Electrokinetic Supercapacitor for Simultaneous Harvesting and

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 8010−8015

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 S Supporting Information *

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 a new type of energy unit for self-powered systems. KEYWORDS: supercapacitor, electrokinetic effect, energy harvesting, self-powered, mechanical energy

1. INTRODUCTION With 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 the last decade, enormous efforts have been devoted to exploit a variety of energy harvesters, such as solar cells,7,8 thermoelectric devices,9−11 and nanogenerators.12−16 Although the energy harvesters are evergrowingly 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 the energy storage function. Recently, mechanical-toelectrochemical process has been proposed by integrating piezoelectric and triboelectric materials in battery systems.17−19 Compared with batteries, capacitive energy storage devices have advantages such that they can operate at high charge/discharge rates over almost unlimited cycles, receiving increasing attention.20 Herein, we develop a new mechanical-to-electrical energy conversion device, a self-charging electrokinetic supercapacitor, for mechanical energy to be simultaneously harvested and stored by using carbon nanotube (CNT)-coated titanium (Ti) mesh as capacitive electrodes and anodic aluminum oxide (AAO) nanochannels as a separator membrane. Energy generation is achieved through the electrokinetic effect in AAO nanochannels under pressure-driven flow, and the © 2018 American Chemical Society

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 of 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 2.1. CNT/Ti Electrode Preparation.2,21 CNT ink was first prepared. Briefly, 3 g of commercial CNTs were dispersed in 120 mL of mixed H2SO4 and HNO3 (H2SO4/HNO3 = 3/1, v/v) solution by sonication for 10 min. Then, the CNT−acid mixture was refluxed while stirring vigorously for 2 h in a 90 °C 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 redispersed 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 °C to reinforce their association. The final CNT loading on the titanium mesh is about 0.2 mg cm−2. 2.2. AAO Preparation.22 The AAO nanochannel array is fabricated through a two-step anodization process. In brief, electrochemical polishing is undertaken for a designed 0.2 mm thick Received: December 7, 2017 Accepted: February 6, 2018 Published: February 6, 2018 8010

DOI: 10.1021/acsami.7b18640 ACS Appl. Mater. Interfaces 2018, 10, 8010−8015

Research Article

ACS Applied Materials & Interfaces

Figure 1. 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 a nearly transparent character. (c) Scanning electron microscopy (SEM) images of the CNT/Ti electrode. The inset indicates a uniform CNT loading. (d) SEM image of AAO nanochannels. The inset shows a cross-sectional view. Scale bars: 1 cm for (b), 200 μm for (c) (200 nm for inset), and 100 nm for (d) (4 μm for inset). aluminum foil (99.999%) before the anodization process, which was performed 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 are shown in Figure 1a. Two pieces of CNT/Ti electrodes were cut into the required shape. The device was constructed using two partial CNT electrodes sandwiching an AAO membrane. The joint of 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 a diameter of 5 mm (effective area of 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. Because the cyclic voltammetry (CV) curves of the capacitive device are nearly rectangular, the areal capacitance can be calculated as23

C=

∮ I dU 2νS ΔU

(1)

where ν is the potential scan rate, S is the area, and ΔU is the potential window. The energy stored in the device is evaluated by

E=

Q2 2C

(2)

where Q is the stored charge 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

η=

E PV

(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

η′ =

I0U0 4Pvf

(4)

where P is the working pressure and vf is the electrolyte flow rate during the electricity generation, I0 and U0 are 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 that consists of two CNT/Ti electrodes and 8011


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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. The inset shows the simulated equivalent circuit. (f) Cycling stability over 10 000 cycles at a scan rate of 5 mV s−1. The inset shows the CV curves at the 1st and 10 000th cycle.

Figure 3. Working mechanism of the self-charging electrokinetic supercapacitor. (a) The measured voltage curve and (b) corresponding mechanism illustration of one charge/discharge cycle under 1 bar as follows: (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 electron-transfer direction in the external circuit. (c) Charging/discharging processes using a periodic pressure (0 and 1 bar). The blue shading in (a)−(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.

