Converting Chemical Energy to Electricity through a Three-Jaw Mini

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Converting Chemical Energy to Electricity through a Three-Jaw MiniGenerator Driven by the Decomposition of Hydrogen Peroxide Meng Xiao,† Lei Wang,*,‡ Fanqin Ji,‡ and Feng Shi*,† †

State Key Laboratory of Chemical Resource Engineering & Beijing Engineering Research Center for the Synthesis and Applications of Waterborne Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China ‡ State Key Laboratory of Inorganic Synthesis and Applied Chemistry, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China S Supporting Information *

ABSTRACT: Energy conversion from a mechanical form to electricity is one of the most important research advancements to come from the horizontal locomotion of small objects. Until now, the Marangoni effect has been the only propulsion method to produce the horizontal locomotion to induce an electromotive force, which is limited to a short duration because of the specific property of surfactants. To solve this issue, in this article we utilized the decomposition of hydrogen peroxide to provide the propulsion for a sustainable energy conversion from a mechanical form to electricity. We fabricated a mini-generator consisting of three parts: a superhydrophobic rotator with three jaws, three motors to produce a jet of oxygen bubbles to propel the rotation of the rotator, and three magnets integrated into the upper surface of the rotator to produce the magnet flux. Once the minigenerator was placed on the solution surface, the motor catalyzed the decomposition of hydrogen peroxide. This generated a large amount of oxygen bubbles that caused the generator and integrated magnets to rotate at the air/water interface. Thus, the magnets passed under the coil area and induced a change in the magnet flux, thus generating electromotive forces. We also investigated experimental factors, that is, the concentration of hydrogen peroxide and the turns of the solenoid coil, and found that the mini-generator gave the highest output in a hydrogen peroxide solution with a concentration of 10 wt % and under a coil with 9000 turns. Through combining the stable superhydrophobicity and catalyst, we realized electricity generation for a long duration, which could last for 26 000 s after adding H2O2 only once. We believe this work provides a simple process for the development of horizontal motion and provides a new path for energy reutilization. KEYWORDS: energy conversion, mini-generator, bubble propulsion, smart device, horizontal locomotion

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magnesium, etc.,17−27 diffusiophoresis28 and self-electrophoresis29,30 from the decomposition of hydrogen peroxide and the Marangoni effect,31−36 et al. With the breadth of research on the locomotion of small objects, various applications have been proposed in the cancer therapy,37,38 protein detection,39 fabrication of miniature robots,40 transportation of cargoes,41,42 purification of objects at micro- and nanoscale,43 water quality testing,44 environmental remediation,45−48 and fabrication of assembly.49−51 In addition to the above applications, the

INTRODUCTION The locomotion of small objects in a horizontal direction is a key element to understanding an insect’s movement on the surface of water and to further the fabrication of a biomimicking mini-robot.1,2 Until now, two strategies have been developed to propel the locomotion of small objects in a horizontal direction. One involves external field,3−5 such as magnetic fields,6−11 electric fields,12,13 light,14,15 or ultrasound.16 The other one is to introduce a material flow, including propulsion by oxygen bubbles from the decomposition of hydrogen peroxide, dioxide carbon bubbles from reaction between calcium carbonate and hydrochloric acid, hydrogen bubbles from replacement reaction of zinc and © XXXX American Chemical Society

Received: January 15, 2016 Accepted: April 20, 2016

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DOI: 10.1021/acsami.6b00550 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Fabrication Process of the Mini-Generator

