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DOI: 10.1039/C6RA06034J (Paper) RSC Adv., 2016, 6, 43844-43854

All-solid-state flexible asymmetric micro supercapacitors based on cobalt hydroxide and reduced graphene oxide electrodes†

Su Chan Lee‡ a, U. M. Patil‡a, Sun Jun Kima, Seokhoon Ahnb, Seok-Won Kangc and Seong Chan Jun*a
aNano-Electro Mechanical Device Laboratory, School of Mechanical Engineering, Yonsei University, Seoul 120-749, South Korea. E-mail: scj@yonsei.ac.kr; Web: http://nemd.yonsei.ac.kr Fax: +82-2-312-2159; Tel: +82-2-2123-5817
bInstitute of Advanced Composite Materials, Korea Institute of Science and Technology, Jellabuk-do 565-905, South Korea
cKorea Railroad Research Institute, 176 Cheoldo bangmulgwan-ro, Uiwang, Gyeonggi-do 437-757, South Korea

Received 7th March 2016 , Accepted 26th April 2016

First published on 27th April 2016

Abstract

In-plane micro supercapacitors (micro-SC) have attracted interest due to their high areal and volumetric capacitance that is dependent upon their electrode structure. This study proposes simply-fabricated micro-SCs based on cobalt hydroxide and electrochemically-reduced graphene oxide (erGO). The Au electron collector was prepared via photolithography, and Co(OH)2 and erGO were deposited on the Au surface via electrodeposition. Using facile two-step fabrication method and cost-effective materials, the prepared micro-SCs exhibit good electrochemical performance in PVA–KOH–KI solid electrolyte. The in-plane interdigitated electrode maximizes the areal capacitance by increasing the facing area between the anode and cathode. The micro-SC presents good power density (100.38 μW cm−2) and a wide potential window due to the electric double-layer properties of erGO and pseudocapacitive performance of Co(OH)2. These fabricated micro-SCs are flexible and can be used in various wearable and small-scale energy storage devices.

1. Introduction

As electrical power becomes more portable, consumers demand power that is available anytime and anywhere. Current energy storage devices are limited in terms of performance, and in order to satisfy this demand, high-performance energy storage devices should be developed in advance.1 Improving the performance of energy storage devices has attracted considerable attention. The lithium ion battery (LIB), the most widely used energy storage device, has significant drawbacks such as low power density and risk of explosion.2,3 Many other types of energy storage devices have been studied to overcome these limitations;4,5 the supercapacitor (SC), for example, is one of the candidates to complement the LIB. SCs have large power density and better stability and safety than LIBs.6 Especially, the asymmetric SC with one pseudocapacitive material and one electric double layer material, is under development and has intermediate characteristics to achieve both advantages of LIB and conventional SC.

High-performance SCs can be used in applications of varying scale, such as integrated circuit (IC) chips, mobile phones, electrical vehicles and electrical power generation systems.7 Micro-scale electric devices such as sensors, actuators, displays and energy-harvesting modules have been widely used in the development of micro electro mechanical system (MEMS) technology. Many types of electric device have been miniaturized with the advancement of MEMS technology. To operate these micro-scale devices, the energy-storage device must be of a similar scale.8 SCs of many sizes have been studied, and the micro supercapacitor, which has a 2-dimensional micro scale interdigitated electrode, has received significant attention due to its superior performance when compared to the conventional sandwich-type SC.9 The micro-SC, which is typically less than a millimeter in scale, is suitable for small-scale applications such as on-chip devices in IC. Micro-SCs can achieve high volumetric capacitance, because the two electrodes are locate in-plane. Interdigitated electrodes increase the areal capacitance of micro-SC by maximizing the facing area between anode and cathode. The structure of the micro-SC is notably different from conventional SCs, and optimized methods and materials for micro-SC have been studied.10–12

The SC has two asymmetric electrodes based on energy-storage mechanism: the electrical double-layer capacitor (EDLC) and the pseudo capacitor.1 The EDLC uses an electrical double layer, electrostatic ion charge separation, on the electrode surface as its energy storage mechanism. This electrical double layer exists within only a few angstroms, so EDLC can store more charge than conventional capacitors.13 The pseudo capacitor, which acts as a battery-like mechanism, uses faradaic charge transfer with fast and reversible redox reaction.14 The EDLC has a greater power density than the pseudo capacitor, and the pseudo capacitor has a greater energy density than the EDLC.

