Directional Flow-Aided Sonochemistry Yields Graphene with Tunable Defects to Provide Fundamental Insight on Sodium Metal Plating Behavior

Transcription

1 SUPPLEMENTARY INFORMATION Directional Flow-Aided Sonochemistry Yields Graphene with Tunable Defects to Provide Fundamental Insight on Sodium Metal Plating Behavior Wei Liu a *, Peiyu Li a, Wenwu Wang b, Ding Zhu a, Yungui Chen a,b *, Songlin Pen a, Eunsu Paek c, David Mitlin a,c * a: Institute of New-Energy and Low-Carbon Technology, Sichuan University, Chengdu, Sichuan, China, b: College of Materials Science and Technology, Sichuan University, Chengdu, Sichuan, China, c: Chemical & Biomolecular Engineering and Mechanical Engineering, Clarkson University, Potsdam, NY, USA * weiliu@scu.edu.cn, *chenyungui@scu.edu.cn, *dmitlin@clarkson.edu 1

2 S1. Detailed Experimental Procedure 1.1 Materials Flake graphite powder (99.95%, metal basis, Aladdin Chemicals) was used as the graphene precursor. All chemical, including N-Methyl Pyrrolidone (NMP, 99.9%, Aladdin Chemicals), filter paper, etc. are received as is from Aladdin Chemicals. 1.2 Apparatus set-up for IPS/AES exfoliation A self-designed apparatus was adopted in current study as shown in Scheme 1. And a photograph of the self-designed apparatus featured as a convergent slit is shown in Figure S1. It is known that disk-like particles are prone to align along the flow with respect to the strain velocity distribution due to elongation or/and shear strain. 1-3 Governed by rheological laws, the suspended graphite platelets can be induced to align along the flow with respect to the slit geometry when flowing through the convergent slit. Simultaneous to the pump-driven cyclic flowing of suspension, sonication shockwave was imposed to the aligned particles with propagation direction perpendicular or parallel to the oriented graphite platelets, denoted as Inplane sonication (IPS) or At-edge sonication (AES) respectively. Figure S1: Experimental set-up of FAS apparatus, 1 Quartz flow channel with convergent-slit configuration, 2 sonication probe, 3 thermocouple, 4 pipeline for cyclic flow driven by a peristaltic pump with green arrows representing flow direction, 5 water bath. 2

3 Rheological principles: Regarding the design and control on the flow induced structures, fluid dynamics are at the core of this methodology. 4-6 Particles suspended in a fluid are prone to align with respect to shear strain when the strain energy associated with the flow overrules their thermal energy. The Peclet number, P e, is a quantification of these variables: P e = 6πηa3 γ k B T (1) Where η is the viscosity of the suspension, a is the characteristic dimension of the particles, γ is the strain rate and k B is the Boltzmann Constant, T is the absolute temperature. The dominator k B T corresponds to the thermal energy of the particles in suspension, which leads to the random Brownian motion and spontaneous isotropic orientation, while the numerator 6πηa 3 γ represents the intensity of flow strain energy, which is the driving force to alignment. In general, if the Peclet number is greater than unity, i.e. the flow strain energy overrides the thermal energy of the suspended particles, orientation of particles is thermodynamically favored. Based on these flow dynamics, several factors such as viscosity of the suspension, characteristic dimension of graphite, and strain rate needs to be known to guide the random to orientated transition. As characterized by rotational rheometer and SEM analysis (Figure S2), the viscosity of the suspension is determined to be 1.91*10-3 Pa s, and the average diameter of the graphite platelets is estimated to be 100 μm. Figure S2: Rheological characteristics of graphite/nmp suspension used in this study (left); SEM micrograph of the graphite platelets employed as a precursor. 3

4 Given the uncompressible Newtonian fluid nature of the graphite/nmp suspension, the shear rate in the slit flow is only correlated with the volume flow rate and the slit geometry. From the engineering perspective, contribution of wall-friction to volume flow can be neglected thus the shear-rate of the flow can be given as follows: γ = 6q v WH 2 (2) Where q v is the volume flow rate controlled by the pump, and W, H are the width and the height of the slit respectively. It should be noted that another important factor should also be considered when using flowdevice to induce particle orientation, which is the transition of laminar flow to turbulent flow. The Reynolds number provides a useful indication as to when the laminar-to-turbulent transition will happen: R e = ρvl η (3) where ρ is the density, V is the flow velocity, and L is the characteristic dimension of the flow tunnel. For the slit flow, L is equal to the distance between the plates (H). For flow in a constrained tunnel, laminar flow occurs when R e < 2300 and the flow becomes fully turbulent when R e > Flow conditions and Yield In a representative run, 2.4g of graphite was dispersed in 80ml NMP to form suspension, which was circulating pumped to flow through the slit. The width and height of the slit are 6mm and 1mm, respectively. Three 6mm long slits with W = 6mm and H = 1mm were designed in parallel with respect to flow direction (as shown in Figure 1 and Figure S1). The volume flow was controlled at ~ 4.5ml/s. Assuming even flow distribution in three slits, the flow velocity (V) was 0.25m/s at each slit with a shear rate (γ ) 1500s -1. The Peclet number, P e is calculated to be over ~10 10, indicating overwhelming dominance of the flow strain energy over the thermal energy, causing highly ordered graphite alignment in the slit. The Reynolds number R e was 130 in the slit, giving fully laminar flow. 4

