3D Printing of Carbon Nanotubes-Based Microsupercapacitors

Jan 17, 2017 - The 3D printer uses a CNTs ink slurry with a moderate ... KEYWORDS: 3D printing, carbon nanotube, microsupercapacitor, continuous dropl...
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3D Printing of Carbon Nanotubes-Based Microsupercapacitors Wei Yu,† Han Zhou,‡ Ben Q. Li,*,§ and Shujiang Ding*,‡ †

Micro/Nano-technology Research Center, State Key Laboratory for Manufacturing Systems Engineering and ‡MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter and Department of Applied Chemistry, School of Science, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China § Department of Mechanical Engineering, University of Michigan-Dearborn, Dearborn, Michigan 48128, United States ABSTRACT: A novel 3D printing procedure is presented for fabricating carbon-nanotubes (CNTs)-based microsupercapacitors. The 3D printer uses a CNTs ink slurry with a moderate solid content and prints a stream of continuous droplets. Appropriate control of a heated base is applied to facilitate the solvent removal and adhesion between printed layers and to improve the structure integrity without structure delamination or distortion upon drying. The 3D-printed electrodes for microsupercapacitors are characterized by SEM, laser scanning confocal microscope, and step profiler. Effect of process parameters on 3D printing is also studied. The final solid-state microsupercapacitors are assembled with the printed multilayer CNTs structures and poly(vinyl alcohol)-H3PO4 gel as the interdigitated microelectrodes and electrolyte. The electrochemical performance of 3D printed microsupercapacitors is also tested, showing a significant areal capacitance and excellent cycle stability. KEYWORDS: 3D printing, carbon nanotube, microsupercapacitor, continuous droplets, additive manufacturing

1. INTRODUCTION The rapid advancement of portable and miniaturized autonomous electronic devices, such as active radio frequency identification tags, remote and mobile environmental sensors, microrobots, and implantable medical devices, continuously generates demands for superior energy storage and power supply that can perform in various forms and conditions.1−4 Microsupercapacitors (MSCs), as emerging energy storage devices for these applications, have received much attention in the research community owning to their attractive advantages such as superior power intensity, good cycling stability, minimal maintenance expenses, wide working temperature range, and desirable safety properties.5−7 In general, the performances of MSCs depend on the electrodes configuration, the manufacturing methods of the electrodes, the properties of their active materials, and the selection of electrolytes.8 Microsupercapacitors can be classified into different structure types, namely, sandwich, roll-like, and interdigital, depending on the configuration of electrodes used.9 The interdigital structure is considered to possess some salient features, such as ease of extension into three dimensions to allow for more electrode materials per unit area and flexibility in adjusting electrode arrangement to minimize the internal impedance by narrowing the interelectrode gaps.9 Recent years have seen considerable efforts devoted to develop MSCs, with the main focus on the use of highperformance electrode materials and three-dimensional structures. Gnerlich et al. fabricated microsupercapacitors ruthenium oxide coated on multimetallic microelectrodes.10 The property © 2017 American Chemical Society

of the obtained MSCs is limited by the thin layer of the electrode material and the large spacing between the electrodes. The interdigital electrodes, based on onion-like carbon, were created by Pech.11 However, the electrophoretic deposition process used cannot be applied to fabricate thicker electrodes. Moreover, the high-temperature process involved in producing onion-like carbon limits the potential application of these devices. Another strategy to obtain MSCs is to pattern MnO2/ onion-like carbon by screen-printing on a flexible substrate;12 unfortunately the process requires masks or stamps, leading to an increase of fabrication costs. From a materials perspective, carbon is of low toxicity and low-cost and is traditionally used for electrodes. In fact, different nanostructured carbon-based materials have been studied for potential electrodes of MSCs; these include activated carbons,13 carbon nanotubes,14 carbide-derived carbons,15,16 onion-like carbon,9 and graphene.17 Among them, carbon nanotubes (CNTs) have been widely studied because they possess a good electrical conductivity and a large surface area.18 Various processes also have been proposed to manufacture CNTs-based electrodes for energy storage applications. Kim et al. reported a fabrication approach with facile selective wettinginduced micropatterning for CNTs-based MSCs. 14 The method appears to offer some useful features including ease Received: October 31, 2016 Accepted: January 17, 2017 Published: January 17, 2017 4597

