Design And Performance Of Anion Exchange Membranes Having Sterically Hindered Organic Cations For Alkaline Fuel Cell

As the core component of alkaline anion exchange membrane fuel cell（AAEMFC）,anion exchange membrane（AEM）is responsible for transferring counter ions（e.g.,OH^-or X^-）as well as separating the cathode from the anode,and its performance is directly related to the efficiency of entire fuel cell.Currently,problems such as low ionic conductivity,poor dimensional stability and low alkaline stability still hinder the development of AEM.In terms of designing certain structure substituents around the organic cations,the alkaline stability of AEMs with sterically hindered organic cations could be improved due to the steric hindrance of substituents protecting the organic cations from being attacked by hydroxide ions.However,polymeric AEMs with sterically hindered organic cations remain new challenges:on one hand,bulky groups on the fixed organic cations may reduce the entanglement interactions in polymer chains,thus leading to the poor film-forming ability;on the other hand,the hydrophobicity of the substituent may deteriorate the hydration performance of membranes,thereby reducing the ionic conductivity of AEM.This paper focused on the key constraints of AEMs with sterically hindered organic cations by following the“structure-performance”relationship of polymer design.As an effective approach,covalent cross-linking between polymer chains could significantly improve the mechanical property of polymeric membranes,and on this basis,the crosslinkers with/without hydrophilic ether groups have been employed for the tuning of the hydrophilicity of these membranes.Besides,with the help of physical chain entanglement between the grafted flexible poly（ethylene glycol）methyl ether（mPEG）segments,the AEMs with sterically hindered organic cations not only exhibited good mechanical properties and high hydrophilicity,but also achieved high ionic conductivity at low ion exchange capacity（IEC）due to the induced microphase separation structure.Moreover,excellent alkaline stability of polyolefin-based AEMs with long side chain steric protection were obtained based on the tight entanglement between flexible aliphatic polymer chains.The specific contents and innovative results are as follows:（1）The prepared imidazole molecules with steric hindrance were grafted to the side chain of poly（2,6-dimethyl-phenylene oxide）s（PPO）s by means of the efficient Menshutkin reaction,then the novel bulky imidazolium cations based crosslinked AEMs were obtained via olefin metathesis reaction catalyzed by a Grubb’s second generation catalyst at room temperature.The existence of the crosslinking network not only guaranteed the mechanical strength of AEMs with bulky cations（tensile strength at maximum load of 20.8～49.9 MPa）,but also the synergy with steric hindrance at C2,C4,and C5 positions of imidazolium made membranes exhibit excellent base stability.Namely,the conductivities of aged crosslinked membranes could still maintain～100%after soaking for up to 960 h in 1 M NaOH solution at 80℃.（2）Series of hydrophilic/hydrophobicity adjustable crosslinked AEMs with bulky imidazolium cations were achieved by thermally-initiated“thiol-ene”click chemistry reaction with using two different dithiol crosslinkers agents.The research results showed that these crosslinked AEMs exhibited easily regulatable water uptake,dimensional stability and ion migration rate as well as ensuring good mechanical properties and alkali resistance due to this design of the crosslinking reaction.In particular,compared with uncrosslinked ionomers,the membrane electrode assembly（MEA）with the crosslinked imidazolium-based PPO ionomers in catalyst layers showed higher peak power density of 200 m W·cm^-2 at a current density of 475 m A·cm^-2 and enhanced short-term life（20 h）at a constant current density of 200 m A·cm^-2.（3）In order to solve the limitations of the crosslinking network on the water uptake and ion transport abilities of membranes,multi-substituted,methoxypolyethylene glycols（mPEG）modified AEMs with bulky imidazolium were prepared by introducing the mPEG structure at the N1 position of steric hindrance imidazole.Through the physical entanglement of hydrophilic polymer chains,the bulky imidazolium-based AEMs with mPEG pendent demonstrated outstanding mechanical flexibility and the elongation at break of hydrated membrane（100%relative humidity）at 20℃ could be as high as 133.3%.Moreover,by virtue of the developed hydrophilic-hydrophobic microphase separation structure from the hydrophilic mPEG group,the membrane with the low theoretical ion exchange capacity（IEC_t）value of 0.84 meq.·g^-1 displayed the highest hydroxide conductivity of 32.3 m S·cm^-1 at 20℃.Excitingly,the cell voltage of MEA could still maintain more than 63.1%of the original at a constant current density of 200 m A·cm^-2 over 100 h.Besides,typical bulky imidazolium cation model compounds with mPEG pendent were synthesized,then their degradation mechanism was revealed as the S_N2 substitution and degradation position mainly occurred at the aromatic ether bond of PPO and linker group by ¹H NMR spectroscopic analysis.（4）Based on the degradation mechanism of PPO-based AEMs having sterically hindered imidazolium cations,a class of high molecular weight fluoropolyolefin was prepared via one-pot Ziegler-Natta copolymerization.Then two new generations of fluoropolyolefin-based AEMs with bulky 9C side chains were achieved by using homogeneous and heterogeneous quaternization methods.These solvent processable fluoropolyolefin AEMs showed better initial fuel cell device performance with a maximum power density of 133 m W·cm^-2 at a current density of 250 m A·cm^-2,which was nearly 4 times higher than that of non-fluorinated polyolefin-based AEMs by hot-pressing technology.In addition,such fluoropolyolefin-based AEMs exhibited excellent alkaline stability due to the existence of polyolefin structure and long alkyl chain spacer of up to 9 carbon atoms,and the loss of OH^-conductivity of membrane was only less than 10.5%after 700 h of aging in 1M NaOH at 80℃.More importantly,impressive device durability of MEA using fluoropolyolefin-based AEMs was observed with no loss of performance over 80 h of testing at a constant cell voltage of 0.3 V,and it was expected to be one durable membrane material with great development potential.