Portable green energy out of the blue: hydrogel-based energy conversion devices

Abstract

To alleviate the escalating global demands for electricity with a low carbon footprint, we can resort to a green energy source that is conveyed by tiny temperature or moisture gradients. A tremendous source of low-grade energy scatters around us and remains unutilized, which is why thermoelectric and hydrovoltaic devices were invented. Our review focuses on a growing trend of implementing hydrogel-based ionic thermoelectric systems and hydrovoltaic devices as they hold the promise of electric outputs that are several times higher than conventional solid-state inorganic counterparts. This is due to the molecular-level tailorable features of hydrogel polymers and their interactions with water and other functional additives, which provide an ideal platform for low-grade heat and water energy harvesting from fundamental and practical perspectives. This review is divided into three sections. We present working principles, engineering concepts, state-of-art designs, and urgent challenges for hydrogel-based (i) ionic thermoelectric systems; (ii) hydrovoltaic devices; and (iii) their hybrids.