Three-Dimensional Microcavity Array Electrodes ... - ACS Publications

Subscriber access provided by UOW Library

Article

Three-Dimensional Microcavity Array Electrodes for HighCapacitance All-Solid-State Flexible Micro-Supercapacitors Jimin Maeng, Young-Joon Kim, Chuizhou Meng, and Pedro P. Irazoqui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03559 • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Three-Dimensional Microcavity Array Electrodes for High-Capacitance All-Solid-State Flexible Micro-Supercapacitors Jimin Maeng,*,† Young-Joon Kim,‡ Chuizhou Meng,ǁ and Pedro P. Irazoqui§ †

Department of Bioengineering, University of Texas at Dallas, Richardson, Texas 75080,

United States ‡

School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana

47907, United States ǁ

GlobalFoundries Inc., East Fishkill, New York 12533, United States

§

Weldon School of Biomedical Engineering, Purdue University, West Lafayette, Indiana

47907, United States *Corresponding Author E-mail: [email protected] (Jimin Maeng)

ABSTRACT: We report novel three-dimensional (3D) microcavity array electrodes for high-capacitance all-solid-state micro-supercapactiors. The microcavity arrays are formed in a polymer substrate via a plasma-assisted reactive ion etching (RIE) process and provide extra 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sidewall surface areas on which the active materials are grown in the form of nanofibers. This 3D structure leads to an increase in the areal capacitance by a factor of 2.56 for a 15-µm-deep cavity etching, agreeing well with the prediction. The fabricated micro-supercapactiors exhibit a maximum areal capacitance of 65.1 mF cm-2 (a volumetric capacitance of 93.0 F cm-3) and an energy density of 0.011 mWh cm-2 (a volumetric energy density of 16.4 mWh cm-3) which substantially surpass previously reported values for all-solid-state flexible microsupercapacitors. The devices show good electrochemical stability under extended voltammetry cycles and bending cycles. It is demonstrated that they can sustain a radiofrequency (RF) microsystem in a temporary absence of a power supply. These results suggest the potential utility of our 3D micro-supercapactiors as miniaturized power sources in wearable and implantable medical devices.

KEYWORDS: Three-dimensional electrode, microcavity array, micro-supercapacitor, all-solid-state, flexible

1. INTRODUCTION Continuous miniaturization of portable consumer electronics and growing interest in wireless biomedical microdevices raise the demand for power sources that are small, lightweight, and can be integrated with other electronic components. Up until today, batteries have been the primary power sources of choice due to their high energy densities. However, batteries generally suffer from drawbacks such as slow charge rate (several hours), limited life time (hundreds to thousands of charge/discharge cycles) and safety concerns associated with using toxic materials. For implantable medical devices, in particular, the battery replacement after

2 ACS Paragon Plus Environment

Page 2 of 23

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


years of usage may pose an economic burden as well as a potentially high risk from surgery to the patient. In recent years, micro-supercapacitors have drawn special attention as a new class of microscale power sources that can complement or potentially replace batteries and electrolytic capacitors.1-7 This is due mainly to their high power density, fast charge-discharge rate, long cycle life as against batteries, and high energy density as against electrolytic capacitors. Recent studies in the micro-supercapacitor field have focused on improving the power/energy densities or rate capabilities through exploring a wide variety of new electrode materials and their composite/hybrids.8-15 Another promising approach to enhance the micro-supercapacitor performance is employing new device architectures.16-25 In particular, three-dimensional (3D) electrodes utilizing 3D frame structures such as trenches,21 pillars,22-23 channels,24 and walls25 have been proved effective to increase the areal capacitance (i.e. capacitance per footprint) by creating additional sidewall surface areas on which active materials can be deposited. However, the 3D electrodes in these works are formed on a rigid host substrate (i.e. silicon) and such devices are not well suited for emerging flexible applications such as wearable and implantable wireless sensors.26 All-solid-state flexible micro-supercapacitors are a class of energy storage devices that are compatible for flexible applications.3, 5, 6, 9, 27-34 They use polymer materials both for a substrate and an electrolyte such that the fabricated devices are compact and flexible. However, studies on 3D electrodes are relatively scarce for all-solidstate flexible micro-supercapacitors due mainly to lack of compatible microfabrication techniques for creating 3D frame structures on polymer materials. This is in contrast to the well-established 3D micromachining techniques on silicon.35 Here, we report a novel use of 3D microcavity array electrodes for all-solid-state flexible micro-supercapacitors as a new means to achieve a high areal capacitance. The microcavity arrays are formed in a polymer substrate via an RIE process and create extra sidewall surface areas on which the active materials are grown in the form of nanofibers. This leads to an 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

increase in the areal capacitance by a factor of 2.56 for a 15-µm-deep cavity etching, agreeing well with the prediction. The fabricated micro-supercapacitors exhibit a high areal capacitance of 65.1 mF cm-2 (a volumetric capacitance of 93.0 F cm-3) and an energy density of 0.011 mWh cm-2 (a volumetric energy density of 16.4 mWh cm-3). These values are substantially higher than those previously reported for all-solid-state flexible micro-supercapacitors. The devices show good electrochemical stability under extended voltammetry cycles and bending cycles. It is demonstrated that they can sustain an RF microsystem in a temporary absence of a power supply, showcasing their potential for a miniaturized power source in wireless wearable and implantable microsystems. The design, fabrication, characterization, and application demonstration are detailed herein.

