Double-layer carboxymethyl chitosan/sulfoethylated cellulose/cellulose nanofiber foams with a photothermally enhanced ionic concentration gradient for hydrovoltaic energy harvesting.

The generation of electricity from water, which contains enormous amounts of energy, has attracted significant attention in recent years. However, the development of devices consisting of materials with the required mechanical stability, suitable structures, and wide ion concentration gradients for operation in direct contact with water remains challenging. Herein, we report the preparation of three-dimensional foam structures based on covalent cross-linking and freeze-drying using cellulose and chitosan derivatives as raw materials. Carbon nanotubes (CNTs), which served as the photothermal component, were uniformly dispersed among the cellulose nanofiber (CNF) to realize photothermally enhanced heterogeneous water-enabled electricity generation (HWEG). The output performance of the assembled HWEG was improved by tailoring the ion concentration gradient and widening the moisture gradient induced by photothermal conversion. Under solar irradiation of 1 sun, the HWEG delivered an open-circuit voltage and short-circuit current of 186 mV and 0.5 μA, respectively. Owing to the scalability and practicality of the device, the class of three-dimensional structural materials based on sustainable biomass developed in this study is suitable for use in hydrovoltaic energy harvesters.