1. Inkjet-Printed Graphene for
Flexible Micro-Supercapacitors
L.T. Le1, M.H. Ervin2, H. Qiu1, B.E. Fuchs3, J. Zunino3, and W.Y. Lee1
1
Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, NJ 07030, USA
2
U.S. Army Research Laboratory, RDRL-SER-L, 2800 Powder Mill Road, Adelphi, MD 20783, USA
3
U.S. Army Armament Research, Development and Engineering Center, Picatinny Arsenal, NJ, 07806, USA
E-mail: wlee@stevens.edu
Abstract — Here we report our multi-institutional effort in advances in mW-scale energy harvesting from mechanical
exploring inkjet printing, as a scalable manufacturing vibration and other sources [2-4], we envision the possibility
pathway of fabricating graphene electrodes for flexible of inkjet printing a micro-supercapacitor and integrating it
micro-supercapacitors. This effort is founded on our recent with a printable energy harvester on an implantable
discovery that graphene oxide nanosheets can be easily biomedical device. Such a self-powered implant does not
inkjet-printed and thermally reduced to produce and pattern have to be surgically removed from the patient’s body due to
graphene electrodes on flexible substrates with a lateral the cycle life limitation associated with a rechargeable
spatial resolution of ∼50 µm. The highest specific energy battery.
and specific power were measured to be 6.74 Wh/kg and
2.19 kW/kg, respectively. The electrochemical performance However, to a large extent, integrated flexible micro-
of the graphene electrodes compared favorably to that of supercapacitors do not exist in the marketplace today due to
other graphene-based electrodes fabricated by traditional miniaturization challenges associated with conventional
powder consolidation methods. This paper also outlines our fabrication methods such as screen printing and spray
current activities aimed at increasing the capacitance of the deposition of electrode materials. In contrast to these
printed graphene electrodes and integrating and packaging techniques, inkjet printing offers (1) the ability to precisely
with other supercapacitor materials. pattern inter-digitized electrodes with a lateral spatial
resolution of ∼50 µm; (2) direct phase transformation from
Index Terms – Graphene, Graphene oxide, Inkjet Printing, liquid inks to heterogeneous nanoscale structures in an
Supercapacitor, Flexible Electronics additive, net-shape manner with minimum nanomaterial use,
handling and waste generation; and (3) rapid translation of
I. INKJET-PRINTING FOR MICRO-SUPERCAPACTIORS new discoveries into integration with flexible electronics
using commercially available inkjet printers ranging from
There is a tremendous need for rechargeable power sources desktop to roll-to-roll. Some of these transformative
that have long cycle life and can be rapidly charged and attributes are captured in our concept device design (Fig. 1).
discharged beyond what is possible with rechargeable
batteries. Electric double layer capacitors, commonly
referred to as “supercapacitors,” are promising in terms of
providing fast charge/discharge rates in seconds while being
able to withstand millions of charge/discharge cycles in
comparison to thousands of cycles for batteries [1].
Supercapacitors utilize nanoscale electrostatic charge
separation at electrode-electrolyte interfaces as an energy
storage mechanism. This mechanism avoids faradic
chemical reactions, dimensional changes, and solid-state
diffusion between electrodes and electrolytes, and
consequently provides long-term cycle stability and high
specific power. For high capacitance, electrodes are
typically fabricated of electrically conductive materials such
Fig. 1. Flexible micro-supercapacitor concept.
as activated carbon with high surface area.
II. GRAPHENE AS IDEAL ELECTRODE MATERIAL
While many supercapacitor research efforts are currently
aimed at developing supercapacitors for electric vehicle In order to increase capacitance, significant efforts are being
applications, there is also another exciting opportunity to made to explore carbon nanotubes (CNT) and graphene as
develop micro-supercapacitors for the rapidly emerging ideal electrode materials with their theoretical surface areas
flexible electronics market. For example, with recent of 1315 m2/g and 2630 m2/g, respectively [5,6]. Also, their

2. chemical stability, high electrical and thermal conductivity, spherical ink droplets without clogging nozzles at a lateral
and mechanical strength and flexibility are attractive as spatial resolution of ∼50 µm. For example, the dot structure
electrode materials. However, for inkjet printing, these in Fig. 2c was produced with 20 printing passes to show that
nanomaterials as well as activated carbon nanoparticles are drop-to-drop placement and alignment could be repeated to
hydrophobic, and thus segregate in water even at very low increase thickness. Also, the average distance between the
concentrations (e.g., 5 ppm for single-walled CNT) unless center locations of two neighbouring droplets could be
surfactants are added or their surfaces are functionalized. adjusted to form continuous films. The overlap spacing of
However, the use of surfactants and surface modification 15 µm was used for the film shown in Fig. 2d.
