Thermopower Wave-Driven Hybrid Supercapacitor Charging System

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Thermopower wave-driven hybrid supercapacitor charging system Dongjoon Shin, Hayoung Hwang, Taehan Yeo, Byungseok Seo, and Wonjoon Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11334 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 2, 2016

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ACS Applied Materials & Interfaces

Thermopower wave-driven hybrid supercapacitor charging system

Dongjoon Shin, Hayoung Hwang, Taehan Yeo, Byungseok Seo, and Wonjoon Choi*

School of Mechanical Engineering, Korea University, Seoul 136-701, Korea

*Author to whom correspondence should be addressed: Wonjoon Choi E-mail: [email protected], Phone: +82 2 3290 5951, Fax: +82 2 926 9290.

Keywords: thermopower wave; exothermic chemical reaction; electrical energy generation; thermal transport; combustion; supercapacitor

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Abstract The development of new energy sources and harvesting methods has increased with the rapid development of multiscale wireless and portable systems. A thermopower wave (TW) is a potential portable energy source that exhibits a high power density. TWs generate electrical energy via the transport of charges inside micro- or nanostructured materials. This transport is induced by selfpropagating combustion. Despite the high specific power of TWs, the generation of energy by TWs is transient, making a TW device a one-time use source, which is a critical limitation on the further advancement of this technology. Herein, we first report the development of a hybrid supercapacitor charging system driven by consecutive TWs to accumulate multiple amounts of energy generated by the repetitive combustion of the chemical fuel. In this study, hybrid layers composed of a supercapacitor (polyvinyl alcohol/MnO2/nickel) and solid fuel layer (nitrocellulose film) were fabricated as one integrated platform. Combustion was initiated by the ignition of the fuel layer, resulting in the production of electrical energy, attributed to the potential difference between two electrodes, and the transport of charges inside one of the electrodes. Electrical energy could simultaneously and directly charge the supercapacitor, and the discharged voltage could be significantly increased in comparison with the voltage level before the application of a TW. Furthermore, the application of multiple TWs in succession in the hybrid supercapacitor charging system successfully allowed for stack voltage amplification, which was synchronized to each TW. The results of this study could be used to understand the underlying phenomena for charging supercapacitors with the variation of thermal energy, and to advance the application of TWs as more efficient, practical energy sources.


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1. Introduction Currently, the harvesting and storage of energy is attracting significant interest, attributed to the increase in portable applications. Next-generation platforms such as drones, wearable devices, and electric vehicles inevitably require wireless power sources with a high energy capacity. Recent developments in micro- and nanotechnologies have resulted in the rapid growth of these platforms, with various methods available for harvesting or releasing energy and extending the maximum capacity of usable energy.1 The potential of energy harvesting methods that utilize the surrounding environment has been explored, including the application of photovoltaic,2 piezoelectric,3 triboelectric,4 and thermoelectric.5 Such methods can collect energy from specific environments, including those with a light source, mechanical vibration, or a temperature gradient.6 Moreover, batteries and supercapacitors have been developed as platforms for the storage of energy.7 These sources can supply energy over a long period, while they require the charging process from the external energy sources.

Recently, thermopower waves (TWs), as a high-energy-density source, have been reported as a means of actively generating energy using chemical fuels, without electrical charging from external sources.8 Combustion has been widely utilized for converting chemical energy into mechanical energy in diverse conventional fields.9 On the other hand, TWs, which are realized with the use of hybrid composites integrating chemical fuels and micro- or nanostructured materials, enable the simultaneous conversion of chemical and thermal energy to electrical energy via the transfer of charges inside core materials by the propagation of combustion waves, triggered by ignition, through composite structures.10-11 Because chemical fuels exhibit a high energy density, they are useful for producing a high specific energy and power per mass or volume.8 Previous 3 ACS Paragon Plus Environment