DOI: 10.1021/acsami.6b13904 ACS Appl. Mater. Interfaces 2017, 9, 4597−4604

Research Article

ACS Applied Materials & Interfaces

and surface tension do not have strict limits associated with other methods presented above. The only requirement for the proposed 3D printing technique is that the printing inks can be extruded out of the micronozzle uniformly and steadily. Another important feature is that the proposed 3D printing technique injects inks in the shape of continuous droplets because of the moderate solid contents of CNTs inks, neither like drop by drop in inkjet printing, nor like the continuous filament in traditional 3D printing. Also, the moderate solid contents are also beneficial in improving the processing efficiency compared to inkjet printing. In what follows, this method is described in detail and is used to produce CNTsbased solid-state MSCs, with a measured specific area capacitance of 2.44 mF cm−2 and outstanding cycle stability.

of processing, exclusion of severe condition, and achievement of a binder-free micropatterning. However, the preparation of interdigital poly(dimethylsiloxane) (PDMS) slabs resorts to soft-lithography, and hence the method is in essence a traditional lithography technology, subject to associated caveats.1 In et al. presented the micropattern of CNTs arrays on polycarbonate sheets with the assistance of the laser.19 The size of the electrodes is restricted by the morphology of the CNTs array. Thus, the technique is difficult to be employed for fabricating thick electrodes. In situ growth is an alternative to attain interdigital electrodes, like vertically grown CNTs with the help of chemical vapor deposition (CVD).20,21 In addition to the low volume density of materials in electrodes produced, the demand for high temperature conditions during the CVD process is an obstacle to the possible applications of the integration with some existing processes for microdevices. The limitations of these methods have inspired us to search for other simple, low-cost and high-throughput methods to fabricate MSCs with high performance. 3D printing, an additive manufacturing technique, offers a promising low-cost fabrication protocol for future MSCs.22 Among the recently developed 3D printing techniques, inkjet printing has been successfully applied to fabricate MSCs, which prints a pattern by propelling droplets of ink on demand. Pech et al. fabricated a carbon-based MSC on the Si substrate by means of the inkjet printing process.13 The ink, made by combining active material (activated carbon) with binder (polytetrafluoroethylene, PTFE) into dispersion medium (ethylene glycol), was printed on prepatterned gold metal. However, the processing efficiency, owing to the low solid contents, poses a main problem that limits the scaled-up application of the inkjet printing. Apart from this, the extrusionbased 3D printing has been successfully employed to make different structure forms with the graphene oxides as battery materials. An interesting feature of this process is that the printing technology allows a broad range of materials to be selected, including ceramics, metal alloys, and polymers.22 Highly concentrated solid contents and shear-thinning behavior are prerequisite conditions for extrusion-based 3D printing, which make the printing inks display gel-like behavior and the extruded inks become fine filaments. Otherwise, moderate solid contents, or lower solid contents, would lead to unacceptable results with structural delamination or distortion. The prerequisites often are difficult to be realized. In the present paper, we propose a painless, cheap, and versatile 3D printing technique to fabricate CNTs-based MSCs. The printing inks, which have moderate solid contents, are prepared by milling CNTs in a solution composed of isopropyl alcohol, ethylene glycol, and the dispersion agent. By the present 3D printing technique, printing inks are extruded via a micronozzle, with printing speeds and micronozzle-to-base distances controlled, and with predesigned base temperature and preprogrammed printing trajectory. One salient feature in this 3D printing protocol is the use of the heated base, which can lower the demand in highly concentrated solid contents in the traditional extrusion-based 3D printing. Also, the application of the heated basal can promote adhesion between the printed features, thereby improving the electrode structure integrity without occurrence of delamination or distortion upon drying. In contrast to inkjet printing and the traditional extrusion-based 3D printing, the 3D printing technique presented here is adaptable to various printing inks in that the characteristics of the inks such as solid content, viscosity,

