| Water stores enormous energy,and currently it is possible to obtain a large amount of energy from liquid water,but gaseous water is still not fully developed and utilized.Compared with liquid water,gaseous water molecules are in a higher energy state and worth deep exploration.With the development of moisture-electricity conversion materials,the hydrovoltaic technology provides an interesting solution for the energy collection from moisture.The surface of hydrophilic and hygroscopic solid material tends to capture ambient water molecules to form adsorbed water,resulting in the reduction of the chemical potential of the water-solid system and the release of latent heat.The adsorption of water molecules on solid surfaces not only changes the volume of the solid,but also generates contact electrification at the water solid interface,providing a solution for obtaining electrical energy from water.This thesis explores the multi-scale relationships between structure and effect in several one-and two-dimensional nanomaterials containing hydrophilic or ionizable functional groups,as well as small molecules and polymer materials in the construction of high-performance and multi-functional moisture-electric conversion materials,and demonstrates successful energy collection based on moisture-responsive actuators and moisture-electric conversion.In the thesis,two energy transmission chains based on moisture have been constructed:from chemical potential energy to mechanical energy to electrical energy,and from chemical potential energy to electrical energy.The moisture-driven generator（MDG）based on two-step energy transfer process and the moisture-electric generator（MEG）based on one-step energy transfer process were designed and prepared,respectively.The adsorption and diffusion model of water molecules,the separation,recombination and migration model of ions have been described in detail,which expands the research value and application prospect of the energy harvesting technology based on moisture.The details are as follows:（1）Preparation and moisture-responsive performances of MXene-based actuatorInspired by the nacre structure of mussels,we developed a polydopamine（PDA）-modified Ti3C2Tx MXene/bacterial cellulose nanofiber moisture response actuator（PDMM/BCNF）by stacking one-dimensional bacterial cellulose nanofibers and Two-dimensional transition metal carbides and/or nitrides（MXene）nanoflakes layer by layer.The actuator can rapidly deform（176°/1.6 s）under the stimulation of moisture,and achieve a high driving force output(6.5 N m-2),which is due to the layered structure between the PDMM nanoflakes and the BCNF nanofiber.The deformation mechanism of the moisture response actuator is related to the moisture gradient formed on the film thickness.The asymmetric water absorption causes the asymmetric expansion on both sides of the film,and leads to the bending deformation of the film.The work in this chapter realizes the conversion of chemical potential energy in water molecules adsorbed on materials into mechanical energy.The excellent comprehensive performance makes this moisture response actuator suitable for the application of electrical switches,robot arms,motors and other equipment.（2）Preparation and moisture-electric conversion performances of MDG based on moisture-responsive actuatorThe moisture-responsive actuator can convert the chemical potential energy in water molecules adsorbed on materials into mechanical energy.In order to further convert this mechanical energy into electrical energy,we first developed a moisture response actuator（PD-rGOMX）with ultra-high conductivity(3693 S cm-1),excellent mechanical performance and moisture sensitivity by interlayer bridging via covalent and hydrogen bonding of MXene and reduced graphene oxide（rGO）nanoflakes by PDA.The actuator can deform rapidly and continuously(>148°s-1),and output a large driving force(8.04 N m-2).Then,we prepared a MDG that can obtain electrical energy from moisture by coupling the actuator with the piezoelectric material polyvinylidene fluoride（PVDF）,and realized the energy conversion process of chemical potential energy to mechanical energy to electrical energy.The MDG can continuously extract the chemical potential energy in the environmental water-solid system and convert it into electrical energy to achieve an instantaneous voltage output of more than 1 V.In addition,the electric energy generated by the MDG can be stored in the capacitor through the rectifier circuit,thus realizing the collection,storage and use of energy.（3）Design,preparation and moisture-electric conversion performances of MEG based on ion gradientA simple,universal and extensible method is adopted to prepare MEG with chemical gradient structure,so as to realize the conversion of chemical potential energy in water molecules adsorbed on materials into electric energy in one step.Firstly,a gradient distribution of citric acid（CA）was successfully constructed in cellulose paper by asymmetric drying.Subsequently,MXene with high conductivity,hydrophilicity and permeability was further used as electrode material to prepare a MEG based on CA gradient.The MEG can harvest energy from moisture directly and effectively,which can generate a continuous voltage of tens of millivolts by ambient humidity,and even to volts(275 m V and 7.6μA cm-2)under asymmetric humidity stimulation,and the maximum power density output is 2.1μW cm-2.The device is not limited in size or shape and can be connected in series or in parallel to quasi-linearly increase the voltage or current output,combined with an energy storage device to achieve uninterrupted production of electrical energy.More importantly,the method can be extended to a variety of organic acids and substrates.（4）Preparation and moisture-electric conversion performance of MEG based on self-sustaining moisture gradient and anisotropic structureA CA-crosslinked BCSNF/rGO composite film（CA-BCSNF/rGO）with anisotropic structure was prepared by stacking one-dimensional insulating bacterial cellulose sulfate nanofibers（BCSNF）and two-dimensional conductive nanoflakes rGO layer by layer,and further covalently crosslinking and protonation using CA.The film has excellent mechanical properties and moisture response sensitivity,and can spontaneously generate built-in humidity gradient in the thickness direction.The MEG prepared by asymmetric assembly of the film achieved continuous DC output of more than 0.54 V and 845μW cm-3.This excellent power output is related to the self-sustaining moisture gradient and unique structural anisotropy in the film,which promotes the separation and migration of charge carriers,weakens charges recombination,and enables closed-loop flow of charge carriers.Owing to the ease of scalability of the MEG,AAA-sized batteries composed of multiple MEG cells are designed as a proof-of-concept demonstration for practical implementation,which will uniquely contribute to advancing the broad application of moist-electric materials in everyday life.These unique findings will have reference significance for the redesign of wet-electric conversion materials,and also provide new insights for the study of the interaction between water and matter. |
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