Charge Transport, Dielectric Relaxations, and Phase Transitions in Ammonium-based Protic Ionic Liquids

Over the past few decades, protic ionic liquids (PILs), a highly promising class of materials, have garnered significant interest owing to their distinctive physicochemical properties and promising applications. As alternatives to conventional solvents and electrolytes, PILs exhibit low volatility, high thermal stability, and excellent ionic conductivity, making them ideal candidates for a variety of electrochemical applications. In this work, widely studied ammonium-based PILs were chosen as the research focus due to their exceptional properties such as high ionic conductivity. Most previous reports on ammonium-based PILs have overlooked the impact of water and have been limited to a narrow temperature range and small cations with carbon chains fewer than four atoms. Additionally, while the majority of previous research has focused on basic property reports and practical applications, the fundamental mechanisms underlying charge transport, dielectric relaxations, and phase behavior have rarely been studied. As designable solvents, ammonium-based PILs are closely related to their constituents, which can be modified by varying cations or anions, adding excess acids, and substituting H (hydrogen) with D (deuterium) ions. The application of these methods has produced various states of matter, such as ordered crystalline, plastic crystalline, and glassy states, which serve as research platforms for fundamental questions. In this work, particular attention is given on investigating charge transport mechanisms, identifying the origins of relaxation processes, and studying phase behavior over a wide temperature range using various techniques, primarily broadband dielectric spectroscopy (BDS), differential scanning calorimetry (DSC), and X-Ray Diffraction (XRD). In glass-forming liquids, as the temperature decreases, isotropic liquids transform to supercooled liquids, finally entering the glassy state, with the proton hopping mechanism becoming the dominating charge transport over vehicle transport just above and below the glass transition temperature. Here, the VFT dependence of the DC conductivity reflects the temperature dependence of the charge carrier mobility, while the number density of mobile charges remains relatively insensitive to temperature. The low fraction of mobile charge carriers arises from charge immobilization within molecular aggregates. In contrast, in plastic crystals, proton hopping is the dominant mechanism, as verified by deuterium effects and temperature-dependent conductivity. In most materials, the “conduction–free” dielectric loss reveals two distinct relaxation processes, denoted as slow relaxations and fast relaxations. For PILs with diethylmethylammonium or triethylammonium cations, the fast relaxation has been identified as conductivity relaxation, whereas the slow process can be associated with the dynamic glass transition. In contrast, in PILs containing trioctylammonium cations, the fast process corresponds to the structural relaxation, which is coupled to the crossover from sub-diffusive to diffusive charge transport. The slow process, in turn, is associated with much slower charge polarization dynamics occurring on larger length and time scales, due to the aggregation of non-polar carbon chains. Furthermore, plastic crystals often show multiple phase transitions. BDS is also highly sensitive to phase transitions and the temperature coefficient of the permittivity allows to precisely determine the phase transition temperatures in plastic crystals, which can be confirmed by complementary techniques such as XRD and DSC. Another quantity, the temperature dependent apparent activation energy of permittivity can be used to determine the thermal glass transition temperature in glass forming liquids. These fundamental studies deepen our understanding of PILs at the molecular level. By clarifying mechanisms of charge transport, relaxation dynamics, and phase transitions, this research provides both theoretical knowledge and a guide for material choices for practical applications. The acquired insights are crucial for developing novel PILs tailored for specific applications, ultimately paving the way for innovations that optimize their performance in various technological fields.