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 reactions must take place on the electrode to generate continuous electrical current. If capacitive materials are equipped to the electrodes, we can use the streaming potential to charge the electrodes, thus storing the electricity simultaneously, and the device can be named as electrokinetic supercapacitor. 8012


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

impedances W1 and W2 are introduced for the ions’ diffusion in the nanoporous AAO membrane and CNT electrode, whereas R1 and R2 represent the resistances of the electrolyte solution and the leakage resistance, respectively. The device exhibits an ultrasmall leakage current of ∼4 nA by applying a direct current voltage of 0.2 V for 30 min, with a self-discharge rate of 4 h to decrease the open-circuit potential to 0.1 V (Figure S4). Because the electrodes are mainly based on carbon materials that deliver a nearly pure capacitive behavior, the device is very stable during charge/discharge processes (Figure 2f). The outstanding performance on the self-discharge rate and cycle life of the power cell indicates promising potential for practical applications.29,30 Figure 3a,b shows the working process and mechanism of the electrokinetic supercapacitor: (i) electrolyte flow through the AAO nanochannels under pressure induces an open-circuit voltage of about 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 the charging process is due to the fact 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 opencircuit 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. Because the streaming potential responds quickly to the pressure-driven flow, the four-step process can be simplified to a 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

As shown in Figure 1c, the CNT was uniformly coated on Ti mesh and the sparse Ti wire scaffold almost has no influence on the pressure-driven electrolyte flow. The AAO membrane containing the nanochannels is a key component for streaming potential/current generation, as well as the separator for the capacitive device. The AAO membrane used in this study is fabricated by a two-step anodization process, which is nearly transparent (Figure 1b), with a thickness of about 8 μm and an average pore diameter of ∼70 nm (Figures 1d and S1). In the capacitive device, the AAO membrane is sandwiched between two CNT/Ti mesh electrodes that 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. The energy storage performance of the device was first studied. From the cell configuration for energy storage, the device demonstrates a substantially supercapacitor-like behavior. 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 the 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 the working electrolyte for the following experiments. Figure 2c,d records the nearly rectangular-shaped CV curves at different scan rates 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 s−1, 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 8013


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4. CONCLUSIONS In summary, we have proposed a new type of mechanical-toelectrical process, in which a self-charging 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 vibrations, from human activities to ocean motions. After charging fully under 2.5 bar, the device stores a 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 of power cell shows promising potential applications in the self-powered system.

the device but also the actual results for harvesting and storage electricity from mechanical motion. Figure 3d shows the calculated charge stored for different pressures during the 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 ∼0.4 mC cm−2, which is much larger than that of the thermaldriven and piezoelectric-driven capacitive energy storage devices under similar measured conditions (∼60 μC cm−2).31,32 In addition, the power cell delivers an energy density of ∼40 μJ cm−2, near to the energy densities using the 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 that of the device using Ag/AgCl electrodes (Figure S7), which are of the same order of magnitude as that of electrokinetic batteries.33 We have employed AAO membranes with different lengths in the capacitive device, and the evaluated efficiency is shown in Figure S8. It should be noted that the electrolyte resistance outside the nanochannels generally increases when the nanochannel length decreases due to the concentration polarization effect under the pressuredriven electrolyte flow;34 thus, as the length increases, the efficiency enhances gradually. On further lengthening 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 the 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 determine their electrokinetic performance,35 which is vital for the mechanical-to-electricity conversion process, and the recently reported porous silica,36 twodimensional MoS2 nanopores,37 boron nitride nanotubes,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 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 (the effective pressure is ∼0.6 bar), the devices can be fully charged within 5 min, resulting in ∼2 V open-circuit 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.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18640. 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, selfdischarge rate of the electrokinetic supercapacitor, experimental setup, efficiency evaluation of the selfcharging electrokinetic supercapacitor, and electrokinetic battery using Ag/AgCl electrodes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peihua Yang: 0000-0003-0178-6896 Jun Zhou: 0000-0003-4799-8165 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51672097, 51606082, 51322210, and 61434001), the National Program for Support of TopNotch Young Professionals, the program for HUST Academic Frontier Youth Team, the China Postdoctoral Science Foundation (2015M570639, 2017M610471, and 2017T100549), the Fundamental Research Funds for the Central Universities (HUST: 2015MS004), and the Director Fund of WNLO. The authors are grateful for the facility support of the Center for Nanoscale Characterization & Devices, and WNLO-HUST and the Analysis and Testing Center of Huazhong University of Science and Technology.

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REFERENCES

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