Wet Transfer of Inkjet Printed Graphene for Micro-Supercapacitors on

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Wet Transfer of Inkjet Printed Graphene for Micro-Supercapacitors on Arbitrary Substrates Szymon Sollami Delekta, Mikael Östling, and Jiantong Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01225 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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ACS Applied Energy Materials

Wet Transfer of Inkjet Printed Graphene for MicroSupercapacitors on Arbitrary Substrates Szymon Sollami Delekta, Mikael Östling and Jiantong Li* KTH Royal Institute of Technology, School of Electrical Engineering and Computer Science, Electrum 229, SE-164 40 Kista, Sweden.

Corresponding Author *E-mail: [email protected]

Keywords: Graphene; wet transfer; inkjet printing; micro-supercapacitors; arbitrary substrates

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Abstract

Significant research interest is being devoted to exploiting the properties of graphene but the difficult integration on various substrates limits its use. In this regard, we developed a transfer technique that allows the direct deposition of inkjet printed graphene devices on arbitrary substrates, even 3D objects and living plants. With this technique, we fabricated microsupercapacitors which exhibited good adhesion on almost all substrates and no performance degradation induced by the process. Specifically, the micro-supercapacitor on an orchid leaf showed an areal capacitance as high as 441 µF cm-2 and a volumetric capacitance of 1.16 F cm-3. This technique can boost the use of graphene in key technological applications such as selfpowered epidermal electronics and environmental monitoring systems.


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Changing the way electronic devices coexist and interact with the environment is an ambitious goal which drives the development of disruptive technologies across many fields like wearables,1 bioelectronics,2 health monitoring3 and epidermal electronics.4 The appeal of the universal and non-intrusive integration of an electronic device onto any surface - 3D or flat, curved, movable, biological - enables quite a few new applications.5–7 The merging of electronics and plants could be used, for example, to regulate their growth and function8 or to collect signals from the surroundings for environmental monitoring systems.9 Wearable energy harvesting would also be more widespread if photovoltaic cells and piezoelectric systems were conformal and compatible with any surface.10 A common thread in this field is the need for thin, compact and flexible energy storage solutions11 and, in this regard, supercapacitors have clear advantages over batteries12 along with high power density, long cycle life and safety.13 Many of these applications are made possible by the use of graphene, often labeled the "wonder" material thanks to its record properties like high thermal and electric conductivity, large surface area and high intrinsic tensile strength.14 Nonetheless, preserving these qualities on different substrates is still a major concern in the case of graphene monolayers.15 Commonly grown via chemical vapor deposition (CVD) and transferred,14 they are vulnerable to defects, degradation of electrical properties by contamination and accidental damage during transfer.16,17 Since they are prone to tearing with the slightest disturbance, polymer-supported transfer methods surfaced to prevent the physical damage on graphene at the expense of possible polymer contamination causing higher sheet resistance and decreased carrier mobility.18 Moreover, most CVD graphene transfers require post-patterning on the target surface by semiconductor fabrication processes, a shortcoming which restricts the application of graphene to clean-room and vacuum compatible, rigid and planar substrates like silicon wafers.19 These challenges in depositing graphene on various substrates were the main


