Biomechanical Energy Harvesters Based on Ionic Conductive Organohydrogels via the Hofmeister Effect and Electrostatic Interaction

Abstract

The recent use of cryoprotectant replacement method for solving the easy drying problem of hydrogels has attracted increasing research interest. However, the conductivity decrease of organohydrogels due to the induced insulating solvent limited their electronic applications. Herein, we introduce the Hofmeister effect and electrostatic interaction to generate hydrogen and sodium bonds in the hydrogel. Combined with its double network, an effective charge channel that will not be affected by the solvent replacement, is therefore built. The developed organohydrogel-based single-electrode triboelectric nanogenerator (OHS-TENG) shows low conductivity decrease (one order) and high output (1.02−1.81 W/m 2 ), which is much better than reported OHS-TENGs (2− 3 orders, 41.2−710 mW/m 2 ). Moreover, replacing water with glycerol in the hydrogel enables the device to exhibit excellent long-term stability (four months) and temperature tolerance (−50−100 °C). The presented strategy and mechanism can be extended to common organohydrogel systems aiming at high performance in electronic applications.

Hydrogels as Soft Ionic Conductors in Flexible and Wearable Triboelectric Nanogenerators

“…[ 54 ] This indicates the matched internal resistance of the device was only 3.3 MΩ, which is tens or hundreds times less than that of common reported TENGs. [ 55–58 ] Similar results have been observed from other H‐TENGs as well, [ 59–64 ] with intriguing applications such as wireless real‐time communication, large capacitors charging, temperature sensing, as well as human motion monitoring. The stronger mechanical adhesion and better reliability between the triboelectric layer and hydrogel resulted in the better charge induction.…”

“…Apart from the formation of hydration ions, the added salts in PAM/PVA hydrogels have been reported to play more essential roles. Wu et al [63,82] have described the salting-out phenomenon caused by the Hofmeister effect that enabled hydrogels to form intermolecular hydrogen bonds; the electrostatic interaction resulted in the formation of sodium/lithium bonds. Thus, Na + and Cl − ions interacting with polymer chains ( Figure 5f) may produce a stable channel for charge transfer.…”

“…Given the emergence of organohydrogel-based TENGs with excellent anti-freezing and non-drying properties, the conductivity enhancement of organohydrogels has also been studied recently. Wu et al [63] have proposed the double network structure with sodium bonds based on the electrostatic interaction to construct stable charge channels in the organohydrogel. As a result, the conductivity of the PAM/PVA/NaCl organohydrogel in Figure 8c was much higher than that of the PAM/NaCl and PAM/PVA organohydrogel.…”

“…f) Schematic illustration of the synthesis process of PAM/PVA/NaCl hydrogels. Reproduced with permission [63]. Copyright 2021, American Chemical Society.…”

Hofmeister Effect and Electrostatic Interaction Enhanced Ionic Conductive Organohydrogels for Electronic Applications

“…This indicates again the dominant effect that contributes to the high conductivity of the AV/NaCl organohydrogel is no longer the Hofmeister effect but other factors, for instance, the sodium bonds described in our paper. [19] Therefore, it is important to systematically study the potential electrostatic interaction occurring in the hydrogel synthesis process. Since both lithium and sodium bonds can be formed by electrostatic interaction, [25] LiCl, NaCl, and KCl are introduced to the AV systems.…”