Anion exchange membrane fuel cells (AEMFCs) are clean energy conversion devices that are an attractive alternative to the more common proton exchange membrane fuel cells, because they present the advantage of not using noble metals as catalysts for the oxygen reduction reaction (ORR). Unfortunately the low durability of anion exchange membranes (AEMs) in basic conditions limits their use on a large scale. The International Laboratory "Ionomer Materials for Energy" (LIME) group has extensively worked on synthesis of ionomers applying different strategies to mitigate the damaging effect of alkaline media on anion exchange membranes: the delocalization of the positive charge, the introduction of a long chain to separate the charge and the backbone [1], the introduction of a second phase, backbone without ether groups, and recently the use of biomass to prepare more sustainable materials for fuel cells. In this presentation, after a short overview on the progress made on anionic membranes based on commercial polymers, such as polysulfone (PSU) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) [2], I will focalize on the synthesis of the new ionomer poly(alkylene biphenyl butyltrimethyl ammonium) (ABBA) with a backbone devoid of alkaline-labile C-O-C bonds and with quaternary ammonium groups grafted on long side chains [3]. The ionomer was achieved by metalation reaction on 2-bromobiphenyl, followed by the introduction of the long chain with 1,4-dibromobutane. The precursor was polycondensed and then quaternized using trimethylamine (TMA). The reaction is efficient, well controllable and easy to modulate. The ability to form stable solutions over several months, combined with the spacing of the positive charge from the backbone, the flexibility due to the quaternary carbon in the matrix and the high IEC values such as 2.5 meq/g, allows us to consider this ionomer a good candidate as an electrode binder and AEM. Other synthesis of backbones without ether linkage was explored e.g. a crosslinked poly(vinylbenzylchloride-co-hexene) copolymer grafted with N,N-dimethylhexylammonium groups [4]. The copolymerization was achieved by the Ziegler–Natta method, employing the complex ZrCl₄ (THF)₂ as a catalyst. The resulting aliphatic ionomer showed good alkaline stability, after 72 h of treatment in 2M KOH at 80 °C the remaining IEC of 76% confirmed that ionomers without ether bonds are less sensitive to a S_N2 attack. The ionic conductivity of blended membrane with polyvinyl alcohol (PVA) at 25 °C in the OH⁻ form was 29.5 mS/cm. The last part of presentation will be focus on a more sustainable fuel cell, based on heterocycle building blocks exploiting biomass resources.

References

[1] R.A. Becerra-Arciniegas, R. Narducci, G. Ercolani, E. Sgreccia, L. Pasquini, M.L. Di Vona, P. Knauth, Model Long Side-Chain PPO-Based Anion Exchange Ionomers: Properties and Alkaline Stability, Journal of Physical Chemistry C 124(2) (2020) 1309-1316.

[2] P. Knauth, L. Pasquini, R. Narducci, E. Sgreccia, R.A. Becerra-Arciniegas, M.L. Di Vona, Effective ion mobility in anion exchange ionomers: Relations with hydration, porosity, tortuosity, and percolation, Journal of Membrane Science 617 (2021) 118622.

[3] R. Narducci, R.A. Becerra-Arciniegas, L. Pasquini, G. Ercolani, P. Knauth, M.L. Di Vona, Anion-Conducting Polymer Electrolyte without Ether Linkages and with Ionic Groups Grafted on Long Side Chains: Poly(Alkylene Biphenyl Butyltrimethyl Ammonium) (ABBA), Membranes 12(3) (2022) 337.

[4] R.A. Becerra-Arciniegas, R. Narducci, G. Ercolani, L. Pasquini, P. Knauth, M.L. Di Vona, Aliphatic Anion Exchange Ionomers with Long Spacers and No Ether Links by Ziegler-Natta Polymerization: Properties and Alkaline Stability, Molecules 27(2) (2022) 395.