2. EXPERIMENTAL 2.1. Device Architecture and Materials Our 3D micro-supercapacitors are implemented on a flexible polymer substrate (parylene-C) and composed of a current collector layer, conducting-polymer nanofiber electrodes (polyaniline, PANI), and a polymer electrolyte (polyvinylalcohol, PVA/H2SO4) (Figure 1).

Figure 1. Architecture of proposed all-solid-state flexible micro-supercapacitors with 3D microcavity array electrodes.


Page 4 of 23

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The nanofiber electrodes are grown on the surfaces of the vertically etched microcavity arrays and encapsulated by the electrolyte, resulting in high-density areal capacitances. We use parylene-C as a base substrate due primarily to its flexibility and biocompatibility.36 Besides, parylene-C is transparent, chemically inert, and compatible with the standard thinfilm microfabrication processes. These properties enable parylene-C as a platform material for a wide variety of wearable and implantable applications.37-42 PANI is used for the active electrode material because of its high theoretical specific capacitance, favorable electrochemical characteristics, and high degree of process compatibility to polymer substrates.3, 6, 12, 43 PVA/H2SO4 gel is used for electrolyte as it provides compact encapsulation as well as good wettability to PANI nanofibers. The total thickness of the devices including the entire components (i.e. substrate, current collector, electrode, and electrolyte) does not exceed 40 µm. The all-polymer and low-profile features make these devices ideal candidates for ultrathin and flexible energy storage applications. 2.2. Microcavity Array Design The key strategy for improving the areal capacitance of our micro-supercapacitors is to increase the electrode surface area by creating 3D microcavity arrays. In such an architecture, the “capacitance gain” (i.e. ratio of 3D capacitances to 2D capacitances) is determined by the geometry of these arrays. Therefore, careful design of the microcavity array is essential for achieving the maximum capacitance gain. To maximize the electrode surface area, we maximize the total number of the cavities by minimizing the diameter of each cavity and the gap between them. Here, the diameter of each cavity is designed as 15 µm to allow enough volume to fully accommodate the laterally growing PANI nanofibers inside the cavity. The gap between each cavity is designed as 5 µm due to the lithographical limit. Consequently, we deploy a total of 5616 cavities within a 2 mm × 2 mm interdigital electrode footprint. The height (i.e. etch depth) of the cavity is another important parameter that governs the 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

capacitance gain. While a deeper cavity is favorable for achieving a higher capacitance gain, it also entails unfavorable outcomes including an increased substrate thickness, a prolonged etching time, and an increased manufacturing cost. On balance, we design the cavity height to be 15 µm. One important advantage of this design model is that it provides nearly accurate prediction on the capacitance gain as a function of the cavity height (Figure S1a and S1b) based on a geometrical calculation (Equation S11-S18). For example, the capacitance gain for 15 µm-deep cavity arrays with a 15-µm diameter and a 5-µm gap is predicted to be 2.57. 2.3. Fabrication The fabrication of our 3D all-solid-state flexible micro-supercapacitors is based on the standard cleanroom microfabrication processes (i.e. thin-film deposition, photolithography, wet/dry etching), in combination with electrochemical polymerization of active electrode materials and polymer electrolyte coating. Figure 2 (Figure 2a-2f: bird’s-eye view; Figure 2a'-2f': cross-sectional view) illustrates the simplified fabrication sequence.


Page 6 of 23

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Simplified fabrication process of 3D micro-supercapacitors in bird’s-eye view (a-f) and cross-sectional view (a'-f'). (a/a') Deposition of parylene-C film on silicon carrier wafer. (b/b') Formation of microcavity array via reactive ion etching (RIE). (c/c') Formation of current collector (Ti/Au). (d/d') Polymerization of PANI nanofiber electrodes. (e/e') Film release from carrier wafer. (f/f') PVA/H2SO4 gel electrolyte coating. First, a 20 µm-thick parylene-C film is deposited on a silicon carrier wafer (Figure 2a and 2a'). The parylene film is patterned and selectively etched through a plasma-assisted RIE process to create 3D interdigital electrodes with embedded microcavity arrays (Figure 2b and 2b'). Then, a current collector layer is formed by thin-film (0.5 µm) Ti/Au deposition and patterning (Figure and 2c'). PANI nanofiber electrodes are electrochemically polymerized on the pre-defined area on the current collector layer (Figure 2d and 2d'). Parylene film is released from the carrier wafer to achieve a freestanding thin-film device (Figure 2e and 2e'). Finally, PVA/H2SO4 gel electrolyte is coated (Figure 2f and 2f'). More fabrication details are described in Supporting Information. Importantly, this fabrication method allows wafer-scale 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