during supercapacitor electrode fabrication is generally not
desired, since they can significantly decrease capacitance.
In contrast to CNT and graphene, the recent “re-discovery”
and commercial availability of hydrophilic graphene oxide
(GO) at a reasonable price presents a unique opportunity to
develop and use GO as an ideal ink with stable dispersion in
pure water (up to 1 wt %) [7]. Although GO itself is not
electrically conductive, it can be thermally, chemically, and
photothermally reduced to graphene [8]. As shown in Fig. 2,
we have recently found [9] that GO, stably dispersed in
water at 0.2 wt %, can be inkjet-printed using a bench-scale
inkjet printer (Fujifilm Dimatix DMP2800) and
subsequently reduced at a moderate temperature of 200°C in
flowing N2 as a new means of producing and
micropatterning electrically conductive graphene electrodes.
Fig. 2. Inkjet Printing: (a) ink formulation based on stable GO
dispersion in water, (b) ink droplets jetted by piezoelectric nozzles,
(c) SEM image of a graphene dot printed on titanium substrate, and
(d) SEM image of continuous graphene film on titanium. From
Reference [9].
At room temperature, the viscosity and surface tension of the
water-based GO ink at 0.2 wt% were measured to be 1.06
mPa•s and 68 mN/m, respectively, and were similar to those
of de-ionized water (0.99 mPa•s and 72 mN/m). The
physical properties of the GO ink were outside of the ranges
recommended for normal inkjet printing (e.g., 10-12 mPa•s Fig. 3. Initial electrochemical performance: (a) cyclic voltammetry
and 28-32 mN/m). Nevertheless, as shown in Fig. 2b, we measured at different scan rates (b) specific capacitance retained
found that manipulating the firing voltage of piezoelectric over 1000 charge/discharge cycles at a constant scan rate of 50
mV/s and (c) Ragone plot. From Reference [9].
nozzles as a function of time was effective in generating

3. Titanium foils from Sigma Aldrich (100 µm thick, 99.99%
purity) was used as an example of flexible substrate and
current collector for our initial electrochemical
characterization. Electrochemical performance was
evaluated by cyclic voltammetry (Fig. 3a) and galvanostatic
charge/discharge. Two identical electrodes were clamped
with a Celgard separator. 1 M H2SO4 was used as the
electrolyte. The specific capacitance of the graphene
electrodes was measured to be 48-132 F/g in the scan range
of 0.5 to 0.01 V/s. As shown in Fig. 3b, 96.8 % capacitance
was retained over 1000 cycles. The specific power and
energy density of the graphene electrodes are plotted in Fig.
3c.
Table 1. Comparison of electrochemical performances
As compared in Table 1, the capacitance of the graphene
electrodes was similar to that reported for other graphene
electrodes prepared by conventional powder-based methods
in the absence of any pseudocapacitance materials added to
the electrodes [5,10,11]. However, the power density of
IPGEs was considerably lower than that of CNT-based
electrodes which has been reported as high as 100 kW/kg
[12,13]. The lower power density of the graphene electrodes
may be partly explained by the lack of: (1) interconnectivity
among 2D graphene nanosheets for electron conduction and
(2) 3D mesoscale porosity for ion conduction. Nevertheless,
the initial performance of the inkjet-printed is promising,
and is expected to be further improved by optimizing
printing and reduction conditions and by optimizing its 3D
morphology.
