Meta-kinks are key to binder performance of poly(arylene piperidinium) ionomers for alkaline membrane water electrolysis using non-noble metal catalysts†

Abstract

Anion-exchange membrane water electrolysis (AEMWE) is a key technology for the production of green hydrogen at high current densities without the necessity of noble metal catalysts. AEMWE technology does not only rely on chemically stable and highly hydroxide-conducting membranes, but also on ionomer binders, to which additional criteria apply related to swelling, mechanical properties, gas permeability and porosity to form a triple phase boundary with catalyst particles on top of an membrane electrode assembly (MEA). Here, we investigate seven poly(arylene piperidinium)s (PAPs) with different ratios of meta-/para-terphenyl building blocks as binders for non-noble NiFe-LDH catalysts. We first analyze the materials comprehensively in pristine form and subsequently as binders. With increasing content of meta-terphenyl, specific surface area, water uptake, swelling ratio and ion-conductivity increase continuously, with the latter ranging from 145 to 216 mS cm⁻¹ at 80 °C. We elucidate binder performance from rotating disk electrode experiments of oxygen evolution reactions (OER) catalysed by nickel–iron layered double hydroxides (NiFe-LDH) under AEMWE working potentials. Here, an increasing content of meta-kinks leads to improved catalyst utilization, superior OER performance and improved electrode stability. Finally, AEMWE single cell tests show a strong improvement in current density when altering binders from exclusively para- to meta-terphenyl in the polymer backbone. Current densities as high as 1000 to 1700 mA cm⁻² at 1.8 V and 3000 mA cm⁻² at 2.0 V are measured for the binder with exclusive meta-terphenyl kinks. The results highlight the role of the binder for AEMWE performance as well as the importance of its individual optimization aside from membrane properties.