Novel Application Field of High-Voltage Electrostatic Flocking: Three-Dimensional Electrodes

Due to its high specific surface area and conductivity, electrostatic flocking technology demonstrates significant potential in manufacturing three-dimensional batteries. These three-dimensional electrodes can be used in fields such as supercapacitors, wearable devices, sensors, and other applications, characterized by high energy density and exceptional cycling longevity, offering a novel approach for producing high-performance electrodes.

[9,104]In 2008, Patrissi and his team employed electrostatic flocking to fabricate three-dimensional carbon fiber array cathodes (Fig. a), which remains the earliest reported application of electrostatic flocking technology in the field of three-dimensional electrodes [9,104]. The high electrolyte permeability cathode prepared by this process demonstrated higher power density and a broader power range compared to planar cathodes under all flow rate conditions, with particularly outstanding performance at the maximum flow rate of 200 mL/min.

Researchers Li et al. utilized electrostatic flocking technology to fabricate a bilayer composite electrode based on manganese dioxide (MnO₂) nanofibers (Fig. b) [9]. This electrode was embedded in a graphene/polyvinylidene fluoride (PVDF) composite coating surface. The symmetric supercapacitor based on this electrostatic flocking electrode demonstrated exceptional performance: an energy density of up to 12.1 Wh/kg, and an ultra-high cycling retention of 109.4% after 10,000 cycles. Compared to traditional doped electrodes, the electrostatic flocking MnO₂ electrode exhibited a larger area in the cyclic voltammetry (CV) curves, indicating a higher specific capacitance. Furthermore, the specific capacitance of this electrode remained relatively stable after 10,000 cycles (Fig. c).

Li et al. [104] combined electrostatic flocking technology with textile substrates to develop 3D stretchable fabric electrodes (Fig. d). These electrodes exhibited outstanding energy storage performance, with excellent elasticity and stability under tensile stress. The resulting 3D structure not only increased the electroactive surface area available for charge storage but also maintained its capacitance during repeated stretching, highlighting the compatibility of this technology with dynamic applications involving inevitable mechanical deformation. This ensures sustained performance in wearable electronics and flexible energy storage devices. Notably, electrodes fabricated using electrostatic flocking technology not only possess superior stretchability but also feature a high-friction surface, making them an ideal choice for textile electrodes in wearable sensors.

[8,54,106] Takeshita et al. successfully fabricated conductive electrodes by implanting silver-coated fibers into polyurethane cubes using electrostatic flocking technology, and applied them to the development of wearable electrocardiogram (ECG) monitoring systems (as shown in Fig. e) [8]. The electrode resistance exhibited gradient variations along the xyz directions. When using bulk web electrodes, no motion artifacts were observed even during minor body movements such as breathing. The measured ECG signals were accurate and entirely consistent with traditional medical detection results. Furthermore, the bulk electrostatic flocking electrodes developed in this study effectively addressed the discomfort issues caused by excessive skin contact pressure associated with conventional electrodes. Even under a contact pressure of 500 Pa, they continued to provide precise ECG data. By arranging electrodes and wires fabricated using electrostatic flocking technology on clothing (Fig. f), multi-lead ECG monitoring could be achieved [54]. More notably, as the length of the fiber-forming fibers increased within a specific range, motion artifacts in ECG testing were significantly reduced (Fig. g) [7].

a) Schematic diagram of the early method for preparing electrodes by electrostatic flocking; reproduced with permission. [105] Copyright 2008, The Electrochemical Society.
b) Schematic diagram of doped and electrostatic flocked manganese dioxide electrodes; reproduced with permission. [66] Copyright 2022, John Wiley & Sons.
c) Specific capacitance versus cycle number curve for the assembled supercapacitor (SSC); reproduced with permission. [66] Copyright 2022, John Wiley & Sons.
d) Schematic diagram of the interaction mechanism and structural formation process of the 3D stretchable fabric-based electrode; reproduced with permission. [104] Copyright 2020, Elsevier.
e) Resistance variation curves of CFE under compression along the x, y, and z axes; reproduced with permission. [8] Copyright ©2022, Author(s).
f) Schematic diagram of a wearable device for multi-lead ECG measurement; reproduced with permission. [54] Copyright ©2019, Author(s).
g) ECG signals measured by two types of flocked electrodes; reproduced with permission. [7] Copyright ©2024, Author(s).

References
[7] T. Takeshita, M. Yoshida, T. Kobayashi, Y. Takei, Sensors and Actuators A, 2024.
[8] T. Takeshita, M. Yoshida, Y. Takei, A. Uchi, A. Hinoki, H. Uchida, T. Kobayashi, Scientific Reports, 2022, 12.
[9] M. Lu, L. Ming, International Journal of Energy Research, 2022, 46.
[54] T. Takeshita, M. Yoshida, Y. Takei, A. Uchi, A. Hinoki, H. Uchida, T. Kobayashi, Scientific Reports, 2019, 9.
[104] X. Li, J. Wang, K. Wang, J. Yao, H. Bian, K. Song, S. Komarneni, Z. Cai, Chemical Engineering Journal, 2020, 390, 124442.
[105] C. J. Patrissi, R. R. Bessette, Y. K. Kim, C. R. Schumacher, Journal of The Electrochemical Society, 2008, 155.
[106] M. Liu, M. He, M. Liu, Y. Zhao, L. Dong, H. Wu, X. Liu, Z. Zhao, Y. Feng, D. Chen, SPIE Optical Engineering + Applications, San Diego, California, USA, 2014.