III. CHALLENGES AND CURRENT ACTIVITIES
The fundamental scientific challenge for this research stems
from the lack of understanding of and experience with
graphene and GO as new nanoscale building blocks for 3D
assembly. For example, our initial results show that we are
currently utilizing less than 12% of the theoretical
capacitance possible with graphene (i.e., 132 out of 1104 F/g
for H2SO4 electrolyte). We are currently exploring a concept
of adding nanospacers to control the stacking behavior of
conformal graphene nanosheets and therefore to increase
specific surface area and capacitance. Also, as illustrated in
Fig. 4, we are focusing on droplet coalescing as an important
printing parameter that: (1) will determine optimum printing
speed and (2) can be used to create disordered 3D assembly Fig. 4. Overlapped droplet spacing of: (a) 5 µm (b) 25 µm and (c)
of graphene nanosheets as another means of controlling the 15 µm. (d) illustration of nozzle and substrate movements during
conformal stacking behavior of the nanosheets. inkjet printing.

4. We observed the significant effect of droplet overlap spacing the specific electrolyte development and packaging issues
on the formation of continuous boundaries which appear as and challenges associated with realizing micro-
“white” lines in the SEM images (Figs. 4a-c). As evident supercapacitors that can be integrated with flexible
from these SEM images, the average distance between the electronics.
boundaries corresponded well to the overlap spacing of
neighboring droplets used to prepare these graphene thin
films. At a high magnification (Fig. 4c), graphene sheets
appeared more wrinkly and less uniform at the boundaries
than in areas between the boundaries. The results suggest
that we may be able to control and use these boundaries as a
mechanism to produce more disordered 3D assembly of the
nanosheets.
Fig. 4d illustrates the 3D operation of multi-nozzle printing.
d1 and d2 are the overlap spacings between two neighboring
droplets, which can be controlled as low as 5 µm in the x-
and y-directions, respectively. During typical operation, the
printhead moves in the x-direction to place the first row of
droplets for a specified distance. When the printhead comes
back to its original x location, the substrate stage moves in
the y-direction so that the printhead can place the second
row of droplets. In addition to the spacing parameters, there
are several key time variables to consider from a scaling
perspective. t1 is the time between placing two neighboring
droplets in the x-axis direction with the controllable range of
∼0.5 ms, t2 is the time it takes for the printhead to be ready
to print the next row droplets in the y-direction (e.g., ∼10 s
for 1 cm x-direction motion). t3 is the time between placing
the two layers of droplets in the z-direction (e.g., ∼4 min for
1 cm2).
The effects of these variables on the development of
boundaries with GO ink are being evaluated. Once we are
able to understand and control the formation of continuous
boundaries, the new processing/structure knowledge may be
used to: (1) assess surface area and capacitance
enhancements associated with morphology tailoring and (2) Fig. 5. Islands formation as a function of substrate hydrophobicity:
scale fabrication using bench- and industrial scale printers (a) hydrophobic surface of as-received Kapton and (b) hydrophilic
while controlling electrode morphology. surface of treated Kapton.
On the concept device fabrication and demonstration fronts, IV. CONCLUSIONS
we have undertaken several activities. Kapton (DuPont) is Hydrophilic GO dispersed in water was found to be a stable
initially chosen as a flexible substrate material. Inkjet ink for inkjet printing of GO with the lateral spatial
printing of the GO ink on as-received Kapton substrate resolution of 50 µm. Subsequent thermal reduction of the
surface resulted in the formation of islands of about 1 to 2 printed GO produced electrically conductive graphene
mm (Fig. 5a). After the substrate surface was treated with electrodes with promising initial electrochemical
potassium hydroxide for 3 h, the island formation was performance for flexible micro-supercapacitor applications.
considerably reduced (Fig. 5b). This change was attributed
to the spreading of hydrophilic ink droplets on the Kapton
surface becoming hydrophilic with the treatment. For ACKNOWLEDGMENT
current collector, a commercially available silver The authors thank the U.S. Army - ARDEC for funding this
nanoparticles CCi-300 ink (Cabot Inc.) is selected. This ink project under the contract of W15QKN-05-D-0011.
contains 20 nm silver nanoparticles suspended in a mixture
of ethanol and ethylene glycol. We are evaluating several REFERENCES
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