Predicting ion specific capacitances of supercapacitors due to quantum ionic interactions.

Area of Science:

• Physical Chemistry
• Materials Science
• Electrochemistry

Background:

• Ion specificity, or Hofmeister effects, are observed in physical chemistry measurements but are not explained by classical electrostatic theories.
• These theories neglect ionic dispersion forces, which significantly influence ion adsorption and electrochemical behavior.

Purpose of the Study:

• To develop and apply a theoretical framework explaining ion specificity in supercapacitors.
• To incorporate ionic dispersion energies into a modified Poisson-Boltzmann approach for accurate capacitance calculations.

Main Methods:

• Applied ab initio quantum chemical methods to determine ion sizes and polarizabilities.
• Modeled graphite electrodes using optical dielectric spectra.
• Calculated capacitance (C=dσ/dψ) based on predicted electrode surface charge (σ) and potential (ψ).

Main Results:

• Capacitance of positively charged graphite electrodes was enhanced by over 15% compared to purely electrostatic calculations, with PF6(-) showing a >50% increase.
• The Hofmeister series for capacitance enhancement on positive electrodes was PF6⁻ > BF4⁻ > ClO₄⁻ > BrO₄⁻ > IO₄⁻.
• PF6⁻ and BF4⁻ decreased capacitance on negatively charged electrodes by ~15%, while perhalates showed minimal enhancement, highlighting asymmetric interactions.

Conclusions:

• Ionic dispersion forces drive ion adsorption, leading to ion-specific capacitance enhancements.
• The asymmetric impact of these forces results in unequal capacitances for positive and negative electrodes.
• Supercapacitor capacitance should be reported as two values to account for these asymmetric interactions.