Fully Stretchable Textile Triboelectric Nanogenerator with Knitted

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Fully Stretchable Textile Triboelectric Nanogenerator with Knitted Fabric Structures Sung Soo Kwak, Han Kim, Wanchul Seung, Jihye Kim, Ronan Hinchet, and Sang-Woo Kim ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b05203 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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Fully Stretchable Textile Triboelectric Nanogenerator with Knitted Fabric Structures Sung Soo Kwak1,⊥, Han Kim1,⊥, Wanchul Seung1, Jihye Kim1, Ronan Hinchet1, and Sang-Woo Kim1,2,* 1

School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU),

Suwon 440-746, Republic of Korea, 2SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea

Corresponding Author *E-mail: [email protected] (S.-W. Kim)

ACS Paragon Plus Environment

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ABSTRACT

Harvesting human-motion energy for power-integrated wearable electronics could be a promising way to extend the battery-operation time of small low-power-consumption electronics such as various sensors. For this purpose, a fully stretchable triboelectric nanogenerator (STENG) that has been fabricated with knitted fabrics and has been integrated with the directly available materials and techniques of the textile industry is introduced. This device has been adapted to cloth movement and can generate electricity under compression and stretching. We investigated plain-, double-, and rib-fabric structures and analyzed their potentials for textilebased energy harvesting. The superior stretchable property of the rib-knitted fabric contributed to a dramatic enhancement of the triboelectric power-generation performance owing to the increased contact surface. The present study shows that, under stretching motions of up to 30 %, the S-TENG generates a maximum voltage and a current of 23.50 V and 1.05 µA, respectively, depending on the fabric structures. Under compressions at 3.3 Hz, the S-TENG generated a constant average root-mean square power of up to 60 µW. The results of this work show the feasibility of a cloth-integrated and industrial-ready TENG for the harvesting of energy from human biomechanical movements in cloth and garments.

Keywords: knitted fabric; triboelectric nanogenerator; stretchable power source; self-powered system; wearable electronics


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The rapid progress of wearable electronics has initiated a paradigm shift in the lives of humans. The demand for wearable energy sources is increasing, and this is greatly relevant to the Internet of Things (IoT) that enables access to environmental data in real time.1 However, due to the lightweight nature, flexibility, and integration requirements of wearable electronics, conventional lithium-ion batteries are not very suitable because of their limited capacity, lesser flexibility, lesser stretchability, and lesser biocompatibility. Consequently, the development of sustainable and self-powered systems is highly desirable for the wearable electronics of the future. Biomechanical energy is one of the most common and abundant forms of mechanical energy that can be generated on a daily basis by humans. Scavenging the wasted biomechanical energy from human motions could solve the wearable-electronics power-supply issue, and this has resulted in the great amount of current corresponding attention. 2,3 The development of triboelectric nanogenerators (TENGs), based on the coupling effects between triboelectrification and electrostatic induction through contact separation or relative sliding motions, has provided a highly feasible, affordable, and practical approach for the realization of mechanical-movement-based self-powered systems.4–9 Until now, TENGs have been investigated for the conversion of external mechanical energy into electrical-power sources and have been developed to enhance power-generation performances through processes like increases of the contact area, surface-morphology modification, and surface-treatment functionalization.10,11 These processes, however, have not fully satisfied the wearable-electronics energy requirements, and until now, the TENGs designed to harvest biomechanical movements are still suffering from lacked stretchability and durability.12,13 Lastly, the recent progress in mechanical-energy harvesting has led to the investigations of the TENGs that are based on fabric structures for harvesting energy from various kinds of human


