High-Performing and Stable Wearable Supercapacitor Exploiting rGO

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Letter

Highly performant and stable wearable supercapacitor exploiting rGO aerogel decorated with copper and molybdenum sulphides on carbon fibers Alessandro Pedico, Andrea Lamberti, Arnaud Gigot, Marco Fontana, Federico Bella, Paola Rivolo, Matteo Cocuzza, and Candido Fabrizio Pirri ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00904 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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ACS Applied Energy Materials

Highly Performant and Stable Wearable Supercapacitor Exploiting rGO Aerogel Decorated with Copper and Molybdenum Sulphides on Carbon Fibers Alessandro Pedicoa, Andrea Lambertia,b*, Arnaud Gigota,b, Marco Fontanab, Federico Bellaa,b, Paola Rivoloa,b, Matteo Cocuzzaa,c, Candido Fabrizio Pirria,b a

Politecnico di Torino, Dipartimento di Scienza Applicata e Tecnologia (DISAT), Corso Duca Degli Abruzzi, 24, 10129 Torino, Italy b Istituto Italiano di Tecnologia, Center for Sustainable Future Technologies, Corso Trento, 21, 10129 Torino, Italy c CNR-IMEM, Parco Area delle Scienze, 37a, 43124 Parma, Italy * corresponding author, e-mail: [email protected]

Abstract Herein the concomitant synthesis of a MoS2 and Cu7S4-decorated graphene aerogel is reported. The material is fully characterized and used as active material to coat carbon fiber electrodes for the fabrication of a fiber-shaped supercapacitor. The device provides excellent capacitance values warranting stable performance even at high bending angle conditions. Moreover, a photo-curable resin is selected as smart packaging material to overcome stability problems usually affecting this class of devices. Noteworthy, superior stability is demonstrated with a retention of almost 80% of the initial capacitance after one month. Flexible supercapacitors were also coupled with third generation solar cells to successfully demonstrate the fabrication of wearable, portable and integrated smart energy devices.

Keywords: supercapacitors, wire-shape, graphene, metal-dichalcogenides, wearable electronics

In the recent years, the development of wearable electronics (i.e. integrated sensors, flexible electronics, etc.) led to the birth of a new class of devices for textile industries, called “electronic textile” (e-textile).1 The obtained progresses pushed forward the research to supply the increasing necessity of proper energy storage systems. Standard batteries are not a suitable solution because they are bulky, have a short cycle life, are commonly not flexible, and are intrinsically complicated to be connected to energy harvesting devices to fabricate autonomous energy systems, i.e. the so-called portable power packs.2-4 Supercapacitors have 1 ACS Paragon Plus Environment

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been identified as promising alternative among the energy storage devices for wearable electronics. The simplicity of the working mechanism and the limited fabrication constraints with respect to batteries allow higher compatibility of this class of energy storage devices with wearable electronics.5 Indeed, several works in the literature demonstrate the possibility to integrate supercapacitors directly into smart textiles, by exploiting fiber-shaped configuration.6-8 In this work, we reported the fabrication and characterization of a highly performant fibershaped supercapacitor exploiting carbon fibers current collectors coated with graphene-based active materials coupled to a quasi-solid electrolyte. Carbon fiber bundle and more complicated yarns composed of carbon nanotubes have been recently reported as suitable support for the fabrication of lightweight, flexible and stretchable power systems.8-9 In order to further improve the electrochemical performance of the proposed electrode, pseudocapacitive materials can be exploited in addition to carbon-based materials allowing a synergistic effect between the pseudocapacitive materials and carbon-based electronic doublelayer capacitors (EDLCs).10 MoS2 and other transition metal-dichalcogenides (MoTe2, VSe2, ReS2, WS2) have recently attracted huge attention as highly performant materials in combination with graphene support for supercapacitor application.11-12 Moreover, the increasing demand of environmental friendly and cost effective active materials puts the spotlight on copper sulphides as promising sustainable semiconductor materials for energy applications, thanks to their low-cost preparation and their unique properties derived from copper deficiencies.13 Among them, Cu7S4 nanostructures emerged as competitive candidate materials for practical application in supercapacitors.14-15 For this reason, herein graphene aerogels containing homogeneously dispersed MoS2 and Cu7S4 nanostructures were optimized and tested as active materials for wearable supercapacitors applications. Physicochemical characterizations were performed to analyze the electrodes composition and their structural/morphological properties. Carbon 2 ACS Paragon Plus Environment

