Evaluation of Cation and Backbone Chemistries for Electrospun and Crosslinked Nanofiber Composite Anion Exchange Membranes

Introduction

     The anion exchange membrane fuel cell (AEMFC) has been the subject of increasing research since the mid-2000s, because its alkaline working environment does not require platinum to catalyze the rate-limiting oxygen reduction reaction. Identifying a benchmark membrane with high hydroxide ion conductivity that is strong and ductile with good chemical stability continues to be a primary objective of AEMFC research.

     Park and Pintauro have demonstrated that a dual-fiber electrospinning method can produce a membrane with both good conductivity and robust mechanical properties [1]. A crosslinking step was successfully introduced which allowed for the use of very high IEC ionomers with no water solubility issues [2]. In both studies, the ionomer was a polysulfone with tetramethyl ammonium fixed charge sites and the uncharged polymer was polyphenylsulfone.

     In this presentation, a series of different anion exchange membrane compositions and structures, based on dual nanofiber electrospinning, will be described in terms of the method for membrane preparation and the properties of the final film.  For all membranes, an interconnected network of ionomer fibers will be embedded in an uncharged polymer matrix where the relative amounts of each component are varied.  The ionomer fibers will employ either a crosslinked polysulfone or crosslinked poly(phenylene oxide) backbone polymer with either tetramethyl ammonium, 1-methylimidazolium, or 1,2-dimethylimidazolium fixed charge cations. Each film was evaluated in terms of room temperature in-plane hydroxide ion conductivity, gravimetric water swelling, mechanical properties, and chemical stability in hot water and hot 1M KOH solution.

 Experimental

     Commercial UDEL® P-3500 polysulfone (PSF, MW = 83,000 g/mol, from Solvay) was chloromethylated/iodomethylated and poly(phenylene oxide) (PPO, MW = 30,000 g/mol, from Sigma Aldrich) was brominated using standard procedures. These functionalized polymers were used as ionomer precursors in the nanofiber composite membrane fabrication scheme. The uncharged reinforcing polymer for the composite films was RADEL® poly(phenylsulfone) (PPSU, MW = 60,000 g/mol, from Solvay).

      Electrospun fibers of either brominated PPO or chloromethylated/iodomethylated PSF were crosslinked with either 1,6-hexanediol or hexamethylenediamine [2]. After crosslinking, fiber mats were exposed to chloroform vapor which softened the PPSU fibers and allowed them to fill the void space around ionomer fibers, thus forming a dense and defect-free membrane. Film soaking in solutions of trimethylamine, 1-methylimidazolium, or 1,2-dimethylimidazolium converted the films into anion exchange membranes with a specific type of fixed charge group. Membranes were soaked in 1 M KOH and then in de-gassed water before testing.

 Results and Discussion

     Four membranes of different compositions are listed in Table 1. The effective IEC of the membranes (taking into account the uncharged PPSU material) ranged from 1.8-2.3 mmol/g.  Crosslinking prevented ionomer dissolution in water. All membranes were mechanically robust in the wet and dry states.

                                            Table 1 – Properties of nanofiber composite AEMs in water at 25^oC

Sample	Ionomer Fiber IEC (mmol/g)	In-plane Conductivity (mS/cm)	Gravimetric Swelling (%)
1	2.80	52	118
2	3.11	65	144
3	3.58	72	152
4	3.65	58	95

                Sample 1: Diol-crosslinked polysulfone/PPSU; 65 wt.% ionomer with tetramethyl ammonium  groups

                Sample 2: Diamine-crosslinked polysulfone/PPSU; 65 wt.% ionomer with tetramethyl ammonium groups

                Sample 3: Diamine-crosslinked PPO/PPSU; 65 wt.% ionomer with 1,2-dimethylimidazolium groups

                Sample 4: Diamine-crosslinked PPO/PPSU; 50 wt.% ionomer with tetramethyl ammonium groups

 

References

1. Park, A. M.; Pintauro, P. N. Electrochem. Solid State Lett. 2012, 15, B27
2. Park, A.M.; Turley, F.E.; Wycisk, R.J.; Pintauro, P.N. Macromolecules, 2014, 47 (1), 227.

Acknowledgements

This work is supported by the National Science Foundation (Grant CBET-1032948) and by the Army Research Office (Contract No. W911NF-11-1-0454).