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motions. Particularly, a fabric-type TENG is an ideal design for the harvesting of the biomechanical energy from cloth deformations and frictions.14–16 Still, a greatly improved stretchability, optimized device structure, and enhanced output performance are required to increase the biomechanical-energy harvesting of the TENG and its practical applications. It is well known that a knitting technique as one of the most representative clothing technologies can make the high stretchability and the aesthetic property of textiles depending on the knitting patterns.17–19 From the overall knitting properties, the knitted-fabric characteristics are determined by the fiber selection and the fabric-surface properties that can be controlled by the choice of the basic loop of the fabric and the geometrical configuration of the yarn.20 As for the power-output performance of TENGs, to increase the amount of the induced triboelectric charges, materials with a larger electron-affinity difference can be selected, or the material contact area can be enlarged.21–23 The present paper delivers a report on a fully stretchable TENG (S-TENG) that has been fabricated with knitted fabrics and that shows a high stretchable property even without the intrinsic stretchability of the constitutive fibers and the large contact area that are due to microscaled knitted yarns. To address both the durability and industrial-compatibility issues of previous studies that are because of the complex and fragile nanostructures,3,24,25 a much simpler constitutive fiber structure that is composed of triboelectric materials is considered here. The materials are strong and already available in the textile industry, while the fibers are more robust. As a result, the proposed highly durable devices were created using the available industrial machinery. For the selection of the best device structure, the relationship between the stretchability and the contact area of the diverse knitted structures was first analyzed, and the best fabric structure that optimizes the S-TENG power output was selected. The knitted


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structures showed a high stretchability that enables the S-TENG device to undergo cloth stretching up to a 30 % strain in the transverse direction. The electrical-output performance of the S-TENG was systematically characterized through an investigation of the contact-area effects depending on the stretching conditions and the knitted structural designs. The measured alternating-current (ac) voltage and current peaks reached 23.50 V and 1.05 µA, respectively, for a 10 × 10 cm² device under a 30 % strain at 1.7 Hz. These performances correspond to a generated root-mean square (RMS) voltage and current of 5.30 V and 29 µA/m², respectively, thereby representing the power-supply ability of the proposed S-TENG device.

RESULTS AND DISCUSSION For the investigation and fabrication of a robust S-TENG that is compatible with the current textile industry, the material selections were carefully considered, and they are based not only on the available materials in the textile industry, but also on the triboelectric series (Figure S1) that indicates whether a material is triboelectrically positive or negative. The electrical conductivity of silver (Ag) is high and it is neutral in the triboelectric series. Besides, polytetrafluoroethylene (PTFE) is not only widely used in the textile industries, but it is also very negative in the triboelectric series. For these reasons, Ag and PTFE were selected as the electrode and negative triboelectric materials, respectively.26–31 The S-TENG is constructed as a double-arc-shape device of a 10 × 10 cm² size, and it consists of the knitted PTFE and Ag fabrics on the top and bottom layers with an Ag electrode in the middle, as shown in Figure 1a and S2. Both yarns, the PTFE (198 denier) and the Ag (280 denier) yarns, were knitted into fabrics with a gauge of 12


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(needle/inch) using raw materials (produced by the Swicofil AG and X-Silver industrial companies, respectively). Hereafter, such technical knitting-technique nomenclature will be used for characterizing the STENG device, as shown in Figure 1b. To investigate the exact loop dimensions of the knitted fabrics, field emission scanning electron microscopy (FE-SEM) measurements were carried out, as shown in Figure 1c. The lateral dimensions of the unstretched loop of the PTFE plain-knitted fabric pattern show a wale width and a course height of approximately 1520 µm and 2660 µm, respectively, and a yarn diameter of 300 µm. To show the effect of the knitted structure on the fabric’s mechanical behavior and to determine the suitable knitting design that maximizes the stretchability and the contact area, three different types of knitted structure were prepared. The following three structures that are widely used in the commercial textile industries were chosen: plain knit, double knit, and rib knit. Then, fabrics with differences in the surface properties such as the morphology, roughness, density, and stretchability were fabricated. Tilted and top-view images of the plain-, double-, and rib-knitted fabrics are shown in Figures 1d-f. In the experiments, all of the loop dimensions for the three different knitted structures are varied due to the different loop connections (see Table S1). The yarn integrity enables the loops to deform with interdependent relations and this allows the knitted fabrics to express excellent stretchability. It is important to know the stretching and recovery characteristics of the fabrics depending on the knitting structures in a determination and attainment of the maximum stretching of the STENG device, and consequently, the future practical applications are extended.32 Photo images of each knitted structure under stretching tests are shown in Figure S3. All of the knitted fabrics were fabricated using the same gauge (12-gauge) to characterize both the stretching and surface properties. First, the variation of the stretching property of each knitted structure is different. The


