Journal of Power Sources 358 (2017) 93e100
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Journal of Power Sources
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Enhanced hydrogen storage in sandwich-structured rGO/Co1 xS/rGO hybrid papers through hydrogen spillover
** *** Lu Han a, 1, Wei Qin b, 1, Jiahuang Jian a, Jiawei Liu b, Xiaohong Wu a, , Peng Gao c, , * Benjamin Hultman d, Gang Wu d, a School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China b School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China c College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou, Zhejiang 311121, China d Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260, United States highlights graphical abstract
A binder-free sandwich-structured
rGO/Co1 xS/rGO hybrid paper was obtained.
The Co1 xS was immobilized in be- tween the rGO sheets by the chemi- cal “bridges”. The hydrogen storage ability of rGO was enhanced by 10 through spill- over effects.
article info abstract
Article history: Reduced graphene oxide (rGO) based two-dimensional (2D) structures have been fabricated for elec- Received 18 January 2017 trochemical hydrogen storage. However, the effective transfer of atomic hydrogen to adjacent rGO sur- Received in revised form faces is suppressed by binders, which are widely used in conventional electrochemical hydrogen storage 4 May 2017 electrodes, leading to a confining of the performance of rGO for hydrogen storage. As a proof of concept Accepted 9 May 2017 experiment, a novel strategy is developed to fabricate the binder-free sandwich-structured rGO/Co1 xS/ rGO hybrid paper via facile ball milling and filtration process. Based on the structure investigation, Co1 xS is immobilized in the space between the individual rGO sheets by the creation of chemical Keywords: “ ” e e Hydrogen storage bridges (C S bonds). Through the C S bonds, the atomic hydrogen is transferred from Co1 xS to rGO e Reduced graphene oxides accompanying a C H chemical bond formation. When used as an electrode, the hybrid paper exhibits an Cobalt sulfide improved hydrogen storage capacity of 3.82 wt% and, most importantly, significant cycling stability for Hydrogen spillover up to 50 cycles. Excluding the direct hydrogen storage contribution from the Co1 xS in the hybrid paper, Binder-free electrode the hydrogen storage ability of rGO is enhanced by 10 through the spillover effects caused by the Co1 xS modifier. © 2017 Elsevier B.V. All rights reserved.
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Wu), [email protected] (P. Gao), [email protected] (G. Wu). 1 These two authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2017.05.026 0378-7753/© 2017 Elsevier B.V. All rights reserved. 94 L. Han et al. / Journal of Power Sources 358 (2017) 93e100
1. Introduction effects caused by the Co1 xS modifier.
The ever-growing demand for hydrogen energy storage appli- 2. Experimental details cations in electric vehicles and other energy storage devices has spurred significant researches in the development of high capacity 2.1. Preparation of rGO and Co1 xS electrochemical hydrogen storage materials [1,2]. There is a consensus that a breakthrough in capacity could be achieved by The rGO used in this work was purchased from Shen Zhen BRO combining transition metals with carbon-based materials [3]. Nano Technology Co., Ltd. The Co1 xS compound was prepared with Among the many candidates is graphene, a two-dimensional (2D), a high energy ball mill (FRITSCH PULVERISETTE-7). The ball milling one atom-thick carbon layer, which has been highly touted due to procedure used was based on an improved method from our pre- its excellent chemical stability, superior electrical conductivity, and vious work [18]. Briefly, the analytically pure Co and S powders high surface area. The key issue for utilizing the unique properties without further treatment were mixed at the mole ratios of 1:1, and is in assembling transition metals and graphene into a layer by layer the ball-to-powder weight ratio was 10. The mixtures were ball- nanostructure. Compared to the three-dimensional (3D) bulk milled using a high energy ball mill at the speed of 700 rpm for structure, the 2D nanostructure