Aliphatic Anion Exchange Ionomers with Long Spacers and No Ether Links by Ziegler–Natta Polymerization: Properties and Alkaline Stability

Abstract

In this work we report the synthesis of poly(vinylbenzylchloride-co-hexene) copolymer grafted with N,N-dimethylhexylammonium groups to study the effect of an aliphatic backbone without ether linkage on the ionomer properties. The copolymerization was achieved by the Ziegler–Natta method, employing the complex ZrCl4 (THF)2 as a catalyst. A certain degree of crosslinking with N,N,N′,N′-tetramethylethylenediamine (TEMED) was introduced with the aim of avoiding excessive swelling in water. The resulting anion exchange polymers were characterized by 1H-NMR, FTIR, TGA, and ion exchange capacity (IEC) measurements. The ionomers showed good alkaline stability; after 72 h of treatment in 2 M KOH at 80 °C the remaining IEC of 76% confirms that ionomers without ether bonds are less sensitive to a SN2 attack and suggests the possibility of their use as a binder in a fuel cell electrode formulation. The ionomers were also blended with polyvinyl alcohol (PVA) and crosslinked with glutaraldehyde. The water uptake of the blend membranes was around 110% at 25 °C. The ionic conductivity at 25 °C in the OH− form was 29.5 mS/cm.

A Short Overview of Biological Fuel Cells

“…Many strategies were proposed to decrease the degradation of AEMs, which were related to the attack of positive ionic groups and labile bonds on the polymer backbone, such as ether links, by hydroxide ions [158] and to increase the long-term performance [142], including (i) change of the polymer backbone, especially without ether bonds [159][160][161], (ii) introduction of long side chains to separate the positive charge from the matrix [162][163][164], (iii) delocalization of the positive charge [149,165], (iv) cross-linking to improve the hydrolytic and mechanical stability [166][167][168], (v) formation of composite ionomers, generally obtained by mixing nanoparticles such as layered double hydroxides (LDH), SiO 2 , and TiO 2 or forming nanoparticles via sol-gel routes [169][170][171][172][173][174]. Despite performances similar to PEMs, sometimes obtained with unrealistic testing conditions, the durability issues remain a primary challenge for the commercialization of AEMs [140], but some very encouraging results were reported recently [175].…”

Anion-Conducting Polymer Electrolyte without Ether Linkages and with Ionic Groups Grafted on Long Side Chains: Poly(Alkylene Biphenyl Butyltrimethyl Ammonium) (ABBA)

“…The E2 elimination produces an alkene when a hydrogen is available in β position [ 27 ]. Different strategies are proposed to mitigate the damaging effect of alkaline media [ 2 , 15 , 28 , 29 , 30 , 31 , 32 ], including a delocalization of the positive charge [ 10 , 29 ], a protection by steric hindrance [ 10 , 33 ], an introduction of a long chain to separate the charge from the backbone [ 28 , 34 , 35 ], a different architecture of the polymer backbone [ 31 , 36 , 37 , 38 ], or the formation of composites with inorganic materials stable in alkaline environments [ 39 , 40 ]. Theoretical studies by Pivovar and co-workers show that the most stable side chain length corresponds to four to five CH 2 groups [ 41 ]; most recently the comparison between benzyltrimethylammonium and long alkyl-tethered quaternary ammonium (hexyltrimethylammonium) shows a better chemical stability of the latter in alkaline conditions [ 42 ].…”