High performance aliphatic-heterocyclic benzyl-quaternary ammonium radiation-grafted anion-exchange membranes

Abstract

Anion-exchange membranes (AEM) containing saturated-heterocyclic benzyl-quaternary ammonium (QA) groups synthesised by radiation-grafting onto poly(ethylene-co-tetrafluoroethylene) (ETFE) films are reported. The relative properties of these AEMs are compared with the benchmark radiation-grafted ETFE-g-poly(vinylbenzyltrimethylammonium) AEM. Two AEMs containing heterocyclic-QA head groups were down-selected with higher relative stabilities in aqueous KOH (1 mol dm-3) at 80°C (compared to the benchmark): these 100 μm thick (fully hydrated) ETFE-g-poly(vinylbenzyl-Nmethylpiperidinium)- and ETFE-g-poly(vinylbenzyl-N-methylpyrrolidinium)-based AEMs had as-synthesised ion-exchange capacities (IEC) of 1.64 and 1.66 mmol g-1, respectively, which reduced to 1.36 mmol dm-3 (ca. 17 – 18% loss of IEC) after alkali ageing (the benchmark AEM showed 30% loss of IEC under the same conditions). These down-selected AEMs exhibited as-synthesised Cl- ion conductivities of 49 and 52 mS cm-1, respectively, at 90°C in a 95% relative humidity atmosphere, while the OH- forms exhibited conductivities of 138 and 159 mS cm-1, respectively, at 80°C in a 95% relative humidity atmosphere. The ETFE-g-poly(vinylbenzyl-N-methylpyrrolidinium)-based AEM produced the highest performances when tested as catalyst coated membranes in H2/O2 alkaline polymer electrolyte fuel cells at 60°C with PtRu/C anodes, Pt/C cathodes, and a polysulfone ionomer: the 100 μm thick variant (synthesised from 50 μm thick ETFE) yielded peak power densities of 800 and 630 mW cm-2 (with and without 0.1 MPa back pressurisation, respectively), while a 52 μm thick variant (synthesised from 25 μm thick ETFE) yielded 980 and 800 mW cm-2 under the same conditions. From these results, we make the recommendation that developers of AEMs, especially pendent benzyl-QA types, should consider the benzyl-Nmethylpyrrolidinium head-group as an improvement to the current de facto benchmark benzyltrimethylammonium headgroup

Anion Exchange Membranes Based on Polystyrene-Block-Poly(ethylene-ran-butylene)-Block-Polystyrene Triblock Copolymers: Cation Stability and Fuel Cell Performance

“…69 Thus, based on all the above discussion and results from other authors, we can conclude that the hydroxide ion conductivity of SEBS35-TMA is consistent with the range reported by others, and sufficient for applications in alkaline fuel cells and electrolyzers.…”

Importance of balancing membrane and electrode water in anion exchange membrane fuel cells

“…Therefore, engineering solutions have been explored in a number of studies, including running commercial systems at very low current density [16], pressurizing the gas streams, or even feeding condensed water through the cathode [10] e none of which are tenable long-term solutions to high performing AEMFCs. Compared to many modern AEMs (Table 1), radiation-grafted ETFE-based AEMs have been reported to have high conductivity [13,18] and high water back diffusion rates [15,17], which may be utilized to alleviate the water gradient that is intrinsic to operating AEMFCs. However, high water back diffusion risks the introduction of new variables to be considered, including the possibility for cathode flooding.…”

The first anion-exchange membrane fuel cell to exceed 1 W cm−2 at 70 °C with a non-Pt-group (O2) cathode

“…However, the open circuit voltages (OCV) were o0.95 V, which are much lower than the 41.0 V OCVs obtained with AEMFCs containing thicker ETFE-based RG-AEMs. 6,8 This OCV data suggests that reasonably high gas crossover rates across the thin ETFE-AEM and that moving to even thinner ETFE-based RG-AEMs would be unwise. Interestingly, the Ag/C cathode even outperformed the Pt/C cathode when supplied with CO 2 -free air at a cell temperature of 70 1C: the Pt/C cathode yielded only a peak power density of 650 mW cm…”