2. EXPERIMENTAL SECTION 2.1. Materials. Carbon nanotubes (CNTs, TNIM) and the dispersion agent (TNADIS) were obtained from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Science. Isopropyl alcohol (IPA), ethylene glycol (EG), poly(vinyl alcohol) (PVA, Mw ≈ 95000 g mol−1), and H3PO4 (85 wt % aqueous solution) were obtained from Aladdin Chemistry Co. Ltd. 2.2. CNTs Inks Preparation. CNTs inks with varying solid loadings of 6, 7, and 8 wt % were designed by thorough ball milling of IPA, the dispersion agent, CNTs power, and EG. First, the dispersion agent was evenly dispersed in IPA. Then, CNTs were added to the asprepared solution, where the dispersion agent accounted for 30 wt % of CNTs weight; and until CNTs were adequately infiltrated, ball milling was employed in order to obtain homogenic and printable CNTs inks. The first stage lasted for 6 h at 450 rpm speeds. After that, EG was added to the as-prepared inks and ball milling continued to mix into homogenized CNTs inks. The inks had a IPA to EG volume ratio of 5:1. 2.3. 3D Printing. 3D printing was executed on a 3-axis positioning stage, whose motion is controlled by a preprogrammed patterning procedure. CNTs inks were stored into a separate syringe (5 mL in size), which was fixed to a dispensing micronozzle (60 μm in diameter). The ink flow was controlled by an air-powered dispensing system. Applied printing pressures varied from 10 KPa to 30 KPa and move speed was set at 6 mm s−1. Glass plate, treated by C4F8 gas to make it hydrophobic, was selected as the basal to bear the extruded inks, and the micronozzle-to-glass plate distances ranged from 30 to 90 μm. While printing, the glass plate basal was heated to 80 °C to promote the solvent removement. After printing, the interdigital electrodes were annealed to remove the residuary solvent at 120 °C for a night using a vacuum oven. 2.4. Microsupercapacitors Packaging. The PVA-H3PO4 electrolyte solution yield was as follows.14 300 mL of deionized water, containing 20 g PVA, was vigorously stirred under 90 °C until to be transparent. The solution was taken out and kept completely cool; and then, 16 g of H3PO4 was injected into the solution above and kept stirring at ambient temperature until forming a homogeneous solution. The PVA-H3PO4 electrolyte solution was cast onto the interdigital electrodes and dried by airing. The dried PVA-H3PO4 film was lifted off the glass plate substrate, which will make the interdigital electrodes off together with it. For ease of testing, an adhesive copper tape was applied to connection with the interdigital electrodes assisted by silver paint. In contrast, MSCs with no thermal treatment were also packed in the same process. 2.5. Characterization of Electrode Structures and of Ink Properties. The viscosity study of CNTs inks was conducted on a Kinexus rheometer. All experiments were conducted using 20 mm steel flat plate geometry. The experiments were conducted at 25 °C throughout the process. The apparent viscosity was obtained as a function of shear rate (1−100 s−1). Contact angle measurements were collected using a contact angle meter (OCA20) with 10 μL inks. The microscopic morphological characteristics of the interdigital electrodes were examined on a scanning electron microscopy (SEM, S-8010, 4598

DOI: 10.1021/acsami.6b13904 ACS Appl. Mater. Interfaces 2017, 9, 4597−4604

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Figure 1. (a) TEM image of CNTs used for 3D printing. (b) Raman spectra of CNTs used for 3D printing.