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fibers and NaCl/PVA gel were selected as current collectors and quasi-solid electrolyte, respectively. Once devices were assembled, electrochemical characterizations were performed by cyclic voltammetry and charge-discharge measurements. Bending tests were carried out to evaluate the flexibility of the as-fabricated wearable supercapacitors. Moreover, the fibershaped device was coupled to a third-generation solar cell (dye-sensitized solar cell, DSSC) included in a button to demonstrate the possibility of integrating the energy harvesting device with the storage one for wearable applications. Finally, a suitable polymer formulation was selected for the packaging of the assembled device: a UV-curable difunctional siloxane was chosen, in order to optimize the barrier properties, sealing time and mechanical stability. The graphene and graphene/metal-sulphide nanocomposite, on which the selected active materials are mainly based, were obtained by means of a one-pot hydrothermal synthesis.16 In particular, the graphene hydrogel decorated by Mo/Cu-containing nanostructures was synthetized by adding phosphomolybdic acid, L-cysteine and a Cu wire as Mo, S, and Cu precursors, respectively, to an aqueous graphene oxide (GO) dispersion. After freeze-drying, a hybrid graphene/MoS2/Cu7S4 (GMC) aerogel structure was obtained and used as active materials for electrodes preparation. The bare graphene aerogel was obtained by subjecting the sole GO slurry to the same above described procedure, in absence of other precursors. Figure 1 provides results from the FESEM characterization of the obtained hybrid material after the freeze-drying process. The porous 3D architecture of the material (Figure 1a) is typical of rGO aerogels16 and it is ideal for impregnation with the electrolyte during operation as supercapacitor electrode. Figure 1b-c-d provides a direct visualization of the morphology of the nanostructures decorating the aerogel, alongside with the elemental composition obtained by EDX analysis (Figure 1e-f). Cu/S-containing nanostructures exhibit a planar morphology with thickness of approximately 30 nm; micrometric agglomerates, with “desert rose” morphology, are also visible. As revealed by EDX analysis, micrometric Mo/Scontaining structures are present and they exhibit a clear nanostructuration of the surface. The 3 ACS Paragon Plus Environment


preliminary localized chemical information obtained by EDX was integrated with results from XRD and XPS characterization, in order to allow phase identification through structural and chemical arguments.

Figure 1. a) Low-magnification FESEM images of the obtained aerogel. False color FESEM images b) and c) showing the decoration of the aerogel with Cu and Mo containing structures. d) High-magnification FESEM image of the morphology of copper sulphide structures. The original grayscale images can be found in the supporting information. Representative EDX spectrum is provided for Mo-containing structures (e) and Cu sulphides (f).

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Figure 2a reports XRD spectrum of rGO/metal sulphide composite. The weak and broad peak located at 2θ ~28° is assignable to the (002) feature of rGO. The spectrum of the rGO/metal sulphide composite displays the typical features of the monoclinic phase Cu7S4 according to JCPDS card N° 23-958. Indeed, the peaks located at 2θ = 26.6, 29.8, 31.2, 34.1, 37.9, 46.8, and 48.9° correspond to the (16,0,0), (8,0,4), (18,2,1), (20,0,1), (10,10,3), (0,16,0), and (8,8,6) diffraction planes of Cu7S4, respectively.

