Polyoxometalate/Comb Copolymer Hybrid Proton Exchange Membranes

Currently,the development of clean and efficient new energy technologies has become the key to promote the transformation of the traditional energy industry to green and low-carbon.Proton exchange membrane materials play a crucial role in new energy technologies such as fuel cells,flow batteries and electrolyzers,determining the conversion efficiency and lifetime of these energy devices.However,proton exchange membranes are faced with the problem of trade-off among important properties such as proton conductivity,mechanical properties and active-species permeation.The current common solution is to modify proton exchange membranes,especially hybrid strategies based on inorganic functional additives,to develop high-performance proton exchange membranes that meet the demands of new energy technologies.There are numerous hybrid membranes derived from introducing ceramic oxides or carbon materials into proton exchange membranes,but these additives exhibit distinct physicochemical properties from polymer matrix,such as incompatibility with polymer components and mismatch with the size of polymer ionic nanophases.These differences can lead to the destruction of the continuity of proton conduction channels and a decrease in proton conductivity.Polyoxometalates（POMs）are a kind of inorganic nanoclusters with nanoscale size and precise structure,which have excellent proton conductivity,and are often used as functional additives to prepare hybrid proton conducting materials.This dissertation aims to combine POMs with comb copolymers with rich self-assembled structures,regulate the microphase structure of the membranes through molecular level hybrid assembly strategy,improve the comprehensive performances of the membranes,and establish an understanding of the relationship between microphase structure and macroscopic performances.The detailed research contents of this dissertation are as follows:First,a novel comb copolymer PMS-g-PVP composed of poly（4-methylstyrene）（PMS）and polyvinylpyrrolidone（PVP）is designed and synthesized using the RAFT controlled grafting method.In this design,the neutral PMS main chain serves as the mechanical support,the polar PVP side chain serves as the proton conduction,and phosphotungstic acid（PW）is introduced as the high proton conductor and nano-reinforcer.PW/PMS-g-PVP hybrid proton exchange membrane is prepared by solution casting method.PW is immobilized in the PVP domains through the electrostatic interaction.This interaction increases the thermodynamic incompatibility between the hydrophilic and hydrophobic domains,enabling the transition of the microphase structure from a short-range ordered lamellar structure to a long-range ordered lamellar structure.Moreover,PW can provide additional protons while also contain abundant surface oxygen atoms,serving as sites for continuous proton hopping.Based on the PW conduction characteristics and the ordered lamellar structure of PMS-g-PVP,the efficient proton conduction channels can be constructed,which increases the proton conductivity of hybrid membranes by two orders of magnitude.In addition,PW can play a role in electrostatic crosslinking and nanoreinforcement,increasing the modulus of the hybrid membranes by 5.6 times.This research utilizes electrostatic interaction between POMs and comb copolymers to induce the ordered transformation of polymer microphase structure and successfully prepares hybrid proton exchange membranes with efficient lamellar proton conduction channels.Second,we have expanded the POMs hybrid comb copolymers system and prepared PEG covalently grafted silicotungstic acid(GSiW₁₁)hybrid Nafion proton exchange membranes.The PEG chains of GSiW₁₁ can form dense hydrogen bonds with the sulfonic acid groups of Nafion side chain through ether oxygen atoms,generating strong complexation,thereby achieving stable immobilization of GSiW₁₁ in the Nafion ionic nanophases and solving the problem of anionic POMs easily losing from the Nafion matrix.This molecular level hybridization strategy targeting the Nafion ionic nanophases not only maintains the microphase structure of Nafion and the connectivity of the hydrophilic channels,but also achieves the enrichment of hydrated protons in the centre of the channels,improving the efficiency of proton transport.Meanwhile,GSiW₁₁ can dissociate additional protons and provide abundant proton hopping sites,which helps to improve the proton conductivity of hybrid membranes.The multiple complexation between GSiW₁₁ and sulfonic acid groups also increases the mechanical strength of the hybrid membranes.By combining molecular dynamics simulation,we analyzed the relationship between the microphase structure and macroscopic properties of hybrid membranes at the molecular level.This research effectively enhances the complexation between POMs and Nafion ionic nanophases by covalently grafting organic anchoring groups onto POMs,achieving stable immobilization of POMs in Nafion and improving the comprehensive performances of hybrid membranes.Third,we designed fluoroalkyl chains covalently grafted silicotungstic acid(FSiW₁₁)with excellent compatibility with Nafion based on the unique molecular structure of the fluorocarbon main chain of Nafion,and introduced FSiW₁₁ into the Nafion ionic nanophases to prepare high-performance hybrid proton exchange membranes.FSiW₁₁ exhibits amphiphilicity,with its hydrophobic fluoroalkyl chain thermodynamically compatible with perfluorinated main chain of Nafion,while silicotungstic acid is hydrophilic.This amphiphilicity allows FSiW₁₁ to accumulate at the interface between hydrophobic fluorinated phase and hydrophilic ionic nanophase of Nafion,and to achieve stable immobilization by inserting fluoroalkyl chains into fluorinated phase.This hybrid strategy maintains the microphase structure of Nafion and the connectivity of hydrophilic channels,while improving the proton conductivity of Nafion and making it more dense,significantly enhancing the screened ability to hydrogen and vanadium ions of the hybrid membranes.Fuel cells and vanadium redox flow batteries equipped with hybrid membranes exhibit excellent power density and energy efficiency,respectively.This research utilizes the good compatibility between fluoroalkyl chains and Nafion to achieve stable immobilization of POMs,effectively improving the proton conductivity and selectivity of Nafion.In summary,this dissertation prepared a series of hybrid proton exchange membranes by combining POMs with different types of comb copolymers through non-covalent interactions.These membranes integrate the high proton conductivity,nano-enhancement properties of POMs,and microphase separation properties of comb copolymers,exhibiting enhanced proton conductivity,selectivity and mechanical properties.This dissertation systematically studies the regulation of microphase structure,hybrid assembly methods,and performance improvement mechanisms of hybrid membranes,elucidating the relationship between the microphase structure and macroscopic properties of hybrid membranes.The relevant results can provide references for the design of high-performance proton exchange membrane materials.