Designing flexible CNT/CNF films with highly light-absorbing for solar energy harvesting: Seawater desalination, photothermal power generation and light- driven actuators

Green cellulose- based solar light capturing composite films made of carbon nanotubes (CNT) and cellulose nanofibrils (CNF), with high photothermal conversion performance, were designed and developed. Thanks to the strong electrostatic interactions between carboxyl- rich CNFs (obtained from 2, 2, 6, 6-tetramethylpiperidine-1-oxyl (TEMPO) mediated oxidation), and cationic-modified CNTs using cetyltrimethylammonium bromide, the resultant CNT/CNF composite (FTC) films are uniform and exhibit excellent mechanical properties. More importantly, the effect of CNT/CNF ratio in the FTC films on their photothermal conversion capacity and coefficient of thermal expansion (CTE) was investigated. The FTC film with 50 % CNT incorporation shows both high photothermal conversion capacity and high negative CTE, and its application for solar energy harvesting was explored, including seawater desalination, photothermal power generation and light- driven actuators. The FTC film exhibits high light-induced desalination capability, with water evaporation rate reaching 1.23 Kg m-2 h-1 at an irradiance of 1 kW m-2 (evaporation efficiency of 84.5 %) and ion concentrations in purified water decreasing by 4 orders of magnitude. The FTC film can also be assembled into photothermal power system, achieving an output voltage of 310 mV at an irradiance of 3 kW m-2. With the assistance of a boost module, it can be used to re-charge a cell phone. A light-driven FTC/polytetrafluoroethylene (PTFE) double- layered actuator was also designed. Benefiting from the negative CTE of the FTC film, the actuator has a high sensitivity and achieves a bending curvature of 3.3 cm-1 at an irradiance of 0.74 kW m-2. This study presents a scalable, yet simple strategy to design green/sustainable cellulose- based solar energy harvesting devices that can have multiple applications.