Figure 2. a) XRD spectrum of the rGO/metal sulphide composite. XPS high-resolution scans of the b) C1s, c) S2p, d) Mo3d/S2s, e) Cu2p, f) Cu LMM regions of the photoelectron spectrum.



Interestingly, typical features of MoS2 nanostructures are not observable, suggesting an inhibition of the long range ordered arrangement of rGO and MoS2 layers, as already reported in literature.16 XPS characterization was exploited for two main goals: i) proof of the successful reduction of the GO, ii) identification of the metal sulphide phases decorating the aerogel. At first, a survey spectrum was acquired, confirming the chemical composition in accordance with EDX analysis (see Supporting Information). Afterwards, high-resolution scans (Figure 1b-e) were obtained, since they provide chemically sensitive information through shift in the binding energy of the detected peaks. Specifically, peak deconvolution of the C1s region of the photoelectron spectrum shows little contribution from oxygen-containing functionalities, proving the successful reduction of graphene oxide though the hydrothermal treatment (see supporting information for the C1s region of the starting GO powder). Concerning the identification of the nanostructured phases decorating the aerogel, the significant regions are reported in Figure 2c-f. The presence of sulphides is confirmed by deconvolution of the S2p region, where the two doublets at lower binding energy side (S2p3/2 161.5 eV – S2p1/2 162.7 eV; S2p3/2 163.1 eV – S2p1/2 164.2 eV) are compatible with copper and molybdenum sulphides in accordance with the literature,17-18 while the doublet located at ≈168 eV is due to oxidized forms of Sulphur.19 Concerning Mo-containing sulphide, further proof is provided by the Mo3d doublet (Mo3d5/2 229.3 eV – Mo3d3/2 232.4 eV), which corresponds to MoS2.11 The Mo3d doublet at higher binding energy (Mo3d5/2 232.5 eV – Mo3d3/2 235.7 eV) is ascribed to MoO3.20 The combined analysis of the Cu2p and Cu LMM Auger regions provides information for the identification of copper-containing phases through the evaluation of peak positions and the calculation of the modified Auger parameter (α = kinetic energy (Cu LMM) + binding energy (Cu3p3/2)).21


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In this case, the overall shape of the Cu2p and Cu LMM regions, the position of the Cu2p3/2 peak (932.5 eV) and the modified Auger parameter α = 1850.2 eV point to the presence of copper sulphide species.17, 20, 22 The obtained GMC material was then used for electrode preparation as schematically described in Figure 3a. The carbon fiber bundles selected as current collectors were dipcoated into the slurry obtained mixing the 3D GMC aerogel with poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as binder in aqueous solution and, after drying in air, assembled into parallel-wire device exploiting gel-polymer electrolyte both as separator and ionic medium (additional details can be found in the experimental section).



Figure 3. Scheme of the electrode fabrication process (a). Picture (b) and FESEM images (c-e) of the wireshaped electrodes. Picture (f) and FESEM image (g) of the assembled device.

Figure 3b-e depicts the FESEM images, collected with different magnifications, of the carbon fibers coated with the active material before and after assembling, together with some digital photographs. Finally, the picture and the FESEM cross section image of the assembled device are shown in Figure 3f-g. The electrochemical characterization of the flexible devices was conducted by means of cyclic voltammetry (CV) and galvanostatic charge/discharge measurements. The concentration of active material into binder solution was increased up to the maximum amount allowed by the fabrication approach. Indeed, we observed that for the dip-coating procedure, 10% of active material into PEDOT:PSS aqueous solution represents the maximum amount of filler in order to preserve the suitable viscosity of the slurry. By further increasing this value, the slurry starts to aggregate, thus inducing a peeling off effect on the coating layer due to the low adhesion. Figures 4a and 4b show, respectively, the CVs at 500 mV/s and the charge-discharge profiles at 500 µA/cm of the supercapacitors fabricated increasing the amount of GMC into the slurry from 0.1 wt.% up to 10 wt%. The specific capacitance of the device increases almost linearly by increasing the GMC concentration (see Figure S4). Figure 4c shows the characterization at different scan rates of the most performant device (10 wt.% of GMC). Moreover, considering that one of the advantages in using wireshaped device lies in the possibility to exploit the huge lengths of yarn in the fabrics (a shirt contains some kilometers of cotton wire) we decided to investigate the dependency on the performance of the device according to its length. It results that, even if in a short range of dimension due to the limitation of the homemade fabrication procedure, the electrochemical properties have a linear dependence on the length of the device, as shown in Figure S5. 8 ACS Paragon Plus Environment

