The Application Of Polymer-polyoxometalates Nanocomposites As Proton Conductors

In order to achieve peak carbon dioxide emissions and carbon neutrality goals,it’s imperative to develop renewable energy which demands high-performance energy storage devices.The advantages of hydrogen energy in centralized,long-term and large-scale scenarios make it an irreplaceable part of the new energy market.As a clean and efficient electrochemical power generation device,fuel cell is the mainstream of hydrogen energy utilization at this stage.Proton exchange membranes（PEMs）are not only the membrane between cathode and anode isolating gas and preventing short circuit,but also undertake the task of efficient proton transfer in device,which have important effects on the performance and stability of fuel cell.A large group of well-defined nanoclusters,polyoxometalates（POMs）,are formed by linking early-transition-metal oxide polyhedrons through shared corners,edges and planes.Resulting from the strong dissociation of counterions,protic POMs clusters demonstrate strong acidity which enables them great potential as proton conductors.However,the proton conduction mechanism of POMs,in which protons are delivered through hydrogen-bond networks formed by water of crystallization,leads to the dependence of POMs’proton conductivity on environmental factors,limiting the application of POMs in proton conductors.Improving the conductivity stability and mechanical strength of POM-based conductors is critical to ensure its commercial application.The excellent thermal stability and viscoelasticity of polymers make them good candidates to replace the hydrogen-bond networks of water in POMs and further construct new proton transport path to realize high conductivity,high stability and good mechanical performance of nanocomposites simultaneously.With the background above,we build poly（ethylene glycol）-polyoxometalate（PEG-POM）nanocomposites based on the non-covalent interaction between PEG and POM,which are found to possess excellent proton conductivity and unique mechanical property.Therefore,further studies are conducted on these systems.The in-depth study involving microstructure and proton conducting mechanisms reveals the existing of solid-like decoupling mechanism,which points out the path of promoting proton conductivity and mechanical performance simultaneously.Based on these results,we further enhance the composites’mechanical performance by increasing PEG’s molecular weight and constructing co-crystal structure,respectively.Two solid state proton conductors with excellent anhydrous proton conductivity are prepared.Detailed contents are as follows:（1）Phosphotungstic acid(PW₁₂)clusters can be fully dissolved in the melt of PEG400with high concentration（ca.70 wt.%）,forming clear,stable solutions when cooled to room temperature for the hydrogen bonding and electrostatic interaction between the two components at molecular scale.Originated from the dynamics of PEG chains and high concentration of H⁺in system,the conductivity of PEG400-70%PW₁₂ nanocomposites can reach as high as 1.01×10^-2 S cm^-1 at 80 ^oC,45%relative humidity（RH）.Meanwhile,the PEG400-70%PW₁₂nanocomposites behave like solid with negligible flow（～273 Pa s）due to the volume filling effect of PW₁₂ clusters and the dynamic cross-linked hydrogen bonding and electrostatic interaction between two components,guaranteeing the safety of PEG400-70%PW₁₂ as electrolytes.Upon exerting high-speed shear forces(>32 s^-1),the PEG400-70%PW₁₂nanocomposites flows with continuously decreased viscosity.The shear thinning property enables the nanocomposites good processibility and the wettability of electrolytes to electrodes under typical high shear rate processing condition.（2）Three types of POMs with identical structure and size（～1 nm）but different charge densities are mixed with PEG400,respectively,by non-covalent interaction.The confinement effect of POMs clusters on polymer due to the strong attraction between POMs and PEG and the volume filling effect of POMs clusters both contribute to the high viscosities of PEG-POM nanocomposites,which are analyzed quantitatively by modified Vand’s viscosity theory.The increasing of viscosities can be observed with the rising charge densities of POMs due to the increasing confinement strength on PEG substrates from POMs particles.Fractional Walden rule is further applied to analyze the relation between viscosity and proton conductivity of PEG400-POM nanocomposites.And the transition from liquid-like coupling proton conduction mechanisms to solid-like decoupling mechanism with increasing POMs contents is identified.（3）Due to the strong non-covalent interactions with poly（ethylene oxide）（PEO）,polyoxometalates are able to form stable nanocomposites with high molecular weight PEO and fully inhibit its crystallization at high content of POM（70 wt.%）,facilitating the fast dynamics of PEO chains/segments,as evidenced from dielectric spectroscopy studies.The faster dynamics of PEO chains and high concentration of H⁺ensure the fast proton transportation in PEO matrix and the improvement of the nanocomposites’anhydrous proton conductivities.With POMs’loading ratio approaching 70 wt.%,the nanocomposite’s proton conductivity reaches as high as 6.86×10^-3 S cm^-1 at 100 ^oC in anhydrous（N₂）environment.The nanocomposites’mechanical performance can be further optimized upon the tuning of PEOs’molecular weight and finally,flexible,self-supported anhydrous proton conductor PEO600K-70%PW₁₂ can be obtained benefiting from the physical cross-linking networks between PEO and POM clusters and the entanglement of high molecular weight PEO.Therefore,the super-capacitor devices assembled with PEO-PW₁₂ nanocomposites perform well under extreme conditions.（4）Silicotungstic acid(Si W₁₂)clusters can form simple cubic co-crystal structure with PEG for the non-covalent interaction between them,in which the conformation of PEG chains transforms into Zig-zag from helix conformation due to the space confinement effect.The change of PEG conformation results in the increase of end-to-end distance and influences the structure and size of PEG segmental relaxation units,leading to the reduced main chain diffusion and enhanced segmental relaxation process.The changes in dynamics and contribution of high concentration H⁺in nanocomposites enable the high conductivity of PEG400-Si W₁₂ co-crystals.Specifically,the proton conductivity of PEG400-80%Si W₁₂ co-crystal reaches 8.91×10^-3 S cm^-1 at 110 ^oC,anhydrous environment.Meanwhile,the co-crystal structure ensures the flexible mechanical property of PEG-Si W₁₂ nanocomposites with Young’s modulus as high as 59 MPa.