Freestanding Ion Gels for Flexible, Printed, Multifunctional

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Freestanding Ion Gels for Flexible, Printed, Multifunctional Microsupercapacitors Donghoon Song, Fazel Zare Bidoky, Ethan Secor, Mark C Hersam, and C. Daniel Frisbie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20766 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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ACS Applied Materials & Interfaces

Freestanding Ion Gels for Flexible, Printed, Multifunctional Microsupercapacitors Donghoon Song,† Fazel Zare Bidoky,† Ethan B. Secor,‡ Mark C. Hersam,‡ and C. Daniel Frisbie*,† †Department

of Chemical Engineering and Materials Science, University of Minnesota,

Minneapolis, Minnesota 55455, United States ‡

Department of Materials Science and Engineering, Northwestern University, 2220 Campus

Drive, Evanston, Illinois 60208, USA KEYWORDS: freestanding ion gels, self-aligned printing, pristine graphene ink, multifunctional devices, flexible foldable microsupercapacitors

ACS Paragon Plus Environment

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ABSTRACT: Freestanding ion gels (FIGs) provide unique opportunities for scalable, low-cost fabrication of flexible microsupercapacitors (MSCs). While conventional MSCs employ a distinct electrolyte and substrate, FIGs perform both functions, offering new possibilities for device integration and multifunctionality while maintaining high performance. Here, a capillarity-driven printing method is demonstrated to manufacture high-precision graphene electrodes on FIGs for MSCs. This method achieves excellent self-alignment and resolution (width: 50 μm, interdigitated electrode footprint: 1 mS cm1),

excellent mechanical strength (>10 kPa) and flexibility, versatile solution processability, and

outstanding electrochemical stability.1-4 Based on these benefits, ion gels have been explored for diverse electronic and energy applications including transistors/circuits,5-12 actuators,13 sensors,14 light emitting cells,15-16 nanogenerators,17 supercapacitors (SCs),18-20 and microsupercapacitors (MSCs).21-23 For example, in transistor/circuit applications, IGs provide a high-capacitance gate electrolyte insensitive to IG thickness, an enabling feature for robust fabrication of low-voltage (1 MPa achieved, integration of these materials with advanced fabrication techniques has not yet been demonstrated. In particular, patterning of diverse functional materials on the FIG surface, for example by liquid-phase printing methods, presents a key


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technical challenge due to surface wetting and homogeneity of the gels. Here, precision patterning of colloidal inks is demonstrated using a self-aligned capillarity-driven technique to enable high fidelity printing directly on FIGs. This method is demonstrated in detail for graphene-based MSCs requiring complex interdigitated electrodes (IDEs), resulting in high performance and fabrication yield. Moreover, the generality of the technique is demonstrated for alternative electrode materials, including multi-walled carbon nanotubes (MWCNTs) and the conducting polymer PEDOT:PSS. Finally, several opportunities for multifunctionality are demonstrated, exploiting the FIGs for light emission and transistor gating in addition to energy storage, while maintaining versatile print-andplace integration. Emerging interest in portable and wearable electronics has generated surging demand for reliable, high capacity, and miniaturized energy storage. In this regard, MSCs are attractive due to their long cycle life, rapid charge/discharge, high power density, high round trip efficiency, and potential for low-cost fabrication.24-25 MSCs store electrical energy in an electric double layer (EDL) at the interface between an electrolyte and interdigitated coplanar electrodes,24-25 commonly composed of graphene,22-23, 26-33 CNTs,34 or conducting polymers,35 among others.36-42 Liquidphase printing methods are highly promising for scalable, low-cost fabrication of MSCs (i.e., rollto-roll fabrication), but present technical challenges for materials integration with high reliability.43 To address this challenge, a self-aligned, capillarity-based printing method known as SCALE (selfaligned capillarity-assisted lithography for electronics) is adapted for FIG materials, enabling precision printing of conductive inks in complex patterns to employ the FIG as both electrolyte and substrate for multifunctional MSCs for the first time.23, 44-45


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RESULTS AND DISCUSSION In the SCALE process, a topographically patterned substrate is fabricated with capillary microchannels to guide and constrain ink movement. A functional ink deposited into reservoirs by inkjet printing is drawn into the channels by capillary forces, leading to precise, high resolution features. To date, this process has been applied to traditional substrate materials, with robust mechanical properties but without additional functionality. FIGs present a promising opportunity for the SCALE process, in that multifunctionality can be embedded into the substrate for a diverse scope of applications. The FIGs comprise commercially available chemicals, namely the polymer poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) and an ionic liquid, 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI), the chemical structures of which are shown in Scheme 1a. Scheme 1b shows the full process schematic for MSC fabrication, which begins with the preparation of an imprinted FIG substrate. For this initial step, a master template was first prepared by patterning the MSC electrodes, composed of capillary channels and reservoirs, onto a pristine Si wafer using standard micropatterning methods, followed by spincoating with a fluoropolymer (CYTOP, Asahi Glass Co., Ltd.). A liquid UV-curable adhesive (NOA73, Norland Products Inc.) was then poured on the resulting template and evenly pressed using a PET substrate. Upon UV light exposure through the PET, the NOA73 was cured with the embedded raised patterns and adhered to the PET (see Experimental Methods for more details). The resulting imprinted NOA73/PET substrate provides a robust, flexible, and transparent mold, as shown in Figure S1, and is thus well-suited for roll-to-roll production formats.


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Scheme 1. (a) Chemical structures of FIG film components, P(VDF-HFP) polymer and EMITFSI ionic liquid. (b) Schematic drawing for the MSC fabrication process including imprinting the FIG using the mold, inkjet printing of the pristine graphene ink, and photoannealing.

The imprinted FIG was then fabricated on the mold, using a dilute solution of FIG components in acetone to facilitate spreading and conformal coating of the gel. The FIG solution (polymer/ionic liquid/acetone at 1:4:10 w/w/w) was prepared by stirring at 60 °C overnight, dropcast over the mold (~0.1 mL cm-2), and dried at ambient conditions for a day. This procedure avoids common deposition methods such as spin-coating that result in substantial waste of valuable


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materials, and results in a uniform and freestanding FIG film (thickness of ~200 μm), as shown in Figure 1a and Figure S2a. Although thinner films (

Freestanding Ion Gels for Flexible, Printed, Multifunctional

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