implementation of multiple micro-supercapacitors on a single platform with possible series or parallel device connections, enabling fulfillment of various power demands. It is expected that our approach using RIE-assisted 3D microcavity array electrodes can also be applied to other commonly used flexible polymer substrates such as liquid crystal polymer (LCP),6 polyimide,44 polyethylene terephthalate (PET),5,34 and polyethylene naphthalate (PEN).45

3. RESULTS AND DISCUSSION Figure 3a shows an image of the wafer-scale fabricated 2 mm × 2 mm sized all-solid-state micro-supercapacitors on a single, freestanding parylene film. The interdigital electrodes of these micro-supercapacitors possess thousands of embedded 3D microscale cavities. Figure 3b shows a magnified scanning electron microscopy (SEM) image featuring the microcavities. The PANI nanofiber electrodes grow nearly uniformly on the cavity walls as well as on the top surfaces (Figure 3c-3e). This allows reliable estimation of the areal capacitances based on our design model.


Page 8 of 23

Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Fabricated devices. (a) Photo image of fabricated 3D all-solid-state flexible microsupercapacitors on freestanding parylene film. (b)-(d) SEM images of microcavity array electrodes: (b) before PANI nanofiber deposition and (c)-(d) after PANI nanofiber deposition for 25 min (88 cycles) and 35 min (124 cycles), respectively. (e) Magnified SEM image of PANI nanofibers.

To evaluate the electrochemical performance of the 3D all-solid-state micro-supercapacitors, a series of characterization including cyclic voltammetry (CV), galvanostatic charge-discharge, and electrochemical impedance spectroscopy (EIS) are carried out. 2D reference devices (i.e. no microcavity) are also fabricated and characterized under the same conditions for the sake of comparison. Here, PANI nanofiber electrodes were electrochemically deposited through 200 CV cycles at a scan rate of 100 mV s-1 (Figure S2). The three pairs of redox peaks in the CV curve during the deposition clearly indicate the presence of pseudo-capacitive PANI, agreeing well with other reports.6 Figure 4a shows the CV responses of the all-solid-state micro-supercapacitors at a scan rate of 20 mV s-1. The CV curves exhibit a nearly rectangular shape, indicating a good capacitive behavior. The area under the CV curve of the 3D device is larger than that of the 2D device, suggesting a higher capacitance. Figure 4b shows the 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

galvanostatic charge-discharge responses at different current loads. The resulted triangular curves are in a nearly symmetric and linear shape. The cycle time of 1-8 min obtained herein illustrates the fast-charging characteristic of supercapacitors.

Figure 4. Electrochemical characterization of 3D micro-supercapacitors. For comparison, data for 2D reference devices (i.e. no microcavity) are also shown. (a) CV responses at 20 mV s-1. (b) Galvanostatic charge-discharge responses. (c) Areal capacitances extracted from CV responses at various scan rates. (d) Ragone plots. (e) Nyquist plots from 0.1 Hz to 40 kHz. 10 ACS Paragon Plus Environment

Page 10 of 23

Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The inset shows a zoom-in view of the high-frequency region above 1 kHz. (f) Capacitance retention for 1000 CV cycles at 100 mV s-1.

Capacitances are extracted based on the CV responses at various scan rates from 5 to 100 mV s-1 (Figure 4c). The 3D micro-supercapacitors exhibit the highest areal capacitance of 65.1 mF cm-2 (corresponding to a volumetric capacitance of 90.3 F cm-3) at 5 mV s-1, which is higher than other recently reported all-solid-state supercapacitors (2.5-45 mF cm-2) by a factor of 1.45 to 25.31-33 Importantly, the maximum areal capacitance achieved from the 3D devices is higher than that from the 2D devices (25.4 mF cm-2) by a factor of 2.56. This result agrees almost perfectly with the predicted capacitance gain of 2.57, validating our design model. It is shown that the areal capacitances drop as the scan rate increases from 5 to 100 mV s-1. This is attributed to uncompleted redox reaction and charge diffusion over a short period of scanning time at fast scan rates. The relatively lower capacitance drop in the 2D devices (30.7 %, as against to 47.9 % in the 3D devices) is believed to be due to the shorter total charge diffusion path in the 2D electrodes. Energy density and power density are key factors that provide information about operational range of supercapacitors. The Ragone plots in Figure 4d present the evaluated energy densities and power densities of our devices. It is clearly seen that the overall energy and power performance of the 3D devices is superior to that of the 2D devices. This is directly attributed to the higher areal capacitance of the 3D devices resulted from the microcavity array structure. The maximum energy density achieved for the 3D devices is 0.011 mWh cm-2, corresponding to a volumetric energy density of 16.4 mWh cm-3. This value is higher than those reported for most recent all-solid-state micro-supercapacitors (0.1-1.8 mWh cm-3) by an order of magnitude or two.5, 28-34 The 3D devices exhibit a maximum power density of 2.68 mW cm-2, corresponding to a volumetric power density of 3.83 W cm-3. This is one of the highest volumetric power density values among the comparable all-solid-state microsupercapacitors.5, 28-34 Such aspects of superior volumetric energy and power densities are 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

