Inkjet-Printed Graphene for Flexible Micro-Supercapacitors L.T. Le 1 , M.H. Ervin 2 , H. Qiu 1 , B.E. Fuchs 3 , J. Zunino 3 , and W.Y. Lee 1 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 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 m 2 /g and 2630 m 2 /g, respectively [5,6]. Also, their