Preparation And Characterization Of Quaternary Phosphonium Anion Exchange Membranes

Anion exchange membrane fuel cells (AEMFCs) have been demonstrated to offer enhanced kinetics for oxygen reduction in alkaline environment, which can obviously increase the fuel cell efficiency and allow using non precious metals as the electrode catalyst. However, the poor stability and mechanical strength of the anion exchange membranes (AEMs) restrict their lifetime and practical application in AEMFCs. The durability and mechanical strength of AEMs in the strong alkaline medium need to be improved without losing conductivity evidently, which is a challenge for the development of the AEMs.The widely used AEMs are quaternary ammonium (QA) based membranes. However, the low thermal stability and poor tolerance to alkali result in the degradation of this type of AEMs, which have been the limitation of its development and application. The quaternary phosphonium (QP) aromatic polymer membranes exhibit improved durability and mechanical strength towards the strong alkaline solution.As comparison, both quaternary phosphonium and ammonium AEMs were synthesized based on polysulfone via chloromethylation and quaternization separately. With the same chloromethylation degree of the polysulfone, the obtained quaternary phosphonium polysulfone hydroxide (QPPSU-OH) showed a conductivity of1.6to13.8mS cm-1at a temperature range from20℃to80℃. It was apparently higher than that of quaternary ammonium polysulfone hydroxide (QAPSU-OH), i.e.1.0to6.0mS cm-1at the same conditions. The tensile strength of QPPSU-OH was25.6MPa at room temperature, which was higher than that of QAPSU-OH (19.7MPa). The tolerance of QPPSU-OH membrane towards alkali was investigated by immersing it in1M KOH at60℃. The result of the tensile strength indicated that this membrane was stable as a tensile strength of21MPa was maintained after soaking in the alkaline solution for276h.The process of the chloromethylation of polymers generally conducts around60h. Moreover, the chloromethylation reagen, i.e. chloromethyl methyl ether, is highly toxic. In this work, the bromination reaction was used instead of chloromethylation to prepare the quaternary phosphonium AEMs based on poly(aryl ether ketone)s by using a low toxic reagent N-bromosuccinimide. The bromination reaction was performed only for about4h. The initiatory experimental result indicated that a conductivity of14mS cm-1could be reached by the membrane of quaternary phosphonium poly(aryl ether ketone)s hydroxide (QPPAEK-OH) at room temperature. The result suggested that this membrane is potential to be used as electrolyte in alkaline fuel cells.