Cationic polymer membranes that conduct free anions comprise an enabling area of research for alkaline membrane fuel cells and other solid-state electrochemical devices that operate at high pH. The ...synthesis of anion exchange membranes based on a poly(phenylene) backbone prepared by a Diels−Alder reaction is demonstrated. The poly(phenylene)s have benzylic methyl groups that are converted to bromomethyl groups by a radical reaction. Cationic polymers result from conversion of the bromomethyl groups to ionic moieties by quaternization with trimethylamine in the solid state. The conversion to benzyltrimethylammonium groups is incomplete as evidenced by the differences between the IEC values measured by titration and the theoretical IECs based on 1H NMR measurements. The anion exchange membranes formed from these polymers have hydroxide ion conductivities as high as 50 mS/cm in liquid water, and they are stable under highly basic conditions at elevated temperatures.
The backbone stability of benzyl-trimethyl ammonium (BTMA) functionalized polyaromatics was investigated in two structurally differing polymer architectures; quaternized poly(arylene ether) (PAE) and ...poly(phenylene) (PP). FTIR analysis indicated the cleavage of aryl-ether linkages in quaternized PAEs under high pH environments, while no backbone degradation in quaternized PP was observed. The backbone degradation of PAEs not only significantly reduced the mechanical properties of the membranes, but also negatively impacted hydroxide conductivity. Membrane electrode assemblies (MEAs) using both PAE and PP membranes showed good initial alkaline membrane fuel cell performance. However, the PAE MEA displayed larger performance losses and failure after only 55h, due to a mechanical breach. No catastrophic failure of the PP MEA occurred after 300h, which further confirmed the stability of the polymer backbone.
► Systematically investigated the polymer backbone degradation for poly(arylene ether) anion exchange membranes. ► Proposed the backbone degradation mechanism from FTIR and GPC results. ► Compared the degradation rate of polymer backbone and cation functional group. ► Addressed the impact of polymer backbone degradation on polymer properties and fuel cell performance.
Alkaline stability of anion exchange membranes (AEMs) is an essential requirement for the practical application of alkaline membrane fuel cells. In this study, we investigate the alkaline stability ...of Diels–Alder polyphenylenes (DAPPs) under various stability testing conditions. Structural analysis and membrane properties of the DAPPs indicated that different chemical structural changes of quaternized DAPPs occur depending on test conditions. Benzyltrimethylammonium-functionalized DAPPs degraded rapidly via nucleophilic benzyl substitution under a relatively mild condition at 80 °C with 4 M NaOH, whereas DAPPs with hexyltrimethylammonium (HTMA DAPP) exhibited much better alkaline stability under the same condition. While only cross-linking of unreacted alkyl bromides of HTMA DAPP occurred at 80 °C, we observed the degradation of the cations via β elimination after long-term testing. At higher temperature or reduced relative humidity (RH) conditions, for example, 160 °C with 8 M NaOH or RH 10% at 100 °C, the rapid degradation of the cation in HTMA DAPP occurred via nucleophilic methyl substitution. These results suggest that accelerated stress tests may not be a quick and straightforward alternative to prolonged alkaline stability tests of AEMs.
Modern electrochemical energy conversion devices require more advanced proton conductors for their broad applications. Phosphonated polymers have been proposed as anhydrous proton conductors for fuel ...cells. However, the anhydride formation of phosphonic acid functional groups lowers proton conductivity and this prevents the use of phosphonated polymers in fuel cell applications. Here, we report a poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) that does not undergo anhydride formation and thus maintains protonic conductivity above 200 °C. We use the phosphonated polymer in fuel cell electrodes with an ion-pair coordinated membrane in a membrane electrode assembly. This synergistically integrated fuel cell reached peak power densities of 1,130 mW cm
at 160 °C and 1,740 mW cm
at 240 °C under H
/O
conditions, substantially outperforming polybenzimidazole- and metal phosphate-based fuel cells. Our result indicates a pathway towards using phosphonated polymers in high-performance fuel cells under hot and dry operating conditions.
Hydroxide anion conducting solid polymer membranes, also termed anion exchange membranes, are becoming important materials for electrochemical technology, and activity in this field, spurred by ...renewed interest in alkaline fuel cells, is experiencing a resurgence. Solid polymer anion exchange membranes enable alkaline electrochemistry in devices such as fuel cells and electrolyzers and serve as a counterpoint to proton exchange membranes, of which there is a large body of literature. For their seeming importance, the details of transport in alkaline exchange membranes has not been explored thoroughly. In this work, a chloromethylated polymer with a polysulfone backbone was synthesized. 1H NMR spectroscopy was performed to determine the chloromethyl content and its position on the polymer structure. The chloromethylated polymer was solution cast to form clear, creasable films, and subsequent soaking of these films in aqueous trimethylamine gave benzyltrimethylammonium groups. The resulting anion exchange membranes swell in water and show varying degrees of ionic conductivity depending on their ion exchange capacity. The water mobility in the anion exchange membranes was greater than in previously studied proton exchange membranes; however, the transport properties in these new materials were lower than what might have been expected from the water behavior. This comparison gives some insight as to future anion exchange membrane design objectives.
Perfluorosulfonic acids such as Nafion are industrial standard cation exchange ionomers for polymer electrolyte membrane fuel cells because of their high gas permeability, hydrophobicity, and ...inertness to electro-chemical reaction. In this research, pentamethylguanidinium functionalized, perfluorinated hydroxide conducting ionomers for alkaline membrane fuel cells were prepared and characterized. The alkaline stability of the ionomers largely depended on the adjacent group that connected the cation; Sulfone guanidinium functionalized ionomer degraded almost completely after soaking in 0.5 M NaOH at 80 °C for 24 h, while phenylguanidinium functionalized ionomer did not degrade under the same conditions for 72 h. Spectroscopic data and density functional theory calculation suggested that the stability of the phenylguanidinium ionomer was greatly improved by charge delocalization of the formed resonance structure. Alkaline membrane fuel cells using the resonance stabilized perfluorinated ionomer in the catalyst layers on quaternized polyphenylene membrane showed excellent performance (ca. maximum power density = 466 mW/cm2) and promising stability (ca. Tafel slope degradation rate = 225 μV/dec h) at 80 °C under H2/air conditions.
1H high resolution magic angle spinning (HRMAS) NMR spectroscopy was used to characterize the solvent environments in a series of poly(phenylene)- and poly(phenylene alkylene)-based anion exchange ...membranes (AEMs). Multiple water and methanol environments were resolved in the membranes under HRMAS NMR. This allowed the self-diffusion rate constants to be evaluated for each different solvent environment as a function of the membrane identity, ion exchange capacity, water content, and sample temperature. These ionomers have been designed to function as binders within the catalyst layers of direct methanol fuel cells. In such applications, it is desirable to maximize the diffusion of the fuel (methanol) as well as the solvated ions to increase power output. To that end, the flexibilities of the backbone and the cationic side chains have been varied with the expectation that greater polymer mobility will lead to improved permeability. For the two types of AEMs investigated, it was observed that the methanol self-diffusion rates were preferentially reduced with respect to the water diffusion rates. It was also shown that the water diffusion rates within the AEMs were the largest at high water concentration, as observed in membranes containing the hexamethylene chain spacer in both the polymer backbone and the trimethylammonium (TMA+) cation-containing side chains.
PDF