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drive for developing printing technologies of graphene films, which provide direct patterning during deposition.14,20 Among these, inkjet printing stands out with advantages like non-contact patterning, micro-scale resolution and low cost.21 Even though the technology has broader substrate compatibility, choosing a substrate with suitable surface properties and resistance to postprinting annealing is still a complex matter.22 In this work, we present a novel technique which enables the production of graphene patterns and devices on a wide range of substrates by combining inkjet printing with a wet transfer method, as shown in Figure 1. First, the graphene ink is formulated23 (see Experimental Methods, SI) and a pattern is inkjet printed on a copper substrate (Figure 1a). After the ink is dried and thermally annealed, the substrate containing the graphene pattern is placed in a FeCl3 solution and let float. Once the copper substrate is fully dissolved in solution (Figure 1b), the free-standing graphene pattern is moved to de-ionized water. Finally, by submerging the target substrate in the water and bringing it close to the surface, the graphene film can be caught onto the substrate (Figure 1c). Throughout the transfer process, the graphene pattern floats on the surface of water (or aqueous solution) minimizing any accidental damage. The absence of steps like etching, lithography or other harsh processes is a significant benefit over traditional CVD graphene transfer techniques and allows the deposition of devices on a number of flat surfaces like plastics, paper, wood, leather; 3D substrates like cylinders, spheres and cubes and other unconventional and delicate substrates like fruits and plants. Worth mentioning is that such structures have strong adhesion to the target surface, avoiding the need of any intermediate substrates and adhesives. Because of their thinness, the graphene patterns have excellent conformability to the morphology (the local valleys and peaks) of the substrates, thus increasing their contact area and hence their adhesive force.24


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Figure 1. The wet transfer process of inkjet printed graphene. a) Graphene flakes are inkjet printed on copper foil and thermally annealed. b) The copper substrate is etched in a FeCl3 solution and the floating graphene film is moved first to a cleaning HCl solution, then to de-ionized water. c) The graphene film is transferred onto a lemon by submerging the latter under the floating graphene film and raising it to the water surface. d) SEM image of the inkjet printed and transferred graphene film showing the protruding graphene flakes. Graphene patterns can be transferred onto many objects, including e) the leaf of a poinsettia; f) a petal of an orchid and g) an acrylic sphere.

Besides, since the printed graphene flakes have exceptionally strong cohesive bonds after annealing, the graphene patterns are robust and hence no carrier polymer is needed during our transfer processing, resulting in films of graphene flakes free from polymer residue and damage from polymer removal. An example of the result can be observed in the inset in Figure 1c, with the printed honeycomb pattern adhering firmly to the rugged surface of a lemon, chosen as a substrate because of its surface roughness, 3D nature and susceptibility to harsh solvents and to intrusive processes. Also, in case the pattern to transfer is open (e.g. the interdigitated structure of


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the micro-supercapacitors in Figure 1e to 1g), few supporting lines can be printed with a polyimide (PI) ink as a first step to retain its shape until it is deposited on the target. Our technique addresses some typical limitations of inkjet printing of graphene: (1) the graphene inks often require thermal annealing (at temperatures higher than 200°C)25 or photonic annealing20,26 for removing the stabilizing polymer; (2) ink rheology and printing parameters need to be tailored for every substrate and (3) surface properties of the substrate like wetting and surface energy often need to be engineered in order to obtain desired pattern geometry and resolution.27,28 In our case, all the ink treatment procedures are performed on copper before transfer and hence neither post-annealing nor substrate-dependent ink tailoring or surface engineering is needed, considerably extending the applicability to a broad range of substrates. Besides, it is also possible to accurately transfer the graphene patterns onto a predefined position. To assess the transfer accuracy, we transferred graphene patterns onto their target position defined by inkjet printed gold frames and measured their misalignment. Out of several transferred samples, the misalignment was always smaller than ~300 µm on SiO2 (see Transfer misalignment estimation, SI, Figure S1, Figure S2 and Table S1), demonstrating that our transfer technique is also suitable for many applications in macroelectronics. From the scanning electron microscopy (SEM) images shown in Figure 1d and Figure S3, the protruding graphene flakes, typical of inkjet printed graphene, are clearly visible and contribute in increasing the surface area of the film.21 In general, no cracks or breaks were observed throughout the surface confirming their great structural stability during the transfer process. The morphology of the transferred graphene films on SiO2 is similar to their as-printed counterparts (Figure S3a and S3b) without observable residue induced by the transfer process. They also appear to obtain the morphology of the underlying substrates: e.g. graphene on rippled paper exhibits the same ripples on its surface (Figure S4), confirming a good adhesion between


