Horizontally Asymmetric Nanochannels of Graphene Oxide Membranes for Efficient Osmotic Energy Harvesting

Reverseelectrodialysis (RED) directly harvests renewable energyfrom salinity gradients, and the achievable potential power heavilyrelies on the ion exchange membranes. Graphene oxides (GOs) are considereda solid candidate for the RED membrane because the laminated GO nanochannelswith charged functional groups provide an excellent ionic selectivityand conductivity. Yet, a high internal resistance and poor stabilityin aqueous solutions limit the RED performance. Here, we develop aRED membrane that concurrently achieves high ion permeability andstable operation based on epoxy-confined GO nanochannels with asymmetricstructures. The membrane is fabricated by reacting epoxy-wrapped GOmembranes with ethylene diamine via vapor diffusion, overcoming theswelling properties in aqueous solutions. More importantly, the resultantmembrane exhibits asymmetric GO nanochannels in terms of both channelgeometry and electrostatic surface charges, leading to the rectifiedion transport behavior. The demonstrated GO membrane exhibits theRED performance up to 5.32 W center dot m(-2) with >40%energy conversion efficiency across a 50-fold salinity gradient and20.3 W center dot m(-2) across a 500-fold salinity gradient.Planck-Nernst continuum models coupled to molecular dynamicssimulations rationalize the improved RED performance in terms of theasymmetric ionic concentration gradient within the GO nanochanneland the ionic resistance. The multiscale model also provides the designguidelines for ionic diode-type membranes configuring the optimumsurface charge density and ionic diffusivity for efficient osmoticenergy harvesting. The synthesized asymmetric nanochannels and theirRED performance demonstrate the nanoscale tailoring of the membraneproperties, establishing the potentials for 2D material-based asymmetricmembranes.