Figure 2. Schematic illustration of 3D printing of carbon nanotubes-based MSCs. (a) Schematic of the 3D printing process with a working distance h. (b) Optical images of 3D printing inks. (c) Apparent inks viscosity as a function of shear rate. (d) As-obtained interdigital electrodes. (e) The polymer gel electrolyte was casted and solidified. (f) A packaged MSCs. Hitachi), a laser scanning confocal microscope (LSCM, OLS4000, Olympus), and a step profiler (XP-2, AMBIOS); and the roughness gage of the interdigital electrodes was investigated by a laser scanning confocal microscope (LSCM, OLS4000, Olympus). A CHI 660D electrochemical workstation was applied to characterize the electrochemical properties of MSCs in the two-electrode mode under ambient condition.

distance (h) between the micronozzle and glass plate is monitored by a camera, as shown in Figure 2a. The as-obtained electrodes, after evaporation (see the inset of Figure 2a), consist of randomly distributed CNTs inks, allowing more effective electronic transport of the electrolyte. The present 3D printing process is based on the ejection of a stream of continuous ink droplets. At first, the printing inks were extruded from the micronozzle under the applied pressure and the static fluid pressure, obeying the law of conservation of mass, momentum, and energy. The feed rate of printing inks is mainly affected by the nozzle diameter, ink level, applied pressure, ink density, and ink viscosity. After ejected out of micronozzle, ink drops were forced to deform under the tensile force, compressive force, gravity force, viscous forces, and surface tension until they were separated from the micronozzle. The compression and gravity forces impel the ejected inks to spread on the basal, whereas viscous and surface tension forces tend to counterbalance the spreading. As the micronozzle moves to print, the ejected inks suffer from stretch and thus tend to travel along the direction of the micronozzle movement. Once out from the micronozzle, the ejected inks continued to spread under viscous forces and surface tension opposed the motion, and gravity force aided flow along the surface. At the same time, the ink dies out through solvent evaporation that accelerated by the heating from the basal, thereby forming the printed features.

3. RESULTS AND DISCUSSION 3.1. 3D Printing Process for Microsupercapacitor Fabrication. The TEM image and Raman spectra of CNTs used for 3D printing are given in Figure 1. For the present applications, the CNTs present an average diameter of 50 nm and a length of 10−20 μm (see Figure 1a). Raman spectroscopy of CNTs is revealed in Figure 1b, which presents two obvious peaks at around 1328 cm−1 (D-band) and 1582 cm−1 (G-band). Figure 2 shows the 3D printing technique with the CNTs inks for fabrication of MSCs. Different from other 3D printing processes where the materials are frozen or extruded out at room temperature,22,23 the present 3D printing technique is by rapidly evaporating the dispersed phase by the heated basal. Specifically, CNTs inks are injected from the micronozzle controlled by an air-powered fluid dispenser and form a line of continuous droplets (see Figure 2a), which are neither the discrete droplets of inkjet printing,13 nor the continuous filament of traditional 3D printing.22 The work 4599

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3.2. Contact Angle Analysis of CNTs Ink Slurry. Apart from the benefit of detachment, the C4F8 gas treatment modifies the surface structure of the glass plate and contributes to an increase of the contact angles between the glass plate base and CNTs inks, which is beneficial in obtaining a narrower 3D printing structures.27 For this study, the C4F8 gas treatment was conducted in a plasma etching machine for 40 s, and the contact angles for various CNTs loading inks were measured, with results presented in Figure 3. Apparently, the contact

