Inkjet-Printed Graphene for Flexible Micro-Supercapacitors

L.T. Le1, M.H. Ervin2, H. Qiu1, B.E. Fuchs3, J. Zunino3, and W.Y. Lee1 1Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, NJ 07030, USA

2U.S. Army Research Laboratory, RDRL-SER-L, 2800 Powder Mill Road, Adelphi, MD 20783, USA 3U.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 exploring inkjet printing, as a scalable manufacturing pathway of fabricating graphene electrodes for flexible micro-supercapacitors. This effort is founded on our recent discovery that graphene oxide nanosheets can be easily inkjet-printed and thermally reduced to produce and pattern graphene electrodes on flexible substrates with a lateral spatial resolution of ∼50 µm. The highest specific energy and specific power were measured to be 6.74 Wh/kg and 2.19 kW/kg, respectively. The electrochemical performance of the graphene electrodes compared favorably to that of other graphene-based electrodes fabricated by traditional powder consolidation methods. This paper also outlines our current activities aimed at increasing the capacitance of the printed graphene electrodes and integrating and packaging with other supercapacitor materials. Index Terms – Graphene, Graphene oxide, Inkjet Printing, Supercapacitor, Flexible Electronics

I. INKJET-PRINTING FOR MICRO-SUPERCAPACTIORS

There is a tremendous need for rechargeable power sources that have long cycle life and can be rapidly charged and 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 as activated carbon with high surface area. While many supercapacitor research efforts are currently aimed at developing supercapacitors for electric vehicle applications, there is also another exciting opportunity to develop micro-supercapacitors for the rapidly emerging flexible electronics market. For example, with recent

advances in mW-scale energy harvesting from mechanical vibration and other sources [2-4], we envision the possibility of inkjet printing a micro-supercapacitor and integrating it with a printable energy harvester on an implantable biomedical device. Such a self-powered implant does not have to be surgically removed from the patient’s body due to the cycle life limitation associated with a rechargeable battery. However, to a large extent, integrated flexible micro-supercapacitors do not exist in the marketplace today due to miniaturization challenges associated with conventional fabrication methods such as screen printing and spray deposition of electrode materials. In contrast to these techniques, inkjet printing offers (1) the ability to precisely pattern inter-digitized electrodes with a lateral spatial resolution of ∼50 µm; (2) direct phase transformation from liquid inks to heterogeneous nanoscale structures in an additive, net-shape manner with minimum nanomaterial use, handling and waste generation; and (3) rapid translation of new discoveries into integration with flexible electronics using commercially available inkjet printers ranging from desktop to roll-to-roll. Some of these transformative attributes are captured in our concept device design (Fig. 1).

Fig. 1. Flexible micro-supercapacitor concept.

II. GRAPHENE AS IDEAL ELECTRODE MATERIAL

In order to increase capacitance, significant efforts are being made to explore carbon nanotubes (CNT) and graphene as ideal electrode materials with their theoretical surface areas of 1315 m2/g and 2630 m2/g, respectively [5,6]. Also, their

chemical stability, high electrical and thermal conductivity, and mechanical strength and flexibility are attractive as electrode materials. However, for inkjet printing, these nanomaterials as well as activated carbon nanoparticles are hydrophobic, and thus segregate in water even at very low concentrations (e.g., 5 ppm for single-walled CNT) unless surfactants are added or their surfaces are functionalized. However, the use of surfactants and surface modification 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 and 28-32 mN/m). Nevertheless, as shown in Fig. 2b, we found that manipulating the firing voltage of piezoelectric nozzles as a function of time was effective in generating

spherical ink droplets without clogging nozzles at a lateral spatial resolution of ∼50 µm. For example, the dot structure in Fig. 2c was produced with 20 printing passes to show that drop-to-drop placement and alignment could be repeated to increase thickness. Also, the average distance between the center locations of two neighbouring droplets could be adjusted to form continuous films. The overlap spacing of 15 µm was used for the film shown in Fig. 2d.

Fig. 3. Initial electrochemical performance: (a) cyclic voltammetry measured at different scan rates (b) specific capacitance retained over 1000 charge/discharge cycles at a constant scan rate of 50 mV/s and (c) Ragone plot. From Reference [9].

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 of graphene nanosheets as another means of controlling the conformal stacking behavior of the nanosheets.

Fig. 4. Overlapped droplet spacing of: (a) 5 µm (b) 25 µm and (c) 15 µm. (d) illustration of nozzle and substrate movements during inkjet printing.

We observed the significant effect of droplet overlap spacing on the formation of continuous boundaries which appear as “white” lines in the SEM images (Figs. 4a-c). As evident from these SEM images, the average distance between the 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) scale fabrication using bench- and industrial scale printers while controlling electrode morphology. On the concept device fabrication and demonstration fronts, we have undertaken several activities. Kapton (DuPont) is initially chosen as a flexible substrate material. Inkjet printing of the GO ink on as-received Kapton substrate surface resulted in the formation of islands of about 1 to 2 mm (Fig. 5a). After the substrate surface was treated with potassium hydroxide for 3 h, the island formation was 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 current collector, a commercially available silver nanoparticles CCi-300 ink (Cabot Inc.) is selected. This ink contains 20 nm silver nanoparticles suspended in a mixture of ethanol and ethylene glycol. We are evaluating several electrolytes for electrochemical compatibility with inkjet-printed silver. For packaging, we are exploring a heat-sealing approach using heat-sealable Kapton FN as a primary method and soft-lithography as an alternative option. These initial investigations are expected to uncover

the specific electrolyte development and packaging issues and challenges associated with realizing micro-supercapacitors that can be integrated with flexible electronics.

Fig. 5. Islands formation as a function of substrate hydrophobicity: (a) hydrophobic surface of as-received Kapton and (b) hydrophilic surface of treated Kapton.

IV. CONCLUSIONS

Hydrophilic GO dispersed in water was found to be a stable ink for inkjet printing of GO with the lateral spatial resolution of 50 µm. Subsequent thermal reduction of the printed GO produced electrically conductive graphene electrodes with promising initial electrochemical performance for flexible micro-supercapacitor applications.

ACKNOWLEDGMENT

The authors thank the U.S. Army - ARDEC for funding this project under the contract of W15QKN-05-D-0011.

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