, which may buffer the mechanical strain during lithium intercalation/de-intercalation. The comparatively better performance also suggests that the Ag/graphene self-binding structure is stable during electrochemical cycling. However, to obtain bet-

Conclusions

We demonstrate a general method using in situ reduced graphene to prepare noble metal (Pt, Au, Ag)/graphene composite papers with large demension. This is a convenient and environmentally friendly synthesis approach. It is found that the in situ reduced graphene layers are easily re-bonded to confine the metal particles between them. The resulting composite papers are quite flexible, with a tunable microstructure and relatively good electrical conductivity. Evaluated as the binder-free anodes for lithium ion batteries, the 300 C-annealed Ag/graphene paper shows good electrochemical performances. A reversible charge capacity of 689 mAh/g can be obtained at a current density of 20 mAh/ g, with good cyclic capability. However, for binder-free paper electrode applications, more research effort is needed to improve the reversible charge capacity at high current rates. Our studies open up the feasibility of 3D macro-scale selfbinding graphene-noble metal particles, promoting graphene-based materials much closer to real technological applications. For example, if carefully tailored, the paper could also be applied as the catalyst-current collector integrated electrodes for fuel cells or lithium air batteries without complex fabricatation processes. In addition, the excellent composite capability of the graphene layers, can be used to ‘‘bind’’ nano-particles even micro-particles, and induce interesting properties of metal particles in the graphene layers.

Acknowledgments Financial support from the National Natural Science Foundation of China (No. 21103109) and Shanghai Science and Technology Development Funds (No. 10dz2250900) are greatly appreciated.

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.carbon. 2012.05.053.

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