well illustrated in the Ragone plot (Figure S4) where our result locates in the upper right corner. The comparison of our devices with state-of-the-art all-solid-state flexible microsupercapacitors is summarized in Table S1. The impedance characteristics of the fabricated micro-supercapacitors are investigated by EIS analysis. The Nyquist plots in Figure 4e show a close similarity in impedance behavior between the 3D and the 2D device. The vertical slopes at the low-frequency region in both devices indicate a good capacitor behavior. The equivalent series resistances (ESR) obtained from the x-intercept of the plots are around 268 Ω and 273 Ω, or equivalently, 6.02 Ω cm2 and 6.15 Ω cm2, for the 3D and the 2D device, respectively (inset of Figure 4e). These values are reasonable compared to the previously reported conducting polymer or metal oxide based micro-supercapacitors.3, 5-6 Cycle stability of the supercapacitors is critical for their long-term use in practical applications. As a measure of cycle stability, we evaluate capacitance retention of our devices through 1000 CV cycling at 100 mV s-1 (Figure 4f). It is shown that the capacitance keeps increasing up to 109.1 % of its initial value during the initial 55 cycles, which is ascribed to electrochemical activation of the electrodes. The increase of capacitance at early cycling stage is commonly observed in other conducting polymer or metal oxide electrodes.19, 46 The capacitance decreases back to its initial value during the next 55 cycles, and then gradually decreases toward the last cycling test. The final capacitance after 1000 cycles is 85.7 % of its initial value, demonstrating good retention of capacitance. It is possible to achieve an even higher retention rate by reducing the thickness of PANI nanofiber electrodes.6, 47 After 1000 cycles, the CV profile still retains a roughly rectangular shape without significant distortion (Figure S3a). The impedance spectrum remains similar with an increase of ESR by 14.4 % (Figure S3b).


Page 12 of 23

Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Bending stability of 3D micro-supercapacitors. (a) A test device is attached on a Kapton tape (inset) and manually bent 1000 times with an average bending radius of ≈1 mm (bending angle of ≈145 °). (b) Changes in capacitance and resistance values as a function of bending frequency. (c) CV curves at 20 mV s-1 and (d) Galvanostatic discharge curves at 10 µA for multiple device bending of 0, 50, 100, 200, 300, 500 and 1000 times.

In practical wearable or implantable applications, the devices might undergo repeated bending due to the curvature at the implant site and the continuous motion of body. To prove feasibility for such applications, we characterize the bending stability of our 3D all-solid-state micro-supercapacitors. A 2 mm × 2 mm device under test is attached on a Kapton tape (Figure 5a) and manually bent 1000 times at an average frequency of 1 bending/s with the average bending radius of ≈1 mm (bending angle of ≈145 °). The changes in capacitance and ESR are 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

recorded at various bending cycles (Figure 5b); the 1000 cycles of bending resulted in relatively small changes both in capacitance and ESR (13.4 % and 4.8 %, respectively). These results correspond to the mild distortion of the CV and the galvanostatic charge-discharge responses during these bending cycles (Figure 5c and 5d).

Figure 6. Powering demonstration of an RF transmitter using 3D all-solid-state microsupercapacitors. (a) Schematic of transmitter system coupled with 3D micro-supercapacitors. (b) Test setup. (c) CV curves for the four-in-series micro-supercapacitors used in Figure 6b, at 20 mV s-1. Comparisons are shown for series connection of different number of cells. (d) Transmitter output spectrum measured at on-state. (e)-(f) Transient measurements with abrupt 14 ACS Paragon Plus Environment

Page 14 of 23

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


power elimination after fully charging micro-supercapacitors up to 3.2 V: (e) Supercapacitor voltage retention. (f) Transmitter output waveform.

