Toward Enhancing Wearability and Fashion of Wearable Supercapacitor with Modified Polyurethane Artificial Leather Electrolyte
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KeywordsArtificial leather Neutral electrolyte Wearable supercapacitor Fluorescence
Practically wearable, easily transferrable, and fluorescent artificial leather supercapacitor was fabricated by combining energy storage technology with leather garment industry, solving the intrinsic problem of wearing comfortability in conventional yarn and textile supercapacitors.
Polyurethane as an important artificial leather is modified to be ion conductive by the incorporation of ionic groups and non-hazardous sodium chloride. The modified polyurethane artificial leather serves as a polyelectrolyte simultaneously.
The intrinsically fluorescent artificial leather supercapacitor is easily transferrable from any arbitrary substrates to form various patterns, enabling multifunctionalities of practical wearability, fashion, and energy storage.
Wearable energy storage devices are a critical element in personalized wearable electronics [1, 2, 3, 4]. There is no doubt that textiles are most wearable due to their good comfortability and texture etc., which eventually arise from the micron size of polymeric filaments (< 5 µm) [1, 5]. When a bunch of such thin filaments are twisted to yarns and then further weaved/knitted to textiles, the softness as well as the concomitant comfortability and texture are well maintained. Although many yarn and textile supercapacitors [6, 7, 8, 9, 10, 11] have been developed and become one mainstream of wearable devices [12, 13, 14, 15, 16], this conventional form loses the comfortability and texture due to the incorporation of polyelectrolyte. As the thickness of polyelectrolytes (far larger than 100 µm) is much higher than the aforementioned comforting filament size (< 5 µm), these yarn and textile supercapacitors reported so far give rise to practical wearability challenges. Thus, the widely used form of yarn and textile supercapacitors fundamentally attributes to the limitation of insufficient wearability, which impose a considerable requirement on reforming wearable supercapacitors.
The reformed wearable supercapacitor should maximize wearability. This requires maximum reservation of traditional textiles. Inspired by the configuration of leather garment, we propose an artificial leather supercapacitor as an alternative approach. By the design of supercapacitor in the layer of artificial leather on a leather garment, traditional textiles underneath can be totally reserved. This achieves the same comfortability and wearability as of common leather garment. This makes the artificial leather supercapacitor an excellent reform for wearable supercapacitors.
Notably, compared with polyvinyl alcohol (PVA), polyurethane (PU) is widely used as an artificial leather. Nevertheless, pristine PU is not ion conductive and cannot work as an electrolyte. In this paper, PU is modified to be ion conductive by the incorporation of ionic groups in aqueous solvent, and non-hazardous sodium chloride (NaCl) is used to further improve the ionic transportation, avoiding the potential harm of acid to human. There are no functional groups on the backbone of the modified PU that restrain free sodium ions, thus providing decent ion conductivity. Moreover, PU is easily transferrable from arbitrary substrates and intrinsically fluorescent. These are not exhibited by the widely used PVA-based electrolytes and provide potential fashion garment application. As a proof-of-concept study, a supercapacitor sleeve is fabricated by using large carbon nanotube (CNT) sheet electrodes deposited with polypyrrole (PPy) and the modified PU as both electrolyte and artificial leather, which emits near-blue fluorescence and powers a light-emitting diode.
3.1 Modification of Water-Based PU and Fabrication of the Artificial Leather Supercapacitor
Carboxyl groups were introduced into the water-based PU (wPU), and various NaCl (0.025–0.25 M) and dyes (10 µm) were added into the resultant ionic wPU (iwPU) dispersion under vigorous stirring for 0.5 h. The as-mixed dispersion was dried at room temperature to form miwPU films. Then, CNT sheets were electrodeposited with PPy up to 1.2 mg at 0.8 V versus Ag/AgCl up to 5 min at 0 °C, in an electrolyte solution (30 mL) containing 0.1 M p-toluenesulfonic acid, 0.3 M sodium toluenesulfonate, and 15 µL pyrrole monomer. Besides serving as the suppliers of p-toluenesulfonate, they can stabilize the pH of the solution, which is a key factor in the PPy electropolymerization. Pyrrole monomers were purified by distilling before electrodeposition. Two PPy@CNT sheets were on each side of the miwPU polyelectrolyte film. Finally, two miwPU artificial leathers were deposited on these two sheets.
3.2 Electrochemical Characterization
4 Results and Discussion
4.1 Modification and Physicochemical Properties of PU Artificial Leather as a Polyelectrolyte
4.2 Electrochemical Characterization
Deformation stability is definitely required for practical wearing applications. Our supercapacitor experiences a series of deformation test, such as being folded at 0°, 45°, 90°, 135°, 180°, and twisted. All CV curves overlap almost completely during the whole deformation process (Fig. 3e), revealing the excellent device flexibility required for wearable electronics due to the good mechanical property of the PU gel  (Fig. S2). Moreover, another important feature for practical wearable applications is being impervious to water. By sprinkling waterproof spray onto the miwPU artificial leather supercapacitor, the CV curves are almost identical before and after water sputtering (Fig. 3f), suggesting a potential solution to all wearable devices with water-compatible polyelectrolytes.
4.3 Wear the Fluorescent miwPU Artificial Leather Supercapacitor Sleeve
Two to four supercapacitors are assembled both in parallel (Fig. S10a, b) and in series (Fig. S10c, d) for practical applications. The charge/discharge time and thus the overall capacitance increase linearly with the number of supercapacitors in parallel. Similarly, the overall capacitance linearly decreases with the reciprocal of the number of supercapacitors in series. The combination of good flexibility and scalability of supercapacitors validates the wearable application. We fabricate a supercapacitor sleeve by using in-series and in-parallel assemblies, which displays the fluorescence effect (Fig. 4d) and powers a light-emitting diode (Fig. 4e).
In summary, polyurethane artificial leather is modified with carboxyl groups and non-hazardous sodium chloride to serve as a polyelectrolyte and meanwhile maintains the intrinsic property of fluorescence effect. The artificial leather supercapacitor using sheet electrodes exhibits excellent flexibility, pattern diversity, scalability, and high compatibility with artificial leather industrial processing techniques. As a demonstration, a fluorescent supercapacitor sleeve is fabricated by using these supercapacitors to power a light-emitting diode, realizing the energy storage, fluorescence capability, and wearability. Inheriting from the great wearability of artificial leather garment, the artificial leather supercapacitor has no problem of the practical comforting wearability of textiles, thus providing a reform of the mainstream yarn/textile-based supercapacitors and creating considerable potential for more practical applications of wearable electronics.
The authors appreciate ZG Wang and TF Hung for experimental support. The Startup Funding of Harbin Institute of Technology (Shenzhen) (DD45001015), NSFC/RGC Joint Research Scheme (Project N_CityU123/15), the Science Technology and Innovation Committee of Shenzhen Municipality (JCYJ20130401145617276 and R-IND4903), City University of Hong Kong (PJ7004645), and the Hong Kong Polytechnic University (1-BBA3) supported this work.
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