Assessing the influence of different cation chemistries on ionic conductivity and alkaline stability of anion exchange membranes

Polysulfone (PSF) backbones were functionalized with reactive chloromethyl groups for preparing thin film anion exchange membranes (AEMs) with fixed benzyl quaternary cations. Three different cation chemistries of varying basicity were evaluated: 1,4-dimethylpiperazinium (DMP+), trimethylammonium (TMA(+)), and trimethylphosphonium (TMP+). The water uptake, ionic conductivity, and stability in alkaline media of these AEMs were assessed with both chloride and hydroxide counteranions. The results obtained revealed that the basicity value of the free base conjugate of the functionalized quaternary cations correlated well with gains in ionic conductivity. Cation basicity also correlated well with the alkaline stability of cations with the same inorganic atom, but was not an appropriate heuristic for comparing alkaline stability across cations with different inorganic atoms. The alkaline stability studies indicated that the primal degradation pathway of the TMA(+) cation differed from that of the TMP+ cation (direct nucleophilic attack versus ylide formation). PSF with TMA(+) and DMP+ cations were demonstrated to show alkaline fuel cell performance that reflected their respective ionic conductivity values.