exhibits significant advantages, 6 h under an Ar atmosphere. Finally, the powders were collected such as larger surface area and effective electron transport chan- and analyzed. nels, which can further improve the electrochemical performance [4]. A series of transition metal (Ru, Pd, Pt, Ca, Ni and so on) 2.2. Preparation of the rGO/Co1 xS/rGO hybrid paper modified rGO materials have been fabricated [5e9]. Though these materials have enhanced electrochemical properties, they lack The Co1 xS and rGO were first blended together with the weight direct interconnection, causing a decrease in cycling stability. The ratios of 1:1, then ball milled in a ZrO2 vessel at the speed of decrease in cycling stability is mainly because the rGO sheets 700 rpm for 10 h under an Ar gas atmosphere. The high energy ball cannot effectively confine nanoparticles within the hybrid due to milling process is a suitable method that facilitates the Co1 xS their intrinsic geometrical characteristics [10,11]. anchoring and distributing in carbon matrix due to it is a particular To address this issue, there is a need to provide high activation process for perturbing of surface-bonded species by high pressure energy to the reaction system and obtain stable chemical link to enhance kinetic and thermodynamic reactions between solids (rather than weak “physical” Van der Waals interactions). More- [19,20]. After that, the rGO/Co1 xS/rGO composite was added into over, it is noteworthy that such chemical “bridges” are favorable for 100 ml distilled water and ultrasonicated using a probe sonicator atomic hydrogen transferred from transition metal atoms to adja- for 60 min. The rGO/Co1 xS/rGO hybrid paper was obtained by cent carbon surfaces via spillover and surface diffusion [12], vacuum filtration of well scattered solution through a membrane resulting in better hydrogen storage performance [13]. However, in filter. After drying at room temperature, the hybrid paper was reality, it is difficult to implement, because of the challenges in collected. partly breaking the CeC bonds without distorting the entire structure of rGO. As a result, providing extremely high energy and 2.3. Preparation of binder-free electrode introducing another nonmetal element to link them may be a possible way to avoid the distortion and breakage of the carbon Nickel foams were washed with acetone and distilled water, supporter. Using inorganic compounds formed by the transition then dried under vacuum condition at 60 C for 2 h. The rGO/ metal elements mentioned above would be the first choice to Co1 xS/rGO hybrid paper was then sandwiched between nickel consider. Recently, we have used a novel high energy ball milling foams and pressed under a pressure of 5.0 MPa. The conventional method to obtain a Co9S8-graphene electrode material, in which rGO/Co1 xS/rGO electrode with binder was prepared via plastering the addition of Co9S8 greatly increased the hydrogen storage ca- paste containing the rGO/Co1 xS/rGO hybrid and polyvinylidene pacity. However, due to the relatively large Co9S8 particle size, fluoride (PTFE) at weight ratios of 1:1 and 2:1 on nickel foams. when assembled into the electrode, we have to use binders, which not only restrict hydrogen spillover later, but also reduce the cycle 2.4. Materials Characterizations stability [14]. As a result, it still remains a great challenge to decrease the size of the used inorganic compounds to the nano- Raman scattering spectra was performed on a confocal Raman meter scale and construct a unique, highly electrically conductive, spectroscopic system (Jobin Yvon Labram HR800). X-ray photo- and chemically stable hybrid material, which could be assembled as electron spectroscopy (XPS, ESCALAB Mark II) with Al K-a X-ray an electrode without binders for the electrochemical storage of source was performed to analyse the elemental composition of hydrogen by spillover. For this purpose, Co1 xS would be a suitable samples. The morphologies of all samples were obtained by scan- starting material that facilitates the S atoms bonding with C atoms ning electron microscopy (SEM, JEOL JSM-6700F) and transmission due to it having the highest activity among all cobalt sulfides electron