Laser-Printed In-Plane Micro-Supercapacitors: From Symmetric to

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Laser Printed In-plane Micro-supercapacitors: From Symmetric to Asymmetric Structure Gui-Wen Huang, Na Li, Yi Du, Qing-Ping Feng, Hong-Mei Xiao, Xing-Hua Wu, and Shao-Yun Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15922 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Applied Materials & Interfaces

Laser Printed In-plane Micro-supercapacitors: From Symmetric to Asymmetric Structure Gui-Wen Huang,a Na Li,a Yi Du,a,c Qing-Ping Feng,a,* Hong-Mei Xiao,a,* Xing-Hua Wu,a, c and Shao-Yun Fub a

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, No. 29

Zhongguancun East Road, Beijing 100190, P. R. China b

c

College of Aerospace Engineering, Chongqing University, Chongqing 400044, China

University of Chinese Academy of Sciences

ABSTRACT: Here we propose and demonstrate a complete solution for efficiently fabricating in-plane micro-supercapacitors (MSCs) from symmetric to asymmetric structure. By using an original laser printing process, symmetric MSC with reduced graphene oxide (rGO) / silver nanowires (Ag-NWs) hybrid electrodes was facilely fabricated and a high areal capacitance of 5.5 mF cm-2 was achieved, which reaches the best reports on graphene-based MSCs. More importantly, a “print-and-fold” method has been creatively proposed that enabled the rapidly manufacturing of asymmetric in-plane MSCs beyond the traditional cumbersome technologies. α-Ni(OH)2 particles with high tapping density was successfully synthesized and employed as the pseudocapacitive material. Consequently, an improved supply voltage of 1.5 V was obtained and an areal capacitance as high as 8.6 mF cm-2 has been realized. Moreover, a demonstration of a miniaturized MSC pack was performed by multiply folding the serially Ag-NW-connected MSC

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units. As a result, a compact MSC pack with high supply voltage of 3 V was obtained which can be utilized to power a LED light. These presented technologies may pave the way for the efficiently producing of high performance in-plane MSCs, meanwhile offer a solution for the achievement of practical power supply packs integrated in limited spaces.

KEYWORDS: micro-supercapacitors, asymmetric supercapacitors, in-plane supercapacitors, laser printing, paper-based 1. INTRODUCTION The rapid development of portable and miniaturized electronics has increased the demand for small-sized and on-chip energy storage units.1-2 Thus, micro energy storage devices with inplane structure are highly preferred because they are thin, lightweight, flexible, and easy to be integrated into on-chip systems.3 Under the circumstances, thin-film batteries have been commercially produced as the power sources for miniaturized electronics.4-5 However, the current available thin-film batteries still suffer from several disadvantages, such as short cycle life, low power density, poor low-temperature kinetics, and safety concerns.6-7 Recently, in-plane micro-supercapacitors (MSCs) have attracted booming attentions as potential alternative to thinfilm batteries for they possess higher power densities and faster charge/discharge rates than the batteries.6 Furthermore, the in-plane electrode finger array design of the in-plane MSCs makes them promising in direct on-chip integration with microelectronics because they are thinner and more flexible than the sandwich-structured batteries.8 Therefore, the in-plane MSCs can be assumed as the most suitable candidates for energy storage components of miniaturized electronics.