To increase the performance of the SC, asymmetric SC (ASC), EDLC, and pseudo capacitor hybrids have been developed by many groups.15 The ASC can maximize the performance of a SC due to the synergistic effects between the large power density of the EDLC and the large energy density of the pseudocapacitor. The aqueous-electrolyte-based SC has excellent power density due to the ionic conductivity of its electrolyte. In addition to its performance, the aqueous electrolyte is eco-friendly and offers better safety than organic electrolytes, though its limited potential window is a significant drawback. A recent approach for overcoming the potential window problem is combing two different materials with natural polarized potential as positive and negative. Consequently, ASC offers not only high energy density and power density, but also wide working voltage.

In this study, we present an all-solid-state flexible asymmetric micro-SC based on Co(OH)2 and erGO. Our micro-SC was made using a simple fabrication method and cost-effective materials. The micro-SC was fabricated in two simple steps of photolithography and electrodeposition. The photolithography, which is a widely used fabrication method in the semiconductor industry, can produce accurate and uniform large-area samples. Electrodeposition enables the desired material to be precisely deposited only on the target electrode surface via a simple process. We demonstrate that simple and affordable micro-SCs can exhibit comparable performance with other micro-SCs.

2. Experimental

2.1 Materials

Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), polyvinyl alcohol (PVA), and sodium sulfate (Na2SO4) were purchased from Sigma Aldrich, Korea. Potassium iodide (KI) and potassium hydroxide (KOH) was obtained from SAMJUN chemicals, Korea. Graphite was purchased from Bay Carbon (USA). Graphene oxide was synthesized via a modified Hummer's method, and all other chemicals for the modified Hummer's method were obtained from Sigma Aldrich, Korea.16

2.2 Electrode fabrication process

A micro-SC electrode was prepared in a two-step fabrication method, as shown in Fig. 1. The interdigitated electrode was patterned via a conventional photolithography process similar to that presented in literature on polyimide (PI) substrates.17 A 5 nm Ti layer and 50 nm Au layer were then deposited by using an E-beam evaporator. A potentiostatic electrodeposition technique was used to produce Co(OH)2 and erGO on the Au/Ti/PI electrode surface at room temperature. The conventional three-electrode cell configuration was used for electrodeposition with the Au/Ti/PI electrode as a working electrode, a platinum wire as counter electrode, and a saturated calomel electrode (SCE) as reference electrode. For the deposition of Co(OH)2, potentiostatic electrodeposition was performed at a potential of −0.94 V versus SCE for 120 s with 0.1 M Co(NO3)2·6H2O.18 The same method was used for the GO, at −1.2 V versus SCE for 140 s with 2 mg ml−1 GO solution and 0.1 M NaSO4 as electrolyte solution.19 GO solution was synthesized via modified Hummer's method.16 Following the electrodeposition process, samples were dried at 80 °C.
image file: c6ra06034j-f1.tif
Fig. 1 Schematic illustration of the sample fabrication method.

2.3 Preparation of solid electrolyte

The PVA–KOH–KI solid electrolyte was prepared based on the method by Haijun et al.20 4.5 g PVA was dissolved in 40 ml DI water and heated at 90 °C to become transparent. 4.5 g of KOH was then dissolved in 20 ml DI water and added to PVA solution. The 2.7 g KI was added to the mixed solution, and the solution was heated at 90 °C during 30 min.

