Theoretical Description of the Structural Characteristics of the Quaternized SEBS Anion-Exchange Membrane Using DFT

Abstract

For four conforming structures of the quaternized polystyreneblock-poly(ethylene-ran-butylene)-block-polystyrene membrane (QSEBS), (a) tetramethylammonium hydroxide (TMA + OH − ), (b) benzyltrimetylammonium hydroxide, and (c and d) QSEBS segments with 1 and 2 side chains (DBQSEBS), spatial distribution, bond distances, and charge-density profiles were obtained with density functional theory (DFT) and compared with structural simulations of DBQSEBS for two different hydration levels. Results for the TMA + OH − showed that its constituent ions stay metastable in the vicinity of each other and are joined by donor−acceptor interactions. Also, simulations of the other conforming structures show that, in the absence of water, spatial distribution as well as charge-density profiles of trimethylammonium hydroxide do not change with respect to isolated TMA + OH − , demonstrating that the QSEBS chain is a thermodynamically stable backbone to support the functional group, which is in agreement with the literature. When hydrated, simulations of DBQSEBS for water uptake of 4 show that there is a partial dissociation of hydroxide ions due to donor−acceptor interactions acting competitively on them. For water uptake of 6, this dissociation is completed, and hydroxide ions conform to hypercoordinated structures similar to the square-planar arrangement described for pure water medium, but with some structural differences associated with location, type, and interactions among the molecules involved.

Hybrid anion exchange membranes with adjustable ion transport channels designed by compounding SEBS and homo‐polystyrene

“…The PS phase not only is responsible for ion conduction but also provides chemical/mechanical stability to these AEMs while the rubbery PEB phase, which accounts for a large proportion of SEBS, has no contribution to these two factors. As a result, the SEBS AEMs show low dimensional stability in water because the ammonium‐functionalized PS phase has high water uptake weakening the mechanical strength 39–43 . This can be typically addressed by cross‐linking the functionalized PS phase, which improves the mechanical properties at the cost of conductivity and solubility 44–46 .…”

“…As a result, the SEBS AEMs show low dimensional stability in water because the ammonium-functionalized PS phase has high water uptake weakening the mechanical strength. [39][40][41][42][43] This can be typically addressed by cross-linking the functionalized PS phase, which improves the mechanical properties at the cost of conductivity and solubility. [44][45][46] For example, Jeon and coworkers cross-linked the PS units with 1,6-hexanediamine, which, significantly reduced water uptake (e.g., 155% vs 28%) and enhanced their mechanical properties.…”

Density functional theory study on the degradation of fuel cell anion exchange membranes via removal of vinylbenzyl quaternary ammonium head group

“…The inclusion of the polymer backbone in the DFT calculations can validate whether the backbone contributes to the AEM degradation. DFT studies of polystyrene and quaternised poly (styrene‐ethylene‐butylene‐styrene) (QSEBS) with BTMA functionality proved the stability of styrene‐based polymers against backbone degradation . However, theoretical studies of polyaromatic‐based backbone tethered with BTMA functional group revealed backbone degradation due to a lower barrier energy of OH − initiated aryl‐ether scission than the nucleophilic substitution reaction .…”

“…DFT studies of polystyrene [37] and quaternised poly (styrene-ethylene-butylene-styrene) (QSEBS) with BTMA functionality proved the stability of styrene-based polymers against backbone degradation. [38] However, theoretical studies of polyaromatic-based backbone tethered with BTMA functional group revealed backbone degradation due to a lower barrier energy of OH − initiated aryl-ether scission than the nucleophilic substitution reaction. [39] This agrees with the systematic study of Mohanty et al [40] on different polymer backbones and concluded that all carbon-based repeating units (ie, absence of aryl-ether bonds) are promising candidates for AEM that exhibits long-term alkaline stability.…”

Studies on a novel anion‐exchange membrane based on chitosan and ionized organic compounds with multiwalled carbon nanotubes for alkaline fuel cells

“…However, its electrochemical performance, especially ionic conductivity, still has a big disparity with those of PEM, so new membranes for low‐temperature AEM fuel cells urgently need to be found and utilized. To date, much effort has been put into AEM development, and lots of promising candidates have been researched for use in alkaline fuel cells, including HIPTPES‐OH, PPO–TPQP, PBI/IL‐GO, CTS‐LCRD, QSEBS, and radiation grafted [VPSIm][OH] . However, these membranes generally have various disadvantages, such as high expense, complex quaternization process, low OH − conductivity, instability in alkaline media, high fuel permeability, and poor mechanical and thermal stability.…”