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Distinguished by structure, the MSCs can be divided into symmetric and asymmetric types. For the symmetric MSCs, the active materials in positive and negative electrodes are the same. The symmetric MSCs mainly work based on the electric-double-layer (EDL) mechanism, which perform a high degree of recyclability since the charge-storage process is non-Faradaic and no chemical reactions are involved. Carbon-based materials, including active carbon,9 graphene,10 and carbon nanotubes11 are commonly used active materials in symmetric MSCs due to their large surface areas, excellent electrical properties and good stability. However, although the EDL symmetric capacitors can achieve rapid charge storage, their capacitance are relatively low, which is not satisfactory for practical applications. Therefore, in order to improve the capacitance, asymmetric MSCs have been designed by introducing pseudocapacitance to the system.12-13 The pseudocapacitance refers to the capacitance coming from the reversible redox reactions occurring at or near the surface of redox-active materials, which leads to a much higher charge storage value than the EDL capacitors. Conducting polymers including polyaniline,14-15 polypyrrole,16-17 polythiophenes,18-19 and transition-metal oxides or hydroxides such as MnO2,7,20 RuO2,21 Co(OH)222 and Ni(OH)223-25 are typical electrode materials used in pseudocapacitors. Since the pseudocapacitive materials suffer from relatively poor rate capability, in the asymmetric MSCs, a hybrid system consisting of EDL and pseudocapacitive materials is generally employed. In the hybrid system, EDL materials are used in one electrode to offer a fast power supply while pseudocapacitive materials are arranged in another electrode to serve as energy source.26-27 In this way, the asymmetric MSCs could achieve superior overall performance of balanced high energy and power densities as well as high supply voltage, which makes them promising candidates for the energy storage units utilized in miniaturized electronics.


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For electronic units which are expected to be widely applied, the fabrication process is a key factor in determining whether they can be abundantly and cost-effectively produced.6,28 Various technologies have been employed for patterning the electrode finger arrays of in-plane MSCs. For instance, photolithography29 and plasma-etching30 process have been respectively used in fabrication of in-plane MSCs. However, because of the necessity of introducing sacrificial templates during processing, which have to be removed after fabrication, these technologies were proven to be cumbersome in building micro electrode finger arrays. By contrast, direct printing technologies, such as inkjet printing and screen printing, are effective methods for directly patterning the active materials without the requirement of sacrificial templates, and successful applications have been demonstrated.6,28 But, there are two major challenges for these printing techniques: one is that the active materials have to be converted into printable inks, which means lots of inactive additives would be added to achieve the printability; the other is that for the present available printing technologies, it is difficult to directly fabricate the asymmetric electrode fingers with alternate arranged structure,8 namely the high performance asymmetric inplane MSCs still have to be fabricated through relative complicated processes. Therefore, a simple, efficient and low cost technology for the building of in-plane MSCs especially for that with asymmetric structures is highly desired. In this work, an effective solution for the high efficient manufacturing of in-plane MSCs from symmetric to asymmetric structure is proposed. A modified laser printing process is developed for the rapid patterning of the active materials without the requirement of extra additives. Furthermore, based on this technique and a creative “print-and-fold” process, in-plane MSCs with asymmetric structure can be facilely fabricated, which offers a simple and efficient technology for the production of asymmetric in-plane MSCs. In detail, for the symmetric in-


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plane MSC, reduced graphene oxide (rGO)/ silver nanowires (Ag-NWs) hybrid was used as the electrode materials. The rGO/Ag-NWs hybrid system was chosen because i) the one-dimensional Ag-NWs could work as spacers to reduce the stacking of the two-dimensional rGO effectively and enable the fully utilization of the active material;31 ii) the forming of interpenetrating network with Ag-NWs would bring the hybrid electrode excellent electrical conductive property, which avoids the utilization of traditional current collector layer and simplified the total structure. To further improve the overall performance, asymmetric-structured MSC was designed by using α-Ni(OH)2 as the pseudocapacitive material. Specially, the α-Ni(OH)2 was synthesized through a controlled crystallization method which possessed not only higher capacitance than the commonly used β-Ni(OH)2 but also a high tapping density of 1.37 g cm-3 and rich nanostructures based on the regular spherical appearance. Finally, a miniaturized MSC pack was demonstrated by multiply folding the MSC units connected by Ag-NW circuits. As a result, a tandem MSC pack with high supply voltage of 3 V was obtained and a LED light was powered to prove its practicability. This provides new insights on achieving energy storage packs that can be integrated in small spaces. 2. EXPERIMENTAL SECTION 2.1 Fabrication of symmetric MSC. The rGO was purchased from Chengdu Organic Chemicals Co. Ltd., China. The Ag-NWs were synthesized through a modified solvothermal method according to our previous works32-33 with average diameter of ~100 nm and length of ~20 µm. The Laser printing was conducted with a commercial laser printer (CLP-680ND, Samsung, Korea). The paper substrate in this work is the commercial printing paper (70 g m-2, Gold East Paper Co. Ltd., China). The printing patterns of electrodes were designed by using a software of CorelDraw 9. The laminating process was conducted on a hot laminating machine


