Screen-Printing of a Highly Conductive Graphene Ink for Flexible

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Functional Nanostructured Materials (including low-D carbon)

Screen Printing of a Highly Conductive Graphene Ink for Flexible Printed Electronics Pei He, Jianyun Cao, Hui Ding, Chongguang Liu, Joseph Neilson, Zheling Li, Ian A. Kinloch, and Brian Derby ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04589 • Publication Date (Web): 07 Aug 2019 Downloaded from pubs.acs.org on August 7, 2019

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ACS Applied Materials & Interfaces

Screen Printing of a Highly Conductive Graphene Ink for Flexible Printed Electronics

Pei He*,,†,‡,§ Jianyun Cao, ‡, § Hui Ding, ‡ Chongguang Liu, ‡ Joseph Neilson, ‡ Zheling Li, ‡ Ian A. Kinloch, ‡ and Brian Derby*,‡

†

School of Physics and Electronics, Central South University, Changsha 410083, Hunan,

P.R.China. ‡

School of Materials, University of Manchester, Oxford Road, Manchester M13 9PL, U.K.

§These

authors contributed equally

Email: [email protected]; [email protected]

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ABSTRACT Conductive inks for the future printed electronics should have the following merits: high conductivity, flexibility, low cost and compatibility with wide range of substrates. However, the state-of-the-art conductive inks based on metal nanoparticles are high in cost and poor in flexibility. Herein, we reported a highly conductive, low cost and super flexible ink based on graphene nanoplatelets. The graphene ink has been screen printed on plastic and paper substrates. Combined with post-printing treatments including thermal annealing and compression rolling, the printed graphene pattern shows a high conductivity of 8.81 × 104 S m−1 and good flexibility without significant conductivity loss after 1000 bending cycles. We further demonstrate that the printed highly conductive graphene patterns can act as current collectors for supercapacitors. The supercapacitor with printed graphene pattern as current collector and printed activated carbon as active material shows a good rate capability up to 200 mV s−1. This work potentially provides a promising route towards the large-scale fabrication of low cost yet flexible printed electronic devices. KEYWORDS: Graphene ink, screen printing, flexible, printed electronics, supercapacitor


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INTRODUCTION The development of appropriate printing methods and compatible inks represents an emerging methodology for the production of large-area and novel printed electronic devices.1 Printed electronics shows great potential for a number of applications including touch screen displays,2 solar cells,3 flexible displays,4 sensors,5 photodetectors,6 organic light-emittingdiodes (OLEDs),7 electronic paper (E-paper),8 radio-frequency identification (RFID) tags,9 energy conversion and storage10-11 and electronic textiles.12 Through formulating inks containing functional materials, a variety of electronic components can be produced through solution phase printing processes such as: inkjet, screen, flexographic and gravure printing.13

A core requirement for most (perhaps all) electronic devices is the presence of interconnecting highly conductive tracks. Thus, the development of high quality inks for the printing of highly conductive materials is critical to the adoption of printed electronics as a viable manufacturing process.14 A promising conductive ink should fulfil the requirements of good printability, long time stability, low cost, and compatibility with a range of substrates, as well as good resistance to flexure and ability to withstand some extensional strain.15

To date, most conductive inks for printed electronics are based on dispersed metal nanoparticles, such as gold, silver and copper,15 due to their high electrical conductivity. Among these, silver nanoparticles are most commonly used because their relative inertness allows heat treatment under standard atmospheric conditions. However, the price of silverbased paste is over 920 USD/kg and in the trend of upwards due to the increasingly scarce of the raw silver.16 Copper is a lower cost alternative metal with high conductivity; however, copper nanoparticles are easily oxidized, which is a challenge when integration with other electronic components that require annealing treatments. Thus their lower material cost is



offset by the need for more complex and expensive processing.14 Although other materials, such as conductive polymers,17 and carbon nanotubes (CNTs)18 have been formulated in conductive inks for printed electronics, the properties of printed tracks based on these materials are not competitive to metal inks. Thus, there is an urgent requirement for new conductive materials to be developed as printable inks.

Graphene, a two-dimensional allotrope of carbon, is a promising material for conductive inks due to its high intrinsic conductivity, good mechanical properties, and scalable production capability.19 Graphene can be readily produced from natural graphite at a large scale by chemical oxidation,20 electrochemical oxidation,21 exfoliation in liquid phase through ultrasonication,22 electrochemical exfoliation,23 high-shear mixing24 and microfluidization.25 The exfoliated graphene flakes can be dispersed directly in a number of organic solvents or cosolvent mixtures without additives22,

26-28

or, alternatively, using water or other common

solvents with stabilizing polymers or surfactants.29-31

During the last few years, graphene-based inks have been used to fabricate conductive components for a range of demonstrator applications in printed electronics using inkjet printing,32-39 gravure printing,40 screen printing,25, 41-45 transfer printing,46 and 3D printing.47 While inkjet, transfer and gravure printing techniques offer high resolution graphene patterns,40 screen printing is a practical option when low resistance is required, because it can produce thick films (µm range) in a single pass, reducing processing times and allowing easy integration with high-throughput manufacturing processes.

