Scalable Fabrication and Integration of Graphene Microsupercapacitors through Full Inkjet Printing Jiantong Li,*,†,‡ Szymon Sollami Delekta,† Panpan Zhang,‡ Sheng Yang,‡ Martin R. Lohe,‡ Xiaodong Zhuang,‡ Xinliang Feng,*,‡ and Mikael Ö stling† †
School of Information and Communication Technology, KTH Royal Institute of Technology, Electrum 229, 16440 Kista, Sweden Center for Advancing Electronics Dresden (cfaed) and Department of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany
‡
S Supporting Information *
ABSTRACT: A simple full-inkjet-printing technique is developed for the scalable fabrication of graphene-based microsupercapacitors (MSCs) on various substrates. Highperformance graphene inks are formulated by integrating the electrochemically exfoliated graphene with a solvent exchange technique to reliably print graphene interdigitated electrodes with tunable geometry and thickness. Along with the printed polyelectrolyte, poly(4-styrenesulfonic acid), the fully printed graphene-based MSCs attain the highest areal capacitance of ∼0.7 mF/cm2, substantially advancing the state-of-art of all-solid-state MSCs with printed graphene electrodes. The full printing solution enables scalable fabrication of MSCs and effective connection of them in parallel and/or in series at various scales. Remarkably, more than 100 devices have been connected to form large-scale MSC arrays as power banks on both silicon wafers and Kapton. Without any extra protection or encapsulation, the MSC arrays can be reliably charged up to 12 V and retain the performance even 8 months after fabrication. KEYWORDS: microsupercapacitors, full inkjet printing, electrochemically exfoliated graphene, large-scale integration, polyelectrolyte electrochemical exfoliation.17,18 Meanwhile, versatile fabrication techniques have been developed to directly or indirectly pattern the electrode materials in interdigitated structures to construct the MSC electrodes,3 such as lithography,19 printing,20−22 laser scribing,9,10 electrode conversion,2 and electrochemical deposition.23,24 However, these techniques still suffer from poor scalability and/or cost inefficiency. For example, the lithography2,19 and laser scribing9−11 techniques begin with the global deposition (throughout the substrate) of electrode material films, followed by patterning them into interdigitated structures via etching or laser scribing. These protocols involve complicated processing and significant material waste.9 Besides, all the established scalable techniques2,9 are actually restricted to the fabrication of only electrodes and current collectors. The typical manual assembly of electrolytes still severely impairs the overall scalability of the full devices. As a matter of fact, so far only small-scale (500 μm), the conductivity becomes sufficiently high, but the long diffusion length also reduces the capacitance. These two effects lead to an optimal finger width of around 500 μm for the fully printed MSCs. Besides, for the MSCs with narrow fingers, the inactive areas, i.e., the interfinger gaps (∼200 μm), become considerable so that the areal capacitance averaged over the overall device area is even lower (Figure 2f). As a result of the three factors (electrical conductivity, diffusion length and interfinger gaps), the areal capacitance (per device area, Figure 2f) and hence the current density in the CV profiles (Figure 2d) increase with increasing finger width. In the following studies, all the individual MSCs have the same geometry and lateral dimension (4 pairs of fingers and each finger is 500 μm wide, 2 mm long and interspaced by 200 μm). 8251
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Figure 3. Electrochemical performance of fully printed MSCs on glass of different electrode thickness (printed with different numbers of printing passes). (a) Photograph of the MSCs. From left to right, the numbers of printing passes are in sequence 30, 25, 20, and 10. Accordingly, the devices are denoted as 30L, 25L, 20L, and 10L, respectively. In each sample, the fingers are 500 μm wide, 2 mm long, and interspaced by ∼200 μm. (b) The dependence of electrode thickness on the number of printing passes. (c) EIS of the 30L device, complex plan plot of the minus imaginary part of impedance (−Z″) against the real part of impedance (Z′) for frequencies ranging from 10 mHz to 200 kHz. The inset shows the high-frequency region. (d,e) CV profiles for all the devices at the scan rates of 10 mV/s (d) and 100 mV/s (e). (f) Areal capacitance against scan rate for the MSCs (extracted from the CV profiles). (g) GCD curves of the 30L device at different current densities. (h) GCD curves of the different devices at the current density of 10 μA/cm2. (i) Areal specific capacitance against current density for the MSCs (extracted from the GCD curves).
