Triboelectric nanogenerators based on hydrated lithium ions incorporated double-network hydrogels for biomechanical sensing and energy harvesting at low temperature

Conductive hydrogels with flexibility, anti-freezing ability and functionality have garnered significant attention in developing of advanced triboelectric nanogenerators (TENGs). Herein, a tough, adhesive, and anti-freezing ionic double-network conductive hydrogel (referred as Glu/P(HEA-co-AA)-Fe/LiCl) was prepared by choosing glucan as the first network, Fe3+-COO- metal coordination crosslinked poly(2-Hydroxyethyl acrylate-co-acrylic acid) (P(HEA-co-AA) as the second network, which were crosslinked by multiple hydrogen bonds in the presence of hydrated lithium ions ([Li(H2O)n]+). The hydrogel exhibited excellent mechanical properties (toughness of 16.26 MJ/m3, tensile stress of 4.15 MPa, tensile strain of 1213%, compressive stress of 2.98 MPa under 80% compressive strain), strong adhesion (30.84 kPa for Ecoflex), and satisfactory conductivity (0.43 S/m) even at -18°C. The TENGs based on Glu/P(HEA-co-AA)-Fe/LiCl hydrogel (GP-TENGs) proved its applicability to display excellent electrical output performance (VOC = 182.42 V, ISC = 2.61 μA, and QSC = 63.37 nC) at 25°C, outstanding fatigue resistance (> 5000 consecutive taps), and long-term anti-freezing ability (a VOC of 163.58 V after one-month cryostorage). Furthermore, GP-TENGs can be utilized as biomechanical sensor with ultra-low pressure detection limit (0.016 kPa) and high-pressure sensitivity of 0.336 kPa-1 (0.016-1.6 kPa), capable of accurately detecting various human motions, also as energy harvesting devices to power small electronic devices with self-powered systems operating at -18°C. This study developed a strategy for building an anti-freezing hydrogel with high strength and strong adhesion, further expanding the potential applications of hydrogel-based sensors and energy harvesting systems in low temperature environment.