5 A 6mm-diameter sonication probe (SCIENTZ-II, Ningbo Xinzhi Co.td) operating at khz was used to impose shockwaves to the oriented suspension. A pulsing sonication sequence of 20s on followed by 10s off was adopted to avoid overheating of the probe. The cycling flow device along with the sonicator were immersed in the water bath, which was monitored by a thermal couple and kept at room temperature. The dwell time (τ) of the suspension at every cycle in the slit was τ = 0.006m = 0.024s, and the suspension cycle time (t) was t = 80ml = 17.7s. 0.25m/s 4.5ml/s Given that the suspension was subject to sonication shockwave for 0.024s for each flow cycle (17.7s), the actual sonication action time of the suspension can be calculated from the experimental sonication time (720min) as = 0.65min. After AES or IPS 3 treatment, the suspensions were centrifuged (3000 rpm) for 15min to isolate exfoliated flakes from remnant thick graphitic platelets Per Beer-Lambert law (UV-Vis absorption spectra), 7 after a single synthesis run the graphene concentration is estimated to be 59 μg/ml, which is achieved by applying 0.65 min of sonication. This is equivalent to 90 μg/ml min graphene production rate, which is on par with/higher than prior graphite liquid phase sonication exfoliation studies. 7-9 The final graphene yield of a single run is estimated at 0.2 wt.% of the starting graphite precursor, also comparable with prior reports. While liquid phase exfoliation yields are intrinsically much lower than for reactive chemical - thermal methods such as Hummer's, 10, 11 they do have the inherent advantage of leaving the remnant material fully reusable. 12, 13 The un-exfoliated graphite is centrifugally separated and may be readily employed in the next fabrication run. The graphite may also be cycled for longer times within the flow loop reactor without process interruption, giving higher overall graphene yield. In principle, 100% of the graphite particles could be exfoliated into graphene, e.g. by extending the sonication time, replacing the dispersant, and re-cycling graphite residuals. The yield of graphene by sonication-based exfoliation method is dependent on sonication time, precursor characteristics, solubility of solvent and surfactant, etc. Therefore, it is difficult to precisely evaluate and compare the energy requirements for this process versus other techniques. It is also reasonable to argue that the industrial health and safety costs for scaled-up sonication 5

6 methods are significantly lower than for traditional routes such as Hummer's, because sonication avoids the use of highly aggressive chemicals and does not generate toxic off-gasses. 6

7 S2. Supplementary data Figure S3. A representative SEM image of AES-G on Cu foil electrode. Figure S4. Two Equivalent Circuit models employed in this study. The double RC model accounts for the second semicircle which appears in IPS-G after 10 cycles between 2.5 and 0.01 V vs. Na/Na +. 7

8 Figure S5. Cycling charge - discharge behavior and the corresponding impedance rise of AES-G (a) - (c) and IPS-G (d) - (f) tested in the high voltage range V vs. Na/Na +, at 30 ma/g. The lower voltage cutoff is significantly above the equilibrium Na plating potential of 0 V. 8

9 Figure S6. Post-cycled SEI covered Na metal surface of AES-G. (a) SEM micrograph and the associated EDXS elemental-maps of F, Na, O and C. The F map is uniform in intensity. (b) and (c) High resolution C 1s, O 1s and F 1S XPS spectra. 9

10 Figure S7. Post-cycled SEI covered Na metal surface of IPS-G. (a) SEM micrograph and the associated EDXS elemental-maps of F, Na, O and C. Arrows drawn on the F map highlight F- rich regions in the SEI layer. (b) and (c) High resolution C 1s, O 1s and F 1S XPS spectra. Suppl References 1. Goodwin, J.; Hughes, R., Rheology for Chemists an Introduction. The Royal Society of Chemistry Gunes, D. Z.; Scirocco, R.; Mewis, J.; Vermant, J., Flow-Induced Orientation of Non- Spherical Particles: Effect of Aspect Ratio and Medium Rheology. J. Non-Newton. Fluid. 2008, 155,

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