2.4 Preparation of micro-SC

A micro-SC was prepared by following the procedure in Fig. 1. The symmetric Au/Ti/PI electrodes were prepared and only one electrode was coated with Co(OH)2 and erGO for the half-cell test. One side of the electrode was coated with Co(OH)2 and erGO was deposited on the other side for the Co(OH)2//erGO micro-SC. The as-prepared solid electrolyte was covered over the electrode surface of the all-solid micro-SC, and then the sample was dried at 50 °C to remove excess water in the solid electrolyte.

2.5 Characterization

The molecular and atomic structure characterization was performed via X-ray diffraction (XRD, Smartlab, Rigaku). Chemical bonds were investigated using X-ray photoelectron spectroscopy (XPS, k-alpha, Thermo. U.K.), and the lattice vibration and structure of graphene were analyzed via Raman spectroscopy (LabRam Aramis, Horriba Jovin Yvon). The morphology of the active material was measured using a Field Emission Scanning Electron Microscope (FE-SEM, JEOL-7001F, JEOL Ltd.), and electrochemical measurements were carried out via a conventional three-electrode cell configuration and two-electrode system using Ivium n Stat (HS technology). A three-electrode system was used for the half-cell test with a micro-SC electrode as the working electrode, a platinum (Pt) wire as a counter electrode, and Ag/AgCl as a reference electrode in 1 M KOH. A full hybrid cell test with liquid electrolyte was carried out with asymmetric electrodes in 1 M KOH.

3. Results and discussion

3.1 Fabrication of electrodes

The micro-SC electrode was fabricated using a simple two-step process consisting of photolithography and electrodeposition. Au/Ti/PI was prepared via a photolithography process, and the size of the details are shown in Fig. 1. Co(OH)2 was coated onto the Au/Ti/PI electrode surface via potentiostatic electrodeposition at constant negative potential (−0.94 V versus SCE). The reaction of Co(OH)2 electrodeposition, which consists of electrochemical and precipitation reactions, can be expressed as follows:
 
NO3− + 7H2O + 8e− → NH4+ + 10OH− (1)
 
Co2+ + 2OH− → Co(OH)2. (2)

The Co(OH)2 was formed by a reaction between Co2+ and OH−, which was produced by reaction (1). GO was electrochemically reduced via the typical potentiostatic method at a different potential of −1.2 V versus SCE. The possible reaction of the reduction process can be proposed as follows:21

 
GO + aH+ + be− → erGO + cH2O. (3)

3.2 Structural characterizations

The X-ray diffraction (XRD) pattern of Co(OH)2 on the Au/PI substrate is shown in Fig. 2. Not including the Au (38.2°, 44.6°, and 64.9°)22 and PI (14.5°, 21° and 26.3°)23 substrate peaks, there is α-Co(OH)2 peak at 11.3°, which is indexed as (001), in agreement with literature.24,25 The (001) plane is related to the basal plane of layered α-Co(OH)2.26 Electrodeposited Co(OH)2 is green in color, which indicates that our Co(OH)2 is layered – α-Co(OH)2.27 The erGO has less crystallinity than Co(OH)2 and the XRD peak positions of typical GO and reduced GO are similar to PI, so the erGO peak can not be distinguished on Au/PI substrate. The X-ray photoelectron spectroscopy (XPS) spectrums of Co(OH)2 and erGO are shown in Fig. 3. The curve shape in the Co2p spectra of Co(OH)2 agrees well with literature.24,28,29 The Co2p spectra exhibits two distinct peaks at 780.88 eV and 796.78 eV, corresponding to the 2p3/2 and 2p1/2 transition, respectively. These peaks are oriented from the spin–orbit peaks of Co2+.30,31 The shake-up satellite peaks, which are usually observed in 3d transition metal materials, appear at a higher binding energy area (786.08 eV and 802.78 eV, respectively) than the 2p3/2 and 2p1/2 peaks.32 The satellite peaks can support that Co3+ ions were formed on the surface.33 There are obvious hydroxide groups (531.58 eV) with shoulder water peak in the O1s spectra of Co(OH)2, confirming that hydrous Co(OH)2 is well formed. The high-resolution C1s peak is presented in Fig. 3(c), which has three peaks at C[double bond, length as m-dash]C (285.26 eV), C–O (287.20 eV), and C[double bond, length as m-dash]O (288.57 eV). The C–O peak has significantly less intensity than the C[double bond, length as m-dash]C peak, indicating that our GO was successfully electrochemically reduced.34 The C–O and C[double bond, length as m-dash]O peaks of erGO are low in intensity than it for pristine GO (see Fig. S1†), confirms the electrochemical reduction of GO. The electrochemical reduction is also confirmed via Raman spectroscopy, with results shown in Fig. 4. The erGO has two prominent peaks, the G and D peaks.35 The G peak (at 1592 cm−1) is related to first-order scattering of the E2g mode in a hexagonal carbon lattice and degree of graphitization.36 The D peak (at 1349 cm−1) corresponds to a structural defect of the sp2 domain.37 The resultant electrochemical reduction of GO is confirmed by increase in ID/IG ratio.38 Prepared erGO has an ID/IG ratio of 1.05, which is larger than that of pristine GO (0.94) as shown in Fig. S2.† The change of this value agrees with other literature on reduced GO.39,40 The Co(OH)2 has two Raman peaks at 513 cm−1, which correspond to the CoO (Ag) symmetric stretching mode and A1g modes of the Co(OH)2 crystalline phase, respectively.33
image file: c6ra06034j-f2.tif
Fig. 2 The X-ray diffraction (XRD) results of Au/PI, Co(OH)2/Au/PI, and erGO/Au/PI electrodes.