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In order to gain a deep understanding of the improvement obtained by the addition of the different active materials into the slurry discretizing the various contributions, we characterized our devices from the simplest to the most complex configuration: a symmetric device made by carbon fibers alone, the fibers coated with PEDOT:PSS and then separately adding rGO, MoS2 and GMC. From Figure 4d it is clear how, at a fixed scan rate of 100 mV/s, CV allows to compare the different active materials, returning a higher value of specific capacitance scaling with the complexity of the active material coating. Taking a look to the results, the optimized device shows a specific capacitance two orders of magnitude greater than the starting device (from 0.14 up to 10.3 mF/cm). This boost can be linked to two different contributions: i) the increased area offered by the rGO-based slurry, as compared to the bare carbon fibers (increasing the active area, indeed, it is possible to allocate more charges in the electrical double layer, strongly improving the final capacitance of the device); ii) the metal-sulphides active sites present on the decorated rGO, providing Faradaic contributions to the final capacitance of the device. The obtained values are remarkable if compared with results previously reported in the literature concerning the storage properties per unit length of the wire (see comparison with literature in Figure 4e).23-33 26



Figure 4. CVs (a) and charge-discharge curves (b) of GMC supercapacitors varying the GMC concentration into the slurry; CVs varying the scan rate for the 10% GMC device (c), comparison of the 100 mV/s CVs for the different enrichment steps of the slurry (d). Histogram of the specific linear capacitance23-33 26 (e) and Ragone plot23, 26, 28-29, 31, 33-35 (f) compared to literature results.

Being the values of capacity obtained from charge and discharge curves strictly in accordance with the ones obtained from cyclic voltammetry, we decided to use charge-discharge method to estimate the maximum specific energy and power of our devices (see supporting information). The calculated specific energy and power are shown in Figure 4f (Ragone plot), where the excellent performance of our fiber-shaped device is compared with other wearable supercapacitors.23, 26, 28-29, 31, 33-35 10 ACS Paragon Plus Environment

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Figure 5. Bending (a) and time (b) stability of the wire-shaped supercapacitor. Pictures (c) and photocharge/discharge measurement (d) of the integrated DSSC/supercapacitor system.

In order to evaluate the performance of the proposed wire-shaped supercapacitor for its exploitation as flexible energy storage device, we characterize the electrochemical response of the device under mechanical deformation at various bending radii. Figure 5a shows the capacitance retention of the device at different bending angles. The capacitance remains close to 100% even with a complete fold of 180 degrees demonstrating remarkable bending stability, even when compared to literature results obtained by more complicated architectures based on carbon nanotube yarns.9 Moreover, considering the sealing problems intrinsically present in the fabrication of a wireshaped electrochemical device, a suitable resin was selected as packaging material. Indeed, this class of devices is usually affected by the evaporation of the liquid component of the gel electrolyte resulting in the fast degradation of the performance over time. The literature survey evidences that no aging study was never conducted for long period making difficult to evaluate the potential application of these devices in real condition. For this reason, we 11 ACS Paragon Plus Environment