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(V350, Rongda, China) at 160 ℃ with speed of 2 m/min. For a typical process, a certain amount of rGO was firstly dispersed in deionized water by a tip sonicator to form the rGO dispersion liquid. Afterwards, it went through a vacuum filtration process mixing with the Ag-NW aqueous dispersion to obtain the hybrid film on the filter membrane, which was then transferred onto the PI film under the assistance of vacuum and got ready for being selectively adhered by the toner patterns. For the gel electrolyte, 2 g polyvinyl alcohol (PVA, Mw ~ 67,000) was dispersed in 80 mL deionized water at room temperature under stirring. Then 8 g of KOH was added into the solution and the mixture was heated to 90 ℃ and kept stirring till the PVA was completely dissolved. 2.2 Synthesis of α-Ni(OH)2. First of all, 0.5 M ammonia solution was added in a threenecked flask. Then, mixed aqueous solution of ammonia (1 M), NiSO4 (1 M), NaOH (2 M) and Al2(SO4)3 (0.125 M) were dropped to the flask under vigorous stirring. Control the reaction solution pH value at 10.75 ± 0.05 through adjusting the adding speed of the mixed solution. The reaction was taking place at temperature of 60 °C and lasted for 24 h, then the product was aged in the reaction solution for 17 h at 60 °C. Subsequently, the product was centrifuged, washed and completely dried. Afterwards, 100 mL ammonia solution (1 M) together with 20 g α-Ni(OH)2, and 3 g CoSO4·7H2O were added in 200 mL water and dispersed. 1M NaOH solution was then dropped into the mixed dispersion at 50 °C till the red color faded. After kept in the solution for 3h, the product was washed and re-dispersed in alkaline solution. Then 8 mL of 30 % H2O2 was dispersed into the solution and left for two hours. Finally, the product was washed till neutral and dried. 2.3 Fabrication of asymmetric MSC. The anode electrode of the asymmetric MSC was fabricated same as the symmetric one. For the fabrication of the cathode electrode, certain


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amount of α-Ni(OH)2 was dispersed in Ag-NW aqueous solution to form a mixture dispersion. Then it went through a vacuum filtration process to obtain the hybrid film on the filter membrane, which was subsequently transferred onto the PI film under the assistance of vacuum and got ready for being selectively adhered by the cathode electrode toner pattern. After printing, the paper substrate was folded along the middle line between the cathode and anode electrodes and the electrode fingers were alternately arranged in a plane between the paper substrates. Finally, the gel electrolyte was drop-cast between the electrodes and left overnight to obtain the asymmetric in-plane MSC. 2.4 Characterizations. The morphology of the samples were observed by a transmission electron microscope (TEM, JEM2100, Japan) and a scanning electron microscope (SEM, Hitachi S-4800, Japan) at 10 kV in secondary electron scattering mode. The optical microscopy images were taken by Olympus STM6 Optical Microscope (Japan).The crystalline phase of the αNi(OH)2 was characterized by an X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.154 nm) between 2θ = 5° and 2θ = 80°. The cyclic voltammetry and charge-discharge curves were obtained by using an electrochemical workstation (CHI 760E, China). The resistance of the electrodes were measured using a Keithley SourceMeter 2400 (USA). The tensile tests of the MSC were performed on an Instron 5882 universal machine with a 500 N load cell at tensile speed of 2mm min-1. 3. RESULTS AND DISCUSSION The morphology of the rGO sheets was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as shown in Figure 1a and the inset. The SEM image shows the typical wrinkle morphology of the rGO and the few layers structure can be distinctly caught the sight from the TEM image. Figure 1b presents the SEM and TEM characterizations of


