Cation- and Anion-Conducting Membranes for Fuel Cell and Electrolysis Applications Prepared Via the Acid-Base Blend Concept
Authors: Kerres Jochen 
Published: February 2020

In this study the synthesis of low-T and high-T proton-conducting blend membranes and of anion-exchange blend membranes utilizing the acid-base blend concept by usage of the following blend components is presented: (i) a cation-exchange polymer, (ii) an anion-exchange polymer, and (iii) a stable basic polymer such as a polybenzimidazole (PBI, F6PBI or PBIOO) as the stabilising matrix.
For the preparation of low-T proton-conductive blend membranes, DMAc solutions of a sulfonated cation-exchange polymer in molar excess were mixed with a DMAc-dissolved halomethylated polymer, a tertiary amine or alkylated imidazole, and a DMAc-dissolved polybenzimidazole. During membrane formation by solvent evaporation quaternization of the halomethylated polymer took place. A blend membrane composed of a partially fluorinated aromatic sulfonated polymer, F6PBI, and a partially fluorinated aromatic anion-exchange polymer with pendent 1-ethyl-2-methylimidazolium anion-exchange groups was investigated in a SO2-depolarized electrolyser and yielded an excellent performance (operation temperature 95°C, 0.5 Acm-2@0.85V) which was markedly better than that of a Nafion®115 membrane (operation temperature 95°C, 0.4 Acm-2@0.92V), even after voltage stepping [1].
High-T proton-conducting blend membranes were prepared by mixing DMAc solutions of an excess PBI amount with a halomethylated polymer, a sulfonated polymer and a tertiary N base. During membrane formation, following reactions took place: quaternization of the halomethylated polymer with the tertiary N base to formation of an anion-exchange polymer, ionic cross-linking of the anion-exchange polymer´s cationic groups with the anionic groups of the sulfonated polymer, and a minor extent of covalent cross-linking by reaction of residual halomethyl groups of the halomethylated polymer with N-H groups of the PBI blend component, yielding base-excess blend membranes which were subsequently doped with phosphoric acid to achieve H+-conductivity. As an example, DMAc solutions of F6PBI (70 wt%), a partially fluorinated aromatic anion-exchange polymer with pendent 1-ethyl-2-methylimidazolium anion-exchange groups (15 wt%) and a sulfonated PPSU polymer (15 wt%) were mixed together. After solvent evaporation the blend membrane was doped with PA (H3PO4, PA doping degree 167%) and operated in a high-T fuel cell at T=140°C. The blend membrane showed excellent performance (0.5 A/cm-2@0.5 V) and good longevity (fuel cell experiment stopped after 400 hrs without membrane failure) [2].
Anion-exchange blend membranes were prepared mixing DMAc solutions of a halomethylated polymer (40-60 wt%), a PBI (30-50 wt%), a tertiary N base, and a sulfonated polymer (5-15 wt%). During solvent evaporation, formation of the anion-exchange polymer and of ionical and covalent cross-links took place. As an example, DMAc solutions of polyvinylbenzylchloride, PBIOO and sulfonated PPSU were mixed with tetramethylimidazole. After solvent evaporation, an anion-exchange blend membrane was obtained which was composed of 66.35% anion-exchange polymer, 26.15% PBIOO and 7.5% sulfonated PPSU. After KOH treatment (90°C, 1M, 10 days) the membrane exhibited a Cl- conductivity of 65 mScm-1@90°C@90% r.h. (IEC=2.9 meq OH-/g) which is an excellent value for anion-exchange membranes.

References

[1] R. Peach, H. M. Krieg, A. J. Krϋger, J. JC Rossouw, D. Bessarabov, J. Kerres, Int. J. Hydrogen Energy 2016, accepted
[2] J. Kerres, V. Atanasov, Int. J. Hydrogen Energy 2015, 40, 14723-14735