Construction Of Cellulose-Based Nanofluidic Membranes And Study On Salinity Gradient Energy Conversion

With the increasing demand for energy and the growing pollution from fossil fuels,the research and development of clean,efficient and renewable energy is of great significance to the sustainable development of humankind.Salinity gradient energy is a Gibbs free energy,known as"blue energy",that comes from seawater with high salt concentration and fresh water with low salt concentration.In recent years,nanofluidic membranes with charged nanocrystalline channels have attracted much attention in high-performance salinity gradient energy conversion applications due to their unique ion transport properties.However,due to its low charge density and single pore structure,achieving high selectivity and high throughput ion transport in the reported nanofluidic membranes is difficult,which limits their salinity gradient energy performance.The cellulose chain contains a large number of hydroxyl groups,and its surface charge density can be adjusted effectively by various modification methods.At the same time,cellulose fibers of various sizes can be prepared by mechanical and chemical treatment,which is helpful in realizing the size adjustment of the nanochannel.In addition,cellulose contains hydroxyl groups that can form hydrogen bonds,thus enhancing the mechanical properties of the system.In this paper,two-dimensional/three-dimensional cellulose-based nanofluidic composite membranes were constructed with two-dimensional nanosheets and three-dimensional conducting polymers by taking advantage of the chemical properties and mechanical properties of cellulose.The selective ion transport mechanism was explored,the salinity gradient energy acquisition and conversion properties were tested,and the practical application potential and operational stability of the membranes were evaluated.The main research contents and conclusions are as follows:（1）Porous MXene/nanocellulose composite membrane and salinity gradient energy conversion.Porous MXene nanosheets（PMXene）with abundant nanopore structure were prepared by mild H₂O₂ oxidation etching method,and CNF prepared by 2,2,6,6-tetramethylpiperidin-1-oxy（TEMPO）mediated oxidation was combined by vacuum filtration to construct PMXene/CNF two-dimensional nanofluidic composite membranes.The ion transport characteristics and salinity gradient energy conversion performance were investigated.The introduction of PMXene reduces the ion transport barrier.The energy barrier of K⁺in PMXene/CNF nanochannel is 12.3 k J·mol^-1,which is lower than that of MXene/CNF(15.38 k J·mol^-1).As an intercalator,CNF expanded the layer spacing between MXene nanosheets and significantly increased ion flux.Meanwhile,TEMPO-oxidized CNF contained a large number of carboxyl groups,which increased the surface negative charge density and cation selectivity of PMXene/CNF composite nanofluidic membranes.The output power density of the composite membrane can be optimized by changing the content of CNF.At neutral p H and room temperature,the maximum output power density of the PMXene/CNF membrane at a 50-fold KCl concentration gradient is 0.95 W·m^-2,which is 43%higher than that of the MXene/CNF membrane.Further optimization of electrolyte p H value can increase the output power density of the composite membrane to 1.29 W·m^-2.The 36PMXene/CNF components are connected in series to achieve a long time stable voltage output of5.26 V,which can power a small computer.The composite strategy of nanoporous two-dimensional materials and CNF provides a new approach for designing biomas-based nanofluidic membranes with excellent ion transport properties.（2）Construction of three-dimensional AAM/BC double-network hydrogel nanofluidic membranes and salt-difference energy conversion.Using BC hydrogel membrane as the matrix,AAM/BC double network hydrogel membrane with 3D nanoporous structure was constructed by photopolymerizing acrylamide-methyl methacrylate（AAM）under UV irradiation.AAM was filled into the micropore of the original BC hydrogel by free radical polymerization to form a nanoion transport channel with high electronegativity and interconnection.Because acrylic acid has an abundant carboxyl group,the composite hydrogel membrane has good cation selectivity.The AAM/BC DN hydrogel assembly-based salinity gradient energy conversion device can reach the output power density of 7.63 W·m^-2 at p H 11.Under the action of acid-base neutralization reaction,the power density of the composite hydrogel membrane is further increased to 45.5 W·m^-2,which can effectively transform the salinity gradient energy in the highly alkaline pulping black liquor into electric energy,providing a new idea for the high-value utilization of industrial waste liquor.（3）Construction of reverse electrodialysis system based on BC/conductive polymer composite nanofluidic membranes and its salinity gradient energy generation performance.Firstly,BC was prepared by TEMPO oxidation and etherization respectively to obtain negatively charged BC membrane（NBC）and positively charged BC membrane（PBC）.Further,the negatively charged polysodium p-styrene sulfonate（PSS）and polydopamine（PDA）were synthesized into NBC and PBC nanofiber membranes by in-situ polymerization.Finally,the negatively charged NBC/PSS and positively charged PBC/PDA composite membranes were obtained by vacuum filtration.When PSS and PDA were filled in NBC and PBC nanofiber networks,not only the ion transport channels with nanometer size were obtained,but also the surface charge density of the nanofluidic membranes was improved,and the selective ion transport was promoted.Salinity gradient energy conversion performance of the nanofluidic composite membranes was enhanced.By optimizing the content of the carboxyl group and the concentration of SS and DA in the in-situ polymerization process,the maximum output power density of NBC/PSS and PBC/PDA composite membranes can reach 0.99 W·m^-2 and 1.14 W·m^-2,respectively,under the simulated50-fold Na Cl concentration gradient.The output power density can be as high as 1.86 W·m^-2 by assembling two composite membranes with opposite charges into a reverse electrodialysis device.A reverse electrodialysis device with 30 units in series can achieve a high output voltage of 2.5 V and power small electronic devices.The composite strategy of cellulose and conductive polymer has essential reference significance in constructing energy conversion devices with high ion selectivity and high salinity gradient energy performance.