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the Ag-NWs. The Ag-NWs was synthesized by a modified solvothermal method with an average diameter of ~100 nm and length of ~20 µm.33 Figure 1c schematically illustrates the “spacer” effect between rGO and Ag-NWs. By forming interpenetrating network, Ag-NWs work as spacers to prevent stacking of the rGO sheets and meanwhile bring favorable electrical conductivity to the hybrid electrode, so that the traditional extra current collector can be omitted. Figure 1d shows the schematic diagram of the manufacturing process of the symmetric in-plane MSC via laser printing method. In the process, firstly, the designed MSC patterns were printed on a piece of paper by a common laser printing machine to obtain the toner patterns (the left image of Figure 1d). Secondly, a fusing process was used to gain the active electrode material coating. The rGO and Ag-NWs in aqueous solution were vacuum filtrated to form hybrid layer and transferred onto the polyimide (PI) film. The PI film and the printed paper were then simultaneously sent to go through a hot laminating process (the middle image of Figure 1d). As the printed toner is thermoplastic, during the hot laminating, the printed patterns were instantly fused by the heating silicone rollers and the rGO/Ag-NW hybrid would be selectively adhered by the melted toner, resulting in the forming of rGO/Ag-NW finger patterns on paper. The printed pattern possesses a low areal resistance of 0.029 Ω sq-1, which shows the good electrical property of the composite electrode. Finally, a gel electrolyte of poly(vinyl alcohol) (PVA)/KOH was drop-cast on the electrodes and allowed to solidify. After that, an all solid state symmetric inplane MSC was obtained (the right image of Figure 1d). It worth noting that although the cellulose-based paper may hydrolyze in alkaline aqueous medium, no obvious hydrolyzing was observed in this work since the alkaline electrolyte was in gel form and solidified subsequently after coating. In fact, the tensile strength of the MSC even increased instead of decreasing after electrolyte coating as shown in Figure S1, which can be attributed to the introducing of PVA.


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Figure 1e and f show the digital photographs and the SEM images of the surfaces and crosssections of the finger patterns before and after active material coating, respectively. As can be

Figure 1. a) SEM and TEM (inset) images of rGO. b) SEM and TEM (inset) of Ag-NWs. c) Schematic drawing of the “spacer” effect between rGO and Ag-NWs. d) Schematic diagram of the fabrication process of the paper-based in-plane symmetric MSC via laser printing. e) and f) respectively showing the digital photographs, the surface and cross-section SEM images of the printing pattern before and after rGO/Ag-NW coating.


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seen from the digital photographs, the rGO/Ag-NW hybrid was uniformly embedded in the patterned areas and formed finger electrodes with resolution as high as the pristine toner pattern,which indicates the high accuracy of the fabricating approach. The SEM images present the hybrid coating of the rGO/Ag-NW on the surface with interpenetrating network and the layered structure can be clearly seen in the cross-sectional view image. To examine the electrochemical properties of the obtained symmetric MSC, cyclic voltammetry (CV) measurements were carried out at various scan rates. In the tested MSC, the areal densities of the rGO and Ag-NWs in the electrode were 0.25 mg cm-2 and 0.5 mg cm-2, respectively. Figure 2a-d show the CV curves of the symmetric MSC with scan rates from 50 mV s-1 to 1000 mV s-1. Different from the MSCs based on pure carbon material, whose typical CV curves are symmetric without obvious peaks,3,6,9 a few peaks can be observed in the CV curves of the rGO/Ag-NW hybrid MSC especially at slow scan rates. These peaks correspond to the redox reactions of the Ag-NWs of positive electrode in KOH electrolyte, which could be described by the following equations:34-35 2Ag + 2OH- ↔ Ag2O + H2O + 2e-

(1)

Ag2O + 2OH- ↔ 2AgO + H2O + 2e-

(2)