Several studies have reported the screen printing of graphene-based inks based on either reduced graphene oxide44-45,

48

or exfoliated pristine graphene.35-37 The measured sheet


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resistance, Rs, of screen printed graphene patterns obtained using these inks were in the range 2 - 100  □−1.25, 37, 41 However, the conductivity of these printed graphene patterns are in the range of one to a few hundred S cm-1, which is much lower than that of polycrystalline graphite (1250 S cm−1).49 This is due in part to the difficulty in fully removing the binder or surfactant components of the inks from the printed patterns. The binder or surfactant enhances the stability and printability of graphene inks, but it coats the graphene flakes with low conductivity material,50 which greatly increases the contact resistance between overlapped graphene flakes. Another reason is the process used to break down the source graphite material to few-layer flakes or monolayers of graphene leads to a relatively small mean graphene flake size, typically < 1 µm,24-25, 33 which introduces a large number of sheetto-sheet junctions in large area printed graphene patterns.

To increase the conductivity of printed graphene patterns, it is necessary to enlarge the size of graphene flakes and minimize the amount of stabilizer used in the final ink formulation. Although the highest conductivity of printed pristine graphene is based on binder-free graphene inks,9 the adhesion between graphene and the substrate is generally weak in the absence of binder. Here, we present a highly conductive ink based on the Graphene nanoplatelets with a large original mean flake diameter of ~ 19.3 µm and polymer binder less than 1% in weight. The printed graphene patterns on flexible substrate show a high conductivity of 8.81 × 104 S m−1. In addition, we have demonstrated that the printed highly conductive graphene patterns can be used directly as current collector to replace the traditionally used metallic current collector for electrochemical energy storage devices. The use of printed highly conductive graphene pattern as current collector potentially offers a low cost, flexible, and one-stop option for both organic and aqueous electrolyte systems.



EXPERIMENTAL SECTION Graphene ink preparation: Graphene nanoplatelets (GNPs) (xGnP, Grade M25) was purchased from XG Sciences (XG Sciences Inc., Lansing, MI, USA). Polyvinylpyrrolidone (PVP) with average molecular weight of 1300,000 was purchased from Sigma-Aldrich (Gillingham, Dorset, UK). Briefly, 0.5 g PVP was added into a 80 ml glass beaker containing 50 ml of ethylene glycol (EG, Sigma-Aldrich) under continuous stirring for 2 hours. Then 5 g of GNPs was dispersed in the mixture with further stirring for 10 mins. The viscous slurry was then placed under a tip sonicator (Q700 Sonicator, QSonica, Newtown, CT, USA) at 80% power for 15-30 min. During the sonication, the ink container was cooled by ice water. The obtained viscous dispersion was used for further experimentation.

Activated carbon ink preparation: Briefly, 2.4 g activated carbon (YEC-8B, Yihuan Carbon Inc., Fuzhou, China) and 0.6 g carbon black (Carbon black, acetylene, 50% compressed, Alfa Aesar, Heysham, UK) were added in a solution of ethyl cellulose (EC) (2% w/v dispersion, EC, Fisher Scientific, viscosity 10 cP, 5% in toluene/ethanol 80:20, 48% ethoxy) in 20 ml terpineol (Sigma-Aldrich) in a mixing cup. The mixing cup was then located in a speed mixer (DAC 150 FVZ-K, FlackTek Inc., Landrum, SC, USA) at 2000 rpm for 30 min, and a black ink was obtained.

Screen printing of graphene ink: Screen printing was performed using a 10 in. × 8 in. hardwood screen printer (Screenstretch Ltd, Manchester, UK) with emulsion screens and swiss polyester screen printing mesh (155 mesh count, thread per inch, ~ 100 micron opening). The screen printing frame was placed on the substrate separated by a PDMS spacer with thickness of  2 mm. A polyurethane squeegee ( 45° angle with the mesh) was employed to brush the ink at a speed  60 mm s−1. Polyimide (PI, 70 µm), Polyethylene 6 ACS Paragon Plus Environment

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terephthalate (PET, OHP Laser Printer Film, 100 µm), and paper (Lenzingpapier, 80 g m−2) were employed as the substrates for printing. All the substrates were used as-received. The printed graphene patterns were then dried in an oven at 80 C for 2 h. For graphene patterns with more than one printing layer, the printed pattern was annealed at 80 C for 30 min after each printing pass.

Post annealing and rolling compression process: After drying, the patterns printed on the PI substrate were heat treated on a hotplate (C-MAG HS10, IKA, Wilmington, NC, USA) at temperatures of 100, 120, 150, 200, 250, 300, 350 and 400 C for 30 min. For PET and paper substrates, the printed graphene patterns were treated at 120 C for 2 h. A compression rolling process was performed using a MSK-HRP-MR100A roller (MTI Corporation, CA, USA) at a speed of 22 mm s−1.

Fabrication and characterization of printed supercapacitors: Supercapacitors were fabricated on PI films. First, the as-prepared graphene ink was printed on PI substrates through a screen mask with 5 passes. The printed graphene patterns were annealed at 120 C for 10 min between each printing pass to remove the solvents. After printing, the graphene patterns were annealed at 350 C for 30 min to further remove the polymer binder and then compressed to obtain high conductivity. For graphene/AC electrode, the AC ink was printed on the top of graphene patterns with a screen mask. For AC electrode, the AC ink was printed directly onto the PI substrate with a screen mask. In terms of traditional metal current collector, AC was printed directly on Ti foil (titanium foil, 0.032 mm thick, annealed, 99.7% metal basis, purchased from Alfa Aesar). The printed patterns were annealed at 120 C for 30 min to remove the solvent. The printed graphene, graphene/AC, and AC patterns with 1 cm  1 cm dimensions were directly used as electrodes for the supercapacitor device fabrication. 7 ACS Paragon Plus Environment


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Symmetric sandwich structured capacitor were assembled from two pieces of printed electrodes with a piece of PTFE filter as a separator and 1 M H3PO4/ polyvinyl alcohol (PVA) gel as the electrolyte. All electrochemical experiments were carried out using an electrochemical workstation, Ivium Stat (Ivium Technologies, Eindhoven, Netherlands).