image file: c6ra06034j-f3.tif
Fig. 3 X-ray photoelectron spectroscopy (XPS) characteristics: (a) Co2p spectra of Co(OH)2, (b) O1s spectra of Co(OH)2, and (c) high-resolution C1s spectra of erGO.

image file: c6ra06034j-f4.tif
Fig. 4 Raman spectrum of Co(OH)2 and erGO.

3.3 Morphology study

The morphology of Co(OH)2 and erGO was investigated using FE-SEM, as shown in Fig. 5. The SEM measurement was carried out after half-cell coating. Only the right side of the electrode, which is slightly brighter than the left side, is coated by Co(OH)2 in Fig. 5(a). EDS analysis (Fig. S3†) confirms, that the coated material has cobalt and oxygen elements, and this result agrees with the characterization results. The Co(OH)2 was coated on the Au electrode surface via electrodeposition, proving that the electrodeposition method yields high accuracy and selectivity. Fig. 5(b and c) shows high-magnitude SEM images of Co(OH)2. The Co(OH)2 exhibits microflake shapes with an average thickness of 10 nm, and was uniformly distributed over the electrode surface. The erGO also coated the right side of the electrode, as shown is Fig. 5(d). EDS analysis (Fig. S3†) reveals that the coated electrode contains carbon. The interval space of two electrodes, which is part of the PI substrate, has carbon in its polymer chain that appears in the EDS image as a high-density carbon area. The typical wrinkle of erGO was observed in high-magnification images (Fig. 5(e) and f). The erGO sheets were deposited over the Au electrode surface horizontally, and the micro-SC full cell was fabricated and examined using FE-SEM. Fig. S4(a–c)† show the Co(OH)2//erGO full cell, in which each electrode is shown in a different color. The right electrode was coated with Co(OH)2 and the left with erGO, as confirmed via EDS analysis: the right electrode contains cobalt without carbon and the left contains carbon in erGO without cobalt. The shapes of the active materials were observed via SEM image and EDS analysis, and their elements were studied.
image file: c6ra06034j-f5.tif
Fig. 5 SEM image of (a) Co(OH)2 on an Au electrode with (b and c) a high-magnification image, (d) erGO on an Au electrode, and a (e and f) high-magnification image of erGO.

