Electrokinetic Supercapacitor for Simultaneous Harvesting and

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Electrokinetic Supercapacitor for Simultaneous Harvesting and Storage of Mechanical Energy Peihua Yang, Xiaopeng Qu, Kang Liu, Jiangjiang Duan, Jia Li, Qian Chen, Guobin Xue, Wenke Xie, Zhimou Xu, and Jun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18640 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Materials & Interfaces

Electrokinetic Supercapacitor for Simultaneous Harvesting and Storage of Mechanical Energy Peihua Yang, Xiaopeng Qu, Kang Liu, Jiangjiang Duan, Jia Li, Qian Chen, Guobin Xue, Wenke Xie, Zhimou Xu, and Jun Zhou* Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, Hubei, China KEYWORDS: supercapacitor, electrokinetic effect, energy harvesting, self-powered, mechanical energy

ABSTRACT: Energy harvesting and storage are two distinct processes that are generally achieved using two separated parts based on different physical and chemical principles. Here we report a self-charging electrokinetic supercapacitor that directly couples the energy harvesting and storage processes into one device. The device consists of two identical carbon nanotube/titanium electrodes, separated by a piece of anodic aluminum oxide nanochannels membrane. Pressure-driven electrolyte flow through the nanochannels generates streaming potential, which can be used to charge the capacitive electrodes, accomplishing simultaneous energy generation and storage. The device stores electric charge density of 0.4 mC cm-2 after fully charging under pressure of 2.5 bar. This work may offer a train of thought for the development of new type energy unit for self-powered systems.

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1. INTRODUCTION With the consideration of developing renewable energy solutions to cast aside combustion of fossil fuels, harvesting energy as electrical energy is of great importance and motivates the research of advanced technologies.1-6 During last decade, enormous efforts have been devoted to exploit a variety of energy harvesters, such as solar cells,7-8 thermoelectric devices9-11 and nanogenerators.12-16 Although the energy harvesters are ever-growing prosperous, the energy storage elements are usually separately treated and virtually neglected taken into account with energy harvesters, which are significantly important to the entire energy management system. Generally, batteries and supercapacitors take in charge of energy storage function. Recently, mechanical-to-electrochemical process has been proposed by integrating piezoelectric and triboelectric materials in battery systems.17-19 Compare with batteries, capacitive energy storage devices have advantages that can operate at high charge/discharge rates over almost unlimited cycles, receiving increasing attentions.20 Herein, we develop a new mechanical-to-electrical energy conversion device, self-charging electrokinetic supercapacitor, for mechanical energy simultaneous harvesting and storage, by using carbon nanotubes (CNT) coated titanium (Ti) mesh as capacitive electrodes and anodic aluminum oxide (AAO) nanochannels as separator membrane. Energy generation is achieved through electrokinetic effect in AAO nanochannels under pressure-driven flow, and the generated energy can be directly stored in the capacitive CNT/Ti electrodes. The power cell shows the ability to generate and store charge density of 0.4 mC cm-2 under pressure about 2.5 bar. The device is easy to scale up, as well as operated in microelectronic and micro/nanofluidic fields, showing the promising ability to utilize a broad range of mechanical vibrations and forces. 2. EXPERIMENTAL SECTION


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2.1. CNT/Ti electrode preparation.2, 21 CNT ink was firstly prepared. Briefly, 3 g commercial CNTs were dispersed in 120 mL mixed H2SO4 and HNO3 (H2SO4/HNO3 = 3/1, V/V) solution by sonication for 10 minutes. Then the CNT-acid mixture was refluxed while stirring vigorously for 2 hours in 90 oC oil bath, and cooled at room temperature after the refluxing process. The CNTs extracted from the residual acids were purified by repeating the processes of diluting with deionized water, centrifuging and decanting the solutions. When the liquid supernatant was neutral, the treated CNT was re-dispersed in deionized water to form CNT ink. For the electrode design, commercial titanium mesh was dipped in the CNT ink, and then heated at 160 oC to reinforce their association. The finally CNT loading on the titanium mesh is about 0.2 mg cm-2. 2.2. AAO preparation.22 The AAO nanochannels array is fabricated through two-step anodization process. In brief, electrochemical polishing are taken to a designed 0.2-mm-thick aluminum foil (99.999%) before the anodization process, which was taken under a temperature of 5 °C and voltage of 18 V in the mixture solution of ethyl alcohol and perchloric acid with the volume ratio of 4:1. The first anodization is carried out in oxalic acid solution for 4 h at the temperature of 5 °C and voltage of 40 V, followed by the 6.1 wt% H3PO4 + 1.5 wt% CrO3 mixed solution to remove the oxide layer for 8 h. The second anodization lasts for 30 min with the same anodizing parameters, and the pore-widening process is carried out for 18 min in 6 wt% H3PO4 solution at room temperature. The length of the AAO membrane can be controlled by the second anodization time. 2.3. Electrokinetic supercapacitor assembly. The device structure and components is shown in Figure 1a. Two pieces of CNT/Ti electrode were cutting into required shape. The device was constructed using two partially CNT electrodes sandwiching an AAO membrane. The joint of