The area capacitances of the devices were calculated from the charge-discharge curves according to the following equation: CA = I∆t/(S∆V)

(1)

here S is the geometric area of the electrode (cm2); I is the discharge current density (mA cm−2); ∆t is the discharge time (s); ∆V is the cell voltage excluded the voltage drop (V).

Characterization methods: The morphology of the GNP flakes and printed graphene patterns was characterized using an atomic force microscope (AFM) (Dimension 3100, Bruker, Billerica, MA USA) and a scanning electron microscope (SEM) (Philips XL30 FEG, Eindhoven, Netherlands). The rheological properties of the inks was measured using a TA Discovery HR-3 rheometer (TA Instruments, New Castle, DE, USA) with a cone-plate geometry (100 µm truncation gap and 60 mm diameter, 2°). The contact angle of graphene ink on different substrates was recorded by a Drop Shape Analyzer DSA100 (KRÜSS GmbH, Hamburg, Germany). Raman spectra were obtained using a Renishaw 2000 Raman spectrometer system (Wooton-under-Edge, UK) equipped with a 633 nm laser. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Axis Ultra DLD XPS spectrometer (Kratos Analytical Ltd, Manchester, UK). The thermogravimetric analysis (TGA) was measured using a TA Q500 system (TA Instruments) under ambient air conditions. The thickness of the printed patterns was obtained using a Dektak XT stylus surface profiler (Bruker AXS GmbH, Karlsruhe, Germany). All the sheet resistance was


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measured using a Jandel four-point-probe station (Jandel Engineering, Linslade, UK) equipped with a 2182A nanovoltmeter and a 6220 current source (Keithley Instruments, Cleveland, OH, USA). RESULTS AND DISCUSSION Screen printing of graphene inks. The graphene ink used in this study contains GNPs at a concentration of 100 mg/ml with average lateral size of 7.3±3.4 µm and thickness of 10-15 nm (Figure S1 and Figure S2), dispersed in EG as the solvent using PVP with mean molecular weight of 1300 KDa as the organic binder and rheological agent to stabilize the ink. PVP was chosen as the stabilizer and binder due to its ability to disperse graphene flakes and has been used in printable conductive graphene inks formulation.51-54 EG was selected as the solvent due its low vapour pressure and good solubility for PVP. Figure 1a and b illustrates the screen printing process used with the graphene inks in this study. The PVP dispersed in EG shows Newtonian fluid behaviour, while the graphene ink shows shear thinning with a viscosity of 0.65 Pa.s at a shear rate of 10 s-1 (Figure 1c). The resulting ink contains certain population (20%) of flakes with size larger than 10 µm (Figure 1d and e, Figure S1b). (a)

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0.1

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Figure 1. (a) Schematic illustration of the screen printing process used with the graphene inks. (b) A cross section schematic of the screen printing process during printing. (c) The measured viscosity of polymer solution and GNP-ink under different shear rates. (d) SEM image of the dried GNP-ink, scale bar = 10 µm. (e) higher magnification image of the region outlined on (d), scale bar = 2 µm.

In order to produce high quality conductive tracks after screen printing, it is necessary to control the drying of the ink and use a post-printing heat treatment to remove all traces of residual solvent along with the PVP binder phase. To check the wettability of the ink, we measured the ink contact angle on PI, PET and paper substrates. The recorded contact angle images are shown in Figure S3. The contact angle of graphene ink on PI, PET and paper substrates are 51.5±4.2°, 54.7±3.8°, and 61.1±2.5°, respectively, suggesting good wettability of graphene ink over these substrates. After printing on PI substrates, graphene patterns were firstly dried at 80 C for 2 hours to partially remove the solvent, then further annealed in air at a temperature range of 100 to 400 C to investigate the influence of residual solvent and PVP binder to the electrical resistance. Figure 2a shows that the sheet resistance of the printed graphene patterns decreases with the increasing of annealing temperature (Ta). With Ta < 150 C, there is a slight reduction in Rs to 34.1  □ -1. When Ta is further increased to above 250 C, there is a significantly greater influence of annealing temperature on the sheet resistance. This transition is correlated to the boiling point of the EG solvent at ~ 197 C. At 350 C, Rs reaches 13.3  □−1, then decreases slightly to 12.1  □−1 at 400 C.


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Figure 2. (a) Sheet resistance of the printed graphene films as a function of the annealing temperature for a fixed annealing time of 30 min, showing effective polymer decomposition at 300 C. (b) Variation of sheet resistance with annealing time for a fixed annealing temperature of 350 C. (c) Raman spectra from the starting GNP powder, as-printed and annealed (350 C) GNP pattern (d) High-resolution XPS spectra of the GNP powder, as-printed and annealed (350 C) GNP pattern illustrating the changes in the C 1s peak.