generation of electrical power from the mechanical energy of a moving object through Faraday’s law has been shown to be a popular field and is a simple prospect for the development of horizontal motion, thus, providing a new way for energy reutilization. In previous reports on the generation of electrical power from horizontal mechanical energy, magnets were integrated into the upper surfaces of a device capable of selfpropulsion.52,53 The locomotion of the device leads to the movement of magnets relative to solenoid coils located above, thus, generating electromotive forces. Matsui and co-workers fabricated a microelectric generator that moved in circles and induced the cyclic locomotion of magnets under a solenoid coil, thereby producing electromotive forces. Similarly, Mitsumata et al. used an amphiphilic polymer gel to propel a generator and induced an electromotive force. The motions of the above generators were propelled by the Marangoni effect induced by the release of a low-surface-energy compound. However, because of the specific property of a traditional surfactant, the motion generated by the Marangoni effect is usually not sustainable.54 When the air/water interface is saturated with surfactant molecules, the release of additional surfactants does not further the locomotion of generators. Therefore, it remains a challenge to produce a sustainable motion. A possible strategy to overcome this challenge is to replace the Marangoni effect with another type of driving force. The self-propulsion strategy based on Pt−H2O2 systems has been widely used in the research field of motions of small objects for many potential applications. Electricity generation through the motions based on bubble propulsion is a new possible application. Therefore, we wondered whether we could apply this chemical propulsion to settle the unsolved challenge of unsustainable locomotion for electricity production. Chattopadhyay has realized the conversion from mechanical energy of a moving magnetic bot in the vertical direction to electricity through the classical Faraday’s law.55 However, the output of the induced voltage was at the hundreds of nanovolts level, which may be caused by the restricted flux changing rate and remarkable fluidic dynamics induced by the limited scale of the moving objects. Normally, the parameter of output voltage is significant for electricity transportation and further use, resulting in the high voltage meaningful for practical use. Therefore, herein, we applied the bubble propulsion generated from the decomposition of hydrogen peroxide to a mini-generator system and realized the energy conversion from chemical form to mechanical power and then to electricity. When the generator was placed on a solution of hydrogen peroxide, it autonomously

began to rotate. The integrated magnets passed the coil area and induced electromotive forces; thus, the energy conversion from chemical form to electricity was realized. Although H2O2 does not commonly exist around us, the established principle should be versatile to a wide range of available bubble systems, such as water photolysis and biological fermentation. The current Pt−H2O2 is a model system to demonstrate the feasibility of this strategy and could provide possible solution to collect energy from environment for useful electricity generation.

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RESULTS AND DISCUSSION The fabrication of the mini-generator is shown in Scheme 1. The mini-generator consisted of three parts, that is, a dragreducing superhydrophobic rotator with three jaws, three motors composed of a platinum catalyst integrated with the side surfaces of the rotator jaws, and three magnets to provide the magnetic flux. First, the superhydrophobic rotator was formed by successively depositing gold aggregates on the surface of a shaped rotator and modifying HS(CH2)11CH3. The combination of sufficient roughness and a low-surface-energy compound resulted in a superhydrophobic surface with a contact angle of 151° (Figure S1).56−58 Second, the motor was prepared through the successive electrochemical deposition of successive gold and platinum aggregates on a gold plate. The platinum-covered surface (Figure S2a) provided a large amount of attachment sites for a second deposition of platinum particles (Figure S2b). These steps maximized the reactivity of the catalyst used to decompose the hydrogen peroxide and enhanced the decomposition rate to produce oxygen bubbles, which served as the driving force for locomotion. Third, three magnets were attached to the upper surface of the three jaws with a piece of double-sided adhesive tape at a distance of 22 mm from the center of the generator. Moreover, we used a copper pillar through a hole in the generator center to fix the location of the generator, while it was rotating on the water surface. After obtaining the above generator, we wondered whether the introduced bubble propulsion could provide a powerful driving force for the rotation of the generator. Therefore, we placed the mini-generator on the surface of an aqueous solution of hydrogen peroxide with a concentration of 10 wt % and used a copper pillar to fix the location of the generator. As seen in Movie S1, once placed on the solution surface, the generator immediately began to rotate anticlockwise with a large number of bubbles moving on the water surface. To calculate the velocity of the generator, we set 0 s as the point when the B


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ACS Applied Materials & Interfaces

Figure 1. (a) Photograph of the device. Scale bar =1 cm. (b−f) Snapshots of the rotating process of the mini-generator propelled by the decomposition of H2O2. Scale bar = 2 cm.

