Researchers from HSE MIEM, in collaboration with colleagues at the RAS Institute of Solution Chemistry, have modelled the behaviour of ionic liquids within charged carbon nanopores ranging in width from 1 to 15 nm and assessed the mobility of both their cations and anions. The scientists observed that an increase in anion size resulted in higher mobility, whereas cations exhibited the opposite trend of reduced mobility with an increase in size. A better understanding of ionic liquids will enhance their use in supercapacitor technology. The study has been published in Journal of Molecular Liquids and supported by a grant from the Russian Science Foundation (RSF).
In its solid state, salt exhibits a structure in which its charged particles (ions) are arranged in a crystalline lattice. However, when salt, such as table salt, is heated to a temperature of around 800°C, its crystal lattice disintegrates, allowing the ions to move freely and ultimately transforming the solid crystals into a liquid state. Referred to as ionic liquids, such salts hold promise as an alternative to electrolyte solutions in supercapacitors, which are electrochemical devices used for energy storage.
The advantage of using ionic liquids lies in the heightened mobility of their ions compared to solid electrolytes with a crystalline lattice. Compared to solutions, ionic liquids also have a greater ion concentration, because their ions are not separated by the solvent’s molecules. Furthermore, owing to their structural characteristics, ionic liquids maintain a liquid state even at low temperatures, which is crucial for the manufacture of supercapacitors used in digital communication devices, consumer electronics, hybrid electric vehicles, and other applications.
Ionic liquids are employed as electrolytes in conjunction with porous electrodes. To use them effectively, it is essential to understand and consider the structural and electrical properties they manifest within the small spaces of the electrode pores. A team of Russian scientists modelled the behaviour of four ionic liquids to clarify the influence of ion substitution on the liquids' properties. The simulation was performed in nanoscale pores with charged carbon walls measuring between 1 and 15 nm, using [EMIM]+ and [OMIM]+ cations as well as [BF4]- and [NTf2]-anions.
The researchers investigated the impact of ion substitution on the diffusion coefficient, which characterises ion mobility: a higher diffusion coefficient corresponds to increased mobility. When ion mobility is high, the ionic liquid becomes a viable medium for efficient charge transfer.
Measuring the diffusion coefficient of an ionic liquid within a nanopore is not possible through experimental means. However, by modelling the dynamics of molecules and even atoms interacting with each other, we can 'peek' into the processes occurring within the substance. We can conduct a 'virtual experiment' by solving Newton's equations for each individual atom and molecule. Subsequently, we can employ statistical physics methods to compute the properties of interest, such as diffusion and electrical conductivity coefficients, pair correlation functions, angular distributions, and more.
As anticipated by the scientists, the diffusion coefficient of the [EMIM]+ cation, a positively charged ion, was found to be greater than that of the [OMIM]+ cation, which exhibited reduced mobility due to the lengthy alkyl chain constraining its movement. The opposite was observed when modelling negatively charged ions: as the anion's size increased, its mobility also increased. The scientists suggest that smaller anions may exhibit a stronger binding to the cation, causing a decrease in their mobility.
This concept can be visualised using the analogy of two charged spheres, one large and the other small. If we distribute the same amount of charge over the surface, the charge density on the smaller sphere will be higher. Hence, a smaller anion will form stronger bonds with the charged pore walls and cations. Consequently, a reduction in size leads to a decrease in mobility.
The diffusion coefficients of cations were typically observed to be higher than those of anions, even when the cation had a larger radius and molar mass. Thus, cations are more effective at carrying charge than anions, regardless of their size. The liquid [EMIM][NTf2], with ions displaying higher diffusion coefficients, demonstrated the highest electrical conductivity. At the same time, despite having lower ion diffusion coefficients, [EMIM][BF4] exhibited higher electrical conductivity when compared to [OMIM][NTf2]. The scientists attribute this to an elevated concentration of charge carriers.
Полученные данные интересны для электрохимических приложений, которые используют при разработке суперконденсаторов. Сейчас мы изучили основные особенности влияния химической структуры катиона и природы аниона на транспортные свойства ионных жидкостей в условиях ограниченной геометрии. В будущем мы планируем также рассмотреть новые ионные жидкости с примесями различных растворителей и дополнительно вычислить не изученный в этом исследовании параметр вязкости.