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studies on TWs have focused on understanding the underlying physics of the energy conversion and the propagation of combustion within carbon nanotubes.11-13 The enhanced generation of energy has been regarded as a decisive task for utilizing TWs in portable applications. The use of core micro- and nanostructured materials with high Seebeck coefficients at high temperature, such as Bi2Te3,14, Sb2Te3,15 ZnO,16-17 CuO,18 and MnO2,19 has been reported to significantly improve the maximum voltage generated, which ranges from 500 mV to 3 V. Furthermore, the types of chemical fuels and defects in the core materials in hybrid composites affect the absolute magnitude of the energy generated from TWs.20-22 In addition to energy generation, the intrinsic manipulation of combustion waves along core materials has been investigated for superadiabaticity,23 as well as the amplification of the reaction24 and transformation of the phase and structure of micro- and nanostructured metal oxides.25-26 More recently, to improve the efficiency and sustainability of TWs, Strano et al. reported sustainable power sources using TWs, which produced energy from both the thermoelectric effects of the surrounding environment and charge transfer inside the core materials.27

Even though significant advances have been made toward the generation of energy from TWs with respect to the maximum voltage, materials, fuels, and utility, the development of TWs for applications is hindered by a critical limitation that needs to be resolved: The chemical fuel surrounding the core materials is consumed after initiating combustion waves, making devices using TWs one-time-use platforms. Hence, there have been intrinsic demands to extend the usable duration of TWs.

Herein, we first report the development of a TW-driven hybrid supercapacitor charging system for


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accumulating multiple amounts of energy generated by repetitive combustion within one integrated platform (Figure 1). Because of the integration of the capacitor and direct initiation of TWs on one electrode, thermal energy gradient by the propagation of combustion produced thermoelectric potential, which induced direct charge transport, within the supercapacitor. At the same time, the energy generated with a pulse shape, which is typical for TWs was used to directly charge an electrical double layer, while the TWs induced a rapid redox reaction in the active material of a supercapacitor. This system accumulated charges from multiple TWs in succession in identical structures. The transient energy generated by the TWs was accumulated and gradually increased the charge of the capacitors, without an additional electric circuit. This hybrid system enabled the successful demonstration of a stacking voltage in a supercapacitor by the use of consecutive TWs. Furthermore, the optimal structures for supercapacitors, which were composed of MnO2 and nickel electrodes, were investigated to reduce the energy loss and efficiently collect the energy generated by multiple TWs. The hybrid systems will resolve the intrinsic limitation of TWs, one-time use energy source based on chemical reaction. The results of this study will be helpful in realizing the use of TWs, which still remain in the development stage, as portable energy sources, without the need for electrical charging from external sources.

2. Results and Discussion 2.1. Fabrication of Supercapacitor Charging System Typically, TWs give rise to sinusoidal or pulse-shaped electrical waves because the high-energy reaction front passes through the core materials between two electrodes and induces the reversal of the thermal gradient.10 The transient generation of electrical energy exhibits an intrinsic 5 ACS Paragon Plus Environment


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limitation in that the energy from TWs cannot be easily accumulated without the use of additional devices such as thermoelectric modules.27 To integrate charging platforms based on multiple TWs, hybrid structures between the supercapacitor (which is charged by the energy from TWs) and chemical fuel layer (which initiates combustion) were designed as a single integrated platform (Figure 1). In this design, the TWs directly generate pulse-shaped electrical waves, and the corresponding energy transport during multiple combustion events results in a cumulative charge and the formation of an electrical double layer, as well as a rapid redox reaction.

As shown in Figure 2a, the supercapacitor for the TW-driven hybrid charging system is composed of a pair of layered structures consisting of a nickel electrode, a MnO2 layer, polyvinyl alcohol (PVA) electrolyte gel, and cotton paper. Nickel foil was used to prepare the electrodes, which were in direct contact with a combustion source, because it was stable near the combustion temperature induced by the propagation of the reaction front (up to 800 °C). The deposition of MnO2 on the nickel electrode resulted in the formation of an active layer inside the supercapacitor. MnO2 is one of the ideal candidates for an active material because it is a very common, cost-effective metal oxide, with properties suitable for supercapacitor applications and high voltage TW-based devices.19, 28-30 Because MnO2 has been extensively applied in other applications such as Libatteries, its intrinsic properties are widely recognized.31-32 Moreover, MnO2 is a non-toxic, ecofriendly, stable material; hence, it can be easily integrated in hybrid platforms.33 Thus, MnO2 was selected as the active material for the TW-driven hybrid supercapacitor charging system. Electrodeposition led to a homogenous coating of MnO2 on the nickel electrodes.33 A cycled electric field between 0.15 and 0.65 V was applied to the nickel foil at room temperature, and the electrodeposition time was responsible for controlling the thickness (or amount) of the MnO2 layer


