Construction of cellulose 3D network composite membrane supported by hydroxylated boron nitride with high surface charge density to achieve high-efficiency osmotic energy harvesting

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Ion selectivity and mechanical persistence are critical properties of membranes to harvest osmotic energy. Two-dimensional nanofluidic membranes have been studied to enhance those properties, whereas they are usually constrained due to unsatisfactory mass transfer performances. Herein, a ternary three-dimensional membrane (T-CNF/PVDF/BN-OH composite membrane, abbreviated as CPBN) with distinct mechanical strength and high surface charge density (−2.65 mC·m−2) for sustainable osmotic energy harvesting is designed. In CPBN, negatively charged TEMPO-oxidized cellulose acts as a scaffold and constructs a 3D network structure together with the hydroxylated boron nitride (BN-OH) via a self-assembly process. The affluent nano-constrained ion channels effectively enhance the controllability and effectiveness of ion transport. The abundant ionized carboxyl groups extremely facilitate the selective transportation of the cations, while the BN-OH also assist this process, promoting the one-way transmission of cations in the channel. Additionally, the participation of PVDF polymer forms a network interlock structure, which significantly improves mechanical performances and stability in water. The membrane demonstrates a high power density (10.6 W/m2), measured in minimal testing areas, and an average power density around 1 W/m2 in larger areas, with an energy conversion efficiency of up to 30.7 %. This study provides a promising exploration of replacing traditional two-dimensional layered structure with three-dimensional network structure of cellulose for osmotic energy harvesting.