The significant decrease of Rs above 250 C is possibly due to the decomposition of the PVP polymer binder. From TGA results (Figure S4), PVP polymer begins to decompose and char at around 270 C. PVP can transform into a polyamide-polyene structure at temperatures > 250 C, which in turn converts to a thin layer of amorphous carbon at T > 300 C,55 the resulting - stacking of the residual products provides an efficient charge transport bridge between graphene flakes. This hypothesis correlates with the much lower sheet resistance of graphene films measured when Ta > 300 C. Although the lowest sheet resistance was found with 400 C, the GNP powder degrades at temperature higher than 350 C due to its reaction 11 ACS Paragon Plus Environment


with oxygen in the air atmosphere. Therefore we selected 350 C as the optimum postprinting annealing temperature for further investigation. Figure 2b shows that, for Ta = 350 C, annealing for as short as 20 min is sufficient to achieve the minimum sheet resistance. Based on the measured track thickness, t, (Figure S9), the conductivity of the printed graphene film annealed at 350 C is 5.12 ± 0.21 × 103 S m−1, according to the equation  = 1/ (Rst).

To further characterize the chemical and structural changes of the printed graphene films after annealing, Raman spectroscopy and XPS were performed on the source GNP powder, as-printed and annealed graphene films. As shown in Figure 2c, the GNP powder shows a typical spectrum of D, G, and 2D peaks, with the D/G ratio of about 0.15, which is similar to that of expanded few-layered graphene reported previously.41 The as-printed graphene film shows the same typical peaks as the graphene powders but the D/G ratio increases to 0.43 with broad D and G peaks, which can be attributed to the increasing density of edge defects within the GNPs planes after the sonication process during ink formulation, and the Raman signal from the polymer binder (Figure S5). After thermal annealing, the features in the Raman spectrum of the graphene film shows a reduction of D/G ratio to about 0.24, which is similar to that of the initial GNPs powder, indicating a reduced defect density in the graphene flakes accompanying the removal of the polymer.

The XPS results can be interpreted in a similar manner to the Raman data. As shown in Figure 2d, the C 1s XPS spectrum of the GNPs powder shows a high sp2 carbon content and relatively low C-O/C=O character. However, the as-printed graphene film also exhibits features consistent with PVP ((C6H9NO)n) in addition to that from GNPs. This includes higher O and N content, in which the N 1s peak at ~ 400 eV indicates the presence of C-N 12 ACS Paragon Plus Environment

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bonding (Figure S6). Following thermal annealing, the presence of the C-O/C=O bond weakens, while only a minor N 1s peak at ~ 400 eV is observed, indicating the decomposition of PVP. The remaining minor components of N 1s seen in the spectrum might be residual decomposition products of PVP, such as polyamide-polyene fragments.55 Moreover, the C/O ratio from the annealed graphene film is 33.8 (Figure S7), which is higher than that of asprinted graphene film (26.8), indicating the decomposition of oxygen functional groups in the PVP. Overall, from electronical, Raman and XPS analysis of the printed graphene film, it is concluded that Ta = 350 C is sufficient to decompose the PVP and improve the interfacial contact between graphene flakes.

The surface of the as-printed graphene films is relatively rough and reveals a loosely bonded flake microstructure (Figure 3a). This indicates a low film density, which is confirmed by SEM images of film cross-sections shown in Figure 3c. Low film densities were seen with films after annealing at all temperatures (Figure S8). These low density films contain small flake-flake contact areas and thus show relatively high resistance. (b)

(a)

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(d) 2 µm

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Figure 3. SEM images of the surface morphologies of as-printed (a) and rolling compressed (b) graphene patterns. SEM images of cross-sections of as-printed (c) and rolling compressed (d) graphene patterns. The dashed lines indicate the outline of the graphene layers. The insert in (d) shows a higher magnification image.



To improve the physical contact between the graphene flakes, the films were compressed through a rolling process. Figure 3 displays the SEM surface and cross section morphologies of printed pattern before and after compress. On comparing Figure 3c and d, it is clear that the rolling process decreases the film thickness significantly, leading to dense films with well contacted graphene flakes. The thickness of the compressed graphene film is 2.5 ± 0.2 µm, which is about 17% of the thickness of the annealed film (14.7 ± 2.9 µm) (Figure S9). Meanwhile, the higher density of the graphene films after rolling improves the electrical properties of the printed film. As shown in Figure 4a, the sheet resistance decreases by about 60% as compared to the printed and annealed films (Figure 2a), reaching a lowest value of 5.3  □−1 for Ta = 350 C, corresponding to an electrical conductivity of 7.48 ± 0.28 × 104 S m−1 (Figure 4b), which is a factor of  15 greater than that of the printed and annealed films. (b)

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Figure 4. (a) Sheet resistance and film thickness of the printed graphene patterns before and after compression at an annealing temperature of 350 C. (b) Electrical conductivity of the printed graphene patterns before and after compression at annealing temperature of 350 C. (c) Sheet resistance of as-printed and compressed graphene patterns at an annealing temperature of 350 C with the increasing number of print repetitions. (d) Change of thickness and electrical conductivity of the


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compressed graphene patterns on PI substrates for increasing numbers of printing passes, showing relatively stable conductivity after two print repetitions. (e) Evolution of sheet resistance of the compressed graphene patterns printed on PET and paper substrates for increasing numbers of print repetitions. (f) Map of conductivity and highest process temperature for previous reported conductive graphene-based patterns by a range of printing processes, including C-RGO, T-RGO, and graphene. Isolines present the product volume resistivity ( □−1 mil−1) corresponding to the conductivity at 4 × 103 and 4 × 104 S m−1, respectively.