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PTFE plain-, double-, and rib-knitted fabrics could recover their original shape after approximately 10, 20, and 30 % strains in the transverse direction, respectively. The results demonstrated that the rib-knitted fabric distinguishes itself for the greatest stretchability in the transverse direction. Simple geometrical models of the plain-, double-, and rib-knitted fabrics are depicted in Figures 2 to show a comparison of their structures during the stretching motions. In the case of the plain-knitted fabric, the loop head contacts the legs of the adjacent loops. Although it is possible to extend the plain fabric in both the wale and course directions,33 its extensibility is lower than those of the double and rib structures, and this is because the contacts between each loop are maintained even during the stretching motions. Therefore, compared with its original state, its potential in the increasing of the contact surface during the stretching is lower, leading to almost the same poststretching contact surface. The experiment results showed that the PTFE plain-knitted fabric can endure approximately 10 % of strain in the transverse direction and recover itself with no plastic deformation, as shown in Figure 2a. Meanwhile, the double-knitted fabric is much more stretchable, and it is worth noticing that its high stretchability is a product of its double-layered structure, which is composed of purl- and knit-side layers. The region between both layers, called the middle region, exists for the purpose of not only the connecting of the loops, but also the enhancement of the stretchability. Indeed, the middle region is free from the other loops at rest, while the contacts are tied up with each other in the case of the plain-knitted fabrics. In the results, the PTFE double-knitted fabric showed approximately 20 % of stretchability in the transverse direction, which is larger than that of the plain-knitted fabric, as shown in Figure 2 (b). The schematic image of the stretched rib-knitted fabric is depicted in Figure 2 (c). The rib-knitted fabric is also composed of purl- and knit-side layers and includes a middle region. But, in comparison with the double-knitted fabric of Figure 2 (b), the


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rib fabric is composed of two loops on each side, and both sides overlap while the middle region is folded and hidden at rest. As a consequence, the rib-fabric stretchability is even larger, and the potential in terms of the increasing of the contact surface during the stretching is higher. As a result, the rib-knitted fabric stretched up to approximately 30 % of the strain in the transverse direction, and this represents the most stretchable knit structure in the samples of the present study. For an improved understanding of the different fabric properties regarding the triboelectriccharge generation, calculations of the contact area of the knitted fabrics depending on the morphology and the stretching property were performed. Here, the proposed concept of an enlarged-area ratio represents the ratio of the stretched elemental loop area to the unstretched planar yarn area using the cross-sectional view. For the estimation of the enlarged-area ratio, a simplified schematic image of the elemental loop of the rib-knitted fabric is depicted in Figure 2d. The contact and noncontact regions were distinguished from each other, and the dimensions and shapes were considered to obtain the quantitative values. Then, the calculations of the enlarged-area ratio were carried out by integrating the elemental loop areas and the planar yarn areas over the whole fabric, as shown in Figure S4 and Table S2. The values of the enlarged-area ratio depending on the strain are plotted in Figure 2e. An increasing trend, where the contact areas of all of the experimented fabrics were increased with the applied strain until a different elastic-deformation limit was reached, became evident. In addition, greater enlarged-area ratios are evident in the sequential order of the rib-, double-, and plain-knitted structures with the values of 2.39, 1.77, and 1.27 at the maximum stretched state, respectively. Notably, since the properties of the fabrics such as the yarn density and the structural dimensions differ between the


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knitted structures even though all of the knitted samples were knitted using the same gauge, the enlarged-area ratio varied with each knitted structure. In another approach, the contact area that is shown in Figure 2f and is based on the top-view photo images of the stretched knitted fabrics (Figure S3) through the differentiation of the black and white pixels representing the empty parts and the PTFE, respectively, was extracted. The contact area was calculated by multiplying the total stretched area of the fabric by the PTFE pixel density for various strains (see Table S3). Although the PTFE-pixel density was decreased in the unit area during the stretching, the total contact area was increased due to the increased total surface (Figure S5). In the results, the trends of the calculated total contact areas as a function of the strain increased for all of the plain-, double-, and rib-knitted fabrics, which is similar to the trends of the enlarged-area ratios. At the maximum stretched states, the contact area reached the maximum values of 108, 163, and 180 cm2 for the plain, double, and ribbed fabrics, respectively. It became evident that the mechanical and surface properties of the ribknitted fabric are not only superior compared with the plain and double fabrics, but it is also implied that the potential of the rib-knitted fabric for the generation of triboelectric charges is greater. The working principle of the S-TENG is based on the coupling of the triboelectric effect and the electrostatic induction.34–36 Because the PTFE-fabric electron affinity is high, it becomes negatively charged after it contacts the conducting lower electron affinity of the Ag fabric. Note that the amount of generated charges varies with the knitted structures due to their different surface properties, such as the morphology and the contact area. The variation of the gap between the PTFE and the Ag fabrics under the driving of the external forces induces an electrical-potential difference between the electrodes, which drives the electrons to flow through


