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Electrochemistry Communications 13 (2011) 355–358
Contents lists available at ScienceDirect
Electrochemistry Communications
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Graphene supercapacitor electrodes fabricated by inkjet printing and thermal
reduction of graphene oxide
Linh T. Le a, Matthew H. Ervin b, Hongwei Qiu a, Brian E. Fuchs c, Woo Y. Lee a,⁎
a
Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, NJ 07030, USA
b
U.S. Army Research Laboratory, RDRL-SER_L, 2800 Powder Mill Road, Adelphi, MD 20783–1197, USA
c
U.S. Army Armament Research, Development and Engineering Center, Picatinny Arsenal, NJ, 07806, USA
a r t i c l e i n f o a b s t r a c t
Article history: Graphene oxide nanosheets, stably dispersed in water at 0.2 wt.%, were inkjet-printed onto Ti foils and
Received 10 January 2011 thermally reduced at 200 °C in N2, as a new method of fabricating inkjet printed graphene electrodes (IPGEs)
Received in revised form 21 January 2011 for supercapacitors. The specific capacitance of IPGE ranged from 48 to 132 F/g, depending on the potential
Accepted 24 January 2011
scan rate from 0.5 to 0.01 V/s using 1M H2SO4 as the electrolyte. The initial performance of IPGEs compares
Available online 2 February 2011
favorably to that reported for graphene electrodes prepared by other fabrication methods. This new finding is
Keywords:
expected to be particularly useful for designing and fabricating inter-digitized electrode arrays with a lateral
Graphene oxide spatial resolution of ~ 50 μm for flexible micro-supercapacitors.
Graphene © 2011 Elsevier B.V. All rights reserved.
Supercapacitors
Electrodes
Inkjet printing
Flexible electronics
1. Introduction Unlike these carbonaceous nanomaterials, graphene oxide (GO) is
hydrophilic and can be easily dispersed in water at relatively high
Electric double layer capacitor (“supercapacitor”) electrodes are concentrations of up to 0.2% [10]. Although GO is not electrically
generally fabricated of electrically conductive and high surface area conductive, it can be thermally [11], chemically [12] and photothermally
materials (e.g., activated carbon) required for high capacitance [1–3]. [13] reduced to graphene. In this communication, we report for the first
Recently, there has been a significant interest in exploring carbon time, to our best knowledge, the feasibility of inkjet printing GO
nanotubes (CNT) and graphene as ideal electrode materials with their dispersed in water and the subsequent thermal reduction as a new
theoretical surface areas of 1315 and 2630 m2/g, respectively [4–8]. avenue for fabricating graphene supercapacitor electrodes.
Also, their chemical stability, high electrical and thermal conductivity,
and mechanical strength and flexibility are attractive as conformal 2. Experimental
electrode materials particularly for flexible supercapacitors. However,
for inkjet printing, these nanomaterials as well as activated carbon GO dispersed in water at 2 mg/ml was purchased from a
nanoparticles are hydrophobic and thus segregate in water even at commercial source (Cheap Tubes). The average dimensions of GO
very low concentrations (e.g., 5 ppm for single-walled CNT) unless were reported by the supplier to be 500 nm × 500 nm × 0.8 nm. The
surfactants are added or their surfaces are functionalized [9]. The use as-received GO solution was sonicated for 15 min followed by filtering
of surfactants and surface modification during supercapacitor elec- with a 450 nm Millex syringe filter before loading into a printhead
trode fabrication is generally not desired, since they can function as cartridge. The viscosity and surface tension of the GO ink were
dielectric films to: (1) increase junction resistance between particles, measured at ambient conditions using a Viscolab 450 viscometer
(2) impede electrolyte access to the electrode surface and (3) conse- (Cambridge Viscosity) and a DeltaPi tensiometer (Kibron), respec-
quently decrease capacitance. We have found that CNT and activated tively. Ti foils (100 μm thick, 99.99% purity, Sigma Aldrich) were
carbon nanoparticles dispersed in water even at ppm levels are mainly used as an example of a flexible substrate and current
basically not jettable due to nozzle clogging. collector. The substrates were cleaned with acetone and de-ionized
water several times prior to printing.
A commercial Dimatix Material Printer DMP 2800 inkjet printer
(Fujifilm Dimatix) was used to print the GO ink. This inkjet printer
⁎ Corresponding author. Tel.: + 1 201 216 8307; fax: + 1 201 216 8306. utilizes 16 microfabricated piezoelectric nozzles for on-demand and
E-mail address: woo.lee@stevens.edu (W.Y. Lee). programmable generation of 10 pL microscopic ink droplets. The
1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.elecom.2011.01.023

3. Author's personal copy
356 L.T. Le et al. / Electrochemistry Communications 13 (2011) 355–358
inkjet-printed GO samples were reduced in flowing N2 at 200 °C for the GO surface (Fig. 1a). At room temperature, the viscosity and
12 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 water
graphene electrodes were characterized by Nikon C-BD115 optical (0.99 mPa s and 72 mN/m). The physical properties of the GO ink
microscopy (Nikon Instrument) and Zeiss Auriga FIB-SEM scanning were outside of the ranges recommended by the manufacturer for
electron 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 the
voltammetry (CV) and constant current charge/discharge measure- firing voltage of the piezoelectric nozzles as a function of time was
ments 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 increased
block using a Celgard 3401 membrane (Celgard) as a separator and the voltage to the maximum over 5 μs to force rapid pressure buildup
1M H2SO4 electrolyte in order to make constant current charge/ in the nozzles for droplet ejection. In the second segment, we
discharge measurements as a full, though unpackaged device. Two decreased the voltage at a slower rate of over 28 μs to cutoff droplet
samples 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 of
3. 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 diameter
months due to the presence of hydrophilic functional groups [14] on of ~50 μm. For example, the circular GO dot shown in Fig. 1c was
Fig. 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 dot
printed 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 electrochemical
evaluation.

4. Author's personal copy
L.T. Le et al. / Electrochemistry Communications 13 (2011) 355–358 357
produced 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 characterization
and (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 printing
resolution of ~ 50 μm. The droplets were overlapped at a spacing of and was not caused by the reduction step. The island formation was
15 μm between the center locations of two neighboring droplets to also observed to be much less pronounced on more hydrophilic
print 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 densely
sufficient 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 networked
whereas that of the thermally reduced GO film in N2 at 200 °C (i.e., over an average distance of ~ 20–30 μm. At high magnification
IPGE) was measurable at less than ~ 1 MΩ. Also, the color of the GO (Fig. 1f), the graphene sheets appear to be more wrinkled and
film changed from light brown to black upon thermal reduction. These stacked less uniformly at these boundaries than in the areas within
observations were consistent with the prior finding of Zangmeister the boundaries. This morphological development was observed on
[11] 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 curves
photoemission 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 current
charge/discharge curves, (d) specific capacitance as a function of voltage scan rates and (e) Ragone plot.

5. Author's personal copy
358 L.T. Le et al. / Electrochemistry Communications 13 (2011) 355–358
capacitive behavior. The specific capacitance decreased from 125 to Acknowledgment
121 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 project
that the charging/discharging curves were fairly linear, again under the contract of W15QKN-05-D-0011.
demonstrating capacitive behavior. Also, at the device level, the
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