The 3D printing process presented above requires an appropriate ink. For the present application, the CNTs ink slurry was prepared as the required ink for 3D printing. To manage ink evaporation and union in the 3D printing so that a desired structure is formed, a mixed solvent of IPA (boiling point, 82.45 °C) and EG (boiling point, 197.3 °C) is used. The IPA evaporates during printing to allow partial consolidation of the printed features to ensure their structural integrity, while the EG serves as a moisturizer which endorses adhesion between two adjacent layers. During 3D printing, the glass plate basal was heated to 80 °C (close to the boiling point of IPA) to expedite the IPA removal and strengthen the combination among layers. The presence of high boiling point solvents, EG, helps to maintain sufficient liquid of the deposited material to form a perfect union between 3D printing layers. The combined action of IPA and EG results in a structurally stable transition between each layer, a vulgar challenge for most of the 3D printing technologies.23 The fluidity of the ink slurry is of crucial importance to ensure the continuous and uniform extrusion in the present 3D printing process. To better understand the effect of viscosity and to obtain the desired rheological behavior for 3D printing, CNTs inks with different solid loadings of 6, 7, and 8 wt % were designed, and the apparent viscosity of these CNTs ink slurries was measured as a function of shear rates, as presented in Figure 2c. Apparently, shear thinning behaviors can be seen in the measurement results of every ink slurry, and the range of apparent viscosities is between 10−1 Pa·s at 1 s−1 and 3 × 10−1 Pa·s at 1 s−1, which is less than those of traditional 3D filamentary printing (103−104 Pa·s at 1 s−1) but much more than those of inkjet printing (10−3 Pa·s at 1 s−1).24,25 As expected, a higher solid loading produces a more viscous slurry. A more viscous slurry permits a shorter travel distance upon loading on the basal. As a consequence, the more viscous slurry is beneficial in printing a slenderer and continuous structure, a feature desirable of 3D printing. Nevertheless, an ink with too high a viscosity would make it difficult to prepare a welldispersed slurry and to be extruded out. For the present study, a CNTs ink with a solid loading of 8 wt % is found to be preferable for the 3D printing of microsupercapacitors. The final structures of electrodes (Figure 2d) were printed layer-by-layer by controlling the motion in three-axis directions of the 3D printer with a rate of 6 mm s−1. After that, a heat treatment was applied to enable a thorough removal of the residuary solvent. The gel-state PVA-H3PO4 electrolyte then was selected based on a combination of factors such as the relatively excellent electrochemical performances, excellent mechanical properties, and good security features.22 The printed electrodes were combined with the PVA-H3PO4 electrolyte while at liquid state, and a solid-state supercapacitor is produced once the electrolyte was dried (see Figures 2d and 2e).14 In the present study, a sufficient amount of PVA-H3PO4 solution was used so that a continuous PVA-H3PO4 film was formed. This allows a 3D printed microsupercapacitor to be peeled off the basal in one piece (Figure 2e, f). The interdigital electrodes of the MSCs were connected with an adhesive copper tape assisted by silver paint and reinforced with the Kapton tapes to ensure good electrical contact. In addition, in order to facilitate the detachment of the assembled electrodePVA-H3PO4 film, the glass plate base is treated with C4F8 gas, allowing a molecular film to be formed between the basal and the electrolyte.26

Figure 3. Contact angles for various CNTs loading inks on the gastreated basal: (a) 6 wt %, (b) 7 wt %, and (c) 8 wt %. (d) The contact angle for 7 wt % CNTs content on the pristine basal.

angles increase gradually from 47.4° to 52.3° (Figure 3a-c) as the CNTs contents increase. In contrast, without the C4F8 gas treatment, the contact angle is significantly lower; in fact a contact angle of 16.3° was obtained with the CNTs content of 7 wt % (Figure 3d). Based on the contact angle measurements, a higher CNTs content ink has a larger contact angle. Moreover, the basal with the C4F8 gas treatment appears to give a larger contact angle and a poorer wettability than those of the pristine basal, which can furnish the extruded inks with a limited travel distance upon loading on the basal. Based on the contact angle analysis, the CNTs ink with a solid loading of 8 wt %, combined with gas-treated basal, is considered suitable for the 3D printing. 3.3. Meterage of Structural Dimensions of 3D Printing Features. With a working distance of 60 μm set between the nozzle and the base plate, the 3D printing process creates an interdigitated pattern of electrodes in a single pass. The height and width of structures printed with inks of various CNTs contents were recorded as a function of the driving gas pressure. Examination of Figures 4a and 4b indicates that at 8 wt % CNTs content the height and width of 3D-printed features in a single pass steadily increases with an increase in the applied gas pressure from 10 KPa to 30 KPa. The height grows from 6.10 to 9.97 μm, and the width grows from 162.88 to 304.78 μm. The as-obtained heights are much higher than the magnitude of inkjet printing, which is usually at a nanometer level.28 With the speed of micronozzle movement kept unchanged, a decrease of CNTs contents to 7 and 6 wt % increases both the height and the width of printed features with the same applied gas pressures. This is because more inks can 4600

