Anion conductive poly(2,6-dimethyl phenylene oxide)s with clicked bulky quaternary phosphonium groups

Abstract

A series of anion exchange membranes (AEMs) having pendent bulky quaternary phosphonium groups (based on the tris(2,4,6-trimethoxyphenyl)phosphane) (TPP-x and LTPP-x) have been designed and prepared by Cu(I)-catalyzed click chemistry for H2/O2 alkaline fuel cell application. In spite of numerous attempts, the high degree of functionalization (DF) copolymer displayed very poor film forming ability. Thus, the tough and transparent membranes were obtained only at IEC level as low as ~ 1.0 meq./g. The as-obtained TPP and LTPP AEMs having bulky phosphonium groups showed lower water uptake than that of the clicked CQA and LCQA membranes based on quaternary ammonium groups in spite of their similar IEC values. Lower ion conductivities were observed for all of the AEMs due to the lower water uptake. However, the bulky phosphonium groups could protect efficiently the core atom in organic cations against hydroxide attack and thus induced excellent alkaline stability of AEMs even at high NaOH concentration of 10 M at 80 °C for 200 h, although possibly SN2 substitution degradation was occurred according to the 1H NMR results. Interestingly, these AEMs with bulky quaternary phosphonium groups had not initial performance, e.g. no open circuit voltage (OCV) when it was employed as membrane in fuel cell probably because no efficient phase boundary forming between the catalyst layer and membrane resulting from the poor compatibility between poly(2,6-dimethyl phenylene oxide)s (PPOs) backbone and bulk phosphonium groups, although they were served as an ionomer successfully for the fuel cell in catalyst layer. This new finding that contrasts material alkaline stability and device operability is extremely important and gives us directions for new polymer designs for high performance anion exchange membrane for alkaline fuel cells.