Self-powered Performance Of Nano Porous Alumina Based On The Flow Of Water Droplet And Its Application

As a new form of power generation,water/moisture enabled electricity generation（WEEG）can utilize the water resources in nature and the moisture in life to produce electricity,which provides a new way to utilize clean power,and also provides a new energy model for building a self-powered system.Aluminum oxide（Al₂O₃）has stable performance and there are no complex functional groups on its surface,which is one ideal candidate for WEEG.It has been reported that aluminum oxide nanoparticles interacting with water show superior power generation performance.It is necessary to explore the diversity of power generation forms of this material to meet the needs of multiple application scenarios.In this thesis,nano porous alumina structures are prepared by vacuum filtration and direct compression methods,based on which the water droplet induced electricity generation devices were successfully fabricated.The device prepared by vacuum suction filtration has an optimal alumina layer thickness of 1.06 mm,and the optimal working condition is applying 8μL of deionized water to the system at a time interval of 10 min.Under this condition,the device can generate a peak output voltage of 4 V and the peak short circuit current is about 18μA.The maximum power output is about 14.8μW which can be achieved when the external resistance is 400 kΩ.A single voltage pulse generated from a single device responding to one single drop of water can drive a commercial green LED to work for 2 s.The device has good cycle stability,and can work for more than200 cycles without significant deterioration in the electricity generation performance.The optimum comprehensive performance of the nano porous Al₂O₃ based self-powered device prepared using direct compression is obtained when the compression pressure is 6 MPa and the thickness of Al₂O₃ layer is 1.3 mm.Applying water drops to this device can also obtain an open circuit voltage of 4 V.Increasing the operation temperature of the device can help shorten the time interval of applying water droplets and improve the power generation efficiency.The electricity generation performance based on water diffusion in a parallel direction is studied.When the distance between the two electrodes in the horizontal direction is small,two sequential voltage pulses will be generated,and the polarity of the second voltage pulse may change.The reason is that the diffusion rate of water droplets on the surface of the Al₂O₃ layer and in the bulk material is different,resulting in two voltage signals induced by surface flow and volume flow.Through experimental verification and theoretical exploration,it is proved that the power generation mechanism of the nano porous Al₂O₃ structure is based on the streaming current（streaming potential）which is one of the electrophoresis phenomenon,that is,when water flows through the nano channel in the Al₂O₃ layer,the electrical double layer generated by adjacent aluminum oxide nanoparticles overlaps strongly,and the counter ions outside the shear plane in the diffusion layer are dragged by the water flow to generate streaming current,The movement of counter ions leads to the separation of positive and negative ions which lead to the generation of streaming potential.The influence of different process parameters on electrical output performance is based on the relationship between the wetting spreading state of water in porous Al₂O₃,and the length and number of nano channels.Based on the characteristics of electricity generation process and the electrical signal characteristics of the device,a heavy water sensor was developed using the stacking layer of Al₂O₃ nanoparticles prepared by direct compression.The detector uses the electrical signal to identify heavy water.Two voltage pulses are sequentially generated as one water droplet was applied to the sensor,i.e.,V_sharp and V_broad,characteristics of which are developed into four diagnostics to distinguish between H₂O and D₂O.V_broad originates from the streaming potential generated from water flowing through the Al₂O₃ film and V_sharp derives from the charge separation process as droplet is dispersed from the pipette tip.The sensing mechanism is based on spatially localized variations in the surface potential of alumina,induced by isotopically substituted water molecules,combined with the effect of isotopic composition on charge transfer to the sensor.Two main diagnostics still hold when changing Al₂O₃ film thickness,materials of electrode pairs and work temperatures.It has been verified that the device can be used to detect H₂¹⁸O and can detection limit of D₂O is 100 ppm.A sensing system using the amplitude of V_broad and the time dependent characteristics of V_sharp has been developed which successfully senses the presence of H₂O and D₂O.It is found that the water droplet enabled alumina based self-power device has some rectifier property,which realizes the transition from the natural response induced by water droplet to the response driven by applied voltage.A self-powered triode has been developed,which is characterized by signal amplification effect and bistable signal rectification.The amplification effect comes from the ion polarization effect caused by the input voltage and the circuit amplification effect.The bistable property is related to the electrical viscosity of the liquid flowing in the nanochannel.