The Hydroxide Conductivity and Chemical Stability of a Polymerized Ionic Liquid Diblock Copolymer for Alkaline Fuel Cells Using Rotating Disk Electrode

Currently, minimal effort has been devoted to the characterization of ionic transport in alkaline exchange polymers when integrated into the catalyst layer as the ionomer for alkaline exchange membrane fuel cells (AEMFCs). The interface between ionomer and catalyst greatly affects AEMFC performance and therefore a detailed understanding of this interface is invaluable for the optimization of the AEMFC catalyst layer morphology. In this study, a polymerized ionic liquid (PIL) diblock copolymer, poly(MMA-b-MUBIm-OH), comprised of an IL component (1-[(2-methacryloyloxy)undecyl]-3-butylimidazolium hydroxide) (MUBIm-OH) and a non-ionic component (MMA) was used to understand hydroxide anion transport in a thin film configuration as an analogue to the ionomer/catalyst interface in an AEMFC catalyst layer. Rotating disk electrode (RDE) experiments in hydroxide-poor alkaline electrolytes (pH = 10 - 12) were used to measure hydroxide anion transport through thin films of the PIL diblock copolymer interfaced with polycrystalline platinum (Pt). Diffusion limited currents of the oxygen evolution reaction (OER) at varied rotation rates were lineally extrapolated to determine numerical values of the hydroxide diffusivity. This technique also provides a unique opportunity to study the chemical and electrochemical stability of AEMs under applied potential in contrast to standard methods (i.e., high pH chemical degradation), where potential control is not feasible. The electrochemical stability of the imidazolium-based polymer was studied at high alkaline conditions (pH > 13) at various temperatures (23 - 60 °C) and applied potential within the operational range of AEMFCs (i.e., V = 0.6 - 1.2 V) using cyclic voltammetry and tracking the hydroxide conductivity over time. The results presented here provide important insight into the ionic transport and electrochemical stability of alkaline exchange ionomers interfaced with metal electrocatalysts, which would prove instrumental for the optimization of the AEMFC catalyst layer morphology.