Liquid-metal-elastomer foam for moldable multi-functional triboelectric energy harvesting and force sensing

Abstract This article reports on a moldable soft porous composite of liquid metal alloy (LMA) and Ecoflex-0030 elastomer for the first time, where random pores in the composite behaves as tiny triboelectric nanogenerators (TENGs). The triboelectric foam produced a maximum peak-to-peak short-circuit current (I SC ) of ∼466 nA (charge density = ∼35 μC/m 2 ) and open circuit voltage (V OC ) of ∼78 V for a sample of size 5  cm × 5  cm × 1 cm, where the output current is ∼20% higher than previously reported triboelectric foams based on PDMS and lead zirconate titanate (PZT)/carbon nanotube (CNT) of equivalent area. In addition, surface texture further increases the foam's softness and enhance the charge generation leading to a 36% increase in triboelectric charge (charge density = ∼48 μC/m 2 ). A soft monolithic shoe insole is prepared, which produced a range of triboelectric responses in the order of μA in response to different modes of human motion. Upon jogging, the porous shoe insole with 3 wt parts of LMA in Ecoflex matrix, produce an instantaneous power of ∼2.6 mW (instantaneous power density = 13 μW/cm 2 ). In addition, the insole also shows capacitive response to deformation, enabling the sensor to perform in-motion force and weight measurement.