Nanocellulose-Based Asymmetrically Structured Functional Materials And Their Valorization

The impact of humidity on agricultural production,medical care,aviation and other areas cannot be ignored.Humidity sensors,as devices capable of converting humidity changes into visual signals,have received a great deal of attention from researchers.Water is also the largest carrier of energy.The emergence of moistelectric generator extracts energy from water by converting the chemical potential energy in the diffusion of water molecules into electrical energy.As a green and renewable biomass materials,oxidized cellulose nanofibers(CNF)have strong adsorption capacity for water molecules,which shows unique advantages in the preparation of humidity sensors and moist-electric generator.However,the main challenges with existing cellulose-based humidity sensors and moist-electric generator are as follows:1)external power supply is required for the sensitization and sensing realization;2)the resulting open circuit voltage is low and the electrical output is unstable,etc.In order to solve the above problems,a series of nanocellulose-based asymmetric functional materials were designed and prepared in this study for enhancing the transport rate of water molecules on their surfaces to achieve the purpose of constructing high-performance nanocellulose-based moist-electric generators and humidity sensors.The effects of the functional elements on the structure and properties of nanocellulose-based multidimensional asymmetric materials and the effect of asymmetric structure on moist-electric generation and humidity sensing performance have been investigated in depth.The moist-electric generation and sensing mechanism of nanocellulose-based multidimensional asymmetric materials was elucidated.The role of asymmetric structural features in the enhancement of moist-electric generation and sensing performance was revealed.Also,the application of nanocellulose-based asymmetric materials in various fields such as motion detection,breath detection,energy supply and electromagnetic shielding were comprehensively investigated.A new idea is proposed for the preparation of high-performance nanocellulosebased humidity sensor and moist-electric generator The details of the study are summarized below.(1)In order to solve the problem of low sensitivity of deformation-type humidity sensor,an asymmetrically patterned CNF/graphene oxide(GO)composite film was constructed by the combination of vacuum filtration and surface imprinting.The structure and properties of asymmetrically patterned CNF/GO composite films were tuned.The results showed that thhe asymmetrically patterned CNF/GO composite film could detect humidity changes when there was a humidity difference between its two sides,and the water molecule adsorption difference would cause an asymmetric volume expansion on both sides of the film,resulting in directional deflection of the film.Molecular dynamic simulations confirmed that within a certain range,the carbonyl group on the GO surface enhanced the interaction force between the CNF/GO composite film and the water molecules,increasing the deflection rate and the deformation degree of the composite film.Also,the hydrophilic functional groups on the composite film surface could specifically recognize water molecules and the grid pattern provided a limited channel for water molecule transport on the film surface,which together could significantly increase the water molecule transport rate on the film.Finite element analysis showed that the presence of single-sided patterning also increased the deformation of the composite film under the same stress.As a result,the asymmetrically patterned CNF/GO composite film exhibited linear response,stability,cyclic response,and fast response(3-5 s)to change in the RH range of 25.0%-85.4%at a GO content of 7.5 wt%.Asymmetrically patterned CNF/GO composite films also showed excellent potential for applications in non-contact sensing,the bionic blade field,and selfpowered sensors.It could produce a maximum open circuit voltage of 286 mV as a moist-electric generator.(2)To solve the problem of low moist-electric generation performance of CNF/GO composite film,CNF/GO/MXene composite film with gradient structure was constructed by layer-by-layer suction filtration.The factors influencing the structure and performance of CNF/GO/MXene film moist-electric generators,and their action mechanisms were investigated.The results show that the increasing GO and MXene content reduced the hydrophobicity of the composite film but allowed for the gradual formation of a tight lamellar structure within the film.The difference in hydrophilicity between the two sides of the film and the presence of the lamellar structure inside the film significantly increased the water molecule/ion transport rate within the film.At present,the use of MXene,a highly conductive sheet material,helped to reduce the Joule loss in the energy conversion process during moist-electric generation and contributed to the moist-electric generation performance of the CNF/GO/MXene films.The thickness of the CNF/GO,/MXene film also had a significant effect on its moist-electric generation performance.The CNF/GO/MXene moist-electric generator had the best moist-electricity generation performance when the GO and MXene content was 50 wt%and the film thickness was 15 μm.Measuring only 1.5×1.5 cm2,the CNF/GO/MXene moist-electric generating unit was created with an open circuit voltage of 652.27±29.24 mV and an output current of 4.35±0.65 μA.The output signal could be further extended by integrating several power generation units in series.The CNF/GO/MXene composite film with moist-electric generation characteristics was also suitable for distinguishing the breathing state of the human body,sensing the finger touch and as a power supply unit for timers and LED lights.