The effect of annealing and compression on the adhesion of printed graphene pattern on PI substrate was investigated by a Scotch tape peeling test (Figure S10). Sheet resistance increase (∆R) after Scotch tape peeling was measured and used as an indication of adhesion. The printed graphene patterns on PI substrates with annealing at 80, 200 and 350 °C showed an increased resistance of ~11.7 %, 7.1%, and 10.6%, respectively. While for the printed patterns after compression, the electrical resistance showed an increase of ~5.6%, 2.9%, and 5.9% for annealing temperature of 80, 200 and 350 °C, respectively. The results indicate that totally removing of solvent at 200 °C benefits the adhesion while decomposing of polymer binder at 350 °C worsens the adhesion. In total, compression enhances the adhesion, especially with the presence of polymer binder.

To further decrease the sheet resistance of the printed graphene patterns, we increased the number of printing passes. As shown in Fig 4c, the sheet resistance of the printed graphene films decreases with the increase of number of printing passes. The printed patterns after thermal annealing and compressive rolling process show a lowest sheet resistance of ~ 1  □ −1

at 5 printing passes. As shown in Figure 4d, the thickness of patterns increases linearly

with printing passes, with thickness addition of around 2.3 µm for each additional repetition. The conductivity of compressed patterns appears to increase after the second layer is printed, it then remains approximately constant and is measured to be 8.81 ± 0.34 × 104 S m−1 after 5 15 ACS Paragon Plus Environment


printing passes (~ 11.4 µm). The initial reduction in conductivity is probably associated with multiple passes leading to a more consistent printed track height.

To validate the versatility of the ink and the printing process on different substrates, we printed graphene patterns on PET and printer paper using a lower temperature anneal, Ta = 120 C, followed by the compressive rolling process (see Figure S11 for images of SEM surface morphologies of graphene patterns on PET and paper substrates). As shown in Figure 4e, the sheet resistance of the printed patterns on PET and paper substrates after a single printing pass was Rs = 7.9 and 8.6  □ −1, respectively. For both substrates this decreased further to 1.6 and 1.5  □ −1 after 5 printing passes. The thickness of the printed patterns on PET substrates is t ≈ 12.1 µm after 5 printing passes (Figure S12a), leading to a conductivity of 5.18 ± 0.16 × 104 S m−1. The thickness of printed patterns on paper substrates could not be measured due to the intrinsic roughness of the paper surface and the penetration of graphene flakes into the paper substrate (Figure S13). The conductivity of printed patterns on PET substrate shows a high value of 4.39 ± 0.09 × 104 S m−1 even for a single printing pass (Figure S12b), which is comparable to that of both high temperature annealed50 or binder-free printed graphene patterns.9

Figure 4f compares the conductivity of printed graphene-based patterns on flexible substrates at a range of maximum processing temperatures, taken from a number of reports in the literature, including thermal reduced GO (T-rGO),56-57 chemical reduced GO (C-rGO),34, 37, 5860

and pristine graphene,25-26, 28, 33, 40-41, 43, 50, 61-65 see the detailed data in ESI Table S1. The

dashed lines represent isolines of the product volume resistivity  □ -1 mil-1, corresponding to the conductivity of 4 × 103 and 4 × 104 S m−1, respectively. The shaded areas represent the flexible substrates (PET, paper, and PI) used in the literature. It can be seen that our printed 16 ACS Paragon Plus Environment

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graphene patterns exhibit the highest conductivity compared with previously reported graphene-based conductors, even at a relatively low processing temperature (120 C). Note that the volume resistivity of our graphene patterns on PET and PI substrates is 0.78 and 0.46  □

−1

mil−1, respectively, which can meet the requirements of flexible electronics

applications even for OPV and RFID antennas. For printed graphene lines in micrometre scale (line width from 140 to 220 µm), the conductivity increases significantly with the increase of line width and reaches a value of 4.41 ± 0.22 × 104 S m–1 at a line width of 220 µm (Figure S15). This conductivity and resolution can meet the demands of flexible electronics, including printed potentiometers, resistors, heaters, and LED attach applications.

In addition to high conductivity, conductors require good mechanical durability to confirm their suitability for flexible electronics. To further characterize the mechanical behaviour of the graphene patterns, we conducted bending tests on the printed patterns on PI, PET, and paper substrates with post annealing and compressive rolling. As shown in Figure 5a, the printed graphene patterns on all three substrates exhibit no observable loss in relative resistance (R/R0) after deformation through various radii of bending from ~ 18 to 2.5 mm. The evolution of electrical performance of the printed patterns was also measured with the increase of bending cycle under a constant bending radius. As shown in Figure 5b, there is no observable degradation in the electrical resistance of the printed patterns after 1000 bending cycles at a bending radius of 3 mm. In contrast, the printed silver patterns show clear increase in the relative resistance after 1000 bending cycles (Figure S16). These mechanical tests show the high flexibility of the printed graphene patterns. To investigate the influence of bending to the morphology of the thin films, we measured SEM images of graphene patterns before and after bending cycles, as shown in Figure S14. The printed graphene patterns on PI and PET substrates have almost no breakages after the bending test. Some cracks appear in 17 ACS Paragon Plus Environment


the paper-based pattern due to the mechanical deformation, but they have little effect on the electrical resistance. (a)

(b)

1.4

1.4 PI PET Paper

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0.8

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3

6

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Paper substrate

Figure 5. Normalized resistance of the screen-printed graphene patterns on flexible substrates (PI, PET and paper) that measured after bending deformation: (a) after 1000 cycles with different values of bending radius and (b) at a constant bending radius of 3 mm over a range of bending cycles up to a maximum of1000. Printed graphene patterns on PET (c) and paper (d) substrates used as conductive tracks to connect a LED light, displaying bright illumination when powered by a 3V cell battery with different bending and twisting shapes. Scale bars in (c) and (d) are 1 cm.