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the external load. Therefore, the repeated contact and separation of the PTFE and the Ag fabrics generates AC signals.31,37 Figure 3 shows the fabrics under the different stretching-strain levels 10, 20, and 30 % in the transverse direction. Upon the commencement of the stretching of the fabric, the top, middle, and bottom knitted layers became closer until contact was made, as shown in Figures 3a, 3b, and 3c, generating the first electrical pulse, as shown in Figures 3d and 3e, or two small pulses if the top and bottom layers were not exactly synchronized. Here, the contact surface of the rib-fabricstructured S-TENG is higher because of its particular double-zig-zag knitted structure that generates a higher total triboelectric-charge amount, and so a higher voltage peak is reached compared with the other fabrics. Then, with further stretching, the contact area between each part was increased to a greater extent, generating more triboelectric charges and a second and broader voltage peak. Again, the rib fabric here can extend more than the other fabrics, enhancing its charge generation, whereby a higher voltage peak can be generated. When releasing the force, the reverse process occurs similarly. Because of the different knitted structures, the fabric contact areas are different and show different electrical peaks at the point of contact. In addition, because of their different stretching abilities, a discrepancy is shown during the stretching, as shown in Figure S7, leading to different overall performances. As a result of this mechanism, the proposed S-TENG can operate in the following three modes: first, by contacting each layer; second, by stretching the layers; and finally, a contact and stretching through the combining of these two modes. Thus, this design is very well integrated and adapted for the harvesting of all textile movements. To investigate the surface effect of the knitted fabrics,11 the surface-charge density was first calculated using analytical equations, and this was followed by the performance of finite element


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method (FEM) simulations of the open-circuit (OC)-potential distribution for the three different S-TENGs using the COMSOL Multiphysics software (Comsol, U.S.A.). Under the OC conditions, the electrical-potential difference (open-circuit voltage: ܸை஼ ) that is generated by the vertical contact and separation of the S-TENG layers is defined as follows:38

V୓େ =

(1)

஢∙୶ கబ

,

where σ is the surface-charge density, x is the gap distance between the middle electrode and the triboelectric layers, and ߝ଴ is the vacuum permittivity. Since the VOC is proportional to the surface-charge density, it was expected that the electrical performance could be enhanced under sufficient friction by the enlarging of the effective contact area.39–40 By facilitating contact between a PTFE film and the Ag using Equation (1), the surface-charge density of the triboelectric PTFE-textile layer at approximately 220 µC/cm2 was estimated. Then, using this value, the triboelectric effects among the three different S-TENGs were simulated and compared. Figure 4a presents the cross-sectional views of the OC electrical-potential distribution across the plain, double, and rib S-TENGs after the contact and separation. The simulated electricalpotential differences between the electrodes of the three S-TENGs were plotted as a function of the strain in Figure 4b. The potential differences that were generated by the plain, double, and rib S-TENGs are 41.2 V, 45.6 V, and 50.6 V, respectively, when they were stretched up to their maximum rates of 10, 20, and 30 %. During the stretching beyond 10 %, the electrical potential that was generated by the double S-TENG is higher than that of the plain device because of the enlarged contact area. Furthermore, according to the same principle, the electrical potential of the ribbed S-TENG reached 50.6 V at the 30 % strain, which is superior to those of the other STENGs. Such an increase of the rib-fabric electrical potential is due to the increased total triboelectric charge that was generated, which is itself due to the combined enhanced surface at


