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While most supercapacitors are limited in their performance by the stability of the electrolyte, using neat ionic liquids (ILs) as the electrolyte can expand the voltage window and temperature range of operation. In this study, ILs with bis(trifluoromethylsulfonyl)imide (Tf2N) as the anion were investigated as the electrolyte in onion-like carbon-based electrochemical capacitors. To probe the influence of cations on the electrochemical performance of supercapacitors, three different cations were used: 1-ethyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium and 1,6-bis(3-methylimidazolium-1-yl). A series of electrochemical characterization tests was performed using cyclic voltammetry (CV), galvanostatic cycling and electrochemical impedance spectroscopy (EIS). Diffusion coefficients were measured using EIS and correlated with quasielastic neutron scattering and molecular dynamics simulation. These three techniques were used in parallel to confirm a consistent trend between the three ILs. It was found that the IL with the smaller sized cation had a larger diffusion coefficient, leading to a higher capacitance at faster charge-discharge rates. Furthermore, the IL electrolyte performance was correlated with increasing temperature, which limited the voltage stability window and led to the formation of a solid electrolyte interphase on the carbon electrode surface, evident in both the CV and EIS experiments.

Transport of electrolytes in nanoporous carbon-based electrodes largely defines the function and performance of energy storage devices. Using molecular dynamics simulation and quasielastic neutron scattering, we investigate the microscopic dynamics of a prototypical ionic liquid electrolyte, [emim][Tf2N], under applied electric potential in carbon materials with 6.7 nm and 1.5 nm pores. The simulations demonstrate the formation of dense layers of counter-ions near the charged surfaces, which is reversible when the polarity is reversed. In the experiment, the ions immobilized near the surface manifest themselves in the elastic scattering signal. The experimentally observed ion immobilization near the wall is fully reversible as a function of the applied electric potential in the 6.7 nm, but not in the 1.5 nm nanopores. In the latter case, remarkably, the first application of the electric potential leads to apparently irreversible immobilization of cations or anions, depending on the polarity, near the carbon pore walls. This unexpectedly demonstrates that in carbon electrode materials with the small pores, which are optimal for energy storage applications, the polarity of the electrical potential applied for the first time after the introduction of an ionic liquid electrolyte may define the decoration of the small pore walls with ions for prolonged periods of time and possibly for the lifetime of the electrode. [ABSTRACT FROM AUTHOR]

Abstract: The diffusion of methane confined in nano-porous carbon aerogel with the average pore size 48Å and porosity ∼60% was investigated as a function of pressure at T =298K using quasi-elastic neutron scattering (QENS). The diffusivity of methane shows a clear effect of confinement: it is about two orders of magnitude lower than in bulk at the same thermodynamic conditions and is close to the diffusivity of liquid methane at 100K (i.e. ∼90K below the liquid–gas critical temperature T C ≈191K). The diffusion coefficient (D) of methane initially increases with pressure by a factor of ∼2.5 from 3.47±0.41×10−10 m2 s−1 at 0.482MPa to D =8.55±0.33×10−10 m2 s−1 at 2.75MPa and starts to decrease at higher pressures. An explanation of the observed non-monotonic behavior of the diffusivity in the confined fluid is based on the results of small-angle neutron scattering experiments of the phase behavior of methane in a similar carbon aerogel sample. The initial increase of the diffusion coefficient with pressure is explained as due to progressive filling of bigger pores in which molecular mobility in the internal pore volume is less affected by the sluggish liquid-like molecular mobility in the adsorbed phase. Subsequent decrease of D, is associated with the effect of intermolecular collisions, which result in a lower total molecular mobility with pressure, as in the bulk state. The results are compared with the available QENS data on the methane diffusivity in zeolites, metal organic frameworks, and porous silica as well as with the molecular dynamics simulations of methane in nano-porous carbons and silica zeolites. [Copyright &y& Elsevier]

We used quasielastic neutron scattering (QENS) to studythe dynamicsof phenanthrenequinone (PQ) on the surface of onion-like carbon (OLC),or so-called carbon onions, as a function of surface coverage andtemperature. For both the high- and low-coverage samples, we observedtwo diffusion processes; a faster process and nearly an order of magnitudeslower process. On the high-coverage surface, the slow diffusion processis of long-range translational character, whereas the fast diffusionprocess is spatially localized on the length scale of ∼4.7Å. On the low-coverage surface, both diffusion processes arespatially localized; on the same length scale of ∼4.7 Åfor the fast diffusion and a somewhat larger length scale for theslow diffusion. Arrhenius temperature dependence is observed exceptfor the long-range diffusion on the high-coverage surface. We attributethe fast diffusion process to the generic localized in-cage dynamicsof PQ molecules, and the slow diffusion process to the long-rangetranslational dynamics of PQ molecules, which, depending on the coverage,may be either spatially restricted or long-range. On the low-coveragesurface, uniform surface coverage is not attained, and the PQ moleculesexperience the effect of spatial constraints on their long-range translationaldynamics. Unexpectedly, the dynamics of PQ molecules on OLC as a functionof temperature and surface coverage bears qualitative resemblanceto the dynamics of water molecules on oxide surfaces, including practicallytemperature-independent residence times for the low-coverage surface.The dynamics features that we observed may be universal across differentclasses of surface adsorbates. [ABSTRACT FROM AUTHOR]

We investigate the extent to which the alkyl chain on the cation of an imidazolium-based neat room-temperature ionic liquid influences mobility and electrochemical behavior in nanoporous supercapacitors. Changing the cation chain length from an ethyl (n = 2) to a butyl (n = 4) to a hexyl (n = 6) group affects the electrolyte dynamics and their accumulation densities under dynamic charge-discharge processes. We relied on molecular dynamics (MD) computational simulations and classical density functional theory (cDFT) calculations of our system to reinforce the experimental results obtained from electrochemical measurements and quasi-elastic neutron scattering (QENS). We contrast the different dynamics of ionic liquids in bulk and confined states and demonstrate the effect of the cation dimension on resulting arrangements of positive and negative ions in pores. We correlate these fundamental properties with device performance metrics in an effort to properly tailor high-performance carbon supercapacitor electrodes with non-flammable and electrochemically stable electrolytes. [ABSTRACT FROM AUTHOR]

Quasielastic neutron scattering of H2 and D2 in the same nanoporous carbon at 10-40 K demonstrates extreme quantum sieving, with D2 diffusing up to 76 times faster. D2 also shows liquidlike diffusion while H2 exhibits Chudley-Elliott jump diffusion, evidence of their different relationships with the local lattice of adsorption sites due to quantum effects on intermolecular interactions. The onset of diffusion occurs at 22-25 K for H2and 10-13 K for D2. At these temperatures, H2 and D2 have identical thermal de Broglie wavelengths that correlate with the dominant pore size. [ABSTRACT FROM AUTHOR]