3.4 Electrochemical measurement

3.4.1 Electrochemical characterizations of Co(OH)2 and erGO. Layered Co(OH)2 is one of the most representative pseudocapacitive materials. Co(OH)2 has high theoretical specific capacitance (3458 F g−1), though it can be fabricated by simple process. Reduced GO is promising EDLC material due to its high electrical conductivity, large surface area, and good stability. Only one side of our prepared Au/PI symmetric electrode was coated with Co(OH)2 and erGO, respectively. The electrochemical performances of these single electrodes were tested using a half-cell test. Cyclic Voltammetry (CV) was measured to analyze the electrochemical surface reactions and capacitive behavior during the charging–discharging process. Fig. 6(a) shows the CV curves of Co(OH)2 with various scan rates from 10 to 200 mV s−1 in 1 M KOH. Co(OH)2 has distinctive redox peaks related to a reversible faradaic reaction based on pseudocapacitive property. This faradaic reaction originates from a change in the cobalt oxidation state, and can be described as follows:
 
Co(OH)2 + OH− ↔ CoOOH + H2O + e− (4)
 
CoOOH + OH− ↔ CoO2 + H2O + e−. (5)
image file: c6ra06034j-f6.tif
Fig. 6 Electrochemical characterizations of Co(OH)2 and erGO half cells, (a) cyclic voltammetry (CV) curves of Co(OH)2 within an optimized potential window of −0.2 to 0.5 V (versus Ag/AgCl) in aqueous 1 M KOH at scan rates of 10–200 mV s−1, (b) galvanostatic charge–discharge (CD) curve of Co(OH)2 at constant current densities from 1.637 to 8.193 mA cm−2, (c) CV results of erGO within an optimized potential window of −1 to 0 V (versus Ag/AgCl) in aqueous 1 M KOH at scan rates of 10–200 mV s−1, (d) galvanostatic CD result of erGO at constant current densities from 0.655 to 3.273 mA cm−2, (e) comparison of CV curves between Co(OH)2 and erGO at a scan rate of 200 mV s−1, and (f) changes in areal capacitance versus current density in Co(OH)2 and erGO.

The CV curve of Co(OH)2 exhibits an anodic peak at ∼0.2 V corresponding to the charging process, the oxidation of Co(OH)2 to CoOOH. The cathodic peak at −0.02 V is related to the discharging process and reduction of CoOOH to Co(OH)2. Fig. 6(b) presents the galvanostatic charge–discharge (CD) curve of Co(OH)2 at various current densities from 1.6 mA cm−2 to 8.2 mA cm−2. The CD curve is useful to analyze and measure charge storage behavior. The shape of the CD curve in Co(OH)2 is similar to typical pseudocapacitive material in the literature.41–43 A distinct plateau region is observed, which originates from pseudocapacitive redox property in agreement with the CV result. In addition, Co(OH)2 has relative less IR drop. This phenomenon can be explained by the layered Co(OH)2, which is uniformly deposited on the Au electrode surface via electrodeposition, so whole Co(OH)2 can readily react with the electrolyte and the electron transfer between Co(OH)2 and Au can be carried out efficiently. Fig. 6(c) shows the CV curve of erGO with diverse scan rates from 10 to 200 mV s−1 in 1 M KOH. Reduced GO is typically referred to as an EDLC material; however, our erGO has a small redox peak. The erGO retains a small amount of oxygen containing functional groups following the reduction process. This residual functional group produces a weak redox peak due to the redox reaction of OH−. At the higher scan rates, the CV curve shapes of erGO become rectangular in shape, as shown in Fig. S5.† The galvanostatic CD curve of erGO is presented in Fig. 6(d) at various current densities from 0.66 mA cm−2 to 3.27 mA cm−2. The shapes of the CD curves have similar ideal EDLC CD characteristics (linear shape). Interestingly, the CD curves of erGO exhibit less curvature; this curvature can be derived from the redox peaks of functional groups, which corresponds with CV results. A comparison of the Co(OH)2 and erGO CV curves is shown in Fig. 6(e). Each active material has a different potential window: Co(OH)2 has a potential window of [−0.2 V to 0.5 V] (vs. Ag/AgCl) and erGO has a potential window of [−1 V to 0 V] (vs. Ag/AgCl). By combining two potential window materials, a high-voltage SC can be achieved. Co(OH)2 has a larger capacitance than erGO because of the reversible redox processes it can undergo. These phenomena also can be observed in Fig. 6(f), which presents areal capacitance versus current density. The areal capacitance was calculated as follows:

 
image file: c6ra06034j-t1.tif(6)
where I is the current density (mA), Δt is discharging time in galvanostatic CD test, A is the electrode area, and ΔV is the potential window. The maximum areal capacitance of Co(OH)2 is 112.52 mF cm−2 at 1.6 mA cm−2 and that of erGO is 2.0 mF cm−2 at 0.65 mA cm−2. The electrochemical impedance spectroscopy (EIS) results are presented in Fig. S6.† The EIS was measured at an open circuit potential (OCP) within 250 kHz and 0.1 Hz frequency range. The charge transfer resistance (Rct) of Co(OH)2 and erGO is significantly less (5 and 6.5 Ω, respectively), resulting in a uniform deposition of active materials on metal current collectors. The EIS curve shape of erGO more closely resembles a vertical line than Co(OH)2, corresponding to ideal EDLC property.42

3.5 Asymmetric supercapacitor devices in aqueous electrolyte

The asymmetric supercapacitor is being actively studied due to its wide potential window and high energy density. Although the conventional supercapacitor can achieve higher power density, achieving a wide potential window with high energy density is difficult. In contrast, the asymmetric supercapacitor, which is fabricated using pseudocapacitive and EDLC materials, can obtain a wide potential window based on the potential difference of each electrode material. In such an asymmetric supercapacitor configuration, pseudocapacitive materials act as an energy source and EDLC materials as a power source. In this study, Co(OH)2//erGO asymmetric supercapacitor was prepared with high capacitance, a wide potential window and high power density.

The Co(OH)2//erGO asymmetric supercapacitor (CG-ASC) was electrochemically analyzed. Fig. 7(a) shows the CV results of CG-ASC with various scan rates from 10 to 200 mV s−1 in 1 M KOH. The CV curves have symmetric redox peaks, which originate from the quasi-reversible faradaic electron transfer process of OH− ions. The operating potential window of Co(OH)2 ranges from −0.2 to 0.5 V (vs. Ag/AgCl) and that of erGO from −1 to 0 V (vs. Ag/AgCl). By combining these two materials, the CG-SC has a wide operating potential window from 0 to 1.4 V. Energy density is proportional to the square of voltage, so the CG-SC can achieve a high energy density by increasing the potential window, even though the capacitance of CG-SC is not that much high. Furthermore, CG-ASC has advantages in terms of practical application. Each electric device has its own minimum operating voltage, CG-ASC can be used for devices with higher voltage. Fig. 7(b) presents the galvanostatic CD results of CG-ASC at diverse current densities form 1.72 mA cm−2 to 4.01 mA cm−2. The shape of the curves is non-linear, which is a pseudocapacitive property. The CD curves also exhibit a distinct plateau region similar to the Co(OH)2 half-cell result, indicating that the CG-ASC operates as a asymmetric supercapacitor. The areal capacitance versus the different current densities of CG-ASC is presented in Fig. 7(c). The maximum areal capacitance of CG-ASC is 14.15 mF cm−2 at 1.64 mA cm−2.


image file: c6ra06034j-f7.tif
Fig. 7 Electrochemical studies of asymmetric supercapacitors in aqueous 1 M KOH electrolyte, (a) CV curves of a Co(OH)2//erGO asymmetric supercapacitor within an optimized respective potential window of 0 to 1.4 V in aqueous 1 M KOH at scan rates of 10–200 mV s−1, (b) galvanostatic CD curve of a Co(OH)2//erGO asymmetric supercapacitor at constant current densities from 1.718 to 4.010 mA cm−2, (c) and changes in areal capacitance versus the current density of asymmetric supercapacitor.
3.5.1 Asymmetric supercapacitor devices with solid electrolyte. Although solid electrolyte has drawbacks such as less ionic conductivity and high resistance when compared to liquid electrolyte, solid electrolyte is more suitable for a practical device because it can be more easily packaged without leakage. Usually, solid electrolyte consists of PVA, structural host polymer, and electrolyte materials such as H2SO4, H3PO4, and KOH. The ionic conductivity of electrolyte is significant for asymmetric supercapacitor performance, though typical solid electrolyte has less ionic conductivity due to the high resistance of PVA. In this study, KI was added to the solid electrolyte to increase the ionic conductivity. KI initiates the iodide/iodine redox processes, improving the ionic conductivity and redox reaction of the asymmetric supercapacitor.