Wireless powering of medical devices is key to their successful deployment in clinical applications in that it allows circumvention of problems caused by wires, being the primary source of infection, failure, manufacturing cost, and discomfort to the patient. Also, wireless powering can potentially replace the use of battery which often determines the size of the devices and limits their life time. The main advantage of integrating micro-supercapacitors in a wirelessly powered device is to sustain the device operation even when there is an intermittent disturbance or absence in external power sources in a dynamic application environment. To demonstrate such a capability, our micro-supercapacitors are coupled to an in-house RF transmitter system48 and wirelessly powered. Figure 6a shows the schematic of this oscillator-less transmitter system with supercapacitors. In this demonstration, antennas have been used to transmit and receive high power wireless signals. Once the signal is received from the antenna, a rectifier circuit converts the incoming RF signal to a DC voltage. Then a voltage regulator conditions the DC voltage to a fixed, stable power source. The transmitter requires two voltage sources for operation: 0.5 V for the power amplifier and 1.4 V for the other blocks. Here, a low dropout linear regulator (LDO, ADP 171, Analog Devices) regulates the voltage from the rectifier to a constant level. The current consumption and the power consumption of the power amplifier (PA) in this scheme are measured to be 0.22 mA and 0.27 mW, respectively. Figure 6b shows the experimental setup. To handle a sufficiently high voltage up to 3.2 V, we fabricate a micro-supercapacitor bank with four series-connected cells. It is then coupled to the transmitter output and fully charged up to 3.2 V. Here, the average capacitance of each supercapacitor is measured to be 400 µF at 100 mV s-1, resulting in a net capacitance of 100 µF for the capacitor bank. The CV responses of these supercapacitors are exhibited in Figure 6c. As shown, a variety of power demands can be met by connecting different number of supercapacitor cells in series. Figure 6d is the spectrum of 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the transmitter output recorded at on-state (i.e. when RF-powered by an external source), showing a peak transmit signal at 457.7 MHz. When the external power source is eliminated abruptly, the micro-supercapacitors come into effect and sustain the transmitter for extra 238.8 ms during which a voltage drop of 2 Volts occurred (Figure 6e and 6f). This duration is 26.3 % of the theoretical value (909.1 ms) where an ideal (i.e. purely capacitive) 100 µF capacitor is assumed (Equation S10). The faster voltage drop than theory is due to the net ESR of ≈1.2 kΩ superimposed by the four series-connected supercapacitors. The periodic ripples shown after the 238.8 ms also indicate the effect of the ESR. This demonstration shows the capability of our micro-supercapacitors sustaining a power hungry system during a temporary absence of the external power source. If required, the sustain time can substantially be prolonged by employing supercapacitors with larger sizes (i.e. higher capacitances). For example, it is expected that 5 cm × 5 cm devices will sustain the same transmitter system for 2.5 min. These larger supercapacitors can theoretically sustain an ultra-low power (e.g. 19 µW) microsystem49 for longer than 33 min.

4. CONCLUSION In summary, we report a new type of high-capacitance all-solid-state flexible microsupercapacitors with novel 3D microcavity array electrodes. The 3D microcavity arrays are shown to improve the areal capacitance by a factor of 2.56 for a 15-µm-deep cavity etching, and consequently, very high specific capacitances (65.1 mF cm-2, 93.0 F cm-3) and energy densities (0.011 mWh cm-2, 16.4 mWh cm-3) are achieved. These values substantially surpass those previously reported for other all-solid-state micro-supercapacitors. An accurate design model for the 3D electrodes is provided and the experimental results agree well with the model. Our devices exhibit good electrochemical stability under extended voltammetry cycles and bending cycles, offering potential for use in chronic wearable and implantable energy 16 ACS Paragon Plus Environment

Page 16 of 23

Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


storage applications. The demonstration of our micro-supercapacitors powering an RF transmitter showcases their feasibility for applications in wirelessly-powered microsystems. The fabrication method suggested herein is also applicable to other commonly used polymer materials, and thus opens up a new avenue for fabricating a wide variety of flexible energy storage devices.

ASSOCIATED CONENT Supporting Information. Experimental details on the device fabrication/characterization and supplementary figures/data are provided in Supporting Information.

ACKNOWLEDGEMENTS This work was partially funded by the Defense Advanced Research Projects Agency (DARPA) MTO under the auspices of Dr. Jack Judy through the Space and Naval Warfare Systems Center, Pacific Grant/Contract No. N66001-11-1-4029. The authors acknowledge that the major part of the fabrication in this work was carried out in Birck Nanotechnology Center, Purdue University, West Lafayette, IN, USA.

REFERENCES [1]

Chmiola, J.; Largeot, C.; Taberna, P. L.; Simon, P.; Gogotsi, Y. Monolithic Carbide-

Derived Carbon Films for Micro-Supercapacitors. Science 2010, 328, 480-483. [2]

Pech, D.; Brunet, M.; Durou, H.; Huang, P. H.; Mochalin, V.; Gogotsi, Y.; Taberna, P.