To further demonstrate the conductivity and flexibility of the graphene patterns on these substrates, we fabricated a series of printed graphene tracks with bending and twisting shapes on the PET and paper substrates (Figure 5c and d). A red light-emitting-diode (LED) light was connected to the graphene tracks by double sided copper tape. The red LED light displayed bright illumination when the circuit was powered by a 3 V battery. The luminance of LED light shown almost no visible change when the printed patterns were bended by hand (Video S1 and Video S2), which illustrates the applicability of our printed graphene patterns for flexible electronics. Moreover, graphene patterns with different architectures have been


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printed on PET and paper substrates to demonstrate the ability of high-resolution patterning. Optical microscope images and photographs of the printed patterns are shown in Figure S17.

Printed graphene patterns as collector for supercapacitors. Current collector is an essential component in in energy storage devices, including batteries and supercapacitors; it mechanically holds the electrode material and conduct electricity from the electrode material to the electrode lead.66 The current collectors of commercial batteries and supercapacitors are made of metals such as Al and Cu for organic electrolyte and Ni, Ti and stainless steel for aqueous electrolyte. These metal current collectors are heavy, bulky, expensive and tend to corrosion after long-term service, while Development of highly conductive carbon based current collector potentially offers a one stop option to replace metal current collector in both organic and aqueous energy storage systems.66-67

To further demonstrate the applicability of these highly conductive printed graphene patterns in a variety of flexible electronic applications, we fabricated symmetric metal current collector-free supercapacitors using the printed graphene patterns on a PI substrate as current collector and the printed AC layer on top of the graphene layer as active material. The asprinted graphene/AC electrodes were assembled into a supercapacitor device with a piece of PTFE filter paper as separator and 1 M H3PO4/PVA gel as electrolyte. As a control experiment, symmetric supercapacitors based on printed graphene electrodes (without AC as active material), printed AC electrodes (without graphene as current collector) and printed AC on Ti foil substrate (traditional current collector) were also fabricated and tested.

Figure 6a schematically illustrates the structure of the solid-state supercapacitor based on the printed graphene/AC electrodes. Figure 6b compares the CV curves of different



supercapacitors at a scan rate of 100 mV s−1. As expected, the AC electrode without current collector shows small current response (0.14 mA cm-2 at 0.4 V for positive scan) while the electrode with traditional metal current collector (AC on Ti foil) shows much higher response current (2.66 mA cm−2 at 0.4 V for positive scan) and nearly ideal rectangular characteristic. For the supercapacitor based on printed graphene electrode, the CV curve already exhibits certain value of response current (0.7 mA cm-2 at 0.4 V for positive scan) due to the double layer capacitance from the graphene flakes. Hence, after the printing of AC active layer, the CV curve of graphene/AC electrode shows slightly larger responsive current (3.55 mA cm−2 at 0.4 V for positive scan) than that of the AC on Ti foil electrode, indicating the potential of highly conductive printed graphene pattern as current collector.

Further, as shown the Figure 6c, the CV curves of the graphene/AC supercapacitor are able to retain an acceptable rectangular characteristic even at high a scan rate of 200 mV s−1. Galvanostatic charge-discharge curves for the supercapacitor (Figure 6d) show a typical triangular shape with nearly linear geometry. These characteristics of both CV and galvanostatic charge-discharge curves indicate a typical capacitive response of the supercapacitor based on the printed graphene/AC electrodes.


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(b)

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I (mA cm2)

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0 20 mV s1 50 mV s1 100 mV s1 200 mV s1

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(d)

Retention (%)

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I (mA cm2)

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Single cell Four cells in series

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500

1000

1500

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2000

-1.5

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Figure 6. Electrochemical performances of the metal current collector free supercapacitors based on screen printed graphene/AC electrodes. (a) A schematic diagram of the solid-state supercapacitor. (b) CV curves of supercapacitors based on graphene, AC, Graphene/AC and AC on Ti foil electrodes at a scan rate of 100 mV s−1. (c) CV curves from the supercapacitor based on Graphene/AC electrodes at various scan rates. (d) Galvanostatic charge-discharge curves of supercapacitors based on Graphene/AC electrodes at various current densities. (e) Comparison of area capacitances at various discharge current densities for supercapacitors based on graphene, Graphene/AC and AC on Ti foil electrodes. (f) CV curves of Graphene/AC supercapacitor before and after bending deformation over 5 and 8 mm bending radius at a scan rate of 20 mV s−1. (g) Cycling stability of Graphene, Graphene/AC, and AC on Ti foil supercapacitors. (h) CV curves of single cell and four cells in series. (i) Red LED illuminated by the four cells in series by using commercial cables. (j) A red LED illuminated by the four cells in series by using the printed graphene lines on PET with different bending shapes as cables. Scale bars in (i) and (j) are 5 cm.

