Polyoxometalate-zwitterion Supramolecular Hybrid Proton Conductors

Polyoxometalates（POMs）are a class of metal-oxygen nanoclusters that possess atomically precise structures.They display a wide range of modifiability,stable redox properties,super acidity,and delocalized multi-charge distribution.Therefore,POMs have potential applications in the energy field,particularly in proton conduction.Recent research on proton conductors has primarily focused on incorporating polyoxometalates（POMs）with polymer or framework materials.Alternatively,an alternative approach involves controlling the topological morphology of POMs through covalent bonding to modulate their conductivity.Efficient spatial assembly of POMs and on-demand preparation remain challenging.This dissertation proposes building a supramolecular hybrid proton conduction system through the electrostatic recombination of zwitterionic ligands on the POM surface.This approach enables modulation of the spatial phase structure of the proton transport channels and optimization of the mechanical processing properties of the inorganic clusters.Additionally,the reversible bonding characteristics of electrostatic interactions facilitate the dynamic relaxation motion of functional groups,thereby maintaining fast proton transport.This dissertation presents a novel approach to electrostatic interaction with POM by utilizing zwitterions as end-group functional groups of organic ligands.Unlike traditional electrostatic interactions（e.g.,electrostatic interaction between POM and quaternary ammonium salts）,the electrostatic substitution between the zwitterion and POM does not remove protons from the system.Instead,it binds them in the ionic phase region and promotes their decoupling.We have successfully developed several proton-conducting materials with distinct characteristics,including POM-based flexible eutectic electrolytes,POM superlattice room-temperature ionic liquid crystals,and high-temperature stable POM ionic liquid crystals.These materials were created by reasonably modifying the topological morphology of organic ligands.The details of this dissertation are as follows:POM-based flexible eutectic electrolyte:The study used the concept of hydrogen-bonding-induced deep eutectic design to modify the structure of the conventional betaine-type zwitterion.This involved liquefying two crystalline solids,the hydroxylation-rich zwitterion（HPS）and silicotungstic acid hydrate（Si W）,after they were co-blended.By mixing two solid powders without the aid of solvents,a flowable liquid can be formed.The resulting complexes can be transformed into a flexible eutectic by controlling the relative humidity,highlighting the advantages of environmentally friendly and simple material synthesis.Structural analysis revealed that the increase in the system’s entropy is responsible for the formation of Si W-n HPS（n=2,4,6）.The melting point of HPS and Si W is reduced due to the disruption of their ionic lattice by multiple interactions.Moreover,the eutectic formed by the combination of Si W,which acts as an electrostatic cross-linking center,and HPS,which acts as a hydrogen bonding cross-linking center,exhibits remarkable mechanical properties such as good ductility and tensile strength.The proton conductivity is significantly enhanced through a combination of decoupled zwitterions and hydrogen bonding interactions.The hydroxylated POM-zwitterion eutectics have shown an improvement in conductivity by three orders of magnitude under high-temperature anhydrous conditions.Density functional theory indicates that the enhanced conductivity of the system is attributed to the hydrogen bonding of the HPS terminal hydroxyl group.This study proposes a method to enhance proton transfer from POM to the sulfonic acid group by leveraging the interaction between the POM and HPS terminal hydroxyl group.The resulting flexible proton conductor can serve as a promising electrolyte for high-performance POM-based applications.POM-based superlattice room-temperature ionic liquid crystal:The ligand IDBS,which has zwitterionic end groups,was combined with Si W using electrostatic forces to create a solid substance under constant humidity.The ternary complex Si W-n IDBS-m HPS（where n and m represent the molar ratio of the complex）was formed by adding the eutectic-inducing ligand HPS.Rheological tests showed that HPS reduces the complex’s modulus and causes a transition from a solid to a liquid state.The ternary complex system exhibits a contradictory equilibrium where the eutectic-induced entropy increases while the structure-induced entropy decreases.However,with rational adjustment,the POM complexes can achieve a unification of both liquefied and ordered properties,resulting in the first room-temperature ionic liquid crystal based on POM.The high degree of order in the entropy-reduced system is due to theπ-πconjugation of the biphenyl-nitrile moiety in the structure-inducing ligand.In the entropy-increasing system,the reduction in orderliness occurs due to the disturbance caused by the various interactions of electrostatic-hydrogen bonds between POM and HPS.Additionally,the ternary complexes centered on POM display microphase separation of zwitterions.In the entropy-increasing system,the decrease in the degree of order arises from the perturbation of the multiple interactions of electrostatic-hydrogen bonds between POM and HPS.Besides,the ternary complexes centered on POM exhibited microphase separation of zwitterions.Due to differences in the chemical properties and topology of zwitterions,those with similar properties will aggregate spontaneously on one side,forming an intercalated distribution.In a humid environment,this distribution assembles into a superlattice structure,allowing for the refined reorganization of different zwitterions.The introduction of HPS-induced superlattice structure improves proton conduction properties by up to 23 times,while POM-based room-temperature liquid crystal states improve proton conduction up to127 times.This research demonstrates a method for constructing POM-based room-temperature liquid crystal systems,which expands the means to regulate the assembly of zwitterionic materials with greater precision.High-temperature stable POM ionic liquid crystal:Polymer-grafted nanoparticles Si W-4IPS_n（n represents the degree of aggregation）were synthesized through the modification of zwitterions at the end of a polymer chain that can be adjusted in length.The resulting Si W-4IPS_n exhibits a hexagonal columnar phase structure that is stable across a broad range of polymerization degrees.Theoretical simulations demonstrate that during the annealing process,polymer chains undergo dynamic rearrangement on the POM surface.Isotropic spherical particles gradually transform into amphiphilic fan-shaped particles due to the multicharged delocalization property of POM and the unsaturated nature of electrostatic interaction.This dynamic rearrangement of the ligands leads to an extremely high degree of order in Si W-4IPS_n.In situ structural characterization reveals that Si W-4IPS₁₉ exhibits excellent order stability over a wide temperature range and transforms into a liquid crystal state at temperatures above 85°C.This ordered stability can be further enhanced to 250°C by hybridizing low-charged POMs(e.g.,Si W-PW-8IPS₁₉).In terms of anhydrous proton conductivity,the proton,sulfonate,imidazole,and Si W₁₂O₄₀^4-were found to be in electrostatic equilibrium by Fourier transform infrared spectroscopy.As the temperature rises,the sulfonate binding gradually releases the protons,resulting in increased proton conductivity after the liquid crystal transition.The combination of electrostatic cross-linker Si W and zwitterionic polymer ligands in Si W-4IPS_n leads to a synergistic enhancement of structural stability and conductivity at high temperatures.This study marks the first application of POM ionic liquid crystals in anhydrous proton transport and highlights the unique advantages of POM in constructing high-performance liquid crystal electrolytes.The results provide valuable insights for designing ion transport systems for energy and electronics applications.This dissertation explores the incorporation of eutectic-induced effects and ionic self-assembly design concepts into the POM-zwitterion composite system.The research systematically studies hybridized ligand molecules,system entropy balance regulation,supramolecular interaction drive forms,and assembly structure transformation.The dissertation’s research content energetically explores the design principles of zwitterion-enhanced proton conduction,broadens the application of POM-based proton-conducting materials,and establishes a methodological basis for constructing functionally directed proton conductors.