Under the background of continuous increasing chemical energy consumption and energy demand, the development of sustainable and environmentally friendly new energy sources have attracted intensive interest in modern scientific research. Osmotic energy is a renewable green energy source that produces predictable and stable energy output, as opposed to solar and wind power. The nanopore energy conversion system based on converting fluid mechanical energy into electrical energy and generated by the water pressure difference between solutions of different concentrations is called osmotic energy. The clean energy source can be captured through some membrane technologies, such as osmosis or reverse electrodialysis. With the development of nanofluidic science and membrane technology, osmotic energy can be harvested by controlling ion transport in limited nanochannel membrane. The ionic selectivity and high quality fluxes given by the nanofluids can greatly facilitate the collection of osmotic forces. It is still necessary to explore how to break through the relationship between power density and pore density through the design and control of material characteristics.
We design a nanofluidic device that can harvest osmotic energy and rectify ionic transport by directly prepared with a nanoporous Cellulose nanofibers (CNF) membrane and a conical variable-channel porous polyethylene terephthalate (PET) substrate membrane. The CNF nanochannels membrane with abundant carboxyl groups and hydroxy groups dominates the ionic transport properties; while the used PET membrane provide conical nanochannels substrate, which forms an asymmetric nanofluidic diode by integrating with the CNF nanochannels membrane and features geometry and surface charge polarity. The large number of surface negative charges in CNF are formed on the interface of membrane, which contribute to ultrahigh ionic rectification and notably anion selectivity. Afterwards, this anion selective nanofluidic diode with asymmetric bipolar structure are introduced into a concentration gradient driven energy harvesting device, which contribute for dissolving traditional reverse electrodialysis and increasing output power density. The several obvious advantages of this membrane is observed: (1) what counts is the explored CNF heterogeneous membrane can be easily fabricated with environmentally friendly characteristic; (2) its power density reaches a maximum value of 0.96 Wm⁻² by using artificial simulation power generation of seawater and river water; and (3) the impressive rectification ratio 562 of the CNF/PET is 71 times more than one of the PET because surface charge governed ion transport behavior at basic condition. Such a nanofluidic device can trigger further energy harvesting device, which construct of CNF/PET heterogeneous membrane.
 

Concentration-gradient-driven,Cellulose nanofibers,Heterogeneous membranes,Asymmetric nanochannels