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each component was sealed by epoxy resin (A/B = 1/1, V/V). Eventually, the area for electrolyte flow was controlled in a circle with diameter of 5 mm (effective area about 0.2 cm2). 2.4. Characterizations. The morphology and structure of the materials were characterized by scanning electron microscopy (SEM, FEI Nova Nano450). The electrical and electrochemical measurements were conducted with ECLab (VMP-300) and CHI660E electrochemical workstations. The electrochemical impedance was measured from 50 mHz to 5 MHz with a potential amplitude of 10 mV. In the charging and discharging process of the capacitive device under pressure condition, an external load resistance was connected between the electrodes. 2.5. Calculations. Since the CV curves of the capacitive device are nearly rectangular, the areal capacitance can be calculated as:23 =

∮

(1)

where v is the potential scan rate, S is the area, and ∆U is the potential window. The energy stored in the device is evaluated by: =

(2)

where Q is the stored charges and C is the capacitance. The energy stored in the capacitive device is converted from electrolyte flow, the efficiency of the mechanical-to-electrical energy conversion process is calculated by: =

(3)

where P is the working pressure and V is the electrolyte volume flowing through the AAO membrane during the charging process. The efficiency of the electrokinetic battery using Ag/AgCl electrodes is calculated by:

=

(4)


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where P is the working pressure and vf is the electrolyte flow rate during the electricity generation, I0 and U0 is the short-circuit current and open-circuit voltage, respectively, which is recorded from the current-voltage curves. 3. RESULTS AND DISCUSSION Figure 1a illustrates the schematic structure of the self-charging capacitive device which consists with two CNT/Ti electrodes and an AAO membrane. The pressure-driven electrolyte flow through nanochannels has been extensively studied, which could produce streaming potential/current under pressure gradient, providing effective means for converting mechanical energy into electricity.24-27 A drawback of the traditional electrokinetic batteries is that a continuous pressure must be kept across the nanochannels, and redox reaction must take place on the electrode in order to generate continuous electrical current. If capacitive materials are equipped to the electrodes, we can use the streaming potential to charge the electrodes, thus store the electricity simultaneously, and the device can be named as electrokinetic supercapacitor. As shown in Figure 1c, the CNT was uniformly coated on Ti mesh, and the sparse Ti wires scaffold almost has no influence to the pressure-driven electrolyte flow. The AAO membrane containing nanochannel is a key component for streaming potential/current generation, as well as the separator for capacitive device. The AAO membrane used in this study is fabricated by twostep anodization process, which is nearly transparent (Figure 1b), with a thickness of about 8 µm and average pore diameter of ~70 nm (Figure 1d and S1). In the capacitive device, the AAO membrane is sandwiched between two CNT/Ti mesh electrodes which can act as scaffolds, and the stressed area is uniform and small (0.2 cm2), thus the device can work well under pressure driven electrolyte flow.


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The energy storage performance of device was firstly studied. From cell configuration for energy storage, the device demonstrates substantially supercapacitor-like behavior. Cyclic voltammetry (CV) curves of the device in various NaCl solutions are presented in Figure 2a at a scan rate of 5 mV s-1. The capacitances are calculated in Figure 2b, and the parabola-like curve may be determined by the Debye length of electrical double layer, which is dependent on electrode potential and electrolyte concentration.28 As dilute electrolyte benefits streaming potential (Figure S2), we chose 10-7 mol L-1 NaCl solution as working electrolyte for the following experiments. Figure 2c and 2d records the nearly rectangular shaped CV curves at different scan rate and symmetric charge/discharge curves at various current densities, respectively, suggesting the excellent capacitive behavior of the device. At a scan rate of 2 mV s1

, the capacitance of the device is about 2.2 mF cm-2. The average power density is in the range

of 0.5-3 µW cm-2 (Figure S3). As shown in the Nyquist plot (Figure 2e), two distinct parts including a deformed semicircle in the high frequency region and a sloped line in the low frequency region are presented. In the circuit model shown in the inset of Figure 2e, Warburg impedance W1 and W2 are introduced for the ions diffusion in nanoporous AAO membrane and CNT electrode, while R1 and R2 represent the resistances of the electrolyte solution and the leakage resistance. The device exhibits an ultrasmall leakage current of ~4 nA by applying a DC voltage of 0.2 V for 30 minutes, with a selfdischarge rate of 4 hours to decreased the open-circuit potential to 0.1 V (Figure S4). Since the electrodes are mainly based on carbon materials which deliver nearly pure capacitive behavior, the device is very stable during charge/discharge processes (Figure 2f). The outstanding performance on self-discharge rate and cycle life of the power cell indicates promise potential for practical applications.29-30


