Double-gradient-structured composite aerogels for ultra-high-performance moisture energy harvesting

Abstract

Currently, the introduction of a gradient structure in moisture-electricity generators (MEGs) has been shown to significantly improve their output performance. However, due to the limitations of material systems and methodologies, it is impossible to combine the gradients of ion density and hydrophilicity in one system, which is necessary to realize high-power and long-duration MEGs. Herein, aerogel-based MEGs with double-gradient structures were prepared via self-assembly and freeze-drying using single-walled carbon nanotubes (SWNTs) and polymer dendritic colloids. The biomimetic ‘core–shell’ structure with SWNTs on the outer surface of the polymer dendritic colloids enhanced the direct contact and interaction between water and SWNTs. The ultra-high ion density gradient (from −30.2 to +27.6 mV) formed by abundant SWNTs before and after surface modification drove the diffusion of carriers and improved the power of MEGs. Moreover, a hydrophilic gradient of the water contact angle from 32.1° to 110.8° was formed in the aerogel due to the different solubilities of various materials in dilute hydrochloric acid, ensuring the long-term stable transport of water and carriers. Based on the above-mentioned design, MEGs illustrated an industry-leading energy density (165.23 μW h cm⁻²), power density (32.59 μW cm⁻²), and duration (120 h) by adjusting their working environment. Density functional theory simulation and Kelvin probe force microscopy demonstrated the charge generation mechanism of MEGs. Besides, MEGs also illustrated potential applications in humidity sensing, low-power device power supply and seawater desalination, providing an inspiring approach for energy harvesting.