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(see the Experimental Section “Deposition of MnO2 on Nickel Foils as Electrodes for Capacitors”). For instance, as shown in Figure 2b, with an electrodeposition time of 3 min, the color was brighter than that observed at 1 min, which was attributed to the thick MnO2 layer, as well as the intrinsic nanostructure. After completely depositing the MnO2 layer, a PVA-based electrolyte gel was dropped on the MnO2-deposited nickel electrodes, and cotton paper, serving as a separator, was placed above the electrolyte gel. Two electrodes (cotton paper/PVA/MnO2/nickel) were fabricated, and KAPTON tape was applied to the edges of the electrodes, completing the fabrication of freestanding capacitors with strong mechanical stability, which endured the TWs produced by combustion (Figure 2c) (see the Experimental Section “Packaging of Gel-type Electrolyte for Capacitors.”).

Before constructing the hybrid charging platform, it was imperative to understand the characteristics and performance of the supercapacitors. Hence, scanning electron microscopy (SEM) was employed to investigate the surface morphology and inner structure of the MnO2 layer. As shown in Figures 2d and 2e, at electrodeposition times of 1 and 3 min, respectively, the randomly oriented MnO2 nanorods were uniformly coated on the nickel electrodes. This nanostructured MnO2 coating, with a nanorod morphology, significantly increased the total surface area, which directly affected the capacitor performance. The network of nanorods increased the total effective area of the electrical double layer, which in turn increased the charge transport and total capacitance. With respect to the electrodeposition time, the length and thickness of the MnO2 nanorods observed at 3 min were greater than those observed at 1 min, while the MnO2 nanorods sufficiently covered the entire nickel surface. Electrodeposition for 3 min was estimated to provide a larger amount of MnO2 on the nickel surface than that at 1 min. On the other hand, the total



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capacitance is highly dependent on the flow of current through an active material, as well as its exposed surface area. In this regard, the thick layer of MnO2 produced by a long electrodeposition time may serve as a resistance to the current flow, resulting in a decrease in the effective surface area, as an ion double layer, rather than exhibiting positive effects. Energy-dispersive X-ray spectroscopy (EDS) provided evidence of the composition of the nickel electrodes, following the deposition of the MnO2 layer on them. As shown in Figure 2f, the atomic percentage values for oxygen, manganese, and nickel were 20.20, 5.18, and 74.63%, respectively. Even though the relative ratio between manganese and oxygen from the EDS peaks was approximately 1:4, the deposited layer was regarded as MnO2, because the excess oxygen was attributed to the oxidized nickel foil.

2.2. Electrochemical Performance of Supercapacitor Charging System Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted to investigate the electrochemical performance of the supercapacitors (Figure 3). Figures 3a and 3b show the CV plots obtained at different scan rates (5, 10, 25, 50, and 100 mV/s) under the different electrodeposition times of 1 and 3 min, respectively. The maximum specific capacitance values for the supercapacitors were 101.4 F/g and 96.8 F/g for the electrodeposition times of 1 and 3 min, respectively. According to the previously mentioned SEM analysis results, the thick MnO2 layer obtained at an electrodeposition time of 3 min had a specific capacitance that was less than that observed at an electrodeposition time of 1 min, whereas the total capacitance observed at an electrodeposition time of 3 min was greater than that observed at an electrodeposition time of 1 min. Such conflicting results are attributed to the total amount and surface area of the active material. The larger surface area from the random network of thin MnO2


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nanorods might have produced the higher specific capacitance at the electrodeposition time of 1 min, whereas the longer deposition time of 3 min might have produced the larger total capacitance. In both cases, the CV curves in the range of 5–50 mV/s exhibited rectangular shapes, without any specific peaks (Figure 3a and 3b); this observation verified that no unexpected chemical reaction occurred during the CV measurement, and the test sample behaved like an ideal supercapacitor. Furthermore, Figures 3c and 3d show the EIS curves based on the galvanostatic measurements obtained for layers with electrodeposition times of 1 and 3 min, respectively. Based on these measurements, ESR (Equivalent Serial Resistance) obtained at electrodeposition times of 1 and 3 min were approximately 1.44 Ω and 2.03 Ω, respectively. These values were sufficient to supply an independent platform for consecutive TWs because the supercapacitor would not significantly affect the generation of energy via the transport of charge through the nickel electrodes under transient combustion.