The evolution of area capacitance with the increase of current density (Figure 6e) demonstrates that the graphene/AC electrode is able to deliver an area capacitance of 36.6 mF cm−2 at 0.5 mA cm−2, and maintain at 21.7 mF cm−2 at a high rate of 10 mA cm−2. The area 21 ACS Paragon Plus Environment


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capacitance of graphene/AC electrode is comparable with that of the AC on Ti foil electrode, especially at low rate. Due to the double layer capacitance of the GNPs, the printed graphene electrode shows an area capacitance of 6.0 mF cm−2 at 0.5 mA cm−2, and maintains at 3.0 mF cm−2 at 10 mA cm−2.

Figure 6f demonstrates the mechanical flexibility of the supercapacitor based on printed graphene/AC electrodes. As shown, the CV curves (20 mV s−1) from the supercapacitors in their flat undeformed state and those measured when deformed with a bending radius of 8 and 5 mm are almost overlapped, suggesting good flexibility and excellent resistance to deformation. We further tested the cycling stability of the supercapacitor based on graphene/AC electrode, As control experiments, the cycling stability of supercapacitors based on printed graphene and printed AC on Ti foil electrodes was also tested. As shown in Figure 6g, the supercapacitor based on AC on Ti foil electrodes retains ~100% capacity after 2000 cycles, suggesting the AC is highly stable during cycling. However, the printed graphene supercapacitor loses 15% capacitance after 2000 cycles. Since the printed graphene acts as both active material and current collector in the electrode, the insertion and de-insertion of ions between the graphene flakes will dissociate the integrity of the electrodes, causing degradation of the conductivity and thus capacitance. Measurement of the sheet resistance of the graphene electrode after cycling confirmed this assumption, the sheet resistance increases from 6.85 ± 0.25 to 8.34 ± 0.24 Ω □ −1 (Note: 1 M H3PO4 aqueous solution was used as electrolyte for this control experiment on the purpose of easy disassemble of the cell).

In

terms of the graphene/AC supercapacitor, the capacitance retention is around 80% after 2000 cycles, which is slightly lower than the graphene capacitor (85%). As the AC is stable during cycling according to the control experiment, the capacitance loss of graphene/AC


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supercapacitor is possibly due to the degradation of the graphene layer and also the interface between graphene and AC.

We further connected four single cells of the supercapacitors in series, which could operate at a voltage of 3.2 V. The CV curves of the single cell (0.8 V) and four cells in series (3.2 V) both show relatively good rectangular characteristic (Figure 6h). As a demonstration, a red LED was illuminated by the as-fabricated four supercapacitors connected in series (Figure 6i). More interestingly, using the highly conductive printed graphene tracks on a PET substrate as the connector, the red LED has been successfully illuminated even when the connecting films are deformed to different shapes (Figure 6j). These results indicate the printed highly conductive graphene is able to work as the current collector for supercapacitor electrodes, thus has potential to be applied in various types of electrochemical energy storage and conversion systems.

CONCLUSION In summary, we have demonstrated a simple method for fabricating highly conductive graphene patterns on flexible plastic and paper substrates using a screen printing process. The graphene ink is prepared by dispersing commercial few-layered graphene nanoplatelets in EG as the solvent, and PVP as the stabilizer. The screen-printed graphene patterns on PI substrates exhibited high electrical conductivity of ~ 8.81 × 104 S m−1 after post annealing and a compressive rolling process. The printed graphene patterns showed outstanding mechanical stability on both plastic and paper substrates, suitable for flexible electronic applications. With the highly conductive graphene patterns as the current collector, solid-state supercapacitors on flexible substrates were fabricated successfully and showed an area capacitance of ~ 40 mF cm−2, as well as good rate capability and flexibility. Overall, this



work provides a scalable and low cost method for the practical manufacturing of a highly conductive graphene inks for printed and flexible electronic applications.

ASSOCIATED CONTENT Supporting information The supporting information is available free of charge on the ACS Publications website. SEM images and flake size distribution of GNP flakes; AFM images and height profiles for the GNP flakes; Images for contact angle of graphene ink on PI, PET, and paper substrates; TGA analysis of the GNP powder, PVP and GNP ink dried at 120 °C; Raman spectrum of PVP film; XPS N 1s spectra and carbon to oxygen ratio of GNP powder, as printed graphene film, and printed graphene films after annealing at 350 C; SEM images of printed graphene patterns annealed at different temperatures; Surface profiles of annealed graphene patterns before and after the rolling compressing process; Images and change in resistance for printed graphene pattern after Scotch tape peeling test; SEM images of the graphene pattern with 5 printing passes after rolling compression; Thickness and electrical conductivity of compressed graphene patterns on PET substrates for increasing numbers of printing passes; SEM images of cross section of printed graphene patterns with 5 printing passes on paper substrates after the rolling compressing process; SEM images of the printed graphene pattern before and after 1000 bending cycles; Images for different printed graphene patterns on PET and paper substrates; Comparison of printed graphene-based patterns on flexible substrates from the literatures.

AUTHOR INFORMATION Corresponding Authors *[email protected] 24 ACS Paragon Plus Environment

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*[email protected]

Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. §These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors are grateful to the Engineering and Physical Sciences Research Council (EPSRC), grant reference EP/N010345/1, EP/L012022/1, EP/K016954/1 and EP/L020742/1 for supporting this work. The authors would like to thank Dr. Ben Spencer and Dr. Adam Parry for their assistance in measurements.