(equivalent to ∼0.7 A/cm3 for the 30-layer devices), confirming the high charge/discharge rate. The areal capacitance extracted from the GCD curves (Figure 3i) is consistent with that from the CV profiles (Figure 3f), but obviously decays slowly with the current density (up to 50 μA/cm2). The devices also exhibit acceptable cycling stability of ∼77% capacitance retention after 11,000 GCD cycles (Figure S5), leakage current of ∼50 nA after being biased at 1 V for 10 h (Figure S6a), and selfdischarge time of ∼1 h (Figure S6b). Furthermore, the device performance shows negligible variation on different substrates (Figure S7). All these results strongly suggest that our fully printed MSCs achieved the comparable electrochemical performance to most of the reported graphene-based MSCs (0.4−2 mF/cm2) fabricated with other (not printable) techniques.6 In particular, the areal capacitance is significantly higher than that of the “partly printed” graphene MSCs reported to date, including ∼0.04 mF/cm2 for printed MSCs based on shear exfoliated graphene21 and ∼0.1 mF/cm2 for directly sonication-exfoliated graphene.22 In most cases, the voltage and current provided by one single MSC are not sufficient to power practical circuits. It is often
Next, we fabricated MSCs with different electrode thicknesses. The reliable printing allows us to overwrite the graphene electrodes for up to 30 layers without deforming the patterns (Figure 3a). A further increase of the printing layers (passes) may cause occasional shorting of some neighboring fingers and reduce the yield of device fabrication. As shown in Figures 3b and S4, the electrode thickness increases almost linearly with the number of printing layers at the rate of ∼25 nm per printing layer, giving rise to a film thickness around 750 nm for the 30 layers. The corresponding equivalent series resistance is about 200 Ω cm2, as indicated by the electrochemical impedance spectra (EIS, Figure 3c where the overall device area is 21.7 mm2). For all the fully printed MSCs with different printing layers, the CV profiles (Figure 3d,e) exhibit good rectangularity at different scan rates. The areal capacitance continuously increases with electrode thickness (Figure 3f), with the highest areal capacitance (over the overall device area) reaching ∼0.7 mF/cm2 at the scan rate of 10 mV/s. In addition, the galvanostatic charge/discharge (GCD) curves (Figure 3g,h) show nearly ideal triangular shape without any visible voltage drop9 even at the current density as high as 50 μA/cm2 8252
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Figure 4. Small-scale integration of fully printed MSCs. (a−c) Photographs of 4 parallel-connected MSCs (4P) on glass (a), 4 series-connected MSC (4S) on glass (b), and 4 parallel rows of 4 series-connected MSCs (4S × 4P) on Kapton (c). The series connection (local connect) between two neighboring MSCs is realized by a short printed EEG line (1 mm long and 1 mm wide), as indicated in (b). The parallel connection (global connect) between two rows of MSCs is realized by a printed silver line, as indicated in (a,c). (d−f) CV profiles at the scan rate of 100 mV/s and (g−i) GCD curves at the current of 5 μA of the three MSC arrays (4P, 4S, and 4S × 4P). All the MSCs in the arrays have the same lateral geometry as those in Figure 3, and the EEG electrodes are printed with 10 printing passes. For comparison, the corresponding performance of one single cell (10L) is also presented in (d−i).
needed to connect multiple MSCs in series and/or in parallel to form a “bank” with a specific voltage and capacitance.9 Thus, our full printing technique provides a facile solution for such connection. Figure 4 shows the small-scale connection of 4 MSCs in parallel (Figure 4a), in series (Figure 4b), and in a mixed combination (Figure 4c). In contrast to most literature research where the MSCs are connected via external conductive tapes, our full printing technique can precisely control the deployment of electrodes and electrolytes and allows to connect two neighboring MSCs in series through short (∼1 mm long) printed graphene lines as local connects and to connect those in parallel through printed silver lines as global connects (Figure 4a,c). The CV profiles (Figure 4d−f) and GCD curves (Figure 4g−i) show that the 4 parallel-connected MSCs on glass (4P, Figure 4a) deliver about 4 times higher capacitance (Figure 4d,g) than a single device while retaining the same operating voltage window. The 4 series-connected MSCs on glass (4S, Figure 4b) increase the voltage window from 1 to 4 V, while the capacitance becomes about one-fourth of a single device (Figure 4e,h). For the MSC array (Figure 4c) comprising 4 parallel rows of 4 series-connected MSCs (denoted as 4S × 4P) on Kapton, the operating voltage window increases by 4 times, while the capacitance remains unaltered (Figure 4f,i). All of these suggest the excellent scalability of the fully printed MSCs.
