Abstract
The influence of spatial confinement on the thermally excited stochastic cation dynamics of the room-temperature ionic liquid 1-N-butylpyridinium bis-((trifluoromethyl)sulfonyl)imide ([BuPy][Tf_2N]) inside porous carbide-derived carbons with various pore sizes in the sub- to a few nanometer range are investigated by quasi-elastic neutron spectroscopy. Using the potential of fixed window scans, i.e. scanning a sample parameter, while observing solely one specific energy transfer value, an overview of the dynamic landscape within a wide temperature range is obtained. It is shown that already these data provide a quite comprehensive understanding of the confinement-induced alteration of the molecular mobility in comparison to the bulk. A complementary, more detailed analysis of full energy transfer spectra at selected temperatures reveals two translational diffusive processes on different time scales. Both are considerably slower than in the bulk liquid and show a decrease of the respective self-diffusion coefficients with decreasing nanopore size. Different thermal activation energies for molecular self-diffusion in nanoporous carbons with similar pore size indicate the importance of pore morphology on the molecular mobility, beyond the pure degree of confinement. In spite of the dynamic slowing down we can show that the temperature range of the liquid state upon nanoconfinement is remarkably extended to much lower temperatures, which is beneficial for potential technical applications of such systems.
Highlights
Room-temperature ionic liquid electrolytes in combination with porous carbon electrodes are promising candidates for electric double layer capacitors (EDLCs), which are commonly referred to as super- or ultracapacitors [1,2,3,4,5]
Nanoconfinement of ionic liquids is known to change the phase transition behavior [31,32]. To address these fundamental issues, we present a quasielastic neutron scattering investigation on the selfdiffusion dynamics of a room-temperature ionic liquid confined in the nanopores of carbide-derived carbons (CDCs) as a function of pore size over a wide temperature range
A further qualitative overview of the temperature-dependent activation of dynamic processes is given by the averaged intensity over the Q range between 0.44 Å−1 and 1.90 Å−1, that is depicted in the back panel of Fig. 3, as well as in Fig. 4 for the bulk [BuPy][Tf2N] and confined inside the nanopores of the CDCs with different pore sizes
Summary
Room-temperature ionic liquid electrolytes in combination with porous carbon electrodes are promising candidates for electric double layer capacitors (EDLCs), which are commonly referred to as super- or ultracapacitors [1,2,3,4,5] These capacitors benefit from high energy densities on a par with those of batteries along with the superior power densities of conventional capacitors and offer efficient electric energy storage and conversion systems for a variety of applications. Nanoporous carbons distinguish themselves by a welltunable pore size, shape, and surface chemistry They possess a high electrical conductivity, good (electro-) chemical stability, and a large specific surface area, making them well-suited electrode materials [9]. These systems are a very active field of research, the actual technical implementation as high-performance supercapacitors remains
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