Note that although there are redox reactions taking place on Ag-NWs, the electrical conductive property of the hybrid electrodes won’t be influenced dramatically because the partly transformed AgO also possesses high electrical conductivity. Figure 2e presents the chargedischarge curves of the symmetric in-plane MSC at various currents. From the discharge curves, the areal capacitance (CA, mF cm-2) was calculated by the following equation: CA = I × t / ∆V ×S, where I, t, ∆V and S are the discharge current, discharge time, potential window and the electrode


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area, respectively. Furthermore, the areal power density (PA, mW cm-2) was also calculated according to the equation of PA = I × Vaverage / S, where the I is discharge current, Vaverage is the average potential value of the discharge curves, S is the area of the electrode. The CA and PA values at different currents are shown in Figure 2f, from which it can be seen, the symmetric rGO/Ag-NW MSC exhibits a high CA of 5.5 mF cm-2 at the current of 0.5 mA. This value is comparable to the best report till now on graphene-based MSCs.6 Meanwhile, a high PA of 0.45 mW cm-2 and energy density of 0.388 µW h cm-2 were achieved at the discharge current of 3 mA.


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Figure 2. a-d) CV curves of the paper-based symmetric MSC with scan rates ranging from 50 to 1000 mV s-1. e) Charge-discharge curves with different currents. f) Areal capacitance and areal power density as functions of applied currents. In order to further improve the energy and power performances, an asymmetric MSC with pseudocapacitance was designed and fabricated. In the asymmetric MSC, α-Ni(OH)2 synthesized through a controlled crystallization method was used as the pseudocapacitive material, which holds higher theoretical capacitance than the traditional β-Ni(OH)2.24,36 By


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accurately controlling the nucleation, growth and secondary crystallization processes, α-Ni(OH)2 with regular spherical morphology (Figure 3a) and solid internal structure (Figure S2 of Supporting Information) was obtained. As a result, a high tapping density of 1.37 g cm-3 have been realized which ensured the high energy density and good operability of the material. Besides, Figure 3b presents the rich nano-lamellar on the surface, whose special structures would lead to the large surface area and facilitate the fast redox reactions. Figure 3c shows the XRD pattern of the as-synthesized Ni(OH)2, which confirms the high purity α crystal phase of the product. Furthermore, Figure S3 in Supporting Information exhibits the Fourier transform infrared (FTIR) spectrum of the α-Ni(OH)2, where lots of hydrones can be found in the interlayer that playing the role of stabilizing the wide lattice spacing. CV and typical discharge curves of the α-Ni(OH)2 are presented in Figure S4 and S5, reflecting its good electrochemical performances. Notably, Figure S6 shows the cycling stability test of the α-Ni(OH)2 and a high capacity retention of 87 % after 100 cycles has been demonstrated. For the fabrication of the asymmetric in-plane MSC, a creative method of “print-and-fold” was proposed. As schematically described in Figure 3d, cathode (α-Ni(OH)2/Ag-NW) and anode (rGO/Ag-NW) materials were firstly printed serially on one piece of paper by the laser printing process introduced previously. The obtained α-Ni(OH)2/Ag-NW electrode presents a areal resistance of 0.066 Ω sq-1. Then the paper substrate was folded along the middle line between the cathode and anode patterns, and as a result, the electrode fingers were alternately arranged in a plane between the paper substrates. This architecture design could increase the availability of the active materials, for the edges of the electrodes are exposed to electrolyte and close to each other, which is particularly important for the two-dimensional electrode materials like graphene.8 At the same time, since there is no separator between the two electrodes, the alternately arranged


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electrodes could avoid the risk of short-current. After drop-casting the gel electrolyte between the electrodes and letting it solidified, an asymmetric structured in-plane MSC was obtained. In this way, the asymmetric MSCs can be fabricated facilely, making it a simple and efficient manufacturing method compared to the traditional cumbersome technologies. Figure 3e shows the digital photograph of the printed cathode and anode electrodes on paper, presenting the high resolution of the electrode patterns. Figure 3f and 3g give the SEM images of the surface and cross-sectional view of the α-Ni(OH)2/Ag-NW electrode, respectively. From which it can be seen that the α-Ni(OH)2 particles were uniformly embedded in the Ag-NW networks, ensuring the fast charge transfer in the electrode.