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(41) Arapov, K.; Rubingh, E.; Abbel, R.; Laven, J.; de With, G.; Friedrich, H. Conductive Screen Printing Inks by Gelation of Graphene Dispersions. Adv. Funct. Mater. 2016, 26, 586593. (42) Xu, Y.; Schwab, M. G.; Strudwick, A. J.; Hennig, I.; Feng, X.; Wu, Z.; Müllen, K. Screen-Printable Thin Film Supercapacitor Device Utilizing Graphene/Polyaniline Inks. Adv. Energy Mater. 2013, 3, 1035-1040. (43) Hyun, W. J.; Secor, E. B.; Hersam, M. C.; Frisbie, C. D.; Francis, L. F. High-Resolution Patterning of Graphene by Screen Printing with a Silicon Stencil for Highly Flexible Printed Electronics. Adv. Mater. 2015, 27, 109-115. (44) Zhang, D. W.; Li, X. D.; Li, H. B.; Chen, S.; Sun, Z.; Yin, X. J.; Huang, S. M. Graphene-Based Counter Electrode for Dye-Sensitized Solar Cells. Carbon 2011, 49, 53825388. (45) Qian, M.; Feng, T.; Ding, H.; Lin, L.; Li, H.; Chen, Y.; Sun, Z. Electron Field Emission from Screen-Printed Graphene Films. Nanotechnology 2009, 20, 425702. (46) Song, D.; Mahajan, A.; Secor, E. B.; Hersam, M. C.; Francis, L. F.; Frisbie, C. D. HighResolution Transfer Printing of Graphene Lines for Fully Printed, Flexible Electronics. ACS Nano 2017, 11, 7431-7439. (47) Secor, E. B.; Gao, T. Z.; Dos Santos, M. H.; Wallace, S. G.; Putz, K. W.; Hersam, M. C. Combustion-Assisted Photonic Annealing of Printable Graphene Inks Via Exothermic Binders. ACS Appl. Mater. Interfaces 2017, 9, 29418-29423. (48) Overgaard, M. H.; Kuhnel, M.; Hvidsten, R.; Petersen, S. V.; Vosch, T.; Norgaard, K.; Laursen, B. W. Highly Conductive Semitransparent Graphene Circuits Screen-Printed from Water-Based Graphene Oxide Ink. Adv. Mater. Technol. 2017, 2, 1700011. (49) Grisdale, R. O. The Properties of Carbon Contacts. J. Appl. Phys. 1953, 24, 1288-1296. (50) Secor, E. B.; Gao, T. Z.; Islam, A. E.; Rao, R.; Wallace, S. G.; Zhu, J.; Putz, K. W.; Matuyama, B.; Hersam, M. C. Enhanced Conductivity, Adhesion, and Environmental Stability of Printed Graphene Inks with Nitrocellulose. Chem. Mater. 2017, 29, 2332-2340. (51) Wajid, A. S.; Das, S.; Irin, F.; Ahmed, H. S. T.; Shelburne, J. L.; Parviz, D.; Fullerton, R. J.; Jankowski, A. F.; Hedden, R. C.; Green, M. J. Polymer-Stabilized Graphene Dispersions at High Concentrations in Organic Solvents for Composite Production. Carbon 2012, 50, 526-534. (52) Santra, S.; Hu, G.; Howe, R. C.; De Luca, A.; Ali, S. Z.; Udrea, F.; Gardner, J. W.; Ray, S. K.; Guha, P. K.; Hasan, T. Cmos Integration of Inkjet-Printed Graphene for Humidity Sensing. Sci. Rep. 2015, 5, 17374. (53) Dodoo-Arhin, D.; Howe, R. C. T.; Hu, G. H.; Zhang, Y. H.; Hiralal, P.; Bello, A.; Amaratunga, G.; Hasan, T. Inkjet-Printed Graphene Electrodes for Dye-Sensitized Solar Cells. Carbon 2016, 105, 33-41. (54) Juntunen, T.; Jussila, H.; Ruoho, M.; Liu, S.; Hu, G.; Albrow‐Owen, T.; Ng, L. W. T.; Howe, R. C. T.; Hasan, T.; Sun, Z.; Tittonen, I. Inkjet Printed Large‐Area Flexible Few‐ Layer Graphene Thermoelectrics. Adv. Funct. Mater. 2018, 28, 1800480. (55) Borodko, Y.; Lee, H. S.; Joo, S. H.; Zhang, Y.; Somorjai, G. Spectroscopic Study of the Thermal Degradation of Pvp-Capped Rh and Pt Nanoparticles in H2 and O2 Environments. J. Phys. Chem. C 2010, 114, 1117-1126. (56) Kong, D.; Le, L. T.; Li, Y.; Zunino, J. L.; Lee, W. Temperature-Dependent Electrical Properties of Graphene Inkjet-Printed on Flexible Materials. Langmuir 2012, 28, 1346713472. (57) Huang, L.; Huang, Y.; Liang, J. J.; Wan, X. J.; Chen, Y. S. Graphene-Based Conducting Inks for Direct Inkjet Printing of Flexible Conductive Patterns and Their Applications in Electric Circuits and Chemical Sensors. Nano Res. 2011, 4, 675-684. 28 ACS Paragon Plus Environment

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Screen-Printing of a Highly Conductive Graphene Ink for Flexible

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