Figure 2. Induced electromotive forces of the mini-generator under a solenoid coil with 1000 turns: (a, b) potential-time curve and (c) current−time curve.

attributed to the macroscopic scale of the moving device. Compared to the current system, the nano- or microgenerators based on the similar mechanism led to a finite magnetic field strength and relative enhanced fluid drag, resulting in a decreasing flux changing rate and consequently weak electricity. To clearly present the recorded electromotive forces, we display the results from the first 100 s of the recorded electromotive forces curves, beginning when the mini-generator began to rotate smoothly (Figure 2b). From the results shown in Figure 2b,c, we found that the recorded potential and current curves consisted of negative and positive peaks, which corresponded to the entrance and exit of the magnets from the coil area, respectively.61,62 On average, the mini-generator exhibited a high induction frequency of approximately four-thirds of a second for one pair of negative−positive peaks, thus, ensuring a high output of induced electromotive forces. This was attributed to the enhanced propulsion of the bubble propulsion generated from the decomposition of hydrogen peroxide. The negative−positive peaks was, at this moment, thought to be interpreted with Faraday’s law. On the basis of Faraday’s law, the induced potential is negatively proportional to the magnetic flux per unit time. Therefore, the induced electromotive force should consist of pairs of negative and positive peaks when the magnets in the upper surface of the jaw passed under a solenoid coil. The negative peak is induced when magnet enters the coil area, and the positive peak is generated when the magnet exits the coil area.

generator began to rotate smoothly (Figure 1a) and captured the motion of the generator every other 1 s (Figure 1b−e). We found the generator rotated 90° per one second (∼39 mm s−1), that is, 4 s for one cycle, showing a high propulsion efficiency for the decomposition of hydrogen peroxide. The high velocity was attributed to the effective decomposition of hydrogen peroxide by the platinum catalyst. When the catalyst contacted the solution, it began to catalyze the decomposition of hydrogen peroxide to produce water and oxygen gas. Then, the produced oxygen accumulated as bubbles, which diffused and generated a spray of bubbles that caused the rotation of the generator at the air/water interface.59,60 Although we realized high velocity for the rotation of the mini-generator, it still remained a question whether we could convert this mechanical energy into electricity through Faraday’s law of induction. Therefore, we fixed a solenoid coil with 1000 turns right above the moving trajectory of the magnets at a distance of 10 mm; the two ends of the coil were connected to an electrochemical station (Figure S3). The coil was made of a flexible copper wire with a diameter of 0.15 mm. The obtained solenoid coil had an inner diameter of 24 mm and an external diameter of 26 mm. The induced electromotive forces were recorded right before the generator was propelled by the decomposition of hydrogen peroxide (10 wt %). As a consequence, an induced potential of ca. ±1.0 mV was obtained (Figure 2a). The induced electromotive force of the current system was larger than that of nano- or microgenerators was C


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Figure 3. Influence of the concentration of H2O2 on the induced electromotive forces: (a) 2%, (b) 4%, (c) 6%, (d) 8%, and (e) 10% in weight; and (f) summarized results.

Because the mechanical energy of the rotating generator was generated by the chemical decomposition of hydrogen peroxide, the concentration of the reactant played a key role in the conversion process. Therefore, we investigated the influence of the concentration of hydrogen peroxide on the rotation of the mini-generator and the energy conversion process. To obtain a high output of the induced electromotive force, we fixed the number of turns in the solenoid coil at 5000. As shown in Figure 3a−e, a grouping of three pairs of negative−positive peaks revealed the induced electromotive forces that corresponded to the entrance−exit of three magnets when passing through the coil area (For more details, please refer to Movie S2). Additionally, the value of the positive peaks was equal to that of the negative peaks, which demonstrated that the exit velocity of magnet was the same as the entrance velocity. Moreover, for a constant number of coil turns, the concentration of hydrogen peroxide played an important role in the induction of electromotive forces. With increasing concentrations of hydrogen peroxide (2%, 4%, 6%, 8%, and 10%), the current peak value increased (2.8, 4.2, 5.8, 8.5, and 10.5 μA, respectively), as well as the frequency of the negative− positive peaks. Through a similar recording method, we also investigated how the concentration of the hydrogen peroxide affected the induced potential, and the results are shown in Figure S4. To clearly analyze the role played by the concentration of hydrogen peroxide during the energy conversion, we calculated and summarized the current and potential results, as shown in Figure 3f. The calculations showed that both the induced current and potential were positively related to the concentration of hydrogen peroxide. This phenomenon could be interpreted through Faraday’s law of electromagnetic induction. On the basis of the law, the induced electromotive force is negatively proportional to the rate of change for the magnetic flux and the number of turns of solenoid coils, that is:

ε(t ) = − n

dΦ(t ) dt

(1)

where ε is the electromotive force, n is the turns of the coil, and Φ is the magnetic flux. dΦ(t ) is the rate of change for the dt magnetic flux in unit time. In the current system, this rate was determined63−66 by the rotating velocity of the mini-generator, which was positively related to the decomposition rate and concentration of hydrogen peroxide, that is: dΦ(t ) ∝ v ∝ k H2O2 ∝ c H2O2 dt

(2)

where v is the rotational velocity of the mini-generator, and kH2O2 and cH2O2 are the decomposition rate and concentration of hydrogen peroxide, respectively. With increasing concentrations of hydrogen peroxide (cH2O2), the number of oxygen bubbles generated from the decomposition of H2O2 in the unit interval (kH2O2) increased, and the mini-generator was driven to rotate with an increasing velocity (v). This led to an increase in the rate of change for the magnetic flux and, consequently, the electromotive forces, which were recorded as the potential. The current was the combined result of voltage and resistance, and the resistance of a single solenoid coil was constant. Therefore, the current showed a dependency on the concentration of hydrogen peroxide similar to that of the induced potential. In addition to the induced electromotive forces, the rotating velocity of the mini-generator also determined the cycles the mini-generator rotated at unit intervals, that is, the frequency of induced electromotive forces. As shown in Figure 4, the frequency of the induced electromotive forces increased with an increase in hydrogen peroxide concentration. This phenomenon was interpreted as follows: With the increase in concentration, the mini-generator rotated with a greater velocity, and it required a reduced amount of time to finish one cycle; thus, an increase in the amount of cycles performed D


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%, and varied the turns of the coil from 1000 to 9000 in intervals of 2000. As shown in Figure 5a−e, as the number of turns in the solenoid coils increased, the frequency of the induced electromotive force did not show an obvious change, with ∼48 pairs of positive−negative peaks per minute. Additionally, at a constant concentration of hydrogen peroxide, the coil turns played a key role in the induction of electromotive force; with an increase in coil turns (1000, 3000, 5000, 7000, and 9000), the potential peak value increased (0.9, 2.2, 4.0, 8.0, and 12.0 mV, respectively). The induced potential peak value was nearly proportional to the number of turns in the coil, as shown in Figure 5f (red columns). This could be interpreted with Faraday’s law of electromagnetic induction. On the basis of Equation 1, the induced electromotive force was the combined result of the turns of the coil and the absolute value of the rate of change for the magnetic flux when passing the coil area. When the concentration of hydrogen peroxide was fixed (i.e., 10 wt %), the rotational velocity of the generator could be regarded as constant, leading to a constant rate of change for the magnetic flux, that is, dΦ(t ) . dt Therefore, Equation 1 could be transformed into

Figure 4. Frequency of the magnets on the mini-generator passed the coil area in the hydrogen peroxide solution with different concentration.

per unit interval (e.g., 1 min). However, when the concentration of the hydrogen peroxide was higher than 10 wt %, the reactive oxygen generated from the decomposition process of H2O2 damaged the superhydrophobic surface of the mini-generator, leaving a hydrophilic surface of the minigenerator and consequently a high motion resistance (For the rotating details of a hydrophilic device, please refer to Movie S3);53 therefore, the sustainability of the energy conversion process was impacted. Therefore, a concentration of 10 wt % was optimal and was chosen to investigate another factor. After demonstrating the positive effect of the concentration of hydrogen peroxide, we wondered whether we could enhance the output of the electromotive force by regulating the number of turns in the solenoid coil. To investigate the influence of the turns of the coil on the induced electromotive forces, we chose the optimal concentration of hydrogen peroxide, that is, 10 wt

P = Cn

(3)

where P is the peak value of the induced electromotive force, n is the turns of the solenoid coil, and C is the absolute value of the rate of change rate for the magnetic flux, which is constant for a fixed hydrogen peroxide concentration. From Equation 3, we concluded that the absolute value of the induced potential was proportional to the turns of the coil, which was identical to the experimental results. Moreover, although the induced potential increased with the number of coil turns, the corresponding current remained almost constant showing only a small decreasing trend (14−10 μA; Figure S5 and the blue columns in Figure 5f). This phenomenon was the combined result of the induced potential and the resistance