Figure 3. a-b) SEM images of the as-synthesized α-Ni(OH)2 with different magnifications. c) XRD pattern of the as-synthesized α-Ni(OH)2. d) Schematic drawing of the fabrication process


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of the asymmetric in-plane MSC. e) Digital photograph of the as-fabricated cathode and anode electrodes on paper. f-g) SEM images of the surface and cross-section of the α-Ni(OH)2/Ag-NW electrode. CV experiments were performed at various scan rates to evaluate the electrochemical property of the asymmetric MSC. In the tested sample, the areal densities of the α-Ni(OH)2 and Ag-NWs in the electrode were 1.5 mg cm-2 and 0.5 mg cm-2, respectively. Figure 4a-d display the CV curves with scan rates ranging from 50 mV s-1 to 1000 mV s-1, from which relatively complicated redox peaks can be found. Besides that corresponding to the redox reactions of AgNWs discussed above, the additional peaks could be attributed to the extraction and insertion of proton in Ni(OH)2 as described in the following equation:23,37-38 Ni(OH)2 + OH- ↔ NiOOH + H2O + e-

(3)

These introduced redox reactions enable the large charge storage of the system. Furthermore, it worth noting that compared with the symmetric MSC, the voltage window of the asymmetric MSC could be extended from 0-1 V to 0-1.5 V due to the raised potential difference between the Ni(OH)2 and rGO electrodes.39-40 The improvement of the supply voltage means higher power could be achieved during application. Figure 4e displays the charge-discharge curves of the asymmetric MSC at different currents. It can be found that obvious discharge plateaus have been shown in the discharge curves, which are typical symbols indicating the existence of pseudocapacitance. Also, it can be seen that the values of IR drops of asymmetric MSC are more obvious than that of symmetric one in Figure 2e, which can be contributed to the higher resistance of the Ni(OH)2 electrode than the rGO electrode (0.066 Ω sq-1 compared to 0.029 Ω sq-1). The CA and PA values at different currents are presented in Figure 4f. For the asymmetric


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MSC, a CA as high as 8.6 mF cm-2 was achieved at the discharge current of 0.5 mA, showing 56 % improvement compared to the symmetric MSC. However, the capacitive ability of the symmetric MSC at large currents is still better than the asymmetric MSC, which can be attributed to the faster charge storage in the EDL electrode than in the pseudocapacitive one. Benefiting from the raise of the supply voltage, a high PA of 0.73 mW cm-2 and energy density of 0.669 µW h cm-2 at applied current of 3 mA have been realized, showing respectively 62% and 72.4% increments compared to that of the symmetric MSC. This large improvements in PA and energy density are significantly important for the practical application and the values are competitive among the reported in-plane MSCs as listed in Table S1 in Supporting Information.8,41-43


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Figure 4. a-d) CV curves of the paper-based asymmetric MSC with scan rates ranging from 50 to 1000 mV s-1. e) Charge-discharge curves with different currents. f) Areal capacitance and areal power density as functions of applied currents. To work as the power source in an electronic system, a pack is generally required in which the energy storage units are connected in parallel or in series to meet the power and energy requirements by adjusting the capacity or potential ranges. For in-plane MSCs, parallel