L.; Simon, P. Ultrahigh-Power Micrometre-Sized Supercapacitors Based on Onion-Like Carbon. Nat. Nanotechnol. 2010, 5, 651-654.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[3]

Wang, K.; Zou, W.; Quan, B.; Yu, A.; Wu, H.; Jiang, P.; Wei, Z. An All-Solid-State

Flexible Micro-Supercapacitor on a Chip. Adv. Energy Mater. 2011, 1, 1068-1072. [4]

El-Kady, M. F.; Kaner, R. B. Scalable Fabrication of High-Power Graphene Micro-

Supercapacitors for Flexible and On-Chip Energy Storage. Nat. Commun. 2013, 4, 1475. [5]

Si, W.; Yan, C.; Chen, Y.; Oswald, S.; Han, L.; Schmidt, O. G. On Chip, All Solid-

State and Flexible Micro-Supercapacitors with High Performance Based on MnOx/Au Multilayers. Energy Environ. Sci. 2013, 6, 3218-3223. [6]

Meng, C.; Maeng, J.; John, S. W. M.; Irazoqui, P. P. Ultrasmall Integrated 3D Micro-

Supercapacitors Solve Energy Storage for Miniature Devices. Adv. Energy Mater. 2014, 4, 1301269. [7]

Dong, X.; Guo, Z.; Song, Y.; Hou, M.; Wang, J.; Wang, Y.; Xia, Y. Flexible and

Wire-Shaped Micro-Supercapacitor Based on Ni(OH)2-Nanowire and Ordered Mesoporous Carbon Electrodes. Adv. Funct. Mater. 2014, 24, 3405-3412. [8]

Beidaghi, M., Wang, C. Micro-Supercapacitors Based on Interdigital Electrodes of

Reduced Graphene Oxide and Carbon Nanotube Composites with Ultrahigh Power Handling Performance. Adv. Funct. Mater. 2012, 22, 4501-4510. [9]

Niu, Z.; Zhang, L.; Liu, L.; Zhu, B.; Dong, H.; Chen, X. All-Solid-State Flexible

Ultrathin Micro-Supercapacitors Based on Graphene. Adv. Mater. 2013, 25, 4035-4042. [10]

Liu, W.; Feng, Y.; Yan, X.; Chen, J.; Xue, Q. Superior Micro-Supercapacitors Based

on Graphene Quantum Dots. Adv. Funct. Mater. 2013, 23, 4111-4122. [11]

Meng, Y.; Wang, K.; Zhang, Y.; Wei, Z. Hierarchical Porous Graphene/Polyaniline

Composite Film with Superior Rate Performance for Flexible Supercapacitors. Adv. Mater. 2013, 25, 6985-6990. [12]

Xiong, G.; Meng, C.; Reifenberger, R. G.; Irazoqui, P. P.; Fisher, T. S. Graphitic Petal

Micro-Supercapacitor Electrodes for Ultra-High Power Density. Adv. Energy Mater. 2014, 4, 1300515. 18 ACS Paragon Plus Environment

Page 18 of 23

Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[13]

Xia, X.; Chao, D.; Fan, Z.; Guan, C.; Cao, X.; Zhang, H.; Fan, H. J. A New Type of

Porous Graphite Foams and Their Integrated Composites with Oxide/Polymer Core/Shell Nanowires for Supercapacitors: Structural Design, Fabrication, and Full Supercapacitor Demonstrations. Nano Lett. 2014, 14, 1651-1658. [14]

Wu, Z.; Parvez, K.; Winter, A.; Vieker, H.; Liu, X.; Han, S.; Turchanin, A.; Feng, X.;

Müllen, K. Layer-by-Layer Assembled Heteroatom-Doped Graphene Films with Ultrahigh Volumetric Capacitance and Rate Capability for Micro-Supercapacitors. Adv. Mater. 2014, 26, 4552-4558. [15]

Moon, Y. S.; Kim, D.; Lee, G.; Hong, S. Y.; Kim, K. K.; Park, S. M.; Ha, J. S.

Fabrication of Flexible Micro-Supercapacitor Array with Patterned Graphene Foam/MWNTCOOH/MnOx Electrodes and Its Application. Carbon 2015, 81, 29-37. [16]

Ren, J.; Li, L.; Chen, C.; Chen, X.; Cai, Z.; Qiu, L.; Wang, Y.; Zhu, X.; Peng, H.

Twisting Carbon Nanotube Fibers for Both Wire-Shaped Micro-Supercapacitor and MicroBattery. Adv. Mater. 2013, 25, 1155-1159. [17]

Su, Z.; Yang, C.; Xie, B.; Lin, Z.; Zhang, Z.; Liu, J.; Li, B.; Kang, F.; Wong, C. P.

