Preparation And Properties Of Micro-separated High-temperature Polymer Proton Exchange Membranes

At present,with the gradual exhaustion of non-renewable energy sources such as fossil fuels,the development of clean energy technologies has become an urgent need for sustainable social development.Energy conversion and storage technologies with high energy density are essential for the transition to a sustainable and clean energy future.High-temperature proton exchange membranes（HT-PEMs）have attracted much attention as key materials for energy technologies such as fuel cells and supercapacitors.HT-PEMs need to possess both excellent mechanical strength and high proton conductivity to satisfy their stable and efficient proton conduction under high temperature and low humidity conditions.However,HT-PEMs face the challenge that it is difficult to improve the mechanical and conductive properties together.In general,the introduction of crosslinked structures can enhance the interchain interactions of polymers at the molecular scale,thereby improving the mechanical properties of membranes.On the other hand,the construction of a microphase separation structure can promote the functional decoupling of the proton conduction phase region and the mechanical support phase region at the micro-nano scale,which is beneficial to the simultaneous improvement of the performance of both.However,the chain assembly behavior is difficult to control due to the limited mobility of polymer chains in the crosslinked state.Therefore,it is difficult to construct microphase-separated structures in crosslinked polymer membranes,and the development of suitable solutions is urgently needed.In response to the above problems,we developed strategies such as covalent micro-crosslinking,non-covalent crosslinking,in situ enhanced crosslinking to construct phosphoric acid（PA）-adsorbed HT-PEMs with a microphase-separated structure,starting from the regulation of the segmental mobility of the polymer crosslinked network.The fine regulation of the position,strength and dynamics of the crosslinking point in the crosslinking network makes it possible to drive the self-assembly of the chain segments through the polarity difference after adsorption of PA to form HT-PEMs with bicontinuous structure.This improves the thermal stability,mechanical properties,and proton conductivity of HT-PEMs.The applications of these membrane materials in high temperature fuel cells and high temperature supercapacitors are also explored.The main research content of this paper includes the following three parts:First,based on the rearrangement of segments in covalent micro-crosslinked polymer networks,we proposed a strategy of PA induced self-assembly of polymer networks to construct bicontinuous HT-PEMs.We prepared a homogeneous micro-crosslinking network of incompatible brominated poly（ether-ether-ketone）（Br-PEEK）and poly（4-vinyl pyridine）（P4VP）with low crosslinking density.By controlling the average length of the P4VP segment between adjacent crosslinks to be greater than the length of the Kuhn segment of P4VP,this endows the P4VP component with a certain chain mobility.When the micro-crosslinked network adsorbs PA,the polarity difference between P4VP and PEEK increases,which drives the self-assembly of the P4VP segment and promotes the transition of the crosslinked membrane from a homogeneous phase to a bicontinuous phase.The XPS results confirmed the covalent crosslinking of Br-PEEK with P4VP.The SAXS and AFM-IR data confirmed the formation of a bicontinuous microphase structure.It should be pointed out that the size of the bicontinuous phase region can be finely tuned by changing the crosslinking density.Moreover,the proton conductivity of the membrane increases with decreasing phase region size,exhibiting a confinement-enhanced effects.In addition,TGA and DMA tests show that reducing the size of the domains can effectively improve the thermal stability and mechanical properties of the membranes.The optimized PEM-PA-3membrane exhibits a high storage modulus of 39.3 MPa and a proton conductivity of70 m S cm^-1 at 150°C.The high-temperature supercapacitors（HT-SCs）assembled with the best comprehensive performance HT-PEM and activated carbon electrodes exhibit a good capacitance of 138.0 F g^-1(110.4 m F cm^-2)at 150°C.It still maintains a high Coulombic efficiency of 98.3%after 2500 GCD cycles at 150°C.Second,based on the electrostatic crosslinking function of polymetallic oxygen clusters,we developed a