Figure 5. Influence of turns of the solenoids on the induced electromotive forces: (a) 1000, (b) 3000, (c) 5000, (d) 7000 and (e) 9000 turns; and (f) the summarized results. E


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Figure 6. Sustainability evaluation of the energy conversion system propelled by the decomposition of H2O2: (a) original and (b) improved system.

and 9960 s. Therefore, we added to the initial fuel an additional 10 mL of 30 wt % H2O2. The middle section of the results shown in Figure 6 clearly revealed that once the hydrogen peroxide solution was added, the mini-generator restarted the energy conversion process. The obtained potential was almost equal to that observed before the cessation. Furthermore, we attempted to restart this process for a second and third time by adding fuel to the initial solution. The latter half of the plot shown in Figure 6a indicated that additional fuel could always recover the energy conversion process over a duration of 30− 40 min. Compared with similar work on the conversion of mechanical energy into electricity, the current system provided a more sustainable process. To recover the energy conversion process, other scientists replaced the motor with a new one; in the current system, we could recover the energy conversion process by simply adding fuel. However, the induced electromotive force sharply reduced during the first 1000 s. To clarify the dominant factors of the output voltage, we considered possible reasons responsible for the potential value and the sustainability of electricity generation: the concentration of the H2O2 solution, the durability of both the superhydrophobic property, and the Pt catalyst. As shown in Section S8 in the Supporting Information, we concluded that the sharp decrease was caused by the superhydrophobic coating and the Pt catalyst. To solve these problems, we improved the current system through replacing the modification of lowsurface-energy coating from the sensitive thiol to the relatively chemically inert silane and through changing the deposited platinum aggregates to an abraded platinum plate. With the improved durability of superhydrophobicity and catalyst, we performed the experiment of electricity generation, which could

of the coil, and it was interpreted as follows: With an increase in the solenoid coil turns, the induced potential increased proportionally; in contrast, because the external diameter increased with additional winding, the resistance of the coils was larger than the multiple of the proportional increase in potential. Therefore, the induced current exhibited a small tendency to decrease, as the number of turns in the solenoid coil increased. Having demonstrated the possibility of the bubble propulsion by hydrogen peroxide and investigated the influence of the concentration and coil turns, we wondered whether the mini-generator could provide a sustainable energy conversion of chemical potential to electricity and provide a steady current output. Therefore, we recorded the energy conversion process for at least 12 000 s with a hydrogen peroxide solution with a concentration of 10 wt %. Once the generator was placed on the solution, it began to rotate, and the integrated magnets passed the under the solenoid coil (with 1000 turns); this induced electromotive forces with a potential of 1.0 mV. As shown in Figure 6a, as the operation preceded the induced electromotive force decreased and was the result of a decreasing rotational velocity, caused by the depletion of hydrogen peroxide in the solution. After 5550 s, the rotation of the mini-generator stopped. The cessation could be interpreted as follows: on the basis of Lenz’s law, the rotating velocity of the mini-generator was a combining result of the bubble propulsion and the obstruction of magnetic field generated by the induced current in the solenoid coils, and the rotation was induced when the bubble propulsion overcame the obstruction of the electromagnetic damping. When the concentration of H2O2 was too low to provide enough propulsion to defeat the electromagnetic damping, the rotation stopped, like 5550, 7330, F


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deposited in a mixture of H2SO4 (0.5 M) and H2PtCl6 (2 mg mL−1) at −200 mV (vs Ag/AgCl) for 800 s. The hierarchical structure of nanoparticles maximized the surface area of the catalyst, enhancing the rate of hydrogen peroxide decomposition to produce oxygen bubbles as the driving force. To solve the catalyst activity problem, we improved the catalyst system as follows: a piece of purchased platinum plate was polished with a No. 600 abrasive paper, followed by washing with ethanol; then the platinum plate was cut into three pieces of plate with a size of 5 mm × 4 mm to adhere onto the jaws of the rotator to propel the motion of the device. Fabrication of the Mini-Generator. Three catalyst plates (5 mm × 4 mm) were adhered to the side surface of the rotator jaws, and three magnets (5 mm diameter and 0.3 T magnetic intensity) were attached to the upper surface of the three jaws with a piece of doublesided adhesive tape at a distance of 22 mm from the center of the generator. When the device was placed on the water surface in a dish (12 cm), it was secured in the middle of the dish by a copper pillar (2 mm diameter) through a hole (2.5 mm) in the middle of the minigenerator.

last for 26 000 s after adding H2O2 for only once, as displayed in Figure 6b.

