Ionomer boosts catalyst layer oxygen transport and membrane ion conduction for fuel cells

Abstract

Anion exchange ionomers employed as electrode catalyst binders and anion exchange membranes are central components for anion exchange membrane fuel cells. Fast oxygen transport in the catalyst binder and high ion conductivity of the ionomer and membrane are essential while designing their molecular structure. Here, we tailor a fluorinated ionomer and elucidate the effect of fluorination on the properties of catalyst binder and membrane. The extraordinary oxygen-dissolving capacity of the fluorinated ionomer improves the local oxygen transport at the catalyst layer triple-phase boundary. Moreover, fluorination enhances the mechanical stability and chemical inertness of the ionomer membrane and promotes its self-assembly to construct well-defined microphase separated morphology by increasing chain thermodynamic immiscibility. The resulting fluorinated membrane shows 1.4–1.8-fold improvements in hydroxide conductivity and mechanical properties compared to the fluorine-free counterpart, as well as exceptional alkaline stability (over 90% hydroxide conductivity retention under 2 M aq. NaOH at 80 °C for 2000 h). Such synergistic improvements in ionomer binder and membrane significantly improve the single-cell performance (1.7 vs. 1.0 W cm⁻² peak power density) and durability (1.8 vs. 2.4 mV h⁻¹ voltage decline rate for 100 h).