N-spirocyclic Based Polysulfone Anion Exchange Membranes For High Alkali Stability And Conductivity

Anion exchange membrane fuel cells(AEMFCs)can achieve efficient utilization of hydrogen energy to meet major national needs.Under alkaline conditions,fuel cells have faster oxygen reduction kinetics and can use non-noble metal catalysts.However,as the key component of AEMFCs,anion exchange membranes(AEMs)have low OH-intrinsic mobility.Cationic groups and polymer main chains are easily degraded under OH-attack,resulting in low ionic conductivity and poor alkali stability.Designing molecular structures of the membrane materials to promote microphase separation can effectively construct OHconduction channels,thereby increasing the OH-conductivity of the membranes.Applying the cationic groups with high alkali stability can significantly improve the alkali stability of the membranes,thereby prolonging the service life of the membranes.The alkali stability of N-spirocyclic quaternary ammonium functional groups is higher than the commonly used imidazole,aliphatic amine and cyclic amino cations due to their constrained ring conformation.However,the high rigidity of the bicyclic structures of the N-spirocyclic rings makes the membranes difficult to form microphase separation,which leads to key problems such as poor membrane-forming properties,difficulty in balancing the conductivity and swelling,low power density of the N-spirocyclic based fuel cells.This paper proposes the design methods of microphase separation structures for N-spirocyclic based alkaline stable and highly conductive polysulfone anion exchange membranes.N-spirocyclic side chain structures with ether oxygen spacers,hydrogen bond diamine ether oxygen spacers as well as hydrophilic hydrogen bond diamine ether oxygen N-spirocyclic/hydrophobic alkyl chains are designed,which regulate the hydrophilic-hydrophobic microphase separation in the membranes.The hydrogen bonds cooperate with the ionic bonds could promote OHconduction,which significantly improve the performance of N-spirocyclic based anion exchange membranes.N-spirocyclic cations have high stability,but the grafting of the N-spirocyclic cations is difficult due to their closed-loop structures.The conventional bromomethyl grafting routes will form N-spirocyclic side chains with benzyl structures,which could lead to the poor alkali stability of the functional groups and polymer main chains.The synthetic routes of hydroxyl-substituted N-spirocyclic cations are proposed to connect the N-spirocyclic cations to the polysulfone main chains through Williamson reaction in this chapter.Then the in-situ thermal crosslinking between N-spirocyclic based polysulfone and chloromethyl polysulfone is used to prepare N-spirocyclic based polysulfone cross-linked membranes(Cr-ASD-PSF and Cr-ASU-PSF)with ether oxygen spacers.The membranes show excellent thermal alkali stability(the conductivity of the Cr-ASU-PSF 1.92 membrane is only reduced by 4.4%after immersed in 1 M KOH at 80? for 720 h),which are better than the N-spirocyclic based anion exchange membranes with benzyl spacers reported in the literatures.The rotation of the ether oxygen bonds makes the N-spirocyclic based polysulfone membranes form microphase separation structures(the ion clusters are about 5.2 nm).The conductivity of the Cr-ASD-PSF 2.05 membrane is 85.7 mS cm-1 and the swelling ratio is 39.6%at 80?.The ether oxygen bonds are easy to rotate,but the bicyclic structures of the N-spirocyclic cations are highly rigid,making it difficult for the uncrosslinked N-spirocyclic based polysulfone with ether oxygen spacers to form membranes separately.Therefore,it is urgent to improve the mobility of the N-spirocyclic side chains.In this thesis,diamine spacers with hydrogen bonds are designed.The commonly used imidazole functional group whose rigidity is much lower than that of N-spirocyclic rings is used to study the regulating effect of diamine spacers with hydrogen bonds on the microphase separation structures and the OH-conduction mechanism.Molecular dynamics simulations show that there are hydrogen bond networks between amine groups and water molecules,which can expand the OH-conduction channels and cooperate with ionic bonds to