Scalable Fabrication of MnO2 Nanostructure Deposited on Free-Standing Ni Nanocone Arrays for Ultrathin, Flexible, High-Performance Micro-Supercapacitor. Energy Environ. Sci. 2014, 7, 2652-2659. [18]

Zhao, X.; Zheng, B.; Huang, T.; Gao, C. Graphene-Based Single Fiber Supercapacitor

with a Coaxial Structure. Nanoscale 2015, 7, 9399-9404. [19]

Shao, M.; Li, Z.; Zhang, R.; Ning, F.; Wei, M.; Evans, D. G.; Duan, X. Hierarchical

Conducting Polymer@Clay Core–Shell Arrays for Flexible All-Solid-State Supercapacitor Devices. Small 2015, 11, 3530-3538. [20]

Niu, Z.; Zhou, W.; Chen, X.; Chen, J.; Xie, S. Highly Compressible and All-Solid-

State Supercapacitors Based on Nanostructured Composite Sponge. Adv. Mater. 2015, 27, 6002-6008. 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[21]

Sun, W.; Chen, X. Preparation and Characterization of Polypyrrole Films for Three-

Dimensional Micro Supercapacitor. J. Power Sources 2009, 193, 924-929. [22]

Wang, X.; Yin, Y.; HaO, C.; You, Z. A High-Performance Three-Dimensional Micro

Supercapacitor Based on Ripple-Like Ruthenium Oxide-Carbon Nanotube Composite Films. Carbon 2015, 82, 436-445. [23]

Beidaghi, M.; Wang, C. Micro-Supercapacitors Based on Three Dimensional

Interdigital Polypyrrole/C-MEMS Electrodes. Electrochim. Acta 2011, 56, 9508-9514. [24]

Shen, C.; Wang, X.; Zhang, W.; Kang, F. A High-Performance Three-Dimensional

Micro Supercapacitor Based on Self-Supporting Composite Materials. J. Power Sources 2011, 196, 10465-10471. [25]

Li, S.; Wang, X.; Xing, H.; Shen, C. Micro Supercapacitors Based on a 3D Structure

with Symmetric Graphene or Activated Carbon Electrodes. J. Micromech. Microeng. 2013, 23, 114013. [26]

Darwish, A.; Hassanien, A. E. Wearable and Implantable Wireless Sensor Network

Solutions for Healthcare Monitoring. Sensors 2011, 11, 5561-5595. [27]

Meng, F.; Ding, Y. Sub-Micrometer-Thick All-Solid-State Supercapacitors with High

Power and Energy Densities. Adv. Mater. 2011, 23, 4098-4102. [28]

Xiao, X.; Li, T.; Yang, P.; Gao, Y.; Jin, H.; Ni, W.; Zhan, W.; Zhang, X.; Cao, Y.;

Zhong, J.; Gong, L.; Yen, W.; Mai, W.; Chen, J.; Huo, K.; Chueh, Y.; Wang, Z. L.; Zhou, J. Fiber-Based All-Solid-State Flexible Supercapacitors for Self-Powered Systems. ACS Nano 2012, 6, 9200-9206. [29]

Xiao, X.; Peng, X.; Jin, H.; Li, T.; Zhang, C.; Gao, B.; Hu, B.; Huo, K.; Zhou, J.

Freestanding Mesoporous VN/CNT Hybrid Electrodes for Flexible All-Solid-State Supercapacitors. Adv. Mater. 2013, 25, 5091-5097.


Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[30]

Xiao, X.; Li, T.; Peng, Z.; Jin. H.; Zhong, Q.; Hu, Q.; Yao, B.; Luo, Q.; Zhang, C.;

Gong, L.; Chen, J.; Gogotsi, Y.; Zhou, J. Freestanding Functionalized Carbon NanotubeBased Electrode for Solid-State Asymmetric Supercapacitors. Nano Energy 2014, 6, 1-9. [31]

Wang, X.; Liu, B.; Liu, R.; Wang, Q.; Hou, X.; Chen, D.; Wang, R.; Shen, G. Fiber-

Based Flexible All-Solid-State Asymmetric Supercapacitors for Integrated Photodetecting System. Angew. Chem. 2014, 126, 1880-1884. [32]

Qian, Y.; Liu, R.; Wang, Q.; Xu, J.; Chen, D.; Shen, G. Efficient Synthesis of

Hierarchical NiO Nanosheets for High-Performance Flexible All-Solid-State Supercapacitors. J. Mater. Chem. A 2014, 2, 10917-10922. [33]

Yun, J.; Kim, D.; Lee, G.; Ha, J. S. All-Solid-State Flexible Micro-Supercapacitor

Arrays with Patterned Graphene/MWNT Electrodes. Carbon 2014, 79, 156-164. [34]

Lee, H.; Hong, S.; Kwon, J.; Suh, Y. D.; Lee, J.; Moon, H.; Yeo, J.; Ko, S. H. All-

Solid-State Flexible Supercapacitors by Fast Laser Annealing of Printed Metal Nanoparticle Layers. J. Mater. Chem. A 2015, 3, 8339-8345. [35]

Kovacs, G. T. A.; Maluf, N. I.; Petersen, K. E. Bulk Micromachining of Silicon. Proc.

