Preparation And Properties Of Low Swelling Anion Exchange Membranes For Fuel Cells

Anion exchange membrane fuel cells(AEMFCs)are drawing more attention than proton exchange membrane fuel cells(PEMFCs)owing to their faster electrokinetics,higher catalyst resistance to carbon monoxide and wider choice of non-precious metal catalysts(e.g.Ni,Co and Ag).As a critical component in AEMFCs,anion exchange membranes(AEMs)play a role in separating the fuel and oxygen and simultaneously transferring hydroxide ions.Better than PEMs,the migration of hydroxide ions inside AEMs starts from the anode to cathode,and is opposite to the diffusion direction of fuel,effectively preventing the fuel crossover of AEMs and facilitating the full utilization of fuels.However,the inherent lower mobility of OH-than H+ makes it hard to improve the conductivity of AEMs to the level of PEMs.This makes it particularly necessary to design efficient polymer structures with high ionic exchange capacity(IEC).Paradoxically,many more hydroxide conduction groups introduced to the polymer electrolytes usually result in high water uptake,excessive membrane swelling and poor mechanical stability.To disentangle the conductivity/swelling dilemma in AEMs,it is essential to fabricate AEMs with well-designed microstructure which promotes the OH-conduction efficiency.Here,we prepared two types(macrocrosslinked-type and side-chain-type)of AEMs,and investigated the structure-property relationship of the membrane.First of all,a series of novel macrocrosslinked imidazolium-based anion exchange membranes(AEMs)with high hydroxide conductivity and dimensional stability were synthesized by crosslinking poly(vinyl imidazole)ionic liquid with bromide-terminated poly(ether sulfone)via Menshutkin reaction.The contiguous imidazolium cations along the polyolefin backbone are found to aggregate and connect to form continuous hydroxide transport microchannels by the introduction of long hydrophobic poly(ether sulfone)chain as evidenced by atomic force microscopy(AFM).As a consequence,a high hydroxide conductivity of 78.5 mS·cm-1 was achieved for the crosslinked PES/PVIIL-0.4 membrane at 80?.Fuel cell test using the PES/PVIIL-0.4 membrane exhibits an open circuit voltage of 1.039 V and peak power density of 109.5 mW·cm-2 at the current density of 190 mA·cm-2 at 60?.Furthermore,a series of fluorine-containing poly(arylene ether sulfone)s with multifunctionalized flexible pendant ammounium cation(DFHF-TQA-x)was synthesized by grafting hydroxyl-bearing trifunctional moieties 2,4,6-tri(dimethyl aminomethyl)-phenol(TDAP)into fluorine-bearing poly(ether sulfone)matrix,followed by functionalization with bromine-bearing(u-bromoalkyl)trimethyl ammonium(Br-n-QA).The incorporation of the multi-functionalized pendant ammounium cation facilitates the aggregation of the ionic clusters leading to the formation of hydrophilic/hydrophobic microphase separation and microchannels.As a result,an enhancement of OH-conductivity in low IEC can be achieved.The as-synthesized AEMs not only possess higher mechanical properties and ion conductivity,but also lower water uptake and swelling than t:raditional AEMs with monofunctional ammonium cation.Specially,the DFHF-TQA-0.75 membrane with the highest IEC of 1.31 meq·g-1 achieves the OH-conductivity of 76.1 mS cm-1 at 80?.Moreover,the membranes also exhibit good alkaline and thermal stability.