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Figure 4. Meterage of structural dimensions on 3D printing features. (a) The effect of the CNTs contents on the height of the printed features. (b) The effect of the CNTs contents on the width of the printed features. (c) The influence of the work distance upon the height of the printed features. (d) The influence of the work distance upon the width of the printed features. Error bars represents a standard deviation of metric data for 3−12 models under various conditions.

layers were printed. The CNTs content was set at 7 wt %, applied gas pressure was set at 10 KPa, and the work distance was 60 μm. Once one layer was printed, the micronozzle would regulate position along the vertical direction to perform the next printing layer on top of the previous one. The as-prepared MSCs with 5 layers printed are provided with 27.6 μm height, 235.7 μm width, and 185 μm gap space. That height accounts for about 89% of the ideal value, while their width is nearly constant and is not subjected to the effect of the print layers; and such a state also has been reported in a previous article.24 3.4. Characterization of Surface Morphologies. After the interdigital electrodes were created, heat treatment (350 °C in air for 3 h) was applied to remove the dispersion agent of CNTs for some samples. It was noticed that delamination of the electrodes from the substrate did not occur during 3D printing, which is evident by examining the SEM images in Figure 5. Figure 5a illustrates the optical image of a 3D printed interdigital electrode, which is rather uniform in dimensions (see Figure 5b). The inset image in Figure 5b displays a high magnification image of the selective region. Figures 5c and e are the enlarged views of the interdigital electrodes without heat treatment. Here it is obvious that CNTs are stacked together with a large randomly distributed porosity. This differs from the samples of the traditional 3D filamentary printing, which exhibits a filament microstructure with reorientation and alignment along the surface.23 Also, the interdigital electrodes are filled with macropores, which allow the permeation and storage of the electrolyte. However, the as-3D-printed CNTs are coated with the dispersion agent, and this coating may prevent good contact between CNTs and the electrolyte to reduce the electrochemical performance of the MSCs. After heat treatment at 350 °C to evaporate the dispersion agent via chemical decomposition, the desired bare CNTs were obtained with the porous structure of the interdigital electrodes unchanged (see Figures 5d and 5f). A more porous 3D

be squeezed out of the micronozzle with a lower CNTs content and viscosity slurry, which would result in more accumulation, the longer flow time, and larger flow scope of CNTs inks on the glass plate basal than those of CNTs content of 8 wt %. Figures 4c and d depict the printed electrode architectures achieved at different work distance under the CNTs content of 7 wt %. For the work distance set at 60 μm, the height of printed features grows from 6.22 to 11.38 μm and the width of printed features grows from 236 to 374 μm, as the applied gas pressure increases. When the work distance is reduced to 30 μm, the droplets streamed out from the micronozzle undergo a larger structural deformation, resulting in a lower height and wider width, than that with the work distance of 60 μm. With the work distance elevated to 90 μm, the ink droplet has more room to relax, thereby resulting in a higher vertical dimension and a decrease of the contact area between the droplets and the basal. Thus, the higher the work distances are, the higher the heights of printed features and the smaller the widths of printed features become and vice versa. In the results below, the work distance is all set at 60 μm unless indicated otherwise. In general, the CNTs ink with a solid loading of 8 wt % printed on the gas-treated basal at the work distance of 90 μm appears to be a good choice to build high aspect ratio electrodes. However, with two other processing factors (i.e., roughness and manufacturing rate) considered above, a CNTs ink with a solid loading of 7 wt % was adopted for the 3D printed results below with the work distance of 60 μm. Moreover, in the text that follows, roughness measurement of 3D printing structures reveals that a lower solid loading yields a larger roughness, which is believed to be conducive to the sufficient contact between the interdigital electrodes and the electrolyte. Lastly, the processing efficiency is one of the major concerns for the heights of the printed features. Comprehensive above factors, CNTs ink with solid loadings of 7 wt % was eventually employed. So with the purpose to acquire the integrally MSCs, interdigitated electrode architectures with 5 4601