We further fabricated large-scale MSC arrays on both silicon wafers (12S × 9P, Figure S8) and Kapton (12S × 12P, Figure 5). The almost rectangular CV profiles (Figure 5c) and triangular GCD curves (Figure 5d) confirm the excellent capacitive behavior of the MSC array, even when being charged up to 12 V. It is worth noticing that the good performance is achieved without any voltage balance that is often used for series connections to protect the individual cells from being over charged.9 This could be ascribed to the small sample-tosample variation which makes the applied voltage distributed almost equally over all the individual MSCs. As shown in Figure 5c, although the 12S × 12P MSC array is expected to yield the same capacitance as one individual MSC, its CV profiles exhibit excellent rectangularity even at a high scan rate of 1000 mV/s. On the contrary, the CV profiles of single MSCs turn to be lens-like shaped once the scan rate is higher than 300 mV/s (Figure 2c). This result highlights that the large-scale MSC arrays deliver higher power density, as confirmed in the areal Ragone plot in Figure 5e. In addition, our fully printed MSCs can retain their functionality for a long time. Even after being stored in air (i.e., without any encapsulation) for 8 months, the 12S × 12P MSC array still exhibited excellent rectangularity in the CV profiles (Figure 5f). As discussed above, it should result from the small amount of hygroscopic ethylene glycol remaining in the final electrolytes that attracts water from the 8253
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Figure 5. Large-scale integration of fully printed MSCs on Kapton. (a,b) Photographs of a 12S × 12P MSC array on Kapton. Because of the transparency, the printed electrolytes are not visible in (a); (b) was captured at a different perspective so that some of the printed electrolytes are visible. (c) CV profiles of the 12S × 12P MSC array at different scan rates with a voltage window of 12 V (measured at one month after the fabrication). (d) GCD curves of the MSC array at the current of 10 μA. For comparison, the corresponding GCD curves at the same current (10 μA) of one single cell (10L on Kapton), and the 4S × 4P array in Figure 4 are also presented in (d). (e) Areal energy density against power density (areal Ragone plot) for a single cell and the 4S × 4P and 12S × 12P arrays on Kapton. (f) CV profiles at the scan rate of 1000 mV/s for the 12S × 12P array measured at different periods after the fabrication. All the MSCs have the same lateral geometry as those in Figure 3, and the EEG electrodes are printed with 10 printing passes. The local and global connects are realized through the same way as Figure 4.
environment so as to enhance the performance of the PSSH electrolyte.32 This merit may provide an effective solution to avoid the need of encapsulation, a fundamental hurdle remaining to overcome in order to enable fully operational MSCs.3 To assess the device performance of on-chip MSCs or other directly integrated MSCs for electronic applications, it is proper and important to normalize the performance to their footprint area.3 However, in order to evaluate the efficiency of various fabrication techniques for the same electrode materials (e.g., graphene), it is still meaningful to compare the volumetric metrics. From the volumetric Ragone plot (Figure 6), it is clear that our devices exhibit fairly high energy density (over 1 mW h/cm3 when the power density is around 0.1 W/cm3), superior to most graphene-based MSCs that are with polymer electrolyte, including those with electrodes made of borondoped laser-induced graphene,36 laser-scribed graphene,9 and thermally reduced graphene oxide.19
Figure 6. Volumetric energy density and power density of the fully printed EEG MSCs and 12S × 12P array compared with some relevant MSCs in the literature with various kinds of graphene electrodes: 5B-LIG is boron-doped laser-induced graphene,36 LSG is laser-scribed graphene,9 MPG is methane-plasma reduced graphene oxide, and TG is thermally reduced graphene oxide.19 All the MSCs from the literature use the polymer electrolytes, poly(vinyl alcohol) (PVA)/H2SO4, except for LSG/IL which uses ionogel as the electrolytes.
