Ponce-González J, Whelligan DK, Wang L, Bance-Soualhi R, Wang Y, Peng Y, Peng H, Apperley DC, Sarode HN, Pandey TP, Divekar AG, Seifert S, Herring AM, Zhuang L, Varcoe J (2016) High performance aliphatic-heterocyclic benzyl-quaternary ammonium radiation-grafted anion-exchange membranes, Energy and Environmental Science 9 (12) pp. 3724-3735
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 ¼m 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 ¼m thick variant (synthesised from 50 ¼m 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 ¼m thick variant (synthesised
from 25 ¼m 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.
This study used Raman spectro-microscopy to investigate the synthesis and degradation of radiation-grafted anion-exchange membranes (RG-AEM) made using 50 ¼m thick poly(ethylene-co-tetrafluoroethylene) (ETFE) films, vinylbenzyl chloride (VBC) monomer, and 1-methylpyrrolidine (MPY) amination agent. The data obtained confirmed the operation of the grafting-front mechanism. VBC grafting times of 1 and 4 h led to low degrees of grafting homogeneity, while 72 h led to extreme levels of grafting that resulted in mechanically weak RG-AEMs due to the excessive H2O contents. A grafting time of 16 h was optimal yielding a RG-AEM with an ion-exchange capacity (IEC) of 2.06 ± 0.02 mmol g-1 (n = 3). An excess of grafting was detected at the surface of this RG-AEM (at least within the first few ¼m of the surface). This RG-AEM was then degraded in O2-purged aqueous KOH (1.0 mol dm-3) for 14 d at 80 °C. Degradation was detected throughout the RG-AEM cross-section, where the Raman data was quantitatively consistent with the loss of IEC. A slight excess of degradation was detected at the surface of the RG-AEM. Degradation involved the loss of whole benzyl-1-methypyrrolidinium grafted units as well as the direct attack on the pendent (cationic) pyrrolidinium groups by the hydroxide anions.
A variety of radiation-grafted cation-exchange membranes (RG-CEM) were synthesised, using a high-dose rate electron-beam peroxidation method, for an initial evaluation of their applicability to reverse electrodialysis cells (RED, a type of salinity gradient ?blue? energy). The RG-CEMs were adequately conductive (to Na+ cations) but without the incorporation of crosslinking co-monomers, the permselectivities were too low (d80%). In contrast, when ETFE-based RG-CEMs were synthesised with incorporation of 10% mol bis(vinylphenyl)ethane (BVPE) crosslinking co-monomer into the styrene-containing grafting mixture, permselectivities of >90% were obtained without a significant decrease in conductivity. The use of BVPE in the grafting mixture also resulted in the RG-CEMs exhibiting enhanced ion-exchange capacities without any increase in water uptakes (cf. uncrosslinked variants). In contrast, the use of less flexible divinylbenzene crosslinker led to prohibitively large decreases in RG-CEM conductivity. This study highlights that the future development of both radiation-grafted cation-exchange and anion-exchange membranes for RED (and other electrodialysis applications) should utilise flexible crosslinkers (such as BVPE) to ensure adequate permselectivities.