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Authors personal copy356 L.T. Le et al. / Electrochemistry Communications 13 (2011) 355–358inkjet-printed GO samples were reduced in ﬂowing N2 at 200 °C for the GO surface (Fig. 1a). At room temperature, the viscosity and12 h using a Microtherm MT furnace (The Mellen Company) in a glass surface tension of the GO ink were 1.06 mPa s and 68 mN/m,tube. The uniformity and surface morphology of the resulting respectively, and were similar to those of de-ionized watergraphene electrodes were characterized by Nikon C-BD115 optical (0.99 mPa s and 72 mN/m). The physical properties of the GO inkmicroscopy (Nikon Instrument) and Zeiss Auriga FIB-SEM scanning were outside of the ranges recommended by the manufacturer forelectron microscopy (Carl Zeiss NTS). normal operation of the printer (i.e., 10–12 mPa s and 28–32 mN/m). IPGEs electrochemical performance was evaluated with cyclic Nevertheless, as shown in Fig. 1b, we found that manipulating thevoltammetry (CV) and constant current charge/discharge measure- ﬁring voltage of the piezoelectric nozzles as a function of time wasments made using a VersaStat 3 system (Princeton Applied Research). effective in generating spherical ink droplets at a velocity of ~7.5 m/s.Two IPGEs printed on Ti substrates were clamped together in a Teﬂon During the ﬁrst segment of droplet generation, we rapidly increasedblock using a Celgard 3401 membrane (Celgard) as a separator and the voltage to the maximum over 5 μs to force rapid pressure buildup1M H2SO4 electrolyte in order to make constant current charge/ in the nozzles for droplet ejection. In the second segment, wedischarge measurements as a full, though unpackaged device. Two decreased the voltage at a slower rate of over 28 μs to cutoff dropletsamples were evaluated to conﬁrm the reproducibility of our results. tails and therefore form spherical droplets. This “waveform function” optimization was performed through real-time observations of3. Results and discussion droplet generation using a built-in video camera. After hitting the Ti foil surface, spreading, and solvent evaporation, The as-received GO ink was observed to be dispersion-stable for each 10 pL droplet produced a disk-shaped GO dot with a diametermonths due to the presence of hydrophilic functional groups  on of ~50 μm. For example, the circular GO dot shown in Fig. 1c wasFig. 1. IPGE ink and morphology: (a) GO dispersed in water at 0.2 wt.% as a stable ink; (b) spherical ink droplets generated by piezoelectric nozzles; (c) SEM image of a circular GO dotprinted on the Ti foil surface after 20 printing passes at a spatial resolution of ~ 50 μm; and (d), (e) and (f) SEM images of IPGE printed on the Ti surface used for electrochemicalevaluation.
Authors personal copy L.T. Le et al. / Electrochemistry Communications 13 (2011) 355–358 357produced with 20 printing passes at 20 min between passes to: It was interesting to observe the island features of ~ 1–2 mm on the(1) build GO thickness sufﬁcient for microscopy characterization IPGE surface (i.e., “white” areas in Fig. 1d). SEM characterizationand (2) show that drop-to-drop placement and alignment could be indicated that there were almost no graphene present in the “black”repeated to increase the GO thickness with a minimum spatial boundaries. The island formation occurred right after inkjet printingresolution of ~ 50 μm. The droplets were overlapped at a spacing of and was not caused by the reduction step. The island formation was15 μm between the center locations of two neighboring droplets to also observed to be much less pronounced on more hydrophilicprint a continuous GO thin-ﬁlm of 1 cm × 1 cm on the Ti surface substrates and appeared to be dependent on the hydrophobicity of the(Fig. 1d). The printing step was repeated 100 times to deposit