Understanding the infrared spectrum of the protic ionic liquid [DEMA][TfO] by atomistic simulations

Abstract

Polymer-electrolyte fuel cells operating at a temperature above 100 °C would markedly reduce issues associated with water management in the cell and allow for a simplified system design. Available electrolytes such as fluoropolymers grafted with sulfonic acid groups or phosphoric acid either rely on the presence of water or they suffer from sluggish kinetics of the oxygen reduction reaction. Here, with experiments and atomistic simulations, we analysed vibrational spectra of the protic ionic liquid diethylmethylammonium triflate ([DEMA][TfO]) as an alternative electrolyte, with the aim to understand the statistical distribution of cations and anions in the electrolyte and the interaction of the H-bond with the surroundings. We present a comprehensive analysis of the infrared (IR) spectrum of [DEMA][TfO]. Special attention is given to understanding the high-frequency modes above 2500 cm⁻¹, which exhibit a double peak feature in the experiment. While this feature can generally be attributed to the N–H vibrations of the cation, the precise mechanism behind the double peak was unclear. In this manuscript we managed to explain the nature of the double distribution, being influenced by different orientations between the DEMAs and TFOs. The correct assignment of observed vibrational modes is enabled by simulations of the ionic liquid as an infinitely extended fluid.