Preparation And Performance Of Novel Polyphosphazene Polymer Grafting Crown Ether Groups For Alkaline Fuel Cells

Recently alkaline membrane fuel cells (AMFC) have attracted more and more attention because they have many advantages over proton exchange membrane fuel cells such as possible utility of non-precious metal electrocatalysts (nickel, silver, etc.) and faster reduction kinetics of oxygen. A hydroxyl ion exchange membrane (HEM) is one of the key components of an AMFC system. However, the typical cationic polymers such as polyether-ether-ketone and poly(aryl ether ether ketone) carrying quaternary ammonium or phosphonium groups were difficult to be prepared anion membranes with high performance because of their regid backbone, low density and chemical decomposition of cationic groups.In the thesis, based on the advantages of polyphosphazene with flexible backbone and potential high density of functional groups, two kind of polyphosphazene polymers grafting dibenzo-18-crown-6 units were successfully synthesized, and the corresponding three serials of membranes were successfully prepared. The main research items in this thesis are as follows.1. Serials of bis(1-phenylethan-l-ol)dibenzo-18-crown-6-functionalized polyphosphazene (PBBEB186P-P) anion membranes with different functionality were prepared from the initial chemical exachlorocyclotriphosphazene (HCCP). Bis(1-phenylethan-1-ol)dibenzo-18-crown-6 (BBEB18C6) was synthesized by the precursor benzo-18-crown-6 after acetylation and subsequent reduction. The corresponding serials of membranes (PBBEB186P-BP) with different functionality were prepared by reaction between polydichlorophosphazene and BBEB18C6/n-pentyl alcohol in different ratios. After PBBEB186P-BP was ion-exchanged by KOH to form (PBBEB186P-BPOH) alkaline polymer, PBBEB186P-BPOH was characterized by FT-IR, GPC and TGA. GPC test indicates that the molelular weight of PBBEB 186P-BPOH is up to 105 magnetitude, and TGA test shows that the PBBEB 186P-BPOH membrane has high thermal stability with onset degradation temperature high above 230℃. Their typical fuel cell performance including swelling ratio, water uptake, ion exchange capability, tension test and ionic conductivity have been evaluated. The highest ionic conductivity of 15.2mS·cm-1 was achieved at 80 ℃. The prepared membranes PBBEB 186P-BPOH show the high chemical stability in 2 M KOH for at least 800 h at 30℃.2. Serials of 4’-hydroxylmethyldibenzo-18-crown-6-functionalized polyphosphazene (PHMDB186P-BP) anion membranes with different functionality were prepared.4’-Hydroxylmethyldibenzo-18-crown-6 (PHMDB 186P) was synthesized by the precursor benzo-18-crown-6 after formylation and subsequent reduction reaction. The corresponding serials of membranes (PHMDB186P-BP) with different functionality were prepared by reaction between polydichlorophosphazene and PHMDB186P/bromophbenol in different ratios. After PHMDB186P-BP was ion-exchanged by KOH to form (PHMDB186P-BPOH) alkaline polymer, PHMDB186P-BPOH was characterized by FT-IR, GPC and TGA. GPC test indicates that the molelular weight of PHMDB186P-BPOH is up to 105 magnetitude, and TGA test shows that the PBBEB186P-BPOH membrane has high thermal stability with onset degradation temperature high above 230 ℃. Similarly, the typical fuel cell performance have been evaluated, and the ionic conductivity of PHMDB186P-BPOH-25 membrane was up to 56.2mS·-cm-1 at 80 ℃. The prepared membranes PBBEB186P-BPOH show the high chemical stability in 2 M KOH for at least 800 h at 30℃.3. A series of membranes of PHMDB186P-BPOH-25 blending polyvinyl alcohol (PVA) in different weight percentages have been preapread. The blending membranes exhibit excellent thermal stability with onset degradation temperature high above 230℃ and good chemical stability. The typical fuel cell performance for the prepared blend membranes including swelling ratio, water uptake, ion exchange capability, tension test and ionic conductivity have been evaluated. The ionic conductivity of blending membrane containing 10w% PVA was up to 60.6mS·cm-1 at 80℃, and tensile strength was up to 24.32MPa. All these properties indicate P-P blend membrane is one of the promising AEM materials for potential applications in alkaline membrane fuel cells.