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Figure 3a and 3b show the working process and mechanism of the electrokinetic supercapacitor: (i) Electrolyte flow through the AAO nanochannels under pressure induces an open-circuit voltage of 0.25 V via the electrokinetic effect. (ii) By connecting an external load, the streaming potential is used to charge the CNT/Ti electrodes. The decrease of voltage during charging process is due to that the induced charges in the external circuit have been transferred to the electrodes, which induces an opposite potential that compensates the streaming potential. (iii) After charging, the external load is disconnected and the pressure is withdrawn. In this process, the streaming potential disappears immediately, and the open-circuit potential is only controlled by the induced charges, thus mechanical energy is converted to electricity and stored in the device. (iv) At the final step, the device can be discharged by connecting external loads or at required current rates. Since the streaming potential responses quickly to the pressure-driven flow, the four-step process can be simplified to two-step process via keeping the external load in the circuit as shown in Figure 3c. Repeating the pressure-driven charge/discharge process over several cycles produces highly reproducible profiles, demonstrating not only the stability of the device but also the actual results for harvesting and storage electricity from mechanical motion. Figure 3d shows the calculated charge stored for different pressure during charging process. As the pressure increases, the stored charges will achieve a saturated value, which is governed by the equation Q=CU, here C is the capacitance and U is the device charged voltage (reaching to a limited value under a certain pressure). At the pressure of 2.5 bar, the charge density reaches at ~0.4 mC cm-2, which is much larger than the thermal-driven and piezoelectric-driven capacitive energy storage devices under similar measure conditions (~60 µC cm-2).31-32 In addition, the power cell delivers


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an energy density of ~40 µJ cm-2, nearly to the energy densities using electrical charging method (~60-85 µJ cm-2 in Figure S3). We evaluate the efficiency of the electrokinetic supercapacitor in converting mechanical energy to electricity. The efficiency is about 0.03% under 1 bar, and it can be improved to ~0.1% via regulating the external load (Figure S6). This value is comparable to the device using Ag/AgCl electrodes (Figure S7), which are on the same order of magnitude of electrokinetic batteries.33 We have employed AAO membranes with different length in the capacitive device, and the evaluated efficiency was shown in Figure S8. It should be noted that the electrolyte resistance outside the nanochannels generally increases when the nanochannels length decreases due to due to the concentration polarization effect under the pressure driven electrolyte flow,34 thus as the length increases, the efficiency enhances gradually. Further lengthen the nanochannel, the channel resistance will enlarge and decrease the energy efficiency. With a 60 µm AAO membrane, the energy efficiency reaches to nearly 0.5%. The efficiency at current stage is somewhat low, to further promote the performance of the electrokinetic supercapacitor, employing better electrode materials and membrane is an effective way, as the efficiency is identified by energy conversion and electricity storage processes. The structure and surface chemistry of the nanochannels determines its electrokinetic performance,35 which is vital for the mechanical-to-electricity conversion process, and the recently reported porous silica,36 2D MoS2 nanopores,37 and boron nitride nanotube,38 etc, may be potential candidates. On the other hand, the storage property of converted electricity is controlled by the capacitive electrodes part, in which high surface area graphene is considered to be a good choice.39 The electrokinetic supercapacitors are easy to scale-up. As shown in Figure 4a, we designed a closed prototype, in which the device and electrolyte are sealed in an organic glass tube by two


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movable rubber plugs. When we press one rubber plug at one side, the electrolyte will flow through the device to another side, and vice versa. The prototypes are scaled-up by connecting them in series, and we have fabricated a 3 × 3 integrated system. The integrated devices can be operated at an electrochemical potential window of 3.6 V, showing good capacitive performance (Figure 4b). As shown in Figure 4c, when we put an 8 kg load on the integrated system (effective pressure is ~0.6 bar), the devices can be fully charged within 5 minutes, resulting ~2 V opencircuit potential. After discharging, an energy of ~ 40 µJ can be released (Figure 4d). The generated voltage and stored energy is enough to power common electronics. 4. CONCLUSIONS In summary, we have proposed a new type of mechanical-to-electrical process, in which a selfcharging electrokinetic supercapacitor is designed for simultaneously converting mechanical energy to electricity and storing the electricity directly. Different from traditional electrokinetic batteries, our devices work under uncontentious pressure, charging under pressure and discharging without pressure, which can be suitable for various low-frequency mechanical vibration, from human activities to ocean motions. After charging fully under 2.5 bar, the device stores charge density of 0.4 mC cm-2, which can be further promoted by improving the electrode materials and membrane. As demonstrated in this study, the new type power cell shows promising potential applications in self-powered system. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Additional characteristic results, such as pore size distribution of the AAO nanochannels, electrokinetic performance of the AAO nanochannels, energy storage performance of the electrokinetic supercapacitor, self-discharge rate of the electrokinetic


