Preparation And Performance Of Poly(Ionic Liquid)/Polybenzimidazole Membranes For High Temperature-Proton Exchange Membranes

Nowadays,fossil energy is rapidly depleting,and the environmental pollution caused by the use of fossil energy has become severe.Hydrogen energy is widely researched as a good substitute for fossil energy,including petroleum.In hydrogen energy applications,the conversion and utilization of energy is an unavoidable problem.Proton exchange membrane fuel cells(PEMFCs),as an energy conversion device,are the most promising energy device for mobile devices such as new energy vehicles et al.High temperature-proton exchange membrane fuel cells(HT-PEMFCs),as one of the most promising fuel cell technologies,have a simplified water/heat management system and promoted catalyst kinetic reaction due to elevated operating temperature.In HT-PEMFCs,high-temperature proton exchange membranes(HT-PEMs) as the core component have become a study focus for researchers.Polybenzimidazole phosphoric acid doped membranes(PA/PBI) are the most promising HT-PEMs because of their good thermal stability,excellent chemical stability and high mechanical properties.Phosphoric acid doping is prerequisite for PA/PBI membranes to achieve high proton conductivity,which requires high phosphoric acid doping levels to obtain satisfactory electrochemical performance,resulting in decreased mechanical properties.Simultaneously,when the phosphoric acid doping level grows higher,more "free acid" will be produced,which are easily lost.Phosphoric acid loss will cause some problems such as membrane performance decreases,membrane size-reduction and electrode corrosion.Therefore,obtaining membranes with high proton conductivity and mechanical properties and improve their phosphoric acid retention capacity is a scientific problem faced by researchers.Poly(ionic liquids)(PILs),as an ionic polymer,contain a lot of ionic structures.By introducing PILs,making the cation portion of PILs with the phosphoric acid molecules to form ion-pairs is a feasible solution of improving phosphoric acid stability.Another strategy is to improve the phosphoric acid molecule migration resistance by form densely cross-linked structure,resulting in improved phosphoric acid retention.(1)A fluorine-containing polybenzimidazole is used as polymer matrix,and an imidazole-type ionic liquid with double bonds is polymerized into PILs by in-situ free radical reaction.Simultaneously,a cross-linking agent is added to construct cross-linking networks,further improving the mechanical properties of membranes.Alkali-acid doping is used to obtains the ion-pairs of PILs and phosphoric acid.To reduce the influence of phosphoric acid doping levels,the phosphoric acid doping concentration were changed to control the phosphoric acid doping level of the membrane.The results prove that the incorporation of PIL improves the proton conductivity of membranes under similar PA uptake.In particularly,the proton conductivity of 6FPBI-PIL 40 membrane increases to 0.069 S cm^-1,higher than that of pristine 6FPBI membrane(0.039 S cm^-1),confirming the promoting effect of PIL on proton conductivity.Simultaneously,the incorporation of PIL significantly improves the phosphoric acid retention of membranes.However,the mechanical properties of membranes are decreased as PIL content increases.(2)In the previous work,PIL composite membranes have shown excellent performance.However,there are many types of PILs.Which PIL is more suitable for HT-PEM?For this issue,three representative ionic liquids([VBIm]Cl,[MPIm]Br and[TPAm]Br)were selected as monomers of PIL,and the norbornene-type polybenzimidazole is used as the polymer matrix.Ionic liquids are polymerized into PIL through in-situ radical reaction to form a cross-linked structure with the polymer chains.The membranes have larger free volume due to the three-dimensional structure of the norbornene monomer,but the incorporated PILs will fill this part of space to varying degrees,affecting the proton conductivity.Among them,Nb PBI-MPIm exhibits the highest proton conductivity(170°C,0.074 S cm^-1),but Nb PBI-TPAm membrane has the highest power density(160°C,385 m W cm^-2).Similarly,the ion-pairs of the PIL and phosphoric acid is obtained by alkali-acid doping,effectively improving the phosphoric acid retention of the membranes.(3)According to the above results,PIL with three-dimensional cation can more effectively improve the proton conductivity.Therefore,we used bulky groups as cation groups of ionic liquid to prepare a diazabicyclic type-ionic liquid with a double bond and a chlorobutyl group([CPDOc]Br₂),which was inserted into polymer chains to prepare poly(ionic liquid)/polybenzimidazole(PBI-CPDOc)cross-linked membranes by in-situ free radical reaction.This PIL can effectively promote proton conductivity of membranes,showing high proton conductivity under low phosphoric acid uptake.Among them,PBI-CPDOc₃₀ with 139%phosphoric acid uptake exhibits a proton conductivity of 0.121 S cm^-1 at 170°C.Although,the cross-linked structure is formed by the cationic groups,weakly improving the mechanical properties of the membranes,which lower than the decrease in mechanical strength caused by the incorporation of PIL.(4)For the problems of decreased mechanical strength in the previous work,we developed dual cross-linking composite membranes to improve the cross-linking degree of membrane.According to the excellent proton conductivity of composite membranes in the previous work,the diazabicyclic groups are still used as cationic groups of ionic liquid,then a siloxane and a double bond are inserted into the ionic liquid.On the order hand,the norbornene-type polybenzimidazole in chapter fourth is used as the polymer matrix to construct the first cross-linking networks with ionic liquid by in-situ radical reaction.Therefore,the siloxane groups of PIL were fixed in the membranes,which form the second cross-linking networks by a hydrolysis reaction under acidic conditions.Because of the first cross-linking networks,the siloxane groups are limited,avoiding structural defects caused by particle aggregation.The Nb PBI-TSPDO₃₀ membrane exhibits a proton conductivity of0.061 S cm^-1 at 170°C,indicating the dual cross-linking networks will not hinder proton conductivity.The phosphoric acid retention capacity of the Nb PBI-TSPDO membranes is increased due to PIL incorporation and dual cross-linking network formation.The phosphoric acid retention of Nb PBI-TSPDO membranes has reached 81%after 400 h test at160°C and 64%after 96 h test at 80°C/40% RH.