microscopy (TEM, HITACHI H-7000). The crystal phases of (Co1 xS, CoS2,Co3S4, and Co9S8) reported in the literature [15e17]. the samples were analysed by X-ray diffraction (XRD, Rigaku D/max Herein, we attempted to prepare a CeS bond-strengthened rGO/ IIIA). Fourier transform infrared (FT-IR, Nicolet iS10) spectra were 1 Co1 xS/rGO hybrid paper. To the best of our knowledge, rGO/ recorded in the range 400e4000 cm to characterize the chemical Co1 xS/rGO hybrid paper has not yet been applied to electro- bonds of the samples. chemical storage hydrogen by spillover phenomenon. The created chemical “bridges” (CeS bonds) between these two materials in- 2.5. Electrochemical measurements crease the direct interconnection and, hence, increase facilitation of hydrogen spillover. Through the CeS bond, the atomic hydrogen The electrochemical measurements were similar to the method can be transferred from Co1 xS to rGO accompanying a CeH reported in Ref. [19]. The electrolyte was 6.0 M KOH aqueous so- chemical bond formation. An increased hydrogen storage capacity lution. The rGO/Co1 xS/rGO negative electrode was charged for of 3.82 wt % was achieved. Excluding the direct hydrogen storage 15 h at a current density of 100 mA g 1 and discharged at 30 mA g 1 contribution from the Co1 xS in the hybrid paper, the hydrogen to 0 V. In order to study the rate capability of the samples, the storage ability of rGO was enhanced by 10 through the spillover negative electrodes were charged at a current density of L. Han et al. / Journal of Power Sources 358 (2017) 93e100 95
1 100 mA g for 15 h, and then discharged to 0 V at current density and the rGO/Co1 xS/rGO hybrid paper (Fig. S3c). The spectrum of 1 of 200e1000 mA g . All of the electrochemical hydrogen storage the rGO/Co1 XS/rGO hybrid paper shows only the peaks assigned to experiments were carried out using the battery test system (LAND S, C, O, and Co, without any other impurity. The high-resolution C 1s CT2001A) at room temperature and ambient pressure. The cyclic spectrum of rGO in Fig. 2a can be fitted to two peaks of CeC voltammetry (CV) curves were obtained via a three-electrode test (284.6 eV) and CeO (286.4 eV) [24,25]. After the composition has cell on an electrochemical workstation (PRINCETON PARSTAT taken place, the C 1s spectrum shows an overlap of the CeC, CeO, 4000). and another peak, revealing the appearance of a new CeS peak (285.6 eV) [26]. The difference between the two samples suggests 3. Results and discussion the ball milling process could produce more defects and bond the activated S and C atoms. In addition, the S 2p spectrum of Co1 xSin Fig. 1 illustrates the fabrication steps of the sandwich-structured Fig. 2b indicates the presence of two main peaks at 162.8 eV and 161.4 eV respectively, which are assigned to cobalt sulfide [27].In rGO/Co1 xS/rGO hybrid paper, wherein the most critical step is to the case of the rGO/Co1 xS/rGO hybrid paper, a small shoulder ensure homogeneous, tight coating of Co1 xS on the surface of rGO related to the SeC bond appears at 163.6 eV, which is in accordance sheets. Typically, the Co1 xS and rGO were first blended together, with the previous result [28]. The Co 2p spectra (Fig. S3d) in the then ball milled in a ZrO2 vessel at the speed of 700 rpm for 10 h under an Ar atmosphere. In the process, large rGO sheets were samples reveal the Co2p 1/2 and Co2p 3/2 spin-orbit peaks, which fi crushed into smaller pieces with much more structural defects. can be assigned to its sul de [29,30]. The slight changes in morphology and location of the main peaks and the shake-up Meanwhile, the activated Co1 xS by ball-milling was readily dispersed on the surface of the rGO, and then partly combined with satellites between the two samples suggest different valence the activated C atoms especially on the surface and defects of rGO. states and ion distribution between different S surroundings of Co During the later composition process, the bounded and activated [29,31]. These results taken together indicate the successful establishment of chemical “bridges” between rGO and Co1 xS Co1 xS particles aggregate and grow rapidly, homogeneously, and through a simple high energy ball milling