non-covalent crosslinking strategy to prepare a dynamically rearranged hybrid polymer network,which can form HT-PEMs with bicontinuous microphase structure under the induction of PA.Based on the interaction of POMs with poly（terphenyldimethylpiperidine）（PTP）and polyvinylpyrrolidone（PVP）,the Keggin type H₃PW₁₂O₄₀（PW）is selected as the crosslinking agent and proton carrier dispersed in the PTP-PW-PVP ternary hybrid membrane.The FT-IR data confirmed electrostatic interactions between the three components.The XRD results indicated that the PWs were uniformly dispersed in the hybrid membranes.TEM and EDS mapping showed that the electrostatic crosslinked network was a homogeneous structure.SAXS and AFM confirmed that after PA adsorption,the homogeneous network increased due to the polarity difference between PTP-PW-PA and PVP-PW-PA,and self-assembled into bicontinuous HT-PEMs.TGA showed that PN2 was structurally stable up to 350°C.Therefore,the PN2-PA after adsorption of PA can be used stably for a long time at160°C.The optimized PN2-PA membrane exhibits a high proton conductivity of 55m S cm^-1（160°C）and a maximum tensile strength of 7 MPa at a PA doping content of only 121 wt%.Applying PN2-PA to a high-temperature supercapacitor,the specific capacitance is 145.4 F g^-1(116.3 m F cm^-2)at 150°C,and the Coulombic efficiency still maintains 98.8%after 3000 cycles at 150°C.PN2-PA was applied to a H₂/air fuel cell with a peak power of 273.6 m W cm^-2 at 160°C.This strategy to construct bicontinuously structured HT-PEMs based on dynamic crosslinked networks is simple and feasible,and can be extended to prepare other functional electrolyte membranes.Third,based on the tunable denaturation of the electrostatic interactions between polymetallic oxygen clusters and polymers,we propose a strategy to enhance the mechanical properties of polymetallic oxygen cluster hybrid networks through in situ reactions within the membrane.This solves the problem that basic polymers cannot be directly crosslinked with polyoxometalates to form membranes,and bicontinuous HT-PEMs with high mechanical strength are prepared.Polyethyleneimine（PEI）and PW are directly precipitated in the solvent due to strong electrostatic crosslinking,and cannot be produced by solution casting.To this end,we chose poly（2-ethyl-2-oxazoline）（POx）,which is stably co-soluble with PW,as the precursor of PEI,and pre-constructed the PTP-PW-POx ternary hybrid membrane.Then POx was converted into PEI in situ by hydrolysis reaction,and the hybrid membrane based on PTP-PW-PEI（TE）crosslinked network was prepared.The membrane can form bicontinuous HT-PEMs through segmental self-assembly induced by subsequent PA adsorption.The FT-IR data confirmed the hydrolysis of POx to PEI and in situ electrostatic crosslinking with PW.The SAXS and AFM results indicated that PA could induce the transition of the hybrid membrane from homogeneous to bicontinuous structure.The enhanced electrostatic crosslinking resulted in a significant improvement in the dimensional stability of the membrane after saturated adsorption of PA.After optimization,the breaking strength of the TE2-PA membrane with excellent comprehensive properties is 11 MPa.The membrane with proton conductivity of 70.4 m S cm^-1 was used for high-temperature supercapacitors with a specific capacitance of 151.6 F g^-1 at 150°C and a capacitance retention of 85.0%after 3000 charge-discharge cycles at high temperature.The above results show that by rationally designing the polymer network structure,the polymer chain can be endowed with a certain ability of kinematic rearrangement in the crosslinked state.These crosslinked networks can serve as precursors for polarity-driven segmental self-assembly,transforming from a homogeneous structure to a microphase-separated structure in the presence of increased polarity differences between components（such as during PA adsorption）.It can be used to prepare bicontinuous HT-PEMs.This type of HT-PEMs has both molecular-scale crosslinking structure and nano-scale microphase structure,which not only enhances the robustness of interchain interactions,but also realizes the decoupling of the functions of different phase regions,thus showing excellent comprehensive performance.These membrane materials not only have good mechanical properties,proton conductivity and thermal stability,but also facilitate large-scale preparation,showing good performance and application potential in high-temperature energy storage and conversion devices.