CONCLUSIONS In conclusion, we have developed a simple full-inkjet-printing technique for the scalable fabrication of graphene-based MSCs on various substrates. This approach greatly increases the areal capacitance of printed all-solid-state graphene MSCs as single devices and enables effective integration of large-scale MSC arrays to attain excellent collective electrochemical performance. We believe that the full printing technique will provide a promising solution to compact and on-chip power sources for future electronics.
EXPERIMENTAL SECTION Electrochemical Exfoliation of Graphene. The EEG was produced according to the procedure described in our previous publication.18 Briefly, two pieces of graphite foil (Alfa Aesar, 0.13 mm thick, product number: 43078) were inserted into 0.1 M (NH4)2SO4 aqueous electrolyte solution as cathode and anode (at a distance of 2 cm), respectively, and biased at a DC voltage of 10 V for the exfoliation. The obtained graphite powder was collected and rinsed through filtration with water and DMF for 3 times each. Finally, the 8254
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ACS Nano powder was dispersed in DMF at a concentration about 2 mg/mL by ultrasonication for 10 min. Graphene Ink Formulation. The centrifugation-assisted solvent exchange technique was used to formulate EEG ink. First, ∼40 mL EEG/DMF dispersion was centrifuged at 3000 rpm for 15 min to remove the big particles. Then the harvested supernatant was centrifuged again at 10,500 rpm for 15 min to separate EEG from DMF. After DMF was removed, 16 mL of mixture solvent of cyclohexanone and terpineol (with volume ratio of 3:1) and 320 mg of ethyl cellulose (viscosity 4 cP for 5 w/v% in 80:20 toluene:ethanol, Sigma-Aldrich, product number: 200646) were added to disperse the sedimented EEG and sonicate for 30 min. Finally, the EEG ink was centrifuged at 2000 rpm for 3 min to further remove big particles. The final ink concentration was about 2.3 mg/mL (Figure S2). Electrolyte Ink Formulation. The PSSH electrolyte inks were prepared by mixing 1 mL of poly(4-styrenesulfonic acid) solution (Mw ∼ 75,000, 18 wt % in H2O, Sigma-Aldrich, product number: 561223) with, in sequence, 0.5 mL of deionized water, 0.5 mL of ethylene glycol (Fluka, product number: 03750), and 0.14 mL of phosphoric acid (≥85%, Sigma-Aldrich, product number: 40278). Device Fabrication. As illustrated in Figure S1, the fabrication of all the MSCs and arrays comprises three steps: EEG printing, EEG annealing, and PSSH electrolyte printing. Both the EEG and PSSH inks are printed using a commercial piezoelectric Dimatix Materials printer (DMP 2800, Dimatix-Fujifilm Inc.) equipped with a 10 pL cartridge (DMC-11610). First, EEG inks are printed at the drop spacing of 25 μm and substrate temperature of 45 °C on different substrates to form the electrodes for MSCs and short local connects for the arrays. Afterward, the samples were annealed on a hot plate in air for 1−2 h at the temperatures around 300 °C for silicon wafers, 380 °C for Kapton, and 400 °C for glass slides, respectively. In need of global silver connects, the commercial silver inks (Cabot Conductive Ink CCI-300, Cabot Corporation) were used to print the long lines to connect the annealed EEG electrodes and annealed at 250 °C for 30 min. Finally, the PSSH inks were printed at the drop spacing of 30 μm and substrate temperature of 45 °C for 20 printing passes (for all the MSCs and arrays) to cover the annealed EEG electrodes and dried at 50 °C on a hot plate for about 50 h to complete the fabrication of allsolid-state fully printed MSCs or MSC arrays. Electrochemical Characterization. All the electrochemical properties of the fully printed MSCs and arrays were characterized using a two-electrode configuration with Princeton Applied Research potentiostat/galvanostat 273A (for the GCD curves and CV profiles of MSCs and arrays when the voltage windows were no more than 10 V) and Biologic VMP2 (for EIS and CV profiles and GCD curves of the 12S × 12P array on Kapton). The areal capacitance CA was calculated either from the CV profiles or the GCD curves. For the CV profiles, CA,CV = ∫ ΔV 0 (IC − ID)dV/(2AvΔV) where IC and ID are the charging and discharging currents, respectively, ΔV is voltage window, A is the device area (except Figure 2e, all the device area in this work includes the interfinger gaps), and v is the scan rate. For the MSC arrays, A is the sum of the area of all devices, i.e., excluding the spaces among devices. For the GCD curves, CA,GDC = I ΔV where I is the discharging AΔt current and Δt is the discharging time. All the areal energy density is 2 calculated as EA = 0.5CA,CV ΔV , and the areal power density is PA = EA/t = vEA/ΔV with t = ΔV/v being the discharging time. The volumetric energy (power) density is calculated from dividing EA (PA) by the graphene electrode thickness.