method. tightly coat on the surface of rGO sheets. As a result, Co1 xSwas incorporated into the space between the individual rGO sheets, and To further demonstrate the structure of the rGO/Co1 xS/rGO the sandwich structure was formed. As shown in Fig. S1, the Raman hybrid, scanning electron microscopy (SEM) and transmission electron microscope (TEM) were employed. As shown in Fig. 3a, the spectra for rGO and the rGO/Co1 xS/rGO hybrid paper display both D band (sp3 defects induced disordering) and G band (sp2 bonded hybrid paper is considered to be the layer by layer structure. Ho- mogeneously distribute Co1 xS is embedded in the space of inter- pairs) peaks [21]. The negative shift and the slight increasing of ID/IG connected and overlapped rGO sheets (Fig. 3a inset). As shown by ratio suggest more defects are formed due to the addition of Co1 xS the TEM image (Fig. 3b and S4), Co1 xS is evenly and uniformly through the ball milling process [21,22]. In addition, the Co1 xS peaks before 1000 cm 1 are reduced, which indicates most of the coated on the surface of rGO sheets. Fig. 3b inset depicts the cor- responding selected area electron-diffraction (SAED) patterns. Co1 xS has been covered by rGO sheets. The above synthesized There are two sets of diffraction patterns which belong to the rGO/Co1 xS/rGO hybrid was dispersed in distilled water, and then ultrasonicated for 60 min. After filtrating and drying, the binder- indexed (100), (101), (102), and (110) planes of hexagonal Co1 xS and rGO, respectively. A high resolution TEM image (Fig. 3c) was free rGO/Co1 xS/rGO hybrid paper was successfully fabricated. taken at the edge of the nanosheets, which presents Co1 xS sand- Fig. S2 and Fig. 1 show the as-filtered rGO and the rGO/Co1 xS/rGO wiched between two rGO sheets. The inter-planar d spacing of hybrid paper, respectively. Moreover, the rGO/Co S/rGO papers 1 x Å could be freely rolled, showing high mechanical flexibility. 1.935 corresponds to the (102) plane of hexagonal Co1 xS crystal, fi The high-energy ball milling formation is a cost efficient process which was con rmed by the later XRD test. In the central position for producing various interesting, solid-state composites with of the sample, it can be observed that the inter-planar d spacing of finely controlled microstructures. The high pressure can perturb 1.935 Å and 2.537 Å corresponds to the (102) and (101) planes of surface-bonded species, leading to enhanced kinetic and thermo- hexagonal Co1 xS crystal (Fig. S4). This further demonstrates the dynamic reactions between solids [19,20,23]. In order to study the Co1 xS not only grew on the edge defects but also between the effect of this process, X-ray photoelectron spectroscopy (XPS) was middle of the rGO sheets. Furthermore, the distribution of Co1 xSin the hybrid was analyzed by EDS elemental mappings (Fig. 3d,e, and conducted for the as filtered rGO paper (Fig. S3a), Co1 xS(Fig. S3b)
Fig. 1. Illustration of the formation process of the binder-free rGO/Co1 xS/rGO hybrid paper. 96 L. Han et al. / Journal of Power Sources 358 (2017) 93e100
and caused slight lattices deformations. Fig. 3h shows a schematic drawing of the rGO/Co1 xS/rGO hybrid paper. The excellent dispersion benefited from the high energy ball milling process, which could facilitate the subsequent Co1 xS anchoring and distribution within the carbon matrix. The sandwiched structure, in which the Co1 xS also act as a spacer between the individual rGO sheets, is favorable for spillover atomic hydrogen from the Co1 xS layer to the surface of rGO receptor. The mechanical properties of the two kinds of papers were evaluated through tensile test as shown in Fig. 3i. Though intro- ducing the Co1 xS on the rGO should resulting in a weaker inter- layer interaction, the rGO/Co1 xS/rGO hybrid paper tested showed a relatively high tensile strength of 30.21 MPa and a strain of 1.79%, which are, to some extent, lower than those of the rGO paper (a tensile strength of 45.80 MPa and a strain of 2.22%). The linear variation indicates that the coated Co1 xS layer is ultrathin and uniform. To explore the mechanism of the rGO/Co1 xS/rGO hybrid paper during hydrogen storage, the charge/discharge curves