propose the exploitation of a siloxane-based photo-curable resin, since it can provide - at the same time - three strong advantages: i) a strong sealing of the device due to the siloxane barrier, ii) suitable mechanical properties thanks to the hybrid siloxane/ethoxylated bulk, iii) a truly fast (≈1 min) deposition and curing process due to the rapidity of UV-induced crosslinking process,36 thus making this approach also suitable for large scale fabrication of hybrid energy devices. Thermogravimetric analysis (TGA) and dynamic mechanical thermal analysis (DMTA) (Figure S6 and S7) traces clearly show the excellent thermal stability (up to 400 °C) and mechanical properties (106/107 Pa) of the UV-produced sealing. The sealing is in the rubbery phase at room temperature, and two glass transition temperatures (Tg) can be seen in the DMTA trace: the first one (‒68 °C) is related to the siloxane segments while the second one (‒42 °C) is related to the ethoxy phase. These Tg values ensure that no phase transitions occur in the temperature operation range of the proposed energy device. Water vapor permeability tests led to a water-vapor transmission rate (WVTR) equal to 36.8 g m−2 (24 h−1), that represents a strong value for a barrier coating. The proposed approach allows to overcome the above cited problems achieving superior long-term stability (see Figure 5b): the fabricated devices retain almost 80% of the initial capacitance after one month. As comparison standard fabrication approaches (no sealing or scotch tape sealing) were implemented resulting in a complete performance degradation after 1 day (for open devices) and a period of 7 days (for scotch tape sealing). As a final point, since supercapacitors have recently been proposed as energy storage element for direct connection with energy harvesting counterpart to produce portable power pack,3 herein photovoltaics was selected as energy harvesting technology to demonstrate the feasibility of devices integration. Dye-sensitized solar cells (DSSCs), in particular, were chosen among third-generation photovoltaics thanks to their interesting high efficiency in indoor environment and their low cost. A self-powered system consisting of two commercial DSSCs (provided by Solaronix) connected in series and a wire-shaped supercapacitor was 12 ACS Paragon Plus Environment

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assembled as demonstrator (Figure 5c). The photovoltaic characterization of DSSCs is shown in Figure S8 in the supporting information. Figure 5d shows the photo-charge/discharge curve or the integrated system: when the sun simulator lamp is switched on, the DSSCs convert light energy and store it into the supercapacitor. After less than 3 s the supercapacitor reaches the potential of 1 V and, after switching off the light, it can be discharged by imposing a discharge current equal to the short circuit photocurrent measured during the illumination step. The resulting triangular profile confirms the good electrochemical performance of the energy storage element. In summary, the fabrication of highly performant and stable fiber-shaped supercapacitor was reported. The selection of rGO aerogel combined with metal-sulphide as active material provides high linear capacitance while the assembling procedure is responsible for the high bending and temporal stability of the proposed device. Moreover, the synthesis and technological steps involved appear highly appropriate for large scale production methods typical of textile industry. Finally, the direct integration of a wire-shaped supercapacitor with solar cells demonstrates the effective integrability of energy conversion and storage units in a lab coat in view of the fabrication of self-powered wearable devices.

Supporting Information Experimental details, additional morphological, thermal, electrochemical and photovoltaic characterizations. Acknowledgements Authors would like to thank Dr. Stefano Bianco and Prof. Elena Tresso from Politecnico of Turin for the useful discussions. A. Pedico and A. Lamberti contributed equally to this work. References (1) Weng, W.; Chen, P.; He, S.; Sun, X.; Peng, H. Smart electronic textiles. Angew. Chem. 2016, 55, 6140-6169. (2) Wen, Z.; Yeh, M. H.; Guo, H.; Wang, J.; Zi, Y.; Xu, W.; Deng, J.; Zhu, L.; Wang, X.; Hu, C.; Sun, X.; Wang, Z. L. Self-powered textile for Wearable electronics by hybridizing fibershaped nanogenerators, solar cells, and supercapacitors. Sci. Adv. 2016, 2, e1600097.



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High-Performing and Stable Wearable Supercapacitor Exploiting rGO

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