different printing passes in Figure 3a. Figure S5: Cycle stability of a fully printed MSC. Figure S6: Testing of self-discharge rate of a fully printed MSC. Figure S7: CV profiles of the printed MSCs on different substrates. Figure S8: Large-scale (12S × 9P) integration of fully printed MSCs on silicon wafer (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Jiantong Li: 0000-0002-6430-6135 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS We acknowledge the Marie Skłodowska Curie International Career Grant cofunded by the Swedish Research Council (no. 2015-00395) and Marie Skłodowska-Curie Actions (INCA 600398) as well as the financial support by the Swedish Research Council through the Framework grant (no. 20146160) and the Olle Engkvist Byggmästare Foundation through the Research grant (no. 2014/799). We also thank the financial support from the German Research Foundation (DFG) within the Cluster of Excellence “Center for Advancing Electronics Dresden” (cfaed) and financed by the Initiative and Networking Fund of the German Helmholtz Association, Helmholtz International Research School for Nanoelectronic Networks NanoNet (VH-KO-606), ERC grant on 2DMATER, UP-GREEN, and EU Graphene Flagship. REFERENCES (1) Beidaghi, M.; Gogotsi, Y. Capacitive Energy Storage in MicroScale Devices: Recent Advances in Design and Fabrication of MicroSupercapacitors. Energy Environ. Sci. 2014, 7, 867−884. (2) Huang, P.; Lethien, C.; Pinaud, S.; Brousse, K.; Laloo, R.; Turq, V.; Respaud, M.; Demortière, A.; Daffos, B.; Taberna, P. L.; Chaudret, B.; Gogotsi, Y.; Simon, P. On-Chip and Freestanding Elastic Carbon Films for Micro-Supercapacitors. Science 2016, 351, 691−695. (3) Amponsah Kyeremateng, N.; Brousse, T.; Pech, D. Microsupercapacitors as Miniaturized Energy-Storage Components for OnChip Electronics. Nat. Nanotechnol. 2017, 12, 7−15. (4) Chmiola, J.; Largeot, C.; Taberna, P.-L.; Simon, P.; Gogotsi, Y. Monolithic Carbide-Derived Carbon Films for Micro-Supercapacitors. Science 2010, 328, 480−483. (5) Lin, J.; Zhang, C.; Yan, Z.; Zhu, Y.; Peng, Z.; Hauge, R. H.; Natelson, D.; Tour, J. M. 3-Dimensional Graphene Carbon Nanotube Carpet-Based Microsupercapacitors With High Electrochemical Performance. Nano Lett. 2013, 13, 72−78. (6) Pech, D.; Brunet, M.; Durou, H.; Huang, P.; Mochalin, V.; Gogotsi, Y.; Taberna, P.-L.; Simon, P. Ultrahigh-Power MicrometreSized Supercapacitors Based on Onion-Like Carbon. Nat. Nanotechnol. 2010, 5, 651−654. (7) Liu, Z.; Wu, Z.-S.; Yang, S.; Dong, R.; Feng, X.; Müllen, K. Ultraflexible In-Plane Micro-Supercapacitors by Direct Printing of Solution-Processable Electrochemically Exfoliated Graphene. Adv. Mater. 2016, 28, 2217−2222. (8) Wu, Z.-S.; Feng, X.; Cheng, H.-M. Recent Advances in GrapheneBased Planar Micro-Supercapacitors for On-Chip Energy Storage. Natl. Sci. Rev. 2014, 1, 277−292. (9) El-Kady, M. F.; Kaner, R. B. Scalable Fabrication of High-Power Graphene Micro-Supercapacitors for Flexible and On-Chip Energy Storage. Nat. Commun. 2013, 4, 1475.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03354. Figure S1: Fabrication procedure of the fully printed MSCs on Kapton. Figure S2: Characterization of the concentration of the final EEG inks. Figure S3: Stroboscopic images of jetted drops of the EEG inks. Figure S4: Profiles of printed EEG electrodes with 8255
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