Xin Zhang, Minmin Li,* Fusheng Zhang, Qiongya Li, Jie Xiao, Qiwen Lin, and Guangyan Qing*
Small 2023, DOI: 10.1002/smll.202304603
https://onlinelibrary.wiley.com/doi/epdf/10.1002/smll.202304603
Balancing energy utilisation and environmental protection is a key requirement for sustainable development, so the search for the development of renewable and clean energy sources has long been a driving force for researchers. Osmotic energy, also known as salt difference energy, is the energy generated by the salinity difference between solutions of different concentrations, which is widely existed in nature at the junction of river water and seawater, and it is a promising "blue energy" with the characteristics of stable source, simple acquisition method, green sustainability and huge storage capacity.
Currently, osmotic energy collection is usually based on reverse electrodialysis (RED) technology, which is highly dependent on ion-selective membranes (ISM). As the core component of osmotic energy harvesting, ISMs can selectively and preferentially transport single or anionic ions across the membrane, thereby spontaneously generating a net ionic current across the membrane salinity gradient. However, RED systems based on commercial ion exchange membranes tend to suffer from high cost, low ion selectivity, insufficient mass transfer and high membrane resistance, resulting in low power output.
In contrast to the conventional perception of long-chain cellulose, needle-like cellulose nanocrystals (CNCs) have received much attention due to their excellent mechanical strength, abundant surface charge, and ability to spontaneously assemble. However, due to the inherent water instability of CNC materials, few studies have utilised CNC to construct osmotic energy harvesting membranes. Based on a series of related research work around CNC carried out by the group in the previous period: ACS Appl. Mater. Interfaces 2019, 10.1021/acsami.9b00471; ACS Appl. Mater. Interfaces 2021, 10.1021/acsami.1c02753; Adv. Funct. Adv. Funct. Mater. 2022, 10.1002/adfm.202204487; Small2023, 10.1002/smll.202207932, where the innovative use of poly(vinyl alcohol) (PVA) and graphene oxide (GO) nanosheets as guest additives was developed and designed to develop and design self-assembled cellulose nanocrystal based crystals (CNC) nanofluidic composite membranes, where flexible long-chain PVA molecules self-assemble with CNC to form a pearl-layered structure, which can significantly enhance the mechanical strength and water stability of the composite membranes. The composite membrane shows good cation selectivity due to the abundant surface negative charges of CNC and GO materials and the large number of nano-network channels in the composite membrane. The optimal power output density of 6.5 W/m2 has been demonstrated at 100 times the salinity gradient, exceeding the commercial standard of 5 W/m2. Excellent output stability was maintained during 25 days of continuous operation.
This work provides a new paradigm for thinking about the development of nanofluidic membranes that can be easily and massively prepared using resourceful and sustainable biomass materials for efficient harvesting and utilisation of osmotic energy.