promote OH-conduction.Adjusting the length of the diamine spacers can optimize the hydrophilic-hydrophobic microphase separation structures.As the length of the diamine spacers increases,the mobility of the side chains increases to promote the aggregation of the cationic groups,while reducing the incompatibility between the side chains and the polymer main chains,which is not conducive to the aggregation of the cationic groups.Therefore,the size of the ion clusters can be used as the criterion for the optimization of the length of the side chains.When the length of the diamine spacers is 6,the imidazolium based PSF-C6-DMIm membrane containing hydrogen bond diamine spacers has large ion clusters(about 11.2 nm),high conductivity(125.8 mS cm-1 at 80?)and low swelling ratio(<20%at 80?).In addition,N-spirocyclic side chains with hydrogen bond diamine ether oxygen spacers are proposed to improve the mobility of the N-spirocyclic side chains,microphase separation,conductivity and mechanical properties.Molecular dynamics simulations show that,compared with the imidazolium side chains with hydrogen bond diamine spacers,the increase of the rigidity of the N-spirocyclic side chains with hydrogen bond diamine spacers and the decrease of their incompatibility with the main chains lead to the decrease of the microphase separation degree.But compared with the N-spirocyclic side chains with ether oxygen spacers,the N-spirocyclic side chains with hydrogen bond diamine spacers have higher chain mobility and entanglement,which significantly promote microphase separation.The polymers have good membrane-forming properties and the membranes have higher conductivity and mechanical properties.Similar to the imidazolium based membranes with hydrogen bond diamine spacers,the PSF-C6-ASD membrane with 6-CH2-spacers exhibits the largest ion clusters(about 8.5 nm)and the highest OH-conductivity(107.1 mS cm-1 at 80?).The chain entanglement of the flexible spacers can significantly improve the membrane-forming properties of the N-spirocyclic polymers,mechanical strength and toughness of the membranes.The elongation at break of the PSF-C6-ASD membrane is 3 times of that of the Cr-ASD-PSF 2.05 membrane.After the PSF-C6-ASD membrane immersed in 1 M KOH at 80? for 720 hours,the retention rates of the conductivity and mechanical strength reach 98.3%and 92.3%,respectively.The peak power density of the fuel cell assembled with PSF-C6-ASD under the optimal test conditions is 149.3 mW cm-2.The N-spirocyclic side chains with hydrogen bond diamine ether oxygen spacers have high mobility.However,the steric hindrance effect of the N-spirocyclic cations leads to large free volume in the membranes.The increase of feed gas velocities could cause a significant decrease of the open circuit voltages(open circuit voltage is only 0.71 V,when gas velocity is 1000 mL min-1)during fuel cell tests,which indicates that the permeation of feed gas in the membranes is serious and limits the power density of N-spirocyclic based fuel cells.The N-spirocyclic based membrane structures with hydrophilic-hydrophobic grafts based on hydrophilic N-spirocyclic side chains with hydrogen bond diamine ether oxygen spacers and hydrophobic alkyl side chains are further designed in this thesis to reduce the swelling and gas permeability of the membranes.Molecular dynamics simulations show that the hydrophobic alkyl side chains can not only promote the aggregation of rigid N-spirocyclic groups and microphase separation,but also fill the free volume in the membranes,which is beneficial to reduce the permeation of feed gas.The N-spirocyclic based membrane with optimized n-octylamine hydrophobic chain length has better conductivity-swelling balance(the OH-conductivity and swelling ratio are 136.2 mS cm-1 and 15.4%at 80 ?,respectively;the tensile strength is 28.5 MPa),which makes the fuel cells maintain high open circuit voltages(>1.0 V)at high gas velocities.The power density of the fuel cell can reach up to 850.1 mW cm-2 under the optimal test conditions,which is much higher than other N-spirocyclic based fuel cells reported in the literatures,greatly improving the application potential of the high alkaline stable N-spirocyclic based anion exchange membranes.