Electrokinetic ion rectification in the coupling asymmetric charged nanochannel

Ion transport in nanochannels attracts increasing attention with the advance of nanotechnology, where ion rectification has been intensively studied due to its diode-like electrokinetic behavior. However, the electrochemical interface in most ion rectification is not uniform but involves asymmetric-charged characteristics that offer new degrees of electrical double layer (EDL) confinement. Herein, we investigate the ion transport in coupling asymmetric charged nanochannel to evaluate how the asymmetric geometry and surface charge influences the ion rectification performance. The results show that at low surface charge gradient, the coupling effect enhances ion rectification performance. While at high surface charge gradient, an anomalous electrokinetic behavior is observed that coupling effect deteriorates ion rectification performance. Further analysis discloses that the enhancement-deterioration transition threshold depends on the coordination mechanism of asymmetric geometry and surface charge. It is found that the asymmetric geometry shaped nanochannel with low surface charge gradient and homogeneous geometry shaped nanochannel with high surface charge gradient have the better ion rectification performance. This work fills the gap in understanding the coupling asymmetry effect on ion rectification, providing the useful guidance to design nanofluidic energy devices.