(3)In order to further expand the hydrophilic difference on both sides of the film and enhance the moist-electric generation performance of nanocellulosebased films,a CNF-CNF/MXene bilayer film with an asymmetric structure was developed by combining the asymmetric film’s structural advantages,and the effect of MXene addition on the structure and performance of the bilayer film was investigated.The results show that the structural compactness of the CNF layer and the CNF/MXene layer in the CNF-CNF/MXene bilayer is obviously different.Moreover,with the increase of MXene content,the lamellar structure of CNF/MXene layer gradually compacted and the hydrophilicity of CNF surface gradually increased.The anisotropy of the double-layered film structure increases the rate of transport of water molecules and mobile ions across the film,which contributes to its moist-electric generation performance.Also,the size of the generator unit had a significant impact on its electrical output.The larger size of the CNF-CNF/MXene moist-electric generation unit has a higher open-circuit voltage value.With a film size of 1.5×1.5 cm2,the maximum open circuit voltage and output current can reach 997.4±69.0 mV and 11.36±2.81 μA respectively.The moist-electric generator can be used to discriminate various breathing patterns and as a power source to supply commercially available timers and LED lights.In a way,the addition of MXene,which has a high electrical conductivity,gives the bilayer film outstanding electrical conductivity.The CNF-CNF/MXene1:3.5 film with high conductivity is suitable for the following:1)real-time monitoring of human body movements of various amplitudes;2)in the 8.2-12.4 GHz band,the CNF-CNF/MXene1:3.5 achieves a total electromagnetic shielding value of 47.79 dB and a shielding efficiency of 99.998%;3)by adjusting the external voltage,it is possible to achieve the purpose of controlling the surface temperature of the CNF-CNF/MXene1:3.5 film.Due to its flexibility,stability,and repeatability of Joule heating,this double-layer film is well suited for body heat management;and 4)real-time respiration monitoring.Also,the low infrared emissivity of MXene allows the CNF-CNF/MXene composite film to be used as a thermal camouflage material.(4)To improve the stability of CNF-CNF/MXene bilayer moist-electric generation,polyvinyl alcohol(PVA)with high water absorption was introduced and the effect of PVA on the structure and performance of CNF/PVA-CNF/MXene moist-electric generator was investigated.The results show that the two layers of the CNF/PVA-CNF/MXene films are distinctly different in structure yet tightly bound together.The introduction of PVA with a large amount of-OH in the molecular chain contributed to the enhanced hydrophilicity of the CNF/PVACNF/MXene film and provided stability of the nanocellulose-based moist-electric generator electrical signal output.An appropriate increase in the thickness of the structurally dense CNF/PVA layer can significantly improve the moist-electric generation capacity of CNF/PVA-CNF/MXene films.When the overall thickness of the bilayer film is 30 μm,the average open circuit voltage generated is 826.78±25.67 mV.There is a positive correlation between the output voltage of this moist-electric generator and the series number of generating units.The three 1.5x1.5 cm2 moist-electric generating units connected in series can reach an open circuit voltage of around 2 V,which is significantly higher than that of a commercially available battery size 5.It is capable of supplying the timer with a continuous power supply for almost 15 min.At the same time,the CNF/PVACNF/MXene film also has breathing monitoring,tactile sensing and non-contact sensing capabilities.(5)A structurally similar PVA/glycerol-CNF/chitosan(CS)/PVA bilayer hydrogel was designed and developed on the basis of a two-dimensional CNF/PVA-CNF/MXene film.The impact of the asymmetric structure on the structure and properties of hydrogels and aerogels with a three-dimensional network structure was researched.It was discovered that the PVA/glycerolCNF/CS/PVA hydrogels could be constructed by combining the structurally dense PVA/glycerol hydrogel layer with the more porous CNF/CS/PVA hydrogel layer using a simple freeze-thaw process.The introduction of CS increased the density of the hydrogel cross-linked network and contributted to the mechanical strength of the hydrogel.In addition,the asymmetric mechanical strength,adhesion,and bacteriostatic properties of the bilayer hydrogel were given by taking advantage of the asymmetric structure.The introduction of the bilayer structure was also significant for the improvement of the mechanical and moisturizing properties of CNF/CS/PVA hydrogels.Meanwhile,the PVA/glycerol-CNF/CS/PVA bilayer hydrogel and aerogel differed significantly in the level of moist-electric generation.Under the same conditions,the strong water absorption capacity of the aerogel enabled it to generate an open circuit voltage and current of 526.33±12.52 mV and 6.39±0.46 μA,respectively,all of which are much higher than those from the hydrogel.However,PVA/glycerol-CNF/CS/PVA hydrogel could utilize its own moisture to generate electricity spontaneously,and no external humidity source was required.In summary,the nanocellulose-based functional material with asymmetric structure,prepared using planted-derived CNF as a substrate,can have many functionalities,such as humidity sensing,moist-electric generation,and motion monitoring,with the advantages of being simple,environmentally friendly,sustainable,and expandable.The strategy outlined in the present research provides a new pathway for the development of high-performance humidity sensors and moist-electric generators,while it also gives more insights/theoretical basis for the high-value utilization of biomass materials.