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supercapacitor, experimental setup, efficiency evaluation of the self-charging electrokinetic supercapacitor and electrokinetic battery using Ag/AgCl electrodes. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51672097, 51606082, 51322210, 61434001), the National Program for Support of Top-Notch Young Professionals, the program for HUST Academic Frontier Youth Team, the China Postdoctoral

Science

Foundation

(2015M570639,

2017M610471,

2017T100549),

the

Fundamental Research Funds for the Central Universities (HUST: 2015MS004) and the Director Fund of WNLO. The authors thank to the facility support of the Center for Nanoscale Characterization & Devices, WNLO-HUST and the Analysis and Testing Center of Huazhong University of Science and Technology. REFERENCES (1) Kim, S. H.; Haines, C. S.; Li, N.; Kim, K. J.; Mun, T. J.; Choi, C.; Di, J.; Oh, Y. J.; Oviedo, J. P.; Bykova, J.; Fang, S.; Jiang, N.; Liu, Z.; Wang, R.; Kumar, P.; Qiao, R.; Priya, S.; Cho, K.; Kim, M.; Lucas, M. S.; Drummy, L. F.; Maruyama, B.; Lee, D. Y.; Lepró, X.; Gao, E.; Albarq, D.; Ovalle-Robles, R.; Kim, S. J.; Baughman, R. H. Harvesting Electrical Energy from Carbon Nanotube Yarn Twist. Science 2017, 357, 773-778. (2) Xue, G.; Xu, Y.; Ding, T.; Li, J.; Yin, J.; Fei, W.; Cao, Y.; Yu, J.; Yuan, L.; Gong, L.; Chen, J.; Deng, S.; Zhou, J.; Guo, W. Water-Evaporation-Induced Electricity with Nanostructured Carbon Materials. Nat. Nanotechnol. 2017, 12, 317-321.


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Figure 1. The illustration of the self-charging electrokinetic supercapacitor. (a) Structure design of the device. Left side is the schematic of the ion flow inside the AAO nanochannels. (b) Photograph of the CNT/Ti electrode and AAO membrane. The AAO is shown in the red dashed box, revealing nearly transparent character. (c) SEM images of the CNT/Ti electrode, and inset indicates a uniform CNT loading. (d) SEM image of AAO nanochannels. Inset shows a crosssectional view. Scale bars: 1 cm for b, 200 µm for c (200 nm for inset), and 100 nm for d (4 µm for inset).


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Figure 2. Energy storage performance of the supercapacitor. (a) CV curves at a scan rate of 5 mV s-1 in various NaCl electrolytes. (b) Calculated capacitance as a function of the electrolyte concentration. (c) CV curves as scan rates increasing from 2 to 20 mV s-1 in 10-7 mol L-1 NaCl solution. (d) Galvanostatic charge/discharge curves of the device under different current densities. (e) Nyquist plots of the device with the frequency ranging from 50 mHz to 5 MHz. Inset shows the simulated equivalent circuit. (f) Cycling stability over 10,000 cycles at a scan rate of 5 mV s-1. Inset shows the CV curves at 1st and 10,000th cycle.


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Figure 3. The working mechanism of the self-charging electrokinetic supercapacitor. (a) Measured voltage curve and (b) corresponding mechanism illustration of one charge/discharge cycle under 1 bar: (i) electrolyte flow under pressure induces streaming potential, (ii) the streaming potential is used to charge the electrodes, (iii) the external load is disconnected and the pressure is withdrawn for voltage equilibration, and (iv) the device is discharged. The arrows in (ii) and (iv) indicate the electrons transfer direction in external circuit. (c) Charging/discharging processes using a periodic pressure (0 and 1 bar). The blue shading in a, b, and c means the device is under pressure condition. (d) The charge density and energy density stored in the device after charging by pressure. The external load during charge/discharge processes is 100 kΩ.


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Figure 4. Proof-of-concept large-scale integration. (a) Illustration and experimental setup of 3 × 3 devices integration. Scale bars is 2 cm. (b) CV curves at a scan rate of 10 mV s-1 for the serieswound devices, indicating a stable operation potential window of 3.6 V. (c) Measured voltage curve of the devices charging by an 8 kg load for 5 minutes, resulting about 2 V open-circuit potential. The charging resistance is 100 kΩ. (d) The energy stored in the series-wound devices.


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TOC

A pressure-driven self-charging electrokinetic supercapacitor which realizing energy harvesting and storage simultaneously was demonstrated, with promising applications in wide range of selfpowered systems.


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Electrokinetic Supercapacitor for Simultaneous Harvesting and

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