2.3. Working Principle and Realization of TWs in Hybrid Supercapacitor Charging System The hybrid supercapacitor charging system was composed of supercapacitors and a neighboring chemical fuel layer adjoining one electrode (Figure 4a). In this hybrid system, TWs were initiated by contact with a Joule-heated nichrome wire, which was placed on one side of the solid fuel layer. During the propagation of combustion waves, the overall energy generated by the TWs induced a temperature difference between the two nickel electrodes in the capacitors, while the moving reaction front developed a temperature gradient between the starting and ending positions of the combustion waves inside one of the nickel electrodes under the solid fuel layer.



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The temperature gradient along the horizontal direction induced charge transport from the starting position to the moving reaction front via TWs, while the temperature gradient along the perpendicular direction rapidly formed a potential difference between two electrodes in the supercapacitor, based on the thermoelectric effect, because MnO2 is an effective thermoelectric material, with a high Seebeck coefficient, particularly in the high-temperature regime.34 The extended scheme shown in Figure 4a explains the charge transport in the layered structures when TWs are generated. The red plus and blue minus symbols indicate positive and negative thermoelectric potentials, respectively. The potential gradient and corresponding electric field are rapidly formed between the electrodes and electrolyte, attributed to the temperature gradient. The negative potential at the boundary with the electrolyte electrostatically drives and attracts sodium ions from the electrolyte to the surface of the MnO2 layer, which serves as the active material. Next, the intercalation or adsorption of sodium ions from the electrolyte successively reduce the MnO2 to MnOONa with the electrical energy.29 Furthermore, potential differences between the anions and cations in the electrolyte gel result in the formation of an electrical double layer on the MnO2 surface. When the hybrid charging system is consumed, the stored energy in the active material is released with the separation of electrons, MnO2, and Na+, as well as the consumption of the electrical double layer.

On the other hand, the solid chemical fuel layer was derived from collodion. After the solvent in the collodion was completely evaporated, a thin solidified nitrocellulose layer was obtained. Next, a solid fuel film of nitrocellulose with an optimal size was physically cut out with the fabricated supercapacitor and carefully added to one of the nickel electrodes (1st snapshot in Figure 4b) (see the Experimental Section “Integration of Chemical Fuel and Capacitor Platform for TWs”). To


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initiate the TWs, the Joule-heated nichrome wire was gently made to contact one end of the nitrocellulose layer. Self-propagating combustion waves rapidly moved from one side to the other side along one nickel electrode, while simultaneously preserving the reaction front position (2nd– 5th snapshots in Figure 4b). After the complete generation of TWs, the supercapacitor in the hybrid charging system was retained, without any specific change to its physical structure (6th snapshot in Figure 4b).

2.4.

Generation

and

Accumulation

of

Energy

from

TW-driven

Hybrid

Supercapacitor Charging System Figure 5 shows an evaluation of the transient voltage signal for a single TW and the accumulation of energy via multiple TWs in the hybrid supercapacitor charging system. First, Figure 5a shows a representative voltage signal for the hybrid system with the MnO2 layer fabricated with an electrodeposition time of 1 min. According to previously reported studies, the typical voltage traces from TWs exhibit a pulse or sinusoidal shape.10-11 On the other hand, because the TWs in this system could directly charge the supercapacitor under the solid fuel layer during the propagation of combustion, the change in voltage became more gradual, resulting in an increase in the time for the generation of voltage, corresponding to a long discharge time. In reality, the total discharge time in this hybrid system was approximately 10 s, which was 20 times greater than the value of

Thermopower Wave-Driven Hybrid Supercapacitor Charging System

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