were recor- ded. As presented in Fig. 4a, the discharge process corresponds to the H relaxation process. The hydrogen storage capabilities are calculated according to the stored hydrogen released from different samples. In the charge process, the charge potentials exceed 1.46 V quickly and then remain almost constant, which can be attributed to the hydrogenation reaction [32]. In the following discharge process, we attempt to initially isolate the voltage plateaus attrib- uted to the hydrogen stored in different location by varying the adsorption samples. As for rGO, Co1 xS, and Co1 xS/rGO mixture, no spillover is known to occur, thus the voltage plateaus we observed at about 1.11 V and 0.38 V were assigned to hydrogen reacted with the Co1 xS and adsorbed on the surfaces, respectively [18]. For the rGO/Co1 xS/rGO hybrid, we can see the appearance of the spillover voltage plateau shown at 0.50 V [33]. Most impor- tantly, as in Table S2, there is clear information that the storage capacity of the pure Co1 xS is 1.49 wt %. Given the weight per- Fig. 2. XPS spectra and peak fitting of the rGO paper, Co1 xS, and rGO/Co1 xS/rGO centage of Co1 xS in the hybrid paper (50 wt %), however, the direct hybrid paper for (a) C 1s and (b) S 2p. contribution from Co1 xS to the total capacity of 3.82 wt % is halved (only ~0.75 wt %). This would mean that approximately ~3 wt % of the hydrogen storage capacity for rGO/Co1 xS/rGO hybrid derives S5). The results clearly show that the elements of Co and S are from the rGO. As a result, the unique hybrid structure significantly distributed homogeneously, indicating that the Co1 xS coat on rGO enhances the hydrogen storage capacity of rGO by 10 from is uniform. It should be emphasized that even after 1 h of ultra- 0.30 wt % to 3.0 wt %. sonication during the preparation of the sample, very few uncoated We then use FT-IR to provide more definite evidence of spillover rGO sheets are observed, suggesting tight interaction between during the adsorption/desorption cycles. Fig. 4b shows the FT-IR Co1 xS and rGO. spectra of the rGO/Co S/rGO hybrid paper in comparison with fi 1 x Fig. S6 shows the XPS depth pro les for the rGO/Co1 xS/rGO the samples obtained at different voltage plateaus. The peak at hybrid paper. It can be seen that the major elements found on the 1577, 1041 and 635 cm 1 correspond to C]C/CeC, CeO and CeS surface were C, O, Co, and S. After initial sputtering, the O impurity vibrations, respectively [18,34]. When the rGO/Co1 xS/rGO hybrid level decreased slowly. This means the O, which is introduced into paper was subjected to hydrogen absorption (sample A), there was the reaction system through the O in the air adsorbing and reacting an emergence of CeH stretching vibrations at 2922, 2850 cm 1 and with the as-prepared samples, was fully eliminated during XPS SeH stretching vibration at 530 cm 1, signifying the formation of etching. The C, Co, and S atomic percentages had changed with the rGO-H and Co S-H [35]. In the following desorption pro- fi x 1 x x treatment time. Interestingly, the C atomic percentage rst cess, for sample B (obtained from the voltage plateaus at about fi decreased and then increased with the depth pro ling of the ma- 0.50 V), the SeH peak cannot be identified. Moreover, there was a terial by the cluster sputtering, and the percentages of Co and S at disappearance of the CeH peaks in sample C (obtained from the the same depth showed the opposite trend. Therefore, this research voltage plateaus at about 0.38 V). Fig. S7 and S8 show that when a suggests the synthesized rGO/Co1 xS/rGO hybrid was the sand- rGO and Co1 xS/rGO mixture was exposed to the same conditions wich-structure. as the rGO/Co1 xS/rGO hybrid paper, the FT-IR spectra remain Fig. 3f shows the XRD patterns of rGO, Co1 xS, and rGO/Co1 xS/ almost relatively unchanged except for the SeH stretching vibra- rGO. The strong diffraction peaks at 30.6 , 35.1 and 46.9 could be tions at 530 cm 1. These characteristic features indicate the fact indexed to the pure hexagonal Co1 xS (PDF# 42-0826), and a weak that the process of hydrogen adsorption, hydrogen dissociation, diffraction peak centered at 25.6 corresponds to (002) spacing of and subsequent spillover from the Co1 xS to the rGO are due to the rGO sheets [15]. The (102) diffraction peak position of rGO/Co1 xS/ CeS chemical “bridges”. It is noted that these observations are also rGO slightly shifted to a larger value (Fig. 3g), indicating that Co1 xS in agreement with the above studies. is incorporated into the space between the individual rGO sheets To further confirm the electrochemical hydrogen adsorption/ L. Han et al. / Journal of Power Sources 358 (2017) 93e100 97
