Colorful fibers, as the basic building units for textiles and clothing, are widely used in daily life. Thanks to the industrial continuous preparation and dyeing techniques, kilometer-range color fibers were prepared using dyes containing various chemical chromophores, thus enabling large-scale commercial applications of colorful fibers. Recently, with the increasing popularity of wearable electronic products and the rise of the smart clothing concept, smart colors that can be changed on demand are urgently required for use in fiber and clothing to achieve various smart applications, including wearable displays, visual sensing, and adaptive camouflage. Unfortunately, in existing dyeing processes, the dyed fiber colors cannot be changed controllably because of the passive intrinsic properties of the dyes, leading to extreme difficulty in the continuous preparation of smart color-changing fibers and thus limiting applications of these colorful fibers.
Electrochromism offers many advantages of high controllability, low energy consumption, abundant material types and color changes, and thus provides an excellent strategy for the realization of smart colors. However, many shortcomings and difficulties still exist that restrict the practical application of electrochromic (EC) fibers. First and most importantly, because of the complex device structures and immature continuous processing technology, the EC fibers had to be hand-made in the laboratory, which led to limited fiber lengths (approximately 10 cm) that could not meet industrial requirements. Second, in long (e.g., meters in length) EC fibers, it is difficult to realize sufficiently fast electron transfer/ionic diffusion to guarantee color change uniformity because of the long transfer/diffusion pathways. Third, there is a lack of effective protection for the electrolytes and the other active layers in EC fibers. Therefore, direct exposure of EC fibers to air will produce poor environmental stability, which is disadvantageous for long-term practical use. Finally, the preparation method must be suitably generalized to construct various EC fibers based on different EC active materials to allow abundant color changes.
Recently, Prof. Hongzhi Wang led a research group at Donghua University, Shanghai, China, to realize the continuous fabrication of hundreds-of-meters-long EC fibers based on a parallel dual-counter electrode structure. The EC fibers exhibit both great color-changing performance and excellent environmental stabilities (e.g., mechanical, washing, irradiation, and thermal stabilities). The EC fibers can also be knitted into large-area smart color-changing textiles, and implanted into textiles with different patterns, thus demonstrating multiple promising applications, including wearable displays and active camouflage. This work was published in ACS Applied Materials & Interfaces with the title of “Continuously Processed, Long Electrochromic Fibers with Multi-Environmental Stability”.
Figure 1. Scalable preparation of multicolor EC fibers based on parallel dual-counter electrode structure.
The EC fibers were continuously prepared by constructing a set of equipment. Cu@Ni metal wires were used as the conductive electrodes, and the ITO dispersion was brushed on the conductive wire to form a uniform protective layer and further enhance the electrochemical stability of the metal electrodes. After drying, an EC precursor was dip-coated on the ITO layer and transferred into a heater to form a mechanically stable EC layer. Then, two thinner Cu@Ni wires were attached in parallel on two sides of the EC active layer. Finally, melted PE was used to envelop the active layers with a transparent protective layer to obtain EC fibers. Based on different EC materials, the EC fibers showed abundant color changes.
Figure 2. Color-changing uniformity of EC fibers over the long-range.
Most importantly, using the unconventional parallel dual-counter electrode structure, highly enhanced color-changing uniformity was achieved in long-range EC fibers because of the more uniform electric field distribution and fast electron transfer. Compared with the EC effects and the electric field simulations of the EC fibers with parallel dual-counter electrodes and single-counter electrode, the potential field distribution of dual-counter electrode structure was more uniform, thus guaranteeing the uniform color changes of long EC fibers.
Figure 3. Highly enhanced electrochemical stability of the EC fibers.
Figure 4. Environmental stabilities of the EC fibers.
Because of the effective dual protection of ITO and outer polymer layers, the electrochemical and environmental stabilities of the fibers were greatly improved. The ITO protective layer prevented direct contact and electrochemical interaction between the Cu@Ni metal wires and EC active layers. Therefore, the EC effects of the EC fibers maintained almost constant during 300 electrochemical cycles. Because of the protection of the polymer layer, the EC fibers showed stable color changes under different environmental conditions, such as mechanical bending, washing in water, radiation, etc.
Figure 5. Multifunctional applications of EC fibers.
Because of the highly improved EC controllability, uniformity and fiber lengths, the EC fibers were knittable and implantable for large-area smart color-changing textiles. The EC textile color changed to blue by applying a voltage of −1.5 V and recovered to gray when a voltage of 0 V was applied. By using the color changes of the EC textile, it demonstrates the promising potential for adaptive camouflage based on changes in the surrounding environments. In addition to the adaptive camouflage application, EC fibers with various color changes can be implanted/woven into textiles and used to form wearable displays, showing multifunctional applications including safety warnings, fashion clothing, and decorations.
Original link:
https://pubs.acs.org/doi/abs/10.1021/acsami.0c09589
(Source: Hongwei Fan at College of Material Science and Engineering)