Fig. 8(a) presents the CV curves of CG-ASC with various scan rates from 10 to 200 mV s−1 in PVA–KOH–KI solid electrolyte. CG-ASC has complex redox peaks. The shape of the curves is not ideal rectangular, indicating that CG-ASC has pseudocapacitive property. Fig. 8(b) shows the galvanostatic CD curves of CG-ASC at diverse current densities form 0.05 mA cm−2 to 0.31 mA cm−2 in PVA–KOH–KI solid electrolyte. The calculated areal capacitance as compared to various current densities is shown in Fig. 8(c). CG-ASC has a maximum capacitance of 2.28 mF cm−2 at 0.05 mA cm−2. The overall capacitance in the solid electrolyte is lower than that of the aqueous electrolyte. This phenomenon originates from a lower ionic conductivity in the solid electrolyte. Although the current solid electrolyte works, performance improvements are needed to further decrease the gap with the liquid electrolyte. Also, the stability of CG-ASC cells tested by galvanostatic charge–discharge curve for 10[thin space (1/6-em)]000 cycles at 0.49 mA cm−2. Fig. 8(d) show, the areal capacitance values from 1st to 10[thin space (1/6-em)]000th cycles. The areal capacitance is decreased by ∼11% up to 10[thin space (1/6-em)]000 cycles. Hence, the stability of electrode retained to 89% after 10[thin space (1/6-em)]000th cycles. The initial drastic and further small decrement (total ∼11%) in the interfacial capacitance values at 10[thin space (1/6-em)]000 cycles of CG-ASC cells was observed due to the initial loss of active material.


image file: c6ra06034j-f8.tif
Fig. 8 Electrochemical measurements of asymmetric supercapacitors in PVA–KOH–KI solid electrolyte: (a) CV curves of a Co(OH)2//erGO asymmetric supercapacitor within an optimized respective potential window of 0 to 1.4 V in aqueous PVA–KOH–KI solid electrolyte at scan rates of 10–200 mV s−1, (b) galvanostatic CD curve of a Co(OH)2//erGO asymmetric supercapacitor at constant current densities from 0.049 to 0.306 mA cm−2, (c) change in specific capacitance against the current density of asymmetric supercapacitor, (d) and cycling stability of Co(OH)2//erGO asymmetric supercapacitor at a current density of 0.49 mA cm−2.
3.5.2 Applied measurements and overall performance. The series and parallel circuits of multiple samples were prepared to ensure viability in various environments. Fig. 9(a and b) shows the CV and galvanostatic CD results of parallel circuits for one to three CG-ASC samples. The current density of the CV curve was increased as compared to the number of samples shown in Fig. 9(a). The discharge time of the CD curves was also increased in proportion to the number of samples, indicating an increase in capacitance. The three parallel sample circuits exhibit a capacitance approximately three times higher than that of a single sample. Fig. 9(c) presents the CV results of the series circuits. The working voltage of the circuits can be increased by connecting each micro-SC samples in series. The three series sample circuit can endure up to 4.2 V, which is three times larger than that of a single sample (1.4 V). The current density of the series circuits decreased according to the number of samples due to increased resistivity. Fig. 9(d) shows the galvanostatic CD curves of the series circuits. The bending test of CG-ASC confirms the flexibility and mechanical stability of micro-SCs. The convex curvature is a result of various bending radii from 71.5 to 12.5 mm. Regardless of bending radius, the CV results are stable with uniform curve shapes. This measurement confirms the reliability of our micro-SC under mechanical bending and demonstrates its capability for flexible devices.
image file: c6ra06034j-f9.tif
Fig. 9 Circuit test of micro-SC arrays: (a) CV curve of micro-SC arrays in series at a scan rate of 50 mV s−1, (b) galvanostatic CD curves of micro-SC arrays in series, (c) CV results in parallel at 50 mV s−1, and (d) galvanostatic CD curves in parallel.