Fig. 3. (a) SEM images of the rGO/Co1 xS/rGO hybrid paper from cross-sectional view; TEM images of (b) rGO/Co1 xS/rGO hybrid and (c) high magnification of rGO/Co1 xS/rGO; (d) and (e) EDS elemental mapping images of Co and S; (f) and (g) XRD patterns of the rGO paper, Co1-xS, and rGO/Co1 xS/rGO hybrid paper; (h) the best fit schematic model for (c); (i) Stressestrain curve of rGO and the rGO/Co1 xS/rGO hybrid paper. desorption behavior of the rGO/Co1 xS/rGO hybrid paper, the cyclic polarization, three anodic peaks appear, which are attributed to a voltammogram (CV) curve for the charge/discharge cycle was multistep desorption of hydrogen on the rGO/Co1 xS/rGO hybrid recorded as shown in Fig. 4c. In the cycle, one broad reduction peak paper. Interestingly, the three current peaks suggest the three-step of hydrogen is clearly observed at the potential of 1.10 V, corre- adsorption phenomenon of hydrogen. The first current peak sponding to the insertion of H atoms into the layers and the for- at 0.96 V is attributed to the electrochemical oxidation of the rGO/ mation of rGO/Co1 xS/rGO-Hx. During the following anodic Co1 xS/rGO hybrid, which is consistent with previous observation 98 L. Han et al. / Journal of Power Sources 358 (2017) 93e100
Fig. 4. (a) Charge-discharge curves of the rGO, Co1 xS, Co1 xS/rGO mixture and the rGO/Co1 xS/rGO hybrid paper (b) FT-IR spectra of the rGO/Co1 xS/rGO hybrid paper and the samples obtained at different voltage plateaus; (c) cyclic voltammogram curve of the rGO/Co1 xS/rGO hybrid paper.
[32]. Another peak at 0.68 V is considered to originate from two binder-free rGO/Co1 xS/rGO hybrid paper electrode compared with aspects: one is hydrogen spillover from Co1 xS to rGO and the those with PTFE. Fig. 5b demonstrates the rate capabilities of the 1 other is hydrogen entering the space between the Co1 xS and rGO three electrodes under different current densities: 200 mA g , [33]. The remaining peak at 0.48 V is considered to originate from 400 mA g 1, 600 mA g 1, 800 mA g 1, and 1000 mA g 1. It can be the hydrogen adsorbed on the surfaces of the rGO/Co1 xS/rGO found that all the high rate discharge capacities of the three elec- hybrid [36]. These provide complementary explanations for the trodes steeply decline with an increase in the current density. It is above discussion on the discharging process. It is deemed that the obvious that high discharge current density has negative impact on unique design of the sandwich-structured rGO/Co1 xS/rGO hybrid electrodes, such as accelerating the corrosion speed of the com- paper is responsible for the high electrochemical capacity of posites, so the phenomenon is consistent with the previous find- hydrogen storage. To explain the charge/discharge mechanism of ings [37e39]. In addition, the binder-free rGO/Co1 xS/rGO the hybrid, a simple reversible electrochemical reaction scheme is electrode shows much better rate capability than those with PTFE proposed: binder under every investigated current density. For instance, at the discharge current density 200 mA g 1, the rate discharge charac- rGO/Co1 xS/rGO þ xH2O þ xe 4 rGO/Co1 xS/rGO-Hx þ xOH teristic of the binder-free rGO/Co1 xS/rGO electrode (90%) is nearly 10% and 20% greater than that of electrode with 25% (81%) and 50% To evaluate the electrochemical hydrogen storage performance (73%) PTFE. Unexpectedly, even at a very high current density of 1 of the binder-free rGO/Co1 xS/rGO electrode, a cell with a Ni(OH)2/ 1000 mA g , binder-free electrode is still able to deliver high rate NiOOH counter electrode was assembled (see experiment section discharge characteristic of 74%, which is 27% higher than that of for details) and systematic electrochemical