Unusual effects of solvent polarity on capacitance for organic electrolytes in a nanoporous electrode

Abstract

The interplay between ions and solvent molecules inside the nanoporous electrodes of a supercapacitor has not been well understood but could be a fertile ground for new insights into the device's performance. By tuning the dipole moment of the solvent in an organic electrolyte, we find, from classical density functional theory calculations, pronounced oscillation of capacitance with the pore size for a moderately to weakly polar solvent. A quantitative analysis of the electric-double-layer (EDL) structure indicates that the capacitance oscillation shares a similar physical origin to that of an ionic liquid electrolyte: the oscillatory behavior arises from the formation of alternating layers of counterions and coions near strongly charged surfaces. More interestingly, we find that in the large-pore region, the capacitance versus the pore size has a volcano-shaped trend; in other words, there exists a solvent dipole moment that yields a maximal capacitance. These theoretical predictions can be validated with future experiments and highlight the great potential in tuning the organic solvent to achieve optimal performance of EDL capacitors.

Capacitive Energy Storage: Current and Future Challenges

“…Jiang and coworkers showed that the presence of a nonionic solvent decreases the extent of capacitance enhancement as compared to RTIL electrolytes. 179,192,196 This might explain the lack of substantial (if any) capacitance enhancement in Centeno et al work mentioned above. 176 found for a variety of RTILs in slit-like nanopores with atomically smooth walls.…”

“…It also appears that the capacitance enhancement for non-ionic solvent based electrolytes can have a complex dependence on the solvent polarity showing a maximum at certain values of solvent dielectric constant 196. This issue certainly needs further and more systematic investigation by both experiments and atomistic simulations.While many molecular simulations showed consistent trends on the capacitance enhancement for RTIL electrolytes inside nanoporous electrodes, the magnitude of the capacitance enhancement reported from most simulations for slit pores would still appear quantitatively in disagreement with experimental data.…”

Polar-solvation classical density-functional theory for electrolyte aqueous solutions near a wall

“…On the other hand, the electrostatic potential used in the present work for describing many biological systems is small compared to that used in other applications including supercapacitors [72]. Broader PSCDFT applications will require the consideration of higher electrolyte concentrations, asymmetric particle sizes, multivalent ions, and more complex geometries of the EDLs.…”

Charging Ultrananoporous Electrodes with Size-Asymmetric Ions Assisted by Apolar Solvent

“…IV A, the presence of solvent at low voltages makes pores effectively more ionophobobic, but otherwise a system with apolar solvent and symmetric ions behaves qualitatively similar to the otherwise same system without solvent (highly polar solvents can have a more dramatic effect on the capacitance, however, see Ref. [37]). Since the latter setup has been extensively studied by our group 18,19 and by others, 6,38-40 we shall not discuss it in detail here.…”