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rest and the increased contact area under stretching. Both these attributes were derived from the particular knitted structure of the ribbed fabric given its superior performances. The measured output voltage (with a load of 40 MΩ) and current (with a load of 100 Ω) of the S-TENGs under the stretching conditions are plotted in Figures 4c and S6. The results show that the plain and double S-TENGs produced an output voltage and a current peak of 2.00 V and 0.09 µA and 12.25 V and 0.53 µA, respectively. Interestingly, the use of the rib structure led to a much higher increase of the output up to 23.50 V and 1.05 µA compared with the double fabric. These experiment results show a trend that is in sound agreement with the previous simulated data. Therefore, the selection of the knitted structure of the fabric highly boosted the generated total triboelectric charges and the output voltage by approximately 1170 %, which is critical for the achievement of the design of a high-performance wearable and stretchable TENG. Until now, most of the previous research studies on TENGs represented the electrical-output performances as the maximum voltage and current peaks.2,25,41 However, it is difficult to quantify the effective power generation of TENGs because it varies over time and in terms of the mechanical stimulus. Here, the RMS value was used to describe the average constant power that was generated by our devices. The calculated RMS output voltage and current with respect to the knitted structures and strain are plotted in Figures 5a and 5b. All of the experimented knitted fabrics showed an increasing RMS output voltage and current as the strain was increased, which is in sound agreement with the increasing trend of the peak-to-peak outputs (Figure S7). The plain- and double-knitted-fabric S-TENG devices generated an RMS output voltage and a current up to 0.90 V and 0.07 µA and 2.40 V and 0.20 µA, respectively. Concerning the rib-knittedfabric S-TENG device, the RMS-output voltage and the current are higher, with respective


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values of 5.30 V and 0.29 µA, confirming that the rib S-TENG is an excellent candidate for an efficient knitting structure for wearable mechanical-energy harvesting systems. Human motion occurs in various ways through combining, stretching, and compressions; therefore, apart from the characterization of the S-TENG device under the stretching mode in the transverse direction, additional measurements of the electrical performances were carried out in the vertical contact and separation modes (Figures 5c and S8). To investigate the reliance of the output power on the external load, the RMS output voltage and the current of the rib S-TENG were systematically measured under different resistance loads from 104 Ω to 109 Ω. The optimum impedance that maximized the power that was generated by the S-TENG is approximately 5 MΩ, and a maximum average RMS power of approximately 60 µW was measured in the contact–separation mode, as shown in Figure 5d. Moreover, to demonstrate the feasibility of the fabricated S-TENG as a self-powered energy source, commercial light-emitting diodes (LEDs) were integrated with sportswear in the form of a power-supply cloth. Here, 15 green LEDs were instantly illuminated in a series by a 10 × 10 cm² device during running, thereby illustrating the potential of human-motion energy harvesting for powering wearable electronics (Figure S9 and Video S1).

CONCLUSION In the present study, a highly stretchable and wearable knitted-fabric-based S-TENG has been demonstrated using industrial knitting techniques for the harvesting of biomechanical energy from human-body movements. Double-knitted fabric and rib-knitted fabric showed a much superior stretchability, with the latter showing an even greater superiority, and they can endure


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higher transversal strains up to 30 %. The selection of the knitted structure of the fabric highly boosted the generated total triboelectric charges and the output voltage by approximately 1170 %, which is critical for realizing a high-performance wearable and stretchable TENG. Under a 1.7Hz stretching movement, the S-TENG produced an RMS voltage of 5.30 V and an RMS current of 0.29 µA. Under compression, a constant average power of 60 µW could be generated, confirming that the rib S-TENG is an excellent candidate for wearable mechanical-energy harvesting systems. In the near future, it is expected that an integration of the proposed device with a full cloth will increase the generated power, leading to a realization of the powering of cloth-equipped wearable electronic devices through a utilization of continuous human-body movements.