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conditions. It increases with an increase in the applied gas pressures and with a decrease in the CNTs contents. The 3D-printed features with five layers also were tested, and the results are given in Figure 6. The measured roughness of 0.501 μm is larger than that of printed features with one layer. This may be attributed to the fact that as print layers increase in number, interlayer defects increase, leading to a rougher surface morphology. 3.5. Characterization of Electrochemical Properties of 3D Printing MSCs. Figure 7a shows the schematic of a packaged MSC. To determine the electrochemical properties of the MSC, cyclic voltammetry (CV) measurements of the MSCs with and without the heat treatment were made. From Figures 7b and 7c, it is apparent that the CV curves of both MSCs exhibit a rectangular shape, displaying the successful realization of an electrical double layer at the electrode/electrolyte interface and efficient charge propagation across the electrodes. From the measurements, the area capacitances of the MSCs without heat treatment are calculated to be 0.732 mF cm−2 at the scan rate of 100 mV s−1 and 0.81 mF cm−2 at 50 mV s−1, respectively. The heat-treated MSCs, however, deliver the respective area capacitances of 4.25 mF cm−2 at 100 mV s−1 and 4.69 mF cm−2 at 50 mV s−1, which are comparable to the value of other MSCs previously reported.6,7,11,14,17,29 Clearly, such performance improvement comes from the removal of the dispersion agent and better contact between the CNTs and the electrolyte. To further characterize the MSCs, galvanostatic charge− discharge (GCD) measurements were made at the constant current density of 50 μA cm−2 and 20 μA cm−2, and the results are given in Figure 7d and 7e. When operated at 50 μA cm−2 and 20 μA cm−2, the pristine CNTs-based MSCs exhibit the area capacitances of 0.39 mF cm−2 and 0.418 mF cm−2, respectively. This compares with the results obtained for the heat-treated MSCs, which have the larger area capacitances of 2.23 mF cm−2 and 2.44 mF cm−2. This behavior is essentially the same as that with the CV tests. Both CV and GCD measurements indicate that the 3D printed MCs are superior in electrochemical performance over most of the reported CNTsbased MSCs (see the data comparison in Table 1). The voltage−time responses of heat-treated MSCs during charging/ discharging also show triangular-shaped GCD profiles with a longer time duration, derived from electrical double layer capacitance. According to the data above, the heat-treated MSCs carry a power density of 3.72 W cm−3 along with an energy density of 0.12 mWh cm−3, which is compatible with or better than other state-of-the-art MSCs.14,29,33,34 The effect of printed layer numbers on the electrochemical properties of the MSCs were also examined, and the results are given in Figure 7d-e. The measured data clearly shows that an increase in printed layers result in the corresponding enhancement of the area capacitances of the MSCs. Turning to the electrochemical stability of MSCs, the specific discharge capacitance is plotted against the number of the charge/discharge cycles for up to 2000 cycles, as shown in Figure 7f. From these curves, it is clear that the capacitance retains a steady value over the tested 2000 cycles, revealing a good cycle stability. This suggests that the CNTs-based interdigital electrodes manufactured by 3D printing, with cross-linked porous structures, have minimal structural modification or degradation in the course of repetitive charge/discharge cycles. The morphology of cross-linked porous structures thus is capable of coping with the fast and

Figure 5. Morphology of an interdigital electrode: (a) Optical picture of an interdigital electrode; (b) SEM picture of the top view of the interdigital electrodes, and the inset is the enlarged view of the selective region; porous structures of the electrode surface before (c) and after (d) the heat treatment to evaporate the dispersion agent by chemical decomposition; (e) enlarged view of the electrode surface in (c); and (f) enlarged view of the electrode surface in (d).