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and series forms of packs haven been demonstrated.2,6 However, these packs were all obtained by directly arranging the MSC units in plane, then connecting by circuits. This leaded to a large area of the final MSC pack which is contrary to the space-saving intention of the design of inplane MSCs. Here, a creative solution of folding the power units is proposed to obtain the MSC pack with compact structure. In order to verify the feasibility of the solution, a demonstration of a series connecting pack of asymmetric in-plane MSC is presented. Firstly, as shown in Figure 5a, the cathode and anode electrodes of two asymmetric MSC units were printed alternately on paper substrate. Then, Ag-NWs solution was drop-cast and dried to connect the two MSC units in the middle (The areal density of the Ag-NW circuit is 0.5 mg cm-2 and the areal resistance of the obtained circuit is 0.032 Ω sq-1). Afterwards, the paper substrate was triple folded as described in Figure 5b to form the folded connecting MSC units. After casting gel electrolyte between the electrodes, the MSC pack with compact structure was obtained as shown in Figure 5c. The Ag-NW circuit was employed to connect the units in the pack as it can endure the folding without obvious degradation in electrical conductive property, which has been proved in our previous work.44 Figure S7 in Supporting Information shows the optical microscopy of the Ag-NW circuit after folding. Due to the one-dimensional nanostructure and the excellent ductility of Ag-NWs, there is only a crease left with no crack founded on the circuit after folding, which ensures the reliable connection between the units in the folded pack. As the MSC units were serially connected, the total voltage range can be improved. Figure 5d shows the chargedischarge curves of the folded MSC pack. It can be seen that the supply voltage was improved to a high value of 3 V, thus a high power of 6 mW could be achieved under the discharge current of 3 mA. By using this pack, a red LED light could be powered as shown in Figure 5e, indicating the great potential of the MSC pack used as micro power supply. The idea of the folded MSC


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pack maximizes the advantages of the in-plane MSCs and facilitates the achievement of power supply in compact and small size, which offers a new opportunity for the practical application of the in-plane MSCs.

Figure 5. A demonstration of a folded tandem in-plane MSC pack. a) Cathode and anode electrodes were fabricated on one piece of paper in series and the middle of two electrodes were connected by Ag-NW circuit. b) Fold the paper substrate to form the pack containing two tandem asymmetric MSC units. c) The MSC pack was completed after adding electrolyte between electrodes. d) Charge-discharge curves of the pack with different currents. e) The folded MSC pack was used to power a LED light.


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4. CONCLUSION In summary, we have demonstrated a complete solution for efficiently fabricating in-plane MSCs from symmetric to asymmetric structure. By using a modified laser printing process, symmetric MSC with rGO/Ag-NW hybrid electrodes was fabricated and a high areal capacitance of 5.5 mF cm-2 was achieved, which is comparable with the best reports on graphene-based MSCs. Furthermore, a “print-and-fold” method has been creatively proposed that enabled the rapidly manufacturing of asymmetric in-plane MSCs beyond the traditional cumbersome technologies. α-Ni(OH)2 particles with high tapping density were successfully synthesized and αNi(OH)2/Ag-NW hybrid was employed as the pseudocapacitive electrode in the asymmetric MSC. Consequently, an improved supply voltage of 1.5 V was obtained and an areal capacitance as high as 8.6 mF cm-2 has been realized. Moreover, a demonstration of a miniaturized MSC pack was accomplished by multiply folding the serially Ag-NW-connected MSC units. As a result, a compact MSC pack with high supply voltage of 3 V was obtained and used to power a LED light. Owing to these advantages, our work may pave the way for the efficiently producing of high performance symmetric and asymmetric in-plane MSCs, meanwhile offer a solution for the achievement of practical power supply packs integrated in limited spaces. ASSOCIATED CONTENT Supporting Information. Tensile test of the MSC, SEM images, FTIR spectrum, cyclic voltammetry curves, typical discharge curve, cycling stability of the α-Ni(OH)2 particles, optical microscopy image of the Ag-NW circuit before

and after

folding, and comparison of

performances of in-plane MSCs. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * Q. P. Feng. E-mail: [email protected]. * H. M. Xiao. E-mail: [email protected].

ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (nos. 51503213, 11372312, 51373187, 11572321, and 51573200). The authors gratefully acknowledge financial support

received

from

Sino-tech

Tenonscience

&

Technology

Development

Ltd.

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Laser-Printed In-Plane Micro-Supercapacitors: From Symmetric to

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