Block Copolymer/Polyoxometalate Synergistic Hybridization Strategy For Proton Exchange Membranes

The consumption of fossil fuels caused the aggravation of environmental problems and energy crisis.It is urgent to develop advanced renewable energy technologies to promote the low-carbon economy for a sustainable society.Promising energy conversion and storage technologies such as proton exchange membrane fuel cells and vanadium redox flow batteries（VRFBs）have attracted much attention.The key component in determining their efficiency is the proton exchange membrane（PEM）.However,for the most commonly-used PEMs such as the representative perfluorinated membrane,Nafion and the typical non-fluorinated membrane,SPEEK,there is a trade-off among compatibility,proton conductivity and active-species permeation.It is expected to improve the comprehensive performance by introducing functional fillers to PEMs.Nevertheless,most fillers possess high surface polarity and poor compatibility with polymer matrices.Therefore,the fillers can easily aggolomerate in PEMs and result in macrophase separation,which will damage the continuity of the functional nanophases in PEMs and be detrimental to the overall performance.Polyoxometalates（POMs）are a class of molecular-defined ionic metal oxide nanoclusters with a diameter of 1-5 nm and possess a high proton conductivity around0.2 S cm^-1.These advantages make POMs ideal functional modifiers for the precise hybridization of PEMs.However,limited by the high water-solubility of most anionic POMs,they are difficult to be stably immobilized when hybridizing with sulfonated PEMs because of the electrostatic repulsion.Thus,it is still a challenge to leverage the merits of POMs while realizing their stable immobilization in PEMs.In this thesis,we propose a novel synergistic hybridization strategy by using functional block copolymers（BCPs）and POMs to break the imbalance among compatibility,proton conductivity and proton selectivity in the conventional bulk hybridization.With rational design,POMs can be selectively immobilized by a specific block through non-covalent interactions.Moreover,the interfacial-active BCPs with unique self-assembly behaviour can achieve the optimization in microscopic nanostructures and enhance the macroscopic properties of PEMs.The details of this thesis are as follows:First,BCPs composed of polystyrene（PS）and poly（vinyl pyrrolidone）（PVP）,named as PSP were designed and synthesized.Nanocomposites composed of phosphotungstic acid（PW）and PSP were fabricated by solution casting.The non-covalent interactions between PW and PVP can not only assist the immobilization of PW,but also induce a series of morphological transitions from less-ordered lamellar to long-range ordered lamellar and finally to the cylindrical phase with PVP/PW as the continuous phase.The introduction of PW can endow the hybrid with proton conducting properties as well.The N and O atoms in the pyrrolidone rings of PVP can act as proton hopping sites and assist the construction of proton conducting network.The proton conductivity of the PSP/PW nanocomposite is two orders of magnitudes higher than that of the original PSP.Additionally,as the typical inorganic nano-fillers,PW can electrostatically cross-link the PVP domains and increase the storage modulus of the PSP/PW hybrid by 6 times.This research studied the mechanism and the application of BCPs/POMs hybrid,which provides a reference for the construction of nanostructured POMs/polymer hybrid electrolytes.Second,based on the previous work,PSP/POMs were used as additives with a self-assembly ability for the modification of PEMs.Because of multiple non-covalent interactions among SPEEK,PSP and PW,three components showed good compatibility and no macrophase separation occurred in the hybrid membranes.PSP/PW can in-situ self-assemble into ellipsoidal nano-assemblies with a“core-shell”structure and a long-axis of 50 nm in the SPEEK matrix.The hydrophobic PS cores and protonated pyrrolidone rings in the PVP/PW shells can effectively suppress the vanadium crossovers.Additionally,the PVP/PW hydrophilic shells can act as extra proton-conducting regions in coordination with the surrounding proton-conducting nanophase in SPEEK,facilitating the proton transport.The optimization in the nanostructure can bring an overall enhancement in proton conductivity,proton selectivity and VRFB performance.It is the first time to apply the self-assembled nanostructures of BCPs to the modification of PEMs,which provides a reference for the application of self-assembly in energy technologies.Third,fluorinated BCPs（FBC）,composed of a short perfluoroalkyl block and poly（2-methyl-2-oxazoline）were designed and synthesized.Interfacial-active FBC can be well-compatible with Nafion and can act as compatibilizers to decorate the hydrophobic-hydrophilic interfaces of Nafion.The cooperative noncovalent interactions among Nafion,FBC,and POMs can not only electrostatically cross-link the sulfonic acid groups but also stably immobilize POMs in the ionic nanophase.The optimized 1 nm-shrunk ionic nanophase with preserved continuity,abundant proton transport sites,and efficient vanadium screeners can lead to a comprehensive enhancement in proton conductivity,selectivity,and VRFB performance.This study developed a precise hybridization strategy for the ionic nanophase of PEMs based on the interfacial-active BCPs and POMs,which opens a new route to develop advanced membrane materials for various energy devices.In conclusion,by rationally designing functional BCPs,POMs can be stably immobilized through multiple non-covalent interactions and the nanostructures of PEMs can also be optimized.As a result,the enhancement in overall performance of PEMs can be achieved.The BCPs/POMs synergistic hybridization strategy proposed by this thesis can break the trade-off among component compatibility,proton conductivity and proton selectivity,overcoming the disadvantages of the conventional bulk hybridization.This strategy provides a new route for the modification of advanced polymer electrolyte materials in renewable energy devices,and can extended to the design of other functional hybrid materials.