measurements were electrode with 25% PTFE binder (58%). The results above imply that made. The results were compared to rGO/Co1 xS/rGO electrodes the unique sandwiched structure of the binder-free rGO/Co1 xS/ with 25% and 50% PTFE made by the traditional technique. Fig. 5a rGO hybrid can efficiently buffer volume changes and be main- shows the maximum hydrogen storage capacities and the corre- tained well even under high current density tests. sponding cycle stabilities at a discharge current density of A typical photo of the rGO/Co1 xS/rGO hybrid electrode before 30 mA g 1. The raw rGO with a capacity of 0.30 wt % is electro- and after 50 cycles in 6.0 M KOH aqueous solution is given in Fig. 5c. chemically inactive, and the capacity of the original Co1 xSis After 50 cycles, only a small amount of precipitation can be 1.49 wt %. The capacities of electrodes with 25% and 50% PTFE are observed and the hybrid paper maintained its integrity and flexi- 2.99 wt % and 1.68 wt % (calculation based on rGO/Co1 xS/rGO). The bility, which indicates a stable formation. To further confirm the binder-free rGO/Co1 xS/rGO electrode shows much higher capacity structure stability of the rGO/Co1 xS/rGO hybrid upon cycle, we fi than those with PTFE, which could be attributed to the signi cant then investigated the morphology of the rGO/Co1 xS/rGO hybrid by spillover effect. A comprehensive comparison between the rGO/ TEM. As shown in Fig. 5deg, the 2D sheet-like morphology of the Co1 xS/rGO hybrid in our work and other reported hydrogen stor- rGO/Co1 xS/rGO hybrid still remains and the C, Co, and S elements age materials in terms of hydrogen storage capacities is shown in are uniformly dispersed over the entire range without aggregation. Table S4. The rGO/Co1 xS/rGO hybrid can deliver a comparable These results indicate that the sandwich-structured rGO/Co1 xS/ performance when compared to recent hydrogen storage materials. rGO hybrid electrode is stable during the charge/discharge cycles. Moreover, the binder-free rGO/Co1 xS/rGO electrode possesses The excellent electrochemical performance of the binder-free better cyclic stability than those with PTFE. The discharge capacity rGO/Co1 xS/rGO hybrid paper electrode is attributed to the of the rGO/Co1 xS/rGO hybrid paper electrode shows slow fading. A following reasons: (1) The high energy ball mill process strongly high discharge capacity of 3.06 wt % is still achieved after 50 cycles, promotes the formation of an intimate relationship, and results in corresponding to 80% capacity retention relative to the first-cycle the steady formation of chemical “bridges” (CeS bonds), through capacity of 3.82 wt %. Under the identical test conditions, elec- which the hydrogen confined by Co1 xS could subsequently spill- trodes with 25% and 50% PTFE exhibit much faster capacity fading, over to the rGO surface. (2) The Co1 xS layer is very thin, which is and the capacity retentions relative to the first-cycle capacity are beneficial for providing more tunnels for the spillover of hydrogen only 66% and 25%. The existence of inactive and insulating PTFE in the alkaline solution and acts as a buffer zone for releasing the would inevitably decrease the electrical conductivity and the actual stress formed in the hydrogen absorb/dis-absorb process [40]. (3) system capacity, preventing the access of hydrogen atoms to the The absence of non-conducting PTFE binder could also improve the surface of the active materials [14]. electrode's electronic conductivity. These unique characteristics Another outstanding attribute is the rate capability of the lead to the competitive advantages of the binder-free rGO/Co1 xS/ L. Han et al. / Journal of Power Sources 358 (2017) 93e100 99
Fig. 5. (a) Cycle performances and (b) Rate capabilities of the binder-free rGO/Co1 xS/rGO electrode and electrodes with PTFE as binder; (c) Digital photo of: 1 rGO/Co1 xS/rGO hybrid paper and 2 rGO/Co1 xS/rGO hybrid paper after 50 cycles; (d) TEM image of the binder-free rGO/Co1 xS/rGO hybrid after 50 charge/discharge cycles and corresponding elemental mapping images of (e) C, (f) Co, and (g) S. rGO hybrid paper electrode. the National Science Foundation (CBET-1511528 and 1604392).
