The kinetics of thermal processes in imidazole-doped nanocrystalline cellulose solid proton conductor

Abstract

Proton conductors play an important role amongst ionically conducting systems. Being a common phenomenon in liquids, the protonic conduction in solids is rather rare and suffers from low values. However, due to potential applications in modern devices, e.g., fuel cells, sensors, or solid-state-high-density batteries, there is much interest in proton-conducting solids. To make progress on that field, newly synthesized materials must show increased performance in terms of conductivity, thermal stability, and non-toxicity. The unremitting pressure for increasing the protonic conductivity in solids caused the majority of the research is devoted to this aspect. The environment impact of new materials can be controlled by appropriate chemical composition. Therefore, during the synthesis process, components that are neutral to the habitat are used. Here, the chemical composition of nanocomposite solid-state proton conductor under investigation is based on cellulose nanocrystals and imidazole molecules that provide a neutral influence on the environment. The conductive properties of the proton conductor under consideration were the subject of our previous work. In this paper, we are going to investigate the thermal properties and kinetics of thermal processes acting in the proposed nanocomposite proton conductor to determine its thermal stability and application potential. The combined experimental approach of thermal gravimetric analysis and differential scanning calorimetry was used. Based on the obtained results, the activation energies of the decomposition stages were determined. The evolution of the thermal processes with the conversion degree was studied, and the lifetime of the nanocomposite at various external thermic conditions was tested. The kinetic DSC study has confirmed that the imidazole molecules are attached to the glucose ring of the cellulose in two ways, indirectly through the residual water molecules and directly to the hydroxyl group of the cellulose. Obtained results have shown that the stability and durability on the thermal conditions of the compound are still not satisfactory for commercial applications. However, proposed material can work under anhydrous conditions, thus can constitute a possible substitution for materials that need hydrated conditions to work, like Nafion.