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CONCLUSION In conclusion, we have fabricated a mini-generator that can convert chemical energy to the mechanical form and then to electricity. The locomotion of the generator was propelled by bubbles originating from the decomposition of hydrogen peroxide. When the generator rotated under a solenoid coil, the magnets integrated with the upper surface of the generator passed the coil area, resulting in the induction of electromotive forces. We investigated the influence of the concentration of hydrogen peroxide and the turns of the coil on the energy conversion process. We found that the induced electromotive forces were positively proportional to both of these factors. The generator performed best in a solution of hydrogen peroxide with 10 wt % and under a solenoid coil with 9000 turns. Moreover, with the combination of improved durability of superhydrophobicity and catalyst, we realized the electricity generation that could last for 26 000 s after adding H2O2 only once. On the basis of these improvements, the potential value could grow back to the original high value after adding fresh H2O2 solution. The current system solved the nonsustainability problem of the energy conversion process in the Marangoni effect system to some extent and provides a new avenue for the development of a mini-generator that converts mechanical energy to the electrical form. Currently, we are working on harvesting tiny energy in the environment such as pressure difference or sunlight to convert to mechanical motions of devices and further to electric energy based on Faraday’s law of induction. We believe that this methodology will benefit the development of the electricity harvesting in the future practical utility.

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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.6b00550. Characterization of the as-prepared superhydrophobic rotator and hydrogen peroxide-responsive actuator; illustration of experimental setup; induced potential of generator in the H2O2solution with different concentration; induced current under solenoid coil with different turns; and calculation of energy conversion efficiency. (PDF) Movie S1, showing the rapid rotation of the minigenerator. (AVI) Movie S2, showing the rotating process of a hydrophilic device. (AVI) Movie S3, showing the electricity generation process of the mini-generator. (AVI)

EXPERIMENTAL SECTION

Materials. The following reagents were used: H2SO4, HCl, NaOH, ethanol, AgNO3, and hydrogen peroxide (Beijing Chemical Works, Beijing, China); HAuCl4 and H2PtCl6 (Sinopharm Chemical Reagent Beijing Co., Ltd., Beijing, China); n-dodecyltrimethoxysilane (J&K Chemical Ltd., Shanghai, China). Copper foam, which has a porous structure with continuous meshes, was purchased from Shanghai Zhonghui Foam Aluminum Co., Ltd.; and HS (CH2)11CH3 was purchased from Sigma-Aldrich. Platinum plate was purchased from Tianjin AIDAhengsheng Science-Technology Development Co., Ltd. Instruments. Scanning electron microscopy and energy-dispersive X-ray spectroscopy measurements were performed on a ZEISS SUPRA 55 at 20.0 kV. The contact angle was measured with an OCA20 instrument (DataPhysics Instruments Gmbh, Filderstadt). Electrochemical measurements were operated on an electrochemical workstation (PARSTAT 2273), and the models selected to measure the current−time curve and potential−−ime curve are “Amperometric i-t Curve” and “Open Circuit Potential-Time”, respectively. Fabrication of the Three-Jaw Rotator. A piece of copper foam was cut and folded into the shape of a rotator (handmade with a 50 mm diameter) and washed with ethanol, ethanol/water solution (1:1 in volume ratio), and water, successively. The rotator was immersed in a solution of AgNO3 (20 mM) for 3 min, which was followed by washing with water and dried in an oven at 65 °C. Then, the rotator was immersed in an ethanol solution of 1-dodecanethiol (10 mM) for 6 h and dried at room temperature. The combination of a rough surface with a low-surface-energy compound resulted in a superhydrophobic surface. Fabrication of the Platinum Catalyst. A gold layer was first fabricated by electrochemically depositing a gold plate in a mixed solution of H2SO4 (0.5 M) and HAuCl4 (2 mg mL−1) at −200 mV (vs Ag/AgCl) for 1600 s. Then, the platinum was electrochemically

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by NSFC (21374006 and 51422302), the Program of the Co-Construction with Beijing Municipal Commission of Education of China, Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM201617) and Beijing Young Talents Plan (YETP0488).

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