To evaluate the overall performance of our devices, the areal energy density and power density were calculated as follows:

 
image file: c6ra06034j-t2.tif(7)
 
image file: c6ra06034j-t3.tif(8)
where E is energy density, P is power density, and C is the areal capacitance per electrode area (μF cm−2), V is the potential window and t is the discharging time (s). The CG-ASC in 1 M KOH presents enhanced power density (572.83 μW cm−2), while the excellent energy density (11.85 μW h cm−2) is maintained. The increase in power density originates from the EDLC capacitive properties of erGO. The erGO stores charges via non-faradaic fast electron adsorption and desorption at the surface. The capacity of erGO (43.55 mF cm−2) is lower than that of Co(OH)2 (137.71 mF cm−2) because a typical pseudocapacitive material can store more charge than an EDLC devices based on faradaic reactions. However, when the amount of energy storage is viewed in terms of energy density, is proportional to capacity and the square of potential window. Thereby CG-ASC presents a high energy density due to the wide potential window. The solid-state CG-ASC has a lower energy density (0.35 μW h cm−2) and power density (72.00 μW cm−2) due to the lower ionic conductivity of solid electrolyte.

Fig. 10(b) shows a Ragone plot of areal energy density versus power density to compare our device performance with previously-reported micro-SC.44–52 The performance of our devices is competitive, even with our simple fabrication method. CG-ASC present high energy density with relatively high power density when compared with other literature. The detailed specifications of previously-reported micro-SC are summarized in Table S1†.53–58


image file: c6ra06034j-f10.tif
Fig. 10 (a) CV curves with various bending radii, and (b) a typical Ragone plot of energy density and power density.

4. Conclusions

In summary, an in-planar, micro-SC based on cobalt hydroxide and electrochemically-reduced graphene oxide was prepared via a simple and cost-effective fabrication method. Co(OH)2//erGO was successfully constructed on an Au electrode configuration via photolithography and electrodeposition. This fabrication step can achieve high accuracy and consistency and is both straightforward and inexpensive. The double-layer, interdigitated, in-plane electrode structure is a promising way forward for micro-scale SC. The metal current collector at the bottom of the electrode effectively supplies electrons to the active material due to its excellent conductivity. The reaction between the active material and electrolyte is facilitated by the high surface area of the electrode. This structure maximizes the areal and volumetric capacitance of micro-SC. Despite the simple fabrication method, our device produces good electrochemical results: the Co(OH)2/erGO ASC in KOH shows high energy density (11.85 μW h cm−2) with a good power density (572.83 μW cm−2). The all-solid-state Co(OH)2//erGO ASC was produced with a moderate energy density (0.35 μW h cm−2) and power density (100.38 μW cm−2) for application in practical devices. There is a performance gap between aqueous and solid electrolytes due to the lower conductivity of solid electrolytes; further study is needed to reduce this gap. Finally, a micro-scale energy storage device was successfully prepared using a simple process; this device shows excellent potential for use in many applications such as MEMS devices.

Acknowledgements

This work was partially supported by the Priority Research Centers Program (2009-0093823), the Basic Science Research Program (grant number 2013063062), the Korean Government (MSIP) (No. 2015R1A5A1037668) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST), and the Korea Research Fellowship Program funded by the Ministry of Science, ICT and Future Planning through the National Research Foundation of Korea (2015-11-1063).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06034j
‡ These authors contributed equally.
This journal is © The Royal Society of Chemistry 2016