The Electrokinetic Properties Of Nanofluidic Channels And Their Application In Energy Harvesting

The exploitation of clean energy from ambient environment has become paramount to the sustainable development of human civilization because of the ongoing depletion of fossil fuels and ever-growing energy demands.In addition to the extensively studied solar and biomechanical energy,energy that exists in the fluidic system is one of the most popular energy resources due to its large reserves and non-polluting properties.Meanwhile,a new generation of nanoscale power and electronic devices represented by nanoelectromechanical systems are booming,and their application prospects in defense and civilian fields are extremely promising.Such emerging nanoscale devices typically operate at very low operating voltages and very low power consumption,the nano-scale power device can meet this requirement,namely nano-generators.Therefore,based on the engineered advantages of nanochannel geometry and material properties,nanofluidic channel systems have been proposed as new candidates for energy harvesting.Despite significant improvements have been achieved in the past few years,improving nanofluid energy conversion efficiency is still an important challenge for current research work.Mastering the performance of a single nanofluidic device is the cornerstone for improving energy conversion efficiency.In the thesis,the factors affecting the energy conversion of nanofluids are discussed in detail,and the methods to enhance/optimize the energy conversion efficiency are proposed.First,based on energy conversion from mechanical to electric,we described the concept of short channel effect and discussed its effect on electrokinetic energy conversion of nanofluids.The ion selectivity of nanopores due to the wall surface charges is capable of inducing strong coupling between fluidic and ionic motion within the system.This interaction opens up the prospect of operating nanopores as nanoscale devices for electrokinetic energy conversion.However,the very short channel lengths make the ionic movement and fluidics inside the pore to be substantially affected by the ion depletion/accumulation around the pore ends,we define this phenomenon as short channel effect.Based on three-dimensional electrokinetic modeling and simulation,we present a systematic theoretical study of nanopore electrical resistance,fluidic impedance,and streaming conductance under conditions of slippery/non-slippery wall or nanopore with round corner.Our results show that by utilizing the short channel effect and preparing slippery nanopores the energy conversion efciency can be dramatically increased to about9%under large salt concentrations.Secondly,based on energy conversion from Gibbs free energy to electric,we discussed the electrokinetic transport mechanisms in ultra-thin MoS₂ nanopore systems.Recent experiments demonstrated giant osmotic effects induced in a single-atomic-layer MoS₂nanopore by imposing a KCl concentration bias,thereby highlighting the prospect of ultrathin nanopores as power generators.However,the relevant physical mechanism is not clear,thus we discussed the electrokinetic mechanism of ion transport in the MoS₂ nanopore system.By taking membrane surface chemistry into account,we found profound roles of surface charges in and out of the nanopore on the cross-pore ion transport,which shed light on the intriguing experimental observations of a high pore conductance with a large open-circuit voltage in the MoS₂ system.The present work establishes a theoretical model capable of dealing with ultrathin membrane surface charges for evaluating the energy conversion performance of nanopore power generators constructed with two-dimensional materials.Finally,a silica-nanochannel based nanofluidic generator was fabricated based on standard semiconductor manufacturing process.The development of nanofluidic energy harvesting system plays a fundamental role in harvesting osmotic power from Gibbs free energy within salt concentration gradient,which is considered as a future clean and renewable energy source.In this study,a silica-nanochannel based nanofluidic energy harvesting system was fabricated and its output power density could reach 705 W/m² under suitable KCl concentration bias which exceeded—by almost two orders of magnitude—the results obtained by previous work.The enhancement of energy harvesting was mainly ascribed to the appropriate length of nanochannel that makes a good balance between the desirable ion selectivity and the unfavorable large resistance of nanochannel.This high-performance nanofluidic energy devices could be used in a variety of applications,including power biomedical tiny devices or constructing future clean-energy recovery plants.In an all,based on the shape of nanopore/nanochannel,channel length,material and wall roughness,the effects of these factors on electrokinetic properties of nanofluidic channels and energy conversion efficiency were systematic discussed,and we proposed effective strategies for enhancing/optimizing the energy conversion efciency.Meanwhile,a silica-nanochannel based nanofluidic generator was fabricated based on standard semiconductor manufacturing process.