Cobaltocenium-containing polybenzimidazole polymers for alkaline anion exchange membrane applications

Abstract

A polybenzimidazole, containing cobaltocenium on its backbones, was used for anion exchange membranes (AEMs) for the first time. The polymer was synthesized by polymerizing 1,1′-dicarboxycobaltocenium and 3,3′,4,4′-biphenyltetramine in a microwave reactor. Before the polymer fabrication, we studied the alkaline stability of three different cobaltocenium cations—cobaltocenium, 1,1′-dimethylcobaltocenium and 1,1′-dicarboxycobaltocenium—by ¹HNMR and ¹³CNMR spectroscopy and investigated the degradation mechanisms of these cations under alkaline conditions. Then the three corresponding cobaltocenium-containing polybenzimidazole membranes were synthesized, and the relationship between the structure and performance of these cobaltocenium-containing polybenzimidazole membranes was investigated by ¹HNMR spectroscopy, FTIR spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and AC impedance spectrascopy. These AEMs, based on cobaltocenium-containing polybenzimidazole backbones, show high thermal stabilities, good chemical stabilities, comparable hydroxide conductivities, low swelling ratios and good mechanical properties. The 1,1′-cobaltocenium-5,5′-(2,2′-dimethyl)-bibenzimidazole (MCp₂Co⁺OH⁻-PBI) membrane shows the best comprehensive performance in this study. The hydroxide conductivity of the MCp₂Co⁺OH⁻-PBI membrane at 90 °C can reach 37.5 mS cm⁻¹ with a low swelling ratio. Furthermore, the degradation mechanism of the MCp₂Co⁺OH⁻-PBI membrane under alkaline conditions was investigated by ¹HNMR spectroscopy. In summary, our work investigates the degradation mechanisms of the cobaltocenium cations and cobaltocenium-containing polybenzimidazole under alkaline conditions and presents a novel polymer structure for AEM applications.