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graphene and substrate. The Raman spectra in Figure S5 show the same characteristic graphene peaks before and after transfer, suggesting that the graphene quality does not degrade during the transfer process. Because of their high electrical conductivity, intrinsic capacitance and high surface area, films of graphene flakes find applications as electrode material for supercapacitors.29 Our transfer technique of inkjet printed graphene might further broaden its use and provide better integration with surrounding structures or components, with the same mechanical properties and flexibility of the host object. In this regard, with this technique we have fabricated fully-assembled microsupercapacitors (MSCs) with a PVA/H3PO4 gel electrolyte on a multitude of surfaces.


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Figure 2. Electrochemical performance of the inkjet printed and transferred graphene micro-supercapacitors. a) Micro-supercapacitors on four of the substrates used for the electrochemical characterization: polystyrene, paper, birch wood and leather. b) Cyclic voltammetry curves at the scan rate of 100 mV s-1 of micro-supercapacitors on polystyrene, polyvinyl chloride, birch wood, leather, paper and glass. c) Dependence of the areal capacitance on the contact angle of de-ionized water on the substrates. The contact angle values are shown in Table S2. d) Galvanostatic chargedischarge curves of a micro-supercapacitor on polystyrene at current density of 5 µA cm-2. e) Electrochemical impedance spectroscopy curves of a micro-supercapacitor on polystyrene for frequencies ranging from 10 mHz to 200 kHz. The insert shows a detail of the curve at high frequencies.

To characterize their electrochemical performance, we have performed cyclic voltammetry (CV), galvanostatic charge discharge (GCD) and electrochemical impedance spectroscopy (EIS) on each device. The final devices had 550 µm wide and 1.9 mm long fingers interspaced by 200 µm gaps, with an active area of ~0.15 cm2 (excluding finger gaps). The thickness of the transferred


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devices was measured with a profilometer to be less than 400 nm (see Figure S6), about 230 nm thicker than directly printed graphene with the same printing parameters. Such a difference may indicate some volume expansion of the graphene films during the transfer process. The roughness of the transferred graphene electrodes was also measured with atomic force microscopy to be 90 nm (see Figure S7). To demonstrate the reproducibility of the technique, we have also performed CV measurements on as-printed and transferred samples both on SiO2 and glass, showing similar performances without any evident capacitance degradation (Figure S8). The lack of additional redox peaks in the transferred samples provide further evidence that our transfer process does not introduce any detectable contamination for energy storage applications, although more detailed analysis could be needed to locate residual metallic contamination which may impact the applications in integrated circuits.17 Besides SiO2 and glass, the following untreated materials were chosen as substrates: polyvinyl chloride (PVC), polystyrene (PS), polypropylene (PP) with smooth and matte surface, low-density polyethylene (LDPE), polyimide (PI, Kapton), common printing paper, birch wood and leather. The optical images and the results of the electrochemical characterization of all the devices can be found in Figure S9 and S10. The capacitances vary with the underlying substrates (Figure S11a), with the best performing device being on PS (~300 µF cm-2 at 5 mV s-1). Figure 2b provides a summary of typical CV curves at 100 mV s-1 of MSCs on some relevant substrates. The absence of redox peaks proves that the energy is only stored through the electric-double layer capacitance (EDLC) and that no reaction takes place between substrate and device.13 We found that the dominant factor determining the performance of the devices is the wettability of the surface, with the highest capacitances occurring at contact angles between 80° and 100° (Figure 2c). For substrates with contact angles higher than 100°, the adhesion of graphene is not as strong, leading


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to capacitance drop and a less reliable transfer process. A good adhesion of graphene flakes to the substrate is crucial since the gel electrolyte PVA/H3PO4 is drop-casted in liquid form causing the graphene to peel off from the hydrophobic substrate. On the other side of the spectrum, all the MSCs on more hydrophilic substrates with contact angles

Wet Transfer of Inkjet Printed Graphene for Micro-Supercapacitors on

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