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Enhanced Hydrogen Storage in Sandwich-Structured Rgo/Co1-Xs/Rgo Hybrid Papers Through Hydrogen Spillover

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Enhanced Hydrogen Storage in Sandwich-Structured Rgo/Co1-Xs/Rgo Hybrid Papers Through Hydrogen Spillover Journal of Power Sources 358 (2017) 93e100 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Enhanced hydrogen storage in sandwich-structured rGO/Co1ÀxS/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/Co1ÀxS/rGO hybrid paper was obtained. The Co1ÀxS was immobilized in be- tween the rGO sheets by the chemi- cal “bridges”. The hydrogen storage ability of rGO was enhanced by 10Â 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/Co1ÀxS/ rGO hybrid paper via facile ball milling and filtration process. Based on the structure investigation, Co1ÀxS 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 Co1ÀxS 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 Co1ÀxS in the hybrid paper, Binder-free electrode the hydrogen storage ability of rGO is enhanced by 10Â through the spillover effects caused by the Co1ÀxS 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 Co1ÀxS 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 Co1ÀxS 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 Co1ÀxS 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/Co1ÀxS/rGO hybrid paper modified rGO materials have been fabricated [5e9]. Though these materials have enhanced electrochemical properties, they lack The Co1ÀxS 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 Co1ÀxS 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/Co1ÀxS/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/Co1ÀxS/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 Co1ÀxS/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/Co1ÀxS/rGO electrode with binder was prepared via plastering the addition of Co9S8 greatly increased the hydrogen storage ca- paste containing the rGO/Co1ÀxS/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, Co1ÀxS 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 (Co1ÀxS, 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 À1 Co1ÀxS/rGO hybrid paper. To the best of our knowledge, rGO/ recorded in the range 400e4000 cm to characterize the chemical Co1ÀxS/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 Co1ÀxS 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/Co1ÀxS/rGO negative electrode was charged for À À 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 Co1ÀxS in the hybrid paper, the hydrogen to 0 V.
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