PPS-reinforced poly(terphenylene) anion-exchange membranes with different quaternary ammonium groups for use in water electrolysers

The nature, concentration and spatial distribution of quaternary ammonium (QA) groups is instrumental for the performance of anion exchange membranes, primarily by augmenting the ion exchange capacity (IEC). This study compares a series of poly(meta-terphenylene)-based ion exchange membranes (mTPN), which vary in the attached quaternary ammonium group. The IEC of the membranes is 1.97, 2.20 and 3.70 mmol OH⁻ g⁻¹ for membranes functionalized with 1,4-diazabicyclo[2.2.2]octane (DABCO), trimethylamine (TMA), and methylated DABCO (DABCO-Me), respectively. The latter is obtained by methylation of the DABCO-functionalized membrane.

To control swelling, all membranes are reinforced with a poly(phenylene sulfide) (PPS) fiber mat. The highest conductivity, achieved by PPS/mTPN/DABCO-Me, is 140 mS cm⁻¹ in 1 M KOH and 237 mS cm⁻¹ in 4 M KOH. An alkaline stability test over 2 months revealed that membranes with TMA and DABCO have a stable in-plane conductivity, measured in pure water, while the membrane with DABCO-Me showed a strongly increased resistance. On the other hand, a 1 month stability test showed that the through-plane conductivity of the membrane with DABCO-Me in 1 M KOH hardly changed. This indicates that conductivity by absorbed KOH solution is more relevant than the conductivity provided by the quaternary ammonium groups, and promises stable operation in the electrolyser. This is supported by an electrolyser test, where a stable voltage of 1.9 V at 0.25 A cm⁻² was achieved, with no observed change in membrane resistance over a test duration of 100 h (60 °C, 4 M KOH, PGM-free catalyst).