In situ amination of anion conducting solid polymer electrolyte membranes

Abstract

Hydrogen is a viable option for storage and on-spot generation of energy. Alkaline electrolyzers and fuel cells have several advantages over acidic counterparts such as simple fabrication, non-precious metal catalysts and low crossover. It has been shown that crosslinked anion–exchange membranes synthesized by vinyl addition polymerization of norbornene show excellent performance in alkaline electrochemical devices. However, a long reaction time is needed for converting the tethered bromoalkyl moiety in the polymer to a quaternary ammonium head-group because a tertiary amine has to diffuse into the polymer. This amination process is not compatible with the roll-to-roll membrane formation process. In this study, anion exchange membranes have been prepared by in situ amination of the functionalized polymer during membrane casting. The polymers used in this study were also in situ crosslinked with N,N,N′,N′-tetramethyl-1,6-hexanediamine during membrane casting to prevent excessive water uptake. The in situ amination and cross-linking processes were achieved by changing reaction rates through a change in casting solvent and reaction temperature. Precisely controlling the reaction time made it possible to directly cast quaternized membranes on a roll-to-roll timescale, thus avoiding the need for the long-duration, ex situ amination step. Membranes having high ion exchange capacity (3.7 meq g⁻¹) and high ionic conductivity (72 mS cm⁻¹ at room temperature) were prepared using this process. Alkaline electrolyzer performance with these in situ aminated membranes showed comparable performance to membranes prepared by the conventional, ex situ amination method. Finally, the alkaline stability of the membranes was evaluated, and they showed low degradation after ex situ aging in 1.25 M KOH at 80 °C for more than 500 h.