IEEE 1998, 86, 1536-1551. [36]

Schmidt, E. M.; McIntosh, J. S.; Bak, M. J. Long-Term Implants of Parylene-C Coated

Microelectrodes. Med. Biol. Eng. Comput. 1988, 26, 96-101. [37]

Chen, P. J.; Rodger, D. C.; Saati, S.; Humayun, M. S.; Tai, Y. C. Microfabricated

Implantable Parylene-Based Wireless Passive Intraocular Pressure Sensors. J. Microelectromech. Syst. 2008, 17, 1342-1351. [38]

Maeng, J.; Kim, B.; Ha, D.; Chappell, W. J. Parylene Interposer as Thin Flexible 3-D

Packaging Enabler for Wireless Applications. IEEE Trans. Microwave Theory Tech. 2011, 59, 3410-3418.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[39]

Ha, D.; de Vries, W. N.; John, S. W. M.; Irazoqui, P. P.; Chappell, W. J. Polymer-

Based Miniature Flexible Capacitive Pressure Sensor for Intraocular Pressure (IOP) Monitoring Inside a Mouse Eye. Biomed. Microdevices 2012, 14, 207-215. [40]

Salvatore, G. A.; Münzenrieder, N.; Kinkeldei, T.; Petti, L.; Zysset, C.; Strebel, I.;

Büthe, L.; Tröster, G. Wafer-Scale Design of Lightweight and Transparent Electronics That Wraps Around Hairs. Nat. Commun. 2014, 5, 2982. [41]

Kim, Y. J.; Maeng, J.; Irazoqui, P. P. Eyeglasses-Powered, Contact Lens-Like

Platform with High Power Transfer Efficiency. Biomed. Microdevices 2015, 17, 1-9. [42]

Castagnola, V.; Descamps, E.; Lecestre, A.; Dahan, L.; Remaud, J.; Nowak, L. G.;

Bergaud, C. Parylene-Based Flexible Neural Probes with PEDOT Coated Surface for Brain Stimulation and Recording. Biosens. Bioelectron. 2015, 67, 450-457. [43]

Maeng, J.; Meng, C.; Irazoqui, P. P. Wafer-Scale Integrated Micro-Supercapacitors on

an Ultrathin and Highly Flexible Biomedical Platform. Biomed. Microdevices 2015, 17, 1-10. [44]

El-Kady, M. F.; Ihns, M.; Li, M.; Hwang, J. Y.; Mousavi, M. F.; Chaney, L.; Lech, A.

T.; Kaner, R. B. Engineering Three-Dimensional Hybrid Supercapacitors and Microsupercapacitors for High-Performance Integrated Energy Storage. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 4233-4238. [45]

Kurra, N.; Hota, M. K.; Alshareef, H. N. Conducting Polymer Micro-Supercapacitors

for Flexible Energy Storage and Ac Line-Filtering. Nano Energy 2015, 13, 500-508. [46]

Wu, J.; Ouyang, C.; Dou, S.; Wang, S. Hybrid NiS/CoO Mesoporous Nanosheet

Arrays on Ni Foam for High-Rate Supercapacitors. Nanotechnology 2015, 26, 325401. [47]

Sumboja, A.; Tefashe, U. M.; Wittstock, G.; Lee, P. S. Investigation of Charge

Transfer Kinetics of Polyaniline Supercapacitor Electrodes by Scanning Electrochemical Microscopy. Adv. Mater. Interfaces 2015, 2, 1400154.


Page 22 of 23

Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[48]

Kim, Y. J.; Bhamra, H. S.; Joseph, J.; Irazoqui, P. P. An Ultra-Low-Power RF Energy-

Harvesting Transceiver for Multiple-Node Sensor Application. IEEE Trans. Circuits Syst. II: Express Briefs 2015, 62, 1028-1032. [49]

Zhang, Y.; Zhang, F.; Shakhsheer, Y.; Silver, J. D.; Klinefelter, A.; Nagaraju, M.;

Boley, J.; Pandey, J.; Shrivastava, A.; Carlson, E. J.; Wood, A.; Calhoun, B. H.; Otis, B. P. A Batteryless 19 µW MICS/ISM-Band Energy Harvesting Body Sensor Node SoC for ExG Applications. IEEE J. Solid-State Circuits 2013, 48, 199-213.

Table of Contents (TOC) Graphic


Three-Dimensional Microcavity Array Electrodes ... - ACS Publications

Recommend Documents