Soft Materials for Wearable/Flexible Electrochemical Energy Conversion, Storage, and Biosensor Devices

Abstract

Next-generation wearable technology needs portable flexible energy storage, conversion, and biosensor devices that can be worn on soft and curved surfaces. The conformal integration of these devices requires the use of soft, flexible, light materials, and substrates with similar mechanical properties as well as high performances. In this review, we have collected and discussed the remarkable research contributions of recent years, focusing the attention on the development and arrangement of soft and flexible materials (electrodes, electrolytes, substrates) that allowed traditional power sources and sensors to become viable and compatible with wearable electronics, preserving or improving their conventional performances.

Keywords: embedded mediators; enzymatic biofuel cells; flexible lithium batteries.; flexible polymer electrolyte fuel cells; microbial fuel cells; soft materials; wearable devices.

Figure 1

Exploded view of fuel cell components in an air-breathing configuration. Reproduced from reference [93] under CC BY 4.0 license.

Figure 2

Schematic illustration of (a) sketch of the structures and SEM photos of the nanofibers, (b) simulated cross-sections, (c) simulated molecular structures, (d) digital photographs of the Zn/Co-based nanofibers film before and after direct carbonization at 800 °C. Reproduced from reference [95] under CC BY 4.0 license.

Figure 3

Digital photos of (a) a flexible solid-state zinc–air battery (ZAB) with three modules in series; (b–f) 75 red LEDs with “SCNU” shape powered by the ZAB of the panel (a) under bending angles of 180°, 135°, 90°, 45°, and 0°, respectively; (g–l) ZAB under bending angles of 0° powering a 3V LED breastpiece, ZAB as a wearable bracelet, ZAB powering a 3V LED breastpiece at different bending radii of 4, 3, and 2.5 cm, respectively. Reproduced from reference [96] under CC BY 4.0 license.

Figure 4

(a) Digital photo of a Flexible LIB cell that illuminates a blue Organic Light Emitting Diode (OLED) at 4 V; Inset: flexibility of the LIB cell under bending; (b) galvanostatic charge and discharge curvefor the flexible LIB cell. Reproduced from reference [138] under CC BY 4.0 license.

Figure 5

(a) Sketch of the typical components of a flexible Zn–carbon battery; (b) SEM micrographs of MnO₂-cathode containing functionalized MWCNTs (left) and Zn particulate anode (right); (c) Zn–C discharge curves for different cathodes at the same load (8.6% w/w). Panels (b,c) are reprinted from reference [146], copyright (2013), with permission from Elsevier.

Figure 6

(a) Fabrication process of lithium and sulfur electrodes from carbon fabrics (CF); (b,c) digital (scale bar = 3 cm) and SEM (scale bar = 3 µm) photographs of CuCF and NiCF electrodes; (d) CF, NiCF, CuCF resistances and (e) tensile stress–strain curves. Reproduced from reference [172] under CC BY 4.0 license.

Figure 7

Examples of flexible supports for wearable EFCs that can be easily found in the lab: (a) textile fabric; (b) a polymeric filter disk; (c) a medical skin patch; (d) a filter paper disk; (e) a flexible transpirant plastic sheet. Many other flexible/stretchable supports were used in the literature.

Figure 8

Possible architectures for skin EFCs with reservoir: (a) one reservoir, conductive; (b) two separated reservoirs with dielectric separation for the ionic current through the skin—a porous transpirant microlayer at skin/reservoir interphase may be included.

Figure 9

The four components for the advanced formulation of printable inks as the main soft material for wearable EFCs. The role and specific properties affected by each component are reported.

Figure 10

Digital photos of the PANI/CNT/EVA electrode: (a) mass loading, (b) twisting and binding, stretched and released. (c) Polarization curves of the PANI/CNT/EVA electrode under different bending angles. (d) Polarization curves and (e) discharge curves of solid-state symmetric supercapacitors at different scan rates and current densities. (f) Digital photo of the PANI/CNT/EVA flexible symmetric supercapacitors powering a LED. (g) Ragone plots PANI/CNT/EVA supercapacitor. Reproduced from reference [259] under CC BY 4.0 license.