initial surface. Within each island, graphene appeared to be denselysufﬁcient GO for electrochemical measurements. The resistance of the stacked with the appearance of secondary boundaries (i.e., “white”as-printed GO on Kapton was measured by a voltmeter to be inﬁnite lines in the SEM image of Fig. 1e) that were continuously networkedwhereas that of the thermally reduced GO ﬁlm in N2 at 200 °C (i.e., over an average distance of ~ 20–30 μm. At high magniﬁcationIPGE) was measurable at less than ~ 1 MΩ. Also, the color of the GO (Fig. 1f), the graphene sheets appear to be more wrinkled andﬁlm changed from light brown to black upon thermal reduction. These stacked less uniformly at these boundaries than in the areas withinobservations were consistent with the prior ﬁnding of Zangmeister the boundaries. This morphological development was observed on who treated GO at 220 °C in air and conﬁrmed the reduction of other substrate materials.GO to graphene by Fourier transform infrared spectroscopy and X-ray As shown in Fig. 2a, IPGEs exhibited fairly rectangular CV curvesphotoemission spectroscopy. at scan rates in the range of 0.01 to 0.5 V/s which is indicative of 6 (a) 140 (b) Specific Capacitance (F/g) 4 Current Density (mA/g) 120 2 0 100 -2 0.02 V/s 80 0.05 V/s 0.1 V/s -4 0.2 V/s 0.5 V/s 60 -6 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0 200 400 600 800 1000 1200 1400 Potential (V) Cycle number 0.6 (c) 140 (d) Specific Capacitance (F/g) 0.4 120 0.2 100 Potential (V) 0.0 80 -0.2 60 -0.4 40 -0.6 20 -0.8 0 0 200 400 600 800 1000 1200 1400 1600 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Time (s) Scan Rate (V/s) 10 (e) Specific Power (kW/kg) 1 0.1 0.01 0.1 1 10 Specific Energy (Wh/kg)Fig. 2. Electrochemical properties of IPGE: (a) cyclic voltammograms measured at different scan rates, (b) speciﬁc capacitance retained as a function of CV cycles, (c) constant currentcharge/discharge curves, (d) speciﬁc capacitance as a function of voltage scan rates and (e) Ragone plot.
Authors personal copy358 L.T. Le et al. / Electrochemistry Communications 13 (2011) 355–358capacitive behavior. The speciﬁc capacitance decreased from 125 to Acknowledgment121 F/g over 1000 CV cycles at a constant scan rate of 50 mV/s(Fig. 2b) demonstrating 96.8% capacitance retention. Fig. 2c shows The authors thank the U.S. Army — ARDEC for funding this projectthat the charging/discharging curves were fairly linear, again under the contract of W15QKN-05-D-0011.demonstrating capacitive behavior. Also, at the device level, thespeciﬁc capacitance was measured to be 48 to 132 F/g in the scan Referencesrange of 0.5 to 0.01 V/s (Fig. 2d). The energy and power density of  B.E. Conway, Electrochemical Supercapacitors: Scientiﬁc Fundamentals andIPGEs are plotted in Fig. 2e with: (1) the highest energy density Technological Applications, Springer, 1999.of 6.74 Wh/kg achieved at a power density of 0.190 kW/kg and  P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nature Materials 7(2) the highest power density of 2.19 kW/kg at an energy density of (2008) 845–854.  S. Vivekchand, C. Rout, K. Subrahmanyam, A. Govindaraj, C. Rao, Graphene-based1.34 Wh/kg. electrochemical supercapacitors, Journal of Chemical Sciences 120 (2008) 9–13. The electrochemical performance of IPGEs was more or less similar  A. Peigney, C. Laurent, E. Flahaut, R.R. Bacsa, A. Rousset, Speciﬁc surface area ofto that reported for other graphene electrodes prepared by conventional carbon nanotubes and bundles of carbon nanotubes, Carbon 39 (2001) 507–514.  M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors, Nanopowder-based methods in the absence of any pseudocapacitance Letters 8 (2008) 3498–3502.materials added to the electrodes [3,15,16]. However, the power density  A.K. Geim, K.S. 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