METHODS Fabrication. The PTFE (dtex 220, Swicofil Company) and Ag (280 denier, X-Silver Company) were knitted into plain-, double-, and rib-patterned fabrics with a gauge of 12 (needle/inch) on an industrial knitting machine. The structure of the S-TENG is a double-arc shape of a 10 × 10 cm size. It consists of knitted PTFE fabrics for the top and bottom triboelectric layers, and knitted Ag fabrics are used for the electrode in the middle and on the back of the top and bottom triboelectric layers. Each PTFE knitted fabric and Ag knitted fabric of top/bottom layers is attached by sewing at regular intervals in a direction perpendicular to the stretching direction in order not to interfere with stretching deformation. The width of middle layer is slightly shorter than that of the top and bottom layers, and both ends of the three layers are fixed by sewing so that top and bottom layers are slightly bent to have an arc shape.


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Characterization. The Dead Pixel Test MFC Application software (version 1.0, Michael Salzlechner) was used to measure the density of the PTFE yarns from photo images. The voltage signals were measured and recorded using a Tektronix DPO3052 (Tektronix) oscilloscope with a Tektronix P5100A (Tektronix) voltage probe with a 40-MΩ input impedance. The current signals were measured using the SR570 low-noise current amplifier (Stanford Research) with an input impedance of 100 Ω at a caliber of 1 µA/V.


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Figure 1. Knitted-fabric-based fully stretchable TENGs with different knitted structures. (a) 3D image of the S-TENG structure. (b) Nomenclature of the knitted fabrics. (c) FE-SEM image of the fabric loops. Photo images of (d) plain-, (e) double-, and (f) rib-knitted fabrics with the insets of the magnified images.


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Figure 2. Stretching characteristics of the S-TENGs depending on the strain. Schematic images of the stretched structures of the (a) plain, (b) double and (c) rib fabrics at the maximum elastic deformation. (d) Schematic of the elemental pattern of the rib fabric. (e) Enlarged-area ratio and (f) contact surfaces of the plain, double, and rib fabrics depending on the applied transversal strain.


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Figure 3. Schematic images of the working principle of the different S-TENGs. Triboelectriccharge generation process during the stretching (a) plain-, (b) double-, and (c) rib-fabric-based STENGs. (d) Output voltage of the rib S-TENG under a 30 % strain in the transverse direction with a load resistance of 40 MΩ. (e) Single-period graphic of the output voltage generated in the different stretching and releasing steps, and the corresponding RMS output voltage.


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Figure 4. Simulation- and experimental-voltage trends depending on the structure of the STENG. (a) OC electrical-potential distribution that was generated by a plain S-TENG under a 10 % transversal strain, a double S-TENG under a 20 % transversal strain, and a rib S-TENG under a 30 % transversal strain. (b) Simulation results of the OC electrode-potential difference and (c) experiment results of the electrode-potential difference (40 MΩ load) generated by the plain, double, and rib S-TENGs under various transversal strains from 5 to 30 %.


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Figure 5. RMS electrical characteristic of the different S-TENGs under a transversal strain and a vertical compression. (a) RMS output voltage at a 40-MΩ load and (b) RMS output current at a 100-Ω load generated by the plain, double, and rib S-TENGs under various transversal strains from 5 to 30 %. (c) The RMS voltage and the current and (d) the average power generated by the rib S-TENG in a vertical contact mode at 3.3 Hz depending on the external-load resistance.


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ASSOCIATED CONTENT Supporting Information. Additional details include the triboelectric series of the common materials in the textile industry and the typical metals; the photo images of the S-TENG fabrics during the stretching from the 0 to 30 % strains in the transverse direction; a schematic of the enlarged-yarn contact-surface calculation; the contact area of the S-TENG fabrics depending on the strain; the S-TENG maximum current peak when it is stretched; the increasing output tendency of the rib-fabric STENG versus the strain; the output electrical performances of the S-TENG under the vertical contact and the separation actuations; and a snapshot of the 15 lit-up commercial green LEDs of the sportswear-integrated S-TENG on a runner. Movie S1 shows the power generation from the running motion. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (S.-W.K.) Author Contributions ⊥

S.S.K. and H.K. contributed equally to this work.

ACKNOWLEDGMENTS This research was financially supported by the Center for Advanced Soft-Electronics as the Global Frontier Project (2013M3A6A5073177) and the Basic Science Research Program


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(2015R1A2A1A05001851) through the National Research Foundation (NRF) of Korea Grant funded by the Ministry of Science, ICT & Future Planning, and the “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20174030201800).


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