CNTs network is thought to possess a relatively higher specific area capacitance. Inspection of the inset in Figure 5b further reveals that the surface of the interdigital electrodes is not smooth. The roughness of the printed features was measured with the aid of the laser scanning confocal microscope. The results, which are presented in Figure 6, show that the roughness increases with

Figure 6. Roughness measurement of 3D printing structures.

the gas pressure from 0.482 to 0.622 μm for the ink of 7 wt % CNTs content. Under the gas pressure of 10 KPa, a reduction of CNTs content to 6 wt % yields the roughness measured at 0.799 μm, while an increase of CNTs content to 8 wt % results in a reduction of the roughness to 0.417 μm. In other words, the roughness of the printed features is affected by the printing 4602

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Figure 7. Electrochemical performance of 3D printed MSCs: (a) schematic illustration of a packed MSC, (b, c) the CV curves of the packed MSCs, (d, e) the GCD curves of the packed MSCs, and (f) the cyclic stability performance of the packed MSCs.

Table 1. Compare with the Reported CNTs-Based MSCs Performance Carea 1 2 3 4 5

2.75 mF cm−2 0.43 mF cm−2 0.428 mF cm−2 1.0 mF cm−2 4.69 mF cm−2

electrode thickness 5 50 80 10

measuring condition (electrolyte, potential window, and scan rate)

ref

PVA-H3PO4, −0.4−0.4 V, 0.01 V s−1 ionogel electrolyte, 0−3 V, 0.1 V s−1 ionic liquid electrolyte, −0.5−0.5 V, 0.05 V s−1 polymer-gel electrolyte, 0−2.8 V, 1 mV s−1 PVA-H3PO4, 0−1 V, 0.05 V s−1

14 30 31 32 this work

μm μm μm μm 27.6 μm

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efficient migration of H+ ions. On the other hand, a drop of ∼7% in the capacitance of the treated MSCs was detected after about 500 cycles, presumably because the void left by the dispersion agent results in the rearrangement and structural modification of the electrodes. Nonetheless, the capacitance stays essentially unchanged for the remaining 1500 cycles.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.Q.L.). *E-mail: [email protected] (S.D.). ORCID

Wei Yu: 0000-0002-2113-4046 Notes

4. CONCLUDING REMARKS This paper has successfully demonstrated a facile manufacturing process for 3D printing of carbon nanotubes-based microsupercapacitors (MSCs). The present 3D printing technique is unique in that it is easy to apply and environmentally friendly and produces a free-standing electrode. This technique can use a wide variety of inks with various CNTs contents. The widths and heights of the printed patterns can be readily tuned by 3D printing process parameters such as the diameter of the micronozzle, applied printing pressure, CNTs contents, and work distance. Appropriate control of heating at the base is beneficial in facilitating the solvent removal by evaporation and adhesion among printed layers and in improving the structural integrity without delamination or distortion upon drying. As a demonstration, this method was used to 3D print MSCs. The electrochemical tests show that the 3D printed MSCs deliver a specific capacitance of 2.44 F cm−2 and exhibit superior cycle stability and reliable energy storage capacity, suggesting that this minipower source device is a promising candidate for energy storage applications. Our experience also suggests that a variety of other special materials, as long as they are dispersed well in the agent, can be used by the 3D printing technique presented here for energy storage and other applications.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (No. 51273158, 21303131), the Natural Science Basis Research Plan in Shaanxi Province of China (No. 2012JQ6003, 2013KJXX-49), and the Fundamental Research Funds for the Central Universities for financial support. Partial support of this work by the Shaanxi Department of Science and Technology Development (Grant No. 2013KTCQ01-48) is acknowledged. The authors are grateful to the International Research Centre for Dielectric for TEM characterization; Micronano Manufacturing Centre for SEM characterization; Center of Nanomaterials for Renewable Energy (CNRE) for viscous characterization.

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