4. Conclusions Appendix A. Supplementary data
In summary, Co1 xS was incorporated into the space between Supplementary data related to this article can be found at http:// the individual rGO sheets by high energy ball milling and the rGO/ dx.doi.org/10.1016/j.jpowsour.2017.05.026. Co1 xS/rGO hybrid paper was prepared by vacuum filtration. The unique structure and hydrogen storage capacities of the obtained References hybrid paper were studied in detail. The Co1 xS was highly crys- [1] L. Schlapbach, A. Zuttel, Nature 414 (2001) 353e358. tallized and homogenously and tightly coated on the surface of rGO e “ ” e [2] J.A. Turner, Science 285 (1999) 687 689. sheets through chemical bridges (C S bonds). When used as a [3] F. Bonaccorso, L. Colombo, G. Yu, M. Stoller, V. Tozzini, A.C. Ferrari, R.S. Ruoff, cathode, chemical “bridges” (CeS bonds) have been shown to be V. Pellegrini, Science 347 (2015) 1246501. effective for facilitating hydrogen spillover and, hence, hydrogen [4] F. Cao, M. Zhao, Y. Yu, B. Chen, Y. Huang, J. Yang, X. Cao, Q. Lu, X. Zhang, e e Z. Zhang, C. Tan, H. Zhang, J. Am. Chem. Soc. (2016) 6924 6927. storage. Through the C S bond, the atomic hydrogen can be [5] Z. Novotny, F.R. Netzer, Z. Dohnalek, ACS Nano 9 (2015) 8617e8626. transferred from Co1 xS to rGO accompanying a CeH chemical [6] B.P. Vinayan, R. Nagar, S. Ramaprabhu, J. Mater. Chem. A 1 (2013) bond formation, and an increased hydrogen storage capacity of 11192e11199. 1 [7] H. Zhou, X. Liu, J. Zhang, X. Yan, Y. Liu, A. Yuan, Int. J. Hydrogen Energy 39 3.82 wt % was achieved at the current density of 30 mA g . (2014) 2160e2167. Excluding the direct hydrogen storage contribution from the Co1 xS [8] Y. Wang, C.X. Guo, X. Wang, C. Guan, H.B. Yang, K.A. Wang, C.M. Li, Energy in the hybrid paper, the hydrogen storage ability of rGO was Environ. Sci. 4 (2011) 195e200. enhanced by 10 through the spillover effects caused by the [9] Y. Gao, N. Zhao, J. Li, E. Liu, C. He, C. Shi, Int. J. Hydrogen Energy 37 (2012) 11835e11841. combination of the strong interaction between the Co1 xS and rGO. [10] R. Lian, H. Yu, L. He, L. Zhang, Y. Zhou, X. Bu, T. Xu, L. Sun, Carbon 101 (2016) The results show that the fabrication method and the hydrogen 368e376. [11] S. Zhao, Y. Li, H. Yin, Z. Liu, E. Luan, F. Zhao, Z. Tang, S. Liu, Sci. Adv. 1 (2015) storage capacity of the rGO/Co S/rGO hybrid paper deserve 1 x e1500372ee1500372. further exploration for creating versatile hybrid hydrogen storage [12] S. Mukherjee, B. Ramalingam, S. Gangopadhyay, J. Mater. Chem. A 2 (2014) materials. 3954e3960. [13] H. Wang, H. Dai, Chem. Soc. Rev. 42 (2013) 3088e3113. [14] S. He, W. Chen, Nanoscale 7 (2015) 6957e6990. Acknowledgements [15] H. Wang, Y. Liang, Y. Li, H. Dai, Angew. Chem. Int. Ed. 50 (2011) 10969e10972. [16] Z. Shadike, M.-H. Cao, F. Ding, L. Sang, Z.-W. Fu, Chem. Commun. 51 (2015) 10486e10489. This work was supported by the National Natural Science [17] C. Ouyang, X. Wang, S. Wang, Chem. Commun. 51 (2015) 14160e14163. Foundation of China (Grant Nos. 51671074, 51602097, 51572060, [18] W. Qin, L. Han, H. Bi, J. Jian, X. Wu, P. Gao, Nanoscale 7 (2015) 20180e20187. and 51502062) and Excellent Youth Foundation of Heilongjiang [19] A.E. Hannora, J. Non Cryst. Solids 429 (2015) 1e4. e fi [20] C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1 184. Scienti c Committee (No. JC2015010). G. Wu acknowledges the [21] M. Yu, Y. Huang, C. Li, Y. Zeng, W. Wang, Y. Li, P. Fang, X. Lu, Y. Tong, Adv. start-up funding from the University at Buffalo (SUNY) along with Funct. Mater. 25 (2015) 324e330. 100 L. Han et al. / Journal of Power Sources 358 (2017) 93e100
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